本文選題：高壓氮氣 + 氣射流�。� 參考：《中國礦業(yè)大學》2017年碩士論文

【摘要】：氮氣作為惰性氣體,在煤中不容易被吸附存儲;當高壓氮氣在煤層中發(fā)生沖擊致裂后,不會留存大量的膨脹能,不具有突出危險性。隨著我國開采深度的增加,礦井的瓦斯含量隨之上升而滲透率同時下降,通過在煤層中實施高壓氮氣致裂可以提高瓦斯抽采率、降低瓦斯含量。因此,高壓氮氣沖擊致裂的研究對于礦井的安全生產(chǎn)具有重要意義。本文以對高壓氮氣氣射流沖擊過程以及高壓氮氣的準靜態(tài)膨脹作用的研究為基礎,分析了致裂過程中能量的變化,并以之作為理論指導開展高壓氮氣沖擊致裂實驗與數(shù)值模擬研究,獲得了不同工況下高壓氮氣沖擊致裂的相關規(guī)律。實驗系統(tǒng)以高壓氮氣沖擊致裂裝置為核心,主要由高壓氮氣沖擊致裂系統(tǒng)和實時監(jiān)控系統(tǒng)兩部分組成實驗系統(tǒng)。其中高壓氮氣致裂系統(tǒng)包括高壓氮氣壓力源、高壓氮氣沖擊致裂實驗裝置、氣動高壓切斷球閥、試塊以及連接裝置等部分。實驗中采用毫秒級壓力傳送器以及聲發(fā)射監(jiān)測系統(tǒng),對致裂過程中壓力的升降和試塊內(nèi)部的能量等物理量的變化進行監(jiān)測。通過強度測定等實驗確定了不同強度相似材料的配比并確定28天的養(yǎng)護周期;同時通過應力加載的方式來模擬煤層在實際情況中應力場的情況。實驗中采用1L的容器作為氣體的初始體積,分別先對三種強度的試塊進行初始壓力為5MPa、7.5MPa以及10MPa三種工況條件下的沖擊致裂實驗;再改變?nèi)N強度試塊的應力環(huán)境進行初始壓力為5MPa的實驗。從結(jié)果分析得出隨著氣體初始壓力的升高,裂紋從最初只有一條縱向裂紋到隨后以釋放孔為中心呈星狀發(fā)散的裂紋,從垂直于最小主應力方向到均勻分布。同時裂紋的分布于試塊周邊的應力差也有一定關系,應力差小則裂紋均勻分布,應力差大則垂直于最小主應力方向。再此基礎上對于最小致裂壓力進行了初步研究,結(jié)果圈定在2MPa-3MPa之間。根據(jù)對壓力監(jiān)測結(jié)果的分析,整個高壓氮氣致裂過程根據(jù)壓力上升的快慢可以分為氣體射流沖擊階段、起裂致裂階段和動態(tài)止裂階段。從聲發(fā)射監(jiān)測結(jié)果中得出,隨著氣體壓力的上升,試塊致裂時內(nèi)部絕對能量也呈現(xiàn)上升趨勢,尤其是在7.5MPa升至10MPa時能量上升明顯。以理論研究為基礎結(jié)合現(xiàn)場實際,通過ANSYS/LS-DYNA模擬軟件建立模型進行精算并通過LS_PREPOST后處理,對5MPa、7.5MPa和10MPa三種工況下的高壓氮氣沖擊致裂進行模擬。從模擬的運行過程中可以看出,裂隙的發(fā)育情況隨著氣體初始壓力的升高而得到明顯的提升,同時裂隙呈現(xiàn)出先沿軸向再沿徑向擴展的變化規(guī)律,這一規(guī)律在應力云圖中也得到了驗證。通過對應力波傳遞結(jié)果的分析得出,致裂過程中的應力波呈現(xiàn)震動上升,當應力值大于煤層失效應力時,致裂發(fā)生;小于時則保持高應力狀態(tài)直至發(fā)生致裂。
[Abstract]:Nitrogen, as an inert gas, is not easily adsorbed and stored in coal. With the increase of mining depth in China, the gas content in coal mine increases and the permeability decreases simultaneously. The gas extraction rate can be increased and the gas content can be reduced by the application of high pressure nitrogen in coal seam. Therefore, the research of high pressure nitrogen impact cracking is of great significance to mine safety production. Based on the study of the impinging process of high pressure nitrogen jet and the quasi-static expansion of high pressure nitrogen, the energy changes during the process of cracking are analyzed. The experiment and numerical simulation of high pressure nitrogen impact cracking were carried out under the guidance of the theory, and the related laws of high pressure nitrogen impact cracking under different working conditions were obtained. The experimental system is composed of two parts: high pressure nitrogen impact cracking system and real time monitoring system. The high pressure nitrogen fracturing system includes high pressure nitrogen pressure source, high pressure nitrogen impact cracking experimental device, pneumatic high pressure cut off ball valve, test block and connecting device and so on. In the experiment, millisecond pressure transmitter and acoustic emission monitoring system are used to monitor the change of physical quantities such as pressure rise and fall and energy inside the specimen during the process of cracking. The proportioning of similar materials with different strength and the curing period of 28 days were determined by strength measurement, and the stress field of coal seam was simulated by the way of stress loading. In the experiment, 1L vessel was used as the initial volume of gas, and the initial pressure of three kinds of strength samples was tested under the initial pressure of 5 MPA and 7.5 MPA, respectively, and the impact cracking test was carried out under three conditions of 10MPa. Then the stress environment of the three strength specimens was changed and the initial pressure was 5MPa. The results show that with the increase of the initial gas pressure, the crack changes from one longitudinal crack to a star-shaped crack centered on the release hole, from the direction perpendicular to the minimum principal stress to the uniform distribution. At the same time, the distribution of the crack is also related to the stress difference around the specimen. The crack distributes evenly when the stress difference is small, and the stress difference is perpendicular to the direction of the minimum principal stress. On this basis, a preliminary study of the minimum cracking pressure is carried out, and the results are delineated between 2MPa-3MPa. According to the analysis of pressure monitoring results, the whole process of high pressure nitrogen cracking can be divided into three stages: gas jet impingement stage, crack initiation stage and dynamic crack arrest stage according to the speed of pressure rise. From the results of acoustic emission monitoring, it can be concluded that with the increase of gas pressure, the internal absolute energy of the specimen also shows an upward trend, especially when 7.5MPa rises to 10MPa. On the basis of theoretical research and field practice, the model was established by ANSYS/LS-DYNA simulation software and LS_PREPOST post-treatment was used to simulate the impact cracking of high pressure nitrogen under three conditions: 5MPA 7.5MPa and 10MPa. It can be seen from the operation of the simulation that the development of fractures increases obviously with the increase of the initial gas pressure, and the fracture shows a variation law of first axial direction and then radial expansion. This rule is also verified in the stress cloud diagram. Through the analysis of the results of stress wave transmission, it is concluded that the stress wave in the process of crack appears vibration rising, when the stress value is greater than the failure stress of coal seam, the crack occurs, and when the stress value is larger than the failure stress of coal seam, the stress wave will remain in a high stress state until the crack occurs.
【學位授予單位】：中國礦業(yè)大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TD712.6

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