Thermo-Hydro-Mechanical Coupling Numerical Simulation on Mechanical Heterogeneity of Coal Rock

Coal is a kind of rock with the characteristics of soft structure, developed joints, cleats, cracks, and pores, resulting in that its mechanical behaviors are highly sensitive to stress, pore coalbed methane (CBM, gas) pressure and temperature. Thus, due to the variations in stress, pore pressure and temperature caused by the drilling operation, the coal surrounding wellbore can be easily damaged, which would cause serious wellbore instability problems. In this presented work, a stress path of loading axial stress and unloading confining pressure (LAS-UCP) was first determined based on the stress redistribution of the coal surrounding horizontal wellbore in CBM reservoir during drilling process. A series of triaxial compression tests with the LAS-UCP stress path was then conducted to study the effects of axial loading rate, pore pressure and temperature on the mechanical behaviors of coal. The results show that: (1) Under the LAS-UCP stress path, the deformation of coal can be divided into elastic deformation stage, plastic deformation stage, and stress reduction stage. With the decrease in axial loading rate and the increases in pore pressure and temperature, the elastic deformation stage becomes shorter, the plastic deformation stage becomes more significant, stress reduction rate in the stress reduction stage becomes slower, and the coal shows more features of plasticity and ductility. (2) With the increasing axial loading rate, the compressive strength and apparent elastic modulus increase linearly, the absolute values of axial strain, radial strain and volumetric strain at peak stress grow gradually, but the apparent Poisson’s ratio changes irregular. (3) With the increase in pore pressure, the compressive strength, axial strain at peak stress and apparent elastic modulus decrease linearly, the radial strain and volumetric strain at peak stress have no change rule, and the apparent Poisson’s ratio increases gradually. (4) With the increasing temperature, the compressive strength, axial strain at peak stress and apparent elastic modulus reduce gradually, but the absolute values of radial strain and volumetric strain at peak stress, and the apparent Poisson’s ratio increase linearly. The results can not only provide a guidance for safety drilling operation of the horizontal wellbore in CBM reservoir, but also have important significance for other engineering constructions related to coal seam.

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Study on full-scale pores characterization and heterogeneity of coal based on low-temperature nitrogen adsorption and low-field nuclear magnetic resonance experiments

Lu,

Liu,

Zhang

et al. 2024

Sci Rep

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The characteristics and heterogeneity of coal pores are crucial for understanding the production mechanism of coalbed methane (CBM). In this study, coal samples with varying degrees of metamorphism (0.58% ≤ R O , max ≤ 3.44%) were collected. The characteristics of pore development and the heterogeneous properties of pores were revealed through low-temperature nitrogen adsorption (LTNA) and low-field nuclear magnetic resonance (NMR) experiments. The results indicate that pores with varying diameters exhibit favorable development in low-rank coals, along with favorable pores connectivity. The micropores composition of middle-rank coals was found to be 73.56%, however, the connectivity among transitional, meso, and macropores was observed to be poor. In high-rank coals, the proportion of micropores was 92.74%, with numerous micropores being closed or semi-closed. This resulted in inferior connectivity between micropores and transitional pores. As coal metamorphism progressed, the D L 1 (characterizing the roughness of adsorption pores (AP) surface, ranging from 2.13 to 2.45) and D L 2 (characterizing the complexity of AP structure, ranging from 2.56 to 2.77) initially decreased and then increased, whereas the D N (characterizing the heterogeneity of seepage pores (SP), ranging from 2.92 to 2.95) consistently improved. Furthermore, the roughness of pore surface and the complexity of pore structure in AP increased as the specific surface area and volume of pores increased. On the contrary, as the SP content increased, the uniformity of the pore structure improved. When the volume of SP remained constant, the complexity of the pore structure decreased due to increased pore connectivity.

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Thermo-Hydro-Mechanical Coupling Numerical Simulation on Mechanical Heterogeneity of Coal Rock

Cited by 4 publications

References 15 publications

Mechanical heterogeneity characterization of coal materials based on nano-indentation experiments

Mechanical heterogeneity characterization of coal materials based on nano-indentation experiments

Mechanical behaviors of coal surrounding horizontal wellbore during drilling process considering the effects of loading rate, pore pressure and temperature

Study on full-scale pores characterization and heterogeneity of coal based on low-temperature nitrogen adsorption and low-field nuclear magnetic resonance experiments

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