Changes

The term ''reservoir heterogeneity'' is used here to describe the geological complexity of a [[What is a reservoir system?|reservoir]] and the relationship of that complexity to the flow of fluids through it.<ref name=pt06r4>Alpay, O. A., 1972, A practical approach to defining reservoir heterogeneity: Journal of Petroleum Technology, 24, p. 841–848., 10., 2118/3608-PA</ref>

The term ''reservoir heterogeneity'' is used here to describe the geological complexity of a [[What is a reservoir system?|reservoir]] and the relationship of that complexity to the flow of fluids through it.<ref name=pt06r4>Alpay, O. A., 1972, A practical approach to defining reservoir heterogeneity: Journal of Petroleum Technology, 24, p. 841–848., 10., 2118/3608-PA</ref>

Reservoirs are inherently heterogeneous assemblages of depositional facies and subfacies (for more information, see [[Lithofacies and environmental analysis of clastic depositional systems#Clastic depositional lithofacies and environments|Clastic lithofacies]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Carbonate sediments and environments|Carbonate lithofacies]]), each with characteristic and commonly differing [[sediment textures]], [[stratification types]], and [[bedding architectures]]. Variability is compounded by postdepositional alterations of the strata, such as through [[Reservoir quality#compaction|compaction]], [[Reservoir quality#cementation|cementation]], and [[Understanding the geology of a structural play#Reservoir and seal changes|tectonic deformation]]. Geological heterogeneities have been classified in a variety of ways according to their size or scale; the common categories, used here, are ''wellbore, interwell'', and ''fieldwide'' scales of heterogeneity ([[:file:geological-heterogeneities_fig1.png|Figure 1]]).

Reservoirs are inherently heterogeneous assemblages of depositional facies and subfacies (for more information, see [[Lithofacies and environmental analysis of clastic depositional systems#Clastic depositional lithofacies and environments|Clastic lithofacies]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Carbonate sediments and environments|Carbonate lithofacies]]), each with characteristic and commonly differing [[sediment texture]]s, [[stratification type]]s, and [[bedding architecture]]s. Variability is compounded by postdepositional alterations of the strata, such as through [[Reservoir quality#compaction|compaction]], [[Reservoir quality#cementation|cementation]], and [[Understanding the geology of a structural play#Reservoir and seal changes|tectonic deformation]]. Geological heterogeneities have been classified in a variety of ways according to their size or scale; the common categories, used here, are ''wellbore, interwell'', and ''fieldwide'' scales of heterogeneity ([[:file:geological-heterogeneities_fig1.png|Figure 1]]).

Heterogeneities at the wellbore scale affect matrix [[permeability]], distribution of [[Interpreting hydrocarbon shows#Residual shows|residual oil]], directional flow of fluids, potential [[Rock-water reaction|fluid-rock interactions]], and [[Rock-water reaction: formation damage|formation damage]]. Heterogeneities at the interwell scale affect [[Fundamentals of fluid flow|fluid flow]] patterns, [[drainage efficiency]] of the reservoir, and vertical and lateral [[Enhanced oil recovery|sweep efficiency]] of [[Waterflooding|secondary and tertiary recovery projects]]. Heterogeneities at the fieldwide scale determine the in-place hydrocarbon volume, areal distribution, and trend of hydrocarbon production.

Heterogeneities at the wellbore scale affect matrix [[permeability]], distribution of [[Interpreting hydrocarbon shows#Residual shows|residual oil]], directional flow of fluids, potential [[Rock-water reaction|fluid-rock interactions]], and [[Rock-water reaction: formation damage|formation damage]]. Heterogeneities at the interwell scale affect [[Fundamentals of fluid flow|fluid flow]] patterns, [[drainage efficiency]] of the reservoir, and vertical and lateral [[Enhanced oil recovery|sweep efficiency]] of [[Waterflooding|secondary and tertiary recovery projects]]. Heterogeneities at the fieldwide scale determine the in-place hydrocarbon volume, areal distribution, and trend of hydrocarbon production.

[[file:geological-heterogeneities_fig2.png|thumb|{{figure number|2}}Typical vertical stratification and permeability profiles of (a) fining- or thinning-upward and (b) coarsening- or thickening-upward sequences. ''Fining'' and ''coarsening'' refer to average relative grain size of individual laminae and beds, and ''thinning'' and ''thickening'' refer to the relative thickness of Individual laminae and beds.]]

[[file:geological-heterogeneities_fig2.png|thumb|{{figure number|2}}Typical vertical stratification and permeability profiles of (a) fining- or thinning-upward and (b) coarsening- or thickening-upward sequences. ''Fining'' and ''coarsening'' refer to average relative grain size of individual laminae and beds, and ''thinning'' and ''thickening'' refer to the relative thickness of Individual laminae and beds.]]

Elements of wellbore heterogeneities include the pore network ([[Pore system fundamentals|pores and pore throats]]), [[Porosity#Influence of textural parameters on porosity|grain size and composition, grain packing]], lamination and [[bedding styles]], [[sedimentary structures]], lithofacies, and [[vertical stratification sequences]]. These properties can be readily described in a numerical or quantitative fashion because of the usual availability of rock samples and well logs. Rock [[Conventional coring|core]]s provide the best information on lithofacies and stratification sequences, plug or whole core [[porosity]], [[Permeability|permeability]], and [[fluid saturation]] (if [[oil-based drilling mud]] was used during coring). The use of [[Quick-look lithology from logs#Log shapes|log shapes]] for facies recognition, as well as [[Sidewall coring|sidewall samples]], [[Basic open hole tools#Microresistivity|micrologs]], and [[Dipmeter analysis|dipmeter]] tools can also provide indirect information on lithofacies and stratification types. (For more on lithofacies, see [[Lithofacies and environmental analysis of clastic depositional systems#Clastic depositional lithofacies and environments|Clastic lithofacies]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Carbonate sediments and environments|Carbonate lithofacies]]). [[Pore networks]], [[grain size]] characteristics, and [[mineralogy]] can be analyzed by routine [[Thin section analysis|thin section petrography]] as well as by [[SEM, XRD, CL, and XF methods#X-ray diffraction (XRD)|X-ray diffraction]], [[Scanning electron microscopy (SEM)|scanning electron microscopy]], [[capillary pressure]] measurements, and [[petrographic image analysis]] (see [[Reservoir quality]]). Analysis of all or most of these properties is essential for adequate reservoir description because these properties provide the database and thus the foundation for reservoir description at larger scales.  

Elements of wellbore heterogeneities include the pore network ([[Pore system fundamentals|pores and pore throats]]), [[Porosity#Influence of textural parameters on porosity|grain size and composition, grain packing]], lamination and [[bedding style]]], [[sedimentary structure]]s, lithofacies, and [[vertical stratification sequence]]s. These properties can be readily described in a numerical or quantitative fashion because of the usual availability of rock samples and well logs. Rock [[Conventional coring|core]]s provide the best information on lithofacies and stratification sequences, plug or whole core [[porosity]], [[Permeability|permeability]], and [[fluid saturation]] (if [[oil-based drilling mud]] was used during coring). The use of [[Quick-look lithology from logs#Log shapes|log shapes]] for facies recognition, as well as [[Sidewall coring|sidewall samples]], [[Basic open hole tools#Microresistivity|micrologs]], and [[Dipmeter analysis|dipmeter]] tools can also provide indirect information on lithofacies and stratification types. (For more on lithofacies, see [[Lithofacies and environmental analysis of clastic depositional systems#Clastic depositional lithofacies and environments|Clastic lithofacies]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Carbonate sediments and environments|Carbonate lithofacies]]). [[Pore networks]], [[grain size]] characteristics, and [[mineralogy]] can be analyzed by routine [[Thin section analysis|thin section petrography]] as well as by [[SEM, XRD, CL, and XF methods#X-ray diffraction (XRD)|X-ray diffraction]], [[Scanning electron microscopy (SEM)|scanning electron microscopy]], [[capillary pressure]] measurements, and [[petrographic image analysis]] (see [[Reservoir quality]]). Analysis of all or most of these properties is essential for adequate reservoir description because these properties provide the database and thus the foundation for reservoir description at larger scales.  

In [[clastic]] rocks, there is usually a direct relationship between primary depositional lithofacies and reservoir properties and performance. For example, sandstones that become progressively thinner bedded and finer grained stratigraphically upward also become progressively less permeable upward ([[:file:geological-heterogeneities_fig1.png|Figure 2]]) so that during [[Waterflooding|waterflood]], both gravity and higher permeability toward the bottom will pull water down. In contrast, sandstones that become progressively thicker bedded and coarser grained upward also become more permeable upward ([[:file:geological-heterogeneities_fig2.png|Figure 2]]) so that during waterflood, gravity still pulls the water down, but permeability pulls the water up, resulting in better vertical [[sweep]].<ref name=pt06r75>Lassiter, T. K., Waggoner, J. R., Lake, L. W., 1986, Reservoir heterogeneities and their influence on ultimate recovery, in Lake, L. W., Carroll, N. B., Jr., eds., Reservoir Characterization: Orlando, FL, Academy Press, p. 545–560.</ref><ref name=pt06r144>van de Graaff, W. J. E., Ealey, P. S. 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0011/1400/1436.htm Geological modeling for simulation studies]: AAPG Bulletin, v. 73, p. 1436–1444.</ref>

In [[clastic]] rocks, there is usually a direct relationship between primary depositional lithofacies and reservoir properties and performance. For example, sandstones that become progressively thinner bedded and finer grained stratigraphically upward also become progressively less permeable upward ([[:file:geological-heterogeneities_fig1.png|Figure 2]]) so that during [[Waterflooding|waterflood]], both gravity and higher permeability toward the bottom will pull water down. In contrast, sandstones that become progressively thicker bedded and coarser grained upward also become more permeable upward ([[:file:geological-heterogeneities_fig2.png|Figure 2]]) so that during waterflood, gravity still pulls the water down, but permeability pulls the water up, resulting in better vertical [[sweep]].<ref name=pt06r75>Lassiter, T. K., Waggoner, J. R., Lake, L. W., 1986, Reservoir heterogeneities and their influence on ultimate recovery, in Lake, L. W., Carroll, N. B., Jr., eds., Reservoir Characterization: Orlando, FL, Academy Press, p. 545–560.</ref><ref name=pt06r144>van de Graaff, W. J. E., Ealey, P. S. 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0011/1400/1436.htm Geological modeling for simulation studies]: AAPG Bulletin, v. 73, p. 1436–1444.</ref>