Numerical simulation to study the feasibility of using CO2 as a stimulation agent for enhanced geothermal systems

NUMERICAL SIMULATION TO STUDY FEASIBILITY OF USING CO 2 AS A STIMULATION AGENT FOR ENHANCED GEOTHERMAL SYSTEMS Tianfu Xu 1 , Wei Zhang 2 , and Karsten Pruess 1 Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 e-mail: Tianfu_Xu@lbl.gov School of Environmental Studies, China University of Geosciences, Wuhan 430074, China exclusively on the geomechanical aspects of manipulating effective stress to enhance fracture permeability. Research into EGS field systems in different countries has gradually led to the recognition that chemical interactions between injected fluids and rocks can be as important as mechanical ones (Rose et al., 2007; Xu et al., 2009; Portier et al., 2009). Removal of silica and calcite scaling from wellbores is commonly accomplished by injecting strong mineral acids, such as HCl (Mella et al., 2006). Injected strong acid tends to enter the formation via the first fluid entry zone, dissolving first-contacted minerals aggressively while leaving much of the rest of the wellbore untreated. The effect of calcite dissolution using chelating agents is better than mineral acids, but still quite aggressive (Xu et al., 2009). In the present paper, we explore the feasibility of using supercritical CO 2 as a stimulation agent for enhanced geothermal systems (Rosenbauer et al., 2005). This may offer the ancillary benefit of storing some amounts of greenhouse gas in geologic formations. The thermal and geochemical conditions at Desert Peak EGS site were used. We examined ways in which mixtures of water and CO 2 can be injected to enhance porosity and injectivity. We performed thermo-hydrologic-chemical simulations in which the fractured medium was represented by a one-dimensional MINC model (multiple interacting continua; Pruess and Narasimhan, 1985), using the non-isothermal multi-phase reactive geochemical transport code TOUGHREACT (Xu et al., 2006). PROBLEM SETUP Geometric configuration and flow parameters The MINC (multiple interacting continua) method was employed for a fractured geothermal reservoir (Figure 1). MINC is designed to resolve “global” flow and diffusion of chemicals in the fractured rock and its interaction with “local” exchange between fractures and matrix rock. This method was first developed by Pruess and Narasimhan (1985) for fluid and heat flow in fractured porous media. The ABSTRACT A major concern in the development of enhanced geothermal systems (EGS) is achieving and maintaining adequate injectivity, while avoiding the development of preferential short-circuiting flow paths such as those caused by thermally-induced stress cracking. Past researches have tended to focus primarily on thermal and hydraulic stimulation. Recent studies suggest that chemical stimulation may improve the performance of EGS reservoirs. Geothermal injection wells are often drilled into formations containing reactive minerals such as calcite. Injecting aqueous chemical agents such as mineral acids, could be effective for mineral dissolution and porosity enhancement at distances of several meters around a well. An alternative to treatment with strong acids is the use of supercritical (SC) CO 2 as stimulation agent for an aqueous-based EGS. Reactive transport modeling is used to investigate the effectiveness of this method. We used the thermal condition and mineralogical composition from a well of Desert Peak EGS site, to examine ways in which mixtures of water and CO 2 can be injected to enhance porosity. INTRODUCTION The most crucial step in developing enhanced geothermal system (EGS) for commercial production is reservoir stimulation, a process that involves injecting fluids under high pressure through boreholes deep underground, with the objective of increasing in situ fluid pressures to the point where effective normal stress on pre-existing rock fractures becomes small or negative. This will create favorable conditions for fractures to fail in shear, meaning that one or both fracture surfaces will experience translational motion, creating a mismatch between rough surfaces that will enhance reservoir porosity and permeability. In the early days of EGS research (1980-90s), the focus in reservoir stimulation had been almost