Fault slip in hydraulic stimulation of geothermal reservoirs: governing mechanisms and process-structure interaction

TL;DR

This paper demonstrates how advanced physics-based simulation technology can elucidate the complex hydromechanical interactions and fault reactivation mechanisms during hydraulic stimulation of geothermal reservoirs, aiding in understanding permeability enhancement.

Contribution

The study introduces a fully coupled simulation methodology that models flow, deformation, and fault mechanics, providing new insights into fault reactivation processes in geothermal stimulation.

Findings

Simulation captures direct and indirect fault reactivation.

Permeability enhancement is linked to fault slip and dilation.

Strong nonlinearities and couplings are critical in modeling.

Abstract

Hydraulic stimulation of geothermal reservoirs in low-permeability basement and crystalline igneous rock can enhance permeability by reactivation and shear dilation of existing fractures. The process is characterized by interaction between fluid flow, deformation, and the fractured structure of the formation. The flow is highly affected by the fracture network, which in turn is deformed because of hydromechanical stress changes caused by the fluid injection. This process-structure interaction is decisive for the outcome of hydraulic stimulation, and, in analysis of governing mechanisms, physics-based modeling has potential to complement field and experimental data. Here, we show how recently developed simulation technology is a valuable tool to understand governing mechanisms of hydromechanical coupled processes and the reactivation and deformation of faults. The methodology fully couples flow in faults and matrix with poroelastic matrix deformation and a contact mechanics model for the faults, including dilation because of slip. Key elements are high aspect ratios of faults and strong nonlinearities in highly coupled governing equations. Example simulations using our open-source software illustrate direct and indirect hydraulic fault reactivation and corresponding permeability enhancement. While conceptually simple, the examples illustrate the strong hydromechanical couplings and the prospects of physics-based numerical models in investigating the dynamics.