Lawrence Berkeley National Laboratory

Unconfined Class 2 hydrate accumulations in the oceanic subsurface are characterized by mobile saline water zones enveloping the hydrate-bearing formation and by the absence of impermeable layers to vertical flow. In this paper, we evaluate the gas production potential of such deposits using both single-well and five-spot well configurations. Single-well production is based on depressurization-induced dissociation of the hydrates, whereas the five-spot configuration involves both depressurization at the production wells and thermal stimulation at the injection wells. The results of the study indicate that unconfined Class 2 hydrate accumulations are among the most challenging targets for gas production because (a) the absence of confining boundaries limits the effectiveness of depressurization, (b) gas production is accompanied by the production of very large volumes of water, and (c) thermal stimulation, when employed, requires substantial energy inputs. The amount of produced gas is limited in both the single-well and the five-spot configurations, and is significantly smaller than the total volume of gas released in the formation. For the five-spot configuration, hydrate dissociation releases relatively large amounts of gas into the reservoir, but these are not readily recoverable. Gas production is also significantly affected by the initial hydrate saturation in the hydrate-bearing sediment.