Recovery CH₄ Via Thermal Stimulation And CO₂/N₂ Replacement Of Methane Hydrate

The thermal stimulation method and the replacement method are traditional for nature gas hydrate exploitation.However,the conventional heat injection mostly injects hot water into the hydrate layer,so the injected heat is difficult to be quantified and the heat loss is large and the energy utilization ratio is low.The replacement method has the problem of weak methane recovery.Therefore,this paper proposes a method to improve the methane recovery of replacement by multiple heat injection,and explores the process of gas production from hydrate by heat injection method before hydrate exploitation by the combined method of heat injection and replacement.In this paper,heat was injected into the hydrate layer by the electric heating method to achieve higher precision in controlling of the heat injection,and we explored the law of hydrate decomposition and gas production with different power of heat injection at different pressure conditions.The results showed that the higher the heat injection power was,the faster the average gas production rate was.When the power was 100 W at 6 MPa,the gas production rate of methane was the fastest,which was 0.0135 mol/min.Reducing the pressure from 6 MPa to 3 MPa could increase the gas production rate at low heating power.The methane production rate was 0.0031 mol/min at 3 MPa when the heating power was 20 W,while the value was 0.0025 mol/min at 6 MPa.The energy utilization ratio was still low,the maximum energy utilization ratio was approximately 31%at 20 W and the maximum energy return on investment of each group is about 4.3.Pressure reduction before heat injection could improve the energy utilization ratio and the energy return on investment in the process of heat injection,and it plays a significant role at low power heat injection.By lowering the pressure during 20 W heat injection,the energy utilization ratio was increased from 31%to 46%,and the energy return on investment was increased from 4.3 to 6.6.After studying the heat injection process,this paper attempted to improve the methane recovery during the process in natural gas hydrate exploitation by CO₂-rich flue gas?53mol%CO₂+47mol%N₂?replacement process by using electric heating method to pulse heating,and studied the influence of replacement temperature and pressure on methane recovery.The results showed that the higher the replacement temperature and the lower the pressure were,the higher the methane recovery was.The methane recovery could be increased from 40%to55%by pulse heating at 6?and 8 MPa.Further research showed there was no significant effect on the final methane recovery with the initial pulse heating time and pulse heating interval,because they do not effect the decomposition region of hydrate in each heating process,while shortening the duration of each pulse heating or reducing the power of pulse heating can not improve the methane recovery in the replacement process.Finally,this paper analyzed the mechanism of pulse heat injection promoting gas hydrate exploitation by rich CO₂ flue gas replacement.We think that pulse heat injection can destroy the hydrate layer to a certain extent,make the previously relatively tight hydrate layer loose,and increase the contact area between CO₂/N₂ molecules and natural gas hydrate layer.The gas generated by the thermal decomposition of hydrate will diffuse outward,which will also create some small channels in the hydrate layer.It will promote the mass transfer process and increase the replacement area,thus improving the methane recovery in the process of replacement exploitation of natural gas hydrate.