Mechanical characteristics and fracture propagation mechanism of deep coal reservoirs in the Shenfu block

Objective Clarifying the mechanical properties and in-situ stress distributions of coal seams, along with the mechanisms behind their control over the morphologies and propagation behavior of artificially induced fractures, is crucial to the fracturing design, well pattern deployment, and coalbed methane (CBM) production of deep coal seams. Methods This study investigated the Nos. 8 and 9 coal seams of the Taiyuan Formation in the Shenfu block in the northern portion of the eastern margin of the Ordos Basin. Using data from sonic logging, density logging, injection/falloff well tests, and production, this study systematically analyzed the mechanical properties and in-situ stress distributions of the coal seams, as well as rock layers on their roofs and floors. Furthermore, this study revealed the mechanisms behind the controlling effects of the mechanical properties and in-situ stress on hydraulic fractures. Results and Conclusions Key findings are as follows: (1) The Nos. 8 and 9 coal seams and their roofs and floors consist of six assemblages including mudstone-coal-mudstone (77.4%) and sandstone-coal-mudstone (15.5%). (2) The mechanical parameters, calculated based on sonic and density logging, indicate that the coal seams exhibit elastic moduli ranging from 4.83 GPa to 13.69 GPa (average: 6.28 GPa) and Poisson's ratios ranging from 0.31 to 0.41 (average: 0.37). Regionally, they manifest high brittleness in the north and south and high plasticity in the central part. (3) The calculation results of injection/falloff well tests show that the maximum and minimum horizontal principal stresses in the study area range between 31.11 MPa and 39.11 MPa and between 25.78 GPa and 29.94 MPa, respectively. The sonic logging-based calculation results suggest that various stresses decrease in the order of vertical stress (average: 49.12 MPa), maximum horizontal principal stress (average: 39.50 MPa), and minimum horizontal principal stress (average: 33.80 MPa). The difference in minimum horizontal principal stresses between a coal seam and its roof and floor varies between 0 and 12.75 MPa. (4) The simulation results obtained using software Abaqus and Fracpro PT indicate that higher elastic moduli of the coal seams correspond to larger fracture heights, necessitating preventing fractures from crossing layers in the case of a small difference the mechanical strength of coal seams and that of their roofs. The simulation results also suggest that increasing the difference in the horizontal principal stress of coal seams tends to induce individual fractures along the direction of the maximum horizontal principal stress. Furthermore, lower horizontal principal stresses of the coal seams with respect to their roofs and floors contribute more significantly to the formation of longer, lower, and wider fractures in the coal seams, with a minimal possibility of crossing layers. Overall, the findings lead to the conclusion that the primary approaches for enhancing the hydraulic fracturing performance of the Nos. 8 and 9 coal seams in the Shenfu block include a large fracturing scale, temporary plugging within fractures, and the control of net fracture pressures.