One of the most cost-effective ways to increase deliverability and working capacity of depleted reservoirs used for gas storage is to operate at pressures exceeding the discovery conditions (delta-pressuring). The option of delta-pressuring was considered for the first time in Italy by Stogit S.p.A. (Eni Group) as a pilot test for a partially depleted gas reservoir operated for underground storage of natural gas. The reservoir is a sandstone formation capped by a thick clayey layer and confined by a strong lateral aquifer. On the basis of the current capacity of the surface facilities, a maximum operating pressure 7% higher than the initial reservoir pressure was considered during the design phase. Accurate analyses were then performed to dissolve concerns about rock fracturing, cap rock integrity, and gas losses. Seismic surveys confirmed the absence of fractures in the reservoir area, and the position of the spill-point. In order to verify the suitability of the cap rock to retain the stored gas under delta-pressure conditions, laboratory tests were performed to measure the rock threshold pressure, and a 3-D numerical reservoir model was set up both to simulate the pressure variations and to monitor the movements of the gas-water contact during the gas injection/withdrawal cycles. Geomechanical analyses based on 2-D numerical models were also performed to calculate the strain conditions and the deformations of both the reservoir and cap rock during the pressure cycles. A preliminary uncoupled model at the reservoir scale allowed identification of the most critical zones, for which coupled models were subsequently set up. Based on the results of the study, the decision was taken to apply a 7% delta-pressure to the reservoir. An intense monitoring activity, including continuous pressure data acquisitions, static pressure profiles and repeated RST performed on key wells during two years of delta-pressuring, proved the reliability of the calculations and model predictions, allowed control of the field actual operating conditions, and demonstrated the effectiveness of delta-pressuring to safely increase the underground storage capacity at no additional costs

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