Geomechanical effects of gas storage in depleted gas fields

Cyclic gas storage in and withdrawal from subsurface gas fields generate a sequence of loading-unloading cycles in both the sandy permeable formation and the confining clayey caprock. A basic requirement for the safety of the underground gas disposal is that the related stress variation does not jeopardize the sealing capacity of the caprock, especially if the pore fluid pressure should exceed the original in situ value in order to increase the volume of the injected gas. Moreover, as a side effect the reservoir deformation can induce a non negligible cyclic motion of the ground surface, with possible undesired consequences for the existing civil structures and infrastructures. The present communication aims at investigating numerically the expected geomechanical effects of gas storage in a depleted gas reservoir located in Italy. Computer simulations are performed using quite realistic information on the geometrical and geomechanical properties of the field. Two disposal scenarios are assumed where the maximum pore pressure in the injection stage may increase from 120% up to 150% of its original value. The results show that no significant stress changes are caused in the caprock in the most unfavourable scenario as well, thus ensuring the safety of the operation. However, a shear failure condition can be locally attained in the injected formation. Such an occurrence should be carefully considered and possibly monitored, especially if the failure is predicted close to the injecting wells with potential large displacements which could damage the hole casing. Finally, the reservoir deformation can induce a cyclic vertical ground motion up to a few centimetres according to the generated pore pressure fluctuation, the burial depth and the actual rock geo-mechanical properties of the field and the overburden. Such a motion, however, is expected to occur mainly in the elastic range and is almost fully recovered over a complete injection-withdrawal sequence, with little concern for the safety of the existing civil structures.