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PDC Bit Design Optimization and Applications

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Course Credit: 0.15 CEU, 1.5 PDH

This presentation focuses on our recent research work in three parts: 1) the development of a new bit-rock interaction model which is a key part in bit optimization; 2) a new method to layout primary cutters to increase drilling efficiency and stability; 3) a new method to layout backup cutters to extend bit life without sacrificing rate of penetration.

Part 1) A New Bit-Rock Interaction Model

There are three basic elements in bit optimization procedure: design goals, bit-rock interaction model and design variables. The major goals of bit design optimization include extended bit life, high rate of penetration and low bit vibration. In addition, bit steer-ability and walk are usually required to match with BHA systems in directional drilling. The design variables include local variables and global variables. The local variables are related to each cutter such as cutter location, cutter size, back rake angle and side rake angle. The global variables are related to how cutters are distributed on the bit body such as the way of cutter layout, the number of cutters and the number of blades. A reasonable bit-rock interaction model should be able to predict the changes of bit design goals when any design variables are changed. Therefore, the bit-rock interaction model is a key element in bit design optimization.

More than 30 years ago, a bit rock interaction model was developed based on single cutter tests. In this model, cutter forces were proportional to cutter engagement area (area based model). Since then, this area based model has been widely used in PDC bit design. Unfortunately, the calculated forces such as WOB and TOB from area based model could not match with lab tests and field tests, especially when global design variables were changed.

In recent years, research efforts were made to understand what have been missed in the area based model. Two important findings are:
(1) An engagement shape was used in single cutter tests. However, as explained in the presentation, there are three basic cutter engagement shapes for any PDC bit, depending on cutter’s location, cutter density and cutter layout methods. By pre-groove the rock surface to ensure the cutter engagement have three basic shapes, we immediately found cutter forces were highly dependent on the engagement shapes and Glowka’s assumption was valid only for the shape he tested.

(2) The shear motion of each PDC cutter creates rock chips in front of it, but these rock chips were not considered in previous bit-rock interaction model. In fact, the removal of these chips during bit-rock interaction simulation greatly affect the bottom hole pattern which further affects the cutter engagement area and shape. By intergrating these two findings into the new bit-rock interaction model, full bit test results match very well with model predictions.

The new bit-rock interaction model is able to calculate cutter forces, non-cutting element forces and bit forces, including steer forces and walk forces, for any given bit motion.

Part 2) A New Method of Layout Cutters to Improve Drilling Efficiency

Traditionally, cutters are laid out on blades starting from bit center and following a spiral path in bit rotational direction. This method ensures an uniform cutter distribution along bit profile. However, there are two problems related to this method. The fisrt problem is the violation of force balanced condition in transit formation drilling, especially from soft to hard formation transition. A nose cutter alone may cut into the hard formation and may be damaged first. The second problem is the low drilling efficiency as explained in the presentation.

A new cutter layout method based on force-balanced cutter groups are presented. In this method, two or three neighbor cutters form a force balanced group. Cutters are laid out in groups and may start from any location on bit profile. Further, the method ensure any three or four neighbor cutters (depending on the number of blades) along bit profile are force balanced. Model results, lab test results and field applications have confirmed that the new cutter layout may improve drilling efficiency significantly.

Part 3) A New Method of Layout Backup Cutters to Extend Bit Life Without Sacrificing ROP

PDC bit having backup cutters or dual row cutting structures have been used in the industry for decades. The major reason of using backup cutters is to extend bit life. However, after an extensive study of dull conditions of dual row PDC bits, it is found that the backup cutters may have been used unwisely.

There are two challenges in the layout of backup cutters. The first challenge is how to ensure the backup cutters do not cut or only partially cut into formation during drilling when primary cutters do not experience wear or little wear. The second challenge is how to ensure the backup cutters do act as major cutters when the primary cutters experience certain wear.

Model results and field test results have shown that a PDC bit’s life may be significantly extended by backup cutters if the following two conditions are met.

1) all backup cutters have the same critical depth of cut when the primary cutters do not experience wear, so that when the actual depth of cut is over the critical depth of cut, all the backup cutters will engage formation simultaneously;

2) any backup cutter should be located rotationally behind its primary cutter an angle of approximately 150 ~ 250 degrees.

Several case studies are provided in each part of the presentation.

Major References

(1) Chen S., Arfele R., et al. 2013. A New Theory on Cutter Layout for Improving PDC Bit Performance in Hard-and Transit-Formation Drilling. SPE Drill and Comp 28 (4): 338–349.
(2) Chen S., Grosz G., et al, 2015. The Role of Rock-chip Removals and Cutting-area Shapes in Polycrystalline-Diamond-Compact-Bit Design Optimization, SPE Drill and Comp, Dec.,2015, p 334–347.
(3) Chen S. Khlefat Y et al, A New Method of Layout Back-up Cutters to Extend Bit Life without Sacrificing Rate of Penetration, SPE 187644. The paper is prepared for presentation at the SPE Kuwait Oil & Gas Show and Conference held in Kuwait City, Kuwait, 15-18 October 2017.

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Course Chapters

  • 1PDC Bit Design Optimization and Applications - Chapter 1
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Credits

Earn credits by completing this course0.15 CEU credit1.5 PDH credits

Speakers

Dr. Shilin Chen