In hole sizes larger than 349 mm, standard 279 mm drill collars create excess annular clearance and have insufficient axial stiffness for optimal deviation control. Drilling 444 mm surface holes in hard, highly dipped formations often require drilling a smaller diameter pilot hole and reaming to full gauge. Recognizing this shortcoming, Gulf Canada has developed and successfully deployed two large diameter drill collars on a number of foothill wells over the past two years.
The paper presents some of the basics of deviation theory and bottom hole assembly design with reference to Woods and Lubinski. It details the development and field trial of a 381 mm drill collar and the subsequent purchase and application of a 356 mm collar. In addition bottom hole assemblies are recommended and future applications are discussed.
At many foothills locations exists the unfortunate combination of large diameter surface hole (444 mm) and hard, highly dipped formations (20 ° - 40 °). The largest "off the shelf" drill collar available is 279 mm. When applying sufficient weight to drill, these collars do two things:
Deflect further due to columnar loading
Lay on the lowside of the hole due to diametrical clearance and their own weight.
The net effect is to direct the bit away from vertical. This problem is greatly compounded when high formation dips are prevalent, since bits tend to drill perpendicular to bedding planes. This combination usually results in piloting the surface hole with 311 mm bits and reaming out to 444 mm before setting surface casing. The 311 mm bit requires less weight, has less tendency to drill perpendicular to formation strata and the collars used are much closer to hole diameter. However reaming to 444 mm can double the time spent drilling surface hole with additional costs ranging to $150,000 (10 days @ $15,000/Day). There appeared to be a substantial cost saving available with properly sized and applied downhole equipment.
Well bore deviation is caused by factors such as drill string flexibility and forces acting on the bit due to loads applied to the bottom hole assembly (BHA).
In the early 50's Henry Woods and Authur Lubinski1,2,3,4 proposed their pendulum theory based on the following assumptions:5
The bit is like a ball and socket joint, free to turn but laterally restrained.
Drill collars lie on the low side of the hole and will remain stable on the low side of the hole.
The bit will drill in the direction in which it is pushed, not necessarily in the direction in which it is aimed.
Using mechanics of solids principles, a force balance can be drawn for a typical pendulum assembly. (See Figure 1) Resolving the Forces around the bit into the four basic components:
Axial Load - an alternative expression for weight on bit (WOB). This force is the most easily controlled.
Pendulum Force - generated by gravity attempting to pull the unsupported collars back to vertical. In general, this the only force available to drop angle in a wellbore.
