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 fractu- ring. Firstly,based on the statistics of 42 hydraulic fracturing in-situ stress test data of No. 3 coal reservoir in the south- western 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 res- ervoir 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 estab- lished,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 differ- ences 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 fractu- ring 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.