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

TL;DR

This paper develops a 3D mathematical simulation framework to understand and predict unexpected fault reactivations during underground gas storage operations, considering complex geological and fluid interaction factors.

Contribution

It introduces a novel coupled frictional contact mechanics and fluid flow model specifically designed for faulted porous rocks in UGS settings, addressing complex reactivation mechanisms.

Findings

Fault reactivation can occur during primary production due to stress redistribution.

The model explains unexpected fault reactivations observed in practice.

Different fluid storage cycles influence fault activation hazard.

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.