Modelling fluid injection-induced fracture activation, damage growth, seismicity occurrence and connectivity change in naturally fractured rocks

We develop a fully-coupled hydro-mechanical model to simulate fluid injection-induced activation of preexisting fractures, propagation of new damages, development of seismic activities, and alteration of network connectivity in naturally fractured rocks. The natural fracture system is represented by a discrete fracture network. The stress and strain fields of the fractured porous media are solved in the framework of a finite element model, which mimics the damage evolution in rock matrix based on an elasto-brittle failure criterion and simulates the normal/shear displacement of natural discontinuities based on a non-linear constitutive law. The coupled geomechanics and fluid flow processes in the fractured rock are computed honouring essential coupling mechanisms such as pore pressure-induced shear slip of pre-existing fractures, poro-elastic response of rock matrix, and stress-dependent permeability/storativity of both fractures and rocks. We use the numerical model developed to investigate the hydro-mechanical behaviour of two cases of deeply buried fractured rock in response to high-pressure fluid injection, one case with fracture density just below the percolation threshold and the other above the threshold. We observe a strong control of natural fracture network connectivity on the damage emergence, seismicity occurrence and connectivity change in the rock mass subject to hydraulic stimulation. We also highlight the strong poro-elastic effect that tends to drive heterogeneous connectivity evolution of fracture systems during fluid injection. The results of our research and insights obtained have important implications for injection-related geoengineering activities such as the development of enhanced geothermal systems and extraction of hydrocarbon resources.

Automated summary

A fully-coupled hydro-mechanical model was developed to study the impact of fluid injection on fractured rocks, revealing the control of natural fracture networks on damage, seismic activities, and connectivity changes. The strong poro-elastic effect during fluid injection was highlighted, driving heterogeneous connectivity evolution in fracture systems.