Optimization Design of Multi-Factor Combination for Power Generation from an Enhanced Geothermal System by Sensitivity Analysis and Orthogonal Test at Qiabuqia Geothermal Area

Abstract

In order to explore the optimal mining strategy of a fractured Enhanced Geothermal System (EGS) reservoir, we numerically investigated the influence of seven factors on heat production and conducted an optimization analysis of a multi-factor and multi-level combination by an orthogonal test based on the geological data at the Qiabuqia geothermal field. Seven factors were considered, including five reservoir factors (fracture spacing, fracture permeability, fracture permeability anisotropy, matrix permeability, and heat conductivity) and two operation factors (injected section length and injection rate). The results show that injection rate and fracture permeability have the greatest influence on production performance. Different factor combinations have a great influence on the productivity. The multi-factor and multi-level combination optimization is needed, and the optimization scheme of the EGS can be achieved through the orthogonal test and range analysis. The order of influence degree on the power generation is injection rate > fracture permeability > fracture permeability anisotropy > injected section length > matrix permeability > fracture spacing > heat conductivity. The order of influence degree on the coefficient of performance of the EGS is fracture permeability > injection rate > injected section length > fracture permeability anisotropy > matrix permeability > fracture spacing > heat conductivity. For reservoir stimulation, the stratum with dense natural fractures should be selected as the target EGS reservoir. It is not advisable to acidify the EGS reservoir too much to widen the apertures of the natural fractures. Fracture permeability anisotropy will increase pump energy consumption, but this adverse effect can be greatly reduced if the other parameters are well matched. Matrix permeability and heat conductivity may not be used as indicators in selecting a target reservoir. For project operation, the injected section length should be as long as possible. The injection rate plays a major role in all factors. Special attention should be paid to the value of the injection rate, which should not be too large. The appropriate injection temperature should be determined in accordance with the water source condition and the engineering requirement. If a commercial rate (100 kg/s) is to be obtained, the permeability of the reservoir fracture network needs to be stimulated to be higher. Meanwhile, in order to ensure that the production temperature is both high and stable, it is necessary to further increase the volume of the EGS reservoir.