Estimation of permeability from well logs using resistivity and saturation data

An experimental relationship between permeability, water saturation, and rock resistivity has been suggested. This relationship can be used for interpreting any one of these three parameters when the other two are known. For example, the relationship can be used in interpreting permeability from resistivity logs when water saturation is known. This study is based on electrical measurements of core samples of different permeabilities. Brine saturation, S_w, and corresponding rock resistivity, R_t, were used together to express the permeability of the rock. To eliminate the brine-resistivity effect, the rock resistivity was represented by an apparent formation factor, F_a, which is the ratio of the partially saturated rock resistivity to the brine resistivity, R_w. It was observed that the apparent formation factor vs. water-saturation curves for samples of different permeabilities were arranged similarly to capillary pressure curves, and each permeability is represented by a unique curve. Therefore, permeability was expressed as a function of the apparent formation factor and the brine saturation. When fractional brine saturation is 1.0, permeability becomes a function of formation factor only. Water saturation and apparent formation factor are the parameters that can be calculated from well logs. When water saturation and apparent formation factor are known, permeability can be estimated from the suggested relationship. Data points from the oil zone, transition zone, or water zone can be used for computations. Permeability determinations are sensitive to the accuracy of water-saturation data near irreducible water saturations, although at high water saturations, accuracy of resistivity measurements becomes more significant for correct permeability results.