页岩层理方位及强度对水力压裂的影响

Influence of bedding azimuth and strength on hydraulic fracturing in shale

为了研究页岩天然层理倾角及强度等对水力压裂裂纹扩展的影响,采用室内水力压裂实验,通过监测孔直接对裂纹扩展的实时监测和注水压力信息及试件压裂后的剖切,分析层理倾角、强度等对压裂裂纹扩展的影响。实验结果表明:水力压裂过程中,垂直最小地应力稳定扩展的主裂缝遇层理时,层理面与主裂缝初始扩展方向夹角越小,主裂缝越易沿着层理面方向扩展,层理面与主裂缝初始扩展方向夹角越大,主裂缝遇层理面时越易贯穿层理面沿原方向扩展;层理方位,地应力及基质抗拉强度不变,层理的抗拉强度远弱于基质抗拉强度时,主裂缝与层理面相遇后越易沿着层理面方向扩展,层理抗拉强度与基质抗拉强度越相近,主裂缝遇层理时越易贯穿层理沿原方向扩展;层理方位和强度不变,地应力及应力差越大,主裂缝遇层理后越易贯穿层理面沿原方向扩展。

In order to study the influences of shale natural bedding inclination angle and strength on hydraulic fracturing crack growth,indoor hydraulic fracturing experiment was performed.Real-time monitoring of crack growth and water injection pressure information and sample cutting after fracturing were conducted through monitoring holes to analyze the influence of bedding inclination angle and strength on fracturing crack growth.The results indicate that in the hydraulic fracturing process,when the main crack with stable expansion of minimum vertical ground stress is encountered with the bedding plane,the smaller the angle between the bedding plane and the initial fracture expansion direction is,the more easily the main crack will be expanded along the direction of the bedding plane.The greater the angle between the bedding plane and the initial fracture expansion direction is,the more easily the main crack will be expended through the bedding plane in the original direction.When the bedding azimuth,the ground stress and the tensile strength do not change,and the tensile strength of bedding plane is far weaker than the tensile strength of the matrix,the main crack encountering the bedding plane is much easier to extend in the direction of the bedding plane.The closer the tensile strength of bedding plane approaches to that of the matrix,the more easily the main crack will be extended through the bedding plane along the original direction.When the bedding azimuth and strength do not change,the main crack is much easier to expend through the bedding plane along the original direction for the greater ground stress and stress difference.