Multiscale investigation of CH 4 /CO 2 hydrates in sediment: CH 4 recovery and CO 2 storage

• Main Author: Ouyang, Qian
• Format: Book
• Language: English
• Published: Technical University of Denmark 2023
• Subjects: permafrost
• Online Access: https://orbit.dtu.dk/en/publications/b9394b21-bde6-4e2a-be5d-00e190701a4d; https://backend.orbit.dtu.dk/ws/files/360806403/PhD_thesis_-_Qian_Ouyang.pdf

Natural gas hydrates are naturally occurring in sediments of permafrost regions and continental margins. Recovery of natural gas (main CH 4 ) bound in hydrate deposits with large quantities of reserve can meet the increasing global energy demand. CH 4 -CO 2 hydrate swapping is an attractive exploitation technique that can recover CH 4 gas and store CO 2 hydrate. The CH4 trapped in the hydrate is to be replaced by CO 2 . The newly forming CO 2 hydrate can maintain the stability of hydrate-bearing sediments. However, this technique suffers from low efficiencies of CH 4 gas recovery and CO 2 hydrate storage, due to low CO 2 injectivity caused by limited mass transfer within sediments. To improve mass transfer and increase CO 2 sweep area, slow depressurization was conducted after CH4-CO 2 hydrate swapping. This process of exploitation was investigated in multiscale. The purpose of this combined strategy was to trigger the wanted CH 4 -rich hydrate dissociation and CO 2 -rich hydrate formation, thus promoting CH 4 gas recovery and CO 2 hydrate storage. In the first part of microfluidics, the morphological characteristics of CH 4 hydrate and CO 2 hydrates were investigated to provide microfluidic insights into CH 4 hydrate and CO 2 hydrate dynamics in confined space. Paper 1 explored the effects of wettability and gas/water saturation on CH 4 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 CH 4 hydrate stability in hydrophilicity benefits CH 4 gas recovery from hydrate.For CO 2 hydrate formation in microfluidics, the results of morphological observations and Raman spectra confirmed CO 2 storage in states of hydrate and liquid. The sealing effect of CO 2 hydrates was detected by pressure differences among micropores. The storage capacity of CO 2 hydrates in micropores indicated that hydrate-based CO 2 ...