Contents 1.
Introduction.............................................................................................................................1 1.1 Definition of Extended Reach (ERD) Well...........................................................................1 1.2 Objectives of report...............................................................................................................2
2.
OPPORTUNITY IDENTIFCATION......................................................................................3 2.1 Prospect identification...........................................................................................................3 2.1.1 Subsurface survey..........................................................................................................3 2.1.1 Further analysis of opportunity......................................................................................5 2.1.2 Hydrocarbon volume calculation...................................................................................8 2.2 Wells Economics.................................................................................................................10
3.
Life Cycle of a Well..............................................................................................................13 3.1 Planning..............................................................................................................................13 3.1.2 Build Rate....................................................................................................................15 3.1.3 Hole sizing and selection............................................................................................16 3.2 Drilling................................................................................................................................17 3.2.1 Rig Type.......................................................................................................................17 3.2.2 Rotary Drilling Components........................................................................................19 3.3 Drilling Fluid.......................................................................................................................28 3.4 Casing and Cementing.........................................................................................................29 3.5 Drilling Operations.............................................................................................................30
4.
CONCLUSION.....................................................................................................................32
HOW TO DRILL A WELL
EXTENDED REACH WELL 1. Introduction With the end of the era of “easy oil” and with major oil fields being situated in deep waters and difficult terrains, drilling a well becomes a more and more complex matter. Since 1987, implementation of horizontal well technology and extended reach drilling technology has been done to produce oil and gas in a field in the most cost effective way. The use of these new techniques can make old non‐profitable fields profitable, prolong an existing field’s economic life and make new and uncertain field discoveries technically possible. 1.1 Definition of Extended Reach (ERD) Well Specific definition of an extended reach well is considered as a debatable issue. However, the current consensus agrees on the following definition - a well is defined as extended reach if it has a Step‐Out Ratio of 2 or more. Step‐out Ratio is defined as the horizontal displacement (HD) divided by the true vertical depth (TVD) at total depth. But, for most highly deviated wells in deepwater environments, this definition clearly does not fit. Some methods have evolved to categorize wells according to their stepout within different vertical-depth ranges. ERD wells then can be described conveniently as shallow, intermediate, deep, and ultradeep. Other variants are associated with operating in deep water and highpressure/high-temperature environments. The following are other generally accepted definitions of an ERD well: i.
Wells having horizontal displacements greater than twice the well’s true vertical depth,
ii.
yielding inclination angles in excess of 63.4 degrees; Wells which approach the limits of what has been achieved by the industry in terms of
iii.
horizontal displacement; High‐angle directional wells that approach the capabilities of the contracted rig.
It should be noted that to date, there is still no standard accepted definition for an ERD well.
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EXTENDED REACH WELL 1.2 Objectives of report The objective of this report is to: i. Introduce the different elements that are pertinent to drilling ii. Describe some of the variation of these elements when it comes to Extended Reach Drilling iii. Touch on the general drilling operation sequence
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EXTENDED REACH WELL 1. OPPORTUNITY IDENTIFCATION 2.1 Prospect identification The formation of hydrocarbon reserves occur due to geological processes that spans over a large period of time. Organic remains being the main ingredient of this process, forms the gasses and oils that migrated into the reservoir rock which is then trapped there by overlying rock formations with very low permeability. In other words, for a hydrocarbon reservoir to exist we need the following to be available at the same location: 1. 2. 3. 4. 5.
A source rock containing the original organic remains. Pressure and temperature conditions suitable to convert the organic remains into oil and gas. A porous, permeable reservoir rock where the hydrocarbon can accumulate. A migration pathway from the source rock to the reservoir rock for the hydrocarbons. A suitable trap to keep the hydrocarbons in the reservoir rock.
2.1.1 Subsurface survey The first in line to identify these traps are geologists and geophysicists. In the olden days, primary identification was done by interpreting surface features, surface rock and soil types, and perhaps some small core samples obtained by shallow drilling. Currently, surface and rock terrain is still done but with a more sophisticated approach of using satellite images and aerial photographs for limited surface access areas. However, they also use a variety of other methods to find oil such as using:
Sensitive gravity meters & magnetometers - measures tiny changes in the Earth's gravitational and magnetic field that could be caused by flowing oil, Sniffers – electronic nose that can detect the smell of hydrocarbons Seismology – Most common method of detection by creating shock waves that pass through hidden rock layers and interpreting the waves that are reflected back to the surface. In seismic surveys, a shock wave could be created by either of the following:
Compressed-air gun - shoots pulses of air into the water (for exploration over water)
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Seismic Vibrator Truck (Thumper trucks) – Uses hydraulic actuator to vibrate base plate of
the truck, generating rapid massive force also known as “thumps”. (for exploration over land) Explosives - detonated after being drilled into the ground (for exploration over land) or thrown overboard (for exploration over water)
Figure 1. Airgun (left) and illustration of “seismic streamers” (right)
Figure 2. Ilustration of thumper trucks emitting massive force to geophone receivers. The shock waves travel beneath the surface of the Earth and are reflected back by the various rock layers. The type or density of the rock layers passed affects the reflection speed and these reflections are detected by sensitive microphones or vibration. Seismologists interpret the readings for signs of oil and gas traps. Page | 4
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EXTENDED REACH WELL Once geologists find a prospective oil strike, they mark the location using GPS coordinates on land or by marker buoys on water.
2.1.1 Further analysis of opportunity However, knowing a hydrocarbon reserve exist, does not necessarily equate to drilling a well to extract it. Due to the high capital intensity nature of the oil and gas industry, thoroughly researched considerations are made to ensure that all deductions on the reservoir are as precise as possible and it is worth producing. For investors assurance and further confirmation on production worth, the following matters are taken into considerations:
The location of the hydrocarbon reserves Volume of formation hydrocarbon present and their producible amount. Rate of formation hydrocarbon production Uncertainties in determining the above matters.
These questions are answered with the least amount of uncertainties possible by the combined effort of the subsurface team comprising of production geologist, production technologies, reservoir engineer and petrophysicist who obtains and analyze formation and fluid information. Table 1 and Table 2 in the next page contains the type of information acquired and their description. Table 1. Formation information Type of Information Depth Gross thickness
Description The depth or location at which the reservoir is located It is the thickness of the stratigraphically defined interval in which the reservoir beds occur, including such non-productive intervals as
Net Reservoir Porosity
may be interbedded between the productive intervals. It is the volume of the gross thickness that contains reservoir rocks The porosity of a reservoir rock is defined as that fraction of the bulk Page | 5
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EXTENDED REACH WELL volume of the reservoir that is not occupied by the solid framework Permeability
of the reservoir. The rock’s ability to conduct fluids is termed as permeability. The permeability of a rock depends on its effective porosity, consequently, it is affected by the rock grain size, grain shape, grain size distribution (sorting), grain packing, and the degree of
Lithology
consolidation and cementation. Litho is latin for rock. It is used as a gross identification for a rock layer in the subsurface There are 3 major rock types which are sedimentary, igneous and metamorphic rock. Sedimentary rocks are further subdivided into Siliciclastic or carbonate. In Brunei, the type
Velocity
of rock is mostly Siliciclastic. The uniform seismic velocity of a particular homogeneous rock type.
Formation Dip
For a rock thickness h and a single-travel time t, Vfor= h/t Formation dip is an imaginary line constructed down-slope on a sedimentary bed or fault. The dip direction is perpendicular to the strike direction and usually expressed in bearing and an angle of tilt (dip) measured from the horizontal plane to the top of a bed or fault-a dip angle may not exceed 90 degrees. Table 2. Fluid Information
Type of Information Type
Significance To determine the type of fluid in the subsurface whether it is hydrocarbon or water
Distribution Saturation
Saturation controls the mobility of liquids (water and free product)
Pressure Viscosity Density Contaminants
through a porous medium. Energy drive to push hydrocarbon from the subsurface Measure of how resistant a fluid is to flow Mass per unit volume of a substance Contaminants are anything that is foreign to the fluid system and it is
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EXTENDED REACH WELL responsible for over 75% of all system failures hence its monitoring is very important.
Aside from that, there are also other petrophysical information required:
Hole Volume Reservoir pressure Reservoir temperature Perforation interval Top of cement / cement quality Corrosion monitoring Production rate Identification of production issues
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EXTENDED REACH WELL 2.1.2 Hydrocarbon volume calculation The calculation of hydrocarbon volume requires us to know the volume of the formations containing hydrocarbons (the reservoir rock volume), the porosity of each formation, and the hydrocarbon saturation in each formation. Volume of reservoir rock The volume of reservoir rock in a single zone depends on the area of the zone A, and the thickness of reservoir in the zone h. Data obtainable from the seismic data given by the geologist which is the only data not obtained via petrophysical technique.
Figure 3. Illustration of a zone of reservoir rock and formula of calculation Volume of pore in the rock The relative amount of pore space to the bulk volume is denoted by the porosity Φ, where the porosity is the fraction of the bulk volume occupied by pore volume, and is expressed as a fraction or as a percentage;
Φ = Vpore /Vbulk Hydrocarbon Saturation
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EXTENDED REACH WELL In general the porosity is completely occupied by either water and hydrocarbon, where the saturation of the water is Sw, and that of the hydrocarbon is Sh, and Sw + Sh = 1. Hence, hydrocarbon saturation could be computed from this.
Sh = (1 - Sw) Volume of Hydrocarbon in Place (HCIP) Finally, the volume of hydrocarbon as it would be in the subsurface and can be calculated by combining the earlier mentioned formulas:
HCIP= A x h x (N/G) x Φ x (1-Sw) The determination of this value is the primary job of the petrophysicist, and requires a lithological assessment and zonation of the reservoir. At a later stage the petrophysicist may also be called upon to assess the permeability of the reservoir under various conditions. However, the primary function of the petrophysicist is to assess the amount of hydrocarbons initially in place.
Figure 4.. Illustration of HCIP equation derivation.
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EXTENDED REACH WELL HCIP indicates the volume of hydrocarbon when it is in the subsurface where it is subjected to raised temperature and pressure. This would change once it is brought to the surface. Hence, when determining the surface volume, the formation volume factor B o (for oil) and Bg (for gas) is included into the HCIP equation. 2.2 Wells Economics There are many points in the project life cycle where an oil and gas operating company has to decide whether operations remain commercially viable. In fact, only 10% of wells are likely to be successful in frontier offshore exploration areas. In Wytch farm, due to aesthetic and environmental regulation, British Petroleum developed a project that initially called for an artificial island with conventional directional at a cost of $330 million. However, it was replaced instead with an ERD well at less than half the cost (app. $150 million). Although in the above case, an ERD well is considered as more economical, in general, ERD wells are deemed high on the capital intensity scale than conventional wells which averages from $30 to $40 million. The cost to drill and equip a well, any well, varies by such factors as the depth of the well, its general location, and industry economics that drive demand for drilling rigs in the immediate area of the well site. The reason for the enhanced cost in drilling an ERD well is due to the requirement for higher spec equipments such as drilling rig with larger capacity and also the requirement of additional tools or resources due to the complex nature. For example, the drilling mud used in an ERD well is typically Oil Based Mud or Synthetic Based Mud because of their higher friction reduction properties which is less in cheaper water based mud.
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EXTENDED REACH WELL The costs for drilling, completing and servicing a well could be further broken down into three basic elements with subsets (Refer to Figure 3)
Time-dependent costs Depth-dependent costs Fixed costs or Once-off costs
Contract Payment Small Unit Personnel Big Unit Timedependent
Consumables Services Fees Company Overhead
Types of Costs
Equipment Depthdependent Consumables Fixed Cost Once Off Cost Figure 5. Cost Breakdown Overview
Overall well cost may be expressed using the following formula:
Cwdo = Cd + Co
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Cd is the drilling cost or ‘hole-making cost’ (including the cost of each bit used)
Co is all other costs of drilling (e.g. casings, mud, cementing services, logging services, coring services, site preparation, fuel, transportation and completion).
Co can be easily calculated – simply total of each individual costs.
Cd, drilling cost, can be expressed as:
Cd = drilling cost $/m Cb = cost of bit, $/bit Cr = fixed operating cost of rig, $/hr tb = total rotating time, hrs tc = total non-rotating time (e.g. connecting time), hrs tt = total trip time (round trip), hrs ∆D= depth drilled with bit, m/bit
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EXTENDED REACH WELL 2. Life Cycle of a Well The stages by which an extended reach well or also known as ERD well undergoes, is the same to that of a conventional well. However, special consideration to certain aspects within the stages due to the increase of complexity that heightens some issues pre-existing in any well drilling project. The following diagram shows the key stages of the typical oil and gas project life cycle.
Planning
Drilling
Completion
Production
Abandonm ent
3.1 Planning Well planning is usually an iterative process to determine the optimal balance among wellpath, fluid and hydraulic requirements, drillstring design, torque & drag analysis, casing setting depth and others. In other to achieve an optimum well path, the following aspects are considered to be key factors in well planning:
Well Trajectory Build Rate Surveying and Target Sizing
3.1.1 Well Trajectory
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EXTENDED REACH WELL Optimum well trajectory for directional drilling is determined by the identified limiting factors which could be in terms of most critical operations or wellbore characteristic. There are several approaches to trajectory design to achieve long reaches with the fewest possible limitations on other downhole operations. Table 3 is a general comparison of the major options while Figure 6 is a representation of the various trajectory profiles.
Table3. Trajectory design major options Option Multiple Build Profile: Rate of build increases with depth in several discrete steps to tangent angle, hold constant tangent angle Build and Hold: Constant build rate to tangent angle, hold constant tangent angle Double Build: Build‐hold‐build‐ hold trajectory, can use two different BURs in the build sections Under‐Section: Build and hold with deep KOP
Advantages Very long reach, torque/drag values, casing wear
Disadvantages low High tangent angle low
Simple, long reaches achievable, low tangent angle Very long reaches possible with low contact forces in upper build Reducing hanging weight below build section, reduces contact force in build Inverted: Tangent angle above Flexibility for multiple horizontal so the wellbore enters the targets, avoids gas cap reservoir from underneath
Potentially high contact force in build (torque, casing wear) May require deep steering, High second tangent angle High tangent angle, shorter reach
Higher axial (buckling) loads to push string uphill, deep steering required 3‐D: Any of the above with Flexibility to handle anti‐ More curvature means significant azimuth changes collision and multiple target more torque and drag, deep steering may be requirements required, shorter reach
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Figure 6. Illustration of the various trajectory profile 3.1.2 Build Rate Build rate may have a marginal effect on torque and drag levels for very high ratio wells. This is due to the increasing percentage of string weight supported on the low side of the hole resulting in lower tensile forces at surface. However contact forces may be sufficient to promote unacceptable casing wear at the higher build rates. As a guideline, build rates in excess of 2.5 o / 30m may cause concern with respect to high contact forces. If higher build rates than this are planned, the difficulty of achieving a smooth build also has to be considered where an increasing
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EXTENDED REACH WELL percentage of the build will be performed while sliding and not rotating the assembly. Some other important things to note about build rate are:
High Reach/TVD ratio wells may tolerate high BUR because the string tension in the curve is low and may even be in mechanical compression.
Low Reach/TVD ratio wells do not tolerate high BUR since drill string tension in the curve is higher.
High build rates can cause casing wear problems, especially in high Reach/TVD ratio wells where there may be high tensile loads through the build section during trips out of the hole and backreaming.
Low BURs result in lower contact forces. This typically means lower casing wear.
Low tortuosity is also achievable with low BURs. It tends to be more difficult to maintain low tortuosity with a high BUR.
Generally, with lower build rate, more can be achieved while rotating the assembly and thus the chances of achieving the desired smooth build will be greatest.
3.1.3 Hole sizing and selection The majority of ERD wells drilled around the world use a combination of 17½”, 12 ¼”, and 8½” hole sizes (K & M Technologies, 2003). According to K & M Technologies, the reasons for this include the availability of tools and equipment, ability to drill smaller hole sizes, and simply the depth of experience in these sizes. There are instances when different hole sizes are used for example in two-strings well design where considerations could be given to using 13½” and 9 7/8” hole sizes.
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EXTENDED REACH WELL 3.2 Drilling The selection of rig and its components along with the bottom hole assembly, drill string, casing, and cementing designs are governed by the issues that are either specific to ERD wells or simply magnified in comparison to other conventional wells. Aside from that, the procedures involved in drilling are also adapted in order to mitigate any risks involved in such a complex well design. Below are the major contributing factors in equipment and procedure done during drilling:
Directional Well Design, Torque and Drag Limitations, Hydraulics and Hole Cleaning, Vibration and Wellbore Stability, Equivalent Circulation Density Management, as well as Mud Rheology and Solids Control.
3.2.1 Rig Type Drilling rigs are categorized according to whether it is used for land drilling or offshore drilling. This is further subdivided accordingly. (Refer to Figure 7) Bottom Supported Rigs Onshore Trailer mounted Barges
Drilling rigs
Bottom Supported
Platforms Jackup
Offshore
Drill Ship Floating
Semi -submersible tension leg platform
Figure 7. Different types of drilling rigs Extended reach wells challenge the capabilities of the drilling rig more than a conventional directional well of the same measured depth – except for pick‐up loads. Rig and logistics issues Page | 17
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EXTENDED REACH WELL include storage space, setback space, accuracy of load indicators, pump pressure and volume capacity, and top-drive output torque. Factors in choosing rig:
Anticipated formation pressure
Hole and casing programme
Preferred drill string size and weight(s) to be used
Hoisting requirement
Hydraulic requirement
Rotary requirements
Auxiliary equipment required
Figure 8. Different types of offshore rigs and their relative operating depth visual representation
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EXTENDED REACH WELL 3.2.2 Rotary Drilling Components Rotary drilling rigs are used in drilling an extended reached well. They function to create rotation of the drill string and lift the drill stringas well as casings and special equipment into and out of the hole drilled. The main components of a rotary drilling rig are:
Power system Hoisting system Rotating Circulating Figure 9 Illustration of the rotary drilling main components overview
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EXTENDED REACH WELL 3.2.2.1 Power System In most rigs, the power is consumed by hoisting and fluid circulating system. The early drilling rigs were powered by steam. Modern rigs are powered by internal combustion diesel engine and classified as: (i)
Diesel electric type
(ii)
Direct Drive type
For ERD wells, power may be limited, especially in a backreaming scenario where pick‐up, torque and pumps are all operating at/or near their limit. The combined power usage when deep on a long ERD well may thus become an issue because it is often at this point that maximum output levels are required from the mud pump, drawworks and the rotary system. Many of the industry’s rigs that are being utilized for conventional directional drilling do not have the capability to meet these combined output requirements.
Figure 10.
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EXTENDED REACH WELL 3.2.2.2 Hoisting System The main task of the hoisting system is to lower and raise the drill string, casings, and other subsurface equipment into or out of the well. The hoisting equipment itself consists of: 1. 2. 3. 4. 5. 6. 7. 8. 9.
Draw works, Fast line, Crown block, Travelling block, Dead line, Dead line anchor, Storage reel, Hook and Derrick,
Figure 11. Hoisting equipment elements set up.
3.2.2.3 Rotary System The function of the rotary system is to transmit rotation to the drill string and consequently rotate the bit. During drilling operation, this rotation is to the right. The main parts of the rotary system in a conventional well are: Page | 21
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EXTENDED REACH WELL 1. Swivel, 2. Rotary hose, 3. Kelly, 4. Rotary drive (master bushing, Kelly pushing), 5. Rotary table and 6. Drill string For Extended reach drilling, certain parts are either replaced with more specialized ones or additional equipments are added. Rotary table Rotary table rotate clockwise or anti-clockwise and apply torque to the drill string and anything attached to it. Other than that, it supports the entire string on slips when the pipe is not suspended from the hook
Figure 12. Rotary Table In drilling extended-reach wells, drillpipe is rotated not by the rotary table but instead by the topdrive, which travels the length of the derrick and permits drilling with an entire stand of pipe. The topdrive also provides backreaming capacity and the capability to push casing down the well when high drag is encountered. Maximum output from the top drive system is closely related to the maximum torque capacity of the drill pipe used.
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Figure 13. Example of Overdrive from Halliburton. Drill string Design The drill string is an important part of the rotary drilling process. It is the connection between the rig and the drill bit. The components of a drill string are:
Kelly
Drill pipe
Drill collars
Other various pieces of equipment (Bottom Hole Assembly) above the drill bit such as stabilisers and reamers.
A drill string serve to function as Weight-on-bit provider, transmitter of torque to bit, deviation controller, and conduit for drilling fluid. In drillstring design for ERD, the following considerations are taken into account: (1) determining expected loads;
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EXTENDED REACH WELL (2) selecting drillstring components; (3) verifying each component’s condition; (4) setting operating limits for the rig team; and (5) monitoring condition during drilling. In conventional wells, drillstring tension may be a primary concern, but in ERD, drillstring torsion may be the limiting factor. Running normal-weight drill pipe to apply weight to the bit in ERD can lead to buckling of the drill pipe and rapid fatigue failure. Conventional drilling tools are prone to twist-off because of unanticipated failure under high torsional and tensile loads of an extended-reach well. Drillpipe Torque and drag occurred during the drilling of an extended-reach well (ERW), when the drill string is pushed through the long horizontal hole section is a limiting factor for conventional drill pipe technology presented by standard steel drill pipes. High-strength grades are ideal when lighter drill pipe are required to minimize torque and drag in ultra-extended reach wells. Reducing wall thickness also improves hydraulic programs, making it easier to reach drilling targets efficiently. Substantial ERD projects have used and continue to use either or both 6-5/8" and 5-1/2" drillpipe (DP). However, wider diameter drill pipe could also prove to be a better option for ERD wells
Drillpipe connection Additional torsional strength and better hydraulics compared to standard connections providing exceptional performance in difficult well conditions, such as extended reach wells.
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Figure 14. Sample strong connection VAM DS. Bottom Hole Assembly The bottom hole assembly, or BHA, is that portion of the drill string closest to the drill bit it is designed in consideration of the key issues of involved in overall drilling and hole cleaning process. It consists of several components:
Heavy-weight drill pipe, which has the same outer diameter as regular drill pipe but with
thicker walls for greater weight, used as a transition between drill collars and drill pipe. Drill collars, which are heavy, large diameter pipe located above the bit and below the heavy
wall and used to apply weight to the bit. Stabilizers, which are short drill collars with larger diameter blades that are used to control
contact with the borehole wall. Subs, which are devices used to connect various parts of the BHA. Steerable system such as Rotary BHAs, in which the power to turn the bit is supplied by the
rotary table and downhole motor to provide bit power. MWD or LWD tools for monitoring of downhole conditions.
Drill Bit One of the important aspects of rotary drilling is the drill bit. A drill bit is located at the end of a drill string and its core purpose is to cut or bore into the underground formation. Other than that, it also acts as a conduit for drilling fluid circulation which aids in removal of drilled cuttings. There are several types of drill bits which have varying properties that makes them better suited to drill for their respective formation.
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EXTENDED REACH WELL Selection of the best type of drill bit also depends on the performance of the drill bit during drilling the specified formation. These performances will be recorded against the formation type and used for formulating the ideal bit. The performance is measured by: i.
Rate of Penetration (ROP)
ii.
Bit Life (In hours)
iii.
Cost per Unit drilled (Example: Pound per meter, dollars per foot)
There are two major types of drill bit that are commonly used nowadays:
Roller Cone Bits
Fixed Cutter bit
Figure 15. Roller cone and fixed cutter bit.
3.2.1.4 Circulating System A typical circulating system on rotary drilling rig consists of fluid that moves and equipment to move as well as clean and condition fluid. Perhaps the single most important aspect of ERD well planning is ensuring that the rig’s pumps, solids control equipment, drillstring components and selected mud system are adequate to keep the hole clean. Hole cleaning is one of the most crucial areas in successfully drilling an ERD well. Page | 26
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EXTENDED REACH WELL The principle components of the mud circulation system are mud pumps, flow lines, drill pipe, nozzles, mud pits/tanks, mud mixing equipment and contaminant removal (solid control equipments).
Figure 16. Overview of circulating system The parameters which affect hole cleaning more than any are flow rate (determines transport and annular velocity), hole angle (determines mechamism of removal, fluid rheology and flow regime, mud density, rate of penetration drill pipe rotation, geometry, eccentricity, cuttings Size, cuttings Shape, cuttings Density and formation. The mud flow rate is the most important factor for hole cleaning in deviated wells. This is because increased pumping speed equates to the faster cuttings removal out of the hole when coupled with ample rotary speed. Page | 27
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EXTENDED REACH WELL Thus, mud pumps and liner sizes should be selected to ensure a sufficiently high flow rate when drilling ERD wells. Pump pressure is often the limiting factor for achieving the required flow rate. Consideration should thus be given to this during the bottom hole assembly (BHA) design and bit nozzle selection to reduce pump pressure. Ideally, maximum available flow rates should be used for every section of an ERD well, up to the surface pressure or downhole tool limits. 3.3 Drilling Fluid Drilling fluid is essentially a mixture of base fluid and chemical additives. There are generally 3 types of drilling fluid / mud:
Water based mud Oil Based Mud Synthetic Based mud
ERD requires longer hole sections, which require longer drilling times; the result is increased exposure of destabilizing fluids to the well bore. Oil-based muds are superior to water-based muds in ERD as water-based muds may not provide the inhibition, lubrication or confining support of oil-based muds. 3.4 Casing and Cementing At a certain stage during the drilling of oil and gas wells. it becomes necessary to line the walls of a borehole with steel pipe which is called casing. Casing serves numerous purposes during the drilling and production history of oil and gas wells, these include:
Keeping the hole open by preventing the weak format ions from collapsing. i.e. caving of the
hole. Serving as a high strength flow conduit to surface for both drilling and production fluids. Protecting the freshwater-bearing formations from contamination by drilling and production fluids.
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Providing a suitable support for wellhead equipment and blowout preventers for controlling
subsurface pressure. and for the installation of tubing and subsurface equipment. Providing safe passage for running wireline equipment Allowing isolated communication with selectively perforated formation(s) of interest. There are 5 different types of casing used to isolate different depths/zones of formations. They are conductor casing, surface casing, intermediate casing, production casing and liner.
Figure 17. Different types of casings illustration. 3.5 Drilling Operations In Drilling operations, the previously discussed components are brought together to successfully drill a well. Below is an overview of how a well is drilled in general: 1. Preparation of drilling location 2. Rig up 3. Safety and risk assessment (e.g. walk the line) 4. Hammer conductor casing into the ground 5. Install diverter on top 6. Drilling of pilot hole to depth below shallow gas zone
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EXTENDED REACH WELL 7. Open pilot hole using stage opener to the size required for surface casing 8. Run and cement surface casing to surface
Cementing is carried out according to the type of procedure decided upon either single or multiple stage cementing.
Figure 18. a sample of procedure and accessories in cementing
9. Install wellhead and BOP 10. Drill to intermediate casing setting depth
Intermediate casing may consist of several casings
Cuttings are removed, cleaned and re-circulated via circulating system
Drilling fluid in ERD wells are usually highly lubricated
Fast pumping is required for proper hole cleaning at horizontal section
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Constant monitoring of Equivalent Circulating Density would be required during drilling
Due to build trajectory, real time monitoring and rotary steerable system is required for accurate arrival at target.
Bit change out may be required in case of bit wear out during drilling and this requires tripping in and tripping out.
11. Run and cement intermediate casing.
Intermediate casing will be run using a combination of high powered overdrive with proprietary grip and accessories that minimizes torque and drag effects such as non-rotating drill pipe and floating casing.
12. Drill to total depth 13. Run and cement production casing 14. Completion 15. Handover to production Drill 8-1/2" Pilot Hole
Open Pilot Hole to 24"
Run and cement 185/8" casing
Drill 16" hole
12 ¼” Hole Check Trip
SBT/Neutron Log Inside 13 3/8” Casing
Drill 12-1/4" hole section
Run and Cement 133/8" casing
Run and Cement 9 5/8” Casing
Drill 8 ½” x 8 ¾’ hole
Run and Cement 7” Liner
Drill 6” x 6 ½” Hole
Figure 19. Example of a drilling sequence
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EXTENDED REACH WELL 3. CONCLUSION Drilling a conventional well or an extended drilled well follows the same sequence of event from planning to drilling execution. However, due to the issues that are heightened during the latter type of well, either specialized or upgraded form of equipment could be required. Other than that, more careful procedure in terms of matters such as hole cleaning are done. The topic on ‘How to drill an Extended Reach Well’ is a complex matter that would not be completely divulged in a short report.
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