Effect of clay on methane hydrate formation and dissociation in sediment: Implications for energy recovery from clayey-sandy hydrate reservoirs*

Natural gas hydrate (NGH) is an unconventional energy source with high energy density, huge reserves and worldwide distribution. Sand-dominated hydrate-bearing sediments (HBS) are believed to be the most economically and technically feasible category of HBS for exploitation. Clay is one major mineral component of the natural HBS apart from sands. Due to the inhibition effect of clay on thermodynamics and kinetics of hydrate formation, it is challenging to synthesize representative clayey-sandy HBS samples in laboratory, and thus the corresponding fluid production behavior remains poorly understood. In this study, a sample synthesis method dedicated to clayey-sandy HBS (20 wt% clay) was developed. Two synergistic strategies including pre-wetting and multiple-site water injection were employed to enhance hydrate formation kinetics, increasing the final hydrate saturation from 0.285 to 0.473. Water-rich clayey-sandy HBS samples were formed with no dry sediment zones and improved hydrate distribution homogeneity. This optimized HBS synthesis method fills the gap be-tween the conventional HBS synthesis methods and the need to form more representative clayey-sandy HBS samples. Energy recovery and fluid production from sandy and clayey-sandy HBS samples were investigated by depressurizing to 3.0 MPa at 6.0 degrees C. Unique features were observed for hydrate dissociation in clayey-sandy HBS: immediate hydrate decomposition upon depressurization and a great pressure difference (similar to 1.0 MPa) between sediment and wellbore during the production period. This pressure gradient, likely caused by the aggregation and clogging of clays near the wellbore, slowed hydrate decomposition compared to sandy HBS. Additionally, the gas recovery increased from 81.7% to 88.5%, whereas the water recovery substantially decreased from 25.3% to 8.1%. The significant difference in fluid production behavior between clayey-sandy HBS and sandy HBS calls for more thorough investigations into clay-containing HBS to develop specific production strategies for energy re-covery from this type of HBS.

Automated summary

Natural gas hydrate (NGH) is an unconventional energy source with high energy density, huge reserves, and worldwide distribution. Sand-dominated hydrate-bearing sediments (HBS) are the most feasible category for exploitation, however, the presence of clay hinders the formation kinetics of the hydrates, making it challenging to study fluid production behavior. This study synthesized clayey-sandy HBS samples using a new method and observed that the presence of clay led to slower hydrate decomposition and a significant difference in fluid production behavior compared to sandy HBS. The findings call for further investigations and development of specific production strategies for clay-containing HBS.