LARGE-SCALE PHYSICAL MODELING OF WATER INJECTION INTO GEOTHERMAL RESERVOIRS AND CORRELATION TO SELF POTENTIAL MEASUREMENTS

Laboratory measurements of electric self potentials resulting from water injection through a known flow path in a 260 mm cube of Nugget sandstone are used to calibrate a new large-scale testing device that simulates the in-situ conditions at an injection point in a geothermal reservoir. Modeled in-situ temperatures were 20°C and 150°C, while injection pressures were varied from 1 to 1200 kPa. The observed self potential response showed an accurate spatial correlation to the known flow path, with potential differences on the order of 100 mV. A surface contour map of potentials was generated for the sample cube, and the temporal variation of potentials with injection pressure shows good correlation. Results suggest that at higher temperatures, and in the presence of steam, opposing electrokinetic and thermoelectric self potentials, combined with decreased sample resistivity, may counteract to lower the observed potential on the sample surface. Incomplete saturation of the sample at low temperature may produce capacitive effects among the mineral grains leading to a slow decay in observed potentials following the end of injection. A second testing program on Berea sandstone cores at room temperature reinforces these results. This fourpart program isolated the effects of the intact rock, the empty flow pipe, the sand–filled flow pipe, and the quartz sand alone. Streaming potential coupling coefficients for the intact core, the core with empty flow pipe, and the core with sand-filled flow pipe are observed to be approximately constant over a large pressure difference range (up to 1.2 MPa), and are 42 mV/atm, -10 mV/atm, and 42 mV/atm respectively. The coupling coefficient for the test of only coarse quartz sand revealed a logarithmically decreasing coupling coefficient that varied from 60 mV/atm at low pressures to 32 mV/atm at 300 kPa. This testing showed excellent correlation between applied pressure drop and observed streaming potential, and indicates the ability of streaming potentials to identify changes in sample conditions. INTRODUCTION Self-potential, in this case streaming potential (or electrokinetic potential), is a widely recognized method for identifying flow paths through rock and rock/soil matrices (Bogoslovsky and Ogilvy, 1970; Corwin and Hoover, 1979; Wurmstich and Morgan, 1994). This method is based on the existence of an electric double layer at the liquid-matrix interface where a diffuse mobile layer of ions can be effectively dragged away from their adsorbed immobile counterparts under a pore pressure gradient, creating a charge imbalance (Jouniax et al, 1999). Since many minerals have negatively charged surfaces, a diffuse layer of positive ions from the local pore solution weakly bonds to the surface, and can be carried away from the mineral surface yielding a negative anomaly. Areas where water collects (for the above case) would be then characterized by a collection of positive charges, and a positive anomaly (Vichabian and Morgan, 2002). The Helmholtz-Smoluchowski equation describes the relationship between streaming potentials and the pressure gradient under which they move. ∆V = (εζ / μσ) ∆P (1) Where ε is the dielectric constant of the fluid, ζ is the zeta potential, σ is the conductivity of the fluid, and μ is the fluid viscosity (Overbeek, 1952). The quantity (εζ/μσ) is known as the streaming potential coupling coefficient, Cc. Precise quantitative interpretation of streaming potential data can sometimes be difficult, as the coupling coefficient must be previously determined for each specific scenario. Many users of the streaming potential method have relied on a qualitative interpretation of data in which fluid flow paths are identified by contouring data collected on a dense grid of electrodes. Such methodology has been successfully applied to many field problems, most notably the detection of flow paths through earth dams (Ogilvy et al, 1969; Johansson, et al, 2000), and more relevantly, flow detection resulting from hydraulic stimulation of hot dry rock (or enhanced geothermal systems) reservoirs (Kawakami and Takasugi, 1994; Marquis et al, 2002). We analyze the temporal response of the induced potentials, and its relationship with injection pressure. Furthermore, we investigate effects of another type of self potential, the thermoelectric potential, which is a result of a temperature gradient and is commonly observed in active geothermal areas (Corwin and Hoover, 1979). The results presented have been obtained as calibration experiments for large-scale physical testing to investigate damage mechanisms at injection points in geothermal reservoirs. Calibration of the streaming potential response for a known flow path will aid in the interpretation of data for an unknown, possibly chaotic and temporally variable, natural flow path. Temperature effects were modeled by varying sample temperature from 20° C to 150° C. The effect of variations in injection pressure and rate were also analyzed with respect to the observed geophysical response.