ABSTRACT

Two exploratory wells drilled in the Mackenzie Delta area of the Northwest Territories in 1972 by Imperial Oil Limited encountered shallow sand reservoirs containing gas hydrates -- an ice-like mixture of natural gas and water. These hydrocarbon-bearing sands underlie a thick zone of permafrost and are within the required pressure-temperature regime for naturally occurring gas hydrates. A significant increase in the amount of formation gas in the drilling mud occurred during penetration of these hydrate-bearing sands. Although wireline logs and drill cuttings indicated the sands to be very porous, test results showed their permeability to be extremely low. This is characteristic of hydrate-filled reservoirs. Because hydrates decompose at warm temperatures, yielding very high pressures if confined, it is dangerous to attempt to recover hydrated core with a conventional core barrel. Problems due to mud gasification, which occurs while drilling hydrates, can be minimized by using cool mud and by casing off the hydrate zones before drilling to deeper objectives.