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

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Abstract

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 CH4- 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.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages198
Publication statusPublished - 2021

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