The propagation research of hydraulically created fractures in coal seams based on discrete element method

Abstract

Coalbed methane (CBM) is clean unconventional energy that can be exploited with stimulation treatment to realize commercial value, and hydraulic fracturing is the key technology for increasing CBM production. The simulation of hydraulic fractures is an important research content that can guide the engineering practice to achieve the purpose of increasing production. Based on the distribution characteristic of cleats in coal, the research on the propagation of hydraulic fractures is carried out via Discrete element method (DEM). The simulated results show that: hydraulic fractures mainly propagate along the cleats towards the maximum principal stress. The fracture network can be formed due to the intersection of face cleats and butt cleats that can propagate at certain pressure. During the injection process of fracturing fluid, the pressure at injection point decreases with the fracture propagation until the pressure tends to be stable. With the increase of injection rate and fracturing fluid viscosity at the same condition, the maximum aperture of hydraulic fracture increases, while the length of principal hydraulic fracture shortens. Therefore, to achieve the purpose of forming hydraulic fracture network in coal seams with cleats, low viscosity fracturing fluid and low injection rate need be applied to the fracturing technology. As the cleat density increases, the number of branch fractures increases, but the length of principal hydraulic fracture becomes short. Long and narrow hydraulic fractures are easily formed in coal seams with lower cleat density.

Advances in Hydraulic Fracture Propagation Research in Shale Reservoirs

“…It was found that conglomerate particles affect fracture propagation in two ways, penetration and avoidance, and the penetration pressure of conglomerate strata is linearly and nonlinearly related to particle strength and content, respectively. Based on the advantages of the discrete element method in studying discontinuous structures, Lu et al [110] established a discrete element model to simulate the effect of coal seam cuttings on hydraulic fracture propagation and found that hydraulic fractures mainly extend along the direction of maximum principal stress. In addition to the influence of the developmental characteristics of different rocks on hydraulic fracture propagation, other scholars have analyzed the influence of ground stress difference, fracture interaction, tectonic parameters, and other fundamental features on the hydraulic fracture propagation [111].…”