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Workshop on Geothermal Reservoir Engineering Stanford Univ

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PROCEEDINGS, Thirty-First Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 30-February 1, 2006 SGP-TR-179 ON THE FEASIBILITY OF USING SUPERCRITICAL CO2 AS HEAT TRANSMISSION FLUID IN AN ENGINEERED HOT DRY ROCK GEOTHERMAL SYSTEM Karsten Pruess1 and Mohamed Azaroual2 1 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. 2 BRGM - Water Division, 3 av. C. Guillemin, BP 6009, F-45060 Orléans Cedex 2, FRANCE K_Pruess@lbl.gov, M.Azaroual@brgm.fr ABSTRACT Responding to the need to reduce atmospheric emissions of carbon dioxide, Donald Brown (2000) proposed a novel hot dry rock (HDR) concept that would use CO2 as heat transmission fluid, and would achieve geologic sequestration of CO2 as an ancillary benefit. Following up on his suggestion, we have evaluated thermophysical properties and performed numerical simulations to explore the fluid dynamics and heat transfer issues in a HDR reservoir that would be operated with CO2. We find that CO2 is roughly comparable to water in its ability to mine heat from hot fractured rock. CO2 has certain advantages with respect to wellbore hydraulics, where larger compressibility and expansivity and lower viscosity as compared to water would reduce the parasitic power consumption of the fluid circulation system. Chemical interactions induced by CO2 between fluids and rocks suggest a potential for porosity enhancement and reservoir growth. A HDR system running on CO2 has sufficiently attractive features to warrant further investigation. INTRODUCTION Responding to the need to reduce atmospheric emissions of carbon dioxide, Donald Brown (2000) proposed a novel hot dry rock (HDR) concept that would use CO2 instead of water as heat transmission fluid, and would achieve geologic storage of CO2 as an ancillary benefit. Brown noted that CO2 has certain physical and chemical properties that would be favorable for operation of a HDR system. Favorable properties of CO2 emphasized by Brown include the following: • large expansivity would generate large density differences between the cold CO2 in the injection well and the hot CO2 in the production well, and would provide buoyancy force that would reduce the power consumption of the fluid circulation system; • lower viscosity would yield larger flow velocities for a given pressure gradient; and • CO2 would be much less effective as a solvent for rock minerals, which would reduce or eliminate scaling problems, such as silica dissolution and precipitation in water-based systems. Brown also noted the lower mass heat capacity of CO2 as an unfavorable property, but pointed out that this would be partially compensated by the greater flow capacity of CO2 due to lower viscosity. Fouillac et al. (2004) suggested that an enhanced geothermal system (EGS) using CO2 as heat transmission fluid could have favorable geochemical properties, as CO2 uptake and sequestration by rock minerals would be quite rapid at elevated temperatures. The present paper compares thermophysical properties of CO2 and water, and examines pressure and temperature conditions for flow of CO2 in wellbores as well as in reservoirs with predominant fracture permeability. Comparisons are made with the flow behavior of water, in order to identify favorable as well as unfavorable characteristics of CO2 as a HDR working fluid. We also present preliminary considerations on chemical aspects of a CO2-HDR system. THERMOPHYSICAL PROPERTIES Fig. 1 shows the phase diagram for CO2 in the range of temperature and pressure conditions that are of interest for injection into and production from enhanced geothermal systems. The critical point of CO2 is at Tcrit = 31.04 ̊C, Pcrit = 73.82 bar (Vargaftik, 1975). At lower (subcritical) temperatures and/or pressures, CO2 can exist in two different

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