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{{publication
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Predicting reservoir system quality and performance
| frompg = 9-1
| topg = 9-156
| author = Dan J. Hartmann, Edward A. Beaumont
| link = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==Absolute, effective, and relative permeability==
Reservoirs contain water and oil or gas in varying amounts. Each interferes with and impedes the flow of the others. The aquifer portion of a reservoir system by definition contains water as a single phase (100% S<sub>w</sub>). The [[permeability]] of that rock to water is absolute permeability (K<sub>ab</sub>). The permeability of a reservoir rock to any one fluid in the presence of others is its effective permeability to that fluid. It depends on the values of fluid saturations. Relative permeability to oil (K<sub>ro</sub>), gas (K<sub>rg</sub>), or water (K<sub>rw</sub>) is the ratio of effective permeability of oil, gas, or water to absolute permeability. Relative permeability can be expressed as a number between 0 and 1.0 or as a percent. Pore type and formation [[wettability]] affect relative permeability.
==Why k<sub>ro</sub> or k<sub>rg</sub> is less than k<sub>ab</sub>==
A pore system saturated 100% with any fluid transmits that fluid at a rate relative to the pore throat size and the pressure differential. In the drawing below, the absolute pore throat size (A) is noted as the distance between grain surfaces. When a sample contains oil or gas and water (where water wets the grain surface), the pore throat size (B) for oil or gas flow is less than the absolute pore throat size (A). The thickness of the water layer coating the grains is proportional to the S<sub>w</sub> of the rock. In other words, as [[buoyancy pressure]] increases, S<sub>w</sub> decreases and the effective size of the pore throat for oil or gas flow (B) increases.
[[file:predicting-reservoir-system-quality-and-performance_fig9-26.png|thumb|{{figure number|9-26}}See text for explanation.]]
==Interpreting a relative permeability curve==
The diagram below shows relationships between relative permeability curves (drainage and imbibition), [[capillary pressure]], and fluid distribution in a homogeneous section of a reservoir system. The reservoir system rock has a [[porosity]] of 30% and a permeability of 10 md (r<sub>35</sub> = 1.1μ). Laboratory single-phase air permeability is typically used to represent absolute permeability (K<sub>a</sub> when determining relative permeability to oil or water at a specific S<sub>w</sub>.
The figure below depicts three relative permeability curves:
* Water (K<sub>rw</sub>)—similar for both drainage and imbibition tests
* Oil drainage (K<sub>ro-D</sub>)—reflects migrating oil displacing water (decreasing S<sub>w</sub>) with increasing buoyancy pressure (P<sub>b</sub>)
* Oil imbibition (K<sub>ro-I</sub>)—reflects reduction in oil saturation (S<sub>0</sub>) as a water front moves through a rock sample, resaturating it with water (S<sub>xo</sub>)
The curve labeled K<sub>ro</sub> represents the relative permeability of a formation to oil in the presence of varying water saturation (S<sub>w</sub>). The curve labeled K<sub>rw</sub> represents the relative permeability of the formation to water.
Consider points A–D below. Point A, at S<sub>w</sub> = 100%, is the original condition of the sample. Here K<sub>rw</sub> ≈ K<sub>a</sub> (10 md). At point B (S<sub>w</sub> ≈ 90%, S<sub>0</sub> = 10%), oil breaks through the sample, representing the [[migration]] saturation of the sample; K<sub>ro</sub> = 1.0. At point C (S<sub>w</sub> ≈ 50%, S<sub>o</sub> ≈ 10%), K<sub>rw</sub> is less than 1% of K<sub>a</sub> and water, now confined to only the smallest ports, ceases to flow while oil flow approaches its maximum. At point D on the K<sub>ro-D</sub> curve (S<sub>w</sub> ≈ 20%, S<sub>o</sub> ≈ 80%), relative permeability is approaching 1.0 (~ 10 md).
Figure 9-27 is an example of “drainage” relative permeability of a water-wet reservoir. It shows changes in K<sub>ro</sub> and K<sub>rw</sub> as S<sub>w</sub> decreases, as in a water-drive reservoir during hydrocarbon fill-up. “Imbibition” K<sub>ro</sub> and K<sub>rw</sub> have a different aspect, being measured while S<sub>w</sub> increases, as it does during production in a reservoir with a water drive.
[[file:predicting-reservoir-system-quality-and-performance_fig9-27.png|thumb|{{figure number|9-27}}Modified.]]
==Drainage vs. imbibition curves==
The '''drainage curve''' determines from computed S<sub>w</sub> whether a zone is representative of lower transitional (K<sub>rw</sub>> K<sub>ro</sub> - D), upper transitional (K<sub>rw</sub>ro - D), or free oil (K<sub>rw</sub> ≈ 0). The '''imbibition curve''' relates to performance due to filtrate invasion from water injection or flushing from natural water drive.
==Pore throat size and k<sub>r</sub>==
Every pore type has a unique relative permeability signature. Consider the hypothetical drainage relative permeability type curves shown below. Curves A, B, and C represent the relative permeability relationships for rocks with different port types: macro, meso, and micro, respectively. Curve A represents a rock with greater performance capability than B or C. Note how critical water saturation decreases as pore throat size increases. Also note the changing position of K<sub>ro</sub>–K<sub>rw</sub> crossover with changes in pore throat size.
==Critical water saturation==
Critical S<sub>w</sub> is the point where water saturation is so low that no significant water cut can be measured; only hydrocarbon flows from the reservoir. At S<sub>w</sub> higher than critical S<sub>w</sub>, water flows with hydrocarbon. Where S<sub>w</sub> becomes great enough, only water flows.
==Critical water saturation==
The critical S<sub>w</sub> value is different for each port type. Curve A in Figure 9-28 represents rocks with macroporosity. It has a critical S<sub>w</sub> less than 20%. Curve B represents a rock (continued) with mesoporosity. Mesoporous rocks have a critical S<sub>w</sub> of 20–60%. Curve C represents rocks with microporosity. They have a critical S<sub>w</sub> of 60–80%.
[[file:predicting-reservoir-system-quality-and-performance_fig9-28.png|thumb|{{figure number|9-28}}See text for explanation.]]
The table below summarizes representative critical S<sub>w</sub> values for macro-, meso-, and micropore types that correspond to A, B, and C, respectively, in Figure 9-28.
{| class = "wikitable"
|-
! Pore type
! Micro
! Meso
! Macro
|-
| Critical S<sub>w</sub>
| 60–80%
| 20–60%
|
|-
| Length of transition zone
| >30 m
| 2–30 m
| 0–2 m
|}
==See also==
* [[Pore–fluid interaction]]
* [[Hydrocarbon expulsion, migration, and accumulation]]
* [[Characterizing rock quality]]
* [[Pc curves and saturation profiles]]
* [[Converting Pc curves to buoyancy, height, and pore throat radius]]
* [[What is permeability?]]
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Predicting reservoir system quality and performance]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Predicting reservoir system quality and performance
| frompg = 9-1
| topg = 9-156
| author = Dan J. Hartmann, Edward A. Beaumont
| link = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==Absolute, effective, and relative permeability==
Reservoirs contain water and oil or gas in varying amounts. Each interferes with and impedes the flow of the others. The aquifer portion of a reservoir system by definition contains water as a single phase (100% S<sub>w</sub>). The [[permeability]] of that rock to water is absolute permeability (K<sub>ab</sub>). The permeability of a reservoir rock to any one fluid in the presence of others is its effective permeability to that fluid. It depends on the values of fluid saturations. Relative permeability to oil (K<sub>ro</sub>), gas (K<sub>rg</sub>), or water (K<sub>rw</sub>) is the ratio of effective permeability of oil, gas, or water to absolute permeability. Relative permeability can be expressed as a number between 0 and 1.0 or as a percent. Pore type and formation [[wettability]] affect relative permeability.
==Why k<sub>ro</sub> or k<sub>rg</sub> is less than k<sub>ab</sub>==
A pore system saturated 100% with any fluid transmits that fluid at a rate relative to the pore throat size and the pressure differential. In the drawing below, the absolute pore throat size (A) is noted as the distance between grain surfaces. When a sample contains oil or gas and water (where water wets the grain surface), the pore throat size (B) for oil or gas flow is less than the absolute pore throat size (A). The thickness of the water layer coating the grains is proportional to the S<sub>w</sub> of the rock. In other words, as [[buoyancy pressure]] increases, S<sub>w</sub> decreases and the effective size of the pore throat for oil or gas flow (B) increases.
[[file:predicting-reservoir-system-quality-and-performance_fig9-26.png|thumb|{{figure number|9-26}}See text for explanation.]]
==Interpreting a relative permeability curve==
The diagram below shows relationships between relative permeability curves (drainage and imbibition), [[capillary pressure]], and fluid distribution in a homogeneous section of a reservoir system. The reservoir system rock has a [[porosity]] of 30% and a permeability of 10 md (r<sub>35</sub> = 1.1μ). Laboratory single-phase air permeability is typically used to represent absolute permeability (K<sub>a</sub> when determining relative permeability to oil or water at a specific S<sub>w</sub>.
The figure below depicts three relative permeability curves:
* Water (K<sub>rw</sub>)—similar for both drainage and imbibition tests
* Oil drainage (K<sub>ro-D</sub>)—reflects migrating oil displacing water (decreasing S<sub>w</sub>) with increasing buoyancy pressure (P<sub>b</sub>)
* Oil imbibition (K<sub>ro-I</sub>)—reflects reduction in oil saturation (S<sub>0</sub>) as a water front moves through a rock sample, resaturating it with water (S<sub>xo</sub>)
The curve labeled K<sub>ro</sub> represents the relative permeability of a formation to oil in the presence of varying water saturation (S<sub>w</sub>). The curve labeled K<sub>rw</sub> represents the relative permeability of the formation to water.
Consider points A–D below. Point A, at S<sub>w</sub> = 100%, is the original condition of the sample. Here K<sub>rw</sub> ≈ K<sub>a</sub> (10 md). At point B (S<sub>w</sub> ≈ 90%, S<sub>0</sub> = 10%), oil breaks through the sample, representing the [[migration]] saturation of the sample; K<sub>ro</sub> = 1.0. At point C (S<sub>w</sub> ≈ 50%, S<sub>o</sub> ≈ 10%), K<sub>rw</sub> is less than 1% of K<sub>a</sub> and water, now confined to only the smallest ports, ceases to flow while oil flow approaches its maximum. At point D on the K<sub>ro-D</sub> curve (S<sub>w</sub> ≈ 20%, S<sub>o</sub> ≈ 80%), relative permeability is approaching 1.0 (~ 10 md).
Figure 9-27 is an example of “drainage” relative permeability of a water-wet reservoir. It shows changes in K<sub>ro</sub> and K<sub>rw</sub> as S<sub>w</sub> decreases, as in a water-drive reservoir during hydrocarbon fill-up. “Imbibition” K<sub>ro</sub> and K<sub>rw</sub> have a different aspect, being measured while S<sub>w</sub> increases, as it does during production in a reservoir with a water drive.
[[file:predicting-reservoir-system-quality-and-performance_fig9-27.png|thumb|{{figure number|9-27}}Modified.]]
==Drainage vs. imbibition curves==
The '''drainage curve''' determines from computed S<sub>w</sub> whether a zone is representative of lower transitional (K<sub>rw</sub>> K<sub>ro</sub> - D), upper transitional (K<sub>rw</sub>ro - D), or free oil (K<sub>rw</sub> ≈ 0). The '''imbibition curve''' relates to performance due to filtrate invasion from water injection or flushing from natural water drive.
==Pore throat size and k<sub>r</sub>==
Every pore type has a unique relative permeability signature. Consider the hypothetical drainage relative permeability type curves shown below. Curves A, B, and C represent the relative permeability relationships for rocks with different port types: macro, meso, and micro, respectively. Curve A represents a rock with greater performance capability than B or C. Note how critical water saturation decreases as pore throat size increases. Also note the changing position of K<sub>ro</sub>–K<sub>rw</sub> crossover with changes in pore throat size.
==Critical water saturation==
Critical S<sub>w</sub> is the point where water saturation is so low that no significant water cut can be measured; only hydrocarbon flows from the reservoir. At S<sub>w</sub> higher than critical S<sub>w</sub>, water flows with hydrocarbon. Where S<sub>w</sub> becomes great enough, only water flows.
==Critical water saturation==
The critical S<sub>w</sub> value is different for each port type. Curve A in Figure 9-28 represents rocks with macroporosity. It has a critical S<sub>w</sub> less than 20%. Curve B represents a rock (continued) with mesoporosity. Mesoporous rocks have a critical S<sub>w</sub> of 20–60%. Curve C represents rocks with microporosity. They have a critical S<sub>w</sub> of 60–80%.
[[file:predicting-reservoir-system-quality-and-performance_fig9-28.png|thumb|{{figure number|9-28}}See text for explanation.]]
The table below summarizes representative critical S<sub>w</sub> values for macro-, meso-, and micropore types that correspond to A, B, and C, respectively, in Figure 9-28.
{| class = "wikitable"
|-
! Pore type
! Micro
! Meso
! Macro
|-
| Critical S<sub>w</sub>
| 60–80%
| 20–60%
|
|-
| Length of transition zone
| >30 m
| 2–30 m
| 0–2 m
|}
==See also==
* [[Pore–fluid interaction]]
* [[Hydrocarbon expulsion, migration, and accumulation]]
* [[Characterizing rock quality]]
* [[Pc curves and saturation profiles]]
* [[Converting Pc curves to buoyancy, height, and pore throat radius]]
* [[What is permeability?]]
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Predicting reservoir system quality and performance]]