Underground natural gas storage is a cyclical technique that consists in the use of geological structures, such as depleted oil and gas reservoirs, aquifers, and salt domes, to store gas for future consumption as per demand. Although, underground gas storage has proved technical efficiency and economic benefits, research around the world has demonstrated that variations in pressure at the reservoir level have the potential to affect the geological structures, provoking leakage, gas losses and even produce seismicity.
The purpose of this work is to build a 1D-geomechanical model and carry out an analytical study over one of the first underground natural gas storage facilities operated in Australian territory, "The Mondarra Gas Storage Facility", to investigate and analyze the impact that gas injection and withdrawal represent over the geological structure and the prevailing stresses at three different levels of analysis: around the wellbore, over the intact-rock, and considering the worst-case scenario, in which a cohesion-less fault affects the reservoir.
After an extensive analysis of the historical and current data, two 1D-geomechanical models were developed for the field to simulate gas injection and withdrawal situations to explore the effect of pressure variation over the porous medium and the prevailing stresses; then, failure analysis under Mohr-Coulomb criteria were performed to predict the risk of inducing rock failure, fault reactivation or gas leakage during this cyclical process.
The sensitivity analysis to pore pressure showed that the reservoir's depletion caused a change in the original state of stress from "Strike Slip Regime" to "Normal Stress Regime". However, Mohr-Coulomb criteria demonstrated that gas withdrawal leads to more stable conditions at the wellbore-wall and over the intact-rock (main reservoir), concluding that there is not risk of fault reactivation, seismicity, or gas leakage during this activity.
In contrast, gas injection simulation demonstrated that the maximum tangential effective stress exceeds the rock strength, at the wellbore-wall, when reservoir pressure approaches 4130 [psi]. Although, this pressure condition could cause rock failure, further Mohr-Coulomb based analyses at the level of the intact-rock has demonstrated that this failure is more likely to be microfractures and there is low to no risk of operational impact or fracture propagation through the main reservoir.