Coalbed methane mainly exists in coal pores in the state of adsorption. Correct understanding of the pore structure and distribution characteristics of coal is an important basis for studying the porosity, spatial structure, seepage characteristics and coalbed methane recoverability of coal reservoirs. At present, the pore structure and pore size distribution characteristics of rock are mainly evaluated and analyzed by the capillary pressure curve obtained by mercury intrusion analysis and the adsorption desorption curve obtained by low temperature nitrogen adsorption desorption experiment. In view of the fact that coal reservoirs are characterized by fragility, compressibility, pore structure complexity and high heterogeneity compared with conventional reservoirs, this makes the two methods have many disadvantages in coal reservoir application. For example, the low temperature nitrogen adsorption desorption experimental method for the sample pore size test range of 1. 7 ~ 300 nm, can better reflect the distribution of micro and mesopores, but can not reflect the distribution of macropores and fissures, the test range has Limitations; mercury intrusion method is damaged and cannot be reused
The low-field NMR technology test principle is different from the above two methods. The distribution of micro-holes, mesopores, macro-holes and fissures in the coal sample pore system and connectivity are obtained by measuring the T2 relaxation time of the fluid in the pores of coal. And various physical parameters of coal rock. The method has the advantages of fast, non-destructive, rich information and the like.
Low field NMR experiment results
The T2 relaxation time spectrum of the coal sample was obtained by low field nuclear magnetic resonance experiments (Fig. 3). According to the morphological characteristics of the sample T2 spectrum, the samples are mainly divided into two types according to the pore size: one type of micropores is dominant, the mesopores, macropores and fissures are not developed, such as high coal rank samples; another type of sample micropores Large pores or fissures are dominant, and mesopores are relatively undeveloped, such as medium coal samples.
The surface relaxation rate of high coal rank coal is significantly lower than that of medium coal rank coal. The main reasons are as follows: high coal rank coal has a relatively high proportion of micropores, and the pore structure is more complicated, and most of them have "fine flask" type capillary pores. the Lord. Therefore, the surface relaxation rate has a good correspondence with the complexity of the pore structure of the sample and the pore type.
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