| Effective extraction of CBM(coal-bed methane) is a strategy of China’s energy development and demand of China’s coal production safety. China’s coal is always rich in CBM. However, most coal mines have complicated geology conditions and the coal seam permeability is extremely low. It means that traditional technologies to increase the permeability are not effective, which leads to poor extraction and utilization of CBM and accidents.In recent years, as a kind of important measure to increase oil and gas production, hydraulic fracturing technology has been used in underground coal mines to enhance CBM extraction gradually. However, due to many geological differences between coal strata and conventional oil and gas formations, it is not proper to explain the fractures’ initiation and propagation problems in coal seams by oil and gas reservoir’s fracturing theory. At the same time, coal seam has the characters of high ground stress and many joint fissures in the deep conditions, which cause the initiation pressure increase largely; hydraulic fracture propagation is highly vulnerable to interface, coal seams’ roof and floor deformation and limited permeability-increasing range.Hence, to solve the problems which exist in coal mine during hydraulic fracturing, based on research of the structural characteristics of coal, this thesis established initiation criterions for different coal structures; through the analysis of influence by coal rock’s mechanical parameters for hydraulic fracture extension, this thesis revealed the expansion rules when hydraulic fractures meet non-continuous coal-rock interface; through the study of pore pressure’s effect on hydraulic fracture extension, this thesis developed multiple-borehole fracturing technique; relating fracturing parameters were optimized and the field tests were conducted. The main research results are as follows:①Cracks initiation criterions for different coal structure were proposed. Currently, scholars regard coal aselastic-plastic, homogeneous and isotropic body and don’t consider the diversity of coal structure, which had different initiation mechanism. In this study, based on the macroscopic and microscopic characteristics of coal body structure, with the classification method of coal body structure made byJiaozuo Mining Institute,the coal was divided into three categories: primary structure coal, cataclastic coal and granulated coal(mylonitized coal). The elastic mechanics, fracture mechanics and soil mechanics were used to reveal the mechanism of initiation and establish the corresponding criterions under three types of coal body structure.② The expansion rules when hydraulic fractures meet non-continuous coal-rock interface was revealed. Based on the theory analysis and numerical simulation methods, the natural fracture and coal-rock interface were taken as examples to investigate the non-continuous interface’s effect on hydraulic fractures’ propagation. The research results showed: for natural fracture’s influence, angle of interaction between main crack and natural fracture, the horizontal differential principal stress and the development degree of natural fractures are the three main factors that affect direction of hydraulic fracture propagation; the propagation direction of crack tends cross natural fracture in condition of high horizontal differential principal stress and high angle of interaction; besides, with the increase of length of natural fracture,the propagating fracture tends to extend along the natural fracture. As for coal-rock interface’s effect on hydraulic propagation, angle of intersection between coal-rock interface and horizontal section, horizontal crustal stress difference, differences in elasticity modulus of coal and rock and shear strength of coal-rock interface will affect the direction of hydrofracture crack propagation. With the increase in angle of intersection or horizontal crustal stress difference comes the rise of possibility of crack directly crossing coal-rock interface; The bigger the difference in elasticity modulus in coal and rock or smaller shear strength of coal-rock interface,the more evident the trend that crack propagates along the coal-rock interface.③ Pore pressure stress field effect on hydrofracture propagation is researched. Based on porous elastic mechanics and the Griffith strain energy release theory, the influence of pore pressure on hydraulic fracture propagation pressure and orientation were analyzed. Then, numerical simulation software RFPA2D-Flow was further used to analyses the inductive mechanism by pore pressure. Results showed that: pore pressure field can reduce the effective stress in coal and decrease the propagation pressure in coal seam; at the same time, the pore pressure gradient will directly affect the hydraulic crack propagation energy required for propagation. Specifically speaking, the greater the pore pressure means energy required hydraulic crack propagation will be smaller, thereby inducing hydraulic crack propagation along the direction to high pore pressure zone.④ Proposed a multiple-borehole hydraulic fracturing method to control crack propagation in underground coal mines. We developed fracture control technique, which consists of borehole arrangement, borehole sealing and hydraulic fracturing. And then, borehole’s sealing equipments were developed to be used in underground coal mines. Based on Darcy percolation theory, researches were made about the relationship among borehole spacing, pressurized-water pressure and time and the calculation model were built. The comparison experiments were conducted in Fengchun mine and results showed: in the 2 hours of hydraulic fracturing, multiple-borehole fracturing could increase the range by 40 meters compared to traditional fracturing and the propagation pressure decreased. Based on the gas extraction situation, the average single borehole’s extraction amount is 0.037m3/min, which is 4.1 times and 12.3 times compared to traditional hydraulic fracturing and boreholes’ extraction. |
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