Shear dilation of fractures has been recognized as a main mechanism of permeability enhancement by hydraulic stimulation in Enhanced Geothermal Systems (EGSs); however, the interactive role of fracture shear dilation and thermo-hydro-mechanical (THM) coupling processes in long-term heat extraction performance of EGSs remains unclear. In this study, we develop a novel THM coupling model based on the discrete fracture network approach, which can realistically capture important processes including hybrid normal-shear deformation of fractures, thermal expansion of rocks, fluid flow in both fractures and rocks, and heat convection/conduction as well as local thermal non-equilibrium effect and changes in physical parameters of fluid. We quantitatively investigate the effects of fracture network geometries and geomechanical boundary constraints on fracture shear dilatancy, and the resulting heat transfer characteristics of EGSs. Numerical results reveal that shear dilation of fractures can be triggered by transient pore pressurization and thermal stress under anisotropic in-situ stress condition, and would severely engender flow channeling as well as anisotropic heat transfer, which strongly impact the heat extraction performance. The production temperature tends to be overestimated while the thermal production rate may be underestimated, if the shear dilatational behavior is not incorporated. Increased in-situ stress ratio and injection/production pressure would magnify the effects of shear dilation, and lead to considerable enhancement of fracture permeability, eventually resulting in much earlier and quicker temperature drop. Excessive increase of fracture density and the location of injection-production wells parallel to potential channelized flow paths, formed by intersected fractures preferentially oriented for shear sliding, tend to form short circulating flow paths and reduce the heat extraction performance. Our study demonstrates the importance of considering fracture shear dilation and fully-coupled THM behaviors when evaluating the long-term performance and efficiency of heat extraction in EGSs.

裂缝的剪切扩张已被认为是增强型地热系统 (EGS) 中通过水力增产提高渗透率的主要机制；然而，裂缝剪切膨胀和热-水-机械（THM）耦合过程在 EGS 长期排热性能中的相互作用仍不清楚。在这项研究中，我们开发了一种基于离散裂缝网络方法的新型 THM 耦合模型，它可以真实地捕捉重要过程，包括裂缝的混合法向剪切变形、岩石的热膨胀、裂缝和岩石中的流体流动以及热对流/传导以及局部热非平衡效应和流体物理参数的变化。我们定量研究了裂缝网络几何形状和地质力学边界约束对裂缝剪切剪胀的影响，以及由此产生的 EGS 传热特性。数值结果表明，在各向异性地应力条件下，瞬态孔隙加压和热应力可引发裂缝的剪切膨胀，严重地产生流窜和各向异性传热，严重影响排热性能。如果不考虑剪切膨胀行为，则生产温度往往会被高估，而热产率可能会被低估。增加地应力比和注入/生产压力会放大剪切膨胀的影响，并导致裂缝渗透率显着提高，最终导致温度下降得更早和更快。裂缝密度的过度增加和注采井的位置平行于潜在的通道化流动路径，由优先定向为剪切滑动的交叉裂缝形成，倾向于形成短的循环流动路径并降低热提取性能。我们的研究表明，在评估 EGS 中热提取的长期性能和效率时，考虑裂缝剪切膨胀和全耦合 THM 行为的重要性。容易形成短的循环流路并降低排热性能。我们的研究表明，在评估 EGS 中热提取的长期性能和效率时，考虑裂缝剪切膨胀和全耦合 THM 行为的重要性。容易形成短的循环流路并降低排热性能。我们的研究表明，在评估 EGS 中热提取的长期性能和效率时，考虑裂缝剪切膨胀和全耦合 THM 行为的重要性。