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Research Of The Coal Mechanical Properties And Coal Reservoir Fracturing Simulation In Jincheng District, Shanxi Province

Posted on:2010-10-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z F YanFull Text:PDF
GTID:1220360272987940Subject:Marine Geology
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Coalbed methane(CBM), one kind of clean and high-quality energies, can bring various benefits if being took into comprehensive use. Nowadays, vertical well is the mainly CBM development mode. And hydraulic fracturing is the most important means for vertical well to enhance the recovery of CBM. As a result of strong tectonic movement that coal-bearing strata generally undergone in China, coal structure had been great damaged extensively, which usually cause bad fracturing effect and lower production, and hence it is necessary to research the fracturing mechanism of coal reservoir. For these reason, we select the research subject, mechanical properties of coal and coal reservoir fracturing simulation.A series of experiments containing tensile strength, uniaxial compressive strength, triaxial compressive strength and acoustic emission were carried out to the coal collected from No.3 coal seam in Sihe coal mine, south of the Qinshui Basin, and the results indicated that the mechanical properties of coal possess directivity not only in the direction vertical to the coal bedding plane,but also in the direction parallel to the coal bedding plane. The tensile strengths increase with the increase of the saturated water absorption. And the tensile strengths are also influenced greatly by length of the specimen which has negative correlation with the value of the tensile strengths. With the increase of Saturated water absorption, the elastic modulus reduce gradually in the direction vertical face cleat; in contrast the impacts of the Saturated water absorption on the elastic modulus were little in the direction parallel face cleat. The Poisson’s ratio decrease with the increase of uniaxial compressive intensity in the direction vertical face cleat, and similarly, it has little change in the direction parallel face cleat. Under the condition of Uniaxial compression, the characteristics of the acoustic emission count rate curves can be divided into three types:①Burst;②ruptured; and③stable. Under the condition of Triaxial compression, the peak strain of coal also correlate with direction, under the lower confining pressure, the variation of peak strain with the confining pressure when axial of specimens perpendicular to the face cleat are less than that when axial of specimens parallel to the face cleat; whereas under the higher confining pressure, the variation when axial of specimens perpendicular to the face cleat are greater than that when axial of specimens parallel to the face cleat.The results of numerical simulation indicated that, in the non-uniform stress field, under the condition of the isotropic and non-fracture, the breakdown pressure changed with the change of maximum principal stress, minimum principal stress and stress difference; while under the condition of cracks, the breakdown pressure were affected mainly by the crustal stress rather than the variation of the angle between the directions of cracks and the maximum principal stress. In the non-uniform stress field, at given stress condition, the breakdown pressure changed with the change of the angle between the directions of cracks and the maximum principal stress.In fracturing, whether the crack expanding along pre-existing crack or generation new crack depending on the complex factors such as the angle between the directions of pre-existing cracks and the maximum horizontal principal stress, horizontal stress and the maximum working pressure.Under the condition non-fractured and the given ground stress, the fracture stress are close to each other when the mechanical properties of coal is isotropic.
Keywords/Search Tags:coalbed methane, mechanical properties of coal, acoustic emission, stress conditions, breakdown pressure
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