Underground gas storage (UGS) is a worldwide well-established technology that is becoming even more important to cope with seasonal peaks of gas consumption due to the growing uncertainties of the energy market. Safety issues concerning the reactivation of pre-existing faults might arise if the target reservoir is located in a faulted basin, where human activities can trigger (micro-)seismicity events. In the Netherlands, it has been observed that fault activation can occur somehow "unexpectedly" after the primary production (PP), i.e., during cushion gas injection (CGI) and UGS cycles, when the stress regime should be in the unloading/reloading path. To understand the physical mechanisms responsible for such occurrences, a 3D mathematical model coupling frictional contact mechanics in faulted porous rocks with fluid flow is developed, implemented and tested. The final aim of this two-part work is to define a safe operational bandwidth for the pore pressure range for UGS activities in the faulted reservoirs of the Rotliegend formation. Part I of this work concerns the development of the mathematical and numerical model of frictional contact mechanics and flow in faulted porous rocks. A mixed discretization of the governing PDEs under frictional contact constraints along the faults is used. A slip-weakening constitutive law governing the fault macroscopic behavior is also presented. The model is tested in the setting of an ideal reservoir located in the Rotliegend formation. The analyses point out how fault reactivation during PP can lead to a stress redistribution, giving rise to a new equilibrium configuration. When the fault is reloaded in the opposite direction during the CGI and/or UGS stages, further activation events can occur even if the stress range does not exceed either the undisturbed initial value or the maximum strength ever experienced by the formation.

翻译：地下储气库（UGS）是一项全球成熟的技术，在应对能源市场不确定性加剧导致的季节性用气高峰方面变得愈发重要。若目标储层位于断裂盆地中，人类活动可能诱发（微）地震事件，进而引发关于既有断层重新激活的安全问题。在荷兰，观察到断层活化可能在初次开采（PP）后、即在垫底气注入（CGI）和UGS循环期间“非预期”发生，而此时应力状态本应处于卸载/再加载路径。为理解该现象的物理机制，本研究建立、实现并测试了一个耦合断裂多孔岩石中摩擦接触力学与流体流动的三维数学模型。这两部分工作的最终目标是定义Rotliegend组断裂储层中UGS活动的安全孔隙压力运行区间。第一部分涉及断裂多孔岩石中摩擦接触力学与流动的数学及数值模型的构建。采用混合离散方法处理沿断层的摩擦接触约束下控制偏微分方程，同时提出一个控制断层宏观行为的滑移弱化本构定律。模型在位于Rotliegend组的理想储层背景下进行测试。分析指出：初次开采期间的断层再激活可导致应力重分布，形成新的平衡构型；当断层在CGI和/或UGS阶段承受反向再加载时，即使应力范围未超过未扰动初始值或地层曾经历的最大强度，仍可能发生进一步的活化事件。