Gas injection and leakage in layered aquifers

TL;DR

This paper develops a gravity-current model to analyze CO2 injection and leakage in layered aquifers, revealing conditions under which CO2 can leak across seals, impacting secure storage strategies.

Contribution

It introduces a new model accounting for vertical leakage and capillary effects, providing insights into CO2 migration and leakage mechanisms in layered aquifers.

Findings

Capillary pressure buildup is highly sensitive to water film connectivity.

Decreasing water permeability can trigger or increase gas leakage.

CO2 may leak across seals at Sleipner without focused conduits.

Abstract

Carbon dioxide (CO2) injection into saline aquifers is one method of mitigating anthropogenic climate change. To ensure secure storage of this CO2, it is important to understand the interaction of CO2 injection and migration with geological layering. For example, seismic monitoring at the Sleipner pilot project suggests that the injected CO2 is ponding against, and leaking across, a series of thin, intermediate seals. Here, we develop a gravity-current model for weakly compressible, two-phase fluid migration in a system of layered aquifers. Our model includes vertical leakage of both water and gas across seals, where the latter is subject to a capillary entry threshold. We demonstrate that the buildup of capillary pressure is very sensitive to the conductivity and connectivity of water films in the gas region. We identify two associated limiting cases, where gas obstructs water flow either completely or not at all. We then explore the parameters that govern gas leakage and the resulting fluid distributions---demonstrating that this problem involves a complex interplay between pressure dissipation, capillary pressure buildup, and fluid migration. We show that decreasing the relative permeability of water in the gas region can initiate gas leakage or significantly increase the amount of gas leakage. Finally, we apply our model to rock properties expected for Sleipner and show that CO2 injection may build up sufficient capillary pressure to invade the seals---suggesting that, contrary to conventional wisdom, CO2 may be able to leak across the intermediate seals at Sleipner in the absence of a focused conduit.