1. Whirl - General 4. Stick-Slip - General 2. Whirl – Bit Aspects 5. Stick-Slip – Bit Aspects 3. Whirl – Specific Bit Designs 6. Axial Vibration - General
D&M—NSA
rilling
D E
C ngineering
enter
Schlumberger Private
Stability Aspects
Bit Whirl – General
A) Mechanism: Bit whirl is the rotation of the bit off its natural center of rotation in an eccentric manner. Resultant from a combination of excessive side cutting forces and frictional contact with the gauge, the center of rotation moves and creates a momentary center of rotation at a gauge pad of the design. As the bit rotates about this new center, the next blade impacts the wellbore and the center of rotation moves to this point. As this continues, a lobed pattern develops in the wellbore and essentially you obtain a gearing effect between bit and wellbore.
The number of lobes generated is a factor of the number of blades of the design plus one e.g. for a six bladed design you would observe either 7, 13, 19 lobes etc. Whirl type vibrations are generally high frequency, in the range of 10-50 Hz.
B) Identification: This falls into two categories; Identification from parameters and downhole tools whilst drilling, and post evaluation of the drill bit and BHA.
• Increased surface torque
• Large downhole lateral shocks
• Over gauge hole
• Cutter damage on shoulder and gauge
• Blade breakage (worst case)
• Uneven stabilizer wear
-4
7 Lobed pattern for 6 bladed design
Schlumberger Private
C) Effects:
• Reduced bit life = Increased trips
• Extreme bit damage may require junk recovery / milling trips
• Undergauge bits resulting in reaming requirement
• Reduced penetration rates = more rig time
• Poor borehole quality leading to logging / casing issues
• Damage to downhole tools & other components of the BHA
D) Cures: This falls into two categories; Drilling parameters and bit selection.
In terms of parameters, a combination of high RPM and low weight on bit will increase whirl tendency. This is due to the fact that a low depth of cut is established and thus the bit is more susceptible to moving off its natural center of rotation. Gradually increasing weight at a specific rpm will enable you to optimize parameters to combat whirl.
Bit selection will be covered in the following sections.
Schlumberger Private
Stability Aspects
Bit Whirl – Bit aspects
With bit design, there are a number of features which may be found in ‘standard’ PDC bit designs that will aid reducing potential to whirl. Additionally, a number of bit
manufacturers employ concepts so that the overall design is specific to applications where lateral vibration is anticipated.
The general whirl reducing features are as follows:
i) Tracking cutting structure – This is when one or more cutters are positioned in the bit directly tracking a primary cutter i.e. tracking cutters theoretically remove the same formation as the primary cutter. This results in the formation of a distinct borehole pattern, which is grooved or scalloped due to these multiple cutters following the same rotational path. In the scenario that the bit is moved laterally, the cutters bite into these ridges and create a restoring force greater than that pushing the bit laterally. Thus the bit retains rotating around its natural geometric center.
ii) Spiral Gauge – As with spiral blades on stabilizers, the spiraling of gauge pads will increase the contact area between bit and borehole when rotating and thus aid dampening of any initiated bit whirl. The gauge pads should be low friction i.e. gauge protection should be flush with the pad, and ideally, the lead and back angles of the gauge should be chamfered or radiused. Similar approaches have been used with long gauge lengths though of course this leads to detriment in terms of steerability.
iii) Asymmetry – A symmetrical bit design is one that has a blade layout with equal angular spacing between them i.e. a four bladed bit each set at 90 degrees to each other.
This symmetrical layout will fit into a lobed borehole in a regular manner, thus creating a low frequency periodicy that will self perpetuate whirl once initiated. A design that is asymmetric does not possess this regularity and will thus cut into the lobed borehole, forcing the bit to return to its natural geometric center.
A general estimate of a bits asymmetry can be gained by simply looking at a plan view of the design. Obviously, the most likely candidates for symmetrical bits are those
possessing an even number of blades (with exception of a three bladed bit). The designs that are most asymmetric are those termed as ‘porcupine bits’ where cutters are placed irregular on the bit face with random angular positions. These bits are very stable laterally.
iv) Force Balancing – When a PDC bit is drilling, each cutter generates a cutting force in a specific vector. The total forces of the drill bit are a summation of all these individual cutter forces. If, for example, the forces from the cutters on all opposing blades were identical, forces would cancel each other out and the imbalance force would be equal to zero. In a realistic world, this is not the case, thus bits will possess an imbalance (or Out-of-Balance) force, which is usually expressed as a percentage of the weight of bit. This
Schlumberger Private
out of balance force will also have a direction associated with it and is perpendicular to the bits axis.
It is generally accepted that the closer to zero the out of balance force is, the lower the propensity to whirl, though magnitude of whirl related to levels of out of balance force have never really been proven. Additionally, it is the general practice to calculate this force when the bit is theoretically drilling around its geometrical center, which is rarely truly the case when you consider the actual drilling process, particularly the scenario when using a steerable assembly with bends ranging up to 3 degrees. Factors such as formations, parameter changes, and cutter wear must also all be taken into account, along with the bit suppliers modeling software which is NOT uniform through the industry.
Thus if you consider the above, the out of balance force value is only valid for a very specific scenario and can only be benchmarked against other values issued by that bit supplier. As such, you should deeply consider how much emphasis should be placed on the quoted out of balance force when selecting a bit for an application.
Although a common concept surrounding the design of PDC bits, this concept is not shared with that of roller cone bits, as they are commonly unbalanced, primarily due to two key reasons:
• Most soft to medium formation bits are designed so that the teeth on one cone intermesh with that of another for effective cleaning and to entirely cover the bottom of the hole when drilling. As such, the cone shape and number of teeth will vary. Due to this intermeshing, any change to one cone in an attempt to balance the bit will require changing the other cones, which may counteract the balance.
• Cone offset and skew applied to improve gouging in soft formations has an increasing effect on bit imbalance forces. Reducing skew and offset of the cones will have a negative effect on penetration rates.
Due to these reasons, force balancing is not a major concern regarding roller cone bits though Security DBS have just released the ‘Energy Balanced Series’, which looks at both force and volume balancing roller cone designs.
v) Profile – It has been observed in many technical papers (in particular those of Brett and Warren) that a flat, short profile design is more stable due to the fact that it is less sensitive to lateral displacement than a longer, tapered design. A deep cone will also aid stability, as there is an increased amount of formation which would have to be overcome prior to lateral movement occurring. Note, this also could have a negative affect on steerability.
vi) Cutter back rake – An aggressive cutting structure on the shoulder and gauge can lead to a cutting surface that has the tendency to ‘grab’ the formation and initiate whirl. Less
Schlumberger Private
aggressive back rakes (>20 degrees) and / or the use of chamfered cutters will reduce this tendency.
vii) Secondary components – Components located (and usually directly tracking) the PDC cutter known as hybrids, impregs, impact arrestors, etc (depending on bit supplier).
These act as additional contact points as they are usually set just below the tip profile of the cutting structure, thus dampening and aiding stability. Additionally, due to their location behind the cutter and that they are separate from the cutter body, they will also take the impact of any backwards whirl motion and protect the cutters themselves.
Schlumberger Private
Stability Aspects
Bit Whirl – Specific Bit Designs
Bits designed specifically to limit whirl are as follows. Note that these may also incorporate one or more of the prior features, though their overall concept revolves around minimizing whirl potential.
i) Antiwhirl Designs – This technology involves a specific layout of the face cutters and the use of a large, low friction gauge pad. The cutter layout is such that the sum vector of all the cutter forces is directed to this low friction pad, causing the bit to slide at the borehole wall and thus not walk around the hole (causing whirl). Due to the pad width, large variations in cutter forces can be observed without the imbalance force moving off the pad.
The primary drawback is that the low friction gauge is created by removal of cutting elements at both the gauge and shoulder thus producing a significant cutter devoid area that is prone to durability issues. Several of the bit suppliers have approaches to solving this downside via both geometry and cutter placement.
The primary development of Antiwhirl bits was by Amoco, resulting in cutter placement software, which was licensed, to a number of bit manufacturers. The Antiwhirl theory relies on good depth of cut and is thus best suited for rotary assemblies.
ii) Steering Wheel - This was created by, and is specific to Hycalog PDC bits. It involves the creation of a ring of bit body material at gauge to provide 360 degrees circumferential coverage. This centralizes the bit, restricting lateral movement and a reduction in torque fluctuations (thus also good directional bit for PDM). Latter testing revealed that a partial ring provides similar levels of stability with the benefit of increased bit face volume and