Difference between revisions of "Data: sources"

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The mud loggers also collect bags of rock cutting samples at regular intervals while the well is being drilled. These may be used later for biostratigraphic and lithological analysis (Whittaker, 1992).
 
The mud loggers also collect bags of rock cutting samples at regular intervals while the well is being drilled. These may be used later for biostratigraphic and lithological analysis (Whittaker, 1992).
  
 +
==Core data==
 +
The geologist uses core data to provide a sedimentological description and rock property analysis for input to the geological model. Specialist service companies perform the core analysis. Rock properties such as porosity and permeability are measured on core plugs cut from the core. These are about 2.5 or 3.8 cm (1 or 1.5 in.) in diameter and about 2.5–5 cm (1–2 in.) long. The plugs are cut horizontally (i.e., bed parallel) at a frequency of three to four samples per meter or every foot for oil companies that use imperial measurements (Monicard, 1980). Vertical core plugs may also be cut every 1.5 m (5 ft) for example. On occasions, large pieces of full-diameter core are used for measuring rock properties instead of small core plugs. This can be a more meaningful way of establishing the reservoir characteristics for the more complex lithologies such as carbonates.
  
 +
Other members of the subsurface team will also use the core data. The petrophysicist uses core data to calibrate the measurement of rock properties from wire-line log data. The reservoir engineer obtains data for the various reservoir parameters needed to understand the physics of fluid distribution and flow. Properties such as capillary pressure and relative permeability are measured by special core analysis, and this is referred to by the acronym SCAL. The geologist will frequently get requests from the production engineer to provide core samples for laboratory tests. The aim is to ensure that the various downhole chemical treatments do not react with the rock or the pore fluid to plug up the pore space and impair productivity.
  
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{{Fig40
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 +
The core is slabbed once all the samples have been taken and the measurements are complete. It will be cut into three vertical sections down the length of the core. The middle slab is kept as a reference core for further study by the geologist. It is placed in a wooden frame and set in resin or glued to a firm base. This part of the core will be kept as a museum core (Figure 40). The other two sections of the core, referred to as the half cut, are kept for subsequent sampling.
 +
 +
==Coring problems==
 +
Representative cores should be taken from wells throughout the field. The aim should be to have key areas of the field covered. Ideally, all the various reservoir units should be cored. The entire reservoir interval should be cored in at least one well if practical.
 +
 +
Cores are commonly taken at the exploration and appraisal stage, although some of the early production wells may also be cored. It is unusual to take core at the mature stage of field development; however, there may be reasons for doing this if the value of information can be justified.
 +
 +
==Core analysis reports==
 +
The core analysis company will provide two reports once a job has been completed. The first is the core analysis report. This can include the following data: horizontal permeability, vertical permeability, porosity, water saturation, oil saturation, grain density, and sometimes a brief description of the core plug lithology (Table 7). A listing will be provided on a foot by foot basis (or its metric equivalent) of the rock properties measured in the lab (Table 8).
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 +
{{Table 7
 +
 +
{{Table 8
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The depths at which preserved core samples have been picked will also be listed. These are selected pieces of core that are kept to preserve the conditions of the rock as close to those in the reservoir as possible. They may be required for special core analysis such as wettability studies (Bajsarowicz, 1992). One preservation method is to store the samples in sealed jars containing simulated formation brine.
 +
 +
A core gamma log will also be included in a core analysis report. The gamma-ray response is measured along the length of the core in the laboratory. It is used to match up the core depths to the depths on the wireline gamma-ray log run over the cored interval in the reservoir. These can differ from about half a meter to sometimes more than 6 m (18 ft). This is because over a distance of 2000 or 3000 m (6500 or 10,000 ft) within the borehole, the drill string to which the core barrel is attached will stretch under tension a few meters more or less than the wireline to which the log is attached. Also, incomplete recovery of core, particularly unconsolidated core, can lead to discrepancies in the core log. Comparison of the core gamma with the wireline gamma log allows the core-to-log shift to be determined. This is important for matching features in the core to the equivalent log response.
 +
 +
{{Fig41
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 +
The second report received is the core photography report (Figure 41). This is a set of color photographs of the slabbed core. The geologist can keep this in the office as a substitute for a trip to the core storage location to see the actual rock. If any oil is present in the core, the core will also be photographed under ultraviolet light. Any oil-saturated intervals will show up as fluorescent patches on the photographs.
 +
 +
{{Fig42
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==The sedimentology report==
 +
It is good practice to call in an expert sedimentologist to look at the core and to provide a detailed sedimentological report. The report will include a sedimentological log with a detailed description of all the sedimentological features seen in the core (Figure 42). Various details will be noted (Blackbourn, 1990).
 +
 +
These include:
 +
* lithology with graphical lithology column
 +
* graphical representation of grain size variation
 +
* accessory minerals and diagenetic cement
 +
* fossils
 +
* diagenetic features
 +
* sedimentary structures
 +
* bioturbation
 +
* nature of bed contacts
 +
* sedimentary texture
 +
* color
 +
* oil staining
 +
* grain sorting
 +
* induration
 +
* lithofacies
 +
* fractures, faults, and other structural features
 +
 +
A written account of the detailed facies description and interpretation will be provided. Interpretations are made as to the likely sedimentary environment of deposition. A summary of the mineralogy, petrography, porosity types, and diagenetic mineralogy should also be included. The pore scale is also important for the production geologist, especially for an understanding on permeability controls and as to whether there are significant amounts of clay minerals that could potentially cause formation damage during production operations (see [[Problem wells]]). The report should include facies photographs, thin section photomicrographs, and, where appropriate, scanning electron microscopy (SEM) photomicrographs.
 +
 +
==Wireline and LWD logs==
 +
Wireline logs are run in wells to determine the physical properties of the rock and fluids in the borehole (Table 9). From this, a detailed interpretation can be made of the geology and fluid saturations in the reservoir interval. A brief summary of these logs is provided here. For more details, the textbooks by Serra (1984), Rider (1996), and Luthi (2001) can be consulted.
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{{Table 9
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{{Fig 43
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==Gamma-ray logs==
 +
A gamma-ray log measures the natural radiation in the rocks, much of which is emitted by the elements potassium, uranium, and thorium (Figure 43). The geologist typically uses the log to differentiate between sandstone and shale for correlation purposes. Sandstones normally show a lower gamma-ray response than shales. The gamma ray is an excellent tool for this, providing it is used in conjunction with other logs to confirm the lithology response. Care should be taken with the interpretation of the gamma-ray log in some sandstones. Sandstones rich in potassium-rich minerals such as potassium feldspar, muscovite mica, illite, or glauconite can give a high gamma response that is easily mistaken for a shale. A gamma spike at the base of a sand-prone upper shoreface profile can be the result of concentrations of heavy, radioactive accessory minerals by wave winnowing.
 +
 +
==Spectral gamma-ray logs==
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Spectral gamma-ray logs measure the relative contribution of potassium, thorium, and uranium to the overall gamma-ray response. A high potassium content generally indicates the presence of minerals such as potassium feldspar and mica. Thorium is associated with the mineral monazite, a common heavy mineral in sandstones sourced from acid igneous rocks (Hurst and Milodowski, 1996). Uranium is commonly found absorbed onto organic material and clay in marine shales (Serra, 1984).
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Spectral gamma-ray logs are used less frequently than the other types of log, although in certain situations they can pick out features that the other logs will not (Hancock, 1992). For example, the spectral gamma-ray log response can be used to identify a zone of potassium feldspar dissolution in leached sandstone below an unconformity.
 +
 +
==Density and neutron logs==
 +
Density and neutron logs are primarily used for estimating the porosity. Density logs measure the bulk density of a formation, a function of the rock matrix density emitted from the log and the density of the fluids in the pore space, according to the degree by which the energy of gamma rays is progressively absorbed and scattered by electrons in the rock. The principle behind the density log is that, for a rock with a given grain and fluid density, the higher the porosity, the less dense the formation will be. A neutron log bombards the formation with neutrons to detect energy changes as a result of collisions with hydrogen atoms. Hydrogen is found in the water (and oil) molecules filling the pore space. Thus the neutron log gives an indication of the formation porosity (Rider, 1996).
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The logs also have specific geological uses. They can be used to pick out cemented intervals in sandstones. Carbonate-cemented intervals will show a distinctive response on these logs.
 +
 +
==Sonic logs==
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A sonic log measures the time it takes for a sound pulse to travel from a transmitter to a receiver via the formation (Rider, 1996). Sonic logs can be used for measuring porosity but are more commonly used by the geophysicist as they give velocity information for calibrating seismic data. Velocity data allow the geophysicist to convert the time taken for a seismic wave to travel down and back from a specific seismic reflector into an equivalent subsurface depth. The geologist can use sonic logs to pick out coals and poorly consolidated sandstones.
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 +
==Electrical logs==
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Electrical logs measure the resistivity of the rock and its contained fluids to the passage of an electrical current (Rider, 1996). A high-resistivity response within a porous rock is an indication of hydrocarbons. The logs can also help to recognize certain lithologies. Tight cemented intervals will have a high-resistivity response and these can be picked out in combination with the density and neutron log response.
 +
 +
==Nuclear magnetic resonance logs==
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Nuclear magnetic resonance (NMR) logs measure how hydrogen nuclei in a static magnetic field respond to an oscillating radio frequency. The liquid filled porosity, pore size distribution, and volume of movable fluids can be characterized from this. It is also possible to estimate permeability values empirically from NMR log data.
 +
 +
==Dipmeter logs==
 +
Dipmeter logs measure the variation in electrical or sonic response around the circumference of the borehole. From this, formation dip and sometimes the orientation of sedimentary structures can be determined (Bourke, 1992; Cameron, 1992).
 +
 +
==Borehole image logs==
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Borehole image logs give a detailed electrical or sonic map of the borehole wall (Luthi, 1992). This enables geological information such as formation dip, sedimentary structures, faulting, and fracturing to be imaged. The dip and azimuths of these features are measured from the image logs. The logs are especially useful for the structural characterization of heavily faulted and fractured reservoirs. They also show thin beds in reservoir intervals where most conventional logs do not have the resolution to detect them.
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 +
==Formation tester logs==
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Wireline pressure test data in infill wells can provide valuable information on the reservoir performance. The formation tester log contains a probe, which is pushed horizontally against the formation to take a measurement of the reservoir pressure. A small fluid sample can also be taken if required. The pressure measurements are repeated at various depths throughout the reservoir, enabling a pressure-depth plot to be made.
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{{Fig 44
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When these tests are conducted in a virgin reservoir preproduction, it may be possible to define the depth of the free-water level. This will correspond to the intersection of the water and oil (gas) gradients. Postproduction, formation tester data can give information on where the reservoir is separating into zones of different pressures as a result of depletion (Figure 44).
 +
 +
The raw log data will show the rate at which the pressure built up for each test, and a crude assessment of the formation permeability can be made from this (Smolen, 1992a).
 +
 +
==Wireline coring==
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Wireline methods such as sidewall coring allow the retrieval of several short plug-type cores from the borehole wall. A series of wire-attached, hollow steel bullets are fired horizontally into the borehole wall from the wireline tool (Rider, 1996). Sidewall cores are mainly used for lithology determination and biostratigraphic analysis.
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 +
==Checkshot and vertical seismic profiles==
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Checkshots and vertical seismic profiles (VSPs) are used by the geophysicist to record velocity information in a well. A checkshot survey is taken at different depths down the borehole (Hardage, 1992). A log with a geophone for detecting seismic signals is run in the hole at the same time as a seismic source is activated at the surface. The distance between the source and the log is established, and the time taken for the signal to travel to the log is measured. From this, an accurate velocity can be calculated.
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The major difference between a checkshot survey and a VSP is that the VSP data are recorded at a much closer sampling interval down the well. The data can be processed to produce a seismic image of the near wellbore area (Hardage, 1992). The results will be used to tie reflectors on seismic lines to geological features in the well.
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==LWD logs==
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Logging-while-drilling (LWD) logs are run as an integral part of the the drill string a short distance behind the drill bit (typically 1.5–24 m [5–80 ft]). The acronyms MWD (monitoring while drilling) or FEWD (formation evaluation while drilling) are also used. These logs enable reservoir measurements to be taken in real time, that is, while the well is being drilled (Medeiros, 1992). The log signal is sent up the borehole either by mud pulses or by electromagnetic transmission. The log response can be displayed on monitors at the rig site or transmitted back to the oil company office. Most of the capabilities of wireline logs are available in LWD form.
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LWD logs may be used for several reasons:
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* Real time data allow critical decisions to be made before the well has been drilled too far; for example, selection of casing points.
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* The successful run of a suite of LWD logs saves a day or more tying up an expensive rig operation exclusively with wireline logging.
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* They can be run as insurance logs where the need for log data is critical. This can happen in areas where there is a chance that open-hole logs may not be possible because of borehole instability (Meehan, 1994).
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* They are used for steering horizontal wells (see [[Well types]]).
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{{Fig 45
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==Production logs==
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Production logs are run in a producing well to determine downhole flow rates and to evaluate reservoir sweep (Figure 45; Table 10). They give the subsurface team an understanding of how the reservoir is behaving under production. For example, if a well is producing water, the logs can then be analyzed to determine which perforated intervals are sourcing the water. The perforations can then be isolated to restore the well to dry oil production (Smolen, 1992b).
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{{Table 10
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The geologist uses production-log data to determine the flow geology characteristics of the reservoir and to help establish where there may be unswept oil and gas targets.
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==Production well-test data and interference and pulse tests==
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Production well tests are an important for reservoir management because they provide information on flow rates, reservoir architecture, rock properties, and reservoir pressures. A production well test is performed by inducing pressure variations in a well over time. An example of this is where a production well is shut in to conduct a pressure buildup test. Fluid will then move into the pressure sink caused by the production, and the pressure will gradually increase in the well. The pressure data are used to assess the properties of the reservoir and the reservoir fluid around the wellbore by a technique known as pressure transient analysis (Lee, 1992). For instance, the higher the permeability, the more rapidly the fluid moves in and the quicker the pressure builds up.
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Two types of tests can be run to give an idea of interwell communication. Interference tests are set up by assigning one of the wells in a specific sector of the reservoir as an observation well. Then one or a number of wells is produced from or injected into and the pressure response is measured in the monitor well. Pulse tests are a variation on the theme of interference tests. The difference is that the active well is shut in, returned to production, shut in, and so on, in a series of pulses. These tests are especially useful in assessing the communication between injection and production wells (Kamal, 1983).
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Radioactive or chemical tracers can be put into an injection well and nearby production wells will be monitored to see when and where the tracers are back produced (Bjornstad et al., 1990). For example, radioactive tracers have been used in the Endicott field in Alaska to identify communication pathways between injection and production wells. The data were used to assess the validity of the geological correlation for the reservoir (Shaw et al., 1996).
  
  

Revision as of 15:24, 3 June 2015

THIS PAGE IS UNDER CONSTRUCTION


Oil Field Production Geology
Series Memoirs
Part The Geological Scheme
Chapter Sources of Data
Author Mike Shepherd
Link Web page
PDF PDF file (requires access)
Store AAPG Store

A large amount of data is available to the production geologist for reservoir evaluation. Much of the data will have been expensive to acquire, particularly if obtained from wells offshore. For instance, core taken from a drilling operation on an offshore drilling rig may have cost more than a million dollars to recover. There is an obligation to take good care of the data and to make sure that the information is accessible, either as well-organized paper data files or as data on a computer shared drive. Data files stored on a computer should be labeled with the originator's initials, a date, and some idea of the significance of the data, e.g., "MS August 31, 2008, final top reservoir depth map." Well files should be compiled with all the available data collected on a well-by-well basis. Good data management can make all the difference between a project that is well organized and effective, and one that is disorganized and inefficient.

Obtaining data in an oil field environment is expensive; therefore, it is necessary to justify the economics of gathering the information. In the early stage of field life, the value of information is enormous; the data are essential for reservoir evaluation. Later on in field life, it becomes more important to justify the expense of the data. The new information should be gathered on the basis that it significantly improves the project value and reduces the company's investment risk (Gerhardt and Haldorsen, 1989).

Types of data

A production geologist will use data from a variety of sources. These include:

  • mud logging data
  • core data
  • sedimentology and petrography reports
  • outcrop analogs/modern depositional environments
  • wireline-log and logging-while-drilling (LWD) data
  • production-log data
  • well-test data
  • fluid samples
  • production data
  • seismic data

Mud logging data

The mud loggers on the rig site will monitor the drilling parameters during the well operation, and these are summarized graphically as a mud log. The mud log will include a lithology log. This is a depth plot showing in graphical form the percentage of the various lithologies in each cutting sample recovered while drilling the wellbore. A written description will be made for the lithology of the drill cuttings. Accompanying the lithology log is a record of the rate of penetration of the drill bit. This is an indication of lithology; sandstone is normally drilled faster than shale for instance. Any drilling problems encountered or changes in the drilling parameters will be reported in the margins of the mud log. The presence of oil shows will be noted. The gas returns and gas chromatography analysis are monitored and graphed against depth. High gas returns are a sign that a hydrocarbon reservoir may have been drilled. Significant concentrations of the higher alkanes on the gas chromatograph can indicate that an oil zone has been penetrated. The mud log is used as a first pass, qualitative indication of reservoir presence and quality. A more detailed and accurate representation will be available once wireline logs have been run and interpreted.

The mud loggers also collect bags of rock cutting samples at regular intervals while the well is being drilled. These may be used later for biostratigraphic and lithological analysis (Whittaker, 1992).

Core data

The geologist uses core data to provide a sedimentological description and rock property analysis for input to the geological model. Specialist service companies perform the core analysis. Rock properties such as porosity and permeability are measured on core plugs cut from the core. These are about 2.5 or 3.8 cm (1 or 1.5 in.) in diameter and about 2.5–5 cm (1–2 in.) long. The plugs are cut horizontally (i.e., bed parallel) at a frequency of three to four samples per meter or every foot for oil companies that use imperial measurements (Monicard, 1980). Vertical core plugs may also be cut every 1.5 m (5 ft) for example. On occasions, large pieces of full-diameter core are used for measuring rock properties instead of small core plugs. This can be a more meaningful way of establishing the reservoir characteristics for the more complex lithologies such as carbonates.

Other members of the subsurface team will also use the core data. The petrophysicist uses core data to calibrate the measurement of rock properties from wire-line log data. The reservoir engineer obtains data for the various reservoir parameters needed to understand the physics of fluid distribution and flow. Properties such as capillary pressure and relative permeability are measured by special core analysis, and this is referred to by the acronym SCAL. The geologist will frequently get requests from the production engineer to provide core samples for laboratory tests. The aim is to ensure that the various downhole chemical treatments do not react with the rock or the pore fluid to plug up the pore space and impair productivity.

{{Fig40

The core is slabbed once all the samples have been taken and the measurements are complete. It will be cut into three vertical sections down the length of the core. The middle slab is kept as a reference core for further study by the geologist. It is placed in a wooden frame and set in resin or glued to a firm base. This part of the core will be kept as a museum core (Figure 40). The other two sections of the core, referred to as the half cut, are kept for subsequent sampling.

Coring problems

Representative cores should be taken from wells throughout the field. The aim should be to have key areas of the field covered. Ideally, all the various reservoir units should be cored. The entire reservoir interval should be cored in at least one well if practical.

Cores are commonly taken at the exploration and appraisal stage, although some of the early production wells may also be cored. It is unusual to take core at the mature stage of field development; however, there may be reasons for doing this if the value of information can be justified.

Core analysis reports

The core analysis company will provide two reports once a job has been completed. The first is the core analysis report. This can include the following data: horizontal permeability, vertical permeability, porosity, water saturation, oil saturation, grain density, and sometimes a brief description of the core plug lithology (Table 7). A listing will be provided on a foot by foot basis (or its metric equivalent) of the rock properties measured in the lab (Table 8).

{{Table 7

{{Table 8

The depths at which preserved core samples have been picked will also be listed. These are selected pieces of core that are kept to preserve the conditions of the rock as close to those in the reservoir as possible. They may be required for special core analysis such as wettability studies (Bajsarowicz, 1992). One preservation method is to store the samples in sealed jars containing simulated formation brine.

A core gamma log will also be included in a core analysis report. The gamma-ray response is measured along the length of the core in the laboratory. It is used to match up the core depths to the depths on the wireline gamma-ray log run over the cored interval in the reservoir. These can differ from about half a meter to sometimes more than 6 m (18 ft). This is because over a distance of 2000 or 3000 m (6500 or 10,000 ft) within the borehole, the drill string to which the core barrel is attached will stretch under tension a few meters more or less than the wireline to which the log is attached. Also, incomplete recovery of core, particularly unconsolidated core, can lead to discrepancies in the core log. Comparison of the core gamma with the wireline gamma log allows the core-to-log shift to be determined. This is important for matching features in the core to the equivalent log response.

{{Fig41

The second report received is the core photography report (Figure 41). This is a set of color photographs of the slabbed core. The geologist can keep this in the office as a substitute for a trip to the core storage location to see the actual rock. If any oil is present in the core, the core will also be photographed under ultraviolet light. Any oil-saturated intervals will show up as fluorescent patches on the photographs.

{{Fig42

The sedimentology report

It is good practice to call in an expert sedimentologist to look at the core and to provide a detailed sedimentological report. The report will include a sedimentological log with a detailed description of all the sedimentological features seen in the core (Figure 42). Various details will be noted (Blackbourn, 1990).

These include:

  • lithology with graphical lithology column
  • graphical representation of grain size variation
  • accessory minerals and diagenetic cement
  • fossils
  • diagenetic features
  • sedimentary structures
  • bioturbation
  • nature of bed contacts
  • sedimentary texture
  • color
  • oil staining
  • grain sorting
  • induration
  • lithofacies
  • fractures, faults, and other structural features

A written account of the detailed facies description and interpretation will be provided. Interpretations are made as to the likely sedimentary environment of deposition. A summary of the mineralogy, petrography, porosity types, and diagenetic mineralogy should also be included. The pore scale is also important for the production geologist, especially for an understanding on permeability controls and as to whether there are significant amounts of clay minerals that could potentially cause formation damage during production operations (see Problem wells). The report should include facies photographs, thin section photomicrographs, and, where appropriate, scanning electron microscopy (SEM) photomicrographs.

Wireline and LWD logs

Wireline logs are run in wells to determine the physical properties of the rock and fluids in the borehole (Table 9). From this, a detailed interpretation can be made of the geology and fluid saturations in the reservoir interval. A brief summary of these logs is provided here. For more details, the textbooks by Serra (1984), Rider (1996), and Luthi (2001) can be consulted.

{{Table 9

{{Fig 43

Gamma-ray logs

A gamma-ray log measures the natural radiation in the rocks, much of which is emitted by the elements potassium, uranium, and thorium (Figure 43). The geologist typically uses the log to differentiate between sandstone and shale for correlation purposes. Sandstones normally show a lower gamma-ray response than shales. The gamma ray is an excellent tool for this, providing it is used in conjunction with other logs to confirm the lithology response. Care should be taken with the interpretation of the gamma-ray log in some sandstones. Sandstones rich in potassium-rich minerals such as potassium feldspar, muscovite mica, illite, or glauconite can give a high gamma response that is easily mistaken for a shale. A gamma spike at the base of a sand-prone upper shoreface profile can be the result of concentrations of heavy, radioactive accessory minerals by wave winnowing.

Spectral gamma-ray logs

Spectral gamma-ray logs measure the relative contribution of potassium, thorium, and uranium to the overall gamma-ray response. A high potassium content generally indicates the presence of minerals such as potassium feldspar and mica. Thorium is associated with the mineral monazite, a common heavy mineral in sandstones sourced from acid igneous rocks (Hurst and Milodowski, 1996). Uranium is commonly found absorbed onto organic material and clay in marine shales (Serra, 1984).

Spectral gamma-ray logs are used less frequently than the other types of log, although in certain situations they can pick out features that the other logs will not (Hancock, 1992). For example, the spectral gamma-ray log response can be used to identify a zone of potassium feldspar dissolution in leached sandstone below an unconformity.

Density and neutron logs

Density and neutron logs are primarily used for estimating the porosity. Density logs measure the bulk density of a formation, a function of the rock matrix density emitted from the log and the density of the fluids in the pore space, according to the degree by which the energy of gamma rays is progressively absorbed and scattered by electrons in the rock. The principle behind the density log is that, for a rock with a given grain and fluid density, the higher the porosity, the less dense the formation will be. A neutron log bombards the formation with neutrons to detect energy changes as a result of collisions with hydrogen atoms. Hydrogen is found in the water (and oil) molecules filling the pore space. Thus the neutron log gives an indication of the formation porosity (Rider, 1996).

The logs also have specific geological uses. They can be used to pick out cemented intervals in sandstones. Carbonate-cemented intervals will show a distinctive response on these logs.

Sonic logs

A sonic log measures the time it takes for a sound pulse to travel from a transmitter to a receiver via the formation (Rider, 1996). Sonic logs can be used for measuring porosity but are more commonly used by the geophysicist as they give velocity information for calibrating seismic data. Velocity data allow the geophysicist to convert the time taken for a seismic wave to travel down and back from a specific seismic reflector into an equivalent subsurface depth. The geologist can use sonic logs to pick out coals and poorly consolidated sandstones.

Electrical logs

Electrical logs measure the resistivity of the rock and its contained fluids to the passage of an electrical current (Rider, 1996). A high-resistivity response within a porous rock is an indication of hydrocarbons. The logs can also help to recognize certain lithologies. Tight cemented intervals will have a high-resistivity response and these can be picked out in combination with the density and neutron log response.

Nuclear magnetic resonance logs

Nuclear magnetic resonance (NMR) logs measure how hydrogen nuclei in a static magnetic field respond to an oscillating radio frequency. The liquid filled porosity, pore size distribution, and volume of movable fluids can be characterized from this. It is also possible to estimate permeability values empirically from NMR log data.

Dipmeter logs

Dipmeter logs measure the variation in electrical or sonic response around the circumference of the borehole. From this, formation dip and sometimes the orientation of sedimentary structures can be determined (Bourke, 1992; Cameron, 1992).

Borehole image logs

Borehole image logs give a detailed electrical or sonic map of the borehole wall (Luthi, 1992). This enables geological information such as formation dip, sedimentary structures, faulting, and fracturing to be imaged. The dip and azimuths of these features are measured from the image logs. The logs are especially useful for the structural characterization of heavily faulted and fractured reservoirs. They also show thin beds in reservoir intervals where most conventional logs do not have the resolution to detect them.

Formation tester logs

Wireline pressure test data in infill wells can provide valuable information on the reservoir performance. The formation tester log contains a probe, which is pushed horizontally against the formation to take a measurement of the reservoir pressure. A small fluid sample can also be taken if required. The pressure measurements are repeated at various depths throughout the reservoir, enabling a pressure-depth plot to be made.

{{Fig 44

When these tests are conducted in a virgin reservoir preproduction, it may be possible to define the depth of the free-water level. This will correspond to the intersection of the water and oil (gas) gradients. Postproduction, formation tester data can give information on where the reservoir is separating into zones of different pressures as a result of depletion (Figure 44).

The raw log data will show the rate at which the pressure built up for each test, and a crude assessment of the formation permeability can be made from this (Smolen, 1992a).

Wireline coring

Wireline methods such as sidewall coring allow the retrieval of several short plug-type cores from the borehole wall. A series of wire-attached, hollow steel bullets are fired horizontally into the borehole wall from the wireline tool (Rider, 1996). Sidewall cores are mainly used for lithology determination and biostratigraphic analysis.

Checkshot and vertical seismic profiles

Checkshots and vertical seismic profiles (VSPs) are used by the geophysicist to record velocity information in a well. A checkshot survey is taken at different depths down the borehole (Hardage, 1992). A log with a geophone for detecting seismic signals is run in the hole at the same time as a seismic source is activated at the surface. The distance between the source and the log is established, and the time taken for the signal to travel to the log is measured. From this, an accurate velocity can be calculated.

The major difference between a checkshot survey and a VSP is that the VSP data are recorded at a much closer sampling interval down the well. The data can be processed to produce a seismic image of the near wellbore area (Hardage, 1992). The results will be used to tie reflectors on seismic lines to geological features in the well.

LWD logs

Logging-while-drilling (LWD) logs are run as an integral part of the the drill string a short distance behind the drill bit (typically 1.5–24 m [5–80 ft]). The acronyms MWD (monitoring while drilling) or FEWD (formation evaluation while drilling) are also used. These logs enable reservoir measurements to be taken in real time, that is, while the well is being drilled (Medeiros, 1992). The log signal is sent up the borehole either by mud pulses or by electromagnetic transmission. The log response can be displayed on monitors at the rig site or transmitted back to the oil company office. Most of the capabilities of wireline logs are available in LWD form.

LWD logs may be used for several reasons:

  • Real time data allow critical decisions to be made before the well has been drilled too far; for example, selection of casing points.
  • The successful run of a suite of LWD logs saves a day or more tying up an expensive rig operation exclusively with wireline logging.
  • They can be run as insurance logs where the need for log data is critical. This can happen in areas where there is a chance that open-hole logs may not be possible because of borehole instability (Meehan, 1994).
  • They are used for steering horizontal wells (see Well types).

{{Fig 45

Production logs

Production logs are run in a producing well to determine downhole flow rates and to evaluate reservoir sweep (Figure 45; Table 10). They give the subsurface team an understanding of how the reservoir is behaving under production. For example, if a well is producing water, the logs can then be analyzed to determine which perforated intervals are sourcing the water. The perforations can then be isolated to restore the well to dry oil production (Smolen, 1992b).

{{Table 10

The geologist uses production-log data to determine the flow geology characteristics of the reservoir and to help establish where there may be unswept oil and gas targets.

Production well-test data and interference and pulse tests

Production well tests are an important for reservoir management because they provide information on flow rates, reservoir architecture, rock properties, and reservoir pressures. A production well test is performed by inducing pressure variations in a well over time. An example of this is where a production well is shut in to conduct a pressure buildup test. Fluid will then move into the pressure sink caused by the production, and the pressure will gradually increase in the well. The pressure data are used to assess the properties of the reservoir and the reservoir fluid around the wellbore by a technique known as pressure transient analysis (Lee, 1992). For instance, the higher the permeability, the more rapidly the fluid moves in and the quicker the pressure builds up. Two types of tests can be run to give an idea of interwell communication. Interference tests are set up by assigning one of the wells in a specific sector of the reservoir as an observation well. Then one or a number of wells is produced from or injected into and the pressure response is measured in the monitor well. Pulse tests are a variation on the theme of interference tests. The difference is that the active well is shut in, returned to production, shut in, and so on, in a series of pulses. These tests are especially useful in assessing the communication between injection and production wells (Kamal, 1983). Radioactive or chemical tracers can be put into an injection well and nearby production wells will be monitored to see when and where the tracers are back produced (Bjornstad et al., 1990). For example, radioactive tracers have been used in the Endicott field in Alaska to identify communication pathways between injection and production wells. The data were used to assess the validity of the geological correlation for the reservoir (Shaw et al., 1996).




See also

References

External links

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