The effect of clay on initial and residual saturation of hydrogen in clay-rich sandstone formation: Implications for underground hydrogen storage

Abstract

Understanding wettability in rock-brine-hydrogen systems is essential for dependable predictions of capillary/residual trapping in clay-rich sandstone formations. Despite being the most used technique, wettability assessment based on contact angle measurements is confronted with inherent uncertainties that limit its reliability. In contrast, core flooding techniques provide a more direct and realistic picture of wettability and its time evolution. Nuclear Magnetic Resonance (NMR) allows us to evaluate the initial and residual hydrogen saturations and distribution along the core specimen. It is a fast, reliable, and effective way of inferring the impact of wettability on hydrogen migration, and residual trapping in prospective geo-storage rock formations. Recent publications have reported the evaluation of wettability in a brine-hydrogen-rock system where the rock is a clean sandstone (no clays). Here we evaluate the impact of the presence of clays in a sandstone, which has not been reported yet. NMR monitoring was employed to characterize the initial and residual hydrogen saturations in the Bandera Grey (BG) sandstone. To investigate the impact of clay minerals on hydrogen saturation, same rock sample was characterized in its natural state, and after heating it to 700 °C for 12 h in an air environment to burn off clay minerals, During the NMR core flooding experiments, ten pore volumes (PVs) were injected/withdrawn during the drainage/imbibition cycles at a fluid injection rate of 2 mL/min under room temperature and 1000 psi confining pressure. Due to the hydrophilicity of quartz and clay, the tested BG sandstone (clay-rich sandstone) shows a significant residual/trapped saturation (∼3.5% can be reproduce); therefore, clay-rich sandstone may not be ideal for hydrogen storage unless cushion gas is used.The results show that initial and residual hydrogen saturations were slightly changed after firing (from 16% to 18% for initial and from 14% to 13% for residual). This also suggests that the wettability of the BG sandstone-brine-hydrogen system is slightly impacted by clay content and type. We also observed that clay firing at 700 °C has little effect on the porosity and gas permeability of the BG sandstone. Moreover, X-ray powder diffraction (XRD) results showed that quartz content increases from 68.1% to 76.2%, Kaolinite transformed into illite and clinochlore disappeared. The disappearance of chlorite after firing suggests that it is transformed into another clay type.