Abstract
Abstract. Subsurface gas storage in porous media is a viable option to mitigate shortages in energy supply in systems largely based on renewable sources. Fault systems adjacent to or intersecting with gas storage could potentially result in a leakage of stored gas. Variations in formation pressure during a storage operation can affect the gas leakage rates, requiring a site and scenario specific assessment. In this study, a geological model of an existing structure in the North German Basin (NGB) is developed, parameterised and a methane gas storage operation is simulated. Based on the observed storage pressure, a sensitivity study aimed at determining gas leakage rates for different parametrisations of the fault damage zone is performed using a simplified 2-D model. The leakage scenario simulations show a strong parameter dependence with the fault acting as either a barrier or a conduit for gas flow. Furthermore, the storage operation greatly affects the gas leakage rates for a given parametrisation with significant leakage only during the injection periods and thus during increased overpressures in the storage formation. During injection, the peak leakage rates can be as high as 2308 Sm3 d−1 for damage zone permeabilities of 10 mD and a capillary entry pressure of 4 bar. Increasing capillary entry pressure results in a sealing effect. If the capillary entry pressure is scaled according to the damage zone permeability, peak leakage rates can be higher, i.e. 3240 Sm3 d−1 for 10 mD and 0.13 bar. During withdrawal periods, the pressure gradient between a storage formation and a fault zone is reduced or even reversed, resulting in greatly reduced leakage rates or even a temporary stop of the leakage. Total leakage volume from storage formation was assessed based on the 2-D study by considering the exposure of the gas-filled part of the storage formation to the fault zone and subsequently compared with gas in place volume.
Highlights
Worldwide, countries are promoting a transition from conventional to renewable energy sources to mitigate global climate change (IPCC, 2015)
The storage operation was designed to provide enough electricity for one week to offset a complete lack of renewable power generation in the state of Schleswig-Holstein, home to around 2.8 million people
The leakage scenario simulations show that the fault zone intersecting the storage formation can act as either a conduit or a barrier for fluid flow, depending on petrophysical parameters, the fluid flow properties and the current storage operation
Summary
Countries are promoting a transition from conventional to renewable energy sources to mitigate global climate change (IPCC, 2015). Fault systems exist throughout the NGB, including the identified storage sites, which introduces uncertainty regarding the possibility of gas leakage (Oldenburg et al, 2002; Folga et al, 2016) Such a leakage of gas would result in a reduced gas in place (GIP), and in a potential drop in formation pressure, both resulting in a reduced storage capacity and storage self-discharging over time. The effect of existing fault systems on the operation at a potential storage site, i.e. occurring leakage rates and resulting reduction in formation pressure, must be investigated prior to any deployment. This study is aimed at investigating leakage characteristics during a storage operation at a potential gas storage site in the NGB for different fault zone parametrisations. A sensitivity analysis aimed at determining leakage rates for different fault zone parametrisations is carried out on a 2-D slice of the model area
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