Evolution of water content in organic-rich shales with increasing maturity and its controlling factors: Implications from a pyrolysis experiment on a water-saturated shale core sample

摘要

In the present study, a pyrolysis experiment was performed on an immature water-saturated shale core sample, and a set of artificially matured subsamples (maturity with equivalent vitrinite reflectance values of 0.67-3.20%, i.e., EqVRo values) with initial water phases was obtained. The water content of these shale samples and its influencing factors were investigated. The results show that the water content significantly varies with maturity and is closely related to the hydrocarbon generation and expulsion, clay mineral transformation, and pore structure evolution of shales. According to the changes of water content with maturity and the main influencing factors, the evolution of the water content in shales is approximately divided into three stages. At the low maturity stage (0.67% < EqVRo < 0.90%), the water content decreases with increasing maturity, which is mainly controlled by the displacement drainage of liquid hydrocarbons. At the moderately mature stage (0.90% < EqVRo < 1.36%), the water content remarkably increases with increasing maturity, which is mainly due to the generation of clay-associated water. At the high maturity stage (EqVRo > 1.36%), with increasing maturity, the water content remarkably decreases at the wet gas window (1.36% < EqVRo < 1.90%) and slightly decreases at the dry gas window (EqVRo > 1.90%), which is mainly controlled by the displacement drainage of gaseous hydrocarbons. In addition, the pore structure evolution of shale, which changes its water-storage capacity, also has an influence on the water content evolution.