Zheng Zhao

Within a single fluid pressure system, fluid migration occurs between different coal measure reservoirs, and the in situ accumulation of coal measure gas (CMG) represents the ultimate outcome of methane undergoing a series of dynamic migration processes. Clarifying the vertical distribution patterns of in situ fluids within coal measure strata is of critical importance for guiding the co-exploration and co-production of CMG. However, direct testing of such distribution patterns remains challenging. Geophysical logging provides an indirect characterization method by evaluating reservoir permeability and gas content. In this study, based on the physical property testing results of coal measure rock samples and their corresponding logging curves from the southeastern margin of the Ordos Basin, a progressive computational model of logging parameters, porosity, and breakthrough pressure was developed for quantitatively evaluating the sealing ability of coal measure strata. Additionally, a comprehensive predictive model for gas-bearing layer was constructed based on the P- and S-wave velocity of rock samples with varying gas-water saturation levels, the logging parameters of density and neutron from coal measure strata, and the logging results of hydrocarbon content. By combining the logging evaluation results of lithology, breakthrough pressure, and gas content, the coal measure strata in the study area were classified into three fluid pressure systems. The reliability of this classification was validated through the analysis of gas characteristic and reservoir pressure differences among the identified fluid pressure systems.

The coalbed methane (CBM) resource in the southern Qinshui Basin, North China, is abundant. Proper understanding of the geological control factors on CBM enrichment in Anze Block of southern Qinshui Basin is critical to design efficient CBM development plan in this area. Field and laboratory data including well logging, well testing, hydraulic fracturing, and groundwater chemical analysis of were used to evaluate the characteristics of the hydrodynamic and geostress, and then, the combined effect of hydrodynamic and geostress field on CBM enrichment was analyzed. The results show that the total dissolved solids (TDS) of groundwater in the west is generally lower than that in other areas in the block, and the primary formation water is NaHCO₃-type. The geostress field shows a dominated geostress type in this area, and the western block has a lower value than the eastern block. The strong hydrodynamic field with low TDS also has a lower gas content while the weak hydrodynamic field with high TDS has a higher gas content. Both the TDS value and the horizontal stress differences are positively related to the gas content, indicating that inactive hydrodynamics and large horizontal stress difference of coal reservoir is favorable for CBM preservation.

Analysis of the adsorptive behaviour of kaolinite to sodium dodecyl benzene sulphonate (SDBS) at different concentrations can provides a basis for selecting the best concentration. The adsorptive capacity and adsorptive behaviour of kaolinite to SDBS at different concentrations were studied using ultraviolet spectrophotometer, pseudo-first-order adsorption kinetics model, and pseudo-second-order adsorption kinetics model. Scanning electron microscopy with energy dispersive spectrometry (SEM–EDS), X-ray diffraction (XRD), and infrared spectroscopy (FTIR) were used to study the variation characteristics of surface structure, crystallinity indices, and main functional groups on kaolinite before, and after, adsorption. The results show that as the SDBS concentration increase, the adsorptive capacity of kaolinite to SDBS increase. The adsorption process can be accurately fitted by the pseudo-secondary adsorption kinetic model, which means the adsorptive behaviour was mainly chemical in origin. The adsorption of SDBS by kaolinite mainly occurs on the surface. The solidification, lamellar aggregation, and crystallinity index of kaolinite are more obvious after the adsorption of SDBS, but the interlayer spacing of kaolinite did not change to any significant. After the adsorption of SDBS, the intensity ratio of 1000–1008 cm⁻¹ bands changed significantly, indicating the change of the chemical environment, and the adsorptive behaviour was chemical.

Differences in content, distribution, and connectivity of pores and fractures with different sizes in coal lead to different modes of gas migration. An accurate classification of pore–fracture combination types in coal can lay a foundation for studying gas migration. High-pressure mercury intrusion and nuclear magnetic resonance (NMR) experiments were conducted on coal samples collected from the Changping coal mine in Jincheng City, Shanxi Province, and Pingdingshan no. 4 mine in Pingdingshan City, Henan Province, China. The fractal dimensions of pores with different sizes were calculated using the Menger model. By combining the data with T₂ spectra obtained by NMR, critical values for distinguishing diffusion pores from seepage pores–microfractures were determined. In addition, the main parameters affecting development of diffusion pores and seepage pores–microfractures and pore–fracture connectivity were analyzed, and a comprehensive evaluation index system for pores and fractures was established by selecting eight indices. Based on the method combining the analytical hierarchy process with multiparameter superposition, a method for determining critical values, establishing the evaluation index system, and classifying pore–fracture combination types was formed. The pore–fracture combination types in the test coal samples were classified according to the experimental data. The results indicate that the critical values for distinguishing diffusion pores from seepage pores–microfractures based on fractal dimensions obtained through mercury intrusion porosimetry and T₂ spectra obtained by NMR are 72 nm and 2.5 ms, respectively. The studied coal samples can be classified into three combination types, separately characterized by high diffusivity and permeability and poor pore–fracture connectivity; low diffusivity, high permeability, and good pore–fracture connectivity; and low diffusivity and permeability and good pore–fracture connectivity. In the coal samples from the Changping coal mine, diffusion pores and seepage pores–microfractures are developed, while the connectivity between pores and fractures is poor. The coal samples from Pingdingshan no. 4 mine have undeveloped diffusion pores and seepage pores–microfractures but good connectivity between pores and fractures. The research results provide a method for classifying pore–fracture combination types in coal samples taken from different regions.

Exploring the classes and causes of fracturing curves under different combinations of coal structures and stress (CMCSSs) in coal measures can provide direction guidance for parameter optimization during hydro-fracturing. The Shizhuang south block, south central Qinshui Basin (Shanxi Province, China) was investigated. By coupling various methods including core drilling for control, geological strength index (GSI), and multi-parameter fitting of logging, the coal structures along the longitudinal direction in coal measures of 58 coalbed methane (CBM) wells in the research area were partitioned. According to the theory of measuring geostress by using hydro-fracturing method, nine CMCSSs were attained. Fracturing curves were divided into five classes (ten sub-classes) based on the pressure fluctuation amplitude and curve shape. The causes and fracturing effect of different fracturing curves were analyzed by using the theoretical rock mechanics and gas-production conditions, to propose the direction and improvement measures for parameter optimization under different CMCSSs. The result showed that the zig-zag fracturing curve was likely to appear when the proportion of soft coal was less than 30% in the roof and floor of a coal seam and a tensile stress was applied, showing optimal fracturing effect. On condition that the proportion of soft coal was less than 30% under transition or compression stress, wave-like or steadily rising fracturing curve was likely to form, with a moderate fracturing effect. The steady and decreasing fracturing curves exhibited an insignificant relationship with fracturing effect. If the soft coal accounted for more than 70%, when under compression, conventional hydro-fracturing showed the worst fracturing effect. The research result provided a reference for the implementation of targeted improvement measures for increasing yield under different CMCSSs.