Geomechanical and Physical Modeling of Deformation in Underground Gas Storages During Cyclic Changes of Pore Pressure

Abstract

AbstractA method of geomechanical modeling of deformation, fracture and filtration processes in reservoirs of underground gas storages (UGS) aiming at justification of rational and safe operating regimes of production wells is given. The method is based on an analysis of changes in stress–strain state occurring in reservoirs during UGS operation and true triaxial experiments on rock specimens that reconstruct the real stress state occurring in a reservoir during gas withdrawal and injection. Depleted reservoirs of hydrocarbon deposits and aquifers with high porosity and permeability are usually used to create underground storage facilities. Hence, the pressure drawdowns and pressure overbalances used during gas withdrawal and injection are as a rule not much and do not influence significantly on rock destruction and sand production. Geomechanical analysis and tests carried out for a number of UGS demonstrate that the key role in the processes of rock fracture in UGS reservoirs is played by the changes in pore pressure as in the reservoirs as a whole, especially at the stage of the maximal withdrawal. Besides the changes in pore pressure results in changes in the side pressure in the reservoirs, i.e. the effective stresses acting at the rock skeleton become unequal component. The tests allow determining the pore pressure resulting in creep deformation leading finally to rock fracture and sand production. The described method has been applied to the conditions of a particular UGS. The acceptable intervals of changes of pore pressure not requiring special filters and supports in bottom-hole zones for preventing rock fracture and sand production are determined.KeywordsUnderground gas storageSand productionGeomechanicsModelingFractureBorehole