Borehole Preconditioning of Geothermal Wells for Enhanced Geothermal System Reservoir Development

Abstract

Thermal stimulation can be utilized to precondition a well to optimize fracturing and production during Enhanced Geothermal System (EGS) reservoir development. A finite element model was developed for the fully coupled processes consisting of: thermoporoelastic deformation, hydraulic conduction, thermal osmosis, heat conduction, pressure thermal effect, and the interconvertibility of mechanical and thermal energy. The model has been applied to Raft River geothermal well RRG-9, which will be subjected to thermal and hydraulic stimulation as part of a Department of Energy EGS demonstration project. RRG-9 encountered a temperature of ~140oC. The two-dimensional fully coupled model assumes that the material diffusion coefficient for the pressure is spatially dependent. Thus, the model can simulate the effect of healed fractures around a borehole subjected to non-hydrostatic in situ stress fields. In this study, the effects of cooling RRG-9 by 60, 40 and 20oC are evaluated The results indicate that cooling induces a decrease in the pore pressure around the borehole. In addition to ÄT, it was found that the time (t), the borehole pressure (pB), and the angle (è) around the borehole perimeter influence the stresses around the borehole. For example, (1) lower borehole temperature, or (2) lower borehole pressure, or (3) a shorter time, results in higher (more compressive or less tensile) effective radial stresses and lower (less compressive or more tensile) effective tangential stresses around the wellbore. For this case of RRG-9, the maximum tensile (most negative value) effective tangential stress occurs at the borehole wall while the maximum tensile effective radial stress occurs within the formation. The tensile effective tangential stress could start the fracture at the borehole wall while the tensile effective radial stress could cause time-delayed spalling. Note that compression is assumed positive and tension negative.