Study on the Rock Mechanical Properties of Deep Coal Under Different Confining Pressures

ABSTRACT: Fracturing transformation of coalbed methane reservoirs is an important step for efficient production of coalbed methane, and the rock mechanical properties of coal have a significant impact on the formation of coal fractures. Taking deep coal seam cores from a parameter well in Xinjiang as an example, coal quality tests were conducted, and Brazilian splitting tests, uniaxial compression tests, and triaxial compression tests were carried out to analyze the tensile characteristics of coal with different bedding planes and mechanical properties under different confining pressures. The results show that the industrial components of coal are mainly volatile matter and fixed carbon, with low ash content. The microscopic components of coal are mainly vitrinite and inertinite, with an average vitrinite reflectance of 0.79%–0.81%, indicating medium- to low-rank coal. The average tensile strength of deep coal in the direction parallel to the bedding is 0.40 MPa, and in vertical bedding direction is 1.21 MPa. In the perpendicular bedding direction, the fractures formed after coal splitting are more complex, indicating that bedding direction has a significant impact on the tensile strength of coal; Under different confining pressures of 10 MPa, 20 MPa, and 30 MPa, the values of elastic modulus and Poisson's ratio of coal in the parallel bedding direction do not differ much, and the perpendicular bedding direction shows the same characteristics. However, under the same confining pressure, the values of elastic modulus and Poisson's ratio in the parallel bedding direction are greater than those in the perpendicular direction; Under confining pressure, these values are generally greater than those at 0 MPa, but the difference in Poisson's ratio is small, indicating that bedding direction has a more significant effect on the elastic modulus of deep coal. The loading curves of compression tests for deep coal show the same trend, but under confining pressure, coal samples do not have an obvious plastic stage before failure and fail rapidly after entering the plastic stage. During the failure stage, the stress curve drops sharply, exhibiting strong brittle failure characteristics. The greater the confining pressure, the higher the peak strength of the coal. The coal body structure of deep coal in this area is well preserved, with less influence from tectonic movements, belonging to primary structural coal, which is conducive to fracturing transformation of deep coal.