Wellbore Stability Analysis And Technology Design Of Coal-bed Methane Air Drilling In Zhengzhuang Block South Qinshui Basin

The geological structure of Zhengzhuang block in the south of Qinshui basin is simple,and stratigraphic water content is weak,and the coal reservoir pressure is the form normal to slightly undervoltage state.The geological characteristics of coalbed methane in the study area are suitable for air drilling.This paper based on the detailed analysis of geological data,well test data and core parameters in the study area,combined with the theory of air drilling wellbore stability,studing the stability of coalbed methane air drilling in the study area,put forward the measures of the effluent in the process of drilling and design of air drilling process parameters,wellbore structure,drilling method and so on,which are favorable to the stability of the study area.The stability of the surrounding rock is mainly determined by the difference between the maximum and minimum horizontal principal stresses of strata and the mechanical parameters of the rock itself.According to the critical conditions which are calculated by the theoretical model of air shaft stability and compared the results with the rock cohesion value,we can get a conclusion that the coal seam in the air drilling process will shear failure and the upper sandstone wall can be stable.The minimum drilling fluid density that maintain coal seam stability is 0.55 0.57 g / cm³ by calculation.The shale in the study area is hard and dispersible,and the surface fissure is developed,and the hydration reaction occurs with the strata water.And the hydration expansion rate is about 13%.The elastic modulus of the rock becomes smaller,the Poisson's ratio increases,the cohesion and the internal friction angle will be reduced after hydration.The shale is become easy to break.The radial stress decreases and the tangential stress increases first and then decreases,and reaches the maximum value in the borehole wall,and the maximum value is larger than the tangential stress value of the borehole when unhydrated,which leads to the collapse of the wall,which is not conducive to the stability of the shaft the drilling wells.Using FLAC^3D for simulation analysis,the results show that the drilling wells deformation of the fluid-solid coupling is larger than that of the non-fluid-solid coupling.In the case of fluid-solid coupling,the stress value is obviously increased,and the stress concentration occurs in the minimum stress direction which leads to the collapse of the wellbore in the minimum stress direction.These are not conducive to the stability of the borehole wall.Using the theory of rock debris final settlement velocity,the rock debris produced at the bottom of the well are able to carry the wellhead completely when the air flow rate is of 30⁷0 m³ / min and mechanical drilling speed is of 8m /h.According to the stratigraphic characteristics of the study area and the working characteristics of the drilling equipment,it can able to keep air drilling well stable when the pump pressure value is between 1.4 MPa and 2.0 MPa,the bit pressure value is between 15 k N and 30 k N,the rotational speed is 30 45 r / min when drilling into soft stratum and keep 25 40 r / min when drilling into hard stratum.When the quantity of stratum flows out of the water is small,then selecting underbalanced drilling all the whole well.When the quantity of stratum flows out of the water is large,then using conventional water-based mud drilling and air drilling combined with the drilling method.