MIMO Control during Oil Well Drilling

A drilling system consists of a rotating drill string, which is placed into the well. The drill fluid is pumped through the drill string and exits through the choke valve. An important scope of the drill fluid is to maintain a certain pressure gradient along the length of the well. Well construction is a complex job in which annular pressures must be kept inside the operational window (limited by fracture and pore pressure). Monitoring bottom hole pressure to avoid fluctuations out of operational window limits is an extremely important job, in order to guarantee safe conditions during drilling. 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. In this work, a non linear mathematical model (gas-liquid-solid), representing an oil well drilling system, was developed, based on mass and momentum balances. Besides, for implementing classic control (PI), alternative control schemes were analyzed using mud pump flow rate, choke opening index and weight on bit as manipulated variables in order to control annulus bottomhole pressure and rate of penetration. Classic controller tuning was performed for servo and regulatory control studies, under MIMO frameworks.