沁水盆地南部煤层水力压裂裂缝及地应力方向分析

Analysis of hydraulic fracturing cracks for coal reservoirs and in-situ stress direction in Southern Qinshui Basin

煤储层水力压裂裂缝分布特征及地应力方向是煤储层压裂改造施工设计的基础。通过对沁水盆地南部施工的51口煤层气井87条煤层气井水力压裂裂缝微震监测数据和大地电位探测数据进行统计分析,揭示研究区煤储层水力压裂裂缝分布特征,建立水力压裂裂缝扩展与地应力之间关系,确定了研究区地应力的展布方向。研究结果表明,沁水盆地南部压裂裂缝展布方向主要为NEE、NE和近EW向,这3个方向所占比例在55.12%;其次是近SN和NWW—SEE向,其中优势方位为NEE向。微地震监测煤层压裂裂缝长度变化范围为108~452 m,均值224.76 m。大地电位法测试煤层压裂裂缝长度变化范围为40~131 m,主要集中于51~90 m,均值73.94 m。微震监测和大地电位法测试获取的压裂裂缝延伸长度差异性较大,微地震监测裂缝长度明显大于大地电位法监测长度的2倍以上,说明微震监测裂缝长度偏大。根据水力压裂裂缝扩展与地应力之间关系,煤储层压裂裂缝几何形态概括起来有:水平裂缝、垂直裂缝和倾斜裂缝3类。压裂时,在煤储层中形成何种裂缝,取决于地层中垂直应力、最大水平主应力和最小水平主应力三向主应力的相对大小,本区现今地应力作用方向主要以NEE—SWW方向为特征,这些认识为煤层气井水力压裂改造提供了理论依据。

The distribution characteristics of hydraulic fracturing cracks in coal reservoirs and the direction of in-situ stress are the basis for the design of coal reservoir fracturing.The paper statistically analyzes the microseismic monitoring data and geodetic potential data of 87 hydraulic fracturing cracks in 51 CBM wells constructed in the southern Qinshui Basin.The distribution characteristics of hydraulic fracturing cracks in coal reservoirs of the study area are revealed,the relationship between hydraulic fracturing cracks extension and in-situ stress is established,and the distribution direction of in-situ stress in the study area is determined.The results show that the distribution of fracturing cracks in southern Qinshui Basin is dominated by NEE.The main distribution directions are NEE,NE,and near EW,accounting for 55.12%of all distribution directions,then followed by near-SN and NWW—SEE.The length of fracturing cracks,detected by microseismic monitoring,varies from 108 to 452 m,with an average of 224.76 m.The length of fracturing cracks,tested by geopotential method,is between 40 m and 131 m,mainly concentrated between 51 m and 90 m,with an average of 73.94 m.Significant differences exist between the length of the fractures obtained by the microseismic monitoring and by the geopotential method.The length of the fractures detected by the microseismic monitoring is over twice larger than that tested by the geopotential method.According to the relationship between hydraulic fracturing cracks extension and in-situ stress,the coal reservoir fracturing cracks geometry is summarized as horizontal fracture,vertical fracture,and dipping fracture.During the fracturing process,the types of fractures in coal reservoir formed by hydraulic fracturing depend on the relative stress of three principal stresses,namely vertical stress,the maximum horizontal principal stress,and the minimum horizontal principal stress.The current in-situ stress direction in this area is mainly characterized by the NEE—SWW direction.The research results of this paper provide a theoretical basis for the hydraulic fracturing of CBM wells.