Natural gas hydrates attract worldwide attention for extensive reserve. Current exploitation methods of thermal stimulation, depressurization, and inhibitor injection consume much energy, inducing formation instability or marine biological problems. In this work, a novel in situ intermittent heating-assisted CO 2 replacement (IIAR) method was proposed to enhance methane production efficiency of hydrates. Experiments investigated recovery performance with an electrical rod simulating the in situ heating, tested at 275.15−279.15 K and 4.00−13.06 MPa. The results indicated that the presence of N 2 and liquid CO 2 was conducive to CH 4 hydrate exploitation. Rising temperature and decreasing pressure assisted IIAR dissociate partial hydrates and promote further CH 4 −CO 2 /N 2 replacement. The maximum CH 4 recovery percentage was 52.42% obtained at 279.15 K and 8.01 MPa in 48 h. The exploitation time could be shortened by 40−60% using IIAR compared with pure CO 2 and CO 2 /N 2 replacement. The improved recovery performance indicated that IIAR was more competitive than pure CO 2 and CO 2 /N 2 replacement and CO 2 replacement combined with thermal stimulation. Further reduction of exploitation time and improvement of methane production efficiency demonstrated that intensive heating modes of IIAR increased exploitation economy and reduced methane consumption.
Most of the natural gas hydrates on Earth are buried in shallow formations under deep water. Comprehensively understanding the reaction kinetic characteristics of gas hydrate in porous media is very beneficial to the deep exploration of the hydrate accumulation in nature. In this paper, the formation process of CH4 hydrate in porous media was simulated physically, using a reactor that is operating at high pressure and low temperature. The hydrate phase equilibrium and reaction kinetic characteristics at different temperatures, pressures, sand grain sizes, and clay contents were assessed. Based on the determination of relevant hydrate kinetic parameters, a novel mixing-flux hydrate reaction model was proposed, which can be used for numerical simulation of gas hydrate accumulation. The experimental results show that the porous media can make the phase equilibrium of CH4 hydrate shift to the right under the capillary effects on the gas and hydrate phases. Low temperature and high pressure can provide a large driving force for hydrate formation, but large clay content and small sand grain size usually give a negative effect on the CH4 transfer in the porous media. It often leads to a slow hydrate formation rate and hard distinction of pressure drop between hydrate nucleation and growth stages. Based on the experimental results, the hydrate nucleation kinetic parameters were regressed, and the activation energy (E a), as well as the reaction frequency factor (k fo), of hydrate growth were fitted to be 75.45–90.85 kJ/mol and 8.72 × 108–6.02 × 1011 mol/(m2 kPa day), respectively. In the numerical simulation of hydrate accumulation, the hydrate formation process can be described by coupling the low-flux reaction and the high-flux reaction, which consume the CH4 dissolved in water and the free CH4 gas in pores, respectively. This novel mixing-flux hydrate formation model is suitable for the flexible and practical hydrate accumulation simulation, which can consider various gas sources and transfer states in the hydrate reservoir.
CH 4 /CO 2 mixed hydrate forms upon CO 2 gas injection into the CH 4 gas hydrate reservoir. An improved understanding of the dissociation behavior of the CH 4 /CO 2 hydrate system is necessary to increase the yield of CH 4 production and CO 2 storage. In this study, CH 4 /CO 2 mixed hydrates (in bulk and unconsolidated coarse sand) were dissociated using the multistep cyclic depressurization (MCD) method. Visual and kinetic data were collected using a high-pressure reactor and gas chromatography (GC) setup to study the change in morphology and mole fraction of CH 4 and CO 2 in the released gas. The influence of chemicals (methionine, sodium dodecyl sulfate, and methanol) in the aqueous phase and reservoir temperature (below and above 0 °C) on recovery and storage yield was also investigated. This study reported additional CH 4 recovery below the CH 4 hydrate stability pressure when cyclic depressurization was implemented between CH 4 and CO 2 hydrate stability pressures. A rapid increase in CH 4 mole fraction and a decrease in CO 2 mole fraction were observed due to simultaneous CH 4 hydrate dissociation and CO 2 hydrate reformation. This phenomenon was accelerated at high liquid saturation. CH 4 recovery potential was positively correlated with hydrate saturation and for T > 0 °C conditions. Morphology study showed the expansion of hydrate volume during cyclic depressurization, which confirmed hydrate reformation from released water from dissociation. The chemicals affected the mixed CH 4 /CO 2 hydrate synthesis, reformation kinetics, and subsequent CO 2 storage. This study demonstrates a novel application of cyclic depressurization to enhance CH 4 production and improve CO 2 storage. A new hydrate production method is also proposed that includes constantrate depressurization, kinetic inhibitor-based CO 2 injection, and cyclic depressurization.
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