INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS Int. J. Numer. Anal. Meth. Geomech. 2009; 33:1613–1640 Published online 19 February 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/nag.780 A three-dimensional integral equation model for calculating poro- and thermoelastic stresses induced by cold water injection into a geothermal reservoir X. X. Zhou 1 , A. Ghassemi 1, ∗, † and A. H.-D. Cheng 2 1 Department of Petroleum Engineering, Texas A&M University, College Station, TX 77843, U.S.A. 2 Department of Civil and Environmental Engineering, University of Mississippi, MS 38677, U.S.A. SUMMARY Poro-mechanical and thermo-mechanical processes change the fracture aperture and thus affect the water flow pattern in the fracture during the cold water injection into enhanced geothermal systems (EGS). In addition, the stresses generated by these processes contribute to the phenomenon of reservoir seismicity. In this paper, we present a three-dimensional (3D) partially coupled poro-thermoelastic model to investigate the poroelastic and thermoelastic effects of cold water injection in EGS. In the model, the lubrication fluid flow and the convective heat transfer in the fracture are modeled by the finite element method, while the pore fluid diffusion and heat conductive transfer in the reservoir matrix are assumed to be 3D and modeled by the boundary integral equation method without the need to discretize the reservoir. The stresses at the fracture surface and in the reservoir matrix are obtained from the numerical model and can be used to assess the variation of in situ stress and induced seismicty with injection/extraction. Application of the model shows that rock cooling induces large tensile stresses and increases fracture conductivity, whereas the rock dilation caused by fluid leakoff decreases fracture aperture and increases compressive total stresses around the injection zone. However, increases in pore pressure reduce the effective stresses and can contribute to rock failure, fracture slip, and microseismic activity. Copyright 2009 John Wiley & Sons, Ltd. Received 8 July 2008; Revised 7 November 2008; Accepted 27 December 2008 KEY WORDS: geothermal energy; displacement discontinuity; reservoir seismicity; thermoelastic stress; poroelastic stress ∗ Correspondence to: A. Ghassemi, Department of Petroleum Engineering, Texas A&M University, College Station, TX 77843, U.S.A. † E-mail: ahmad.ghassemi@pe.tamu.edu Contract/grant sponsor: U.S. Department of Energy Office of Energy Efficiency and Renewable Energy under Cooperative Agreement; contract/grant number: DE-FG36-06GO95002 Copyright 2009 John Wiley & Sons, Ltd.