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[[file:Box_core32.jpg|thumb|600x|Box of core samples, labelled and stored on site, [http://nc.water.usgs.gov/ccp/2003Etown/photos.html USGS Coastal Carolina Project 2003,] Elizabethtown.]]

[[file:Box_core32.jpg|thumb|300px|Box of core samples, labelled and stored on site, [http://nc.water.usgs.gov/ccp/2003Etown/photos.html USGS Coastal Carolina Project 2003,] Elizabethtown.]]

Retrieval and analysis of cores is essential to all phases of the petroleum industry. Cores offer the only opportunity to obtain intact, vertically continuous samples that allow the visual examination of depositional sequences and variations in reservoir character. Properly analyzed cores provide data available from no other source; these data should provide direct evidence of the presence, quantity, distribution, and deliverability of hydrocarbons. Cores are essential to understanding the nature of the pore system in the potential reservoir unit. The knowledge gained from cores enhances our ability to predict reservoir performance and to select procedures to maximize profitable hydrocarbon recovery.

Retrieval and analysis of [[core description|cores]] is essential to all phases of the [[petroleum]] industry. Cores offer the only opportunity to obtain intact, vertically continuous samples that allow the visual examination of depositional sequences and variations in reservoir character. Properly analyzed cores provide data available from no other source; these data should provide direct evidence of the presence, quantity, distribution, and deliverability of hydrocarbons. Cores are essential to understanding the nature of the pore system in the potential reservoir unit. The knowledge gained from cores enhances our ability to predict reservoir performance and to select procedures to maximize profitable hydrocarbon recovery.

==Types of cores==

==Types of cores==

Continuous coring, first attempted in Holland in 1908,<ref name=pt05r12>Andersen, G., 1975, Coring and Core Analysis Handbook: Tulsa, OK, PennWell Books, 200 p.</ref> is usually most desirable, although many types of specialized core can be obtained.<ref name=pt05r123>Park, A., 1985, Coring, Part 2—core barrel types and uses: World Oil, v. 200, p. 83–90.</ref> Data from continuous core are typically combined with wireline log and formation test data to evaluate productivity. Diagenetically altered sandstones<ref name=pt05r123 /> and thinly laminated reservoirs<ref name=pt05r29>Bradburn, F. R., Cheatham, C. A., 1988, Improved core recovery in laminated sand shale sequences: Journal of Petroleum Technology, v. 40, p. 1544–1546., 10., 2118/18570-PA</ref> require laboratory analysis of large diameter cores to evaluate [[porosity]], hydrocarbon saturation, and net pay. (For more on continuous coring, see [[Conventional coring]].)

Continuous coring, first attempted in Holland in 1908,<ref name=pt05r12>Andersen, G., 1975, Coring and Core Analysis Handbook: Tulsa, OK, PennWell Books, 200 p.</ref> is usually most desirable, although many types of specialized core can be obtained.<ref name=pt05r123>Park, A., 1985, Coring, Part 2—core barrel types and uses: World Oil, v. 200, p. 83–90.</ref> Data from continuous core are typically combined with wireline log and formation test data to evaluate productivity. Diagenetically altered sandstones<ref name=pt05r123 /> and thinly laminated reservoirs<ref name=pt05r29>Bradburn, F. R., and C. A. Cheatham, 1988, Improved core recovery in laminated sand shale sequences: Journal of Petroleum Technology, v. 40, p. 1544–1546., 10., 2118/18570-PA</ref> require laboratory analysis of large diameter cores to evaluate [[porosity]], hydrocarbon saturation, and net pay. (For more on continuous coring, see [[Conventional coring]].)

An alternative to continuous coring is the retrieval of discrete samples from the wellbore face known as sidewall cores. These samples can provide useful details of the lithology, petrology, porosity, [[permeability]], and hydrocarbon content of the formation.<ref name=pt05r155>Toney, J. B., Speiglets, S. L., 1985, Coring, Part 6—sidewall operations: World Oil, v. 201, p. 29–36.</ref> The analytical results can be used to verify log analysis calculations. Selection of sidewall core points after logging allows selective sampling of specific zones.<ref name=pt05r38>Craft, M., Keelan, D. K., 1985, Coring, Part 7—analytical aspects of [[sidewall coring]]: World Oil, v. 201, p. 77–90.</ref> (See also [[Sidewall coring]].)

An alternative to continuous coring is the retrieval of discrete samples from the wellbore face known as sidewall cores. These samples can provide useful details of the lithology, petrology, porosity, [[permeability]], and hydrocarbon content of the formation.<ref name=pt05r155>Toney, J. B., and S. L. Speiglets, 1985, Coring, Part 6—sidewall operations: World Oil, v. 201, p. 29–36.</ref> The analytical results can be used to verify log analysis calculations. Selection of sidewall core points after logging allows selective sampling of specific zones.<ref name=pt05r38>Craft, M., and D. K. Keelan, 1985, Coring, Part 7—analytical aspects of sidewall coring: World Oil, v. 201, p. 77–90.</ref> (See also [[Sidewall coring]].)

==Sampling techniques==

==Sampling techniques==

===Sidewall core analysis===

===Sidewall core analysis===

''Sidewall core analysis'' is performed on cores recovered by any of the sidewall coring techniques. Analysis is limited to basic tests for permeability, porosity, and residual fluid saturation.<ref name=pt05r38 />

''Sidewall core analysis'' is performed on cores recovered by any of the sidewall coring techniques.  

Percussion sidewall cores from hard, well-cemented formations are badly altered during the coring process and generally fail to produce suitable measurements of mechanical and petrophysical properties.<ref name=pt05r38 /> Sample alteration may be reduced through the use of sidewall boring or a hydraulic press to collect sidewall core samples.<ref name=pt05r155 />

Percussion sidewall cores from hard, well-cemented formations are badly altered during the coring process and generally fail to produce suitable measurements of mechanical and petrophysical properties.<ref name=pt05r38 /> Sample alteration may be reduced through the use of sidewall boring or a hydraulic press to collect sidewall core samples.<ref name=pt05r155 />

Analytical procedures vary slightly with sample type. For conventional core plugs and sidewall cores, gas volume is determined by mercury injection. Oil and water volumes are determined by high temperature distillation. Sidewall core measurements are all carried out on one sample. In conventional core analysis, gas volume is determined on one sample while oil and water volumes are measured on a second sample. Significant variations in pore system quality between samples can cause errors in fluid saturation and porosity values.<ref name=pt05r114 />

Analytical procedures vary slightly with sample type. For conventional core plugs and sidewall cores, gas volume is determined by mercury injection. Oil and water volumes are determined by high temperature distillation. Sidewall core measurements are all carried out on one sample. In conventional core analysis, gas volume is determined on one sample while oil and water volumes are measured on a second sample. Significant variations in pore system quality between samples can cause errors in fluid saturation and porosity values.<ref name=pt05r114 />

Gas volume of whole core samples is determined by evacuating the sample and resaturating with water while measuring the weight of water absorbed. With vuggy samples, water injected to refill the gas volume may drain from the surface vugs causing errors in the measured gas saturation.<ref name=pt05r90>Keelan, D. K., 1972, A critical review of core analysis techniques: Journal of Canadian Petroleum Technology, v. 2, p. 42–55.</ref> The oil and water volumes are determined by retorting the core under vacuum. The recovered water volume corresponds to the sum of the pore water and the gas volumes. Low API gravity oils are only partly recovered by this process, thus recovered oil volume may be low.<ref name=pt05r90 /> (For more on determining porosity from core samples, see [[Porosity]].)

Gas volume of whole core samples is determined by evacuating the sample and resaturating with water while measuring the weight of water absorbed. With vuggy samples, water injected to refill the gas volume may drain from the surface vugs causing errors in the measured gas saturation.<ref name=pt05r90>Keelan, D. K., 1972, A critical review of core analysis techniques: Journal of Canadian Petroleum Technology, v. 2, p. 42–55.</ref> The oil and water volumes are determined by retorting the core under vacuum. The recovered water volume corresponds to the sum of the pore water and the gas volumes. Low [[API gravity]] oils are only partly recovered by this process, thus recovered oil volume may be low.<ref name=pt05r90 /> (For more on determining porosity from core samples, see [[Porosity]].)

==Residual fluid saturation==

==Residual fluid saturation==

<gallery mode=packed heights=300px widths=300px>

<gallery mode=packed heights=300px widths=300px>

overview-of-routine-core-analysis_fig1.png|{{figure number|1}}Comparison of water saturation data<ref name=pt05r82 /> from sidewall and conventional cores shows that sidewall core values are almost always higher than conventional core values. Sample values from gas condensate zones are 10 to 15% higher, while values from oil zones are 5 to 10% higher.

overview-of-routine-core-analysis_fig1.png|{{figure number|1}}Comparison of water saturation data<ref name=pt05r82 /> from sidewall and conventional cores shows that sidewall core values are almost always higher than conventional core values. Sample values from gas condensate zones are 10 to 15% higher, while values from oil zones are 5 to 10% higher.

overview-of-routine-core-analysis_fig2.png|{{figure number|2}}Comparison of saturation data<ref name=pt05r82 /> indicate that sidewall cores from gas condensate zones may have measured oil saturation values that are 2% higher than conventional core samples. In oil zones the relationship is less clear. The agreement between oil saturation values In sidewall and conventional cores may vary with such oil characteristics as API gravity.

overview-of-routine-core-analysis_fig2.png|{{figure number|2}}Comparison of saturation data<ref name=pt05r82 /> indicate that sidewall cores from gas condensate zones may have measured oil saturation values that are 2% higher than conventional core samples. In oil zones the relationship is less clear. The agreement between oil saturation values In sidewall and conventional cores may vary with such oil characteristics as [[API gravity]].

</gallery>

</gallery>

Water saturations determined on sidewall cores from gas condensate zones are generally 10 to 15% higher than values from conventional cores. In oil zones, the water saturation from sidewall cores may be 5 to 10% higher relative to conventional core values ([[:file:overview-of-routine-core-analysis_fig1.png|Figure 1]]).

Water saturations determined on sidewall cores from gas condensate zones are generally 10 to 15% higher than values from conventional cores. In oil zones, the water saturation from sidewall cores may be 5 to 10% higher relative to conventional core values ([[:file:overview-of-routine-core-analysis_fig1.png|Figure 1]]).

The agreement between sidewall and conventional core residual oil saturations varies with oil characteristics. When oil gravity is in the range of 35° to 40° API, sidewall core oil saturation values are slightly lower than those obtained by conventional core analysis. As oil gravity and viscosity increase, sidewall core oil saturations become 10 to 20% lower than conventional core saturations.<ref name=pt05r38 /> In gas condensate zones, sidewall cores have measured oil saturations that are equal to or a few percent higher than conventional core values ([[:file:overview-of-routine-core-analysis_fig2.png|Figure 2]]).

The agreement between sidewall and conventional core residual oil saturations varies with oil characteristics. When oil gravity is in the range of 35° to 40° API, sidewall core oil saturation values are slightly lower than those obtained by conventional core analysis. As oil gravity and [[viscosity]] increase, sidewall core oil saturations become 10 to 20% lower than conventional core saturations.<ref name=pt05r38 /> In gas condensate zones, sidewall cores have measured oil saturations that are equal to or a few percent higher than conventional core values ([[:file:overview-of-routine-core-analysis_fig2.png|Figure 2]]).

==Porosity==

==Porosity==

Whole core porosity is usually less than conventional plug porosity because there is a strong tendency to sample the more porous zones preferentially. Whole core samples incorporate tighter parts of the pore system that are frequently excluded from conventional samples. However, whole core porosity may be higher than that determined from conventional analysis when large solution voids are present or when the core is badly invaded by mud solids.

Whole core porosity is usually less than conventional plug porosity because there is a strong tendency to sample the more porous zones preferentially. Whole core samples incorporate tighter parts of the pore system that are frequently excluded from conventional samples. However, whole core porosity may be higher than that determined from conventional analysis when large solution voids are present or when the core is badly invaded by mud solids.

In samples having a porosity greater than 30%, sidewall core porosity is 1 to 2% lower than conventional analysis porosity. This results from slight compaction that occurs during coring. Medium and low porosity percussion sidewall samples, especially from highly cemented rocks, display porosity that is much too high due to fracturing and grain shattering. The deviation between measured porosity and true porosity becomes greater as the actual porosity decreases. Uncertainty caused by systematic variation in sidewall core porosity relative to plug analysis values can be minimized by development of correlations between sidewall core and conventional core values.<ref name=pt05r38 /> (For more on porosity, see [[Porosity]].)

In samples having a porosity greater than 30%, sidewall core porosity is 1 to 2% lower than conventional analysis porosity. This results from slight compaction that occurs during coring. Medium and low porosity percussion sidewall samples, especially from highly cemented rocks, display porosity that is much too high due to [[Fracture|fracturing]] and grain shattering. The deviation between measured porosity and true porosity becomes greater as the actual porosity decreases. Uncertainty caused by systematic variation in sidewall core porosity relative to plug analysis values can be minimized by development of correlations between sidewall core and conventional core values.<ref name=pt05r38 /> (For more on porosity, see [[Porosity]].)

==Permeability==

==Permeability==

==See also==

==See also==

* [[Core description]]

* [[Core description]]

* [[Permeability]]

* [[Permeability]]

* [[SEM, XRD, CL, and XF methods]]

* [[SEM, XRD, CL, and XF methods]]

[[Category:Laboratory methods]]

[[Category:Laboratory methods]]