Investigation on the Kinetics of Natural Gas Hydrate Replacement with CO2/N2 Gas Mixture

Author:Zhang Guo Biao

Supervisor:sun you hong


Degree Year:2019





Natural gas hydrates are found worldwide in marine sediment as well as in and below permafrost regions,which can be regarded as a promising future energy source due to high energy density,global occurrences and large reserves.CO2 replacement is considered as an innovative,safe and eco-friendly method for simultaneously achieving natural gas recovery and CO2 storage,and its successful application in the gas production test form hydrate deposits in Alaska North Slope of USA have confirm the feasibility of methane recovery by CO2swapping method.If safety and environmental protection were considered,CO2 replacement may be a good alternative production strategy for the exploitation of hydrates in the marine environment and permafrost area of China.CO2/N2 replacement has made great progress in thermodynamic feasibility,kinetics,micro-mechanism and other aspects in recent years,however,considering the harsh reservoir environment and complex crystal structure characteristics of natural gas hydrates in China as well as the complex seepage-phase transition during replacement gas injection,there are some problems and deficiencies of the current researches and the application of replacement methods.Base on this,relevant researches on the kinetics of hydrate replacement with CO2/N2 gas mixture have been carried out for the exploitation of natural gas hydrates in the marine environment and permafrost area of China.For a better understanding of(CO2/N2)-CH4 gas exchange phenomenon and the heat flow feature during replacement,the cage-specific guest distributions and the dissociation enthalpy of CH4+CO2+N2 hydrates were studied.The cage occupancies of CH4,N2,and CO2 were calculated by Chen-Guo thermodynamic model,it was found that CO2 molecules were mostly trapped in large cavities of sI hydrates,while N2 molecules competed with CH4 molecules for the occupation of small cavities,meaning that the addition of N2 can enhance CH4 recovery ratio in small cavities of sI hydrates.In addition,with the increase of the N2/CO2 ratio,the occupancy of N2 increases,but the total occupancies of N2 and CO2 molecules decrease,indicating the selection of N2/CO2 ratio is important for the replacement.The dissociation enthalpies of CH4+N2+CO2 hydrates were calculated by the Clausius-Clapeyron equation.It was found that the dissociation enthalpy of the ternary hydrates increased with the increase of CO2 molecules occupancy in large cavities,and the effect of the composition of N2 in gas phase on the dissociation enthalpy is not obvious.During the process of replacement gas injection,CO2+N2 hydrate may be formed under appropriate conditions,which may affect the subsequent replacement.For a better understanding of the kinetics of CO2+N2 hydrate formation and its influence on subsequent replacement,the formation kinetics of CO2+N2 hydrate was studied in a stirred gas-water system,and the seepage-phase transition characteristics of CO2+N2 gas mixtrue in an aqueous sediment were investigated.In a stirred gas-water system,analysis of the gas phase composition and gas consumption,the hydrate formation process of CO2+N2 gas mixture followed two-step kinetics mechanism:basic hydrates containing empty linked cavities were fast formed from CO2 and water,and N2 was slowly adsorbed into the empty linked cavities;In addition,low pressure can decrease the CO2 consumption rate,but increase CO2 content in the hydrate phase,and the high stirring speed can increase the CO2 consumption rate and CO2content in the hydrate phase.In the process of seepage-phase transition,the temporal and spatial evolution of temperature and gas composition was analysed,it is found that the formation of CO2+N2 hydrates followed two distinct kinetic processes,in the fresh water environment,CO2+N2 hydrate growed in a pattern that firstly nucleated to form a hydrate in one place and then gradually expanded,while in the memory water environment,hydrates can nucleate and grow in many places;in addition,the CO2 content in gas phase gradually decreased along the gas seepage direction,and more CO2 consumed in the gas dissolution-hydration reaction,and the location of hydrate formation in different experiments was different,which is caused by the randomness of hydrate nucleation.Most marine gas hydrates reservoirs in China have higher temperatures,and CO2+N2hydrates do not exist in most marine geological environments of China,which strongly indicates that CO2+N2 injection into marine hydrate reservoirs may drive a complex production mechanism rather than pure replacement in earlier investigations.In this study,under the simulated pressure and temperature conditions of marine hydrate sites in Shenhu Area of China,the CO2+N2 injection into CH4 hydrate-bearing sediments was experimentally investigated to assess the feasibility of its potential application to marine hydrates and reveal the underlying mechanism.Results of the experiments indicated that CH4 replaced by CO2 and N2 in hydrates indeed occurred with the help of dissociated CH4 despite the injected CO2+N2gas mixture did not have the ability for replacement.Comparison between CO2+N2consumption and CH4 recovery demonstrated that the kinetics of CH4 recovery from marine hydrates triggered by CO2/N2 injection seemed to follow a two stage pattern:a rapid surface reaction in which decomposition and replacement occurred synchronously was followed by a much slower diffusion controlled replacement stage.Furthermore,the CH4 recovery efficiencies under different experimental conditions were calculated based on gas chromatography(GC)results,it suggested that methane gas was mainly recovered by synchronous decomposition and replacement in initial surface reaction.High temperature and low pressure can lead to the more rapid and intense surface reaction,and ultimately achieved a higher CH4 recovery yield exhibiting an upper limit of 71%in the first 20 hours,which strongly indicated that short-term CH4 recovery trigged by CO2+N2 injection may be an economically feasible production strategy for its potential application to marine hydrates.Natural gas hydrates in the permafrost area of China are structure Ⅱ with complex hydrate composition,which will have a greater impact on gas replacement.Due to the interaction relationship of guest-cage in sII hydrates,the gas exchange of sII hydrates with CO2 or CO2-N2 may follow a unique swapping-pattern,and the gas recovery behavior,the CO2/N2 reaction kinetics,and swapping-pattern will be different in temp-spatial scale.In this study,the replacement of sII hydrates by CO2/N2 was studied to investigate the gas recovery from sII gas hydrate reservoirs.The compositions of the fluid phase in all experimental runs were measured by gas chromatography during replacement.It was found CH4 was the easiest one to be recovered in all experiments while the increase of C3H8 in fluid phase depended on the experimental conditions,which was associated with the replacement pattern of whether the sII mixed hydrates undergoed a structural transformation or an isostructural conversion.Moreover,the analysis of the consumption of CO2/N2 suggested the consumption of CO2 was more previous than N2 during replacement,indicating that CO2 and N2 played different roles in the replacement process.The heterogeneous composition of the final replaced hydrates caused by solid state diffusion during replacement was indirect obtained by coupling gas production measurements with compositional analysis of the hydrate dissociation.It was found that,for sII hydrates replaced by CO2 and CO2-N2,the conversion of sII hydrates to sI hydrates was expected to occur at the surface of the original sII hydrate in the initial stage of replacement whereas the sII isostructural conversion may occur in the inner layers of hydrate particles in the permeation-controlled stage of replacement.The replacement extent of sII hydrates was related with the extent of sII transition to sI.It was found that the injection of liquid CO2 can induce the conversion of most sII hydrates to sI hydrates,with the highest replacement efficiency of about 70%,and the addition of N2 led to the decrease in the portion of sII hydrates transformed to sI hydrates and ultimately lowered the replacement efficiency.In summary,the cage-specific guest distributions and the dissociation enthalpy of CH4+CO2+N2 hydrates,the seepage-phase transition characteristics of CO2+N2 gas mixtrue,and the kinetics of sI and sII hydrate replacement with CO2/N2 gas mixture were systematically studied.,and the present study will help in the design of production strategy and selecting the favorable conditions for enhancing replacement efficiency in field production test,which can lay a theoretical foundation for the exploitation of natural gas hydrate in China using replacement method.