The Iġnik Sikumi Field Experiment, Alaska North Slope: Design, operations, and implications for CO₂−CH₄ exchange in gas hydrate reservoirs

The Iġnik Sikumi Gas Hydrate Exchange Field Experiment was conducted by ConocoPhillips in partnership with the U.S. Department of Energy, the Japan Oil, Gas and Metals National Corporation, and the U.S. Geological Survey within the Prudhoe Bay Unit on the Alaska North Slope during 2011 and 2012. The primary goals of the program were to (1) determine the feasibility of gas injection into hydrate-bearing sand reservoirs and (2) observe reservoir response upon subsequent flowback in order to assess the potential for CO₂ exchange for CH₄ in naturally occurring gas hydrate reservoirs. Initial modeling determined that no feasible means of injection of pure CO₂ was likely, given the presence of free water in the reservoir. Laboratory and numerical modeling studies indicated that the injection of a mixture of CO₂ and N₂ offered the best potential for gas injection and exchange. The test featured the following primary operational phases: (1) injection of a gaseous phase mixture of CO₂, N₂, and chemical tracers; (2) flowback conducted at downhole pressures above the stability threshold for native CH₄ hydrate; and (3) an extended (30-days) flowback at pressures near, and then below, the stability threshold of native CH₄ hydrate. The test findings indicate that the formation of a range of mixed-gas hydrates resulted in a net exchange of CO₂ for CH₄ in the reservoir, although the complexity of the subsurface environment renders the nature, extent, and efficiency of the exchange reaction uncertain. The next steps in the evaluation of exchange technology should feature multiple well applications; however, such field test programs will require extensive preparatory experimental and numerical modeling studies and will likely be a secondary priority to further field testing of production through depressurization. Additional insights gained from the field program include the following: (1) gas hydrate destabilization is self-limiting, dispelling any notion of the potential for uncontrolled destabilization; (2) gas hydrate test wells must be carefully designed to enable rapid remediation of wellbore blockages that will occur during any cessation in operations; (3) sand production during hydrate production likely can be managed through standard engineering controls; and (4) reservoir heat exchange during depressurization was more favorable than expected—mitigating concerns for near-wellbore freezing and enabling consideration of more aggressive pressure reduction.