Numerical simulation of acoustic stimulation for enhancing coal seam gas reservoir permeability

Abstract

Acoustic stimulation method has been recognized as one of the most effective methods for enhancing coal permeability and improving the production amount of gas. Laboratory data has proved that the geometrical parameters (width and length) of coal cleats, which are the crucial pathway for gas migration, could be highly increased by acoustic field stimulation. The main reason for this positive effect is that acoustic energy could induce the mechanical vibration effect around coal cleats. However, the basic mechanism of this effect is still poorly understood due to the limitation of experimental facilities; the progress and details about this effect cannot be directly observed. It is imperative to conduct proper numerical simulation for the technology to improve the understanding of mechanical vibration. And only based on the conclusion of simulation results, the engineering parameters for optimal stimulation effect could be found. This thesis addresses this key issue and demonstrates how exactly the mechanical vibration is induced and affect the original coal cleat system. This thesis focuses on conducting numerical simulation for acoustic wave induced mechanical vibration around the coal cleats. The primary contributions are summarized as follows: (1) The numerical simulation based on the staggered-grid finite differential method (FDM) is applied to simulate 3D wave propagation in a single coal cleat model. Two parameters, shear wave energy (SE) and variable width of cleat (DW), are introduced and implemented in the numerical model to explicitly evaluate the acoustic stimulation effects. The results indicate that these parameters could accurately represent the variations of acoustic energy and geometrical parameters of the coal cleat model. (2) The polarized wave induced wave dynamics in a coal sample with a single cleat model is numerically simulated and analyzed; the energy trapping and dynamic variation of fracture width in coal for the cleat model with three different filled media (i.e. air, water and weak mineral) are numerically analyzed and compared with each other subjected to different incident wave angles, and the optimal stimulation parameters are obtained through such sensitivity analysis. Simulation results show that the mechanical vibration is induced by the energy trapping effect; coal property (i.e. cleat and its filled media) and incident wave angles are crucial for acoustic stimulation to produce physical damages around coal cleats and enhance the permeability of fractured coal samples. (3) The numerical simulation is further applied to simulate the viscoelastic wave propagation in 3D coal sample with a coal cleat system. Two kinds of coal cleat system model, orthogonal cleat sets (OCS) and T-junction cleat sets (TCS), are adopted to investigate the influence of coal cleats special structure for mechanical vibration. A stochastic model building process, which includes Monte Carlo method and von-Karman function, is proposed to better represent the structure of coal cleat system and heterogeneity of coal matrix. (4) Acoustic pressure is measured at cleatsr junctions, interface, interfaces, and coal matrix; the data are recalculated with a certain width of the time window to monitor the vibration history of each position; the effect of cleatsr junction is analysed. The contribution of the incident wave and interface wave to the mechanical vibration are also discussed. The simulation results indicate that the interface wave energy is the major part of vibration effect; averagely the cleatsr junctions experience the strongest vibration. The structure of coal cleats has a major impact for acoustic stimulation, which produces special effects such as weak mechanical vibration, symmetrical effect. Those effects that induced by coal cleat system could further improve the overall stimulation results. In summary, the thesis presents both elastic and viscoelastic wave simulation for acoustic-stimulated mechanical vibration in coal cleat models. The basic mechanism is firstly clarified on a single model and further applied for stochastic cleat system model; the influence of important stimulation parameters is also investigated. This project provides a solid theoretical foundation for acoustic stimulation method. The analyses and conclusions developed in this study are quite helpful for optimizing this technology.