基于Griffith强度理论的煤储层水力压裂有利区评价

水力压裂有利区评价是煤储层压裂改造施工设计的基础。通过对沁水盆地西南部3号煤储层42个水力压裂地应力测试数据统计,系统分析了研究区煤储层地应力分布规律。采用378口水力压裂井资料,基于格里菲斯(Griffith)强度理论计算了研究区煤储层单轴抗拉强度,建立了煤储层破裂压力与最小水平主应力和抗拉强度之间关系和模型,揭示了研究区煤储层可压裂性特征,建立了基于Griffith强度理论的煤储层水力压裂有利区评价方法,对煤储层水力压裂有利区进行了评价。研究结果表明,研究区块3号煤层最大水平主应力14.67~45.05 MPa,平均为29.31 MPa,最大水平主应力梯度为2.00~4.84 MPa/100 m,平均为3.27 MPa/100 m;最小水平主应力10.51~29.09 MPa,平均为18.61 MPa;最小水平主应力梯度为1.44~2.85 MPa/100m,平均为2.09MPa/100 m,煤储层应力和压力均随深度的增加呈线性增大的规律。基于格里菲斯(Griffith)强度理论计算的研究区3号煤储层单轴抗拉强度为0.15~1.10 MPa,在平面上存在一定的差异性。根据单轴抗拉强度值将煤储层可压裂性划分为4类,对于较高抗拉强度区和高抗拉强度区(Ⅲ和Ⅳ),煤储层抗拉强度值大,煤层气井水力压裂改造中起裂压力高,难以进行压裂改造。对于低抗拉强度区和较低抗拉强度区(Ⅰ和Ⅱ),煤储层抗拉强度值小,煤层气井水力压裂改造中起裂压力小,易于进行压裂改造,评价结果与实际水力压裂情况相吻合。

Evaluation of favorable areas for hydraulic fracturing of coal reservoir based on Griffith strength theory

The evaluation of favorable area of hydraulic fracturing is the basis for the design of coal reservoir fracturing.Firstly,based on the statistics of 42 hydraulic fracturing in-situ stress test data of No.3 coal reservoir in the southwestern Qinshui Basin,the in-situ stress distribution law of coal reservoir in studying area was systematically analyzed.Secondly,on the basis of Griffith’s Strength Theory,this paper calculated the uniaxial tensile strength of the coal reservoir in the studying area by using the data collected from 378 hydraulic fracturing wells.Meanwhile,the relationships and model between fracture pressure and minimum horizontal principal stress and tensile strength of the coal reservoir were established,respectively,and the coal reservoir characteristics of fracturability were revealed.A method for evaluating the favorable fracturing area of hydraulic fracturing coal reservoir based on Griffith strength theory was established,and it was used to evaluate the favorable areas of hydraulic fracturing in studying area.The research results show that the maximum horizontal principal stress and gradient of the No.3 coal seam in studying area range from 14.67 to 45.05 MPa with an average of 29.31 MPa,and from 2.00 to 4.84 MPa/100 m with an average of 3.27 MPa/100 m,respectively.The minimum horizontal principal stress and gradient vary from 10.51 to 29.09 MPa with an average of 18.61 MPa,and from 1.44 to 2.85 MPa/100 m with an average of 2.09 MPa/100 m,respectively.Both of the stress and pressure of coal reservoir increase with the increase of depth by linear law.The uniaxial tensile strength calculated by Griffith’s strength theory ranges from 0.15 to 1.10 MPa,which shows that there exists differences in lateral distribution.According to the values of the uniaxial tensile strength,the fracturability of coal reservoir can be divided into four types.For the areas with high tensile strength and higher tensile strength(Ⅲ and Ⅳ),the value of tensile strength of coal reservoir is large,and the initiation fracture pressure in the process of hydraulic fracturing is high,so it is difficult to implement hydraulic fracturing treatment.For the areas with lower tensile strength and low tensile strength(Ⅰ and Ⅱ),both the coal reservoir tensile strength and the initiation fracture pressure are relatively smaller,so it is easy to implement the fracturing treatment in coal seam.The evaluation results are consistent with the practical hydraulic fracturing operations.