Driving mechanisms of quartz wettability alteration under in-situ H2 geo-storage conditions: Role of organic ligands and surface morphology

The wettability of subsurface minerals determines the capillary trapping and fluid transport behaviors, essential for ensuring the safe and efficient hydrogen (H2) geo-storage. This work decouples the impact of the in -situ conditions and the quartz surface properties (methylation degree and roughness) to elucidate the dominant mechanisms of wettability alteration, using molecular dynamic simulation. Our simulation models are capable of reproducing the experimental data of water contact angle on the quartz surface in H2 environment. This work reveals a dual function of methylation degree on the wettability of quartz surfaces: First, it reduces the hydrophilicity of quartz due to the presence of hydrophobic methyl ligands and second, it enhances quartz hydrophilicity by suppressing hydrogen bonds between hydroxyl groups. The former function dominates at high methylation degrees, while the latter prevails at low ones. Also, we observe that the role of salinity and cushion gas on the quartz wettability depends on the quartz surface chemistry. Specifically, the wettability of the fully hydroxylated quartz surface remains largely unaffected by NaCl and CH4, whereas they drastically increase the hydrophilicity of the half -methylated quartz surface. We further reveal that the alteration of quartz wettability with pressure is predominantly driven by two mechanisms: the pinning effect and the H2 -quartz interaction. The dominant mechanism is contingent upon the pressure range, as well as the surface chemistry and morphology. For instance, the dominant mechanism for the pristine and the fully hydroxylated surfaces is the pinning effect at the low-pressure range, while the H2 -quartz interaction mechanism takes precedence at higher pressures. Interestingly, we detect a contact angle jump on the rough surfaces, signifying a transition state of the water film on these surfaces. These findings could reconcile the discrepancies related to the quartz wettability alteration under H2 geo-storage conditions, pivotal for safe and efficient H2 storage.