Sequentially coupled numerical modeling of liquid-propellant-induced in-situ pyrolysis of oil shale

The low thermal conductivity of oil shale restricts heat propagation during in-situ pyrolysis, while reservoir stimulation and heating are commonly treated as separate processes, making it difficult to capture the dynamic alteration of heat-transfer pathways after fracture creation. To address this issue, a sequentially coupled numerical framework integrating liquid-propellant deflagration pressure release, main-fracture propagation, discrete fracture-network construction and mapping, reservoir heat transfer, and kerogen pyrolysis was developed. Deflagration pressure was described using the JWL equation of state, main-fracture growth was driven by net pressure, and the resulting main and statistically generated branch fractures were mapped into a three-dimensional reservoir domain. Subsequent thermal and pyrolysis responses were calculated using Robin-type gas–rock wall heat transfer, bedding-anisotropic heat conduction, and distributed activation energy model (DAEM) kinetics, together with sensitivity analysis of the key thermal-boundary parameters and numerical verification of the heat-transfer implementation. For a single 3 kg charge, the peak gas and net pressures reached 83.78 and 77.73 MPa, respectively, and the main-fracture half-length stabilized at 1.49 m within approximately 0.3 s. The fracture network promoted preferential heat transfer from the hot gas into the reservoir, yielding 138.88 kg of cumulative primary shale-oil generation after 1800 s. Three-stage sequential deflagration substantially increased the axial thermal influence. Increasing the stage spacing from 2 to 8 m expanded the axial extent of the T ≥ 310 °C region from 12.12 to 24.12 m, whereas cumulative primary shale-oil generation decreased from 467.33 to 389.33 kg, demonstrating a trade-off between spatial coverage and inter-stage thermal overlap. These results reveal the sequential mechanism of short-duration fracturing, fracture-enhanced heat transfer, and early-stage pyrolysis, and indicate that liquid-propellant deflagration is better suited as a pre-fracturing and thermal-startup technique coupled with subsequent sustained heating rather than as an independent long-term heat source.