Research On The Evolution Law Of Coal Fracture Under Pulse Loading And Its Application On Gas Extraction

Coalbed methane(CBM) is a kind of high quanlity clean energy. Acelerating the development of CBM industry will be of great importance in protecting national energy safety, optimizing energy mix, reducing gas accidents and reducing greenhouse gas emission. Presently underground CBM extraction is mainly based on boreholes, and there are some problems exsiting such as low gas concentration, small flow rate and fast decay when extracting. It is difficult to improve by the traditional techniques, and mining safety and resource utilization are restricted. Pulse hydraulic fracturing is new method to let CBM extraction be effective. However, researches on the law of coal fracture evolution during pulse hydraulic fracturing still lags behind that of engineering application.In this paper, thereical analysis, numerical simulation and physical experiments are carried out to study the mechanismes of fracture evolution under pulse loading and under the controllings of pulse frequency and pulse pressure based on the engineering application, and the main acheivements are as follows:(1) The law of fracture evolution under pulse loading is investigated. When the sample was loaded by pulse loading, the fractures were went through the process of closure-linear deformation-stable extension of nonlinear deformation-accerlarating extension-fracturing. The speed was smaller than that of static loading, and the formal styles were richer. The sample was destroyed layer by layer at a pressure which is lower than 85% of static loading. A better fracture-network can be formed inside of the sample when it is pulse loading.(2) Pulse frequency loading and coal fracturing experiment system is developed, containing four sub-systems: pulse pressure generater, true triaxial loading, pressure and flow rate controlling and monitoring. Coal fracture evolution experiments can be designed under different conditions such as in-si stresses, loading styles and pulse paremeters. Coal samples and simaliar samples are made, and their mechanical parameters are tested. Simulation borehole is deisned in the samples, and new hole sealing method is invented.(3) The law of fatigue destroy of coal-rock under pulse loading is analyzed. The result of fracture evolution under pulse frequency controlling physical experiment shows that when the frequency is low, the sample is softed, the fracture initial pressure and fracture extension rate are decreased, and micro-fractures propagate and extend, at last complicated frature plane is formed; When the frequency is high, pressure amplitude is high, the fracture initial pressure and fracture extension rate are increased, and the fractured formed are singleness. Variable pulse fracturing is presented based on the controlling of pulse frequency to simulate the formation of fracture net-work.(4) The law of fracture propagation and extension under pulse loading is analyzed. The fracture evolution is changed to more complicated under the comprehensive changes of stactic and pulse stress fields. The result of numerical simulation of pulse loading and static loading shows that the amount of fractures under pulse loading is increased by 20% and the fracture initial pressure is descreased by 85%. The pulse pressure amplitude has an effect on fracture amount and fracture initial pressure. The physical experiment furtherly shows that when the pulse pressure amplitude is lower, the micro-fracures propagate and extend, and the fracturing fuilds have enough time to invade into the coal, and the fracture connection is better. When the pulse pressure amplitude is higher, fracture propagation and extension are singleness, the amount is smaller, and the fracturing plane is single. The method controlling the fracture evolution based on the controlling of pulse pressure amplitude is presented to simulate the formation of fracture net-work.Based on the findings above, pulse hydraulic fracturing technique system is built to form fracture network in coal, and the application results show that the concentration of gas extraction is increased more than 2.22 times, the flow rate is increased more than 2.63 times and the borehole quantity is reduced by 50%. The gas desorption index K1 is reduced to under the critical value, and the safety of roadway driving is ensured.The research findings of this paper have an important significance on the improvement of pulse hydraulic fracturing, the optimization of technique parameters and the improvement of permeability increasment. During the research, 10 papers are published, two of which are indexed by SCI and the others are indexed by EI, 10 patents for invention are applied, 5 of which have been authorized. A provincial prize is also obtained.