Quantitative NMR Technique Study On Methane Hydrate Phase Transition Process

As a kind of clean energy,natural gas hydrate has huge reserves around the world and is considered as a potential source of natural gas in the future.Accelerating the development and utilization of natural gas hydrate is one of the important ways for our country to promote the revolution of energy production and consumption,and build a clean、low-carbon、safe and efficient modern energy system.But because of the accumulation conditions of natural gas hydrate is complicated in nature,besides the fluids distribution and the phase transformation mechanism is unclear in the formation/decomposition process,producing hydrate directly will affect the security and stability of hydrate reservoir,so it is necessary to study the migration law of multiphase fluids during hydrate formation/decomposition process in porous media.In this paper,the formation、decomposition and reformation processes of methane hydrate in porous media were studied systematically by using self-designed low-field nuclear magnetic resonance（NMR）experimental system,and the effects of different factors on the hydrate production rate were analyzed.In hydrate formation process,a variety of NMR measurement methods were used to distinguish multiphase fluids effectively.Besides,the macroscopic quantitative evaluation and the microscopic mechanism were analyzed based on one dimension saturation profile and T₂ distribution.The experimental results show that during hydrate formation process,the temporal and spatial evolution of fluids content distribution and rate were not uniform;Due to the formation of dendrite solid hydrate,the pore space of large and medium pores was gradually partially occupied by the formed hydrate and transformed into small pores,while the pore space of some small pores was gradually completely occupied;The hydrate formation rate decreased with the higher initial water saturation and the larger pore of the porous media,which was related to the higher pore water saturation.In hydrate decomposition process,the effects of different factors on the decomposition rate and icing saturation were analyzed in detail.The experimental results show that the temporal and spatial evolution of fluids content distribution and rate were not uniform,too;the initial water saturation of the reservoir had no effect on the hydrate decomposition,but the uniformity of the reservoir did.Compared with uniform depressurization method and sudden depressurization method,more fluid could be produced based on the staged depressurization method,and a larger staged depressurizing rate in a rational degree could effectively improve the overall production rate and reduce the formation amount of ice.The methane hydrate reformation results show that the average reformation rates of hydrate in porous media increase with higher initial hydrate saturation and residual hydrate saturation,which are related to the content of decomposed fluids and dendritic structure with higher memory effect.The above research results can provide experimental data support for the development of hydrate formation/decomposition kinetics,and further theoretical support for understanding of hydrate accumulation mechanism and the hydrate exploitation experiments.