| Abstract |
This paper presents an analysis of the performance of Enhanced Geothermal Systems (EGS), specifically, reservoirs with subcommercial permeability enhanced by hydraulic stimulation. The performance under consideration here is the net electrical power delivered as a function of time and the parameters in this exercise reflect conditions encountered at the Desert Peak EGS project in the State of Nevada, United States. Preliminary results of this study have been presented at the Twenty-Ninth Annual Workshop on Geothermal Reservoir Engineering at Stanford University; this paper presents further analysis. The analysis was quantified through numerical simulation of three types of EGS set-ups: (a) doublet (an injection and production well pair), (b) triplet (an injector flanked by a production well on each side), and (c) five-spot (an injector at the center and a production well at each corner of a square). The injector and producers communicate through a double-porosity reservoir with a thickness of 1,200m and at a temperature of 210?C. The pre-enhancement porosity and permeability are 2% and 1 millidarcy, respectively. The hydraulic characteristics of the reservoir are assumed to remain constant following enhancement. The thickness of the stimulated zone was varied from 150 to 1,200m, and a range of fracture spacings (from 0.33 to 300m) and fracture permeabilities (from 1 to 100 millidarcy) following enhancement was considered. The spacing between the injector and producers was also varied. The injection water temperature was assumed to be 82?C, which is the temperature of the separated brine available from the existing Desert Peak power plant. The injection rate was dictated, through reservoir simulation, by the production rate assigned to the producers. Production wells were allowed a maximum drawdown of 3.4 MPa and the injection well was limited to a maximum pressure buildup of 6.9 MPa. From the forecast of the production rate and temperature, the gross power available was calculated as a function of time from the First and Second Laws of Thermodynamics; from this, the net power available versus time was calculated, for each scenario, after subtracting the parasitic power needed by injection and production pumps. For each combination of assumed geometry, injector-producer spacing, stimulated thickness, and enhancement level (fracture spacing and permeability), the following criteria of performance were computed: (a) net generation profile (generation versus time), (b) fluid loss as a function of time, (c) net power produced per unit injection rate, and (d) fraction of in-place heat energy recovered. The results indicate that power generation from an enhanced geothermal system, such as at Desert Peak, should be technically feasible under a variety of development scenarios. |