Abstract

Stick slip and bit bounce are common problems while drilling the production sections of oil wells with Polycrystalline Diamond Compact (PDC) bits. The production section often encounters different lithology including highly fractured limestone, sand/sandstone and reactive shale. In some cases, vibrations are so severe that they cause failure in drill string components, loss of expensive tools in the hole; such as bit and logging sources/equipment, bit damage, wellbore instability and reduction in the Rate of Penetration (ROP).

The current paper presents a dynamic model and an active control strategy for reducing stick slip and bit bounce of a drill string with a top drive system. The model includes complete drive systems for both rotational and axial motions, as well as the hoisting system. Also included is the bit/formation interaction, which facilitates the inclusion of the effects of formation characteristics. First, the equations of axial and torsional motions are derived and solved numerically. The numerical results are validated with real data logs recorded while drilling the production sections in Kuwait fields. Second, feedback controllers are designed and incorporated into the model.

Simulation results show that in certain conditions the drill string may experience high values of TOB (Torque on Bit) and WOB (Weight on Bit), which can damage sensitive tools within the drill string, such as, LWD (Logging While Drilling) tools and RSS (Rotary Steerable System). It is demonstrated that implementing only a torsional controller in the system is not effective in mitigating bit bounce phenomena, and an axial controller should be designed and implemented as well. Simulation results show that it is possible to minimize both stick slip and bit bounce and maintain desired drilling conditions with the axial and torsional controllers.

It is expected that, the insight gained by the proposed model will be instrumental for determining and realizing optimal drilling conditions. Therefore, the cost related to drilling a specific well will be reduced, and the life of equipment used to drill the well, such as bit, motor, stabilizer, and drill collars, will be increased.

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