Numerical investigation of fault activation in underground gas storage reservoirs

Underground gas storage (UGS) is a practice that is becoming widely implemented to cope with seasonal peaks of gas consumption. When the target reservoir is located in a faulted basin, a major safety issue concerns the re-activation of pre-existing faults, possibly inducing (micro-) seismicity. Faults are reactivated when the shear component of traction exceeds the acceptable limiting strength. It has been observed that this occurrence can happen "unexpectedly" during the life of a UGS reservoir, i.e., when the actual stress regime is not supposed to reach the failure condition. A numerical investigation has been carried out to shed light in this respect, by analyzing the mechanisms and the critical factors that can be responsible for a fault activation during different UGS stages. The model is applied in a physical context representative of the typical UGS reservoirs located in The Netherlands, in terms of reservoir properties, fault geometry and pressure history. The numerical results show that "unexpected" fault re-activations are likely to occur during UGS when micro-seismicity had been already experienced in the primary reservoir exploitation, even if the pore pressure does not exceed the initial undisturbed conditions.