Abstract
Recent but limited studies have shown that multistep slow depressurization based on mixed CH4/CO2 hydrate dissociation can enhance CH4 recovery and increases CO2 storage after CO2 injection into CH4 hydrate [1,2]. For the first time, the resistivity variation and gas recovery and storage variation was investigated to study the change in hydrate saturation and production/storage yield. Lab-scale CH4 and CO2 rich mixed hydrates were synthesized to mimic the production and injection well scenario. The mixed hydrates were synthesized in sandstone with moderate to high water saturation using two different CH4/CO2 gas mixtures. Furthermore, mixed CH4/CO2 hydrates were dissociated three to six steps based on cyclic depressurization. Pressure, resistivity and gas chromatography data were collected. The presence of two thermodynamic stability zones provided an opportunity for additional CH4 recovery and CO2 storage during mixed hydrate dissociation. Gas and water migration between the injection and production well caused CO2 hydrate reformation, improvement in CO2 sweep area and movement of the CO2 hydrate front toward the production well. Multiple peaks in CH4 recovery and CO2 storage suggest major dissociation and reformation. Peak values were independent of mixed hydrate type. Peaks values of CH4 rich hydrates occurred at high pressure than peak values of CO2 rich hydrates. The slight change in resistance during depressurization below pure CH4 hydrate stability pressure confirms the loss of CH4 hydrate mass recovered by the formation of CO2 hydrate mass. This study discusses the correlation between the change in resistivity and type of guest molecule and its concentration and initial water saturation. The results of this study will be useful to explore the application of slow depressurization for the dissociation of CH4/CO2 mixed hydrates to improve CH4 recovery and CO2 storage.
Highlights
Gas hydrates are ice-like compounds of water and certain gases under high pressure and low temperature
To overcome the challenges associated with depressurization, hybrid techniques combined with other production techniques have been proposed, including hybrid with gas injection [17,18] or hybrid with thermal stimulation
We studied cyclic depressurization based on CH4/ CO2 hydrate dissociation in bulk and unconsolidated sand, with and without additives [2]
Summary
Gas hydrates are ice-like compounds of water and certain gases under high pressure and low temperature. On the other hand, mixed CH4/CO2 hydrate, when depressurized using stepwise dissociation, showed multiple dissociations and reformation events be tween CH4 and CO2 stability pressure This phenomenon was exciting as it led to free water consumption in creating CO2 hydrate, and the risk of water production was reduced [1]. This study assumes that replacement has already occurred and the CH4/CO2 mixed hydrate system has been formed Such systems will be present after injection of CO2 into CH4 hydrates as the distribution be tween injection and production well varies depending on CO2 sweep efficiency. The slow depressurization technique was chosen to exploit two different stability zones such that the CO2 gas phase forms CO2 hydrates while CH4 hydrate dissociated and released water support CO2 hydrate formation. Mass balance analysis based on gas chromatography was performed to calculate the recovery and stor age yield
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