Adsorption Process by Low Field NMR

Adsorption Process by Low Field NMR

Adsorption Process

Natural gas is self-storage. It’s generated during the process of coal formation and exists in the underground coal seams and surrounding rocks in the adsorption and free state.

The data show that the geological evolution or the current geological structure has a huge impact on the exploitation of natural gas. Because the coal-bearing rock series in our country has been preserved through the influence of multi-stage tectonic action, it is very different from other countries. The structure of coal is special, which reduces the permeability of coalbed methane and affects the output of production capacity; at the same time, because coal is self-generated and self-storage, it is completely different from oil and natural gas reservoirs, and many factors restrict its production capacity.

The study found that the total volume of the pores of the coal seam is much smaller than the total content of natural gas, so it can be clearly pointed out that there must be another state of existence of natural gas, that is, the adsorption state, that is to say, in a certain porous medium, a liquid-like liquid is contained. Or gas existing in condensed state, its existence mode is divided into chemical adsorption and physical adsorption. Through various theories and a large number of experiments, it has been proved that the coalbed methane in coal and rock mainly exists in the form of physical adsorption; in addition, desorption means that after the natural gas molecules are adsorbed by coal and rock and other media, they are subjected to thermal motion or a certain Under the influence of this kind of vibration, the natural gas molecules are reactivated, and they are enough to get rid of the adsorption force of the adsorption medium, so that the natural gas molecules can return to the natural gas molecular group in a free state. The study found that the desorption/adsorption of natural gas can be converted into each other under certain conditions.

Natural gas adsorption process

The research shows that the natural gas adsorption is a seepage process first, that is, due to the strong external pressure, the gas molecules of methane flow into the macroporous system and flow, and a CBM film is formed on the surface outside the coal matrix. When the gas molecules pass outside the medium particles, a part of the gas molecules will be strongly adsorbed by the outer surface of the medium, and the adsorbed gas molecules will diffuse into the coal medium through the micropores of the medium; at the same time, the other part will pass through the pores of the medium particles. spread inward.

The natural gas adsorption process includes external diffusion, internal diffusion and surface diffusion. The slow stage, generally the rate of the inner diffusion stage, determines the overall rate of the adsorption process.

Adsorption Process by Low Field NMR

Low-field nuclear magnetic resonance technology is an advanced non-destructive testing technology. The basic working principle of low-field nuclear magnetic resonance technology is to first obtain the nuclear magnetic resonance signal of the object to be measured, and then obtain the nuclear magnetic resonance imaging map or T2 relaxation distribution map according to the relaxation time difference of different components. Low-field nuclear magnetic resonance technology can detect both. It can also detect some physical properties and flow parameters of porous media, as well as the interaction between fluid and porous media skeleton, and study the flow state and distribution law of fluid in it. Low-field nuclear magnetic resonance T2 relaxation distribution technology can According to the measured relaxation time difference, the gas adsorbed in the coal can be analyzed to realize the real-time and dynamic observation of the natural gas adsorption process. The existing flow state in the coal seam and the analysis of various influencing factors in the process of migration and production have better applicability in the analysis and research of the physical properties of the coal seam and the adsorption law of natural gas.

The relaxation time of nuclear magnetic resonance can characterize the molecular mobility of the gas, and the methane gas in different adsorption states can be characterized by the difference in relaxation time. The T2 spectrum curve of a typical coal/shale sample has three distinct peaks, which are adsorbed state, pore bound state, and free state gas in sequence.