A method of Shaly sand correction for estimating gas hydrate saturations using downhole electrical resistivity log data

Estimation of the amount of nonconductive and conductive constituents in the pore space of sediments, using electrical resistivity logs, generally loses accuracy when clays are present in the reservoir. Many different methods and clay models have been proposed to account for the conductivity of clay (for example, the shaly sand correction). In this study, the Simandoux model is employed to correct for the clay effect in order to more accurately estimate gas hydrate saturations.

This study utilizes the fact that the effect of clay on the resistivity of a sediment is manifested in the Archie constants a and m, values of which are generally a = 1 and m = 2 for clean-sand reservoirs. Results of the study indicate that as the clay content increases, a also increases whereas m decreases. On the basis of the relationship between the Archie constants a and m with respect to the clay amount, a method of correcting for the clay effect on the estimation of water saturation is proposed. This method works well if the relationship between porosity and resistivity on a log-log plot is approximately linear and if accurate Archie constants a and m for clean sand are known. However, because of the linearity condition, it is difficult to apply the method to low-porosity reservoirs. Gas-hydrate-bearing sediments generally have high porosities because of their shallow depth of occurrence, so the method can be effectively applied in estimating gas hydrate saturations.