Thickness of gas hydrate stability zone in permafrost and marine gas hydrate deposits: Analysis and implications

摘要

Naturally-occurring gas hydrate, either in deepwater continental margins or in permafrost regions, is a promising unconventional energy resource which has attracted a lot attention from governments and researchers. In this paper, we studied the phase behavior for methane hydrate and mixed-gas hydrate. Four permafrost hydrates and three marine hydrate deposits were selected for analysis. They were Messoyakha (Russia), Mount Elbert (Alaska), Mackezie (Canada), Qilian Mountain (China) in permafrost regions and Blake Ridge (West Coast of US), Shenhu (South China Sea), and Nankai Trough (Japan Sea) in deepwater continental margins. The thickness of the gas hydrate stability zone (GHSZ) was calculated and compared. Our results show that the thickness of the GHSZ in permafrost regions is 1.3 to 4.5 times thicker than that in marine regions due to three factors: lower mudline temperature, smaller geothermal gradients and higher percentage of non-methane gases. The ramifications are that both the gas initially-in-place and the gas production rate will likely be higher in the permafrost environment. In addition, like other unconventional reservoirs, sustained gas production from a gas hydrate reservoir will likely require constantly drilling new wells to replace declining production from existing wells. All these suggest that development of gas hydrate in deep marine environment may be cost prohibitive. On the other hand, the well cost and drilling challenges will be significantly lower in the onshore permafrost environment where the resource density is also higher. Consequently, we see more potential for commercial development of permafrost hydrates than marine hydrates.