Research on vertical structure stability before cementing ultra-deep wellbore by drilling method

To address the problem of vertical stability of ultra-deep wellbore by drilling method, firstly, the results of the equation of critical height of equal section for non-full counterweighted water drilled wellbore are compared with the results of the equation of critical height of the wall of a variable section drilled well; then, calculated the vertical stability of Longgu and Banji wellbores, and investigated the changing law of critical height of wellbores under different self weight of wellbores and height of counterweight water; finally, taking a mine drilling method wind well as an engineering prototype, using numerical computation linear buckling analysis method, calculating and analysing the change rule of vertical stability of the wind well wall structure in different cases of wall self-weight loss and counterweight water height. The results of the study show that the height of counterweight water must be increased with the decrease of the self-weight of the well wall, if the self-weight of the well wall decreases by 2 kN/m³, in order to meet the requirements of suspension sinking, H_cw1 and H_cw2 are increased by 15% and 14%, respectively. Since the center of gravity of the wellbore is higher than the actual variable section wellbore when calculated by the equal section theoretical formula, the result of the equal section theoretical formula is larger than that of the variable section formula, and the result of the equal section theoretical formula for the calculation of the Longgu and Banji wellbore is on the conservative side. Reducing the self-weight of the well wall and appropriately increasing the height of the counterweight water can increase the critical height of the well wall, with the reduction of the self-weight of the well wall, the critical height of the well wall shows a non-linear trend of increase, and the two ways complement each other and gain each other, which can effectively enhance the longitudinal stability of the wellbore. Numerically computed linear buckling analysis can effectively simulate the morphology of the deflection curve and the instability state of the wellbore before cementing by the drilling method, and the eigenvalue of the 1st-order buckling of an air shaft in a mine was increased by a factor of 1.8 after a 20% weight reduction of the well wall. There is a significant difference in the change path of Mises stress with wellbore depth under different weighted water heights. For the vertical stability of the wall structure, the larger the weighted water height is not the better, and the different change paths of stress with wellbore depth under different weighted water heights are the reasons for the optimal weighted water height of ultra-deep wellbore of the drilling method. The results of the study can provide technical support for the subsequent industrial test project of drilling wells by drilling method.