| Natural gas hydrate is a non-stoichiometric solid cage crystalline substance formed by water and small molecular gases（such as H2,CH4,H2S,etc.）under low temperature and high pressure.As a kind of clean fuel with huge reserves,the exploitation of natural gas hydrate has become one of the research hotspots in the world.Among many exploitation methods,depressurization combined with stimulation is considered as one of the most promising methods.As a unique bulk heating method,microwave could heat gas hydrate quickly and efficiently.Besides thermal effect,there may also be fracture-forming and non-thermal effect.Using microwave stimulation to decompose natural gas hydrate would provide the thermal dynamics required for hydrate decomposition,which would be conducive to inhibit the secondary hydrate or promote the decomposition of secondary hydrate.It will guarantee flow channel of gas and water.In this paper,natural quartz sand with main particle size of 106-150μm was used as porous medium to simulate ClassⅠnatural gas hydrate reservoirs.Differential scanning calorimeter（DSC）was used to measure the phase equilibrium conditions of the methane hydrate with three sand-water ratios of 2.5,6,and 8,as well as pure methane without sand.The initial pressure was from 6 MPa to 10 MPa.The points about phase equilibrium temperature and pressure of each experimental group were distributed.They were near the phase equilibrium curve,and increased with the quartz sand content,which were closer to the phase equilibrium curve.When the initial water content was supersaturated（sand-water ratios was 2.5）,the average water conversion rate was 29.0%.When the initial water content was unsaturated,and the average water conversion rate was 86.7%when the sand-water ratios was 6,and 88.6%when it was8.The sample of methane hydrate were synthesized under the condition of sand-water ratio of 6 or 2.5,temperature of-10℃and the pressure of 8 MPa.XRD and Raman under atmospheric pressure and low temperature characterization were conducted.The results showed that the synthesized methane hydrate was S-Ⅰhydrate.In order to obtain more hydrate structures,Ar hydrate samples were synthesized in a diamond anvil cell（DAC）at 0.25～1.48 GPa.Raman characterizations showed the phase transitions occurred at 0.43 GPa,0.66 GPa and 1.05 GPa.The corresponding hydrate structures were S-Ⅱhydrate,S-H hydrate,T-hydrate,and filled ice.The decomposition characteristics of methane hydrate by microwave stimulation were studied under continuous and intermittent microwave heating.Under continuous microwave stimulation method,the decomposition rate of methane hydrate was higher at the initial stage,and the decomposition heat mainly came from microwave volume heating.With the hydrate decomposed,the free water content increased,the microwave penetration depth of the reservoir decreased.The macroscopic heat transfer became the main mechanism of hydrate decomposition in the remote microwave source regions.With the increment of microwave power,the average gas production rate increased from 0.303 L/min to 0.882 L/min.The decomposition time of hydrate decreased with microwave power increased.However,there was a wireless relationship between energy efficiency ratio and microwave power.The maximum energy efficiency ratio was 1.320 at 400 W.Intermittent microwave heating could improve the energy efficiency ratio effectively.When the intermittent heating cycle was 5+1 min,the energy efficiency ratio was 2.103,which was 59.32%higher than that of the continuous microwave heating.Depressurization combined with stimulation is one of the best exploitation methods of natural gas hydrate.When the rapid depressurization method was used,the hydrate decomposition rate increased with the decreased of gas production pressure.When the gas production pressure was 1.5 MPa,the hydrate rapidly decomposed at the initial stage and produced a large amount of water.Due to the serious“Jamin effect”,even though the hydrate decomposition rate was fast,the gas production time would be longer than that of 1.9 MPa because the gas flow was blocked.The piecewise depressurization method can control the decomposition rate of hydrate,which was better for the sensible heat consumption of reservoir.So,it was beneficial to heat storage and reduce the rapid drop of temperature.When microwave stimulation combined with depressurization method,microwave provided extra driving force to increase the temperature of the reservoir,which would accelerate hydrate decomposition and reduce gas production time.The decomposition time of hydrate increased with gas production pressure decreased.The higher of the residual hydrate content,the better of the microwave effect.Considering the delicate balance between average gas production rate and energy efficiency ratio,the optimal gas production pressure of depressurization combined with microwave stimulation was determined to be 1.9 MPa.In order to further enhance the effects of microwave for hydrate decomposition,iron（Fe）powder was added to the reservoir,combined with depressurization or both two methods were used.It would promote the hydrate decomposition by microwave heating.As a strong microwave absorber,iron powder was activated and heated by microwave,resulting in hysteresis loss,which increased its own temperature and transferred heat to the hydrate.The decomposition effect of adding Fe powder group was the best,and the strengthening effect increased by 60.6%.The average gas production rate and the energy efficiency ratio of the depressurization group in the early stage of the decomposition were higher than those of the non-depressurization group,while those of the depressurization group in the middle and late stage were lower than those of the non-depressurization group.This indicated that in gas hydrate decomposition process by depressurization method,microwave stimulation in the early stage of the exploitation could accelerate the decomposition of hydrate.And the decomposition in the middle and late stage could further reduce the energy consumption of the exploitation.Furthermore,the experiments of microwave hydrate decomposition enhanced by different microwave absorbent were studied.C（conductive graphite）,Ti O2 and Fe3O4with powder particle size of 1μm were added into the reservoir as microwave absorbents.The experimental results showed that the average gas production rate and the energy efficiency ratio of C,Ti O2and Fe3O4 groups were increased by 48.5%,24.3%and 93.5%compared with the blank control group,respectively.According to the characterization of powder,Fe3O4 has high dielectric constant and permeability,it is a good wave-absorbing body.Moreover,its loose and porous mass structure is conducive to microwave reflection inside it for many times,so it had the best effect for enhancing microwave decomposition experiment of hydrate.Further,three different particle sizes of Fe3O4＿20nm,Fe3O4＿1μm,Fe3O4＿7μm powder were selected.The results showed that the powder with smaller particle size had higher dielectric constant and permeability due to its larger specific surface area,and the strengthening effect was better.However,too small powder agglomeration due to uneven dispersion would affect the final strengthening effect.The strengthening effect of Fe3O4＿1μm powder added with 0.1 wt.%was the best,which increased by 93.5%compared with blank control group.This paper carried out a system experimental study on the characteristics and laws of gas hydrate decomposition under microwave stimulation.The characteristics and laws of gas hydrate decomposition under microwave stimulation were revealed.The research results could provide theoretical basis and guidance for the process of gas hydrate exploitation in the field. |
