Permeability is a crucial petrophysical parameter for the client's completion design. Traditionally, a continuous permeability log is obtained by applying to log porosity a porosity-to-permeability transform derived from core measurements. This method, although widely used, can result in misleading interpretation when there's a poor correlation between porosity and permeability. In particular, this can happen in the presence of clay. Even a small amount of clay can reduce matrix permeability dramatically.
It has been shown previously that permeability can be more accurately estimated from surface area-to-pore volume ratio. This ratio can be obtained from lithology weight fractions by associating a specific surface area (S0) with each lithology. Quantitative lithology is interpreted in a commercial workflow by using geochemical logs from wireline/LWD tools using Pulsed Neutron Generators (PNG). In general, the S0 values for sand and carbonate are well defined and stable. However, S0 for clay depends largely on clay type and varies from reservoir to reservoir. Without core measurements for calibration, it is common to assume the clay is a mixture of illite and kaolinite, with smaller quantities of smectite and chlorite. Since clay has the most significant effect on permeability, the wrong assumption will surely lead to bias in the permeability estimation.
In modern logging programs, formation pressure tests are commonly included for reservoir characterization. By analyzing pressure behavior versus time in a pretest, one can a direct measurement of fluid mobility. This mobility can be converted to permeability if fluid viscosity is known. We can then use the permeability derived from pretests to calibrate the specific surface area of clay in our permeability log interpretation. Here is the workflow:
Divide pretests into zones in which rock properties are relatively homogeneous.
Using a univariate optimization routine, find the specific surface area of clay that minimizes the sum of squared difference between permeability from pressure pretests and permeability from geochemical logs within the zone.
Use the calibrated S0 for clay to compute a continuous permeability log from quantitative lithology.
The above method was validated through a case study of wireline and LWD logs acquired in a vertical well in a siliciclastic reservoir.
