| Abstract |
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 four-part 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. |