Changes

[[file:May2013BulletinCover.jpg|thumb|400px|Cliff-forming Wingate Sandstone, as seen from Upheaval Dome Road toward the Holeman Spring Canyon, Canyonlands National Park. Photo by Anita Torabi. Courtesy AAPG Bulletin.]]

[[file:May2013BulletinCover.jpg|thumb|400px|Cliff-forming Wingate Sandstone, as seen from Upheaval Dome Road toward the Holeman Spring Canyon, Canyonlands National Park. Photo by Anita Torabi. Courtesy AAPG Bulletin.]]

Pore type, pore geometry, and fluid properties are critical factors affecting [[permeability]]. Sandstone texture directly affects pore type and geometry. Knowing what textures and fluids to expect, as well as what authigenic clays might be present, can help us predict permeability.

[[Pore systems|Pore type]], [[Pore system shapes|pore geometry]], and [[Petroleum reservoir fluid properties|fluid properties]] are critical factors affecting [[permeability]]. Sandstone texture directly affects pore type and geometry. Knowing what textures and fluids to expect, as well as what [[authigenic clay]]s might be present, can help us predict permeability.

==Effects of pore type and geometry==

==Effects of pore type and geometry==

Pore type, defined by pore throat size (i.e., macroporosity), directly controls rock permeability. Pore throat size limits flow capacity. Pore geometry also affects permeability, but not as much. The rougher the surface of the pore, the more difficult for fluid to flow through the pore and the lower the permeability.

Pore type, defined by [[Pore and pore throat sizes|pore throat size]] (i.e., macroporosity), directly controls rock [[permeability]]. Pore throat size limits [[Fundamentals of fluid flow|flow capacity]]. Pore geometry also affects permeability, but not as much. The rougher the surface of the pore, the more difficult for fluid to flow through the pore and the lower the permeability.

==Effects of texture==

==Effects of texture==

Sandstone texture affects permeability as follows:

Sandstone texture affects permeability as follows:

* Decreasing grain size decreases permeability.

* Decreasing [[grain size]] decreases permeability.

* Increasing grain sorting increases permeability.

* Increasing grain sorting increases permeability.

* Increasing grain rounding increases permeability.

* Increasing grain rounding increases permeability.

Use the following rules of thumb for permeability for oil vs. gas reservoirs:

Use the following rules of thumb for permeability for oil vs. gas reservoirs:

* At >10 md, the reservoir can produce oil without [[stimulation]].

* At >10 millidarcies (md), the [[reservoir]] can produce oil without [[stimulation]].

* At >1 md, the reservoir can produce gas without stimulation.

* At >1 md, the reservoir can produce gas without stimulation.

* At 1–10 md, the reservoir probably requires stimulation for oil production.

* At 1–10 md, the reservoir probably requires stimulation for oil production.

[[file:predicting-reservoir-system-quality-and-performance_fig9-61.png|300px|thumb|{{figure number|2}}. Copyright: F. K. North;<ref name=North_1985>North, F. K., 1985, Petroleum Geology: London, Allen & Unwin, 607 p.</ref> courtesy Allen & Unwin.]]

[[file:predicting-reservoir-system-quality-and-performance_fig9-61.png|300px|thumb|{{figure number|2}}. Copyright: F. K. North;<ref name=North_1985>North, F. K., 1985, Petroleum Geology: London, Allen & Unwin, 607 p.</ref> courtesy Allen & Unwin.]]

Pore-bridging clays, like illite, decrease porosity slightly but can destroy sandstone permeability. Discrete particle clay, like kaolinite, lowers porosity and permeability only slightly. [[:file:predicting-reservoir-system-quality-and-performance_fig9-61.png|Figure 2]] compares porosity-permeability relationships for kaolinite-, chlorite-, and illite-cemented sandstones. Note there is no significant change in porosities, but permeabilities range over four orders of magnitude.

Pore-bridging clays, like [[illite]], decrease porosity slightly but can destroy sandstone [[permeability]]. Discrete particle clay, like [[kaolinite]], lowers [[porosity]] and permeability only slightly. [[:file:predicting-reservoir-system-quality-and-performance_fig9-61.png|Figure 2]] compares porosity-permeability relationships for kaolinite-, [[chlorite]]-, and illite-cemented sandstones. Note there is no significant change in porosities, but permeabilities range over four orders of magnitude.

 

[[file:predicting-reservoir-system-quality-and-performance_fig9-62.png|thumb|300px|{{figure number|3}}After Neasham.<ref name=ch09r40>Neasham, J. W., 1977, The morphology of dispersed clay in sandstone reservoirs and its effect on sandstone shaliness, pore space, and fluid flow properties: Proceedings of the SPE Annual Meeting, October 9–12, paper SPE-6858.</ref> Copyright: SPE.]]

[[file:predicting-reservoir-system-quality-and-performance_fig9-62.png|thumb|300px|{{figure number|3}}After Neasham.<ref name=ch09r40>Neasham, J. W., 1977, The morphology of dispersed clay in sandstone reservoirs and its effect on sandstone shaliness, pore space, and fluid flow properties: Proceedings of the SPE Annual Meeting, October 9–12, paper SPE-6858.</ref> Copyright: SPE.]]

[[file:predicting-reservoir-system-quality-and-performance_fig9-63.png|300px|thumb|{{figure number|4}}After Wilson and Pittman.<ref name=ch09r68>Wilson, M. D., Pittman, E. D., 1977, Authigenic clays in sandstones: recognition and influence on reservoir properties and [[paleoenvironmental analysis]]: Journal of Sedimentary Petrology, vol. 47, no. 1, p. 3–31.</ref> Copyright: Journal of Sedimentary Petrology.]]

[[file:predicting-reservoir-system-quality-and-performance_fig9-63.png|300px|thumb|{{figure number|4}}After Wilson and Pittman.<ref name=ch09r68>Wilson, M. D., Pittman, E. D., 1977, Authigenic clays in sandstones: recognition and influence on reservoir properties and [[paleoenvironmental analysis]]: Journal of Sedimentary Petrology, vol. 47, no. 1, p. 3–31.</ref> Copyright: Journal of Sedimentary Petrology.]]

Detrital clays can be part of sandstone matrix or grains. As matrix, detrital clays can obliterate permeability. Detrital grains of clay are often [[Ductility|ductile]] and can be compacted into pore spaces during burial. The percentage of detrital clay in a rock determines permeability. [[:file:predicting-reservoir-system-quality-and-performance_fig9-63.png|Figure 4]] shows different types of detrital clays in a sandstone.

[[Detrital clay]]s can be part of sandstone matrix or grains. As matrix, detrital clays can obliterate permeability. Detrital grains of clay are often [[Ductility|ductile]] and can be compacted into pore spaces during burial. The percentage of detrital clay in a rock determines permeability. [[:file:predicting-reservoir-system-quality-and-performance_fig9-63.png|Figure 4]] shows different types of detrital clays in a sandstone.

==Effect of quartz overgrowths==

==Effect of quartz overgrowths==

In general, as quartz cement precipitates, the pore–pore throat size ratio approaches 1 (Hartmann et al., 1985{{citation needed}}). Throats are reduced less than pore space; therefore, permeability is affected less than porosity. During cementation, the size of the pore spaces between the pore-filling crystals decreases until it approaches the size of the pore throats. Throats become more tabular or sheet-like. Sandstone porosity may be quite low (<5%) and still have some permeability (<10 md) where cemented with quartz.

In general, as quartz cement precipitates, the pore:pore throat size ratio approaches 1 (Hartmann et al., 1985{{citation needed}}). Throats are reduced less than pore space; therefore, [[permeability]] is affected less than porosity. During cementation, the size of the pore spaces between the pore-filling crystals decreases until it approaches the size of the pore throats. Throats become more tabular or sheet-like. Sandstone porosity may be quite low (<5%) and still have some permeability (<10 md) where cemented with quartz. 9See also [[Connectivity and pore throat size]].)

==Effect of fractures==

==Effect of fractures==

Fractures enhance the permeability of any sandstone reservoir. Fractures are especially important for improving the permeability of sandstone reservoirs with abundant microporosity or disconnected dissolution porosity.

[[Fracture]]s enhance the permeability of any sandstone reservoir. Fractures are especially important for improving the permeability of sandstone reservoirs with abundant microporosity or disconnected dissolution porosity.

===Predicting from texture and clay content===

===Predicting from texture and clay content===

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| 1

| 1

| Estimate grain size, sorting, and porosity using the depositional environment. For example, if a reservoir is a beach sand, it should be fine- to medium-grained and well sorted with well-rounded quartz grains

| Estimate grain size, sorting, and porosity using the [[depositional environment]]. For example, if a reservoir is a beach sand, it should be fine to medium grained and well sorted with well-rounded quartz grains

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| 2

| 2

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| 4

| 4

| Determine if authigenic clay is present. If so, what kind: pore lining, discrete particle, or pore throat bridging? Adjust permeability downward according to clay type present.

| Determine if authigenic clay is present. If so, what kind: pore lining, discrete particle, or pore-throat bridging? Adjust permeability downward according to clay type present.

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| 5

| 5

* [[Predicting sandstone reservoir porosity]]

* [[Predicting sandstone reservoir porosity]]

* [[Estimating sandstone permeability from cuttings]]

* [[Estimating sandstone permeability from cuttings]]

==References==

==References==