Soft formations interbedded with hard stringers create a dilemma when selecting a PDC bit to economically complete a section.
The primary objective, particularly for deep-hole drilling, is to complete a section in one bit run to prevent loss of time tripping in and out of the hole.
The selection becomes increasingly difficult when the hard stringers are unpredictably scattered throughout the section. Impacting a hard stringer with an unsuitable drill bit or drilling parameters can lead to catastrophic failure of components.
This can ultimately result in the need to pull out of the hole and change the bit or severely impair bit performance for the rest of the section.
The worst case for this to occur is at the start of a run when only a short interval has been drilled. Catastrophic failure through hard stringers may be caused by the initial collision of the cutters on entry or the increased vibration of the shoulder and gauge cutters when leaving the hard rock.
A known cause of premature bit failure is bit whirl, the backwards whirling motion of PDC bits.
This can be initiated by bit vibration or irregularities in the borehole wall which may cause a component on a bit to grab the formation.
This can result in a PDC bit rotating about an axis which is not its design axis, producing bit whirl. The cutting structure of conventional antiwhirl designs creates an imbalance force that is directed onto a low-friction area of a bit.
The low-friction area cannot grab the formation, so bit whirl cannot be initiated. This drilling theory has been used in gun drilling for many years.1
The approach taken to provide a solution to drilling soft formations interbedded with hard stringers is to combine PDC technologies suited for soft-formation drilling with those for hard-formation drilling.
Typical sonic values for the sandstones, claystones, and shales are 100 μs/ft, decreasing to 80 μs/ft, and in the harder stringers, 50 to 65 μs/ft.
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When drilling in hard-rock applications or through the hard stringers of interbedded formations, penetration rates drop, leading to a low depth of cut.
With conventional PDC bits, this frequently results in highly destructive whirl.
When bit whirl occurs, the cutters are not engaging the formation uniformly, leading to highly inefficient drilling and dramatically reduced penetration rates (ROPs).
This phenomenon often leads to premature cutting structure breakdown as well. The end result is slow and short bit runs that increase the cost of drilling the well.
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