Steam injection in diatomite reservoirs results in permeability changes owing to fracture propagation, and compaction as a result of thermal effects and pressure changes during injection and production. The purpose of this work is to evaluate these coupled thermal, hydrological, and mechanical (THM) processes over several years of cyclic steam injection and production. A single well model in a diatomite reservoir was created to evaluate these processes at a higher resolution near wellbore than used in a 3-D reservoir-scale model.
Simulations include tensile failure, shear failure with simultaneous shear on multiple planes, coupling of porosity and permeability changes with multiphase flow, and diatomite compaction with temperature and effective stress. Initial isotropic horizontal stresses are 1.0375 of vertical (azimuthal average, part of San Joaquin Valley). Injection interval (437-528 m) pressure is fixed, 6.3 MPa, 930 psi (injection), and ~4 MPa (production), with a soak period between injection and production. Three degree dilation on shearing is assumed. To the extent that fracture opening is tensile, fractures close on fluid pressure drop, but shear components remain. Permeability changes due to mechanical failure are simulated using a cubic law.
During injection over multiple cycles, the diatomite surrounding the well is heated to over 250 °C and pressurized by the injected steam. During soak (shut-in) and subsequent production, pressure drops, dropping the boiling point, inducing further vaporization. Geomechanical changes show tensile opening accompanied by a greater amount of shearing. Total shearing increases with each injection cycle, resulting in a greater porosity increase from shearing than tensile opening. Fracture propagation was limited to the diatomite reservoir and did not penetrate the caprock.
Inclusion of an empirical effective stress/temperature diatomite compaction law together with porosity and permeability changes due to mechanical failure more closely models the mechanics of cyclic steaming of diatomite.