Russian, American, and Canadian researchers have described numerous drilling and production problems attributed to the presence of gas hydrates, including uncontrolled gas releases during drilling, collapsed casings, and gas leakage to the surface. When the drill bit penetrates a gas hydrate, under normal drilling conditions, the drilling mud will become highly gasified as the hydrate decomposes. The hydrate adjacent to the wellbore will continue to decompose and gasify the drilling mud as long as the drilling introduces heat into the hydrate-bearing interval. Recent drilling in the Russian Yamburg field shows the potential severity of hydrate-induced drilling problems, which have included gas kicks, blow-outs, and fires. The production of hot fluids from depth through the gas-hydrate-bearing intervals can also adversely raise formation temperatures, resulting in further hydrate decomposition. If the dissociated free-gas becomes trapped behind the casing, reservoir pressures may substantially increase and cause the casing to collapse; in several wells in northern. Alaska, dissociated free-gas has leaked to the surface outside the conductor casing. Remediation of hydrate-related drilling and production problems has generally followed two courses of action:
prevention of hydrate dissociation or
promotion of hydrate dissociation. Both approaches involve regulating mud weights and temperatures to control the dissociation of in-situ gas hydrates.
Gas hydrates are crystalline inclusion compounds formed as ice-like mixtures of gas and water in which gas molecules are trapped within a solid framework of water molecules. In nature, temperature and pressure conditions (figure 1) conducive to the development of gas hydrates are globally widespread in permafrost regions and beneath the sea in sediments of the outer continental margins (figure 2). While methane, propane, and other gases can be included in the clathrate structure, methane hydrates appear to be the most common in nature (Kvenvolden, 1988).
