Unexpected fault activation due to underground gas storage in produced reservoirs. Part II: Definition of safe operational bandwidths

TL;DR

This study develops a modeling framework to identify safe operational pressure ranges in underground gas storage in faulted reservoirs, aiming to prevent unexpected fault reactivation and seismic events.

Contribution

It introduces a detailed numerical model considering fluid effects on mechanical properties to define safe pressure limits for various gases in faulted reservoirs.

Findings

Fault reactivation risk depends on geological and operational factors.

Stored gases influence reservoir mechanical properties, affecting fault stability.

Safe operational bandwidths vary with reservoir conditions and gas types.

Abstract

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 early 20th century, hydrogen has recently been considered as a potential candidate toward a more flexible and sustainable energy infrastructure. Furthermore, these formations can additionally capture gases that contribute to climate change, such as CO2. When such operations are implemented in faulted basins, however, safety concerns may arise due to the potential reactivation of pre-existing faults, which could trigger (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 in previously produced gas reservoirs and define a safe operational bandwidth for pore pressure variation for UGS operations in the faulted reservoirs of the Upper Rotliegend Group, the Netherlands. This paper investigates 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 considering how the presence of stored gases may influence the mechanical properties of the reservoir and caprock, in particular the Young modulus. The study investigates the hazard of fault activation caused by the storage of different fluids for various purposes, such as long-term CO2 sequestration, CH4 and H2 injection and extraction cycles, and N2 injection as cushion gas. The results show how geological configuration, geomechanical properties, and reservoir operating conditions may increase the hazard of fault reactivation.