Changes

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.

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.

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 wireline 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.

[[File:M91Ch6FG40.JPG|thumb|300px|{{figure number|1}}Museum core on display.]]

[[File:M91Ch6FG40.JPG|thumb|300px|{{figure number|1}}Museum core on display.]]

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 ([[:file:M91Ch06FG44.JPG|Figure 5]]).

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 ([[:file:M91Ch06FG44.JPG|Figure 5]]).

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.<ref name=Smolen_1992a>Smolen, J. J., 1992, [[Wire-line formation testers]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 154–157.</ref>

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.<ref name=Smolen_1992a>Smolen, J. J., 1992, [[Wireline formation testers]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 154–157.</ref>

==Wireline coring==

==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 too.<ref name=Rider_1996 /> Sidewall cores are mainly used for lithology determination and biostratigraphic analysis.

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 too.<ref name=Rider_1996 /> Sidewall cores are mainly used for lithology determination and biostratigraphic analysis.

STARTHERE==Checkshot and vertical seismic profiles==

==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.

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.