The study of hydraulic fracture height growth in coal measure shale strata with complex geologic characteristics

The coal measure shale strata (CMSS) is rich in coalbed methane and shale gas. However, different thicknesses and high heterogeneity between the coal and shale typically result in difficultly predicting hydraulic fracture height growth. The feasibility of multi-gas production and the optimal layer for fracturing in CMSS requires a better understanding of fracture vertical propagation mechanism. In this paper, a three-dimensional model of a hydraulic fracture crossing CMSS layer was built according to geological characteristics, based on the rock seepage-stress-damaged coupled finite element method. The influence of geological factors and construction factors on the hydraulic fracture expansion was investigated. The results showed that (1) The physical property and geological conditions presented a huge difference between different lithological rock with various depth; (2) The increase of displacement and fracturing fluid viscosity, the inhibition effect on vertical fracture extension became weak. The augment of displacement was more helpful to the vertical propagation of fracture; (3) Under low permeability difference and small horizontal minimal principal stress difference, fracture length propagated enough distance and fracture height extension gap connected more the production layer. Therefore, the favorable conditions for forming balanced fracture geometry were as follows: permeability difference from 0-3 and stress difference from 1-2 MPa. (4) The 11th layer was chosen as the best fracturing layer in Shanxi Formation. The fracture height was about 26.13 m and the thickness of pay layers accounted for 42.32% of the height. The research results provide theoretical guidance for the optimal fracturing layer and fracturing construction parameters in CMSS.