Study of Simultaneous Hydrogen Gas Stream Purification and CO₂ Storage in Coal Samples using an Experimentally Simulated Huff ’n’ Puff Scheme Combined with the Rate-Transient Analysis, Porosity, and Permeability Core Analysis Method

Hydrogen (H₂ ) production and storage are key strategies that enable the transition to low-carbon energy on a global scale. Steam methane reforming (SMR) is a common process for generating H₂, but SMR produces carbon dioxide (CO₂ ) as a byproduct, which needs to be separated from the gas mixture and sequestered to reduce the carbon footprint. One solution is to simultaneously purify the H₂ component and permanently sequester CO₂ by injecting mixtures of H₂ and CO₂ into depleted subsurface coal through an injection well and producing the gas mixtures through an offset production well. This process was recently simulated and verified experimentally using a coreflooding approach. In the current work, an experimentally simulated huff ’n’ puff (HNP) scheme is applied to evaluate the extent of H₂ purification/CO₂ storage that can be achieved. The rate-transient analysis, porosity, and permeability (RTAPK) method is also used to estimate permeability after each HNP cycle. In the current study, synthetic SMR gases were injected into a coal sample from the Mannville Formation of western Canada, allowed to soak, and produced from the coal sample. Two cycles of injection/equilibration/production of the synthetic SMR gas [volume per-centages: 75% H₂ /15% CO₂ /10% methane (CH₄ )] were performed using RTAPK applied to the coal sample at reservoir temperature; a third cycle was implemented using the purified SMR gas after Cycle 2. For each cycle, the initial injection pressure was between 870 psi and 880 psi, followed by a soaking period of 30–150 hours. After the injection stage of Cycle 1, the gas composition stabilized to ~95% H₂ /2% CO₂ /3% CH₄, demonstrating a simultaneous increase in H₂ component free-gas concentration and decrease in CO₂ and CH₄ component concentrations. A similar result was obtained for Cycle 2. At the start of the production stage for Cycle 1, the produced gas was close to the equilibrated value, but at the end of this period, the produced gas composition was ~86% H₂ /6% CO₂ /8% CH₄, indicating H₂ concentrations were diluted due to the desorption of some CH₄ and CO₂ during production. A similar result was again obtained for Cycle 2 but with a somewhat different (suppressed in H₂ composition) late-stage produced gas composition of ~81% H₂ /7% CO₂ /12% CH₄ . The late production-stage Cycle 2 gas stream was then reinjected to determine if H₂ could be further purified (Cycle 3), but equilibrium compositions obtained after reinjection, and late-stage produced gas compositions, suggest that further purification was not possible. For Cycles 1–3, the amount of injected H₂ that was recovered at the end of the production stage was approximately 20%, 38%, and 46%, re-spectively; the amount of injected CO₂ stored was approximately 96%, 92%, and 79%, respectively. The permeability of the coal sample to H₂, measured before SMR gas injection, was estimated to be ~0.03 md. However, after the SMR gas injection, the permeability of coal sample decreased to ~0.01 md. This proof-of-concept study using the RTAPK method demonstrates that simultaneous H₂ purification and CO₂ storage can be achieved through injection of H₂ /CO₂ /CH₄-containing SMR gases into coal samples and subsequent production of the gas mixture.