Nonlinear Poroelastoplastic Behavior of Geothermal Rocks

Abstract

At present time, advanced geothermal reservoir engineering requires to quantify and predict non-isothermal multiphase fluid flow within deformable porous rocks. Frequently, the geomechanical behavior of these rocks is nonlinear and goes into the plastic range. This non-linearity comprises inelasticity and large, permanent deformations. In order to understand, the true stress-strain behavior of geothermal rocks, thermoporoelasto-plasticity specific techniques are needed. If the rocks were only poroelastic there would be no numerical limits for the values of the principal components of the stress tensor. In such idealized theoretical case, geothermal rocks will never fail. In this work an initial investigation of the general practical aspects of the outlined problem is presented. To identify the processes involved, the flow plasticity theory is introduced and mathematical numerical models are developed and solved using finite differences and finite elements. Both models are able to analyze the geomechanical deformation of geothermal rocks subjected to a nonlinear behavior. The first model is radial and includes the fluid flow, which obeys Darcy’s law, and a poroelastic analysis of the rock. The second model is 3D based on continuum mechanics and flow plasticity theory with poroelastoplastic deformations. In this case, the total stresses that can be applied to geothermal rocks are physically limited by a failure stress criterion. An extended Drucker-Prager failure-yielding criterion is used to represent realistically this limit. Using available field data previously published, the 3D model was applied to compute the poroelastoplastic deformation of a salt dome located in a hybrid oil-thermal reservoir located in the southern part of the Gulf of Mexico, which is related to a deep geothermal aquifer producing hot brine at 160°C and 1284 bar that invades the oil producing wells. The model shows that the vertical deformation of the salt dome is not negligible having an influence in the global reservoir draw-down pressure.