Research on Managed-Pressure Running Casing in Oil and Gas Wells with the Negative Pressure Window

The failure of managed-pressure running casing in oil and gas wells may lead to complex accidents such as overflow or leakage. The technique of using multi-density gradient drilling fluids in wellbores with negative pressure windows (NPWs) is often used to deal with this situation. Therefore, it is vital to analyze the dynamic slurry column structure and calculate the wellbore pressure during casing running. For this issue, the model of transient surge pressure is established during casing running. The calculation equation of the model is proposed, and the calculations of the wellbore pressure are carried out with the exploration of Well LT-X1, located in the Xinjiang oil field. A circulation scheme is designed as follows: Circulate 125 m(3) of drilling fluid with a density of 2.45 g/cm(3) and 155 m(3) of drilling fluid with a density of 2.35 g/cm(3) at a depth of 3560 m. From there, circulate 164 m(3) of drilling fluid with a density of 2.35 g/cm(3) at a depth of 5900 m. Finally, at a depth of 7050 m, circulate 250 m(3) of drilling fluid with a density of 2.30 g/cm(3). The casing running speeds and back-pressure values were designed as follows for the respective well sections: 0-1523 m: 0.160 m/s casing speed, 0 MPa back pressure; 1523-3560 m: 0.160 m/s casing speed, 1.641 MPa back pressure; 3560-5900 m: 0.145 m/s casing speed, 2.427 MPa back pressure; 5900-6674 m: 0.137 m/s casing speed, 4.041 MPa back pressure; 6674-7050 m: 0.124 m/s casing speed, 4.457 MPa back pressure. The results show that optimizing structure of the multi-density gradient drilling fluid with different densities and applying annular back pressure in stages, with the accurate calculation of wellbore pressure, can achieve the goals of leak-proofing and pressure-stabilization. It is concluded that this result may serve as the foundation for managed-pressure running casing technology.