Geomechanical Effect of Hydrate Dissociation-induced Stress Relaxation

The geomechanical behaviour of gas hydrate-bearing sediments is unique. Since gas hydrate exists as a solid in pores, it effectively densifies the host sand and bonds surrounding grains together. As a result, hydrate-bearing sediments exhibit stiffer, stronger and more dilatant behaviour than hydrate-free sediments. The uniqueness of hydrate-bearing sediments becomes more prominent during gas production. Unlike conventional oil and natural gas, gas production from hydrate-bearing sediments involves phase change of the gas hydrate from solid to gaseous. This implies not only that the aforementioned characteristics diminish accordingly to the remaining hydrate in pores, but also that the solid (i.e. hydrate) that has been carrying the effective stresses disappears, resulting in release of the effective stresses. The release of the effective stresses upon hydrate dissociation, referred to as hydrate dissociation-induced stress relaxation, causes stress redistribution as well as plastic deformation. Neglecting the stress relaxation term could therefore lead to inaccurate deformation prediction. This paper presents the formulation for hydrate dissociation-induced stress relaxation and demonstrates the importance of the term for an accurate wellbore deformation prediction.