Prediction of fracture initiation pressure in multiple failure hydraulic fracturing modes: Three-dimensional stress model considering borehole deformation

Abstract

Hydraulic fracturing technology has been widely used as the main method of exploiting coalbed methane (CBM). The core of this technology is the accurate prediction of the fracture initiation pressure. As the exploitation depth increases, the influence of the in-situ stress obviously increases. Because the existing model of calculating the fracture initiation pressure ignores the influence of in-situ stress on the extraction borehole (CBM well) deformation, and the fracture initiation failure mode is single, the fracture initiation pressure of hydraulic fracturing cannot be accurately predicted in the field. In this study, a new model for predicting the fracture initiation pressure in hydraulic fracturing under a three-dimensional stress state was established. For the first time, the deformation of a circular borehole is related to the in-situ stress, and the size of the elliptical borehole can be accurately obtained after deformation. Eleven fracture initiation failure modes for the hydraulic fracturing of coal seams are proposed. The fracture initiation pressure of each failure mode was calculated in MATLAB, and the fracture initiation pressure of the shear failure of the ρz plane (a) was the smallest. The effects of the horizontal maximum principal stress, horizontal minimum principal stress, vertical intermediate principal stress, coal seam dip angle, horizontal azimuth of borehole, and gas pressure on the fracturing pressure were investigated using the proposed model. The coal seam dip angle exerted the greatest influence, and a new hydraulic fracturing construction procedure is proposed: the pump pressure should be gradually increased from the minimum initiation pressure according to the fracture initiation pressure gradient required by each failure mode. A field hydraulic fracturing test was conducted, and the experimental results are in good agreement with the theoretical calculation results. The maximum error was 5.5%, which validates the proposed theoretical model. The accuracy of the proposed theoretical model in the calculation of the fracture initiation pressure increased by 364.4% compared with the classical hydraulic fracturing theory. The results obtained by this study can provide more precise guidance to hydraulic fracturing projects.