Numerical Simulation and Operational Parameter Optimization for Staged Multi-Cluster Fracturing Horizontal Wells in Coal Seam Roof

Abstract

ABSTRACT The staged multi-cluster hydraulic fracturing technique of horizontal well in roof strata of coal seam is a new method for effective developing the broken-soft coal seam. Therefore, based on cohesive model and Bernoulli equation, considering the friction of fracturing fluid flowing through wellbore, perforation and the stress interference during fracture propagation, a finite element numerical model of multi-cluster fracturing of coal seam roof coupling wellbore and formation is established. The effects of perforation diameter, perforation number and injection rate on fracture extension were studied. The results indicated the fracture tip is always located in the roof during fracture dynamic propagation, and coal seam fractures are wider than roof fractures. During multi-cluster perforation in the fracturing section, the fractures initiated from each perforation cluster have competitive propagation. The stress interference between the fractures and the friction of the fracturing fluid make the distribution of the fracturing fluid between the perforation clusters uneven. Increasing injection rate, reducing number of single perforation cluster, and decreasing diameter of perforations hole can facilitate the even propagation of fractures in each cluster, and increasing the stimulated reservoir volume. The influence of the perforation diameter is significantly greater than the injection rate and perforations number. INTRODUCTION China has a wide distribution of broken soft coal seams that have low permeability, and hydraulic fracturing is the key to its efficient development. However, due to their low strength(S), low elastic modulus(E), and high Poisson's Ratio(ν), broken-soft coal seams have high plasticity compared to hard coals or other rocks (Li et al., 2021; Lyu et al., 2020). Long fractures are difficult to form in coal seams and the fracturing sand is severely inlaid when conduct hydraulic fracturing in the plastic broken soft coal seam directly. The stimulation effect is generally poor. In response to this problem, scholars have proposed indirect fracturing technology for coal seam roof. (Li et al., 2014; Olsen et al., 2007; Olsen et al., 2003; Zhang et al., 2018). The technology realizes the efficient extraction of coalbed methane in broken soft coal seam by arranging horizontal wells in the roof strata adjacent to the coal seam, and the fracture extends from the coal seam roof to the coal seam. The field test indicates that the application of this approach to broken soft low-permeability coal seam can significantly improve the efficiency of coalbed methane exploitation (Zhang and Bian, 2015).