Estimating Water Saturation Based on Vp-to-Vs Ratio and Porosity in Shale Gas Reservoirs

Water saturation is very important for estimating free gas content in shale gas reservoirs. And Archie model is too simple to be applied to the shaly reservoirs with conductive minerals. And Vp-to–Vs ratio (VPVS) is capable of calculating gas saturation, which is based on the fact that at a fixed porosity of specific rock skeleton the P–wave velocity decreases dramatically when a small amount of gas is present in fluid filled sand. However, when the gas saturation continues to increase the P–wave velocity changes gradually. In this paper, a new method of estimating total water saturation is proposed for shale gas reservoirs. First the velocities of P–wave and S–wave are calculated with the relevant parameters in shale intervals, then analyzing the effects of water saturation, porosity, permeability, characteristic squirt-flow length, fluid viscosity and sonic frequency on VPVS, and getting the main influencing factors of VPVS. At last the complete empirical formula of water saturation is obtained using the relationship between VPVS, total porosity and total water saturation, as well the core data. And the new method is applied well in YS shale gas field, which is better than the results by Archie.