Modeling porous rock fracturing induced by fluid injection

Abstract

In this paper, we describe and implement a theoretical model that captures the physical processes occurring during hydraulic fracturing of saturated and unsaturated, porous rock. The model is based on a poro-elastic–plastic rheology with a Mohr–Coulomb yield function and unassociated flow rule, and includes frictional hardening, cohesion weakening, damage and mobilized dilatancy effects. In addition, fluid injection is described using the Richard׳s approximation. We numerically implement the model using a finite difference scheme on a staggered grid, and compare the model results with laboratory experiments published in Stanchits et al. (2011) [16]. From various experiments performed by Stanchits et al., we use three different laboratory configurations for comparing our model results and observations: (a) triaxial compression of a drained rock sample, (b) low pressure fluid injection into a drained, critically stressed rock, and (c) high pressure fluid injection into a saturated rock sample. We find good agreement between model and observations for all cases, indicating a proper formulation and implementation of the dominant physical mechanisms acting. The model reproduces experimental observations of macroscopic fracture, fluid front localization, and the stress–strain response curves. Matching observations from laboratory scale experiments establishes a benchmark and a calibrated model for numerically simulating experiments performed at the larger, field scale.