Research On Wellbore Stability Of Complex Structure Branch Well For Coalbed

In recent years,as an efficient and clean energy,CBM production has been increasing.Because of the low pressure and low permeability of coalbed methane reservoir in China,it is often necessary to apply complex structural wells to CMB output.The main hole of complex structure wells was built in the coal seam,and the branch sections are got from sidetracking in the coal seam,but because of coal and rock in the extensive distribution of cleat microstructure,so serious accidents such as collapse and well leakage are often occurred during the construction and production process,so currently the main hole is built on the solid floor,and the branch sections are got from sidetracking in the coal seam floor into coal seam.Branch well in coalbed is easy to be instability due to the special microstructure in coal,and the junction section of the main section and branch section is also instable well,therefore borehole stability evaluation research is necessary for CMB complex branch well.Firstly,for the branch section in coalbed.According to the micro-characteristics in coal cleat,and based on the theory of elastic mechanics,establish calculation models of coalbed equivalent Young’s modulus and coalbed equivalent Poisson’s ratio,then analysis the relationship between equivalent Young’s modulus,Poisson’s ratio and coal matrix,face cleats,butt cleats mechanical parameters.With conclusion that face cleats effect more than butt cleats.Through the establishment of wellbore mechanics model,with the selected Mogi-Coulomb failure criterion and maximum tensile stress criterion,write programs by using MATLAB,to study the effect of face cleat space and face cleat horizontal angle on safe density window.Face cleat spacing increases will increase the safe density window width wellbore stability enhancement with the increase of the safe density window;face cleat horizontal angle and decreased first and then increased.By analyzing the change of ground stress caused by pressure depletion during the process of production,the change of the safety density window caused by different pressure failure is analyzed.Secondly,for the branch section in coalbed floor,this paper established multiple groups of three-dimensional geometric models,simulated by using finite element software in different junction angles,branch orientations,branch hole diameters and the distance of branch wells instability risk,optimization of geometrical parameters of well trajectory and wells,reduce the branch wells junction instability probability.Wellbore instability of complex structure well in coalbed is a complicated problem,and further study is needed.With the continuous development of energy demand and coal gas industry,solving the problem of wellbore instability is more urgent.The conclusion of this study will prevent coal gas exploitation during wellbore instability and provide an important reference.