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In this paper, 12 sandstone samples are collected from the Taiyuan Formation in Qinshui Basin, and sample types using the T2 spectral under LF-NMR saturation and centrifugation conditions are classified. Moreover, single and multifractal models were used to calculate fractal parameters of saturated and centrifugal T2 spectra, and the correlation between different fractal parameters, pore structure, T2cutoff value, and pore permeability parameters was studied. The results are as follows. (1) According to the T2 spectrum curves under centrifugation and saturation conditions, all the samples can be divided into three types. There are significant differences in the uniform pore size distribution. However, the non-uniformity of small pore distribution in type B samples is stronger than that of other types, while heterogeneity of large pore distribution is weaker than that of different types. The centrifugal T2 spectrum curve exhibits both single-fold and multifractal characteristics. The results of a single fractal by using a centrifugal T2 spectrum are consistent with those of a saturated T2 spectrum, indicating that single fractal features by using centrifugal and saturated T2 spectra are consistent. Unlike the single fractal parameters, the correlation between the saturation and centrifugal T2 spectrum's multifractal parameters is weak. This suggests that the physical significance conveyed by the centrifugal T2 spectrum's multifractal parameters differs from that of the saturated T2 spectrum. [ABSTRACT FROM AUTHOR]

Over the last three decades, molecular dynamics (MD) has been extensively utilized in the field of coalbed methane geology. These uses include but are not limited to 1) adsorption of gaseous molecules onto coal, 2) diffusion of gaseous molecules into coal, 3) gas adsorption-induced coal matrix swelling and shrinkage, and 4) coal pyrolysis and combustion. With the development of computation power, we are entering a period where MD can be widely used for the above higher level applications. Here, the application of MD for coalbed methane study was reviewed. Combining GCMC (grand canonical Monte Carlo) and MD simulation can provide microscopic understanding of the adsorption of gaseous molecules onto coal. The experimental observations face significant challenges when encountering the nanoscale diffusion process due to coal structure heterogeneity. Today, all types of diffusion coefficients, such as self-, corrected-, and transport-diffusion coefficients can be calculated based on MD and the Peng-Robinson equation. To date, the MD simulation for both pure and multi-components has reached a situation of unprecedented success. Meanwhile, the swelling deformation of coal has been attracting an increasing amount of attention both via experimental and mimetic angles, which can be successfully clarified using MD and a poromechanical model incorporating the geothermal gradient law. With the development of computational power and physical examination level, simulation sophistication and improvements in MD, GCMC, and other numerical models will provide more opportunities to go beyond the current informed approach, gaining researcher confidence in the engagement in the estimation of coal-swelling deformation behaviors. These reactive MD works have clarified the feasibility and capability of the reactive force field ReaxFF to describe initial reactive events for coal pyrolysis and combustion. In future, advancing MD simulation (primarily characterized by the ReaxFF force field) will allow the exploration of the more complex reaction process. The reaction mechanism of pyrolysis and spontaneous combustion should also be a positive trend, as well as the potential of MD for both visualization and microscopic mechanisms for more clean utilization processes of coal. Thus, it is expected that the availability of MD will continue to increase and be added to the extensive list of advanced analytical approaches to explore the multi-scaled behaviors in coalbed methane geology.

In this paper, the heterogeneity of adsorption pores in middle and high rank coal samples were analyzed by using low temperature N2 and CO2 adsorption technology and fractal theory. The following results were achieved. 1) According to the results of volume and surface fractal dimension, meso‐pores can be classified into Mep‐1, Mep‐2, and Mep‐3, respectively. Micro‐pore can be classified into Mip‐1, Mip‐2, and Mip‐3, respectively. 2) Pore types play an important role in affecting the heterogeneity of meso‐pores. The volume heterogeneity (VHY) of Mep‐1 is simpler than that of Mep‐2 and Mep‐3 in type A samples. However, the VHY of Mep‐1 becomes gradually larger than that of Mep‐2 and Mep‐3 from type A to type B and C. The VHY of open pore in the same diameter is higher than that of semi‐open or closed pore. Meanwhile, the surface heterogeneity (SHY) of types A and B samples is significantly larger than that of type C, the SHY of semi‐open or closed pores is more complicated than that of open pores. 3) Coal rank mainly affects the heterogeneity of micro‐pores. The heterogeneity of type A is always smaller than that of type B and C. The VHY of Mip‐1 is more complicated than that of Mip‐2 and Mip‐3 in the same samples, and the sensitivity of the VHY of Mip‐1 and Mip‐2 to the degree of coal rank is smaller than that of Mip‐3. Meanwhile, the SHY of Mip‐1 and Mip‐2 is simpler than that of Mip‐3 in the same sample, the SHY of micro‐pores remains stable as the pore size decreases, and the affect of coalification level on SHY decreases with the decrease in pore diameter. Full‐scale fractal characterization has enabled quantitative characterization of adsorption pore properties and provided useful information with regards to the similarity of pore features in different coal reservoirs. [ABSTRACT FROM AUTHOR]

Nuclear magnetic resonance (NMR) T2 cutoff value is an important parameter for pore structure evaluation. It is complicated and uneconomical to obtain T2 cutoff value by an experimental method; therefore, it is necessary to explore a prediction method of T2 cutoff value. In this paper, 10 samples of tight gas reservoirs in the eastern Ordos Basin were selected, and then saturation and centrifugal experiments of nuclear magnetic resonance were carried out. On this basis, multifractal theory was introduced to calculate the multifractal characteristics of the NMR T2 spectrum of each sample, and the relationship between multifractal parameters and T2 value was analyzed. The influencing factors of the T2 cutoff value were clarified, and the prediction model of the T2 cutoff value was constructed accordingly. The results show that the T2 spectra of sandstones in the study area can be divided into three types: single steeple peak, double steeple peak, and irregular double peak. The pore diameter of the three types is 1 nm ~ 3×104 nm, 1 nm ~ 104 nm and 1 nm ~ 4×103 nm, respectively. The T2 cutoff value ranges from 9.72 to 35.16 ms. The correlation analysis suggests that the symmetrical fractal dimension difference and symmetrical multifractal dimension ratio (Dmin−Dmax, Dmin/Dmax) shows a positive linear correlation with the T2 cutoff value. The value of T2 cutoff gradually decreases with the increase of the flow zone indicator (FZI). Therefore, three parameters, including symmetrical fractal dimension difference, symmetrical multifractal number ratio, and FZI are optimized, and the prediction model for the NMR T2 cutoff value of sandstone samples in the study area is proposed. The introduction of porosity‐related parameters compensates for the shortcomings of previous T2 cutoff value prediction models. At the same time, the prediction model is proven to be accurate and reliable by testing the measured data of the samples near the study area. The results of this paper can be used for further study of the NMR T2 cutoff value prediction of tight sandstone reservoirs in different areas. [ABSTRACT FROM AUTHOR]