Taking the standard size coal block samples defined by ISRM as research objects, both properties of methane diffusion of coal block under triaxial compressive stress and characteristic influences caused by methane pressure were systematically studied with thermo-fluid-solid coupling with triaxial servocontrolled seepage equipment of methane-containing coal. The result shows the methane diffusion property of coal block under triaxial compressive stress was shown in four-stage as follow, first is sharply reduce stage, second is hyperbolic reduce stage, third is close to a fixed value stage, fourth stage is 0. There is a special point making the reduced rate of characteristic curve of methane diffusion speed become sharply small; the influences of shape of methane diffusion speed characteristic curve caused by methane pressure are not obvious, which only is shown in numerical size of methane diffusion speed. Test time was extended required by appear of the special point makes the reduce rate of methane diffusion speed become sharply small. The fitting four-phase relation of methane diffusion of coal block under triaxial compressive stress was obtained, and the idea is proposed that influences of the fitting four-phase relation caused by methane pressure were only shown in value of fitting parameters.
Most of coal or weak rock is in triaxial stress state near stope of underground coal mining; triaxial stress state apparently has influences on the internal structure of coal or weak rock, and internal structure is the main storage and transport place in coal block. All of these will make the storage and transport characteristics of methane in coal or weak rock be changed because of influences on the internal structure of coal or weak rock. Power of methane flow is methane pressure gradient, which is determined by the change of methane desorption or adsorption caused by the change of stress state in coal or weak rock. And lots of methane stored in coal mass ahead of mining work face, if the coal mass was continued mining, stored methane will be desorbed so that the methane pressure gradient will formed in the coal mass, which will make methane constantly migration to the coal wall and gush into the extractive space at last.
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All tests of this paper were done in State Key Laboratory of Coal Mine Disaster Dynamics and Control belonging to Chongqing University. Thermo-fluid-solid coupling with triaxial servocontrolled seepage equipment of methane-containing coal was the key equipment when tests of this paper were done in the laboratory, which is shown in Figure
Thermo-fluid-solid coupling with triaxial servocontrolled seepage equipment of methane-containing coal.
The briquette samples made by crushed raw coal collected from the mining work face were taken as research objects; the diffusion characteristics of methane when methane pressure is not fixed are studied under fixed triaxial compressive stress state. When the tests were done, the axial stress is fixed in 5.0 MPa and confining pressure is 6.0 MPa, but the methane pressure is changed among 1.0 MPa, 2.0 MPa, and 3.0 MPa (according to gas pressure of work face of mine, and appropriately sized gas pressure gradient is considered, too). Before tests were done, the stress including axial stress and confining pressure was alternately raised until reaching the designed value, and the fixed pressure methane was exerted lasting 24 hours in order to make the coal block adsorbed methane diffuse fully. The fixed pressure methane will be stopped and the control valve was opened in order to make the methane contained in coal block diffusion gush freely from coal block; the properties of methane diffusion speed were monitored from time to time. The tests of every fixed methane pressure level are done from three to five times according to the law of ISRM. The samples used by tests were shown in Figure
Test samples.
According to the designed test program, the smallest unit of mass flow controller is taken as mark of end of methane diffusion test, and characteristics curve of methane diffusion test was divided into two parts taking 1.0 L/min for the boundary in order to get comparable law (the methane diffusion speed of coal block at the beginning of curves is much larger than its size at the later stage; 1.0 L/min is taken as dividing point). The characteristics curve of methane diffusion speed of coal block under fixed triaxial compressive stress state was shown in Figure
Characteristic curve of methane diffusion speed (methane pressure is 1.0 MPa).
Methane diffusion speed > 1.0 L/min
Methane diffusion speed < 1.0 L/min
According to the analysis in Figure
Methane pressure in coal block is formed by free methane; the size of methane pressure not only can be used to show the amount of methane in coal block, but also has a direct impact on methane diffusion speed of coal block. In order to get the influences on methane diffusion speed properties of coal block under triaxial compressive stress caused by methane pressure, the curves of methane diffusion speed of coal block under triaxial compress stress were studied in the different methane pressure according to the designed test program on level of 1.0 MPa, 2.0 MPa, and 3.0 MPa, shown in Figure
Influences of methane diffusion properties caused by methane pressure.
Methane pressure is 1.0 MPa
Methane pressure is 2.0 MPa
Methane pressure is 3.0 MPa
According to the analysis in Figure Influences caused by methane pressure on curves of methane diffusion speed of coal block under triaxial compress stress are not obvious; the four-stage property of the curve is still very obvious as the methane pressure is raised or decreased. Obvious influences are shown in part of the size of methane diffusion speed because of different methane pressure. The lasting time of large methane diffusion speed is extended and the reduction rate of methane diffusion speed is reduced with the raising of methane pressure at the same time. The reason may be that the amount of methane remaining in test system piping is raised and the amount of free methane in coal block is much larger with the increase of methane pressure; all of them will cause the law. Special point that makes the reduction rate of methane diffusion speed of coal block sharply reduce will appear after a long test time with the increase of methane pressure. The reason may be that the amount of methane and the geometric dimensions of pore and crack in coal block are larger if high methane pressure affects them, which will form the methane pressure gradient easily in coal block and make the free methane diffuse outside of coal block and make the absorbed methane in coal block be changed into free methane and diffuse outside of coal block. Test time when curve of methane diffusion speed enters the third phase is extended gradually but the last trend value of methane diffusion speed is similar although the methane pressure is different. The reason may be that the methane source in the third phase of test curve mainly depends on desorbed methane from coal block; the amount of desorbed methane and free methane in coal block is larger when the methane pressure is increased, which makes the time needed by test extended. After the test enters into the third phase, the internal and external environment where the coal block lies are similar to each other; the influence on methane desorbed properties caused by initial methane pressure is not obvious, especially entering into the stage of residual methane desorbing, so that the law is shown in the test curve. Test time when the methane diffusion speed is shown in significant value is basically the same under the condition of used sensors in this paper with the increase of methane pressure, but the reason may be the influence of sensors accuracy. Properties of the methane diffusion speed curve of coal block under triaxial compressive stress can be expressed by the following equation regardless of how to change the initial methane pressure, whose fitting accuracy is larger than 0.9:
The influences caused by methane pressure only are shown in the difference of fitting parameters of the above equations.
The main conclusions can be obtained according to the above tests and analysis as follows. Four-stage properties are very obvious in methane diffusion speed of coal block under triaxial compressive stress, including first is sharply reduce stage, second is hyperbolic reduce stage, third is close to a fixed value stage, fourth stage is 0. There is a special point in test curve of methane diffusion speed of coal block under triaxial compressive stress making the reduction rate of methane diffusion speed sharply decreased in a short time. There is no obviously influences on curve of methane diffusion speed caused by methane pressure, and influences only be shown in size of methane diffusion speed obvious. Equation that properties of the methane diffusion speed curve of coal block under triaxial compressive stress can be expressed was obtained regardless of how to change the initial methane pressure.
The author declares that there is no conflict of interests regarding the publication of this paper.
This paper is supported by the Open Fund (PLN1304) of the State Key Laboratory of Oil and Methane Reservoir Geology and Exploitation (Southwest Petroleum University), National Natural Science Foundation Project of China (51474220), and Open Fund (SKLCRSM13KFB11) of the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology.