Rock-Failure Proximity as a Precursor to Production during In Situ Conversion of Oil Shale: A THMC-Coupled Analysis

Abstract

The in situ conversion process (ICP) of oil shale is governed by tightly coupled thermal–hydraulic–mechanical–chemical (THMC) interactions, yet the evolution of rock-failure proximity within a fully coupled framework remains poorly quantified. This study develops a THMC coupled framework for oil shale ICP that integrates multiphase, multicomponent flow, heat transfer, linear thermo-poroelastic deformation, pyrolysis kinetics, and stress- and reaction-driven porosity–permeability evolution. Tensile and shear failure factors, derived from the tensile-strength and Mohr–Coulomb criteria, are incorporated to quantify the proximity to mechanically induced failure. The coupled spatiotemporal evolutions of temperature, pressure, kerogen decomposition, and both failure factors are systematically analyzed through a sensitivity study of geological and operational parameters. The results demonstrate that appreciable reductions in the failure factors precede the rapid-production stage, indicating that the formation approaches the mechanical failure condition before peak hydrocarbon output. The low-failure-factor zones are localized in the high-temperature, high-pressure region on the outer flank of the heater wells─jointly controlled by the temperature gradient, pore pressure, and effective stress─rather than coinciding with the locations of maximum temperature or kerogen consumption. Horizontal permeability strongly affects cumulative oil production and failure-factor evolution: under low permeability, impeded fluid expulsion promotes pore-pressure accumulation and extensive failure-activation zones. In contrast, cumulative gas production is mainly influenced by kerogen concentration and porosity. Heating temperature and thermal conductivity regulate the gas–oil ratio through secondary cracking, whereas kerogen concentration and porosity govern the overall product scale. This study provides quantitative bases and practical guidance for field-scale ICP development.