3D Analysis of Thermo-poroelastic Processes on Fracture Network Deformation and Induced Micro-Seismicity Potential in EGS

Abstract

This study presents three dimensional (3D) analyses of a fracture network in an Enhanced Geothermal System (EGS) with special emphasis on the role of hydraulic fracture/natural mechanical interactions and coupled thermo-poromechanical processes. The behavior of the system is modeled by coupling a thermo-poroelastic displacement discontinuity (DD) method (for fracture opening and shear, fluid and heat diffusion in the reservoir matrix) with a finite element method (for fluid flow and heat transport inside the fractures). The nonlinear characteristics of fracture opening and shear deformation are taken into account. The resulting method is then used to simulate an injection/production processes into/from a synthetic fracture network consisting of a major fracture intersected by a set of smaller natural fractures. Injection/production into/from the fracture network results in gradual shearing of the fractures that impacts the thermo-hydro-mechanical characteristics of the fracture system. It is also shown that the early micro-seismic events can be associated with fracture slip on smaller connected fractures due to thermal perturbation. Continued injection leads to stress intensity conditions favorable for fracture propagation in shear and tensile modes, which could increase the reservoir surface area and further contribute to seismicity.

Introduction

Design and management of enhanced geothermal system (EGS) and hydrothermal reservoirs can benefit from simulation of coupled fracture deformation and fluid flow - since interactions among fluid and heat flow, and the mechanical response of the fracture and matrix impact reservoir permeability variations and occurrence of seismicity. The coupling between these processes during injection/extraction can be taken into account using linear theory of thermo-poroelasticity. Often, heat transport in a reservoir is dominated by advection. However, when the rock matrix permeability is low (Delaney, 1982) and fluid flows mainly within deformable fractures or a fault, conductive transport in the rock matrix leads to important phenomena related to coupling between temperature, pore pressure, and stress that may result in delayed rock matrix and/or natural fracture failure, potentially producing delayed seismicity. Although the thermo-poroelastic constitutive equations are linear, analytical solutions can be found only for relatively simple geometries and processes (e.g., Tao and Ghassemi, 2010; Ghassemi et al. 2008; Li et al. 1998; Kurashige, 1989) and the solution of problems such as injection /extraction into a deformable fracture network requires numerical modeling even in two dimensions.