Changes

Analyzing air [[permeability]] (K_a and [[porosity]] (Φ) data separately to characterize rock quality can be deceiving. Analyzing K_a and Φ data using the K_a/Φ ratio or the r₃₅ 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_a/Φ ratio or the r₃₅ method yields information about the fluid flow and storage quality of a rock.

Analyzing air [[permeability]] (K_a and [[porosity]] (Φ) data separately to characterize rock quality can be deceiving. Analyzing K_a and Φ data using the K_a/Φ ratio or the r₃₅ 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_a/Φ ratio or the r₃₅ method yields information about the fluid flow and storage quality of a rock.

==Which is better rock?==

==Which is better rock?==

Using K_a 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_a is 10 md. Flow unit 2 is a macroporous, oolitic limestone. Its average Φ is 10% and average K_a is 10 md.

Using K_a 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_a is 10 md. Flow unit 2 is a macroporous, oolitic limestone. Its average Φ is 10% and average K_a is 10 md.

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_a/Φ ratio or r₃₅, 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_a/Φ ratio or r₃₅, flow unit 2 is actually the better rock.

K_a and Φ are standard components of many reservoir engineering wellbore flow performance equations. The K_a/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_a/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_a and Φ are standard components of many reservoir engineering wellbore flow performance equations. The K_a/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_a/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.

In the example on the preceding page, flow unit 1 has a K_a/Φ value of 33 and flow unit 2 has a K_a/Φ value of 100. Even though Φ is greater and K_a is the same for flow unit 1, the lower K_a/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_a/Φ value of 33 and flow unit 2 has a K_a/Φ value of 100. Even though Φ is greater and K_a is the same for flow unit 1, the lower K_a/Φ value indicates its quality is lower than flow unit 2.

 

==K_a/Φ plot==

==K_a/Φ plot==

On the plot below, the contours represent a constant K_a/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_a/Φ values for flow units 1 and 2 presented in Figure 9-16. The position of the clusters relative to the K_a/Φ contours indicates flow unit 2 has higher quality in terms of K_a/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_a/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_a/Φ values for flow units 1 and 2 presented in Figure 9-16. The position of the clusters relative to the K_a/Φ contours indicates flow unit 2 has higher quality in terms of K_a/Φ ratio than flow unit 1.

 

 

==What is r₃₅?==

==What is r₃₅?==