Massimiliano Ferronato

Abstract:

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 is the re-activation of pre-existing faults, possibly inducing (micro-) seismicity. Faults are reactivated when the shear stress exceeds the limiting acceptable 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 expected to reach the failure condition. A numerical analysis has been carried out to cast light in this respect, by investigating the mechanisms and the critical factors that can be responsible for fault activation during the various UGS stages. The reservoir’s geomechanical behavior is simulated by an original elasto-plastic 3D Finite Element (FE) approach where the fault strength is taken into account by means of Lagrange multipliers [1]. The fault geometry is reproduced using special zero-thickness Interface Elements (IE), and the possible activation is controlled by the Mohr-Coulomb failure criterion. The simulations are carried out on a 3D regional-size geological setting, which requires the use of advanced numerical techniques for the solution of the resulting discrete problem [2]. 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 Norg and Bergermeer UGS fields represent the reference for this modelling application. The analysis addresses the role of: (i) the space and time pore pressure gradients in the UGS formation, within the faults bounding/compartmentalizing the reservoir; (b) the poroelastic stress changes with respect to the natural stress regime; (c) the specific geological settings, such as the geometric configuration and the hydro-geomechanical properties of the faults and reservoir. 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.

[1] A. Franceschini, M. Ferronato, C. Janna, P. Teatini. A novel Lagrangian approach for the stable numerical simulation of fault and fracture mechanics. Journal of Computational Physics, 314, pp. 503-521, 2016.
[2] A. Franceschini, N. Castelletto, M. Ferronato. Block preconditioning for fault/fracture mechanics saddle-point problems. Computer Methods in Applied Mechanics and Engineering, 344, pp. 376-401, 2019.