| Coal reservoir permeability and effect of CBM development have very close relationship with geo-stress. In small stress area, coal reservoir permeability is high and CBM productivity is large. In high stress area, coal reservoir permeability is low, which causes the difficulty in CBM productivity. Productivity of CBM wells is controlled by geo-stress of coal reservoir. Therefore, research on geo-stress conditions has an important theoretical and practical significance for high permeability zones of coal reservoir and adoptting reasonable development mode. Based on the southern Qinshui basin, multi-disciplinary theories were used, such as rock mechanics, fluid mechanics, CBM development geology. Through the analysis of geological mechanics, experimental study and theoretical analysis and numerical simulation methods, the basic mechanical properties of coal were analyzed systematically. Combining with in-situ stress condition of the coal reservoirs in the study area, in-situ stress prediction model based on the fault frictional strength was established. Stress field control equation of coal reservoir considering elastic-plastic deformation of coal and seepage field control equation under the nonlinear seepage condition were explored, the multi-physics coupling model of CBM extraction of coal mining area was established. Dynamic change rule of stress field, pore pressure field, strain field and seepage field of of coal reservoir in coal and CBM development process of the coal mining area were revealed by COMSOL Multiphysics coupling finite element software. Based on the fault frictional strength, geo-stress prediction model were established and its influences on coal mining were studied. On this basis, the influence of the coal reservoir stress on productivity of CBM wells and its control mechanism were further analyzed, which provides reasonable theory basis for improving productivity of CBM wells. The main research results and knowledges are as follows:1. Based on southern Qinshui basin, uniaxial compression test of coal rock is carried out, the basic mechanical properties of coal, macro morphology and whole stress-strain of its deformation and failure process are studied, meanwhile the characteristics of acoustic emission-wave velocity in the process of stress-strain is revealed. Results show that the damage of coal rock occurs before peak strength, with the increase of stress, the wave velocity also gradually increases, the damage occurs when the coal rock comes to the peak strength, stress decreases, and wave velocity also shows a trend of decrease, which has a linear positive correlation. Introduced two new concepts of crack initiation point Sc and the fast-growing point Sq, which are the comprehensively judged in the deformation and destruction process of coal rock by the stress-strain and acoustic emission parameters-strain, the two key points provide important precursory information for the deformation and failure process of coal rock.2. Through the systematical analysis of geo-stress data of coal reservoir in the southern Qinshui basin, the relationship and computational model of the minimum horizontal principal stress, the vertical principal stress and the reservoir pressure of coal reservoir is established, geo-stress distribution of coal reservoir is revealed and geo-stress distribution of coal reservoir in Fanzhuang block is evaluated. Within the scope of the block size, geo-stress condition of coal reservoir in Fanzhuang block is divided into low stress distribution area(type I), secondary stress distribution area(type II), high stress distribution area(type III). Due to the smaller buried depth of coal seam in Fanzhuang block, permeability is relatively good, so Fanzhuang block is the favorable area for CBM exploration and development.3. Geo-stress size of coal and rock is controlled by discontinuous surface friction strength of fault and fracture. Based on the fault frictional strength theory, its control on the geo-stress is analyzed, the relationship between the ratio of maximum to minimum principal stress and fault frictional strength was established. Based on the fault classification of Anderson, geo-stress is divided into three stress states, geo-stress calculation model of normal fault, parallel displacement fault and reverse fault considering frictional strength is built up. According to systematical analysis of geo-stress data in southern Qinshui basin, two different stress mechanisms are revealed in different depth of coal reservoir in the study area, when the buried depth is less than 650 meters, the maximum horizontal principal stress is less than strata pressure(vertical stress), the original stress is in the normal fault stress mechanism; when the buried depth is more than 650 meters, the maximum horizontal principal stress is greater than the strata pressure(vertical stress),the original stress may be in the stress mechanism of parallel displacement fault, that means horizontal stress larger in the deep strata.4. On the basis of the mechanical properties research of coal, according to the principle of effective stress, Geshi calculation model of geo-stress is improved. Considering the effect of matrix shrinkage on the effective stress of coal reservoir, dynamic change model of geo-stress of CBM extraction process is established, meanwhile taking Kozeny-Carman equation as a bridge, permeability dynamic model of of coal reservoir under the action of the gas-solid coupling is set up. Through the assumptions about the coal structure, physical property, gas adsorption/desorption, gas seepage and stress conditions, stress field control equation of coal reservoir considering elastic-plastic deformation of coal and seepage field control equation under the nonlinear seepage condition is explored, the multi-physics coupling model of CBM extraction of coal mining area is established,which include: stress field control equation,seepage field control equation and definite condition. Stress field model of coal deformation is set up, which is composed of stress equilibrium equation, geometric equation and constitutive equations. Pore pressure is contained as effective stress in the model, coupling between pore pressure and coal deformation is realized. Meanwhile, seepage field model of coal reservoir is built up, which contains flow continuity equation, flow state equation and the content equation of CBM. Coal deformation is contained in the model, coupling between coal deformation and CBM seepage is realized.Therefore, multi-physics coupling model of CBM extraction more comprehensively reflect coupling mechanism of stress field, strain field and seepage field in coal and CBM development process, which provides basis for efficient coal and CBM development of coal mining area.5. Using COMSOL Multiphysics coupling finite element software, the multi-physics coupling model of CBM extraction in coal mining area is set up, which is composed of plane strain and general PDE mode. Through the research on multi-physics coupling effect of single-hole CBM extraction conditions and multiple-holes CBM extraction conditions under different coal-reservoir parameters and different working conditions, the results show that the model can simulate and calculate the change rules of stress field, pore pressure field, strain field and seepage field in the whole process of drilling CBM extraction, according to comparison analysis on the influence factors of CBM extraction, it is shown that the buried depth, porosity and permeability of coal seam have more obvious effects on CBM extraction, which is influenced smaller by the elastic modulus and extraction negative pressure. In the multi-holes CBM extraction conditions, the change rules of stress field, pore pressure field, strain field and seepage field are roughly same as that in the single-hole CBM extraction conditions. The limit extraction radius of coal seam is slightly greater than 1 meter under the multiple-holes CBM extraction condition. Subjected to extraction time and conditions, the limit equilibrium state is usually hard to reach in the actual production, so hole spacing of CBM extraction should be set within 2 meters. Based on numerical simulation study of different borehole spacings and drilling arrangements, the results show that the smaller the borehole spacing is and the more dense distribution is, the more obvious the reservoir pressure drops and the better CBM development effect is.6. Taking No.3 coal of Shanxi formation of Permian strata in southern Qinshui basin as the geological background, rock deformation caused by coal mining and CBM seepage characteristics is studied in the process of coal and CBM development. Space and time evolvement rules of upper strata stress field, plastic strain field and seepage field are preliminary explored. The results show that unloading area appears along the overburden and the floor rocks of coal goaf after the excavation of coal seam, stress concentration appears on the pillars in the open-off cut and front of working face. Plastic strain area of coal seam roof and floor mainly extends along the dividing line between stress concentration and unloading area extensions. The distribution of saddle shape is eventually formed, which has both high and convex ends and intermediate dips. Stress concentration makes the effective stress in front of working face increase, the porosity of coal seam is compressed, porosity decreases with the working face advancing. Due to coal reservoir permeability is controlled by effective stress, permeability in front of working face decreased with effective stress increasing.7. Based on Darcy’s law and planar radial flow theory of gas seepage, productivity model of CBM wells considering stress sensitivity is deduced, evaluation model of stress sensitivity influenced on productivity of CBM wells is established. Calculation of productivity changing rules of CBM Wells and stress sensitivity analysis are carried out, the results are as follows: stress sensitivity regression coefficients of permeability in No.3 coal seam of Shanxi formation in Fanzhuang block of southern Qinshui basin is 0.26~0.54 MPa-1, with an average of 0.37 MPa-1. The production reduced the magnitude of value(β)of 9 CBM wells respectively are 2.53%~15.84%, 3.00%~35.46%, 4.84%~51.07%, 2.55%~32.71%, 2.84%~34.29%, 5.15%~42.70%, 3.51%~31.56%, 5.13%~34.68% and 7.70%~40.05%. Similarly stress sensitivity regression coefficients of permeability in No.3 coal seam of Shanxi formation in Qinnandong-xiadian block is 0.099~0.115MPa-1, with an average of 0.108MPa-1. The production reduced the magnitude of value(β) of 8 CBM wells respectively are 1.13%~12.16%, 1.55%~20.02%, 0.98%~11.74%, 0.74%~11.41%, 1.07%~19.03%, 0.74%~9.08%, 1.13%~18.96% and 1.34%~17.17%. As can be seen, stress sensitivity of the coal reservoir has a greater effect on the yield of CBM wells.8. With the increase of drawdown pressure, production increasing amplitude of CBM Wells is reduced. When drawdown pressure reaches a certain value, the production of CBM wells is gradually stabilized. Especially in the preliminary stage of CBM extraction, the method using the increase of drawdown pressure to get the maximum production of CBM wells has great blindness. Due to the greater reservoir pressure and stress sensitivity of coal reservior, avoid blindly reducing the bottom hole pressure rapidly by all means. Therefore, formulating reasonable production technology is required, in order to ensure high and stable production of CBM wells. |
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