Study on the extension mechanism of hydraulic fractures in bedding coal

Abstract

This study is aimed at investigating the formation process and extension mechanism of hydraulic fractures (HFs) in bedding coal. To achieve this aim, a numerical model of hydraulic fracturing of bedding coal was established based on the cohesive element. Furthermore, the influences of stress field, bedding dip angle and fracturing fluid displacement on the extension law of HFs in bedding coal were studied, and the formation process and extension mechanism of HFs in bedding coal were analyzed. The following beneficial results were obtained. HFs prefer penetrating the bedding plane when extending towards it at a large angle, while they tend to extend along the bedding plane when the angle is small. Under certain stress field conditions, during hydraulic fracturing, changing the extension direction of original HFs enables HFs to develop into a complex fracture network under the combined effect of bedding structure and stress field. Meanwhile, altering injection displacement can enlarge the extension range of HFs on the bedding plane. The principle can be explained as follows: under the action of continuous unstable pressure waves caused by fluctuant injection displacement, coal is likely to undergo fatigue damage, which further promotes the generation of micro cracks within coal and facilitates the opening and extension of HFs. The pore pressure near HFs increases linearly with the rise of fluid pressure in HFs, decreases linearly with the growth of distance from HFs, and is inversely proportional to the square root of the fracturing fluid’s filtration time. After HFs penetrate the bedding plane, coal is prone to shear failure and further generates secondary fractures under the filtration effect of fracturing fluid. Under the joint effect of fracturing parameters such as bedding, ground stress and displacement, main HFs and secondary HFs can intersect to form a complex fracture network when penetrating the bedding plane.