Model for Fracturing Fluids Entering Soft-Hard Coal Interbedded Strata and Its Application

The effectiveness of hydraulic fracturing is determined by the location of fracture initiation and the propagation. Previous research paid more attention to the control effect of geostress and construction-related parameters on the fracturing and propagation of fractures induced by hydraulic fracturing but ignored the important influence of fracturing fluid flow distribution when soft coal and hard coal interbed. In view of this, a model for the amount of fracturing fluids entering soft-hard coal interbedded strata was established. Meanwhile, the relationship among the strength differences between soft and hard coals, the perforation aperture, the coal seam thickness, the thickness ratio of hard coal, and the ratio of fracturing fluids entering hard coal was analyzed. The effect of the ratio of fracturing fluids flowing into hard coal on fracturing and gas production was discussed. On this basis, the fracturing construction suggestions under different thickness ratios of soft and hard coals were proposed. The results reveal that fracturing fluids always preferentially enter the soft coal with a lower strength, which causes an increased frictional resistance of fracturing fluid migration in soft coal, resulting in the redistribution of fracturing fluids entering coal seams and finally achieving the distribution balance. The strength differences between soft and hard coals, the perforation aperture, and seam thickness are negatively correlated with the proportion of fracturing fluids flowing into hard coal. The thickness of hard coal is positively correlated with that of hard coal; when the reservoir gas production potential is similar and the proportion of fracturing fluids entering hard coal does not exceed 30%, stable daily gas production is unlikely to exceed 800 m3/day; when the proportion of fracturing fluids entering hard coal exceeds 70%, daily gas production exceeds 1,000 m3/day. The results provide theoretical support for optimizing hydraulic fracturing schemes when soft and hard coals are interbedded.