Experimental Studies On Influence Factors And Production Strategies Of Depressurization In Sandy Sediments

Methane hydrate(MH)is a non-stoichiometric clathrate and combination of methane gas and water that form under specific conditions of low temperature and high pressure.It has the characteristics of huge resource reserves,high energy density,and environmental friendliness.Thus,the MH is regarded as one of the most potential unconventional alternative energy sources excepting coal bed methane(CBM),shale gas,and tight gas.To produce gas and recover energy from MH safely and efficiently has been the key focus of the industry,academia,and international research organizations.A few MH dissociation methods including thermal stimulation,depressurization,injection of inhibitors,and CO₂-CH₄ exchange,and other methods have been proposed in the past decade to dissociate MH in geological media.Among them,the depressurization method is an efficient and low energy consumption recovery method in the scientific field and has been widely studied,and this method has a good application prospect.However,there are problems such as presentation of low temperature/freezing point,secondary methane hydrate formation,large-scale water production in the initial MH dissociation stage,low energy recovery efficiency,and deformation during the depressurization dissociation process.Therefore,it is of great significance to study the characteristics of fluid flow,heat transfer,dissociation kinetic behavior,and energy efficiency analysis during the MH depressurization process.This thesis conducts some experiments in the following two aspects,firstly,to methane hydrates with different influence factors(methane hydrate phase saturation and sodium chloride system),the same depressurization mode was used to analyze mass and heat transfer characteristics,triaxial stress change evolution,and the influence of sodium chloride during the depressurization process.Afterward,to methane hydrates with the same reservoir characteristics,two depressurization modes were used in our studies,including the extent of depressurization(various bottom hole pressures,BHPs)and depressurization path(multi-stage depressurization).Furthermore,the mass and heat transfer characteristics,dissociation kinetics,and energy efficiency analysis were analyzed during the depressurization process;These experiments and main conclusions are as follows:(1)A novel tri-axial horizon fixed bed reactor was employed to synthesize MH in quartz sands.The objectives were to identify the effect of several key factors(i.e.water-gas ratio,pressure,temperature,and the presence of NaCl)on the MH formation behaviour(e.g.X_CH4and S_H)and to shed light on the fundamental mechanisms on the formation process of naturally-occurring hydrates in geological media under typical reservoir conditions.The final S_H largely depends on the initial amount of gas present in the system.An initial water-gas ratio larger than the hydration number of methane hydrate(N_H=5.75)increases X_CH4 under the same P and T conditions.A higher P results in a smaller induction time and a faster MH formation rate.P has a positive effect on the S_H and X_CH4 during the 1^st stage of MH formation,while a minimal effect on the final S_H.Increasing T increases the induction time and has a minimum effect on the MH formation rate and the final S_H,which is likely to be attributed to the small range of T investigated in this study.The presence of NaCl(X_NaCl=3.0 wt%)increases the induction time and serves as both a thermodynamic and kinetic inhibitor for MH formation in sandy media,but it does not pose a significant impact on the final S_H and X_CH4.The estimated X_NaCl has an increasing trend during all the MH formation stages and reaches a maximum X_NaCl=5.29%at the end of the 1^st stage.(2)The in-situ depressurization approach under tri-axial conditions was more suitable to the real natural MHBS environment.we examine the effect of NaCl(X_NaCl=3.0 wt%)on gas and water production,dissociation kinetics,and evolution of tri-axial stress during the MH depressurization process.The following conclusions can be drawn from this study.It is noted that the presence of NaCl posed a slightly positive impact on methane recovery.However,the presence of NaCl had a slightly negative effect on the water production.Under the same BHP,the water recovery from MH samples formed under X_NaCl=3.0 wt%is slightly lower than that of the deionized water system.A smaller BHP can increase water production and have a greater deviation value of R_W(?R_W=7.32%at BHP=1.0 MPa)between deionized water and NaCl systems.The evolution of BHP presents a distinct four-stage pattern during the MH in-situ depressurization process,including a sharp stage,a fluctuating stage,a step-wise stage to a stable stage at last.A decreasing BHP increased the gas recovery in the gas reservoir regardless of whether there is NaCl in our experimental system.Most of the gas production was recovered during the fluctuating stage and step-wise stage.In addition,the evolution of confining pressure presents a similar trend to the pressure of the reactor under the MH formation and dissociation processes while the profile of axial pressure shows a step-wise decreasing trend.(3)To methane hydrates with different hydrate phase saturation reservoir characteristics,two depressurization paths(single-stage and three-stage)experiments were carried out.We examine the effect of different hydrate phase saturation(S_H=20%,S_H=40%,S_H=60%,and S_H=80%)on the fluid production,dissociation kinetics,and heat transfer characteristics during the depressurization process.The super-high hydrate phase saturation(S_H=80%)methane hydrate samples were firstly synthesized by the excess-water method,which broadened the methane hydrate samples with different hydrate phase saturations in the lab-scale study.This study found that these different depressurization paths have little effect on the final methane gas recovery ratio.However,a higher methane hydrate saturation led to a greater methane recovery ratio under the same depressurization path and depressurization amplitude.Importantly,two different depressurization paths have little effect on the final water recovery ratio.In addition,the water recovery ratio of methane hydrate with different saturations is maintained at a similar level.Compared with the single-stage depressurization mode,the three-stage depressurization mode increases the minimum temperature during the MH depressurization process.The lower the methane phase saturation,the more obvious the effect.The minimum temperature increased to 4.19?when the hydrate saturation was 20%,and the minimum temperature increased to 1.60?when the hydrate saturation was 80%.(4)This study Synthesized water-saturated MH with consistent high S_H>55%.The recovery ratio of fluid production,dissociation dynamics,and heat transfer were analyzed during the depressurization process under different depressurization amplitudes(BHP=1.5MPa,3.0 MPa,and 4.5 MPa).The study found that gas production always lags behind water production,and water production always ends before gas production regardless of the depressurization amplitudes.In the depressurization(DP)stage,the water recovery ratio is about 90%,and the recovery of methane gas mostly occurs in the constant pressure(CP)stage.Lower bottom hole pressure results in a faster methane production speed and higher methane recovery and water-gas recovery ratios.When the bottom hole pressure,closing to the phase equilibrium pressure(BHP=4.5 MPa),was used to the MH dissociation,the methane recovery ratio and recovery speed decrease significantly,and the final methane recovery ratio drops around 62%.In addition,the greater bottom hole pressure greatly increases the minimum temperature during the MH depressurization process.When the bottom hole pressure is 1.5MPa,the minimum temperature is-0.30?during the MH depressurization process,and the minimum temperature is 5.33?when the bottom hole pressure is 4.5 MPa.(5)Synthesizing water-saturated MH with consistent high S_H>55%and high methane conversion(X_CH4>97.0%)is possible with a lower water-gas ratio of 11.0.We examine the effect of increasing depressurization stages with finer pressure drop on the fluid production behaviour of both gas and water in addition to the energy efficiency of the multi-stage depressurization process including energy efficacy ratio and water-gas ratio.The study found that an increasing number of depressurization stages practically has no effect on the final recovery of gas and water given the same initial and final thermodynamic stages reached.In terms of starting time on gas production,the multi-stage depressurization is more lagging than the single-stage depressurization.Multi-stage depressurization above the methane hydrate phase equilibrium only produces free water from MH accounting for 19–32%of total water production with practically no gas production.The onset of gas production from hydrate dissociation started only when BHP drops below the equilibrium pressure(around 4.6 MPa)at the corresponding temperature.The largest water production stage is when BHP first drops below 4.6 MPa,whereas the stage with the highest cumulative gas production was the last stage when BHP maintained a constant at 3.0 MPa.In addition,Increasing the number of depressurization stages increased the T_min reached in the dissociation process by 2.2?.The multi-stage depressurization can largerly decrease water producion in initial depressurization stage while reducing the water production rate,and assigning the water production process to each depressurization stage.