Thermo-poroelastic Effects on Wellbore Stability and Fracturing in EGS with Reference to Utah FORGE EGS

ABSTRACT: Coupled thermal and poromechanical processes play an important role in many problems of interest in reservoir development such as drilling, stability of boreholes and permeability enhancement in geothermal reservoirs. When rocks are subjected to temperature changes, the bulk solid and the pore fluid experience a volume change. A volumetric expansion can result in significant pressurization of the pore fluid depending on the degree of containment and the thermal and hydraulic properties of the fluid as well as the solid rock. Consequently, a coupling of thermal and poromechanical, and chemical processes develops in geothermal reservoirs. For example, in drilling operations there is a strong coupling between thermal and poromechanical effects that has significant impact on the stress/pore pressure distribution around the wellbore and thus hole failure and fracture initiation. Borehole image logs of the Utah FORGE Well 16-A reveal several zones of closely spaced fractures transverse to the wellbore. In places they occur alongside the usual axial drilling-induced tensile fractures. In this study, a fully coupled thermo-poroelastic model is used to investigate wellbore stability and fracturing with reference to the Utah FORGE and Newberry EGS. The fundamental mechanisms associated with cooling/heating of the rock in the context of drilling in enhanced geothermal systems are first described and then the role of temperature and pore pressure in wellbore failure and fracture is analyzed.