Influence of elastoplastic embedment on CSG production enhancement using graded particle injection

Abstract

A large percentage of coal seam gas (CSG) reservoirs exhibit low permeability and require stimulation to achieve economic production rates. Graded particle injection (GPI) into the natural fracture and cleat system around hydraulically-induced fractures has been proposed and demonstrated by prior laboratory studies as an effective method of enhancing CSG production. However, pore pressure and fracture width decrease with the distance from the wellbore and hydraulic fractures. Previous modelling determined that staging the injected particles with increasing size may sequentially fill the propped cleats and keep them open during production. GPI results in an increase in permeability above the initial value, expands the stimulated zone, and consequently, increases the productivity index. The present work aims to improve our understanding of the GPI process for production enhancement by developing a generalised modelling methodology for GPI into the fracture network. The new model accounts for elastoplastic deformation of fracture surface during particle embedment. This is based on elastoplastic finite element modelling of fracture deformation and flow modelling by the lattice Boltzmann method (LBM). Effects of packing densities, effective stress and material properties are incorporated in the modelling. Results of fracture permeability reduction factor due to particle embedment are then applied to the mathematical model for well productivity prediction. Modelling results indicate that elastoplastic fracture deformation leads to lower permeability and production enhancement using GPI, as compared with linear elastic deformation applied in traditional models. The developed model is applicable to the assessment of potential stimulation outcomes for a range of uncertain reservoir parameters. Better understanding and implementation of GPI schemes can potentially improve post-fracturing results in low-permeability coal intervals substantially.