Underground gas storage is a versatile tool for managing energy resources and addressing pressing environmental concerns. While natural gas is stored in geological formations since the beginning of the 20th century, hydrogen has recently been considered as a potential candidate toward a more flexible and sustainable energy infrastructure. Furthermore, these formations can also be used to sequester environmentally harmful gases such as CO2. When such operations are implemented in faulted basins, however, safety concerns may arise due to the possible reactivation of pre-existing faults, which could result in (micro)-seismicity events. In the Netherlands, it has been recently noted that fault reactivation can occur "unexpectedly" during the life of an underground gas storage (UGS) site, even when stress conditions are not expected to cause a failure. The present two-part work aims to develop a modeling framework to investigate the physical mechanisms causing such occurrences and define a safe operational bandwidth for pore pressure variation for UGS operations in the faulted reservoirs of the Rotliegend formation, the Netherlands. In this follow-up paper, we investigate in detail the mechanisms and crucial factors that result in fault reactivation at various stages of a UGS. The mathematical and numerical model described in Part I is used, also accounting for the effect of geochemical dissolution on reservoir and caprock weakening. TThe study investigates the risks of fault activation caused by the storage of different fluids for various purposes, such as long-term CO2 sequestration, CH4 and N2 injection and extraction cycles, and N2 permanent storage. The results show how geomechanical properties and reservoir operating conditions may increase the risk of fault reactivation at various UGS stages. Finally, operational guidelines for improving secure storage operations are presented.

翻译：地下储气库是管理能源资源和应对紧迫环境问题的多功能工具。尽管自20世纪初以来天然气就被储存在地质构造中，但氢最近被视为构建更灵活、可持续能源基础设施的潜在候选者。此外，这些构造也可用于封存如CO2等对环境有害的气体。然而，当此类操作在断层盆地中实施时，由于既有断层可能被重新激活，可能引发（微）震事件，从而引发安全隐患。在荷兰，最近注意到在地下储气库（UGS）场址的运营寿命期间，断层活化可能“意外”发生，即使在应力条件预计不会导致破坏的情况下。本两部分研究旨在建立一个建模框架，以探究导致此类事件发生的物理机制，并为荷兰Rotliegend地层断层储层中的UGS作业定义孔隙压力变化的安全运行带宽。在这篇后续论文中，我们详细研究了导致UGS不同阶段断层活化的机制和关键因素。使用了第一部分中描述的数学和数值模型，并考虑了地球化学溶解对储层和盖层弱化的影响。本研究调查了为不同目的储存各种流体（如长期CO2封存、CH4和N2注入与提取循环，以及N2永久储存）所引发的断层活化风险。结果表明，地质力学特性和储层运行条件如何可能增加UGS不同阶段断层活化的风险。最后，提出了改进安全储存作业的运行指导原则。