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. Underground Gas Storage (UGS) has become one of the mostwidely used practices to cope with seasonal peaks in energy consumption. Theplanning of any new UGS facility, or its upgrading to increase the workinggas volume and reservoir performance, must be supported by an evaluation ofpossible induced effects on the environment. From a geomechanical point ofview, storage activity results in a cyclic change in stress and deformationin the reservoir rock and the surrounding formations. The main environmentalissues to be accounted for when natural fluid pore pressure is planned to beexceeded are the following: (a) the differential displacements at the landsurface possibly mining the integrity of ground structure; (b) the integrityof the reservoir and caprock; (c) the possible reactivation of faults, ifthe target reservoir is located in a faulted basin; and (d) the verticalupheaval and land subsidence that can impact on the surface drainage networkin low lying coastal areas. We present an original methodology forevaluating the geomechanical safety of UGS activities using an approachderived from what is traditionally applied in the structural design ofbuildings. A safety factor, a margin of security against risks, is definedfor each of the geomechanical issues listed above. First, a 3D FE-IEnumerical model is developed to reproduce the stress and displacement due tothe UGS program under evaluation. Then the reservoir pressure is increaseduntil the “failure” condition is reached allowing to evaluate how far theproject designed condition is from the above limit. The proposed approach isapplied to Romagna, a depleted gas reservoir in Northern Italy converted toUGS, with the aim of investigating the safety of the project to increase thereservoir pressure up to 120 %  pi , where pi is the original reservoir pressurebefore the start of primary production. The 3D geomechanical model has beendeveloped using recent 3D seismic data, land displacements by InSAR, labtests on reservoir and caprock samples, in-situ Modular Formation DynamicTester (MDT) stress tests, and large background information acquired fromother UGS reservoirs located in the same sedimentary basin. The analysisoutcome has revealed that the investigated scenario is safe, with safetyfactor larger than 1, in the range from 1.2 to 4. The most criticalcondition (the smallest safety factor) has been obtained in relation to themechanical integrity of the reservoir formation, under very conservativeconditions (cohesion  =  0, friction angle  =  30 ∘ ).

An engineering approach to quantify geomechanical safety factors in UGS programs