Effect of water saturation on the elastic modulus of coal and sandstone

Abstract

Summary Coalbed methane (CBM) as a new potential energy has been of concern to countries in the world. However, measurements of elastic properties for coals and roof rocks under dry and water saturated condition, despite being very important for the hydraulic fracture propagation of coals, have been very limited. In this paper, the elastic properties of dry and water-saturated coals and roof rocks are measured under different pressure conditions, and the elastic properties also are predicted by DKT model. The results show The fully water saturation increases the P-wave velocity substantially and reduces the S-wave velocity slightly, which suggest that the P-wave is more sensitive to the water. The rocks containing clay minerals would show shear softening after water-saturated, a possible explanation for reduction of the shear modules is driven by multiple competing factors such as change in stiffness of contacts between clay particles and chemical softening of clay particles. DKT model can estimate the shear modulus of water-saturated-rocks better, and must be combined with rock physical analysis when performing fluid displacement for unconventional reservoir rocks, instead of blindly using Gassmann model.