Sensitivity Analysis of Depressurization-Induced Long-Term Production from Class 1 Hydrate Deposits

George J. Moridis and Michael Kowalsky
Lawrence Berkeley National Laboratory, Berkeley, CA

Class 1 hydrate deposits are characterized by a hydrate-bearing layer underlain by a two-phase zone involving mobile gas. We focus on Class 1W accumulations, which involve water and hydrate in the hydrate zone. Dissociation in Class 1W deposits appears to proceed in distinct stages controlled by evolution in the distribution of phases in the reservoir. Capillary pressure effects play a critical role and are responsible for hydrate lensing, i.e., the emergence of distinct banded structures of alternating high-low hydrate saturation, which form channels and shells and have a significant effect on production. We investigate the sensitivity of production to important reservoir conditions and operation parameters. These include the hydrate saturation, the intrinsic permeability of the formation, the capillary pressure, the fluid relative permeabilities, the production rate, the location of the well relative to the hydrate interface, and the rate of wellbore heating. We determine that large volumes of gas can be readily produced at high rates for long times from Class 1W gas hydrate accumulations by means of depressurization-induced dissociation using conventional technology. Class 1W hydrates are capable of contributing in excess of 70% to the gas production rate from the reservoir, and up to 50% to the cumulative volume of the produced gas. The sensitivity of production to the reservoir conditions and operation parameters appears to vary significantly depending on the stage of dissociation in the deposit.