Changes

==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., 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]].)

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., 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]].)

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 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]].)

[[file:overview-of-routine-core-analysis_fig3.png|thumb|{{figure number|3}}Data compiled from 5300 sidewall core samples indicate that sidewall cores from low permeability formations have an indicated permeability greater than that determined from conventional core analysis. Sidewall cores from formations with more than 20 md permeability consistently have a measured permeability that is lower than that from conventional analysis. (After <ref name=pt05r38 />; data from <ref name=pt05r82 />.)]]

[[file:overview-of-routine-core-analysis_fig3.png|thumb|{{figure number|3}}Data compiled from 5300 sidewall core samples indicate that sidewall cores from low permeability formations have an indicated permeability greater than that determined from conventional core analysis. Sidewall cores from formations with more than 20 md permeability consistently have a measured permeability that is lower than that from conventional analysis. (After <ref name=pt05r38 />; data from <ref name=pt05r82 />.)]]