Induced Stress in a Geothermal Doublet System

Abstract

This paper presents the predicted magnitudes of induced thermoelastic and poroelastic stresses and the resulting reservoir deformation, caused by production and injection of water into Groß Schönebeck geothermal Reservoir. The Groß Schönebeck reservoir is a confined aquifer, located at about 4 km depth within the Lower Permian of the North East German Basin. The geological formation is composed from bottom to top of volcanic and siliciclastic rocks. Injection of cold water into a hot water reservoir will contract the rock, however the surrounding rock will constrain this contraction and thermal stress will be induced. This thermally induced stress not only influences the rock mechanical properties but also affects the poroelastic and consequently transport properties of the rock. While geomechanics in conventional reservoir simulator is often governed by change in pore compressibility and permeability as a function of pressure, a coupled mechanical and fluid flow simulator attempts to capture the alterations in reservoir properties (mainly porosity and permeability) due to changes in pressure, temperature and the induced stress and deformation.

In order to predict the thermal effects in reservoir scale, a static model which includes reservoir structure (geological units, faults and induced hydraulic fractures) was created. Thermo-hydro-mechanical analysis of the reservoir was performed for 30 years, the expected life cycle of the reservoir. A transition stress regime between normal faulting to strike-slip faulting is expected in Groß Schönebeck geothermal reservoir; hence different boundary conditions are employed. In particular, porosity and permeability were coupled through the changes in the strain and stress. The induced thermoelastic stress makes the minimum horizontal stress more tensile and pore pressure controls the effective stress. Fracturing would occur if the minimum principal effective stress becomes tensile and equal to tensile strength of the rock. A temperature decrease of 80 °C and an increase of 10 MPa in bottomhole pressure due to water injection, results in a change in minimum horizontal effective stress, such that it exceeds the tensile strength (3.9 MPa) of the rock.