Underground seasonal storage of gas: testing numerical modelling tools with application to i) a deep aquifer-layer, and ii) salt caverns.

Within the framework of the SECURE project, we test modeling techniques used for natural geothermal and volcanic reservoirs and apply them to anthropic underground gas storage facilities. These systems indeed share similar mechanics and physical properties, however gas reservoirs are often extensively monitored, and better imaged. In order to manage fluctuations between gas supply and demand, natural gas can be temporarily stored in different underground storage facilities, such as depleted gas/oil fields, natural aquifers, and salt cavern formations. When properly monitored during storage and withdrawal (production) of gas, these systems provide a unique opportunity to investigate how reservoirs evolve at different time scales, modify the surrounding stress state, produce deformation coupled with diffusion processes, and possibly induce/trigger earthquakes on nearby faults.

In the first case study we addressed within the framework of SECURE project, we take advantage of well constrained reservoir geometry and physical parameters, records of gas injection/production rates, pore pressure variations, and a local seismic catalog at a gas reservoir in Spain. We implement a poro-elastic model to simulate pressure temporal variations, estimate related stress-state variations, and study eventual relationship with small recorded seismic events. The model is based the software POEL by Wang et al., (2003), a semi-analytical physics-based numerical scheme which allows the computation of transient and steady-state solutions in response to pore-pressure variations. Being 2D axisymmetric, POEL drastically simplify the geometry of the reservoir, but it is particularly suitable to link observables such as pressure variations within the reservoir with the physical/mechanical processes occurring in the surroundings.

In the second case study we address the stability condition for salt caverns which has been excavated for salt mining purposes. We make use of 2D discrete-element geomechanical models to compare numerical simulation results with field observations in terms of surface subsidence. With this numerical model we consider different pressure conditions for the fluid (brine) filling the cavity, and return different scenarios for the stability of a salt cavern. Such modeling effort aims at improving our understanding of middle-to-long term stability conditions, for those cavities that have been dismissed after anthropic operations such as salt extraction, but also seasonal gas storage.