Thermo-Poroelastic Analysis of Cold Fluid Injection in Geothermal Reservoirs for Heat Extraction Sustainability

Abstract

ABSTRACT: Geothermal reservoirs are usually deep and have low permeability. Hydraulic fracturing stimulation of the production and injection wells can help increase the heat extraction from these reservoirs. However, injecting cold fluid in the hot formation can cause considerable volumetric changes in reservoir rock and pore pressure distribution in the reservoir. As a result, thermal loading occurs, and local stresses change in magnitudes and directions. Such redistribution of pore pressure, stresses, and temperature may result in an unwanted propagation of the created fractures or failure of pre-existing rock discontinuities and create an unwanted closed loop circle. In this study, we investigate the effect of heat exchange between the circulating fluid and the hot rock on redistribution of local stresses. Our approach employs the thermo-poroelasticity theory to study cold fluid injection in geothermal reservoirs. The displacement discontinuity method (DDM) is used to solve the field equations of the thermo-poroelasticity theory. The DDM is a numerical technique that belongs to the boundary element methods (BEM). This numerical technique is suitable for studying hydraulic fracture (discontinuities in displacement) simulation problems. The thermo-poroelastic theory used here accounts for three fully coupled processes (mechanical, hydraulic, and thermal) in porous media saturated with fluid. Our results show the effect of cold fluid injection on stress magnitude and orientation. Furthermore, the evolution of fracture aperture and cold fluid injection efficiency are discussed. The results presented in this work show that the difference in temperature between the cold treatment fluid and hot reservoir help create a wider fracture in less pumping time. In addition, it can cause a 90° re-orientation of the minimum principal stress direction, which controls the propagation of fractures. Also, cold fluid injection in hot formation creates thermal (tensile) stresses that can be much higher than iso-thermal fluid injection.