Characterization and saturation determination of reservoir metagraywacke from The Geysers corehole SB-15-D (USA), using Nuclear Magnetic Resonance Spectrometry and X-ray Computed Tomography

Abstract

Novel pressure-coring technology, along with nuclear magnetic resonance (NMR) spectrometry and X-ray computed tomography (CT) of cores retrieved from scientific drillhole SB-15-D in the Sulphur Bank area of The Geysers steam field, have enabled the first measurements of indigenous water saturation in a high-temperature, vapor-dominated geothermal system. Two cores for the analyses were obtained using a wireline-retrievable core barrel sealed and pressurized at simulated reservoir pressure during ascent to the surface, and there immediately frozen in dry ice and secured in insulated receptacles to immobilize contained fluids. Immediately upon being thawed at the laboratory, the cores were measured for total water volume by NMR. These values, combined with rock porosities later determined by Boyles Law gas expansion, yielded total water saturations. For the deeper of the two cores (485.2–488.3 m), water saturation was determined by these methods to be 3–13%, but these values probably include some drilling-fluid filtrate. For the shallower core (432.9–435.8 m), filtrate incursion was measurable by CT, since a tritium tracer had been added to the drilling fluid during coring operations. Filtrate accounted for a fifth to three-fifths of the total water in this core, and corrected water saturations ranged from 8 to 25%. Results of subsequent CT-based experiments on the cores suggest that invasion probably took place as a result of drilling-fluid vaporization and convective migration of the vaporized filtrate. This finding (1) implies that much of the hot (about 235°C) cored rock sequence is relatively dry; and (2) independently supports the low water saturation measurements obtained by NMR. The dryness and low water saturations of the SB-15-D cores doubtless reflect the 40-year production history of the Sulphur Bank area, and cannot be considered representative of The Geysers as a whole. As indigenous water saturation will remain a critical variable in the longevity of this resource, it is recommended that our new pressure-coring and saturation-measurement techniques be judiciously applied in other (particularly deeper) portions of the reservoir.