| Coal-bed methane is formed from the coal inside coal rock in the course of the history of geological, which is a clean and high-quality energy. It has been widely used in the aspects of industry fuel, generator fuel, industrial chemicals and domestic fuel. Hydraulic fracturing is the main technique of coal-bed methane exploitation, the difference between the evolutions of coal-bed methane wells cracks the coal rock fracture and the enlargement of the convention rock crack single fracture. Accurately describe the characteristics of the coal-bed methane wells hydraulic fracture the coal rock and the evolution rule of coal bed fracturing are of great value to set up the study of the seepage behavior and solve the technical problem of coal-bed methane exploitation. This essay set to study the geologic characteristic of the coal rock and the distribution characteristics of natural fracture, propose the Fractal and chaos theory to study the evolution rule of the coal-bed methane wells fracture, study the process of the hydraulic fracturing the pore of the coal rock and the evolution characteristic of micro-fracture dynamic fractal damage, establish the evolution chaos characteristic of hydraulic fracture the coal rock fracturing. The main research work of this paper lies in the following aspects:Firstly, considered the special characteristics of coal bed on the aspects of pore system and fracture system, came up with the rule of basing on Fractal theory to describe the complicated pore fracture system of coal bed, established the fractal description method of the pore facture system, utilized box-counting method to determine the fractal dimension of coal bed cleat; use the L-system make morphologically coupling description to the endogen crack, and reverse its Fractal dimension. This is providing a reliable theoretical foundation and scientific basis for the subsequent study of chaotic characteristics of fracture.Secondly, built the damage mechanics model of the borehole wall rock of coal-bed methane wells based on fractal theory, obtained the distribution of borehole wall rock mechanics, and showed the fractal damage characteristics of different regions.Again, built the coal-bed methane wells fracturing damage fracture and the fracture of coal rock fractal evolution model based on the catastrophe theory and energy conservation principle, obtained the standard form of crack and fracture evolution. Based on this, calculated and analyzed the fractal damage evolution rule of coal rock mutations.This work studied the reconstruction method of the phase space of coal rock fracturing fracture evolution system, determined two parameters of the phase space reconstruction:delay time and embed dimension, and laid a foundation for the analysis of chaotic characteristics of the evolution of fracture.Finally introduced the chaos characteristics of coal rock fracturing fracture evolution system:correlation dimension,maximal Lyapunov index and Kolmogorov entropy.By calculating the number of fractures, the correlation dimension of radial stress and circumferential stress of the cusp, maximal Lyapunov index and Kolmogorov entropy,discriminant the chaotic characteristics of fracture evolution. The calculation results show that the fracture evolution behavior based on the fractal theory has the characteristics of chaos.In conclusion, this article is based on the fractal chaos theory to research the evolution rule of coal-bed methane wells fracturing fracture, and is a frontier subject at home and abroad, has important academic value and theoretical significance. This paper revealed the fundamental content of the coal-bed methane wells hydraulic fracturing fracture evolution based on the nonlinear dynamics method, provided a reliable scientific basis for knowing and understanding the distribution of fracture system after wells fracturing and the efficient mining of coal-bed methane. The results of this research has a certain guiding role and realistic significance for the development of coal-bed methane resources and the coal mine gas drainage at home and abroad. |
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