A simulation study of biogeochemical interactions in cyclic underground bio-methanation of carbon dioxide and hydrogen

Abstract

A coupled PHREEQC-MATLAB simulation approach is proposed to investigate the dynamic changes in rock porosity, gas storage capacity, formation water salinity, and reservoir temperature driven by biogeochemical interactions during cyclic underground bio-methanation (UBM) of CO₂ and H₂, and to quantitatively examine how the evolution of these parameters influences CH₄ production efficiency. The results indicate that during the cyclic UBM of CO₂-H₂, the formation water undergoes a dynamic acid-base alternation, leading to periodic mineral dissolution and precipitation with limited impact on rock porosity. Across different mineral systems, the maximum CH₄ production rate remains consistently around 3.6×10⁻³ mol/(L·d) in each cycle. With an increasing number of cycles, under high initial salinity conditions, the metabolic water produced by methanogens can significantly reduce the formation water salinity, gradually enhancing the CH₄ production rate to levels comparable with those under low initial salinity. Additionally, the increased volume of produced water reduces the gas storage capacity of the reservoir. This reduction becomes more pronounced at higher initial CO₂-H₂ pressures, accompanied by a more significant increase in CH₄ production rate increment. Furthermore, the heat generated by methanogen metabolism leads to an increase in reservoir temperature, with the extent of temperature rise significantly influenced by heat loss. If the heat loss is neglected, the reservoir temperature can increase by up to 17.1 °C after five cycles (10 years). When the reservoir has a higher initial temperature, the elevated thermal conditions may reduce CH₄ production efficiency.