Research on numerical simulation method of hydraulic fracturing for deep coal seam in Shenfu block based on mechanical property and in-situ stress

It is important for fracturing design, well pattern deployment and effective development of deep coal seam to clarify the control mechanism of coal mechanical properties and in-situ stress on fracture propagation. Taking the No. 8+9 coal seams of Taiyuan Formation in Shenfu block in the northern part of the eastern margin of the Ordos Basin as the research object, based on the acoustic logging, density logging, injection pressure-drop and drainage data, the mechanical properties and stress distribution characteristics of the coal seam and its roof and floor rock layers were systematically analyzed, and the control mechanism of mechanical properties and in-situ stress on hydraulic fractures was revealed. The results show that: (1) The No.8+9 coal seams and the roof - floor form six combinations, including mudstone-coal-mudstone (accounting for 77.4%) and sandstone-coal-mudstone (15.5%). (2) The mechanical parameters based on acoustic and density logging calculations show that the elastic modulus of coal ranges from 4.83~13.69 GPa (averaging 6.28 GPa), and the Poisson's ratio ranges from 0.31 to 0.41 (averaging 0.37). The region is characterized by high brittleness in the north and south, and high plasticity in the middle. (3) The injection pressure-drop calculation results show that the maximum horizontal principal stress in the study area is between 31.11~39.11 MPa, and the minimum horizontal principal stress is between 25.78~29.94 MPa. The acoustic logging calculation results show that vertical stress (averaging 49.12 MPa) > maximum horizontal principal stress (averaging 39.50 MPa)>minimum horizontal principal stress (averaging 33.80 MPa), and the minimum horizontal principal stress difference between the coal seam and the roof and floor is 0~12.75 MPa. (4) The simulation results of Abaqus and Fracpro PT show that the height of fracture increases with the increase of elastic modulus, and it is necessary to prevent interlayer penetration when the difference in mechanical strength with the roof is small. The increase in horizontal principal stress difference of coal makes it easy to form a single fracture along the maximum horizontal principal stress. The smaller the horizontal principal stress of coal compared to the roof and floor, the easier it is to form longer, lower, and wider fractures in the coal seam, and it is not easy to penetrate the layers. The above results indicate that increasing the fracturing scale, implementing temporary plugging technology, and controlling net fracture pressure are the main ways to improve the hydraulic fracturing effect of No.8+9 coal seams in the Shenfu block.