Thermo-Poroelastic Effects on Permeability Change and Production Optimization in an Enhanced Geothermal Reservoir – Case Study of Deep Upper Jurassic Carbonates in the Bavarian Molasse Basin

Abstract

Optimal stimulation design and long-term sustainable management of enhanced geothermal systems (EGS) can benefit from numerical modelling and simulation of coupled fracture deformation, fluid flow and heat transport. The mechanical response of the fracture-matrix system influences reservoir permeability variations due to cold fluid injection and production processes. We adopt a multidisciplinary approach within the framework of the joint research project ZoKrateS, which comprises extensive completion and stimulation operations to enhance the productivity of a petrothermal reservoir in ultra-deep Upper Jurassic carbonates of the Bavarian Molasse Basin. We develop a 3-D, fully coupled thermo-hydro-mechanical reservoir model to simulate the spatiotemporal thermo-poroelastic response of a stress-sensitive fracture-controlled reservoir. For the 3-D static geomodelling and dynamic finite-element analyses, we use SKUA-GOCADTM and COMSOL Multiphysics®, respectively. The numerical model combines non-isothermal compressible single-phase fluid flow in fractured porous rock and heat transfer with an optimization algorithm for the geothermal energy production task. We consider a nonlinear fracture deformation model to investigate the role of coupled processes on fracture deformation, stress redistributions and production performance. In particular, poromechanical effects of the reservoir are assumed to play an important role based on previous observations made during drilling operations and hydraulic tests, which suggest that matrix reservoir deforms and fractures close up during increasing effective stress, leading to variations in fracture permeability and production performance. We employ structural interpretations of a 3D seismic survey in the license field Wolfratshausen ca. 40 km south of Munich, logging, mud losses, zones of joint calcites and well tests in the 5700 m MD long borehole GEN-1ST-A1 near Geretsried to accurately implement hydraulically active faults and fractures in a 3D reservoir model. We model the present-day, undisturbed 3-D temperature distribution taking into consideration the regional temperature gradient, temperatures measured in the borehole and thermo-physical parameters measured in former, local geothermal research projects. In addition, we develop a 3-D geomechanical model, which considers a most likely strike–slip stress regime in the 4.5 km deep reservoir by choosing adequate boundary conditions in line with the governing stress field and geomechanical data compiled in the previous project Dolomitkluft. We further constrain the geomechanical model by taking into account pressures derived from formation integrity tests (FIT) and stress-limiting conditions. Concerning geothermal energy production optimization, we analyze simulation results to draw conclusions on the controls of the optimal placement of a second well under different cold fluid injection or production strategies. Based on our numerical experiments, we conclude on the role of coupled thermo-hydro-mechanical reservoir processes on fracture deformation, permeability variations and heat transport. We focus on the implications that these coupled reservoir processes have for a long-term sustainable utilization of geothermal energy in a doublet operational scheme.