Unexpected fault activation due to underground gas storage in produced reservoirs. Part I: Mathematical model and mechanisms

TL;DR

This paper develops a 3D frictional-contact–poromechanics framework to study unexpected fault reactivation during underground gas storage in Rotliegend reservoirs. It couples a slip-weakening fracture rheology and a mixed-dimensional finite-element formulation with one-way flow coupling to a reservoir-flow solver, enabling predictive analysis of fault behavior under production, cushion gas injection, and storage cycles. By applying the model to a synthetic Rotliegend-style fault system, the study identifies stress redistribution during primary production as a key driver of reactivation and demonstrates that slip-weakening friction increases the likelihood of fault slip during CGI and UGS phases, even when the regime appears stable. The work provides mechanistic insights and lays groundwork for defining safe operational bandwidths, with Part II extending the analysis to broader storage scenarios (e.g., CO$_2$, H$_2$) and comprehensive sensitivity studies.

Abstract

Underground gas storage (UGS) is a critical technology for managing seasonal gas consumption peaks, increasingly important in the face of market uncertainties. However, safety concerns arise when reactivating pre-existing faults in faulted basins, where human activities may trigger seismic events. Typically, faults are reactivated when shear stress exceeds a critical frictional threshold, but unexpected fault reactivations have been observed during cushion gas injection (CGI) and UGS cycles in the Netherlands, even when the stress regime suggests stability. This two-part study introduces a novel simulation framework to better understand the mechanisms behind fault reactivation in complex settings such as the Rotliegend formation in the Netherlands. A 3D mathematical model coupling frictional contact mechanics in faulted porous rocks with fluid flow allows for predictive analysis of fault behavior. The effect of the storage of different fluids for various purposes, such as the long-term sequestration of CO2, the regular injection and extraction cycles of CH4, and the highly irregular cycles of H2, is investigated with respect to fault activation hazard. The ultimate goal is to define a safe operational bandwidth for UGS activities in faulted reservoirs. Part I of this work presents this comprehensive simulation tool where a slip-weakening constitutive law is introduced to model fault behavior. The approach is designed to address the complex geological setting that characterizes the Rotliegend formation, where multiple factors influence the behavior of fault systems. We succeed in explaining and modeling the occurrence of unexpected fault reactivations. The analysis shows that reactivation during primary production (PP) causes stress redistribution, leading to a new deformed equilibrated configuration.

Figures (20)

• Figure 1: Sketches of two (a and b) "expected" induced seismicity scenarios and one (c) "unexpected". a) Primary production with large pressure drop, b) fluid injection (CO$_2$ sequestration, waste water disposal, fracking) with significant pressure increase), c) pressure in the range already experienced (UGS with $p < p_i$).
• Figure 2: Sketch of the 3D domain $\Omega$ with its boundary, outer normal and inner fracture $\Gamma_f$ (left), made of the top and bottom contact surfaces and the normal direction $\boldsymbol n_f$ (right).
• Figure 3: Slip weakening constitutive laws. From left to right: piecewise linear friction law, exponential friction law and inverse trigonometric friction law.
• Figure 4: Mono-dimensional friction system with 1 degree of freedom. On the left: sketch of the model used in this example. On the right, from the top left, (i) friction strength, (ii) relative displacement, (iii) global system stiffness and (iv) internal energy. Continuous, dotted and dashed line represent linear, exponential and inverse trigonometric slip-weakening formulation, respectively.
• Figure 5: On the left: base Zechstein semblance map of the Norg UGS (in blue) and surrounding area with traces of the bounding faults and localization of the recorded seismic events NAM16. On the right: conceptual map of the Norg field with major and minor faults highlighted in blue and red, respectively.