An Integrated Analytical Technique to Screen Depleted Reservoirs for Underground Gas Storage Suitability

Abstract Underground gas storage (UGS) technique is gaining impetus and is widely implemented due to the global gas demand and consequent variation of gas prices. Due to the upfront investment involved in UGS projects, thorough screening of potential candidate reservoirs followed by economic feasibility analysis is essential to ensure the functionality of the project. A range of technical factors like reservoir containment, gas recovery, pressure regimes throughout the operation, well integrity, available surface equipment, presence of contaminants in the fluid and project safety are considered as a part of the reservoir screening and UGS design workflow. Cushion gas requirement, working gas capacity, and maximum gas injection/ withdrawal requirements are the key operating elements that drive the feasibility and design of a UGS project. Finally, risk evaluation, economic feasibility and policy factors are reviewed to assess the feasibility of the candidate reservoir for a UGS project. The study encompassed evaluation of three depleted fields for UGS candidacy. The two smaller fields offer very limited storage volumes and were deemed as unsuitable due to target extraction and storage rates. The bigger structure was delineated as a potential candidate. Geological investigation affirmed the caprock seal integrity. Formation pressure data confirmed hydraulic connectivity of the reservoir that was continuous to an adjacent structure. However, this connectivity was constrained by a gas-water contact (GWC) that acted as the bottom boundary for the containment. After calibrating the model with a fit-for-purpose objective, the reservoir modelling identified opportunities for maximizing working gas capacity through sensitivity analysis. Well engineering and completion assessments qualified existing wells for conversion to injectors, identify potential workover requirements and propose upgrades to existing equipment. Overall, the reservoir was qualified as good candidate for UGS requiring an additional investment circa $200Mn towards UGS specific surface facility with 30% uncertainty. Based on the meticulous review and integration of outcomes from multiple disciplines, a quick analytical technique based on material balance and production performance prediction was developed. This workflow was carefully implemented to assess the UGS candidacy of a similar reservoir in a nearby field. Further development of the workflow has the potential to serve as quick-look tool and precursor to the detail oriented integrated approach described earlier.