Fracturing processes due to temperature and pressure nonlinear waves propagating in fluid‐saturated porous rocks

Abstract

A model is proposed of rock deformation‐fracturing in the subsurface of hydrothermal systems in response to deep fluid‐rock temperature and pore fluid pressure perturbations, carried upward by hot and pressurized fluid fronts. Since during these episodes of unrest one also has to take into account that rock parameters can evolve, a model of fluid diffusivity change as a function of pore fluid pressure is described. Through reformulating the linear thermoporoelastic equations, rock deformation‐fracturing is thus thought of as being associated with migration of thermomechanical nonlinear waves, which travel upward, associated with an increase in concurrent fluid diffusivity. On dynamical grounds it is assumed that on the boundary of the two superimposed horizons the overlying rock suddenly starts rupturing, caused by the arrival of supercritical water from below, which drives up a pore fluid pressure excess. In this connection, the purpose of this analysis is to investigate the general evolution of the subsurface pressure and temperature fields, assuming that the original signal is itself strong enough to generate fracturing processes of the overburden rock on its arrival. A general formulation provides evidence of nonlinear “thermal waves,” “compensated waves,” and “residual pressure Burgers waves,” that can be found for every value of the system parameters. A mechanical analogy is also presented, which is treated analytically and numerically, allowing one to gain intuitive insight into such complex phenomena. A characteristic of these nonlinear processes is that the resulting timescales (of the order of years for the case of the Campi Flegrei and the Izu Peninsula) can be particularly small, corresponding to quick hyperthermal phenomena during the filtrating movement of fluid toward the Earth's surface.