Enhanced Geothermal Systems (EGS), engineered deep rock heat exchangers, are often touted for their massive untapped renewable energy potential. However, numerous technical and financial challenges must be overcome before the technology is widely deployed. To be viable, EGS must engineer a heat sweep by circulating fluid through a large volume of rock while minimizing fast pathways that create thermal short circuits. Here, we propose a new horizontal EGS well design with partially bridging multi-stage hydraulic fractures (Fig. 1b) to improve heat extraction from hot dry rock (HDR). In order to increase fluid circulation through the stimulated reservoir volume (SRV) between fractures, the hydraulic fractures are created in an alternating pattern between injection and production wells, with each stopping short of connecting the second well. To test this design, we developed a thermal-hydro-mechanical (THM) coupling model and investigated heat extraction performance and reservoir stress evolution during operation. Fractures were modelled as opening contact surfaces with system stresses evolving according to thermal, poroelastic, and fracture opening effects. Based on the model, the temperature performance of proposed EGS design model is compared with the commonly used fully bridging EGS design model (Fig. 1a). An investigation of thermal performance showed that the proposed design obtains higher production temperatures and delays thermal breakthrough by several years compared to a fully-bridging fracture design. It also results in a greater degree of secondary stimulation of the SRV as cold fluids are forced further into the rock matrix. These results indicate that the partially-bridging fracture design is a promising candidate for practical EGS implementation.

工程设计的深部岩石热交换器的增强型地热系统（EGS）通常因其巨大的尚未开发的可再生能源潜力而受到吹捧。但是，在广泛部署该技术之前，必须克服许多技术和财务挑战。为了可行，EGS必须通过使流体循环通过大量岩石来设计热扫除，同时最大程度地减少造成热短路的快速通道。在这里，我们提出了一种新的水平EGS井设计，该设计具有部分桥接的多级水力压裂裂缝（图1b），以改善从热干岩（HDR）中提取热量的能力。为了增加通过裂缝之间的增产油藏体积（SRV）的流体循环，在注入井和生产井之间以交替的方式创建了水力裂缝，每个裂缝都没有连接第二口井就停了下来。为了测试该设计，我们开发了热-水-机械（THM）耦合模型，并研究了采油性能和运行过程中储层应力的演化。裂缝被建模为开放接触表面，系统应力根据热，多孔弹性和裂缝的开放而发展效果。基于该模型，将提出的EGS设计模型的温度性能与常用的全桥接EGS设计模型进行比较（图1a）。热性能研究表明，与完全桥接的断口设计相比，拟议的设计获得了更高的生产温度，并延迟了数年的热突破。当冷流体进一步被迫进入岩石基质时，还会导致SRV受到更大程度的二次刺激。这些结果表明，部分桥接的裂缝设计是实际EGS实施的有希望的候选者。