Investigation of thermal-hydraulic-mechanical coupling model for in-situ transformation of oil shale considering pore structure and anisotropy

The in-situ transformation of oil shale is an intricately complex process involving multiple physical field coupling. Through a series of laboratory experiments, this study reveals the relationship between the anisotropy of pore structure and the anisotropy of physical and mechanical properties in oil shale during the heating process. Results reveal that during heating, pyrolysis-induced parallel bedding macroscopic cracks significantly diminish thermal conductivity in the vertical bedding direction, drastically elevate permeability in the parallel bedding direction, and markedly decrease compressive strength in the parallel bedding direction and elastic modulus in the vertical bedding direction. Subsequently, we firstly propose a thermal-hydraulic-mechanical coupling model for the in-situ transformation of oil shale, which integrates anisotropic thermodynamic damage with a transversely isotropic constitutive model, to investigate the variation patterns of the reservoir temperature field, seepage field, stress field and displacement field during the convective heating process for in-situ transformation. Research findings indicate that: (1) the temperature field expands elliptically from the heating well and disseminates outwardly, achieving the target temperature across the entire reservoir by the 585th day of heating. (2) Permeability changes exhibit pronounced anisotropy and are tightly correlated with temperature fluctuations. (3) The distribution of pore pressure undergoes alterations due to temperature increases, which in turn impacts the heating rate of water vapor. (4) The vertical displacement change of the reservoir cap progresses through four distinct stages: a rapid increase phase, a brief rapid decrease phase, a transitional phase and a continuous decrease phase. Notably, the maximum expansion displacement is 0.056 m, while the maximum compression displacement reaches -0.081 m. This research not only provides significant scientific theoretical support for advancing the development of in-situ transformation technology for oil shale, but also offers reliable scientific evidence for large-scale industrial exploitation of oil shale in the future.