临兴地区深部煤储层地应力场及其对压裂缝形态的控制
In-situ stress field of deep coal reservoir in Linxing area and its control on fracturing crack
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摘要:
地应力是煤储层压裂改造的关键控制因素。基于电成像测井和水力压裂数据，结合Abaqus软件模拟，查明了研究区深部煤储层地应力空间展布规律，探讨了地应力对压裂裂缝延展的影响。结果表明，临兴地区最大水平主应力方向近似呈EW向。地应力的垂向分布具有明显分带性，存在1 200 m 和2 000 m两个临界转换深度。1 200 m以浅，地应力以垂直主应力为主；埋深介于1 200～2 000 m 时，水平主应力占据主导；当埋深超过2 000 m时，地应力再次转换为以垂直主应力为主。研究区深部煤储层地应力主要表现为最大水平主应力＞垂直应力＞最小水平主应(σhmax>σV>σhmin)。地应力的平面分布受埋深和构造2方面控制，埋深导致地应力整体呈南高北低、西高东低的特征；褶皱和断层使得研究区北部地应力呈条带状分布，背斜核部水平地应力较大，向斜核部及断层发育区水平地应力较小。研究区地应力组合特征决定了煤储层压裂裂缝以垂直裂缝为主，局部容易形成一些复杂缝。在天然裂隙存在的情况下，水平主应力差小于3 MPa时，压裂裂缝沿天然裂隙扩展；当水平主应力差超过6 MPa时，压裂裂缝沿最大水平主应力方向扩展；当水平主应力差超过10 MPa时，压裂裂缝基本为沿最大主应力方向的单缝。压裂实例分析表明，压裂裂缝的延伸方向和扩展特征受地应力控制明显。该研究成果以期为深部煤储层的压裂改造提供理论指导。
Abstract:In-situ stress is the key controlling factor for fracturing stimulation of coal reservoirs. Based on the data of electro imaging logging and hydraulic fracturing,combined with Abaqus software simulation,the spatial distribution characteristics of in-situ stress of deep coal reservoir in the study area was identified,and its effect on crack extension was discussed. The results indicate that the maximum horizontal principal stress direction is approximately EW in Linxing area. The vertical distribution of in-situ stress has obvious zonal distribution. There are two critical conversion depths of 1 200 m and 2 000 m,respectively. When the buried depth is less than 1 200 m,the in-situ stress is characterized by vertical principal stress;when the buried depth is between 1 200-2 000 m,the horizontal principal stress dominates;when the buried depth exceeds 2 000 m,the in-situ stress is characterized by vertical principal stress again. The in-situ stress of deep coal reservoir in study area is characterized by σhmax>σV>σhmin. The plane distribution of the in-situ stress is controlled by both the buried depth and the structure. The burial depth causes the in-situ stress to be high in the south and low in the north,and high in the west and low in the east. The horizontal in-situ stress in the part is relatively large,and the horizontal in-situ stress in the syncline core and the fault development area is relatively small. The characteristics of in-situ stress combination in the study area determine that the fracture is dominated by vertical cracks,and some complex cracks are easily formed locally. In the presence of natural fractures,when the horizontal principal stress difference is less than 3 MPa,the fracturing cracks expand along the natural fractures;when the horizontal principal stress difference exceeds 6 MPa,the fracturing cracks expand along the maximum horizontal principal stress direction;when the horizontal principal stress difference exceeds 10 MPa,the fracturing cracks is basically a single crack along the maximum principal stress direction. The analysis of fracturing examples shows that the extension direction and propagation characteristics of cracks are obviously controlled by in-situ stress. The research results provide theoretical guidance for fracturing transformation of deep coal reservoirs.
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