Pore structure evolution model of shale reservoir under different fluid pressures: a case study of Wufeng-Longmaxi Formation, SW China

Multiple studies had been conducted to improve the geological understanding and enrichment patterns of the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin’s complex tectonic belt. However, the influence of different structural conditions on shale gas occurrence and shale reservoirs remained unclear. To address this issue, low-mature shale samples from the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin were selected for thermal simulation experiments, based on the actual burial evolution history of the region. Overmature shale samples with the maximum burial depth were then obtained. Next, based on the maximum burial depth sample, the thermal simulation conditions were adjusted to simulate the evolution of the shale reservoir under different structural preservation conditions. Finally, organic geochemistry, FE-SEM, nuclear magnetic resonance, low-temperature gas adsorption, high-pressure mercury injection, and other experiments were conducted to analyze the characteristics of the shale reservoir and the differences in shale gas occurrence under varying structural preservation conditions. As structural preservation conditions get worse, both the organic and inorganic pore sizes in the shale decrease, and organic pore morphology evolved from round-subround to narrow, elongated shapes. Meanwhile, organic and inorganic pore porosity, organic pore proportion, and fluid pressure within the pores decreased as structural preservation conditions get worse, which suggested that better structural preservation conditions were conducive to maintaining fluid pressure in shale pores, which in turn supported pore preservation. Fluid pressure changes had a greater impact on organic pores. FHH model calculations showed that shale pores formed under poorer structural preservation conditions exhibit more complex pore structures and greater heterogeneity. Finally, a pore structure development model was established based on the characteristics of shale pore structure under different preservation conditions.