Mechanism of Wettability and Pore Water Occurrence State in Overmaturity Shale: A Case Study of Longmaxi Formation Shale in South Sichuan Basin, China

Pore water occupying pore surfaces and filling pore spaces significantly impacts the occurrence and exploration of shale gas. Shale gas exploration and development in China have shifted toward deep to ultradeep marine shale formations in the high-maturity to the overmaturity stage. However, the existing research on shale pore water mainly focuses on shallow to medium-deep shale, and the understanding of the wetting characteristics and pore water evolution of high-maturity-overmaturity marine shale is insufficient. Therefore, this study takes the Longmaxi Formation shale in the Sichuan Basin as an example to carry out contact angles, spontaneous imbibition, and drying-NMR coupling experiments to explore the wettability characteristics and pore water occurrence conditions of shale in the overmaturity stage. Subsequently, an evolution model of wettability characteristics and pore water occurrence characteristics in the overmaturity stage of shale was established. The results show that the maturity degree controls the shale wettability significantly. With increasing maturity, the Longmaxi Formation shale reservoir transforms from water-wet to weakly water-wet. The pore size range of movable water, capillary-bound water, clay-bound water, and hydration water are 6.48–8.47, 2.28–7.12, 1.07–2.28, and 1.07 nm or less, respectively. The controlling model with two stages of wettability on the occurrence of pore water was established. In stage I (R0 = 3.0%–3.8%), due to the effect of organic matter graphitization, graphitized organic matter pores collapsed under the action of ground stress. This leads to a decrease in the area available for fluid adsorption and the percentage of micropores, and the shale water-wettability is slowly weakened, so the percentage of movable water increases and then slowly decreases. In stage II (R0 = 3.8%–4.2%), with a further increase in graphitization, graphite crystals occupy the original microporous volume, resulting in a rapid weakening of shale wettability and a rapid increase in the percentage of movable water.