Isochoric thermodynamic preconditioning for resolving mechanically induced phase change in fractured porous media
Rapid pore-volume changes can trigger phase change on timescales shorter than characteristic transport times and may therefore be skipped by conventional nonlinear solves and adaptive time stepping. We present a persistent-variable framework for transport in fractured porous media, introducing specific volume as an independent variable. Starting from a fully coupled system, we derive volume-based models and recover classical pressure-based formulations by eliminating local thermodynamic variables. To resolve abrupt fracture opening, we introduce a nonlinear preconditioner that assumes instantaneous free expansion and resolves the fluid state through an isochoric equilibrium calculation before advancing the transport problem. The preconditioner applies to both volume- and pressure-based models. In the studied fracture-opening cases, unpreconditioned simulations miss transient vaporization, whereas the preconditioned models do not. Across the investigated aperture range, larger openings produce monotonic increases in gas content, expansion-induced cooling, and durations of transients. Thermal effects alter the phase evolution but have otherwise minor influence on the overall transient duration. Within the proof-of-concept setting, fracture opening generates substantial transient pressure reductions, indicating that geomechanical feedback may become important in fully coupled applications. Pressure-enthalpy and volume-temperature formulations recover identical physical solutions but exhibit different nonlinear robustness, with the pressure-enthalpy formulation proving more robust in some recompression-dominated cases. These results show that equilibrium specifications control the numerical properties of the nonlinear problem rather than recovered physical responses, while isochoric preconditioning connects the nonlinear initialization directly to the underlying thermodynamics.