Dual String Section Milling Tool Design Optimisation Using Advanced Numerical Simulations

Abstract

Well Intervention Plug and Abandon (P&A) operations play an important role at the end of life cycle of a subsea well in safely dismantling the well such that the well no longer poses any environmental hazards. P&A operations for subsea wells require sophisticated and efficient downhole tools that can operate at downhole pressures safely and reduce Non-Productive Time (NPT). A Dual String Section Milling (DSSM) downhole tool removes two adjacent casings in single trip reducing NPT while conducting safe P&A operations is discussed in this paper. This paper’s objectives are two fold first is to utilize advanced computational fluid dynamics (CFD) to improve DSSM tool life span for safe and economical P&A operations and second is to develop methodology for a quick and reliable solution for field personnel by combining CFD results with known closed form hydraulic models. The DSSM tool must maintain a certain pressure drop and flow range to achieve optimum performance. Higher flow rates and pressures can reduce tool life and cause erosion/washouts due to higher flow velocities. To address these issues simultaneously numerical simulations are conducted to optimize the DSSM tool design and operating parameters. To prove validity of numerical results CFD simulation outputs are compared against available test data and a good match is observed. Three dimensional CFD simulations are conducted using pressure based algorithm for numerically solving Reynolds-Averaged Navier-stokes (RANS) equation using second order upwind discretization scheme and k-ε turbulence model. Extensive CFD results are utilized in generating flow coefficients for closed form hydraulics models to formulate pressure drop versus flow rate charts for field usage. CFD simulations are conducted for flow rates between 100 gpm to 700 gpm to understand flow characteristics and pressure drop distribution within DSSM tool. CFD simulations showcased the locations prone to washouts; particularly washouts were observed around 90 degrees bend angles. The washout locations indicated by CFD analysis matched very well against the available test data. The DSSM tool design was then optimized using CFD simulations such that fluid velocities were below washout velocity and also higher annular velocities were maintained to alleviate hole cleaning problem during milling operations. The pressure drop versus flow rates curves obtained using flow coefficients showed correct trends with higher pressure drop for higher flow rates and higher pressure drop for reduced nozzle size. Thus utilizing CFD analysis, optimization and laboratory testing, service life of DSSM tool was increased, thus saving substantially on field NPT.