CHAPTER ONE 1.0 Introduction In our society today, there is an obvious gap between the academia and the industry and this leads to Universities producing what employers refer to as “half-baked graduates”. There is no bridge between the theory that a student learns in the classroom and the industry which he aspires to belong to, as a result he has no hands on skill, no practical knowledge on how to apply what he has learnt and no true knowledge of how the industry works. This leads to a reduced efficiency and productivity from him which is the exact opposite of what an employer expects to get from an employee. As a result, the Federal Government deemed it fit to create theIndustrial Training Fund (ITF), an agency charged with the responsibility of bridging that gap. The ITF in turn introduced the Students Industrial Work Experience Scheme (SIWES) commonly referred to as the Industrial Training program. A program that not only exposes students to the industry so as to gain some practical experience before graduation but also serves as an opportunity to learn about workplace ethics, safety standards of the industry, interpersonal relationship between colleagues, team work but to mention a few. It also gives the student an opportunity to create a link/network with professionals to further ease his search when he graduates. The program was introduced in 1973 with its primary focus being technical disciplines such as engineering, agriculture, some science courses, fine arts and so on with the duration of the training program ranging from six months to one year depending on the institution i.e. polytechnic or university. Though the scheme has undergone some challenges such as funding, supervising, lack of cooperation from some companies, its primary aim still remains to harmonize the theoretical knowledge gained from the University with the actual industrial practice thereby creating better prepared graduates. Also since the training exposes the student to the challenges faced in the industries he is in a better position to apply all that knowledge to create solutions to the problems leading to more practicable innovations and in the long run more technological advances in the nation.
`1
1.1 History of Baker Hughes Incorporated Baker Hughes Incorporated is the product of the 1987 merger of two oilfield-services companies with surprisingly similar histories, Baker Oil Tools and Hughes Tool Company. Both were founded shortly before World War I by aggressive entrepreneurs who won valuable patents. Both embarked on massive worldwide expansion and diversification projects. Baker and Hughes became public companies within ten years of each other. The two rivals experienced the fluctuations of an unpredictable world oil market jarred by political and economic events. Finally, the companies suffered financial slumps in the lean years of the 1980s, leading to their turbulent but successful consolidation and the name Baker Hughes Incorporated. Since then, BHI has acquired and assimilated numerous oilfield pioneers that have developed and introduced technology to serve the petroleum service industry. They include Brown oil tools; Elder and Elder oil tools (completions); Milchem and Newpark (drilling fluids); Exlog (well logging); Eastman, Christensen and Drilex (directional drilling diamond drillbits); Teleco (measurement while drilling); Tri-state and Wilson (fishing tools and services); Aquaness, Chemlink and Petrolite(specialty chemicals), Western Atlas (seismic exploration, well logging); BJ Services Company (pressure pumping). Some acquisitions date back to the early nineteenth century and some are relatively more recent though most of the companies have a combined history dating back to the 1900s. Baker Hughes is one of the world's largest oilfield services companies. It operates in over 80 countries and employs over 58,000-plus employees, in fact it was one of the first oil servicing companies to take residence in Ghana after its discovery of oil in 2008. Baker Hughes Inc. has its headquarters in the America Tower in the America General, Neartown. Houston. It operates world-wide with major offices in Liverpool, United Kingdom, Singapore and Dubai, research and maintenance facilities in Celle, Germany, Lafayette, Louisiana, Houston, Texas, Pescara, Kuala lumpur, and Malaysia. The company is administered in two hemispheres, the eastern hemisphere with five Regions (Europe, Africa, Middle East, Asia, Pacific and Russia) and the Western hemisphere with four Regions (Canada, US Land, US Gulf, and Latin America. These regions are further divided into about nineteen geo-markets to help understand and cater for location specific needs and further increase efficiency and productivity.
`2
Baker Hughes offers a wide variety of oilfield services including drilling and evaluation, completions and production, pressure pumping, reservoir development, drilling fluids, integrated operations, tubular services, process and pipeline services, mud logging, chemical and industrial services, specialty chemicals and so on. BHI is one of the world leaders in the field of oilfield servicing. This is not unconnected to the fact that they work to lower costs, reduce HS&E and economic risk, improve productivity, and increase ultimate recovery for their clients. Major competitors are Schlumberger Limited, Halliburton Company, Smith International, Inc., Weatherford International Ltd., Precision Drilling Corporation, Veritas DGC Inc. etc. Of course the indigenous companies are not lagging behind especially with the increased demand for local content; companies such as Geoplex, Greatwall Nig Ltd. BJ Services is a subsidiary of Baker Hughes that provides pressure pumping services for the petroleum industry; it is one of the more recent acquisitions of Baker Hughes. Pressure pumping services consist of cementing and stimulation services used in the completion of new oil and natural gas wells and in remedial work on existing wells, both onshore and offshore. Oilfield services include casing and tubular services, coiled tubing services, sand control services, water management, production chemical services, and pre commissioning, maintenance and turnaround services in the pipeline and process business, including pipeline inspection. It is no doubt an invaluable addition to the Baker Hughes family.
`3
1.2 Company’s Organizational Chart BOARD OF DIRECTORS 13 DIRECTORS INCLUDING COB
CHAIRMAN OF THE BOARD
PRESIDENT AND CEO
VP: GROUP PRESIDENT D&E VICE PRESIDENT: SALES AND MARKETING, AMERICAS
PRESIDENT: BAKER ATLAS PRESIDENT: HUGHES CHRISTENSEN PRESIDENT: BAKER HUGHES DRILLING FLUIDS
VICE PRESIDENT: CENTRILIFT
CHIEF INFORMATION OFFICER
VP: BAKER PETROLITE
PRESIDENT: GLOBAL PRODUCTS AND SERVICES
VICE PRESIDENT: BJ SERVICES (PRESSURE PUMPING)
PRESIDENT: BAKER OIL TOOLS VICE PRESIDENT: HUMAN RESOURCES
COILED TUBING
CEMENTING
SVP&CHIEF FINANCIAL OFFICER VICE PRESIDENT: TAX VICE PRESIDENT& TREASURER
SVP&GENERAL COUNSEL CORPORATE SECETARY VP: CHIEF COMPLIANCE OFFICEER
VICE PRESIDENT& CONTROLLER
VP: CORPORATE DEVELOPMENT
VP: INTERNAL AUDIT VP: INVESTOR RELATIONS
CEMENTING LABORATORY
`4
1.3 Various Departments and their functions Baker Hughes Inc. is a very large company that cuts across nations, provides employment for millions and provides basically all the oilfield products and services ranging from drilling to completions and production. As a result, it requires a lot of departments in order to run efficiently and keep its standards of service; these departments include 1.
Administration Department
The role of this department is to process and properly file all documents for the rest of the company for future purposes. They are also responsible for looking after the internal communications. 2.
Legal Department
They handle legal issues which may come up in the course of business and offer legal advice on issues pertaining to the company. They also organize trainings and provide employee manuals so as to reduce the risk of potential suits. It consists of qualified lawyers and their legal assistants. 3.
Human Resource Department
This department deals with the management of people within the company. It is responsible for hiring staff, monitoring them and in necessary cases, firing staff. It is also responsible for enforcing company policies. It consists of the human resource manager and his assistants 4.
Accounting Department
This department is charged with the responsibility of looking after the finances of the company. They control money inflow and outflow. They also recommend budgets and estimates to the company. It consists of a team of qualified accountants. 5.
Health, Safety and Environmental (HS&E) Department
Their job is to ensure and enforce safety compliance in all activities performed in the company. This includes safety to human life, company property and the environment. They organize monthly meetings& trainings. It consists of the safety manager and his assistants.
`5
6.
Quality Assurance/ Quality Control Department
This department is in charge of ensuring that products and practices meet up with the accepted industry standards. They organize trainings for employees from time to time. It consists of the quality control officer. 7.
Engineering Department
This department consists of the field engineers and the technical support engineers. Their responsibilities include preparing the design of jobs and going to the field to implement them. They are engaged in onshore and offshore rig work. Vital to this department is the laboratory as they run tests prior to the actual job to ensure accurate results on the field. In the pressure pumping base, the engineering department consists of the cementing engineers, coiled tubing engineers, engineering trainees and the laboratory technicians. 8.
Operations Department
Members of this department are mostly involved in actual operations of the tools on the field and in warehouse activities. They are most times at the field running jobs awarded to the company. It consists of field specialists, operators and trainee engineers. 9.
Maintenance Department
They are responsible for maintenance of tools and equipment on and off the base. They are also charged with the maintenance of every asset of the company. It consists of the maintenance manager and his team. 10.
Research and development Department
Members of this department are charged with innovation of new ideas practicable in the oil and gas industry. The industry is a very dynamic one and as such companies that hope to stay in the game must continually research to develop new technology. It consists of the RDD technologists. 11.
Security Department
As the name implies, they are in charge of securing all company staff and property.
`6
CHAPTER TWO 2.0 Introduction to Cementing The process of drilling a well in order to produce oil and gas is a very delicate one that involves many different operations; each vital in its own unique way, each key to the overall process, every one of them coming together like a jigsaw puzzle to form a perfect picture. Cementing is one of such processes; it involves pumping liquid cement slurry into the annulus between the casing and the wellbore using surface and subsurface equipment and allowing it some time to harden and attain sufficient compressive strength. This operation is vital to the overall well completion because it effectively seals the annulus between the casing string and the drilled hole thereby providing zonal isolation i.e. to exclude fluids (say oil, gas or water) in one zone from fluids in another zone. It also aids in corrosion control and stabilizes the formation and improves pipe strength, as a result if a good cement bond is not attained, further drilling is not possible until the error has been rectified. This operation is done after running in the casing and afterwards, a Cement Bond Log (CBL) is run to evaluate the job and check for channeling; drilling commences immediately or in the case of production casing, completion begins. After drilling the well to its desired depth, the drill pipe is pulled out and a bigger pipe called casing is run into the well. This poses a challenge of how to hold the casing string to the wellbore; then comes the cement. Since there is still drilling mud in the well, a spacer i.e. a fluid compatible with both cement and mud is circulated into the hole after which the cement is then pumped in, a displacement fluid typically follows the cement. The spacer serves to prevent contamination of the cement by the mud or vice versa as they are incompatible. Once there is an indication at the surface that all the cement slurry has entered the annulus, the well is shut in for some time to allow the cement to harden. A number of equipment are used in this process such as the top and bottom plugs, float collar, guide shoe, centralizer ,pumps, jet mixers, batch mixers and so on. The cement slurry used is a mixture of cement, water and additives and its properties and behavior depends on the additives used. Most cement used in the industry are a type of Portland cement and though they are of different classes, class G is most commonly used. The process of cementing is largely dependent on time as wrong timing could lead to flash setting and NPT or recirculating out of hole in cases of cement not setting and NPT.
`7
2.1 Types of Cementing There are two major types of cementing. They are: 1. Primary Cementing Primary Cementing is the introduction or placement of a cement sheath in the annulus between a casing and an open hole or between a casing and a previous casing. It is done on the different casing types – surface, intermediate, production, liner for slightly varying reasons but generally, it is done to provide zonal isolation, protect and support the casing and serve as a support for the borehole. Typically a single slurry designed to specification is used for primary cementing but at times, as a cost saving technique the lead and tail cement slurry is used. Lead Cementing A Lead cement slurry is so called because it is the initial slurry pumped through the casing. It falls at the top of the annulus. It is usually a low quality and low density slurry. Also it has a low cement to water ratio. Extenders are added to lead cement to aid in gelling and increase their yield i.e. increasing the depth of casing with which a limited amount of slurry can be used. It is most times used to decrease cost while still achieving the intended objective. Tail Cementing A tail slurry is the slurry pumped after the lead, thus it ends up cementing the bottom part of the annulus around the casing shoe. This part of the casing requires a firm bonding with the formation, and thus the quality of slurry used is usually high. The tail slurry is usually of higher quality and higher density than the lead slurry and has a high cement to water ratio. Extenders are rarely used with tail slurries as there is really no use. The truth is every cementing engineer hopes to do the primary cementing once and get it right because if not he has to spend more money and incur more downtime or Non Productive Time (NPT) trying to solve the problem. This brings about the second type of cementing.
`8
2. Secondary Cementing This includes i.
Remedial/Squeeze Cementing
As the name implies, the remedial cementing commonly is done to rectify errors from the primary cementing. After primary cementing, a Cement Bond Log (CBL) is usually done to evaluate cement bonding and to check for channels in the cement. If it is found that a section of the well is not well cemented, a squeeze cementing job is carried out. This is done by squeezing cement slurry under pressure through perforations made in the casing at the exact problem spot/hollow section. ii.
Plug Cementing
Plug cementing is the general term for all the cementing processes that require sealing/ plugging the actual wellbore. Plug cementing is of various types depending on the reason for plugging; Plug and Abandon This is done for two main reasons. Either because a new well is found to be unproductive or because a producing well has been depleted sufficiently and is now deemed unproductive. Either ways, the specified length of slurry is pumped in at different intervals to effectively block the well. Plug Kick-off / Side-track This is also done for two main reasons. Either in directional wells to serve as kick off point for deviation or in a case where the original well path of a vertical well can no longer be accessed due to certain problems in the hole like stuck pipe, salt dome and so on and the obstruction must be bypassed, it serves as a kick off point for the sidetrack. iii.
Top up cementing
This is done when the slurry pumped into well stops a few feet short of the desired depth due to unforeseen circumstances. In this case, more cement is mixed and poured directly into the annulus to top up the cementing. `9
2.3 Cement testing Prior to the actual cementing job, laboratory pilot tests are carried out with representative samples of the actual field products – cement, water, additives and under simulated conditions of temperature and pressure. This is to predict accurately the performance and behavior of the slurry to be used on the field since cementing is not an exact science; it serves as a quality assurance, assuring clients that the job will be done accurately barring any complications. Actual field samples are used because cementing is a very delicate process and any slight change in composition of say cement, water or additive could drastically affect the expected results leading to downtime or in extreme cases total loss of casing due to flash setting. Laboratory tests are run according to API specifications.
2.3.1 Cement Additives In the course of drilling, different types of formations are encountered, these formations come with their unique set of conditions; some may be a gas zone prone to gas migration or a lost circulation zone or an over pressured zone or a number of other different conditions that could exist particular to a well. Whatever the case, the cement slurry needs to be designed to accommodate the wide range of conditions that occur in the wellbore. Cement additives are added to the Portland cement to modify or enhance the behavior of the cement system. They ideally allow successful slurry placement between the casing and the formation, rapid compressive strength development and adequate zonal isolation during the lifetime of the well. They are available in both liquid and solid forms though most times, liquid additives are preferred. Though each additive has a primary effect on the slurry i.e. it modifies one aspect of the cement slurry, it hasother effects called secondary effect which could either be helpful orvery unhelpful to the overall picture. At any rate, they must be taken into account when designing the slurry. Another phenomenon which can and does further complicate the picture is that of synergistic effects which is a change in the slurry which results when two additives are in the slurry together, but which will not result from either additive being in the slurry by itself. Other synergisms also occur with more than two additives. There a great number of cement additives typically used in cement slurry design, they include: i.
Accelerators `10
ii.
Retarders
iii.
Foam Preventers
iv.
Extenders
v.
Weighing agents
vi.
Dispersants
vii.
Fluid Loss control agents
viii.
Bonding Agents
ix.
Lost Circulation Materials
x.
Strength Retrogression Materials
xi.
Multipurpose Additives
Others include additives used in specialty cement systems like thixotropicsystems, retarded liquid cement, freeze protected cement etc. i.
Accelerators
Accelerators are added to cement slurries to shorten the setting time and increase early compressive strength development so as to decrease the waiting on cement (WOC) time. Typically inorganic salts are used; these include chlorides, carbonates, silicates, sulphates, etc. however, chlorides are especially preferred because they produce the highest accelerating power. Chlorides used include CaCl2, KCl, NaCl etc. All chlorides are added to the cement slurry by weight of cement (BWOC) with the exception of Sodium and Potassium Chloride which are added by weight of water (BWOW). Calcium chloride is undoubtedly the most efficient andeconomical of all accelerators; regardless of concentration, it always acts as an accelerator. It is normally added at concentrations of1% to 4%BWOC. Its secondary effect is that it causes excess heat and could increase slurry viscosity. Also, results are unpredictable at concentrations exceeding 6% BWOC and premature setting may occur. Its effectiveness reduces in the presence of fluid loss additives. It is compatible with all cement additives. Sodium chloride affects cement slurry in different ways depending upon its concentration. It acts as an accelerator at concentrations up to 10% BWOW, remains neutral at 10% to 18% BWOW, and at concentrations above 18% BWOW causes retardation. Its side effects are that it `11
causes foaming and reduces slurry viscosity. Also it acts as a mild dispersant at all concentrations. It is used for cementing across salt zones or salt domes. Sodium chloride is not a very efficient accelerator, and is used only when calcium chloride is not available at the well site. Potassium chloride acts similar to NaCl but stabilizes shale or clay containing formations. Its effective concentration is 5% BWOW. Though an acceptable accelerator especially in fresh water zones, it is quite expensive and as such is rarely used. ii.
Retarders
Retarders are cement additives used to extend/increase the setting time of cement. There are many materials that could serve this purpose but due to their limitations, they are rarely used. As a result there are only a relatively few chemicals which are actually used as retarders. These include: Lignosulfonates: These are polymers derived from wood pulp. They contain varying amounts of saccharine compound (sucrose). They are the most commonly used retarders in oil well cementing. Typically, the sodium and calcium salts are used. They are effective with all Portland cements and are generally added in concentrations ranging from 0.1% to 1.5% BWOC. Depending on a number of conditions, they are effective to about 250°F (122’C) BHCT. Their effective temperature is increased when they are blended with sodium borate (borax). Their most common side effect is the thinning or dispersing effect which they have on cement slurries. Other retarders include sugars, organophosphates, inorganic compounds (NaCl, ZnO etc.), and cellulosederivatives. It is important to note that each retarder is generally either a low temperature or a high temperature retarder and this must be put into consideration in cement slurry design to achieve good results. iii.
Foam preventers
Many cement additives can cause slurry to foam during mixing coupled with the high rate of shearing. Excessive slurry foaming can have several undesirable consequences one of which is entrainment of air. Entrained air could result in higher than desired slurry density and on hardening of cement, could lead to channeling. Foam preventers are used to prevent all these; they work by lowering the surface and interfacial tension between the cement particles. In well cementing two classes of antifoam agents are commonly used: polyglycol ethers and silicones. `12
Very small concentrations are necessary to achieve adequate foam prevention, usually less than 0.1% BWOW.Poly (propylene glycol) is most frequently used because of its lower cost, and is effective in most situations; however, it must be present in the system before mixing. Silicones on the other hand will defeat a foam regardless of when they are added to the system. Foam preventers are added to all cement slurries and do not affect the chemical behavior of the slurry. iv.
Extenders
Cement extenders are used to accomplish two purposes – to reduce slurry density and to increase slurry yield while still maintaining slurry integrity. A reduction of slurry density reduces the hydrostatic pressure during cementing; this helps to prevent induced lost circulation in case of weak formations with low fracture gradient. They increase yield by reducing the amount of cement required to produce a given volume of set product. This results in greater economy. Different extenders use different mechanisms to achieve their purpose, these include water extending and use of low density aggregates. Commonly used extenders include Clays This refers to materials composed chiefly of clay minerals. Two types of clays are majorly used in oil well cementing: attapulgite (also known as salt gel) and bentonite. Bentonite, commonly referred to as gel is the most frequently used clay-based extender in the industry. It contains 85% of the clay mineral smectite (also called montmorillonite). It works using the water extending mechanism where excess water is added to the cement. When placed in water, it absorbs the water and expands several times its original volume resulting in higher fluid viscosity, gel strength, and solids suspending ability. It tends to improve fluid loss but at high concentrations may slightly reduce the compressive strength development. Free water or settling is rarely experienced when bentonite is in use. It can be added in concentrations up to 20% but is most effective at concentrations ranging from 2-8%. It is majorly used with fresh water slurries as its effectiveness reduces in the presence of salts; this is because salt inhibits its hydration and as such reduces slurry yield. It remains very effective at elevated temperature. It can be dry blended with dry cement but it is generally preferred that it be pre-hydrated in water prior to usage. A slurry containing 2% pre-hydrated bentonite is equivalent to one containing 8% dry-blended bentonite. Complete hydration of a good quality bentonite (no beneficiating agents added) occurs in about 30mins but when rheology is to be carried out, it is pre-hydrated for 2 hours. Bentonite is majorly used in lead slurries where the cement to water ratio is low. `13
Sodium Silicates Silicates work using a different mechanism from that of clay extenders. They absorb the lime in the cement forming a calcium silicate gel which provides sufficient viscosity to allow the addition of excess water. They are available in both solid and liquid forms but in solid forms are known as sodium metasilicate Na2SiO3, the liquid form is also known as water glass. Though they are very effective extenders, they tend to slightly accelerate the setting time of the cement. They lower compressive strengths somewhat, provide some thixotropic properties and aid in free water control. They are majorly used in salt water slurries but in case of use with fresh water, CaCl2 is added as an activator prior to use. Sodium silicate has a temperature limitation of 150° F. Other extenders used include pozzolans (diatomaceous earth, fly ash), silica, commercial light weight cements, light weight particles etc. v.
Weighing agents
These are materials that are used to increase the density of cement slurries so as to maintain high hydrostatic pressures. They are used in cases of high formation pressure, unstable wellbores and deformable/plasticformations.They have little or no effect on thickening time and compressive strength development. One method of increasing density is to reduce the amount of water used but due to its limitations, is rarely used. Alternatively, materials with high specific gravities are used to increase the slurry density. To be acceptable as a weighting agent, such materials must meet the following criteria.
The particle-size distribution must be compatible with the cement.
The water requirement must be low.
The material must be inert with respect to cement hydration, and compatible with other cement additives.
Materials used include Barite, Hematite, Ilmenite, Manganese Oxide, and sand among others. The most widely used weighing agent worldwide is Hematite; this is probably due to its effectiveness with all classes of cement, its fine sized particle distribution and its economic value. Barite on the other hand, though commonly used due to its availability from mud jobs is not as effective as the others; this is due to its additional water requirement.
`14
vi.
Dispersants
They are added to cement slurry to reduce the viscosity of the slurry or to thin the slurry thereby making it easier to pump (pumpable). The most common dispersants used are Sulfonates. Also used are lignosulfonates though they cannot be used at low temperatures since they retard cement systems. The side effect of dispersants is that they could cause free water, sedimentation and segregation. vii.
Fluid loss control agents
Fluid loss is a process whereby the aqueous phase (filtrate) of the slurry escapes into a permeable formation. If fluid loss is not controlled, premature hydration or formation damage could occur leading to job failure. As the volume of the aqueous phase decreases, the slurry density increases and its properties i.e. rheology, thickening time, etc. deviate from that of the original design. If sufficient fluid is lost to the formation, the slurry becomes unpumpable. To control the fluid loss, fluid loss control agents are added to the slurry to obtain fluid loss rate less than 50mL/30min required to maintain adequate slurry performance. Two principal classes of fluid loss additives exist; the particulate materials and water soluble polymers such as HEC and CMHEC, liquid latex is also used. viii.
Bonding agents
Bonding agents are additives added to the cement slurry to increase the bonding of the cement slurry to prevent gas intrusion. They also improve fluid loss. One of the materials used is Styrene-Butadiene which is latex based. ix.
Lost Circulation Materials (LCM)
Lost circulation is the loss of whole fluid to the formation; this is mostly due to problematic formations like vuggy or cavernous formations and highly fractured incompetent formations. Most times, these zones are noticed during drilling so the cementer is better prepared for it and puts this into consideration in his design, hence he adds LCM to his design. Most materials used are bridging materials (granules like gilsonites) which physically bridge over fractures and block weak zones. Also used are the cellophane flakes which form a mat across the face of the fracture. Another LCM commonly used is fibre which tends to block the fractures. Thixotropic cements `15
which are cements that remain liquid while pumping but when no longer subjected to shear, gel and become self-supporting are used in extreme cases. x.
Strength Retrogression materials
Strength retrogression is the term used to describe the breakdown of cement’s compressive strength when the cement is exposed to excessive temperatures. At BHST of 230 degF or higher, cement will over a period of time lose its compressive strength, become permeable and generally be unable to support the casing, provide pressure back-up, or keep corrosive well fluids from boring holes in the casing. This starts eight hours after the cement sets. While there is no way to stop the process, or reverse it (other than by multiple remedial squeeze jobs), strength retrogression can be prevented; this is done by adding strength retrogression materials to the slurry. Materials used include Silica Flour or Coarse Silica. They are added to cement at concentrations ranging from 40to 45% BWOC. xi.
Multipurpose additives
As the name implies, these additives serve many purposes in the cement system; they do not only perform one function. They could serve purposes ranging from accelerating to anti settling. They are quite useful in cementing operations.
2.3.2 Cement testing equipment and methodology To aid in the effective laboratory testing of cement prior to the actual cementing operation so as to approach exactness and accuracy, API has approved a list of equipment and methodology required to carry out these tests. This is in a bid to ensure reproducibility of results anywhere in the world and to standardize the whole process of cement testing. The approved equipment includes: i.
Electronic balance
This equipment is used in accurately weighing samples used in the laboratory. According to API standard, electronic balances should be accurate to 0.1% of indicated load.
`16
Methodology 1. The Container or syringe to be used in measuring is weighed and tarred/zeroed to eliminate its weight. 2. The sample is then measured and weighed as accurately as possible. Calibration Interval: ii.
at least annually.
Constant Speed Mixer
The Constant speed mixer is designed for precise mixing of cement slurry so as to attain a homogenous mixture. It is capable of maintaining two known constant speeds.It has a variable speed, mixingfrom 100 to 21,000 rpm with two preset constant speeds of 4,000 and 12,000 rpm, as established by theAPI. It eliminates slurry property irregularities associated with shear; rate of shear affects thickening time, fluid loss and free fluid of a cement slurry design. The instrument features a digital tachometer that continuously indicates the mixing speed, a speed control for setting the desired speed, and an electronic timer. It works with a 1 litre (I quart) blender cup which is large enough to accommodate the API recommended 600ml laboratory sample. Methodology: 1. After slurry design and accurate measurement of samples, the additives and cement are added to the water with the blender maintaining a constant speed of 4000±200rpm. The cement must be added in not greater than 15secs. 2. The slurry is then allowed to mix properly for 35 seconds using a constant speed of 12000±500rpm.This is done to eliminate slurry irregularities associated with shear.
Fig 1: Constant Speed Mixer `17
Note: The slurry is expected to be used within one minute of mixing according to API standards. Calibration Interval: 6Months. Precaution: The blade for the mixing cup should be weighed prior to use and replaced with an unused blade when 10% weight loss has occurred. If leakage occurs during mixing, the slurry should be discarded and the entire blender blade assembly should be replaced. The right electrical rating should be used as directed by manufacturer. iii.
Atmospheric Consistometer
The Atmospheric Consistometer is used to condition cement slurries to test temperature to enable further testing. It uses its water bath in conditioning of slurries to any temperature from ambient to 200’F by rotating the slurry containers at 150 rpm while continuously shearing the slurry by means of a paddle. Methodology: 1. 470ml of the cement slurry is poured into the slurry cup and the paddle and potentiometer are fixed. 2. The slurry cup is placed in the water bath of the Consistometer. 3. The temperature panel is programmed to the required temperature and the heater is switched on. 4. The motor is switched ONand left for 20 minutes.
Fig 2: Atmospheric Consistometer
`18
Calibration Interval:
Rotational speed
3 Months
Thermocouple
1 Month
iv.
Rotational Viscometer
The Viscometeris used to determine the rheological properties of cement slurry. It is a rotational direct reading viscometer. It consists of concentric cylinders; a rotational sleeve and bob. The sleeve rotates at a constant velocity for each RPM setting. The slurry creates a frictional drag between the sleeve and the bob which is connected to a torsion spring and a dial. The dial will read out proportional to the drag experienced by the bob. The viscometer is capable of rotating at speeds of 600, 300, 200, 100, 6 and 3 rpm though the 600rpm is not used in cement testing because the high speed causes segregation in cement slurries.
Fig 3: Rotational Viscometer Calibration interval: 3 Months for the springs and Rotational speed. Precaution: The bob and sleeve must be checked for centralization before use. The rotor and bob should be thoroughly cleaned after each test. Care should be taken to ensure that the bob shaft does not become bent and that the rotor shaft assembly is not submerged in water. Water may contaminate the bearings, causing excessive friction. v.
HPHT Filter Press
This device is designed to measure the rate at which water will be forced out of a static cement slurry when it contacts a permeable formation. The test incorporates high pressure and specified `19
filters to simulate the pressure drop caused by such a permeable formation. The cell consists of a stainless steel cylinder with removable end caps, which are fitted with O-rings and pressure valve stems. The cell has an opening at the top and bottom; the top opening is used to introduce nitrogen while the filtrate passes through the bottom opening. A 60-mesh screen rests on the bottom cap backing up the 325 mesh screen. The temperature is controlled by a thermostat in the heating jacket, the temperature of the cement slurry is measured using a type J thermocouple mounted in the wall of the Cell and this thermocouple mounted in the heating jacket measures the temperature of the jacket. The test is run at 1000psi as specified by API.
Fig4: HPHT Filter Press
Fig5: Double ended fluid loss cell
Calibration intervals:
Thermometer
1 month
Gauges
12 months.
Precaution: Nitrogen must be supplied in an approved nitrogen gas cylinder and secured to meet safety standards. Due to the high temperature and pressures involved in this test, extreme care must be exercised at all times. All safety precautions must be met, especially in the cell breakdown procedure after the filtration procedure has been completed. vi.
HPHT Consistometer
The Pressurized Consistometer is used to determine the thickening time of cement slurries in strict compliance with API 10A and API RP-10B specifications. In addition, the unit can be used to condition slurries for free water content, rheology, fluid loss, and various other tests. It
`20
consists of a rotating cylindrical slurry cup equipped with a stationary paddle assembly, all enclosed in a pressure chamber. They are operated either by magnetic drive or by packings. Most are capable of exposing cement slurries to a maximum temperature and pressure of 400°Fand 25,000 psi (204°C and 17.5 MPa); however, special units capable of 600°F and 40,000 psi (3 15°C and 280 MPa) are available for deep-well applications. Thermocouples are provided for determining the temperature of the oil bath and the cement slurry. The slurry cup is rotated at a constant speed of 150rpm. The consistency of the cement slurry is indicated and recorded as a DC voltage obtained from a potentiometer mechanism mounted on the paddle shaft, which contains a standardized torsion spring to resist the rotating tendency of the paddle. Operation of the pressurized Consistometer is simple with all the operational controls conveniently located on the front panel. Calibration interval:
Rotational Speed:
3 months
Potentiometers:
1 month
Timer:
6 months
Thermocouples:
1 month
Fig6: Pressurized Consistometer Precaution:Since high temperatures and pressures are used, one must ensure that all safety rules are complied to avoid accidents. `21
vii.
Ultrasonic Cement Analyzer
The Ultrasonic Cement Analyzer (UCA) is used to determine the compressive strength of cement slurries under simulated well conditions. This instrument is designed to measure and record the velocity of sonic waves through cement slurry as a function of time and to convert the measurement via an algorithm to compressive strength. The UCA provides a continuous reading from beginning to end of test and produces a graph to show the transition time from liquid to hard set. It also plots out the transition time. It has a thermocouple to determine and record the temperature and transducer to monitor the sound waves velocity. It is a nondestructive method of testing the compressive strength of the slurry.
Fig7: Ultrasonic Cement Analyzer Calibration interval: vii.
12 months
Pressurized balance
The pressurized balance is used to measure the absolute density of a fluid sample. It consists of a cup mounted on a ruler, a pressuring plunger, and a fulcrum or balance. Although the unit is similar in operation to a conventional mud balance, pressure is introduced to the system so as to eliminate entrained air. The standard scale density may be read directly in units of: lbs/gal, lbs/cuft,sg, psi/1000ft.
Fig 8: Pressurized Mud Balance `22
Methodology 1. Fill the sample cup to a level slightly below the upper edge of the cup (approx. ¼ inch). 2. Place the lid on the cup with the check valve in the down (open) position; push the lid downward into the mouth of the cup until surface contact is made between the outer skirt of the lid and the upper edge of the cup. Ensure that excess slurry is expelled through the check valve. 3. Pull the check valve up in the closed position, rinse off the cup and threads with water, and screw the threaded cap on the cup. 4. Fill the pressurizing plunger by submersing the nose of the assembly in the slurry with the piston rod in the completely inward position. Draw the piston rod upward in order to fill the plunger cylinder with slurry. 5. Push the nose of the plunger onto the mating O-ring surface of the check valve and apply force to pressurize. Ensure that the check valve is in closed position. 6. Rinse the exterior of the cup and wipe properly. 7. Place the instrument on the knife-edge and move the sliding weight right or left until the beam is balanced i.e. when the attached bubble is centered between the two scribed marks. 8. Record the density of the slurry. 9. Use the plunger to release the pressure when testing is complete.; ensure that the check valve is open. 10. Empty the cup and plunger, and then clean thoroughly. Calibration interval: viii.
12 months
Free Fluid Test Apparatus
The free water test is performed in a 250 ml glass graduated cylinder. A foam rubber pad is used to minimize vibration of the cylinder. For horizontal or highlydeviated wells, the graduated cylinder is mounted at a 45-degree angle by means of a free water standand clamp. To prevent any evaporation, the cylinders are sealed by plastic and elastic bands.
`23
Fig 9: Free water stand ix.
Low Temperature Circulator
This instrument is used to lower the temperature of other equipment such as the pressurized Consistometer or the UCA when running low temperature tests. It enables the simulation of low temperature conditions in the well; temperatures as low as32deg.F (0deg.C). It is also used to cool the machines after a very high temperature test.
2.3.3 General Cement tests There are a number of tests carried out on cement slurries under simulated conditions prior to actual field use; each is done to measure a certain property of the cement so as to serve as a quality assurance for the cementer and ensure good results on the field. As such these tests must be carried out according to API specifications. The tests carried out include:
2.3.3.1Rheology and gel strength test Rheology is the study of the flow behavior and deformation of a fluid under applied stress. The viscosity and yield points are calculated from rheological values. The rheology values help to evaluate the slurry mixability and pumpability. Aim To determine the rheological properties and behavior of cement slurries. Material Freshly prepared cement slurry.
`24
Apparatus 6 speed Viscometer, a viscometer cup, Atmospheric Consistometer. Procedure 1. Fill the viscometer cup with the slurry to the 350ml scribed line and place it on the sample cup table; ensure that freshly prepared slurry is used. 2. Raise the viscometer cup and cup table until the slurry level meets the inscribed line on the sleeve and tighten the locking nut ensuring that it covers the two holes below the line. 3. Use the red knob at the top of the viscometer to select gears, and the black switch to opt between low and high speeds. 4. Take the dial readings after duration of 10seconds for the different gears starting from the lowest to the highest and then take the reading again this time starting from the highest to the lowest. At gear 1, obtain the 10seconds Gel and 10minutes Gel at 3 rpm respectively. 5. Condition a freshly prepared slurry using the Atmospheric Consistometer then repeat steps 1-4. 6. Take an average of the two readings gotten then do all necessary calculations at both ambient and test temperature. Plastic Viscosity, PV = (300rpm-100rpm) x 1.5cp Yield Point, YP = PV- 100rpm, lbs/100ft2 Interpretation of result When the ratio at all the speeds is approximately 1, this is an indication that the slurry is a nonsettling, time independent fluid at the average temperature. Ratio values mostly higher than 1 may suggest settling of the slurry at the average temperature. Ratio values mostly lower than 1 may suggest gelling of the slurry. And when significant difference in the readings indicates that the cement slurry is not stable i.e. prone to settling of excessive gelation, adjustments in the slurry design should be considered
`25
2.3.3.2Fluid loss test Fluid loss is a process whereby the aqueous phase (filtrate) of the slurry escapes into a permeable formation. It could lead to several serious problems like shale swelling, flash setting of cement etc. Regardless of whether the slurry is conditioned in a Consistometer or in a stirred fluid loss cell, the fluid loss value is determined under static conditions. Aim The test is carried out to estimate the volume of filtrate lost to the formation. Material Freshly prepared cement slurry, Nitrogen. Apparatus HPHT Filter press, Fluid loss cell, 325-60 mesh screen, Atmospheric Consistometer. Procedure 1. Condition freshly prepared slurry to test temperature using the atmospheric Consistometer for 20mins. 2. Assemble the fluid loss cell ensuring that the mesh, the o rings and the valves are placed correctly. 3. Pour in the conditioned slurry into the fluid loss cell and secure the end cap in the cell. 4. Place the cell in the heating jacket ensuring that the screen is on the bottom and that the pin engages. 5. Connect the nitrogen line, secure it with the retaining pin and apply a differential pressure of 1000, ±50 psig. 6. Open the top fluid loss cell valve to apply and maintain 1000±50psi pressure to the cell. 7. Open the bottom valve (which starts the test) and start the timer. 8. Collect the filtrate using a calibrated measuring cylinder and record the volume to ±1 mL at 30 minutes; if the nitrogen blows through at less than 30 minutes, record the volume and time at which the blowout occurs and do the necessary calculations.
`26
9. Shut down the pressure supply and ensure that you bleed off the pressure totally before dismantling the cell. 10. Calculate the fluid loss and record in cc/30mins. Calculated API Fluid loss = 2xvol cc/30mins. In case of a blowout, Calculated API Fluid loss =2xVt
5.477 cc/30mins. t
Where t is the time of the blowout, expressed in minutes.
2.3.3.3Thickening time test The thickening time of a cement slurryprovides an indication of the length of time that a cement slurry will remain pumpable in a well. It is the time elapsed from the initial application of pressure and temperature to the time at which the slurry reaches a consistency deemed sufficient to make it unpumpable. Two related terms are also used; the pump time – the time at which ideally all pumping operations should be done with and the set time – the time at which the cement slurry is expected to have hardened enough to start building some if negligible compressive strength. Of course the thickening time is the most important parameter in this test. The laboratory test conditions should represent the heating time, temperature and pressure to which cement slurry will be exposed during pumping operations. Aim To determine the thickening time of cement slurries under simulated wellbore conditions. Material Freshly prepared cement slurry, Hydrocarbon oil as specified by API. Apparatus Pressurized Consistometer `27
Procedure 1. Grease and assemble the slurry cup and paddle assembly ensuring that the diaphragms used are really tight and that the collar is holding the paddle stationary. 2. Invert and fill the slurry cup to within 6 mm (1/4 in) of the top, pour out your surface sample. 3. Strike the cup to remove entrained air. 4. Screw in the base plate and make sure slurry is comes out through the centre hole, thenscrew the centre plug (pivot bearing) into the container. 5. Wash off excess cement from the slurry cup. 6. Place the filled slurry container on the drive table in the pressure vessel ensuring that the pins engage in the cup table then switch on the motor. 7. Secure the potentiometer mechanism so that it engages the paddle shaft drive bar properly. 8. Close all pressure valves and open the air supply so as to start filling the vessel with oil. 9. Close the head assembly of the pressure vessel and put in the thermocouple; do not tighten the screw totally so as to remove entrained air in the system. 10. When all air bubbles have escaped, tighten the thermocouple screw. 11. Program the Consistometer to test temperature using the BHCT and heating time and ramp to 500psi. 12. Start the test and switch on the heater, recorder and timer. 13. After the Consistometer has reached the test temperature, ramp to the test pressure. 14. Closely monitor the test and record the pump time, thickening time and setting time; these correspond to 40BC,70BC and 100BC. 15. Shut down the test ensuring all necessary parts are switched off and that all pressure release valves are opened so as to bleed off pressure in the system. 16. Open the slurry cup, discard the slurry and dismantle and clean the cup thoroughly.Special care should be taken to ensure no cement is trapped in any of the threads. 17. If the result gotten is as expected, then the slurry can now be pumped in the field or reconfirmed by the clients, if not, the slurry is redesigned and tested again till adequate results are obtained. `28
2.3.3.4Compressive strength test Compressive strength is the ability of the cement sheath to withstand differentialpressures in the well.A minimum compressive strength of 500psi is required before further operations can commence. There are two methods of testing for compressive strength; the destructive and the non-destructive testing. For the destructive testing, the cement slurry is poured into a cubical mould and cured in a Curing Chamber for 24hrs, and then it is crushed using a Carver Press. In this case, the compressive strength is the pressure it takes to crush the set cement. In the nondestructive testing, sonic speed is measured through the cement as it sets. This is done using an Ultrasonic Cement Analyzer. In present times, the non-destructive testing is rarely used as the NDT is preferred. Aim The test is carried out to determine the compressive strength that can be attained by the set cement under simulated wellbore conditions. Material Freshly prepared cement slurry, water Apparatus Ultrasonic Cement Analyzer (UCA), Slurry cup. Procedure 1. Grease and assemble the slurry cup generously ensuring that you take note of the top and bottom. 2. Place the slurry level gauge at the top of the cup and pour in freshly prepared slurry to the slurry level, then pour in fresh water carefully to the water level. 3. Place the top lid and properly screw it in. 4. Place the slurry cupin the Pressure Curing Chamber and secure it properly; connect the transducer and the thermocouple. 5. Close all pressure valves and open the water supply, allow excess water to flow out through the vent then tighten the thermocouple. `29
6. Program the UCA to test temperature using the BHST and heating time and ramp the pressure slightly,program the software taking note of slurry density. 7. Start the test and switch on the heater. After the test temperature is attained ramp the pressure to 3000psi as recommended by API. 8. Closely monitor the test and record the compressive strength at 8hrs, 12hrs and 24hrs. 9. Shut down the test ensuring all necessary parts are switched off and that all pressure release valves are opened so as to bleed off pressure in the system. 10. Open the slurry cup, discard the slurry and dismantle and clean the cup thoroughly.Special care should be taken to ensure no cement is trapped in any of the threads.
5000
360
18
4500
320
16
4000
280
14
3500
temp uca 2 (°F)
240
200
160
12
10
8
Compressive Strength (psi)
20
Time uca 2 (microsec/in)
400
3000
2500
2000
120
6
1500
80
4
1000
40
2
500
0
0
0 0:00
5:00
10:00
15:00 Time (HH:MM)
20:00
25:00
30:00
Fig 10: A typical UCA plot
2.3.3.5Free fluid and sedimentation test Free fluid is caused by the segregation of water from the cement slurry after placement in the annulus. It can lead to channel formation, gas migration and non-uniform compressive strength of the cement sheath. It is a measure of the slurry stability. It normally occurs hand in hand with `30
solids settling. Since free fluid and sedimentation occurs after placement in the wellbore, the test is done under static conditions and at well conditions. Aim The test is done to determine the stability of cement slurry under static conditions. Material Freshly prepared cement slurry Apparatus 250ml graduated cylinder, Atmospheric Consistometer, syringe Procedure 1. Condition freshly prepared slurryto test temperature using the
Atmospheric
Consistometer for 20mins. 2. Pourthe conditioned slurry into a 250ml graduated cylinder. Stir the slurry with a spatula while pouring to ensure a uniform sample of the slurry. 3. Place the cylinder on a foam pad for vertical wells but in the case of deviated wells place it in the 45deg inclined free water stand . 4. Seal the cylinder to prevent evaporation and leave the test to stand for 2 hours 5. When the time is elapsed use a syringe to remove all the free fluid and record the amount gotten. 6. Discard the slurry and carefully wash the cylinder 7. Express the volume of free fluid as a percentage of the slurry volume used.
2.4
Special Cement Systems
Aside from the conventional cement system used in oil well cementing operations, some special cement systems have been developed to combat certain problems encountered in the well bore; problems such as slurry fallback, lost circulation, micro annuli, cementing across salt formations, and corrosive well environments. Some of these systems are discussed briefly below. `31
Thixotropic Cements Thixotropy is a term used to describe the property exhibited by a system that is fluid under shear, but develops a gel structure and becomes self-supporting when at rest. Thixotropic cement slurries are relatively thin and fluid during mixing and displacement, but rapidly thicken and become self-supportingwhen pumping ceases. Upon reagitation, they become fluid and are once again pumpable but immediately thicken upon cessation of shear. This type of rheological behavior is continuously reversible with truly thixotropic cements. They are mostly used for setting plugs in lost circulation zones since they set up quickly. Harder cement plugs can then be added asneeded. Thixotropic cements are very viscous and have very high strength, this could cause some problems when pumping is temporarily stopped before the cement reaches its target. Common additives include class G cement, Sodium metasilicate, gypsum, dispersant etc. Retarded Liquid Cements These arestorable, premixed cement slurries that can be kept in a liquid state for an extended period, then activated to function as a zonal isolation material. Additives used include class G Portland cement, a set-retarding agent, a dispersant/ plasticizer to provide long term fluidity, and a suspending agent to prevent settling. Upon activation, they yield a finished slurry with properties suitable for well cementing in a wide range of temperature and density conditions. To maintain them, they must be sheared for 5- 10 minutes every 1- 2 days. Ultra-Light Weight Cement These arevery low density cements with very good compressive strength. They are formulated to cement across weak formations where low hydrostatic pressures are required and can be used in wells as deep as 12000ft. They provide superior strength compared to slurries with conventional extenders at the same density and have excellent insulating properties. Additives include class G Portland cement, lightweight silica microspheres and other cement additives. Freeze Protected Cements These are cements that can set at temperatures as low as 20degF (-3degC). They can set in permafrost zones without freezing while attaining high early compressive strength and effective insulation (low thermal conductivity). Permafrost is defined as any permanently frozen `32
subsurface formation; the depths may range from a few feet to 2,000 ft. Below the permafrost, the geothermalgradients are normal.This cement system is a blend of class G Portland cement and gypsumwith Calcium Chloride as a freezing point depressant. Foamed Cements These are used to cement across weak formations where normally extended slurries cannot produce the desired cement density and compressive strengths. They can be formulated and used where adequate compressive strengths are still required in very low density cements. They can minimize slurry loses in cases where activelost circulation is being experienced. Slurry stability is maintained by the foamer or additional stabilizers. Foam cement is produced from cement which has been blended to meet downhole conditions, a foaming agent usually a surfactant and nitrogen as the foam density control agent. The cement is slurried to its designed weight in a mixer, then the foaming agent is added from an additive injection system and then the slurry is foamed by mixing with nitrogen. It is a very economic and effective way of cementing lost circulation zones as opposed to light weight cements which can be quite expensive. Magnesium/Calcium System These are unique acid soluble cements used to seal off and protect producing formation and to stop lost circulation where conventional bridging materials fail. It is a Mixture of magnesium and calcium compounds and other acid soluble inorganic materials. It is a free flowing powder that can be easily mixed to form a smooth pumpable slurry. Unlike normal cement, it hardens by chemical reaction rather than hydration. It requires special additives especially tailored for it to modify its properties. It can be pumped through drill bits nozzles and is compatible with most drilling fluids thus eliminating the need for spacers and washes.
2.5
Compatibility test
Before any cementing operation commences, the well must be adequately prepared; this involves conditioning the mud and circulating it out of hole while cleaning the well so as to remove any mud filter cake on the walls of the well.This is to ensure good cement bonding and effective annular seal because the mud could contaminate the cement and vice versa. The mud is displaced with a spacer or chemical wash or flush depending on the design, after which the cement slurry is `33
pumped into the well. Bonding and cement seal durability is directly related to the efficiency of the displacement process. Because of the importance of this displacement process, tests are carried out in the laboratory to determine the most effective spacer system to be used in the well; the spacer must be compatible with both the mud and the cement. This test is called a Compatibility test. It includes examination of rheology, static gel strength, and wettability.
2.5.1 Spacer preparation This is a fluid used to separate drilling fluids and cement slurries; the spacer displaces the mud and the cement displaces the spacer. It is usually compatible with both the mud and the cement. It can be used alone or with chemical wash or a pre-flush. It can be designed for water based mud, oil based mud or both. It prepares both the pipe and the formation for the cementing operation. Because of the displacement order, the density of the spacer is expected to be slightly higher than that of the mud but lower than that of the cement. Additives For water based mud: foam preventer, surfactant, gel, water and weighing agent. For oil based mud: foam preventer, surfactant, solvent, gel and weighing agent. Note: The basic difference between the spacer for water based mud and that for oil based mud is the use of a solvent in the latter. The concentration of the additives to be used is set as a standard but it varies depending on the density of the spacer to be prepared.The concentration of everything remains constant but that of water and the weighing agent varies with varying density. A typical example is shown below. Spacer
Density, ppg
Foam Preventer
Surfactan t, gal
Gel, lb
For Water Based Mud
10
0.084
1.000
10.2
0.084
1.000
Solvent, gal
Water, gal
Weighing agent, lb
0.50
39.90
36.00
0.50
39.10
64.00
`34
For Oil Based Mud
9
0.084
2.000
0.50
3.00
35.90
36.00
9.5
0.084
2.000
0.50
3.00
35.10
64.00
This design is specifically to prepare a spacer of 1bbl using fresh water. If salt water is used or a different volume is required, concentrations will vary. Apparatus Syringe, Electronic Balance, Constant Speed Mixer, Atmospheric Consistometer, Viscometer. Procedures 1. Accurately measure the additives; for the liquid additives measure in mls using a syringe while for the solid ones measure in grams using an electronic balance. 2. Prehydrate the gel for 30mins maintaining a low speed for the duration. 3. After the 30 minutes, add the other additives ensuring that the mixing order is maintained. 4. Slightly increase the speed and leave for 5 minutes. 5. Check the density using a pressurized Mud Balance and record.
2.5.2 Procedure for compatibility test The rheology of mixtures of cement/mud, cement/spacer and mud/spacer in different ratios is determined. The API recommended ratios are 95/5, 75/25, 50/50, 25/75 and 5/95. The test is done at both ambient and test temperatures. The rheology values gotten are interpreted to determine whether the systems are compatible or not. Usually, cement/spacer and mud/spacer are compatible but mud/cement is not. A wettability test is also done. This is to test the water wetting capability of the spacer. There are different methods and apparatus available but the simplest one is the use of the viscometer sleeve. This is done by shearing 100%mud at 600rpm for 10mins, then shearing 100%spacer with the same parameters and afterwards sprinkling water on the sleeve. If a clean sleeve is
`35
obtained, then the spacer is capable of adequately cleaning the mud from the pipe and well, if not, a different design might have to be recommended.
2.6
Introduction to Stimulation
Stimulation is the general term used to describe a variety of operations performed on a well to improve its productivity and maximize hydrocarbon recovery. Stimulation operations can be focused solely on the wellbore or on the reservoir; it can be conducted on old wells and new wells alike; and it can be designed for remedial purposes or for enhanced production. It is a very vital operation in oil and gas production.
2.6.1 Types of stimulation Oil well stimulation is of two types; matrix acidizing and hydraulic fracturing though most times, matrix acidizing is preferred because of its relatively low cost. In matrix acidizing, acid is squeezed into the matrix of the formation (including the network of pores and pore throats within the rock) at pressures below those that would initiate fracturing. The primary function of matrix acidizing is to remove near-wellbore damage, and return the well to its “original” productivity. In matrix acidizing, acid penetrates into the formation, radially around the wellbore, enlarging flow channels and dissolving pore-space plugging particles.Typically, hydrofluoric acid is used for sandstone/silica-based problems, and hydrochloric acid or acetic acid is used for limestone/carbonate-based problems. Most matrix stimulation operations target up to a ten foot radius in the reservoir surrounding the wellbore. Hydraulic fracturing is a process of creating a fracture by the injection of fluids into a formation at a pressure higher than the parting pressure of the formation. Injection rate has to be high enough and formation permeability to the injected fluid has to be low enough that fluid loss is not excessive so that pressure canbuild up sufficient to fracture the formation or to open existing natural fractures. The variety of materials used includes amongst others: water, acid, special polymer gels, and sand. These cracks are held open by proppants which are basically specialized sands.The creation of acid channels allows for an enhanced conduit to the wellbore from distances in excess of a hundred feet.
`36
2.6.2 General Stimulation tests A number of stimulation tests are carried out in the lab to effectively predict the correct acid system to be used for a specific treatment. They include Dissolution test This test is done to determine the treatment fluid that will effectively dissolve a solid sample from the well bore. The different treatment acid systems are prepared and mixed with the formation sample are desired ratios. The mixture is thenheated to the test temperature using a water bath, after which it is filtered and weighed. The treatment system that gives the most noticeable change is the most effective. Shake out test/ Blend Compatibility test This test is used to determine the most effective combination of additives and the concentration of those additives needed to prevent stable induced emulsions. The treatment fluid and formation fluids are mixed in ratio 1:1 in a test bottle, agitated thoroughly and allowed to stand for 10mins at ambient and test temperatures. If complete separation occurs within10mins time frame, the test is successful. Scale Analysis The test is used to identify scales that are formed in wells, tubings and pipelines. This is done by dissolving the scale in different solvents such as water, diesel and xylene and acid solutions. Emulsion Break test Emulsions sometimes form in producing oil wells and increase the viscosity so much that they stop or restrict production. Recommendations for removal of an emulsion block are based on determinations made of the emulsion this is where laboratory testing comes in. They test todetermine the type emulsion - whether it is oil-in-water, water-in-oil or complex,the percentage of oil, water and solids, and the best treating fluid that can be used to break the emulsions. The treatment fluid and the formation fluid are mixed in ratio 1:1, vigorously shaken and allowed to stand in a glass jar at ambient and test temperatures, The emulsion break time is recorded at time
`37
intervals of 1, 5 , and 10mins. If 100% separation occurs then the test is successful; if not, the treatment fluid has to be redesigned or an alternate one is used. Acid Sludge test This test is used to determine if an acid system will cause the formation of a sludge when used to treat a specific crude oil.
2.6.3 Stimulation additives Additives used in the design of acid system include Iron Control Agents These are used in reducing the effects of dissolved iron.The intermixing of iron-contaminated acid containing inappropriate additives and formation crude oils can to result in many problems in the well bore such assludge precipitation, formation of scale productsetc. Surfactants Surfactants (surface-active agents) are chemicals that are added to an acid blend to alter the surface and/or interfacial tension properties of the acid. This alteration can affect the flow of fluids in the wellbore region in either favorable or unfavorable ways. Where used, surfactants can promote different changes in reservoir rocks and fluids. They can act to create, break, weaken, or strengthen emulsions, reduce acid-induced sludging, create or break foams etc. Corrosion Inhibitors Inhibitors serve to control the rate of dissolution of steel in a well. Corrosion acts on base metals to change them into other types of materials. When a corrosive fluid is in contact with a metal, at any point on that metal surface corrosion is occurring. This is totally undesirable because if the pipes downhole are corroded, it could lead to serious problems during production. Solvents
`38
Solvents are often chosen as the additive to accomplish a number of purposes: to decrease interfacial tension, provide detergency, convert or restore the rock surface to a water-wet condition among others. Scale Inhibitors They are used to inhibit the growth of scale as it precipitates as scale formation could lead to a lot of problems in the well. Friction Reducers and Acid Gellants Friction reducers are those chemicals that, when dissolved in the fluid, reduce the fluid’s frictional pressure drop through well tubing, particularly where high injection rates are required in acid fracturing treatments.
2.6.4 Types of Acid systems The types of acids that can be used are grouped into: Mineral acids, Organic Acids and Acid Mixtures. Mineral acids include HCL, HCL-HF. Organic acids include Acetic Acid and Formic Acid. We also use one shot acid systems and Paravan D can also be used. Though not an acid system, diesel-xylene mixtures are also used as treatment fluids. Aside from the conventional acid systems, we also have the retarded acid system. Stimulation additives can be used to enhance the properties of the acid systems. Different acid systems are compatible with different formations and this must be taken into consideration when designing an acid stimulation job.
2.7
Laboratory safety policies
The oil industry is very particular about HS&E standards. It maintains that in every action, safety must be put into consideration; safety to human life, environment and property. Because of the high exposure to chemicals, high temperature and pressures in the laboratory, special care must be taken in carrying out. Some of these safety precautions are: 1. Ensure that a JSA and HRA is done before any job. 2. Necessary PPE must be worn before handling chemicals. 3. Proper housekeeping must be maintained at all times,everything must be in its place and there should be no crowding of space with materials. `39
4. All laboratory wares and materials should be kept clean. 5. If skin is contacted with chemicals/additives and cement, wash off thoroughly with fresh water. 6. All chemical containers should be properly labeled and sealed. 7. On inhalation of acid, immediately drink water.
`40
CHAPTER THREE 3.1 Problems encountered during SIWES period There is no doubt that the Industrial training experience was very beneficial in more ways than one however, some problems were encountered during the training that could cause a setback to the program. They include: 1.
Placement
It was very difficult to secure a space for the training; this of course was largely due to the number of students searching for IT placement. Despite the fact that I started the search quite early, it was a grueling experience nonetheless and I almost had to resort to settling to for an unrelated industry. This is a trap that a lot of students fall into. 2.
Finance
The monthly stipend I received from my company was barely enough to cater for my needs during the training period; these needs include transportation, feeding, accommodation etc. As a result, the duration of the training was very stressful with me trying to make ends meet. In fact in some companies where I tendered my application, they refused to pay any salary and instead demanded that I pay them. Possible solutions i.
The students should be encouraged to purchase log books and IT letters on time so that they can resume the search for a suitable industry for his/her IT placement.
ii.
The ITF should assist students in securing IT placement so that they can start and finish on time.
iii.
The ITF (Industrial Training Fund) as well as the government should make provision for the payment of the specified amount to the industrial training students on a monthly basis during the training period.
`41
3.2 Relevance of the SIWES program The relevance of the SIWES program to the student and industry alike cannot be overemphasized; they include 1. It serves to bridge the gap between the theory learnt in class and actual practice in the industry. 2. It gives students an opportunity to gain some hands on skills useful in the industry while still in the University. This enhances their understanding and performance in their respective courses of study. 3. It exposes students to a workplace environment, most for the first time in their lives where they can learn about workplace ethics, safety standards etc. 4. It exposes students to the professional method of work and good practices, including use of industrial tools, equipment and machines. 5. It enables students create a network with professionals in his/her field of study. This gives him an edge on graduation. 6. It motivates students to work harder in school so as to secure such a job on graduation. 7. It exposes students to current challenges in the industry that they could work on either as a school or personal project so as to proffer a solution. 8. For some it renews their interest in their course of study; as the popular saying goes “seeing is believing”. 9. It broadens students’ perspective on their field and the jobs available therein. 10. It exposes students to what to expect on graduation thereby making the transition from school to industry easier 11. It enlists and strengthens employers involvement in the entire educational process of preparing universities and other tertiary graduates for employment into industries 12. It is important to note that these only come to play if the student is able to secure an IT placement in an industry relevant to his/her field.
`42
CHAPTER FOUR 4.1 Conclusion The industrial training experience was indispensable to me. It gave me what to look forward to and reason to strive harder to learn in school. It opened up a world of opportunities for me and afforded me an opportunity to learn new things such as workplace ethics, HS&E, QA/QC. It gave me reason to appreciate my lecturers more cause everything I saw, I had been taught one way or the other by them. It exposed me to the world of cementing and how it is used to aid in oil and gas production. It exposed me to the oil and gas industry; its working standards, practices and terms. It was great and I can safely say that the training is the one pillar of academic learning that must be held strong.
4.2 Ways of improving the programme The program is key to the attainment of a degree from a higher institution and is quite beneficial to students that undergo it, however I am of the opinion that some changes must be made for it to achieve its full potential; these being 1. The ITF should take the time to properly orientate and sensitize the students prior to the commencement of the training; they should discuss expectations and brainstorm on how to solve these recurring problems. This would also reduce the issue of students misbehaving in the workplace. 2. The ITF and the government should assist students in securing IT placement in their relevant industries because for all the good the training does, it makes no difference if the student has no work. 3. The ITF should ensure that students undergo the training in an industry relevant to their field of study; this can only be achieved by proper supervision. 4. The industry based supervisors should ensure that they take the time to teach the students properly and equally assess them from time to time. 5. In spite of the large number of students assigned to them, the supervisors should ensure that they have one on one talks with their students and ensure proper follow up.
`43
6. The ITF should ensure that companies allow students to actively participate in the work, of course under supervision and not to become the handymen in the office thereby not gaining anything from the experience.
4.3 Advice for the future participants Prospective IT students should start searching for a place of attachment in a relevant industry as early as possible, so that they can resume training on time and finish up before the next academic session commences. They should focus on what’s important; money is important but they should make sure they learn as much as they can take and then more. They should also make sure they follow all company rules and are on their best behavior throughout the training. IT students should use the opportunity to create a network with professionals in their field that they come across and leave them with a wonderful and lasting impression. Finally, they should make sure that they are a 100% dedicated to their work; it’s just IT but its work all the same and everything counts.
4.4 Advice for the SIWES managers I would firstly thank the SIWES mangers for such an opportunity and give kudos to them. My advice is that they should ensure that they properly sensitize industries and companies on the importance of accepting IT students and the role it plays in the development of the student and if possible with government support, enforce it. They should also ensure that they pay regular visits to the IT students, discuss extensively with them and know their problem areas. Finally, they should actually make an effort to look into the recurring problems faced by IT students and find a solution to them.
`44
`45
