3D Numerical Simulation of Methane Deflagration Fracturing in Vertical Well for Shale Gas Reservoir Based on CDEM
Authors: Wang J. W. , Guo T. K. , Qu Z. Q. , Chen M. , Wang X. D. , Liu X. Q. 
Published: September 2023

ABSTRACT
Methane deflagration fracturing is a kind of reservoir stimulation technology for shale gas reservoirs. A 3D numerical model is established based on the continuum–discontinuum element method (CDEM) to study the fracture propagation laws of methane deflagration fracturing fracture. The linear elastic constitutive is applied to the block element, the tensile-shear composite constitutive considering strain softening is applied to the interface element to simulate rock fracture, and the Landau model is applied to describe the deflagration pressure change. The correctness of the model is verified by comparing it with the blast fracturing experiment of a cement rock sample. Then, the 3D numerical simulations are carried out to study the deflagration fracture mechanism. The results indicate that methane deflagration fracturing forms complex fractures around the borehole, with shear fracture dominating and a few tensile radial fractures. Influenced by the stress difference, the fracture range in the direction σH is larger than that in the direction σh, forming an approximate elliptic cylinder deflagration fracture zone. The effects of rock elastic modulus, Poisson's ratio, peak pressure, and pressurization rate on the reservoir crack and fracture range are positively correlated, while the influence of initial minimum principal stress, stress difference, reservoir tensile strength, and cohesion on deflagration fracturing is negatively correlated. Among them, the most sensitive influencing factors are the deflagration peak pressure and the initial minimum principal stress, and the influences of the reservoir rock physical and mechanical parameters are not significant.

INTRODUCTION
The efficient development of shale gas reservoirs greatly depends on the progress of hydraulic fracturing technology, and in recent years, technical systems such as volume fracturing, factory fracturing of the horizontal well group, and temporary plugging re-fracturing have been gradually formed (Guo et al., 2018; Guo et al., 2014; Li et al., 2020; Liu et al., 2020; Qu et al., 2021; Zhou et al., 2019). However, for deep–ultra-deep marine shale gas reservoirs and continental–marine-continental transitional shale gas reservoirs, the application of traditional hydraulic fracturing technology has encountered bottlenecks due to large in-situ stress or severe hydration damage (Dong et al., 2021; He et al., 2020; Zhao et al., 2021). There is a need to develop reservoir enhancement techniques with high peak pressures and little or no water, such as deflagration fracturing (Middleton et al., 2015; Wei et al., 2018; Zhai et al., 2018). Deflagration fracturing (called high-energy gas fracturing), is first developed and tested by Sandia National Laboratories in the late 1970s (Schmidt et al., 1980). A propellant gas generator is used to control the deflagration process, generating a suitable peak pressure and action time. Compared to hydraulic fracturing, the peak pressure is higher and the action time is shorter.