Research On Key Problems Of Mechanics For Multi-packers Fracturing Strings Pass Ability In Horizontal Well

In the process of horizontal well fracture treatment, whether the multi-packers fracturing string can be successfully tripped into the target layer and picked up is the key to success. When the fracturing string tripped in, the existence of certain friction resistance between fracturing string and borehole requires the axial load in the well head in order to ensure the fracturing string easily tripped in. because the wall of fracturing string is thin and stiffness is small, buckling deformation appears under axial load. Meantime there forms several contact point between fracturing string and casing, which causes the friction resistance increase. The more axial load increased the more contact point comes out, so that the friction resistance much harder and that requires more axial load. By the time the buckling occurs to a certain extent, the axial force can't transmit down, so that the fracturing trapped and can't be tripped in. When fracturing fluid flows back, well head pressure dropped sharply and fracturing fluid flow back to the bore hole, pressure between fractures and the wellbore fluid effects on the casing bullet hole which creating strong stress concentration. The stress concentration leads to the casing plastic deformation or unstability collapsed which causing the casing stuck during fracturing string pick up work. In order to solve the difficult problem of multi-packers fracturing strings pass ability in horizontal well, mechanics analysis of multi-packer fracturing string buckling deformation is innovative developed in this paper. We put forward a new model for friction resistance. Casing deformation research is carried out in the process of fracturing fluid flowback under the condition of different perforation combination, which provides an anticipation method for the fracturing string easily picked up. The main research work and research results can be described in the following respects.Firstly, the mechanical model of fracturing string bending deformation analysis is established according to the bending beam deflection differential equation. After analyzing the bending deflection limit value of fracturing string in the casing, this paper puts forward a new method for determine contact point which afford evidence for friction resistance calculation.Secondly, the critical load problems on the vertical interval, bending interval and horizontal interval where the fracturing string buckling is respectively researched. In vertical interval, considering the effects of the gravity of the string on fracturing string buckling, sine buckling and spiral buckling critical load calculation model of fracturing string is established according to the static equilibrium equations. In bending interval, considering the influence of wellbore bending on the gyrose deformation of fracturing string, sine buckling and spiral buckling critical load calculate model is established under different curvature radius. In the horizontal interval, considering the affection of the gravity of fracture on fracturing string buckling, sine buckling and spiral buckling critical load calculate model of the fracturing string is established on the horizontal interval. Moreover, taking into account the constrained effect of packer on the buckling of fracturing string, the mechanical analysis model of packer section fracturing string buckling is established. A computational method calculated the length of string and the constrained force of fracturing string which abuts the packer has been devised.According the deflection differential equation of flexed beam, when fracturing string occurs helical buckling, a calculation model of fracturing string section maximum bending moment is established, moreover, the calculation method of maximum bending stress in fracturing string section is achieved. Using this method, the variation rules of bending moment and bending stress in different axial force can be calculated, and providing the evidence for fracturing string intensive analysis.Moreover, considering the effect of stiffness action to friction resistance of fracturing string, the calculation model of friction resistance in curved section of horizontal well fracturing string is established according to the static equilibrium equation. Considering the normal pressure exerted from the effect of fracturing string buckle deformation, the calculation model of buckle friction resistance of horizontal well fracturing string is established, which provides a new method for fracturing string tripped in easily.Finally, analyzing the pressure of casing perforation orifice in the process of fracturing flowback, the calculation model of flowback pressure of casing perforation orifice is established. Researching the relation between the stress and the variation on casing perforation orifice by the elastoplasticity theory, the mechanical analyzing model of the relation between flowback velocity and variation of fracturing fluid is established. Analyzing the effect of different perforating combination on the casing deformation, and the correctness of the model can be verified by comparing the results of calculation and simulation. This model can be used as a prediction method for lifting to the well head of the multi-packers fracturing casing. By putting elastic mechanics theory and complex-variable function together, this paper researched stress concentration problem of casing eyehole, calculated the stress concentration factor by using finite element simulation software and analyzed different perforating combination effecting on the casing eyehole stress concentration factor problem.