Hydromechanical Response and Impact of Gas Mixing Behavior in Subsurface CH₄ Storage with CO₂-Based Cushion Gas

Abstract

Power-to-gas (PtG) stores chemical energy by converting excess electrical energy from renewable sources into an energy-dense gas. Due to its higher available capacity compared to surface-based storage technologies, subsurface storage in geological systems is the most promising approach for efficient and economic realization of the PtG system's storage component. For this purpose, methane (CH 4 ) produced by methanation by means of hydrogen (H 2 ) and carbon dioxide (CO 2 ) is stored in a geological reservoir until required for further use. In this context, CO 2 is used as the cushion gas to maintain reservoir pressure and limiting working gas, i.e., (CH 4 ) losses during withdrawal periods. Consequently, mixing of both gases in the reservoir is inevitable. Therefore, it is necessary to minimize the gas mixing region to optimize the efficiency of the PtG system's storage component. In the present study, the physical properties of CH 4 , CO 2 and their mixtures are reviewed. Then, a multicomponent flow model is implemented and validated against published data. Next, a hydromechanically coupled model is established, considering fluid flow through porous media and effective stresses to investigate the mixing behavior of both gases and the mechanical reservoir stability. The simulation results show that, with increasing reservoir thickness and dip angle, the mixing region is reduced during gas injection if CO 2 is employed as the cushion gas. In addition, the degree of mixing is lower at higher temperatures. Feasible injection rates and injection schedules can be derived from the integrated reservoir stability analysis. The methodology developed in the present study allows the determination of optimum strategies for storage reservoir selection and gas injection scheduling by minimizing the gas mixing region.

Influence of Storage Period on the Geochemical Evolution of a Compressed Energy Storage System

“…However, careful system design needs to be considered to not produce CO 2 during the extraction of the working gas (Oldenburg and Pan, 2013). This can lead to pressure losses in the formation that affect working gas recovery (Ma et al, 2019). Also, failure to sequester CO 2 while being used as cushion gas could offset potential tax credits gained by using CO 2 as a cushion gas (Rul Internal Revenue Service, 2009).…”

Thermodynamic and transport properties of hydrogen containing streams

“…Therefore, if the cushion gas and working gas are of different compositions, the accurate quantification of the cushion gas/working gas mixing zone is of paramount importance. Once mixing takes place, the different gaseous components will alter the properties of the gas and introduce significant uncertainty into the expected behaviour of the injected, stored and produced gas, as shown for different gas storage applications 18,19 .…”

Impact of CO2 mixing with trapped hydrocarbons on CO2 storage capacity and security: A case study from the Captain aquifer (North Sea)

“…The impact of gas mixing in subsurface systems have been investigated for different subsurface gas storage applications [25] including, CO 2 and methane [26] and hydrogen systems [27].These studies however have not covered the scale of large CO 2 storage in saline aquifers and depleted gas fields. Mixing in this context refers to the occurrence of direct contact between injected and resident fluids within the storage site, usually with different properties which may alter plume properties, notably its density and thus, may affect the storage characteristics.…”