Direct Visualization of CH4/CO2 Mixed Hydrate Phase Transitions in Porous Media

J. S. Pandey, Ø. Strand, N. von Solms, G. Ersland, S. Almenningen

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Abstract

In this study, we investigate the formation and dissociation pattern of CH4/CO2 mixed hydrate in porous media. Phase transitions of CH4/CO2 mixed hydrate in sediments will continuously take place during recovery of CH4 gas by CO2 injection. The recovery efficiency of CH4 gas depends on the amount of liquid water present in the pore space and the recovery efficiency can be aided by pressure depletion. In this scenario, it is important to evaluate the composition of the hydrate that dissociates and possibly reforms during pressure reduction in the presence of both CO2 and CH4. We formed CH4/CO2 mixed hydrate from gaseous CH4 and liquid/gaseous CO2 to mimic the scenario where a CH4 hydrate reservoir has been injected with CO2. Direct visualization was carried out using a high-pressure, water-wet, silicon-wafer based micromodel with a pore network of actual sandstone rock. Mixed hydrate was formed at reservoir conditions (P = 44-75 bar and T = 1.7-3.6°C) from either a two phase system (liquid water and CH4/CO2 gas mixture) or a three phase system (liquid water, CH4 gas, and liquid CO2). A stepwise pressure reduction method was later applied to record multiple dissociation pressure points for a given mixed hydrate system, and the molar concentration of CH4/CO2 corresponding to each dissociation point was calculated. The effect of hydrate and fluid saturation on fluid flow during dissociation was also analyzed. The results showed that liberated gas during stepwise pressure reduction was trapped by surrounding hydrate, and reformation of CO2 hydrate occurred rapidly when liquid water was present. The reformed CO2 hydrate shielded the CH4 hydrate that was still not dissociated and complete dissociation was accomplished when the pressure was brought below the stability pressure of pure CO2 hydrate.
Original languageEnglish
Publication date2020
Number of pages1
Publication statusPublished - 2020
EventGordon Research Conference on Natural Gas Hydrate Systems - Hotel Galvez , Galveston, United States
Duration: 23 Feb 202028 Feb 2020

Conference

ConferenceGordon Research Conference on Natural Gas Hydrate Systems
LocationHotel Galvez
Country/TerritoryUnited States
CityGalveston
Period23/02/202028/02/2020

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