Fracability evaluation and classification of deep coal reservoirs in the Shenfu block

The deep coal reservoir resources are abundant in the Shenfu block, and conducting the evaluation of deep coal seam fracability in the whole region is an important foundation to realize effective reservoir reconstruction. The No.8+9 coal seams in the Shenfu block are taken as the research objects. Based on the data of logging, well test and drainage, the mechanical properties and in-situ stress characteristics of deep coal reservoirs are analyzed to establish the evaluation index of reservoir fracability, and different types of reservoirs and their production characteristics are compared and analyzed. The results show that: ① The static Elastic modulus and static Poisson’s radio of No.8+9 coal seams in the research area are 7.5 GPa and 0.35, respectively. And the average dynamic Elastic modulus and dynamic Poisson’s radio of No.8+9 coal seams are 6.3 GPa and 0.37, respectively, with significant regional distribution differences. ② The vertical stress of No. 8+9 coal seams ranges from 25.1 to 54.8 MPa, with an average of 49.1 MPa. The maximum horizontal principal stress ranges from 20.4 to 45.2 MPa, with an average of 39.5 MPa. The minimum horizontal principal stress ranges from 17.5 to 40.8 MPa, with an average of 33.8 MPa. The difference in horizontal principal stress ranges from 2.9 to 6.8 MPa, with an average of 5.7 MPa. ③ The fracability index is calculated by the dynamic Elastic modulus, dynamic tensile strength, dynamic compressive strength, dynamic Poisson's ratio, horizontal principal stress difference of coal, and minimum horizontal principal stress difference between the roof and floor of the coal seam. The fracability index of No.8+9 coal seams is between−12.1−17.6, which can be divided into 6 categories. The No.8+9 coal seams have a wide distribution of Class I, Class II and Class V reservoirs. The development effect of gas wells shows that under other similar conditions, the higher the fracability index, the more complete the transformation of coal reservoirs, and the better the gas production effect. However, the mechanical properties, in-situ stress, and compressibility of deep coal reservoirs in the Shenfu block are distributed in a complex plane, and it is necessary to carefully identify and develop targeted and appropriate fracturing transformation plans for segmented areas. The evaluation method of deep coal reservoir fracability is established to provide a basis and theoretical guidance for reservoir classification and transformation and the efficient development of deep coalbed methane in the study area.