This state is reached in hydrocarbon bearing zones when the reservoir will not produce any water. It depends upon the Bulk Volume Water (BVW) which is calculated from water saturation and porosity: BVW = Sw x
φ
When a zone’s bulk volume water values are constant, then the zone is at Sw irr . This is normally computed from cross-plotting Sw and Porosity on charts which have hyperbolic lines indicating constant BVW values.
The fraction of the pore space containing water is known as the water saturation, and is given the notation Sw. The remaining fraction that contains oil or gas is known as the hydrocarbon saturation, Sh, and is determined by 1- Sw, where 1 = 100% f. Sw can be calculated from log interpretation, normally using a combination of resistivity and porosity data.
technical training 2007 2007 WIre Irelin line e Logs & LWD Inte Interpr rpreta etation tion
Chapter 1
Introduction
Chapter 2
Spontaneous Potential Logs
Chapter 3
Gamma Ray Logs
Chapter 4
Resistivity Logs
Chapter 5
Bulk Density Logs
Chapter 6
Neutron Porosity Logs
Chapter 7
Sonic Logs
Chapter 8
Lithology Determination
Chapter 9
Reservoir Evaluation
Chapter 10
Shaly Sand Analysis
Chapter 11
MWD Overview
Chapter 12
LWD Imaging Logs
Chapter 13
Log Witnessing
Appendix A
Vendor Brochures
Appendix B
Log Interpretation Charts Charts
Locating the presence of oil and gas deposits underground is a complex process spanning many months of preliminary research followed by exploration and development drilling. Potential sites for exploration are identified from seismic studies but full evaluation can only be made by drilling wells to see what is actually there. Advances in seismic data collection and interpretation techniques are leading to less uncertainty and greater chances of locating commercial reserves, but the results of the drilling process are ultimately only as good as the interpretation techniques used in the evaluation process. Formation Evaluation can be grouped into four major categories: • Befor fore Dri Drilling ing Seismic Interpretation Offset Data • Duri During ng Dril Drilli ling ng Mud Logs and Wellsite Geology Measurement While Drilling (MWD) Coring • Post - Dr Drilli lling Wireline Logs Production Tests
Whilst advances in seismic processing have been remarkable in recent times the process is still best suited to large scale exploration and field evaluation. Wellsite geology and mudlogging provide geological data while drilling the well but the drilling process and the inefficiences of the hole cleaning process only allows for a largely qualitative and subjective approach. Coring does produce whole rock from which detailed petrophysical analysis and quantitative measurements of porosity, permeability, fluid saturation may be made but cores are normally normally only taken over short intervals in reservoir rocks leaving the majority of the section un-sampled. Petrophysical logging enables large sections of exposed (and sometimes cased) hole to be scanned and variety of geological and reservoir data to be obtained; quantitative analysis can be performed on the data to supplement other information. Historically, petrophysical logging has been called “Wireline Logging”, or even “Electric Logging” but neither of theses terms adequately describe the current range of logging tools or conveyance methods.
The objectives of logging are multiple and varied; depending on the type of well being drilled and the information required. However, we might might try and list some of the required information as follows: • Geol Geolog ogic ical al Cor Corre rela lati tion on Identification of lithology for correlation between wells or to assist general geological evaluation in the current well. Different logging runs taken over the same interval need to be depth matched in order to ensure that we are comparing like with like. Perforating, taking sidewall cores or obtaining pressure information and fluid samples all require accurate internal depth correlation using logs. • Petrology Logs can help to identify lithology, mineral assemblages and pick out features such as bedding, lamination, porosity, permeability, cementation, fractures and facies and depositional environments. • Rese Reserv rvoi oirr Para Parame mete ters rs Logs can identify permeable zones, measure porosity and permeability, identify fluid types and provide information to calculate saturation levels, differentiate between water, oil and gas and determine fluid contact points. Reservoir pressure can be measured and fluids obtained for analysis. • Rock ock Mec Mech hanics Rock strength and the tectonic forces acting upon rocks at depth can be evaluated from logging tools and the information used to help understand drilling and borehole problems. • Geost Geosteer eerin ing g Appl Applica icatio tions ns When MWD and LWD tools are used the information obtained, at the time of drilling, may be used to help drill the well to the required geological target and indeed navigate the reservoir.
In September 1927, Marcel and Conrad Schlumberger, with Henri Doll, recorded the first electrical resistivity log at Pechelbronn, France. This log was actually called a “carottage electrique” or electrical core since it was a quantitative recording of rock properties. The log was hand plotted from point-by-point resistivity measurements. Since then, more than fifty geophysical-type well logs have
been introduced to record the various electrical, nuclear, acoustical, thermal, chemical and mechanical properties of the earth.
Without interpretation, the measurements provided by the various logs are not particularly useful. It takes time, knowledge, and experience to convert the raw r aw data into meaningful and practical information often using sophisticated computer software; the input data consisting of raw well log data, and the output being porosity, hydrocarbon type, type, fluid saturations, and lithology. Logging tools are conveyed into and out of the borehole in a number of ways. Traditionally wireline conveyed tools log boreholes after they have been drilled; the wireline not only conveying the instruments but al so providing the means of data transmission from the tool to the surface equipment. However, borehole conditions often make the use of wireline tools very difficult. High inclinations, high pressures and temperature and unstable borehole conditions can provide severe limitations on the use of wireline tools. Attaching the instruments to jointed drillpipe or tubing can overcome some of thes e issues and the hole is logged whilst tripping the pipe to the surface. A cable attached to the logging tool is strapped to the pipe and reeled in as the string is tripped. Whilst this process does allow high angle and unstable boreholes to be logged the process is very time consuming and, therefore, expensive. The use of coiled tubing can significantly reduce costs as tripping speeds are much higher and the conductive cable can be threaded internally through the coiled tubing eliminating handling time. The use of MWD and LWD logging tools overcomes many of these issues and also enables the hole to be logged very shortly after drilling minimising invasion and other interpretation issues.
The logging company provides the tools, surface equipment and a team of experienced engineers to perform the logging operation, which may take anything from a few hours to t o many days, depending on the nature of the work. The surface logging unit comprises the control functions, surface computer systems, cable drum and winch. The logging tools, which may be up to 30m long are attached to the cable, which is used both for suspension and data transfer, and lowered to the bottom of the borehole. The cable is then pulled out of the hole and the various rock properties are continuously measured. Pulling speeds are dependent on the type of tool being run but are typically around 1800 feet per hour (600m/hr) when radioactive tools such as a gamma ray log are present and can be as much as 6000 ft/hr (1800m/ hr). During the logging process the data is recorded at surface, correlated for depth and corrected for borehole and mud conditions.
Surface Data Ac qu is it io n Sy st em
Mechanical Winching Drum Logging cable
Digital Data Transmission
Downhole Logging Tool
A logging run is typically made at the end of each drilled section, immediately prior to casing being installed. Whilst some tools can make measurements through steel it is beneficial to record basic information over the open-hole section in order to maximise data quality and minimise interpretation difficulties.
Each logging run is identified by a suitable alpha-numeric system to record the type of instrument being used and the actual tools that were run. This is important for calibration and cost management reasons.
Data processing is almost always done by computer, typically in town but increasingly using modern high powered computers at the wellsite. Basic information can be derived by hand using Quick-Look or Shaly Sand methods or by using relatively simple spreadsheets or other processing software.
Many different types of logs, measuring various rock properties may be run at each casing point. Generally the first and intermediate logging runs are performed for lithological evaluation and stratigraphic correlation purposes. Minor hydrocarbon bearing zones may also be identified, together with possible source rock information. Over the main reservoir section the amount of information required is much greater and a full suite of logs covering lithology, porosity, permeability and fluid saturations are required. Additionally there are many other types of tools available for specific purposes, and of helping with the evaluation of cement jobs and other completion operations. The major logs used for routine evaluation of open hole sections are: • Lith itholog ology y Logs Gamma Ray Spontaneous Potential • Resi Resisti stivit vity y (Satu (Saturat ratio ion) n) Log Logss Laterologs Induction Logs Wave Propagation Logs • Porosity Lo Logs Formation Density Log Neutron Porosity Log Sonic Log • Miscel cellan laneous ous Caliper Dipmeter
Repeat Formation Tester Sidewall Cores Cement Bond Logs
FEET
Gamma Ray
Res istivity
10
Caliper IN
20
0.2
Induction Deep OHMM
10
Bit Size IN
20
0.2
Induction Medium OHMM
0
Gamma Ray API
150
Sonic 200 140
200
Sonic Transit Time US/F
Porosity 40 -0.75
0.45
DRHO G/C3
0.25
Neutron Porosity PU
-0.15
PEF 0
1.95
20 Bulk Density G/C3
2.95
5600
5700
5800
5900
Measurement while drilling services have been available since the early 1980s and provide a means of obtaining petrophysical data in real time during the course of drilling the well. This can be of significant benefit when compared to wireline data which is often only available weeks after drilling a particular section. MWD data is very useful in providing additional geological information for the wellsite geologist and helping with geosteering applications in particular.
The logging tools are installed inside special drill collar sections located in the BHA. Powered by downhole turbines or batteries they measure rock properties whilst the well is being drilled and transmit the data to surface by mud pulse telemetry. This data is decoded and interpreted at surface on the wellsite and is available to the drilling engineers and geologists at the same time (and often earlier) as other drill returns logging information. The range of MWD applications has been significantly extended and enhanced over the years and now includes:
•
Gamma Ray
•
Resistivity
•
Density
•
Neutr eutron on Poro Porosi sity ty
•
Sonic
In addition MWD tools also provide real time directional survey data and drilling dynamics information, both of which can be vitally important to the successful drilling of the well.
Both Wireline Logging Operations and MWD tools have to be able to work under a wide range of physical and chemical conditions in and around the borehole. The depth of the hole, bit diameter, borehole erosion, hole deviation, formation temperature, mud weight and type and formation pressures each cause particular problems to the performance of logging logging tools. Calibration and correction for borehole environment variables must be carried out both during and after logging runs in order to ensure that the interpreted results are as accurate as possible. In most cases it is necessary to make multiple measurements with different tools and cross-plot the results to try and minimise the various effects on particular tool response. Once allowance has been made for factors such as borehole temperature and pressure, the key environment effects controlling interpretation are: •
Dril Drilli ling ng Mud Mud Typ Type
•
Mud Mud Inva Invasi sion on Prof Profil ilee
•
Relatio Relationsh nship ip of Pore Pore Water Water to Mud Filtrat Filtratee
•
Bore Boreho hole le Erosi rosion on
•
Tool Tool Dept Depth h of Inves Investi tigat gation ion
One of the most important pieces of reservoir information is porosity. That is, the amount of void space present in the rock expressed as a percentage of total rock volume.
Pore Volume Porosity % = ⎛ -----------------------------------------------⎞ ⎝ Total Rock Volume⎠
× 100
N. B. When used in Quick Look calculations, porosity porosity is expressed expressed as a number between 0 and 1. For example: Porosity (φ)
=
20%
use 0.20
=
8%
use 0.08
Effective Porosity is the amount of porosity able to transmit fluid, and is of vital importance in reservoir evaluation.
Maximum porosity of 48% is obtained in granular sedimentary rocks when perfectly spherical grains of the same grain size are packed in cubic mode. With compaction due to burial grain packing becomes closer and porosities will be reduced to less than 30% in most cases. Where there is significant variation in grain size and with the addition of matrix or cement, porosity values can be further reduced.
Permeability is the ability of the rock to transmit fluid. It is measured in darcy's and usually given the notation k. One darcy is the permeability when a fluid of viscosity 1 centipoise is passed through a 1 cm cube with a differential pressure
of 1 atmosphere. Since this is a relatively large unit of permeability most oil field reservoir permeability is expressed in millidarcy's (one thousandth of a darcy). For granular clastic rocks, grain size is also a key variable in determining rock permeability along with grain shape and sorting. Larger pore throats will allow fluid to pass more easily than smaller sized throats. Both porosity and permeability in carbonates (limestones and dolomites) are less uniform than in granular clastic rocks, being less to do with transportation and grain erosion, and more a product of original sedimentary features (grain type and matrix) and subsequent (often post-depositional) diagenesis. Dolomites are formed by post-depositional percolation of magnesium bearing fluids which causes original calcite (CaCO3) to re-crystallise as dolomite [(Ca.Mg (CaCO3)]. This process normally results in enhanced porosity and is a key factor in the production of carbonate reservoirs. The other major control on porosity in carbonates is fracturing, particularly in Chalks. Whilst primary porosity of Chalks may be very high, being composed mainly of highly spherical calcareous grains, (microscopic coccoliths), permea bilities may be almost zero because of the very small pore throats. Enhancement of both porosity and permeability is required for these rocks to become potential reservoirs. This can be a problem for wireline and MWD interpretation since the resulting secondary porosity may be too large to be evaluated by the logging tool. The main controls on porosity in clastic rocks are: •
Size of available pores
•
Connecting passages between them
When the rock is 100% saturated with one fluid
The ability to transmit a fluid in the presence of another fluid when the two are immiscible.
The ratio of effective to absolute permeability.
Reservoir permeability is not normally available form direct mea surement, either from wireline or MWD tools. Values are computed using mathematical models which use porosity and irreducible water saturation as a means of deriving the permeability. Irreducible water saturation is the amount of porosity that remains containing water in a hydrocarbon bearing zone. Such water is present in isolated pores not connected to the main permeable flow paths, or left adhered to grains by capillary action and is not able to be removed from the rock. In certain cases permeability may be estimated from imaging tools such as NUMAR’s NMRIL, (Nuclear Magnetic Imaging Log). Permeability is usually defined from the Darcy formula:
k =
Q × µ × L A × ∆ p
Where: Q = 1cc volumetric flowrate
µ = 1 centipoise viscosity of flowing fluid A = 1cm2 cross-sectional area
∆ p = 1 atmosphere/cm pressure gradient L = 1 cm length of section
A permeability of one darcy is usually much higher than that commonly found; consequently, a more common unit is the millidarcy, where: 1 darcy = 1000 millidarcy's A practical oil field rule of thumb for classifying permeability is: • poor to fair = 1.0 to 15 md • moderate = 15 to 50 md • good = 50 to 250 md • very good = 250 to 1000 md
• excellent = 1 darcy
Reservoir permeability is a directional property. Horizontal permeability (kH) is measured parallel to bedding planes. Vertical permeability (kV) across bedding planes is usually lower than horizontal. The ratio kH/kV normally ranges from 1.5 to 3. When only a single fluid flows through the rock, the term absolute permeability is used. However, since petroleum reservoirs contain gas and/ or oil and water, the effective permeability for given fluids in the presence of others must be considered. It should be noted that the sum of effec tive permeabilities will always be less than the absolute permeability. This is due to the mutual interference of the several flowing fluids.
Timur Equation
k md =
0.136φ 4.4 2
Swirr
Morris and Biggs
k md =
C φ 3 2
Swirr
Where C is a constant as follows: Gas: 80 Oil: 250
