A Semi-Analytic Transient Approach to Modelling Plasticity and Stimulation of Geothermal Wells

Abstract

In the present work, we introduce a coupled hydro-mechanical semi-analytical model for geothermal boreholes. The solution is based on poro-elastoplastic behavior. The material behavior is defined by non-linear elasticity along with a strain-hardening and softening plasticity model. The model uses a time-dependent flow solution in which the permeability in the reservoir can be easily varied, both in space and time. The solution assumes a homogeneous, isotropic geo-massrock mass, with horizontal isotropic virgin in-situ stresses and a plane-strain condition. We introduce and validate a modification to the Pijnenburg et al. (2019) plasticity model, using the results of their triaxial tests. This model is then applied to the case of injection and shut-in and fall-off. Injection causes stabilization; shut-in destabilization. Further, we show that in the construction stage, the entire reservoir undergoes some inelastic deformation. This is very different from the results of an elastic-perfectly plastic or elastic-brittle-plastic model. This work was part of a project that received funding by the European Union’s Horizon 2020 research and innovation programme DESTRESS under grant agreement No 691728 and GEMex under grant agreement No. 727550.