EVALUATION OF GEOLOGICAL RISKS ASSOCIATED WITH WITHDRAWAL AND STORAGE OF GAS: GEOMECHANICAL MODELING OF A STORAGE FACILITY IN NORTHERN ITALY

The withdrawal and the injection of gas or others fluids from/in porous media in the subsurface involves the deformation of the reservoir by normal compaction and poroelastic perturbations. Such deformations are often accompanied by phenomena of subsidence, instability of the infrastructure, and in some cases also by induced and activated seismic activity, related to variations in the effective stress in the rocks of the reservoir, and in those of seal. In our work we use data from a 26-year leveling-monitoring of land subsidence above a reservoir, managed by Edison ®, consisting of 2 levels, approximately 10 meters thick, separated by clay layers and hydraulically separated, localized to the core of a gentle anticline at the most external structure of the Alpine front in the North East of Italy, at approximately 1400 meters depth. During about 15 years of field development, there has been a relative mean subsidence of about 10 mm, in correspondence to a drop in pore pressure of about 15 MPa in the levels of the reservoir. The reservoir has been converted to natural gas storage site since 1993, and a double monitoring in 2006 showed a relative uplift on the surface topography of about 2 mm, between the extraction phase and the gas injection. The production data, the pressure of the reservoir and the subsidence have been used to derive the variations of the pore volume of the reservoir, allowing to determine its volumetric deformation; this parameter is used as input data to perform a modeling of the deformations induced by the field development and by storage cycling, that takes advantage of the use of the method of Eshelby’s inclusion for inclusions with ellipsoidal shape with elastic parameters different from those of the surrounding material (inhomogeneity problem), with a semi-analytical approach. The elastic fields are derived (stress, strain and displacement) in an infinite space in the neighborhood of the reservoir: the vertical displacement data are used as a control parameter for the modeling and provide a good correlation between surface deformation and strain of the reservoir, both during withdrawal and storage. In addition, through the calculation of the Coulomb Failure Stress, we are able to provide an overview on the areas of the reservoir of major risk during withdrawal and injection. This tool, on the one hand, in the planning stage of the geological risks, allows to determine fairly rapidly and with relative ease the amount of the elastic fields, their variations during different stages of field development. this analysis provide a quantification of the induced phenomena associated with the activities of withdrawal and injection of gas, or fluid, in the subsurface. A further advantage of this method is the ability to quickly manage the monitoring data as soon as they are produced in the geological model prepared.