Rock failure and permeability change is of interest in a number of reservoir geomechanics problems such as induced seismicity and near wellbore mechanics. Temperature and pore pressure variations play an important role in this context and their contributions need be considered. In this work, we develop a fully-coupled thermo-poro-mechanical finite element model with stress dependent permeability and consider the rock shear and tensile failure based on damage mechanics and consideration of heterogeneous rock strength distribution. The model is applied to the problem of injection into a reservoir with reference to reservoir stimulation in geothermal and petroleum reservoirs.
The influence of temperature and pore pressure variations plays an important role in geothermal and petroleum reservoirs. Rock deformation and fracture in response to induced thermo-poroelastic stresses significantly impacts permeability, and in turn, the temperature and pore pressure fields. The subject of rock failure, fracture propagations caused by and fluid behavior has been the subject of many investigations. In particular, the fully coupled behavior of rock deformation and fluid flow has been Biot [1], and others [2-3]; leading to the development of solutions for the effects of chemical and thermal effects [4-7]. However, poroelasticity is appropriate for the elastic region of the rock response. As the effective stress changes in response to e.g., injection or extraction of fluids, the rock can fail in compression or tension. Damage mechanics theory has been developed to describe the result rock failure on mechanical and transport properties and processes in rock using constitutive relationships based on experimental results for flow stress- damage [8-10]. In this paper, we develop a numerical model to study the impact of thermoporoelastic coupling on damage evolution around the wellbore. The damage model used in suitable for brittle rock fracture [9] and has been implemented in a coupled finite element code. The model is applied to study the response of a rock in response to cold water injection with reference to the fracturing and permeability change.
Whereas the coupled pressure and stress problems in the porous media is described with Biot’s (1941) consolidation theory, a poro-thermoelastic approach combines the theory of heat conduction with poroelastic constitutive equations. 2.1. Constitutive equations & Transport equations The governing equations of poro-thermoelasticity have been developed by McTigue, 1986; Palciauskas and Domenico, 1982 by extending the poroelasticity theory to include the effect of temperature [11, 12].
(Equation in full paper)
A damage and permeability model has been proposed by Tang et al. based experiments [9]. According to this model, rock deformation can be divided into an elastic phase and a damage phase which can be described by constitutive relationships. After the stress conditions satisfy the failure criterion, the components of rock begin to fail.
(Equation in full paper)
To implement the progressive damage model and permeability change, we have developed a finite element method for thermo-poroelasticity problems.
