Effect of fracturing on transient pressure fluctuation of tubing in ultra-deep well

Abstract

The tubing is prone to failure during the fracturing process because of the high pressure and massive flow of the fracturing fluid. The fast change in pressure and velocity inside the tubing caused by an instantaneous shift in the flow boundary of the fracturing fluid can lead to tubing failure or possibly fracture, which poses a major risk to the integrity of the wellbore. In the process of high pump pressure and large displacement fracturing in ultra-deep wells, the calculation model of fluid hammer in the fracturing string is constructed in this article in accordance with the instantaneous pump stop condition. The quasi-dynamic boundary conditions of fracturing fluid are also considered. It is discovered how wellhead pressure is affected by pump stop time and fracturing fluid displacement. In this paper, the model is verified based on the field fracturing data of an ultra-deep well and the error between the calculated value and the field value is 1.04%. The simulation results show that the wellhead pressure declines once the pump is turned off, fluctuates close to the equilibrium pressure value, and the magnitude of the fluctuation steadily shrinks until it reaches the equilibrium pressure. The difference between the peak pressure and the stable value is within 5 MPa, and the difference is 2.61 MPa under the fracturing condition of the example well in this paper. The shorter the pump shutdown time, the earlier the inflection point appears, and the greater the pressure mutation value. In the five groups of pump stop time set in this paper, when the stop time is 2.5 s, the peak pressure can reach 80.35 MPa, which is 24.77 MPa higher than the peak pressure when the pump stops for 12.5 s. Proppant content combined with appropriate wellhead pump pressure can reduce the wellhead pump stop pressure under the premise of supporting the formation fracture is not closed. In addition, when the proppant content in the fracturing fluid is high, the additional axial force on the tubing is large and the fluctuation is advanced.