Study on the mechanism of rupture and propagation of T-type fractures in coal fracturing

Abstract

Weak structures like cleats and fractures are abundant in coal rock. It can form a complex fracture system during hydraulic fracturing process. T-type fracture is generally defined as those intersected complex fractures of horizontal and vertical fractures during hydraulic fracturing of the coal bed. T-type fractures can occur during hydraulic fracturing, yet few present theory or experiments are established or carried out to figure out the underlying mechanism of T-type fractures. Compared with horizontal fractures, T-type fractures have a higher fracture conductivity, which is more conducive to the improvement of production. Therefore, it is necessary to establish theoretical models to analyze the initiation and propagation of T-type fractures. We have carried out laboratory simulation of hydraulic experiments on the one hand. On the other hand, we have observed the spatial development of T-type fractures and described the changing pattern of its propagation. The experiment results showed that hydraulic fracture initiated and extended along the vertical direction, and it formed T-type fracture when weak structures opened in the horizontal direction. According to Rupture Mechanics, we have established a calculation model to calculate the rupture and propagation of T-type fractures based on stress intensity factor and analyzed the factors that may affect fracture propagation. The findings of this paper showed that the lengths of vertical and horizontal fractures and the inclination of horizontal fractures can significantly influence the occurrence of T-type fractures. More specifically, longer horizontal fractures, shorter vertical fractures, and smaller horizontal inclination are favorable for the occurrence of T-type fractures. In the current literature, a complete theoretical model for the initiation and propagation of T-type fractures in coal bed hydraulic fracturing is still not available. This paper has established a new theoretical model for T-type fractures in hydraulic fracturing, the results of which serve as a complement and improvement for the extant mechanism study on coal hydraulic fracturing.