PNNL

Abstract

Under high pressure and low temperature conditions small nonpolar molecules (typically gases) can combine with water to form crystalline structures known as clathrate hydrates. Methane (CH4) and carbon dioxide (CO2) form nearly identical clathrate structures (sI), with the CO2 hydrate being thermodynamically favored. Vast accumulations of methane hydrates have been found in suboceanic deposits and beneath the arctic permafrost. Because of the large volumetric storage densities, clathrate hydrates on the deep ocean floor have been suggested as a sequestration option for CO2. Alternatively, CO2 hydrates can be formed in the geologic settings of naturally occurring accumulations of methane hydrates. Global assessments of natural gas resources have shown that gas hydrate resources exceed those of conventional resources, which is indicative of the potential for clathrate hydrate sequestration of CO2. Recovery of natural gas from hydrate-bearing geologic deposits has the potential for being economically viable, but there remain significant technical challenges in converting these natural accumulations into a useable resource. Currently, conventional methods for producing methane hydrates from geologic settings include depressurization, thermal stimulation, and inhibitor injection. Although CO2 clathrates generally are not naturally as abundant as those of CH4, their occurrence forms the foundation of an unconventional approach for producing natural gas hydrates that involves the exchange of CO2 with CH4 in the hydrate structure. This unconventional concept has several distinct benefits over the conventional methods: 1) the heat of formation of CO2 hydrate is greater than the heat of dissociation of CH4 hydrate, providing a low-grade heat source to support additional methane hydrate dissociation, 2) exchanging CO2 with CH4 will maintain the mechanical stability of the geologic formation, and 3) the process is environmentally friendly, providing a sequestration mechanism for the injected CO2. Whereas the exchange production technology would not be feasible without the favorable thermodynamics of CO2 hydrates over CH4 hydrates, this situation yields technical challenges for the technology in avoiding secondary hydrate formation and clogging of the geologic repository. Laboratory-scale experiments have demonstrated the feasibility of producing natural gas and sequestering CO2 using the direct exchange technology in geologic media. These experiments have duplicated numerically using the STOMP-HYD simulator, which solves the nonisothermal multifluid flow and transport equations for mixed hydrate systems in geologic media. This paper describes the design (via numerical simulation) of a pilot-scale demonstration test of the CO2 exchange production and sequestration technology for a geologic setting beneath the arctic permafrost, involving a gas-hydrate interval overlying a free-gas interval (i.e., Class 1 Hydrate Accumulation).