Depleted hydrocarbon reservoirs are attractive targets for short term gas storage with frequent injection and production cycles. Optimum well completion and injection-storage-production design in depleted reservoirs would require understanding of important rock mechanics issues. These include drilling and completion challenges of new wells in low pressure reservoirs accounting for potential rock fatigue due to cyclic injection/depletion and loading and unloading, determination of maximum sustainable storage pressures that would avoid fracturing and fault reactivation. This paper describes a case study from a coal seam gas project considered for a liquefied natural gas plant in Australia and demonstrates a systematic approach for geomechanical risk assessments for short term gas storage in depleted sandstone reservoirs. Geomechanical analyses show that despite a low fracture gradient in depleted reservoirs and the presence of non-depleted overburden rocks, new high angled wells can be drilled safely with a relatively low mud weight in the non-depleted sections and with air in the reservoir section. Fracturing and faulting assessments confirm that the critical pressures for fault reactivation and fracturing of intact rocks are beyond the planned storage pressures and a maximum pressure of 200-300 psi beyond the initial reservoir pressures may be possible from fracturing or fault reactivation aspects. Sand production prediction evaluations indicate that new injection-production wells can be completed with no downhole sand control due to a very low risk of sanding even after considering rock weakening associated with cyclic loading.
Underground gas storage (UGS) is a concept which has evolved to meet the needs of an ever changing gas supply market. Historically, UGS was introduced for the first time in 1915 at an operating gas field in Welland County, Canada [1,2]. Through time, UGS facilities were developed for effectively balancing a variable demand market.
