ABSTRACT:

Enhanced geothermal system (EGS) is the major way to extract heat from impermeable hot dry rocks. As heat mining process in EGS is complex, effective numerical simulation method is required. Lots of EGS simulation models are proposed with different kinds of fracture system using discrete fracture network method or equivalent porous media method. However, the situation that EGS with faults connected have seldom investigated. Thus, a 3D geothermal heat extraction model connected with faults is established, and solved by the finite element method. The proposed numerical model is validated with single fracture heat extraction analytical model. The largest error between two models is 5.63%, which shows the efficiency of the proposed model. The effect of faults on heat extraction in EGS is studied. The results demonstrate that compared with the EGS disconnected with the fault, the EGS with fault get higher production temperature and poor production flow rate. The more faults there are, the more injected fluid is lost, resulting in lowest heat extraction performance. Thus, in hot dry rock heat mining engineering practice, hydraulic fracturing operations should avoid connect with faults to ensure much more injection fluid can be extracted.

1. INTRODUCTION

With the energy crisis and environmental pollution problems more and more serious, increasing attention has been paid to the development of clean energy, especial for the hot dry rock (HDR) (Lund and Boyd, 2016). HDR is a kind of dense, high-thermal geothermal resource buried 3,000 meters underground. In China, the geothermal resource of HDR rock is about 7×1015 MWh, 2% of which can meet the requirement of energy consumption for 3,400 years (Zhu et al. 2015; Huang, 2014).

Enhanced geothermal system (EGS) is developed to extract HDR geothermal resources. Typical EGS is composed of injection well, production well, and fracture network. The wells are connected through fracture network (Olasolo, 2016; Tang et al. 2020). The injected fluid extracts the heat though the fracture network and produces high thermal fluid from production well. The Fenton Hill EGS project in the United States was the world’s first EGS site trial, proving the concept of EGS. France built the world’s first EGS site for commercial application, namely the Soultz EGS site. The generation power reached 1.5MWe. Australia built the world’s first EGS site from private company in Cooper Basin (Lu 2017). Other countries built EGS power stations after another. Unfortunately, most of them eventually shut down, because the fluid loss in the fracture network is dramatic, which is hard for commercial use (Kelkar et al. 2016). Therefore, it is very important to study the heat extraction mechanism of fluid in fracture network.

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