Study on the influence of randomly distributed fracture aperture in a fracture network on heat production from an enhanced geothermal system (EGS)

Abstract

In the attempt to reduce the CO2 emissions to the atmosphere and therefore the dependence on fossil fuels, geothermal energy has started to receive increased scientific interest. With the development of the Enhanced Geothermal System (EGS) technology, extensive geothermal energy applications have become feasible. However, enhanced geothermal reservoirs are usually situated several kilometers below the ground, which makes their experimental investigation challenging. Therefore, numerical models capable of simulating thermohydraulic (TH) effects are an essential additional tool for analyzing geothermal reservoir efficiency. To simulate fluid migration and heat propagation within a fractured geothermal reservoir in EGS, discrete fracture models (DFMs) of the TH processes are widely used. However, the variability of aperture size from one fracture to another is typically ignored in these models. In this work, a discrete fracture model considering variable aperture fractures is presented and used to investigate the performance of a geothermal reservoir in EGS. The outlet temperature and energy production rate are used as the evaluation criteria. Statistically generated fracture networks with different apertures were applied. The fractures are represented as lower-dimensional elements. The fracture apertures are randomly distributed within the networks, but constant for one single fracture. The simulation results show that the coefficient of variation of the DFN apertures strongly affects the performance of the geothermal reservoir. The heat production rate and outlet temperature can be divided into three stages based on the value of coefficient of variation of fracture apertures. The higher variability results in the low heat production rate but high outlet temperature. The investigation on fracture density in turn indicates that the average heat production rate is proportional to the fracture density. However, the effect of fracture density is reduced with an increase of coefficient of variation. Furthermore, the comparison between fracture aperture and fracture density shows that, the increase in mean fracture aperture leads to a higher increase in average heat production rate than an increase in fracture density.