Permeability (k)

The permeability of a reservoir is a measure of how much fluid can flow through a rock for a specified pressure drop. Thus, permeability is a property of the rock, and is independent of the fluid (provided the rock is 100% saturated with that fluid). This means that the absolute permeability of a rock is the same, whether the fluid is gas, oil, or water. What does change between these three fluids is the flow rate per unit pressure drop due to the different viscosities.

In a petroleum reservoir, the rock is usually not fully saturated with a single phase fluid. Generally, saturations in the reservoir rock consist of different amounts of gas, oil, and water. These saturations change the effective permeability of the rock.

Permeability can be measured in a laboratory from core analysis. While this is sometimes done with the core in its native or restored state, the more common method is to clean and dry the core, and measure its absolute permeability (usually to air or nitrogen, but the same value would be obtained if water were used instead). Permeability can also be determined by Pressure Transient Analysis (PTA). However, it must be remembered that the permeability determined from analysis is the in-situ effective permeability of the primary reservoir fluid type, and not the absolute permeability. 

The in-situ effective permeability is usually significantly less than the absolute core-derived permeability by a factor ranging from 2 to 200, depending on the reservoir. In addition, when a permeability is determined from a pressure transient test, it reflects the average permeability of the reservoir within the radius of investigation of the test (often several hundred feet). This is in contrast to a core measurement that represents only a few inches of the reservoir.

In PTA, effective permeability (in-situ) can be determined either by semi-log (radial) analysis, or modeling (matching) pressure data. The results of these two techniques should be consistent; any inconsistencies should be accounted for by reviewing and modifying the information and data provided, and/or modifying the analysis/model accordingly.

When gas, oil, and water are being produced during a test, the effective permeability for each phase can be estimated by assuming that only that fluid phase was flowing.

Note:    In most of the pressure transient equations, the permeability term occurs as a mobility (k / m) or a transmissivity (kh /m) term.