Rock physics modelling of elastic properties of organic shale considering kerogen stress and pore pressure distribution

Abstract

Rock physics model for the elastic properties of organic shale is important for shale oil/gas exploration and production. Up until now, little work has been done to investigate the influence of the distributions of the stress in kerogen (kerogen stress) and the pore pressure on the elastic properties of organic shale. In this work, we used the Kuster- Toksöz (KT) model and Gassmann equation to study such effects. The Gassmann equation was applied for the homogeneous kerogen stress or pore pressure case whereas KT model was suitable for the inhomogeneous case. Four cases with different combinations of kerogen stress and pore pressure distributions, for the elastic properties of organic shale, were analysed and the corresponding models were given. Based on these models, a numerical example was studied. The results showed that the distributions of kerogen stress and pore pressure significantly affect the elastic properties of organic shale. The elastic moduli under the inhomogeneous kerogen stress or pore pressure distributions are larger than the homogeneous case, whose magnitude depends on porosity and kerogen content. Furthermore, the joint effects of kerogen stress and pore pressure distributions are similar to those of kerogen stress due to the much smaller effects of pore pressure. Hence, it is essential to consider the effects of the distributions of kerogen stress and pore pressure when building the rock physics model for the elastic properties of organic shale. This work revealed the importance of kerogen stress and pore pressure distributions on the elastic properties of organic shale and hence is helpful for the shale oil/gas exploration and production.