For conventional reservoirs, matrix permeability is a crucial petrophysical parameter for completion design and reservoir characterization. However, direct measurement of permeability is difficult to obtain through well logging. Traditionally, continuous permeability is estimated from porosity log using relationships derived from core measurements. This method can result in inaccurate permeability values when there's a poor correlation between porosity and permeability.
It has been shown previously that the kΛ equation can accurately estimate permeability when applied to log data. This equation computes permeability from lithology weight fractions by associating a specific surface area (S0) value with each lithology. In general, S0 for sand and carbonate are well defined and stable. However, the specific surface area for clay (S0clay) depends largely on clay type and can be highly variable from reservoir to reservoir. Since clay has the most significant effect on permeability, the wrong assumption on S0clay will lead to strong bias in the interpretation results.
In modern logging programs, formation pressure tests are commonly included for reservoir characterization. By observing pressure behavior versus time in a pretest, one can directly measure fluid mobility at discrete stations. We can then use the measured fluid mobility to calibrate S0clay and compute a continuous permeability log. Here is the workflow:
Zone the well such that the clay is of the same type within the zones.
Using a univariate optimization routine, find S0clay the value that minimizes the sum of squared difference between permeability from pressure tests and permeability from well logs within the zone.
Use the calibrated S0 for clay to compute a continuous permeability log from quantitative lithology.
In this report, we demonstrate that in a complex sandstone reservoir containing interbedded sand and shale, the variability in permeability is fully captured by the continuous permeability log. The calibrated clay S0 ensures that the interpreted permeability matches measured permeability from pressure tests.