Preliminary Assessment of a Geothermal Energy Reservoir Formed by Hydraulic Fracturing

Two 3-km-deep boreholes were drilled into hot granite in northern New Mexico to extract geothermal energy from hot, dry rock. Both boreholes were hydraulically fractured to establish a flow connection. Fracture-to-borehole intersection locations and in-situ thermal conductivity were determined from flowing temperature logs. In-situ measurements of rock permeability and compressibility show a strong dependence on pore pressure. An estimate of the minimum horizontal earth stress was derived from fracture extension pressures and found to be one-half the overburden stress. It was found that fracture growth was remarkably simple to achieve in the low-permeability granite and that these factures appear to be "self-propped." The present connection has a large flow impedance that probably cannot be reduced to a useful level. Establishment of a prototype heat exchange system will require redrilling prototype heat exchange system will require redrilling to intersect directly one of the fractures. Introduction A program designed to demonstrate the feasibility of extracting energy from hot, dry rock has been initiated at the Los Alamos Scientific Laboratory. Basically, it is proposed that man-made geothermal energy reservoirs can be created by drilling into relatively impermeable rock to a depth where the temperature is high enough to be useful, creating a reservoir by hydraulic fracturing, and then completing the circulation loop by drilling a second hole to intersect the hydraulically fractured region, or by drilling into the immediate vicinity of the first fracture and then creating a second fracture that intersects the first one. Thermal power would be extracted from this system by injecting cold water down the first hole, forcing the water to sweep by the freshly exposed hot rock surface in the reservoir-fracture system, and then returning the hot water to the surface where the thermal energy would be converted to electrical energy or used for other purposes. System pressures would be maintained such that only one phase, liquid water, would be present in the reservoir and the drilled holes. The concept is described in more detail by Smith et al., and estimates of heat extraction rates have been reported by Harlow and Pracht and McFarland and Murphy. Briefly, it was Pracht and McFarland and Murphy. Briefly, it was found that a large fracture about 1 km in radius, in hot, 200 to 250C rock, is sufficient to provide an average of 60 MW of thermal power for 20 years. In addition, the thermal contraction associated with the cooling of the rock will result in thermal stress cracking, crating additional porosity and heat-transfer surfaces, so that hundreds of megawatts of thermal power may be provided ultimately. DRILLING AND COMPLETIONS The hot, dry rock concept is being investigated in a series of field experiments at Fenton Hill, located on the west flank of a dormant volcano, the Valles Caldera, in the Jemez mountains of northern New Mexico. In Dec. 1974, the first deep borehole, GT-2, was drilled to a depth of 2.929 km (9,609 ft) in granite, where the temperature was 197C (386F), cased to 2.917 km (9,571 ft), and then fractured in the open hole below the casing. The variation of equilibrium temperature and geology with depth is shown in Fig. 1. The Precambrian crystalline rocks were encountered at 730 m (2,400 ft). From this depth to the bottom the geothermal gradient is reasonably constant, although it is somewhat lower in the gneiss as compared with the deeper formations (44C/km vs 57C/km). The steeper gradient in the sediments above 730 m is caused by their lower thermal conductivity. Based on the conductivity measurements at 2.77 km (9,100 ft), the geothermal heat flow at this site is 0.16 W/sq m, 2 times the worldwide average. SPEJ p. 317