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RESERVOIR CREATION The engineered HDR reservoir region, probably approaching an ultimate volume of 112 cubic kilometer or more, would be created by hydraulically fracturing a deep region of essentially impermeable, hot, crystalline rock using SCCOz instead of water as the fracturing fluid. This would be accomplished by pumping SCCOz from the surface down a high- pressure tubing string, and injecting this fluid into a packed-off (i.e,, pressure-isolated) interval of openhole wellbore for a period of several weeks or more, at a rate in the range of 20 to 40 kus. Initially, as the pressure in the packed-off interval rapidly increases, one or more of the more favorably oriented natural joints intersecting the wellbore would start to open under a combination of tensile (hoop) stresses at the wellbore surface and normal opening stresses from fluid invasion into the somewhat more permeable (than the adjacent sock) hydrothermally sealed natural joints. As pumping continues, these joints would progressively open and interconnect, forming a multiply connected region of pressure-dilated joints in the rock mass surrounding the packed-off wellbore interval, thus creating the fractured HDR reservoir. Based on over 20 years of reservoir testing at Fepton Hill, NM, this opening of an may of natural joints is in stark contrast with the originally envisioned formation of one or more large, near-vertical, penny-shaped fractures created by hydraulic fracturing (Brown, 1995a). Based on the Laboratory’s extensive experience with hydraulic fracturing of deep basement rock using water, there appears to be no limitation to using SCCO2 for similar operations. It should be noted that hydraulic fracturing of sedimentary formations using SCCOz, as reported by Yost et a]. (1994), is now routinely done to increase the productivity of petroleum reservoirs where special reservoir conditions warrant this type of stimulation to minimize formation damage from water-based fracturing fluids. POST-HYDRAULIC-FRACTURINGFLUIDCOMPOSITION IN THE RESERVOIR REGION From laboratory measurements on core samples of Precambrian crystalline rock obtained from depths between 1.2 and 2.8 km at Fenton Hill, a mean in-situ rock mass porosity of 0.9~10-4has been determined (Simmons and Cooper, 1977). In contrast, following reservoir creation by hydraulic fracturing and the accompanying dilation of the ressure-stimulated array of joints, the mean reservoir porosity was about 1 . 2 ~ 1 0 - 3r24.700 m5 of water injected into a pressure- accessible volume of 20 million m3 (Brown et al., 1999)l. Therefore, using an analogy to the deeper Fenton Hill HDR reservoir, the fracture volume occupied by the SCCO2 would be about 13 times greater then the initial microcrack pore volume in the rock mass. For this situation, the SCCO;! would tend lo dissolve almost all of the original pore fluid (essentially a brine), with the mineral constituents previously dissolved in the pore fluid being left behind as mineral precipitates. Figure 4 shows the solubility, at 250°C. of water in SCCOz and SCCOz in water as a function of pressure. For an HDR reservoir with a rock temperature of 260°C at a depth of 4 km, and with a surface injection pressure of 30 MPa, one would anticipate about a 24 mol% solubility of water in SCCO2. This solubility is equivalent to a 10%solubility by weight, which would imply that all the previously existing pore fluid within the microcrack pore structure of the rock would end up being dissolved by the SCCO2 diffusing into the rock mass. COz SEQUESTRATION IN THE ROCK MASS SURROUNDING THE HDR RESERVOIR Again, from experience gained from extensive field testing of the deeper HDR reservoir at Fenton Hill, the fluid loss from a 112 cubic kilometer pressure-stimulated reservoir volume, at a mean reservoir injection pressure of 30 MPa (4350 psi) above hydrostatic, is predicted to be about 3 kg/s for a IO-MW(e) power system, which is equivalent to 100,OOO tons per year. Although not a very large number in absolute terms, over the predicted 20-year lifetime of a suitably engineered HDR reservoir, this diffusional loss of SCCO:! into the rock mass immediately adjacent to the HDR reservoir would be very significant -- about 2 million tons of C 0 2 sequestered deep in the earth for each IO-MW(e) HDR power plant. This is in addition to the 48,000 ton inventory of SCCO:! circulating through the reservoir and the surface power plant for such a 10-MW(e) HDR power system, This leads to an ancillary benefit at the periphery of the HDR fractured region, where the SCCO2 r, would be slowly diffusing outward to the far field from the pressurized reservoir. In the surrounding rock mass, the pre-existing water-filled network of interconnected microcracks would be slowly flushed with SCCO2, leaving behind mineral precipitates which would tend to slowly plug off the microcrack porosity and seal the reservoir boundaries over time -- which, from the normal point of view, are almost impermeable already (with a permeability in the range of several hundredths of a microdarcy). SUMMARY AND CONCLUSIONS In a confined reservoir, which is one of the unique characteristics of a true man-made HDR reservoir, as contrasted with a natural hydrothermal geothermal reservoir, the chemistry and/or < 768PDF Image | GEOTHERMAL POWER PRODUCTION UTILIZING SUPERCRITICALCOz COMBINEDWITHDEEPEARTHCARBONSEQUESTRATION
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