Optimization of Performance Qualifiers during Oil Well Drilling

An optimization analysis of the drilling process constitutes a powerful tool for operating under desired pressure levels (inside operational window) and, simultaneously, maximizing the rate of penetration, which must be harmonized with the conflicting objective of minimizing the specific energy. The drilling efficiency is improved as the rate of penetration is increased, however, there are conflicts with performance qualifiers, such as down hole tool life, footage, vibrations control, directional effectiveness and hydraulic scenarios. Concerning hydraulic effects, the minimization of the specific energy must be constrained by annulus bottom hole pressure safe region, using the operational window, placed above porous pressure and below fracture pressure. Under a conventional oil well drilling task, the pore pressure (minimum limit) and the fracture pressure (maximum limit) define mud density range and pressure operational window. During oil well drilling, several disturbances affect bottom hole pressure; for example, as the length of the well increases, the bottom hole pressure varies for growing hydrostatic pressure levels. In addition, the pipe connection procedure, performed at equal time intervals, stopping the drill rotation and mud injection, mounting a new pipe segment, restarting the drill fluid pump and rotation, causes severe fluctuations in well fluids flow, changing well pressure. Permeability and porous reservoir pressure governs native reservoir fluid well influx, affecting flow patterns inside the well and well pressure. The objective being tracked is operating under desired pressure levels, which assures process safety, also reducing costs. In this scenario, optimization techniques are important tools for narrow operational windows, commonly observed at deepwater and pre-salt layer environments. The major objective of this paper is developing an optimization methodology for minimizing the specific energy, also assuring safe operation (inside operational window), despite the inherent process disturbances, under a scenario that maximization of ROP (rate of penetration) is a target.