Geomechanical Response of Known Permafrost Hydrate Deposits to Depressurization-induced Production

In this simulation study, we analyzed the geomechanical response of two known Class 3 permafrost deposits: the Mallik (Northwest Territories, Canada) deposit and Mount Elbert (Alaska, USA) deposit. Gas was produced from these deposits at constant pressure using horizontal wells placed at the top of the hydrate layer (HL). The depressurization-induced dissociation begins at the well bore, and then spreads laterally mainly along the top of the HL. The depressurization results in an increased shear stress within the body of the receding hydrate, and causes a vertical compaction of the reservoir. However, its effects are partially mitigated by the relatively stiff permafrost overburden, and compaction is limited to less than 0.5%. The increased shear stress may lead to shear failure in the hydrate-free zone that is bounded by the HL overburden and the downward-receding upper dissociation interface. This zone undergoes complete hydrate dissociation, and the cohesive strength of the sediment is low. We determined that the likelihood of shear failure depends on the initial stress state, as well as on the geomechanical properties of the reservoir. The Poisson’s ratio of the hydrate-bearing formation is a particularly important parameter that determines whether the evolution of the reservoir stresses will increase or decrease the likelihood of shear failure.