Extensiwn of two fractures with different characteristics by hydraulic fracturing. Simulation of fracture extension by hydraulic fracturing for the development of hot dry rock. (2nd Report)

At the Fenton Hill Hot Dry Rock Geothermal Project in the United States, more than ten hydraulic fracturing experiments have been conducted to stimulate a hot dry rock reservoir. Two hydraulic fracturing attempts, Exp. 2059 and Exp. 2062, successfully established a large fracture system connecting an injection well, EE-3A and a production well, EE-2. The fracture systems created by the two experiments showed different fracture extension pressures. In order to understand the combined behavior of fractures, thecomputer code FEHM was used to simulate the two layered fracture system. As in part 1 of this series of papers, an exponential relation between fracture aperture and fracture pressure was used. Parameters in the modelwere fitted to simulate specificexperiments.In this paper, the parameters were chosen to fit the data obtained from each experiment described above. The extension of each fracture was then estimated using these parameters at a constant flow rate, 0.8m3/min. Next, the fracture extension is simulated when water is pumped into the combined two-layer fracture system. From the computer simulation, the following results are obtained.1. When two fractures with different characteristics obtained by experiments 2059 and 2062 extend at the same time by pumping water at a fixed flow rate of 0.8m3/min, the higher permeability fracture (Exp. 2059: upper fracture) extends almost at the single fracture rate, while thefracture of Exp. 2062 (lower fracture) which has lower permeability extends at half the single fracture rate. The wellhead fracture extension pressure is only 0.5MPa less than that when upper fracture extends alone.2. By doubling the flow rate and keeping other parameters constant, both fractures extended faster and the lower fracture showed a marked increase in fracture extension rate. The results suggest that if we try to make multiple fractures along openhole section in an effort to create a large artificial reservoir, it is advisable to use the largest flow rate allowable under the given conditions.