Optimization of Enhanced Geothermal Systems under Geological and Reservoir Stimulation Uncertainty

摘要

There are many uncertain parameters when creating an Enhanced Geothermal System (EGS). Uncertain geological parameters include the stress field, location and orientation of pre-existing fractures, rock toughness, and temperature and pressure distribution. In addition, the physical mechanisms governing fracture creation are still being researched. Due to these uncertainties, the reservoir's response to stimulation and operation cannot be predicted with certainty using numerical simulators. With an uncertain reservoir response, it is challenging to make engineering optimization decisions that will greatly improve the profitability of an EGS. This paper explores the optimization of an EGS under geological and reservoir stimulation uncertainty. A base case EGS model is created, using available data for a candidate EGS site in the West Flank of Coso, California. The EGS base case is a horizontal injector well flanked by two producer wells. The injector well is hydraulically fractured to create tensile fractures and open pre-existing natural fractures. Using Monte Carlo simulation, 119 models were generated varying the uncertain earth parameters within ranges reported for the West Flank of Coso site. For each model, fracture stimulation parameters such as mean fracture length, and mean fracture aperture, were randomly selected according to prior uncertainty. Monte Carlo Optimization (exhaustive brute force sampling) was used on these models to optimize four engineering decisions: the well head pressure on the injection well, the pump power on the production wells, the distance between the injector and producers, as well as the spacing between fractures. The engineering decisions were optimized to maximize the present value of the EGS, accounting for the cash flow discount rate and the cost of pumping. Our analysis shows that for the prior uncertainty and conditions used in our model, a fracture spacing of 10 m and a distance between the injector and producers that is less than the mean hydraulic fracture length, increase the present value of the EGS.