Mechanical Responses of Formation and Wellbore Induced by Water Injection for Enhanced Geothermal System

Abstract

Water injection is often needed for enhanced geothermal system (EGS) as the primary thermal recovery is low for many geothermal reservoirs. Water injection causes the variations of temperature, pore pressure and in-situ stress in the swept region. It also could cause wellbore deformation and compromise the wellbore integrity. In this paper, a thermo-hydro-mechanical (THM) fully-coupled model of formation and wellbore is established. The formation is simplified as a poroelastic medium. The casing and cement sheath in injection and production wells are embedded in the formation. The THM responses of formation and the mechanical behavior of wellbore are simulated synchronously in EGS. Results indicate that the pore pressure decreases from injection well to production well and the water temperature rises during flowing into production well. A low-temperature region (LTR) forms around the injection well and expands as the operation continues. Temperature propagates much slower than overpressure. The effective stress decreases around the injection well due to overpressure and thermal contraction. Overpressure and thermal contraction causes a fluctuant deformation of formation. The thermal effect overwhelms the poroelastic effect in the late period. The Mises stress of the casing in injection well reaches to its maximum value soon and then decreases slightly. While, the Mises stress of the casing in production well increases gradually after thermal breakthrough. This research provides a reference for evaluating formation THM response and wellbore integrity in geothermal development.