Toolface Control is widely regarded as one of the greatest challenges when drilling directionally with a Fixed Cutter (FC) drill bit on a Steerable Motor assembly. Toolface offset is proportional to the torque generated by the bit, and by nature, FC bits generate high levels of torque. If large changes in downhole torque are produced while drilling, this will cause rotation of the drill string, and loss of toolface orientation. This results in inefficient drilling and increases risk of bit and downhole tool damage.
This paper examines the effect of varied components of a FC drill bit to determine the key design requirements to deliver a smooth torque response and improved directional performance. This includes review of the results from comprehensive laboratory testing to determine the effectiveness of a number of varied, removable Torque Controlling Components (TCC). These, in combination with specific cutting structure layouts, combine to provide predictable torque response while optimized for high rates of penetration. In addition, unique gauge geometry is disclosed that was engineered to reduce drag and deliver improved borehole quality. This gauge design produces less torque when sliding and beneficial gauge pad interaction with the borehole when in rotating mode.
Field performance studies from within Latin America clearly demonstrate that matching TCC, an optimized cutting structure, and gauge geometry to a steerable assembly delivers smooth torque response and improved directional control. Benefits with regard to improved stability are also discussed.
Successful application has resulted in significant time and cost savings to the operator, demonstrating that Stability and Steerability does not have to result in loss of penetration rate.
Introduction
Since the introduction of the Positive Displacement Motor (PDM) in the 1980's, drill bit manufacturers strived for an optimal design concept for FC bits. The primary objective is a design that can deliver high penetration rates and still give the tool face control required to efficiently deviate the well. The key factor is the difference in aggressivity required for the two operating modes of a motor assembly; sliding and rotating. A steerable motor employs a sufficient bend angle to achieve the planned trajectory in sliding mode. The relative downhole location of this bend (tool face) is held stationary by non-rotation of the string. Rotation of the bit is provided by the mud motor that converts the hydraulic energy of the mud pumped through it to mechanical energy in the form of torque and RPM output to the drill bit. The reactive torque produced by an aggressive FC bit can cause the drill string to twist unpredictably, resulting in loss of tool face. This leads to wasted drilling time associated with reestablishing the desired tool face. It may also lead to stalling of the motor, which can ultimately result in premature failure. However, in rotating mode, the bit is being turned from rotation of both the string and the downhole rotation provided by the mud motor. There are no tool face control concerns thus an aggressive design can be utilized to maximize penetration rate.
