Safety Evaluation of Tubing Strings Under Extreme Fracturing Conditions in Ultra-Deep Wells

Abstract

High pumping pressures and injection rates during acidizing and fracturing can threaten wellbore integrity in ultra-deep wells. Transient loads caused by abnormal events such as bottomhole blockage further increase the risk of tubing-string failure. This study develops a transient water-hammer model to describe pressure evolution after bottomhole blockage. The predicted pressure response is coupled with additional axial loads to establish a multifactor safety evaluation method for tubing strings. The method is applied to a representative ultra-deep well design case. Bottomhole blockage produces pronounced pressure oscillations, and the peak pressure increases with the injection rate. As the injection rate increases from 2.5 to 4.5 m3/min, the maximum pressure fluctuation increases from 18.70 to 35.41 MPa, corresponding to an increase of 89.4%. At 4.5 m3/min, the maximum pressure fluctuation reaches 35.41 MPa, and the maximum additional axial force increases by 87.0% compared with that at 2.5 m3/min. The simultaneous increases in internal pressure and axial load raise the equivalent stress and reduce the internal pressure, tensile, and triaxial safety factors in critical deep sections. Bottomhole blockage should therefore be included as an extreme design condition when selecting injection rates and verifying tubing-string strength in ultra-deep wells.