Cohesive and Adhesive Failure Mechanisms of CO₂/Tetrahydrofuran Structure II Gas Hydrate Crystalline Cores through Hydraulic Yield Strength Measurements

Gas hydrate plugs and deposits pose a challenge to flow assurance in transportation flowlines operating in cold environments. If a plug forms, rapid remediation is highly desirable to mitigate production downtime as well as prevent excessive annealing/densification of the core/plug. One of the most commonly used hydrate plug remediation techniques, one-sided depressurization, can result in a potentially hazardous scenario where the differential pressure across the plug may result in its detachment from the pipe wall and subsequent acceleration down the flowline. These plug projectiles have resulted in equipment damage and personnel injuries and fatalities. Hence, it is important to understand how the depressurization rate and mechanical failure mechanisms impact the safety of operations. Hydrate plug remediation and risk assessment require quantitative information on the mechanical properties of the hydrate. Previous work has focused on examining how the hydrate/solid interface is impacted by system properties, in an aim at establishing estimates of a maximum allowable differential pressure across the plug based on the interface's shear strength. This work seeks to understand how properties of the hydrate plug itself (annealing) as well as the rate of applied stress to the interface impact the ultimate shear strength of the system. A novel high-pressure hydraulic technique is used to apply stress to CO₂/tetrahydrofuran structure II gas hydrate crystalline plugs formed <i>in situ</i> in pipe samples. The resulting shear strength data from the experiments is examined and compared across different levels of hydrate annealing (via water conversion percentage) as well as the rate of applied stress (via hydraulic fluid pumping rate). Two different mechanical failure mechanisms were observed for these plugs: internal cohesive failure (cracking and establishment of channels through the plug without detachment from the wall) and adhesive delamination (detachment of the plug from the wall and subsequent ejection from the pipe). These failure mechanisms were strongly correlated with the pressurization rate. The overall yield strength of crystalline hydrate plugs in a cohesive failure mechanism was observed to be lower than that which underwent an adhesive failure mechanism.