Experimental Study On The Decomposition Characteristics And Mechanism Of Natural Gas Hydrate Formed In Silica Sands

Natural gas hydrates are a huge amount of clean energy which can be used to improve China's energy structure and ease the world's energy crisis.However,the commercial exploitation of natural gas hydrates still faces many problems.At present,researchers mainly carry out experiments for the simulated mining of natural gas hydrates in laboratory.Most of the current studies focus on the effects of porous media type,liquid saturation of porous media,hydrate saturation,decomposition pressure and reactor volume on the decomposition process of hydrates.But the impact of different decomposition methods on reservoir temperature,recovery rate and gas uptake is less studied.There is also lack of a unified understanding of ice formation and the effect of heat injection during the decomposition process of hydrates.In this study,high pressure micro-differential scanning calorimeter?HP-?DSC?has been used to investigate the thermodynamic properties of gas hydrates.And the characteristics and mechanism of natural gas hydrates dissociation in porous media systems have also been studied in depth to reveal the influence of temperature rise,decomposition pressure,heat injection,and ice phase formation on the temperature distribution of the hydrate layer,gas production rate,and gas recovery rate.This study provides a theoretical basis for promoting the development of natural gas hydrate production technology.The main research work in this study are listed as follows:?1?High-pressure micro-differential scanning calorimeter?HP-?DSC?was used to study the phase equilibrium conditions and the dissociation enthalpies of gas hydrates formed from CO₂,CH₄ and their mixtures in the presence of tetra-n-butyl ammonium chloride?TBAC?.It was found that semiclathrate hydrates formed at the stoichiometric TBAC concentration?3.3 mol%?with the CO₂+CH₄ mixture are more stable than that formed at other TBAC concentrations;the semiclathrate hydrates formed in 1.0 mol%TBAC solutions are a mixture of pure TBAC semiclathrate hydrate and TBAC+CO₂+CH₄ semiclathrate hydrate;and the dissociation enthalpy of TBAC+CO₂+CH₄ mixed semiclathrate hydrate formed in 3.3 mol%TBAC solutions increases with the increase of experimental pressure.This trend is not obvious in 5.0 mol%TBAC solutions.?2?The experimental study of gas hydrate formation was carried out in the quartz sands system.It was found that higher hydration conversion rate and shorter reaction time are obtained at lower liquid saturations of quartz sands while gas consumption is the best at 70%liquid saturation.The gas consumption for the experiments using small?0.18⁰.25mm?quartz sands is apparently higher than that using large?0.3⁰.9mm?quartz sands.Gas intake in the sand layer can significantly accelerate natural gas hydrates formation compared with gas intake outside the sand layer.?3?Hydrate decomposition experiments were performed using thermal stimulation method.It was found that the temperature in the reactor has a“platform stage”during hydrate dissociation.Moreover,temperature and pressure profile agrees well with the phase equilibrium curve.The temperature is lower and the decomposition time is longer at the parts where are farther away from the heat exchange surface.In addition,the decomposition rate and recovery rate have a positive correlation with hydrate saturation.?4?Hydrate decomposition experiments were conducted using the depressurization method.It was found that the reservoir temperature and decomposition time decrease with the decrease of the decomposition pressure while the gas recovery rate increases with the decrease of the decomposition pressure.But this trend is not obvious at low pressures.The rapid decomposition stage is longer and the decomposition rate in the stable decomposition stage is smaller when the decomposition pressure is lower.Magnetic valve is used to control the decomposition pressure.Gas production during the decomposition stage is stabilized while the interval of gas production frequency is reduced.Gas production interval and instantaneous gas production rate are increased under higher pressures.The decomposition rate in the depressurization process with ice-phase is faster than that without ice-phase,and the pressure driving force is the main factor of hydrate dissociation.?5?Hydrate decomposition experiments were performed using the depressurization method combined with thermal stimulation.The results indicated that heat injection can accelerate the depressurization process with ice-phase and reservoir temperature recovery during the depressurization experiments at a small pressure driving force.From the point of the cumulative gas production,the effect of heat injection is more obvious at higher decomposition pressures.Heat injection improved the gas recovery and decomposition time at a decomposition pressure of 2.2 MPa,but it turned to be an inhibition effect at 2.6 MPa and 3.0 MPa.The inhibition effect is more apparently at a higher decomposition pressure.As for the decompression process with ice-phase,a small amount of hot water injection will cause the ice-phase increasing compared with non-injection of hot water,which will further block the interstitial space of silica sands and reduce the permeability.Meanwhile,more methane gas will dissolve into the free water,which will prolong the decomposition time.However,the injection of hot water also accelerated the recovery of reservoir temperature.