The Effect of Inorganic Salt Transport and Two-Phase Seepage on Hydrate Wellbore Stability

Huge reserves of methane hydrate have been found in the deep waters, but the decomposition of hydrates in the formation during drilling makes it difficult to drill and exploit hydrate reservoirs. To study the effect of inorganic salts and two-phase percolation on the stability of hydrate wellbore, a multi-field coupling model including chemical-seepage-solid-thermal equations was established. The results show that after the hydrate reservoir was drilled, it began to decompose under inorganic salt intrusion, depressurization and temperature rise. After a short time, the decomposition of hydrate around the wellbore will slow down. This will cause the reduction of the cohesion of hydrate formation, which may increase the lower limit of drilling fluid density window and easily leads to collapse of wellbore. The speeds of these three effects are different. When the influence of inorganic salts and pore pressure drop is relatively slow, the influence of heat transfer is faster. The molecular diffusion in the low-permeability muddy hydrate formation is more obvious, but its speed still can't follow the advancement of hydrate decomposition. To prevent this type of drilling risk, lowering the bottomhole pressure to below the pore pressure is advised, which can help field engineers to prevent the drilling risks.