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

Abstract

Gas hydrate is an ice-like form of water containing gas, in nature mostly methane (CH₄), which requires moderate pressures and low temperatures. The replacement of CH₄ by CO₂, which also forms a hydrate, could allow CH₄ production from hydrate while sequestering CO₂. A number of recent studies have focused on theoretical background, experimental simulations, and engineering approaches related to CH₄-CO₂ exchange in hydrates. We here investigate a key geologic constraint for possible CH₄-CO₂ 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 CH₄ hydrate and hydrate from the resulting gas mix after CO₂ injection. Based on experimental and theoretical studies, we consider 64% a benchmark for maximum achievable CH₄ replacement by CO₂, resulting in a mix of 64% CO₂ – 36% CH₄, in hydrate. Down to a water depth of 1087 m, hydrate from this gas mix is stable within the entire CH₄ hydrate stability field. A gap develops in deeper water with the base of gas hydrate stability (BGHS) for CH₄ being deeper than for the 64% CO₂ – 36% CH₄ mix. In nature, most mechanisms for CH₄ hydrate formation favor high saturation near the BGHS. For an evaluation of possible CH₄-CO₂ exchange, it is therefore important to investigate mixed-gas hydrate stability near the CH₄-BGHS and to identify CH₄ hydrates closer to the seafloor.