Performance analysis of multi-well EGS system based on unique network structure — Voronoi fractures

Abstract

Fracture network is an important factor in the utilization of geothermal energy, its composition and structure have an important influence on the development of geothermal energy. As a component of EGS system, fractures have various forms. Therefore, this paper studies the influence of fracture morphology on reservoir heat extraction. The Voronoi is used to describe the unique network structure and is applied to geothermal development system as a form of fracture network. Therefore, a THM coupled EGS model with Voronoi fractures is established to observe its thermal extraction performance. In present work, the effects of fracture block numbers, well length, well spacing, injection mass flow rate and injection temperature on the thermal extraction performance of EGS system are numerically investigated. The results show that the model has higher production temperature when the number of Voronoi blocks is 77, and the production temperature in the 30th year is 494.01 K. The influence of the change of well spacing on the production temperature of the model is more obvious than that of the change of well length. At the same time, the longer the well length, the smaller the injection pressure, the stronger the connectivity between the injection and production wells, and the narrower the end of the injection well in the temperature diagram. Furthermore, the increase of injection mass flow rate reduces the production temperature of the model, when the injection mass flow rate increases from 80 kg/s to 140 kg/s, the production temperature decreases from 494.01 K to 450.99 K. However, because the increase of the total injection mass flow rate, the large mass flow rate still has a high heat extraction ratio. The higher the injection temperature is, the smaller the pressure difference between injection wells and production wells is. The temperature changes the permeability and liquid viscosity of the reservoir, thereby reducing the pressure difference. In addition, with the increase of injection wells, the production temperature of the model increases, and the injection pressure decreases. However, the pressure difference between injection wells and production wells increases, indicating that the pressure of production wells is large, so the production cost increases. Therefore, in the actual production situation, reasonable selection of geothermal operation parameters should consider the operation cost and the difficulty of development.