Gas hydrate stability for CO2-methane gas mixes in the Pegasus Basin, New Zealand: A geological control for potential gas production using methane-CO2 exchange in hydrates.

Gas hydrate is an ice-like form of water containing gas, in nature mostly methane (CH 4), which requires moderate pressures and low temperatures. The replacement of CH 4 by CO 2 , which also forms a hydrate, could allow CH 4 production from hydrate while sequestering CO 2. A number of recent studies have focused on theoretical background, experimental simulations, and engineering approaches related to CH 4 -CO 2 exchange in hydrates. We here investigate a key geologic constraint for possible CH 4 -CO 2 exchange in sub-seafloor reservoirs, hydrate stability. We analyze seismic data and gas hydrate system models from the Pegasus Basin east of New Zealand, a region with evidence for abundant gas hydrates. Pressure-temperature conditions beneath the seafloor need to be within the stability fields for both CH 4 hydrate and hydrate from the resulting gas mix after CO 2 injection. Based on experimental and theoretical studies, we consider 64% a benchmark for maximum achievable CH 4 replacement by CO 2 , resulting in a mix of 64% CO 2 – 36% CH 4 , in hydrate. Down to a water depth of 1087 m, hydrate from this gas mix is stable within the entire CH 4 hydrate stability field. A gap develops in deeper water with the base of gas hydrate stability (BGHS) for CH 4 being deeper than for the 64% CO 2 – 36% CH 4 mix. In nature, most mechanisms for CH 4 hydrate formation favor high saturation near the BGHS. For an evaluation of possible CH 4 -CO 2 exchange, it is therefore important to investigate mixed-gas hydrate stability near the CH 4 -BGHS and to identify CH 4 hydrates closer to the seafloor. • CH 4 (methane)-CO 2 exchange may allow carbon-neutral production of CH 4 from hydrate. • Stability for hydrates from CO 2 and CH 4 gas mixes beneath the seafloor modeled. • Assumed 64% CO 2 - 36% CH 4 mix as final CO 2 /CH 4 ratio after CH 4 replacement. • For water depth <1087 m, hydrate from this gas mix is more stable than CH 4 hydrate. • In deeper water, CH 4 hydrate partly below hydrate stability field for this gas mix. [ABSTRACT FROM AUTHOR]