Hydrate Stability and Methane Recovery from Gas Hydrate through CH 4 –CO 2 Replacement in Different Mass Transfer Scenarios

CH 4 –CO 2 replacement is a carbon-negative, safer gas production technique to produce methane gas from natural gas hydrate reservoirs by injecting pure CO 2 or other gas mixtures containing CO 2 . Laboratory-scale experiments show that this technique produces low methane volume and has a slow replacement rate due to the mass transfer barrier created due to impermeable CO 2 hydrate layer formation, thus making the process commercially unattractive. This mass-transfer barrier can be reduced through pressure reduction techniques and chemical techniques; however, very few studies have focused on depressurization-assisted and chemical-assisted CH 4 –CO 2 replacement to lower mass-transfer barriers and there are many unknowns. In this work, we qualitatively and quantitatively investigated the effect of the pressure reduction and presence of a hydrate promoter on mixed hydrate stability, CH 4 recovery, and risk of water production during CH 4 –CO 2 exchange. Exchange experiments were carried out using the 500 ppm sodium dodecyl sulfate (SDS) solution inside a high-pressure stirred reactor. Our results indicated that mixed hydrate stability and methane recovery depends on the degree of pressure reduction, type, and composition of injected gas. Final selection between CO 2 and CO 2 + N 2 gas depends on the tradeoff between mixed hydrate stability pressure and methane recovery. Hydrate morphology studies suggest that production of water during the CH 4 –CO 2 exchange is a stochastic phenomenon that is dependent on many parameters.