Dynamic Variation Effects Of Coal Reservoir Permeability And The Response Of Gas Productivity In Southern Qinshui Basin

In order to investgate the dynamic variation of coal reservoir permeability and the dynamic response of productivity during the development process, this thesis discusses the contribution to coal reservoir permeability from physical properties, the geological and engineering factors that influence the dynamic variation of gas wells productivity and permeability, semiquantitative to quantitative characterization of changes in coal bed permeability caused by main factors on the basis of massive test datas, engineering datas and development datas analysis, various theoretical knowledge application of coalbed methane (CBM) geology and development. Then this work builds an evaluation model of the geological and engineering factors that drive dynamic changes in permeability, which in turn drive the dynamic response in terms of productivity. Finally, development proposals which are suitable to the southern Qinshui Basin are proposed. Main conclusions are listed as follows:Pore types of coal reservoir in southern Qinshui Basin are main absorption pores, and the connectivity among them is relatively poor. Meanwhile, fractures are main small ones as C and D types, and the connectivity among them is relatively poor and infilling of minerals. The development features of pores and fractures decide that the permeability of coal reservoir is generally low in southern Qinshui Basin.Main geological factors determine whether CBM wells achieve a high and stable yield, which include buried depth, gas content, critical desorption/reservoir pressure, hydrodynamic conditions, and tectonic positions that CBM wells are in. Main engineering factors include drainage system, fracturing effect, well pattern arrangement and so on. Meanwhile, by the coupling analysis of the relationship between dynamic variation of coal reservoir permeability and geological conditions and development system, it clarifies the geological and engineering factors which influence the dynamic variation of permeability during the development process of CBM. Finally, it builds an evaluation model of the geological and engineering factors that drive dynamic changes in permeability, which in turn drive the dynamic response in terms of productivity. After calculation, the absolute permeability of the coal reservoir decreases by about 10.0% during the development process due to elastic modulus, and decreases 5.5% and 14.3% at buried depths of 500m and 1000m respectively. In displacement pressure experiment, when the displacement pressure is not reach the critical value of pressure drop, coal permeability will not change. But when it reaches, coal permeability drops, and it has an exponential relationship between the pressure drop range and the displacement pressure drop.According to geological features of coal reservoir in southern Qinshui Basin, this paper recommends that the development reservoir should contain some conditions, which include buried depth at 500~700 m, gas content greater than 15 m3/t, critical desorption/reservoir pressure greater than 0.6, and far away from faults. In engineering aspect, we should use under-balanced drilling technology and improvement the drilling fluid before construction; adopt development styles of surface vertical well, horizontal well and multi-branch horizontal well, and for a whole area which has a great geological condition, we can use cluster well pattern form. During the development, this study suggest that a larger pressure drop (about 0.022MPa/d) and stroke (3.0/min) are favorable in the early drainage period, and a smaller pressure drop (0.002MPa/d) and stroke (0.4/min) should be applied when the gas peak appears.