Poroelastic model of gas storage in porous rock layers: a case study from Spain

Within the framework of the SECURE project, we test modeling techniques used for natural geothermal and volcanic reservoirs and apply them to anthropic underground oil/gas storage. These systems indeed share similar mechanics and physical properties, but oil/gas reservoirs are usually more easily and extensively monitored.

Natural gas can be temporarily stored in different underground storage facilities, such as depleted gas fields, aquifers, and salt cavern formations. Gas storage in underground aquifers is a widely used technique for controlled energy storage to manage fluctuations between gas supply and demand. Being carefully 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.

Here 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 poroelastic model to simulate pressure temporal variations, estimate related stress-state variations, and study eventual relationship with small recorded seismic events.

The model is based on a layered poroelastic structure and has been implemented using 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 fluids injection/extraction. 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. Furthermore, we developed POEL to address a common, relevant problem for gas storage in poroelastic layers: the presence of a vertical impermeable fault bounding the reservoir.

Elaborate and detailed numerical modeling techniques (e.g. FEM) are often applied to study gas reservoirs. We show here the possibilities and limitations of a fast, highly flexible and easily formulated tool that can be easily applied to similar cases.