Research on numerical simulation method of hydraulic fracturing for deep coal seam in Shenfu block based on mechanical property and in-situ stress
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摘要: 明确煤力学性质和地应力的分布特征及其对人工裂缝形态和扩展行为的控制机制，对深部煤层压裂设计、井网部署和煤层气资源开发至关重要。以鄂尔多斯盆地东缘北部神府区块太原组8+9号煤为研究对象，基于声波测井、密度测井、注入压降试井和排采资料，系统分析煤层及其顶底板岩层的力学性质和地应力分布特征，揭示力学性质和地应力对水力裂缝的控制机理。结果表明：(1)8+9号煤层与顶底板形成了泥岩-煤-泥岩(占77.4%)、砂岩-煤-泥岩(15.5%)等6种组合；(2)基于声波和密度测井计算的力学参数显示，煤弹性模量在4.83~13.69 GPa (平均6.28 GPa)，泊松比0.31~0.41(平均0.37)，区域上以南北脆性高，中部塑性高；(3)注入压降试井计算结果显示，研究区最大水平主应力介于31.11~39.11 MPa，最小水平主应力变化范围为25.78~29.94 MPa；声波测井计算结果显示，垂向应力(平均49.12 MPa)>最大水平主应力(平均39.50 MPa)>最小水平主应力(平均33.80 MPa)，煤层与顶底板最小水平主应力差在0~12.75 MPa；(4) Abaqus和Fracpro PT模拟结果显示，煤弹性模量越大，裂缝高度相对越大，当顶板与煤层的力学强度差较小时防止穿层；煤层水平主应力差增大，容易沿最大水平主应力形成单一裂缝；煤层水平主应力较顶底板水平主应力越小，易在煤层内形成较长、较低、较宽的裂缝，且不易穿层。研究认为实施较大的压裂规模、缝内暂堵技术和控制裂缝净压力等手段是提高神府区块8+9号煤水力压裂效果的主要途径。Abstract: 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.
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