Exploratory assessment of symbiotic hydrogen storage and CO₂ sequestration in coal seams

Latest studies have demonstrated that significant amounts of hydrogen is adsorbed in coal seams with minimal swelling, which contrasts markedly with CO₂ injection where significant swelling occurs. These observations support a new concept of symbiotic CO₂ sequestration and hydrogen storage in coal seams. CO₂ can be sequestered as a cushion gas because swelling can seal CO₂ perpetually, while hydrogen, as the working gas, can be stored temporarily and extracted when needed. This study developed a novel fully coupled multi-component hydromechanical dual porosity model to exploratorily assess this concept. The model consists of a set of partial differential equations, with coupling achieved through an unconventional effective stress principle that is specific to the gas. This coupled multicomponent model accounts for gas stripping effects between different gas species across the coal matrix-fracture system, the resulting changes in coal effective stress and reservoir properties, as well as competitive adsorption and gas mixing across pore scales—key coupled mechanisms that are not addressed by current models. The model is validated with experimental data and applied to simulate the cyclic operations of a symbiotic hydrogen and CO₂ storage process in coal seams. Model results demonstrate that the symbiotic CO₂ sequestration and hydrogen storage/extraction processes can self-regulate both injectivity for storage and deliverability for production within the coal seam. When CO₂ is injected, it acts as both a gas cushion and a sealing agent due to coal swelling to store hydrogen; when hydrogen is injected, it enhances CO₂ sequestration due to pressure effects. When hydrogen is produced, the deliverability may be enhanced due to CO₂ desorption induced shrinkage. These conclusions support the concept of symbiotic CO₂ sequestration with hydrogen storage and production in coal seams.