Multiscale investigation of CH₄/CO₂ hydrates in sediment: CH₄ recovery and CO₂ storage

Natural gas hydrates are naturally occurring in sediments of permafrost regions and continental margins. Recovery of natural gas (main CH₄) bound in hydrate deposits with large quantities of reserve can meet the increasing global energy demand. CH₄-CO₂ hydrate swapping is an attractive exploitation technique that can recover CH₄ gas and store CO₂ hydrate. The CH4 trapped in the hydrate is to be replaced by CO₂. The newly forming CO₂ hydrate can maintain the stability of hydrate-bearing sediments. However, this technique suffers from low efficiencies of CH₄ gas recovery and CO₂ hydrate storage, due to low CO₂ injectivity caused by limited mass transfer within sediments. To improve mass transfer and increase CO₂ sweep area, slow depressurization was conducted after CH4-CO₂ hydrate swapping. This process of exploitation was investigated in multiscale. The purpose of this combined strategy was to trigger the wanted CH₄-rich hydrate dissociation and CO₂-rich hydrate formation, thus promoting CH₄ gas recovery and CO₂ hydrate storage.

In the first part of microfluidics, the morphological characteristics of CH₄ hydrate and CO₂ hydrates were investigated to provide microfluidic insights into CH₄ hydrate and CO₂ hydrate dynamics in confined space. Paper 1 explored the effects of wettability and gas/water saturation on CH₄ hydrate formation/dissociation in microfluidic chips with hydrophilic or hydrophobic surfaces. The results showed more favorable water diffusion dominated in continuous gas flows of gas-rich hydrophilic pores, resulting in more CH4 hydrate formation. The moderate CH₄ hydrate stability in hydrophilicity benefits CH₄ gas recovery from hydrate.For CO₂ hydrate formation in microfluidics, the results of morphological observations and Raman spectra confirmed CO₂ storage in states of hydrate and liquid. The sealing effect of CO₂ hydrates was detected by pressure differences among micropores. The storage capacity of CO₂ hydrates in micropores indicated that hydrate-based CO₂ storage can be a supplementary option in comparison with the system containing liquid CO₂.

In the second part of multistep depressurization in consolidated/unconsolidated sediments, the characteristics of CH₄/CO₂ hydrate during depressurization and its mechanism for improved exploitation performances were investigated. In the artificial sandstone, Paper 2 studied the visualization of CH₄/CO₂ hydrates by X-ray computed tomography during multistep depressurization for hydrate morphology evolution and fluid migration. The timedependent hydrate morphologies and corresponding pressure variations indicated concurrent mixed hydrate dissociation/reformation. The results showed artificial core with appropriate moderate particle size had sufficient pore connectivity for mass transfer to benefit hydrate exploitation. The key point for efficient exploitation of CH₄/CO₂ hydrates was reducing reservoir pressures in multistep to the value between CH₄ hydrate and CO₂ hydrate stability pressures. In the natural sandstone of consolidated sediment, Paper 3 probed the same strategy of multistep depressurization. The effects of hydrate composition, water saturation and shut-in period were examined and controlled to achieve high efficiency of CH₄ gas recovery and CO₂ hydrate storage, with the indicator of resistivity variation. The optimized values were obtained at ceasing pressure close to CH₄/CO₂ mixed hydrate stability pressure without water production. A positive increase in resistivity variation indicated increased hydrate saturation or improved gas/water distribution in sandstone. In the sandpack of unconsolidated sediments, Paper 4 repeated multistep depressurization to study the dynamic characteristics of CH₄/CO₂ hydrates. The effects of reservoir properties such as CH₄/CO₂ ratio in hydrate, water saturation and reservoir temperature on hydrate exploitation were systematically studied. The results showed that CH₄-rich hydrates in unconsolidated sediments were recommended for exploitation by multistep depressurization. Lower residual water saturation was beneficial to CH₄ gas recovery, while higher value was conducive to CO₂ hydrate storage. Temperature below/above freezing point determined ice formation affecting hydrate exploitation in mass transfer. It was summarized that gas diffusion in pores, contact of CO₂ gas with CH₄ hydrates, and CO₂ gas transforming into hydrate with water were better improved in unconsolidated sediments for more efficient exploitation compared with consolidated sediments.

In the third part of additives coupled with multistep depressurization, the presence of N₂/Air in dilute CO₂ gas and injection of amino acid were investigated. Manuscript 1 employed multistep depressurization to dissociate the mixed hydrates formed after hydrate swapping in bulk-water and coarse sand. The effects of N₂/Air on exploitation were investigated by examining hydrate morphologies and gas compositions. The roles of N₂/Air were confirmed as inhibitors destabilizing CH₄ hydrate and additional exchange guests swapping CH₄ in addition to CO₂. Manuscript 2 tested amino acid injection for CH₄/CO₂ mixed hydrate formation followed by slow depressurization in fine sand. Amino acids slowed CO2 hydrate dissociation and enhanced CO₂-rich hydrate stability for CO2 storage, and Raman spectra and GC results confirmed CO₂-rich hydrate formation. This coupled technique was proved efficient in regaining reservoir pressures and kinetically promoting CO₂ hydrate formation. Based on these multiscale investigations and results of hydrate exploitation performances, a protocol of combination methods including direct depressurization, dilute CO₂ gas injection for hydrate swapping, multistep depressurization, amino acid injection and slow constant-rate depressurization was proposed for efficient CH4 gas recovery and CO2 hydrate storage.