Abstract
Abstract. Underground Gas Storage (UGS) has become one of the most widely used practices to cope with seasonal peaks in energy consumption. The planning of any new UGS facility, or its upgrading to increase the working gas volume and reservoir performance, must be supported by an evaluation of possible induced effects on the environment. From a geomechanical point of view, storage activity results in a cyclic change in stress and deformation in the reservoir rock and the surrounding formations. The main environmental issues to be accounted for when natural fluid pore pressure is planned to be exceeded are the following: (a) the differential displacements at the land surface possibly mining the integrity of ground structure; (b) the integrity of the reservoir and caprock; (c) the possible reactivation of faults, if the target reservoir is located in a faulted basin; and (d) the vertical upheaval and land subsidence that can impact on the surface drainage network in low lying coastal areas. We present an original methodology for evaluating the geomechanical safety of UGS activities using an approach derived from what is traditionally applied in the structural design of buildings. A safety factor, a margin of security against risks, is defined for each of the geomechanical issues listed above. First, a 3D FE-IE numerical model is developed to reproduce the stress and displacement due to the UGS program under evaluation. Then the reservoir pressure is increased until the “failure” condition is reached allowing to evaluate how far the project designed condition is from the above limit. The proposed approach is applied to Romagna, a depleted gas reservoir in Northern Italy converted to UGS, with the aim of investigating the safety of the project to increase the reservoir pressure up to 120 % pi, where pi is the original reservoir pressure before the start of primary production. The 3D geomechanical model has been developed using recent 3D seismic data, land displacements by InSAR, lab tests on reservoir and caprock samples, in-situ Modular Formation Dynamic Tester (MDT) stress tests, and large background information acquired from other UGS reservoirs located in the same sedimentary basin. The analysis outcome has revealed that the investigated scenario is safe, with safety factor larger than 1, in the range from 1.2 to 4. The most critical condition (the smallest safety factor) has been obtained in relation to the mechanical integrity of the reservoir formation, under very conservative conditions (cohesion = 0, friction angle = 30∘).
Highlights
Because of the need to cope with seasonal peaks in energy consumption, the interest to develop new underground gas storage (UGS) facilities is continuously growing
The UGS activities induce changes of the stress and strain fields, with the earlier that remains substantially confined within the reservoir formation and the nearest portion of the surrounding rocks and the latter that spreads to the land surface (Teatini et al, 2011; Castelletto et al, 2013)
From the hydrological-hydraulic point of view, the prediction of the efficiency variation of the surface drainage network possibly caused by the movements of the ground surface due to UGS was simulated by means of the codes HECHMS (USACE-HEC, 2010) and HEC-RAS (USACE-HEC, 2016) developed by the U.S Army Corps of Engineers – Hydrologic Engineering Center
Summary
Because of the need to cope with seasonal peaks in energy consumption, the interest to develop new underground gas storage (UGS) facilities is continuously growing. The widespread use of UGS is followed by a growing concern of Government authorities and Public opinion about its geomechanical issues, especially subsidence and induced (micro-) seismicity. There is a growing need to combine advanced geomechanical analyses with more understandable evaluations. Over the last decades a number of modelling applications were developed to characterize the safety or the possible impact of UGS. A first attempt is made to develop an understandable approach to evaluate the safety of a UGS plant derived from what is traditionally applied in the structural design of buildings. Taking inspiration from this approach, the design and failure configurations as well as the safety factor related to the update of a UGS reservoir are evaluated with the aid of stateof-the-art numerical models
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