Structure Parameters Optimization Of Special Tools And Extracting Ability Study In Directional Well

In the process of extracting casing in directional well, for increasing the compatibility between washover string and casing, washover head with shaft sleeves, centralizer with shaft sleeves and variable direction joint(VDJ) are developed. On the basis of experience, there is still some issues left:(1) the structure of special extracting tools needs to be understood quantificationally by reasonable theory: with too big or small VDJ bending angel, washover head may cut casing off from bottom or top. Reasonable VDJ bending angel is lack of theory to calculate quantificationally; the contact between casing and centralizing blocks on the internal shaft sleeves will generate large contact stress which will affect casing. So it’s necessary to optimize the shape and number of centralizing blocks, and its other parameters. The compatibility between different string assembly and casing damage well can’t be determined: for designing string assembly, we need quantificationally calculate extracting string limited depth, its suitable inclined angel and borehole curvature. Considering above, it will creates more extra work, and brings more difficulty to assembly optimization. Unreasonable construction parameters: unreasonable drill pressure and delivery capacity will slower the process of extracting casing, and lead pressure build-up or some other engineering accidents. So we need establish the whole construction parameters which matche with extracting string.For the above questions, through analyzing the effect of casing, centralizers, the contacting points between washingover tubing and well wall, and variable direction joint on the axial force conducting, this paper establishes an variable force model for describing the compatibility between washover string and casing. And, through simulating the contact stress of washover string-centralizing block-casing combination, the optimization design for the bending angle of variable direction joint is achieved. Moreover, considering that the surface contacting between centralizing and casing can reduce the contact stress, this paper proposes an idea of a cambered centralizing block replacing the original one. Based on the calculation results of variable force, this paper designs the cambered area, cambered length and central angle. Meanwhile, because the existence of centralizing block can reduce the cross section of workover fluid, the jet velocity will increase as a result. There is no doubt that the increase of jet velocity can enhance the migration efficiency of cuttings. Using the numerical simulation method, the effects of centralizing block width and length on the workover fluid’s velocity are analyzed. Based on the analyzing results, the width and length of centralizing block is optimized. Furthermore, through establishing the limit depth calculation model under the condition of casing yield failure not happening, the extracting limit depth, the reasonable range of inclined angle and borehole curvature in casing damage well is obtained. Finally, through establishing the calculating model of bit weight, torque, annular displacement and drilling rate, an construction parameters matching with washover string is achieved.There is no doubt the research results provide an important theoretical guide for choosing the washover string. And, it plays an important significance for the safe extracting casing operation.