The generation of hydrogen in-situ from hydrocarbon reservoirs has emerged as a carbon neutral technology for fossil fuel-based hydrogen production. This technology has been extensively investigated for heavy oil reservoirs through in-situ combustion gasification. This study proposes in-situ hydrogen generation from depleted gas reservoirs and assess graphite gravel packing for selective hydrogen production with underground carbon storage. The viability of this hydrogen generation process was accessed through process simulation, followed by experimental investigation and molecular simulation of the selective production of hydrogen through graphite. Equilibrium and kinetic models reproduced measured effluent fractions, confirming their reliability. The simulation outcomes reveal that higher temperature and steam-to-carbon ratio increase hydrogen yield/purity, whereas high pressure favors methanation. This necessitates elevated temperatures beyond the usual reaction temperature under reservoir conditions. Longer residence time and judicious catalyst loading improve conversion while limiting diminishing returns. Adiabatic simulation yields lower hydrogen purity than isothermal but better reflects field behavior. Reservoir mineralogy governs outcomes as quartz-rich rocks inhibit hydrogen production by steam reforming, while clays/feldspars reported elsewhere can be catalytic. The experimental results showed that graphite can be used as gravel pack in the production well to produce hydrogen and retain carbon dioxide underground. Literature report indicates that high compaction can further enhance separation significantly reducing the carbon emission associated with hydrogen production from fossil fuels.

The wetting behavior of rock/CO₂/brine systems highly impacts the fluid distribution at the pore-scale and multiphase flow at the macroscale and is considered a key parameter controlling the CO₂ residual trapping in geological storage. The effect of wettability on residual trapping is, however, still uncertain as the current literature suggests high discrepancies among the published datasets. Moreover, the dataset for residual trapping observations for non water-wet carbonate rocks is relatively scarce; none of the published studies investigated this aspect in CO₂-wet limestones. Thus, a series of core-flooding experiments was conducted at reservoir conditions for three limestone samples having different wettability states, water-wet, intermediate wet, and CO₂-wet. Wettability alteration of sister rocks was achieved using stearic acid to mimic the wettability alteration in saline aquifers due to the interaction with natural organic compounds. Notably, increasing the hydrophobicity of limestone tends to decrease CO₂ residual trapping efficiency $\sim$ 19% and 37% when the initial water-wetting state shifts to intermediate-wet and CO₂-wet, respectively. This is attributed to the fluid distribution at the pore scale, in particular the wetting layers, and its effect on the CO₂/brine displacement. In case of CO₂-wet rocks, macroscopic CO₂-wetting layers act as flow paths, which reduces the residual CO₂ saturation from 29% (water-wet) to 8%. These findings advocate water-wet rocks as better candidates for CO₂ residual trapping and provide insights into residual trapping-rock wettability correlation pertinent to CO₂ geo-storage.

Subsurface formations typically exhibit heterogeneous wetting characteristics due to the complex pore system, mixed lithology, and prolonged contact with native fluids. This non-uniformity in spatial wettability distribution thus makes the subsurface formations exhibit more complex localized CO₂/brine/rock interactions, introducing uncertainties in estimating trapping capacity and predicting CO₂ plume migration. Field-scale investigation on the role of wettability in CO₂ geo-storage has received limited attention, and previous studies typically assume an internal uniform wettability condition across the whole formation. However, the more realistic scenario of internal wettability spatial variations within a single formation is yet to be thoroughly examined. In this study, a range of experiment-derived wettability-dependent trapping coefficients were utilized to implement the internal wettability heterogeneity in a single formation model, and its impact on CO₂ plume pattern and trapping efficiency was examined. Furthermore, mixed-wet systems with different CO₂-wet fractions were also considered in this study. The results indicate that internal wettability variations result in changes in the local CO₂ saturation pattern and thus impact the overall plume shape and migration. In addition, the internal heterogeneous wettability system exhibits an approximately 35% reduction and an approximately 20% increase in residual trapping capacity in comparison to internal uniform strongly water-wet and uniform weakly water-wet systems, respectively. An increase in the fraction of CO₂-wet regions in the mixed-wet system results in concentrated high-saturation clusters and reduced local CO₂ residual saturation. This further results in reduced residual and dissolution trapping, followed by a linear correlation.