Hydrate Swapping for CO₂ storage & CH₄ Production: Challenges & Innovations

Sedimentary gas hydrate deposits occurring in geological settings in cold regions such as permafrost and deep oceans are considered a clean gas source to meet future energy needs. A large amount of CH₄-rich gas stored in these reservoirs could also leak into the atmosphere due to melting caused by global warming. Therefore, there is an urgent need to extract CH4 gas from these reservoirs on a commercial scale and stabilize the hydrate reservoirs.

CO₂ injection into CH₄ hydrates addresses both of the above concerns with the added benefit of CO₂ storage in the hydrate reservoir. This technique is referred to as “hydrate swapping”. CO₂ gas also tends to form hydrates, similar to CH₄ hydrates. Thermodynamically, CO₂ hydrates are more stable than CH4 hydrates. Therefore, CO₂ injection into CH₄ hydrate would cause spontaneous conversion of CH₄-rich hydrate system to CO₂-rich hydrate system. This is expected to release CH₄ gas for production, store CO₂ in the hydrate, and improve the thermodynamic stability of hydrate-bearing sediments.

Although hydrate swapping is an environmentally friendly, carbon-neutral technique and poses less risk to geomechanical stability, the technique has not yet been applied commercially due to the low CH₄ production volume. This technique suffers from low CO₂ injectivity and low CO₂ sweep range caused by the mass transfer barrier at the gas-liquid interface. The mass transfer barrier reduces the volume of CO₂ that can be injected into the CH₄ hydrate, reduces the gas mobility in the pore space, and may even trap the produced CH₄ gas. Therefore, in this study, we attempted to improve CH₄ recovery and store additional CO₂.

In our first approach, we sought to understand the effect of the mass transfer barrier on CH_4- CO₂ hydrate exchange. To do this, we altered the pore water chemistry by adding chemicals (inhibitors and promoters) to the water in small doses. Paper 1 and Paper 2 focused on the effects of the additives at the core scale to investigate their effect on the extent of CH₄-CO₂ hydrate exchange. The results show that modified pore water chemistry can control the CO₂ mass transfer barrier and provide additional CH₄ recovery and CO₂ storage. We tested the effect of pore water chemistry for both CO₂-rich gas and diluted CO₂ gas.

In the second approach, we combined CH₄-CO₂ hydrate swapping with depressurization. Rapid pressure release (depressurization) was performed before CO₂ injection into the bulk phase (Paper 3), while slow pressure release (stepwise/cyclic depressurization) was performed after CO₂ injection into the porous medium/bulk phase (Paper 4 and Paper 5). In paper 3, two different CO₂ concentrations (pure and diluted) gas were injected into the depressurized CH₄ hydrate in the bulk phase to investigate the change in morphology and the extent of hydrate exchange (Paper 3). On the other hand, CH₄/CO₂ mixed hydrates were slowly depressurized by stepwise and cyclic depressurization (Paper4 and Paper 5). These studies were carried out using a high-pressure micromodel (Paper 4) and a high-pressure cell (Paper 5). It was found that several dissociations and reforming events were found to occur between CH₄ and CO₂ hydrate stability pressures during slow pressure release, leading to CO₂ hydrate reformation. We investigated the effects of hydrate saturation, residual water saturation, and reservoir temperature on recovery and storage yield with and without additives. In another study, we investigated the effect of depressurization on the dissociation of CH₄ hydrates at temperatures below 0 ℃ using high-pressure micromodels (Paper 6). This study showed that the depressurization technique is insufficient to produce CH₄ when the hydrate reservoir temperature is below 0℃.

We touch on two new approaches to improve CH₄/CO₂ hydrate exchange that were previously unexplored through this analysis. We hope that the work performed in this paper is relevant to the advancement of gas ydrate science and technology to recover CH₄ and store CO₂ in an environmentally friendly manner.