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Abstract Toolface Control is widely regarded as one of the greatest challenges when drilling directionally with a FC (Fixed Cutter) drill bit on a Positive Displacement Motor (PDM).
Toolface offset is proportional to the torque generated by the bit. FC bits, by nature, generate high levels of torque. If an external force acting on the bit causes a FC bit to over-engage, a large change in downhole torque is typically produced, which causes rotation of the drill string, and loss of toolface orientation. It is therefore desirable for a FC bit to produce a torque response that does not vary greatly with changes in the external forces applied.
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 evaluation of the results from a comprehensive series of laboratory tests to determine the effectiveness of a number of varied, removable Torque Controlling Components (TCC). The goal was to establish a configuration that would provide predictable torque response to applied weight on bit, allowing cutting structures to be independently optimized for overall higher penetration rates.
A novel gauge geometry was engineered to reduce drag and deliver a smoother borehole. This would provide less torque when sliding and beneficial gauge pad interaction with the borehole when in rotating mode.
Field performance studies 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 within rotary vertical applications are also demonstrated. Successful application has resulted in significant time and cost savings.
Introduction The introduction and development of the PDM marked the way forward for directional drilling in the oil industry 1,2,3. Since the early 1980's, drill bit manufacturers have sought to provide FC drill bits that could provide optimal performance in both the slide and rotate modes of the directional drilling operation with steerable motor assemblies. One such innovation was the addition of unaggressive secondary cutting elements, known as Hybrids, that reduced lateral vibration and torque fluctuations. However, the requirement of the drill bit, particularly in terms of aggressivity, differs notably between the two modes; In rotating mode, the drill bit is being turned from both rotation of the drillstring and the downhole rotation generated by the PDM. Relatively aggressive designs can be used to optimize ROP because there are no toolface concerns in this mode. When sliding, the PDM is rotating the bit downhole without any rotation of the drillstring from surface. This allows for the required toolface to be held stationary to attain deviation. In this mode, the reactive torque generated by an aggressive FC drill bit can cause the drillstring to twist unpredictably, resulting in loss of toolface.
The variance in the hole diameter and geometry in rotate compared to slide mode (as a factor of the bend in the motor) will also place different demands on the gauge geometry of the bit. When rotating, the drill bit will drill an oversize hole and this will result in the gauge closest to the face of the bit rubbing against the side of the borehole due to the bits spiral motion around the hole. However, in slide mode, the contact on the gauge typically shifts to the gauge pad furthest from the cutting structure. In both cases, this can lead to drag and increased torque. A specific gauge geometry configured to the demands placed in both slide and rotate mode would lead to improved borehole quality, smoother torque response, and improved directional performance.
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