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| Analyzing air [[permeability]] (K<sub>a</sub> and [[porosity]] (Φ) data separately to characterize rock quality can be deceiving. Analyzing K<sub>a</sub> and Φ data using the K<sub>a</sub>/Φ ratio or the r<sub>35</sub> method<ref name=ch09r46>Pittman, E., D., 1992, Relationship of porosity to permeability to various parameters derived from mercury injection–[[capillary pressure]] curves for sandstone: AAPG Bulletin, vol. 76, no. 2, p. 191–198.</ref> is much more effective for determining quality. The K<sub>a</sub>/Φ ratio or the r<sub>35</sub> method yields information about the fluid flow and storage quality of a rock. | | Analyzing air [[permeability]] (K<sub>a</sub> and [[porosity]] (Φ) data separately to characterize rock quality can be deceiving. Analyzing K<sub>a</sub> and Φ data using the K<sub>a</sub>/Φ ratio or the r<sub>35</sub> method<ref name=ch09r46>Pittman, E., D., 1992, Relationship of porosity to permeability to various parameters derived from mercury injection–[[capillary pressure]] curves for sandstone: AAPG Bulletin, vol. 76, no. 2, p. 191–198.</ref> is much more effective for determining quality. The K<sub>a</sub>/Φ ratio or the r<sub>35</sub> method yields information about the fluid flow and storage quality of a rock. |
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| + | [[file:predicting-reservoir-system-quality-and-performance_fig9-16.png|thumb|{{figure number|1}}See text for explanation.]] |
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| ==Which is better rock?== | | ==Which is better rock?== |
− | Using K<sub>a</sub> and Φ data separately to characterize reservoir rock quality is misleading. Consider the rocks shown in the SEM microphotographs in the figure below. Flow unit 1 is a mesoporous, sucrosic dolomite. Its average Φ is 30% and average K<sub>a</sub> is 10 md. Flow unit 2 is a macroporous, oolitic limestone. Its average Φ is 10% and average K<sub>a</sub> is 10 md. | + | Using K<sub>a</sub> and Φ data separately to characterize reservoir rock quality is misleading. Consider the rocks shown in the SEM microphotographs in [[:file:predicting-reservoir-system-quality-and-performance_fig9-16.png|Figure 1]]. Flow unit 1 is a mesoporous, sucrosic dolomite. Its average Φ is 30% and average K<sub>a</sub> is 10 md. Flow unit 2 is a macroporous, oolitic limestone. Its average Φ is 10% and average K<sub>a</sub> is 10 md. |
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| Initially, we might think that flow unit 1 is higher quality because it has three times more porosity and the same permeability as flow unit 2. However, in terms of fluid flow efficiency and storage, as shown by the K<sub>a</sub>/Φ ratio or r<sub>35</sub>, flow unit 2 is actually the better rock. | | Initially, we might think that flow unit 1 is higher quality because it has three times more porosity and the same permeability as flow unit 2. However, in terms of fluid flow efficiency and storage, as shown by the K<sub>a</sub>/Φ ratio or r<sub>35</sub>, flow unit 2 is actually the better rock. |
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| K<sub>a</sub> and Φ are standard components of many reservoir engineering wellbore flow performance equations. The K<sub>a</sub>/Φ ratio reflects rock quality in terms of flow efficiency of a reservoir sample. When clastics and carbonates are deposited, they have a close correlation of particle size to the K<sub>a</sub>/Φ ratio. Mean pore throat radius increases as grain or crystal size increases, but modification to grain shape and size tends to “smear” the distribution. | | K<sub>a</sub> and Φ are standard components of many reservoir engineering wellbore flow performance equations. The K<sub>a</sub>/Φ ratio reflects rock quality in terms of flow efficiency of a reservoir sample. When clastics and carbonates are deposited, they have a close correlation of particle size to the K<sub>a</sub>/Φ ratio. Mean pore throat radius increases as grain or crystal size increases, but modification to grain shape and size tends to “smear” the distribution. |
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− | In the example on the preceding page, flow unit 1 has a K<sub>a</sub>/Φ value of 33 and flow unit 2 has a K<sub>a</sub>/Φ value of 100. Even though Φ is greater and K<sub>a</sub> is the same for flow unit 1, the lower K<sub>a</sub>/Φ value indicates its quality is lower than flow unit 2. | + | In [[:file:predicting-reservoir-system-quality-and-performance_fig9-16.png|Figure 1]], flow unit 1 has a K<sub>a</sub>/Φ value of 33 and flow unit 2 has a K<sub>a</sub>/Φ value of 100. Even though Φ is greater and K<sub>a</sub> is the same for flow unit 1, the lower K<sub>a</sub>/Φ value indicates its quality is lower than flow unit 2. |
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| + | [[file:predicting-reservoir-system-quality-and-performance_fig9-17.png|thumb|{{figure number|2}}See text for explanation.]] |
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| ==K<sub>a</sub>/Φ plot== | | ==K<sub>a</sub>/Φ plot== |
− | On the plot below, the contours represent a constant K<sub>a</sub>/Φ ratio and divide the plot into areas of similar pore types. Data points that plot along a constant ratio have similar flow quality across a large range of porosity and/or permeability. The clusters of points on the plot below represent hypothetical K<sub>a</sub>/Φ values for flow units 1 and 2 presented in Figure 9-16. The position of the clusters relative to the K<sub>a</sub>/Φ contours indicates flow unit 2 has higher quality in terms of K<sub>a</sub>/Φ ratio than flow unit 1. | + | On the plot in [[:file:predicting-reservoir-system-quality-and-performance_fig9-17.png|Figure 2]], the contours represent a constant K<sub>a</sub>/Φ ratio and divide the plot into areas of similar pore types. Data points that plot along a constant ratio have similar flow quality across a large range of porosity and/or permeability. The clusters of points on the plot below represent hypothetical K<sub>a</sub>/Φ values for flow units 1 and 2 presented in Figure 9-16. The position of the clusters relative to the K<sub>a</sub>/Φ contours indicates flow unit 2 has higher quality in terms of K<sub>a</sub>/Φ ratio than flow unit 1. |
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− | [[file:predicting-reservoir-system-quality-and-performance_fig9-16.png|thumb|{{figure number|9-16}}See text for explanation.]]
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− | [[file:predicting-reservoir-system-quality-and-performance_fig9-17.png|thumb|{{figure number|9-17}}See text for explanation.]]
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| ==What is r<sub>35</sub>?== | | ==What is r<sub>35</sub>?== |