Control Methods Wellbore Deviation
One of the most fundamental objectives in drilling is to provide a usable wellbore. A key to meeting this objective is to control borehole deviation, minimizing abrupt changes in hole angle so as to allow for running downhole tools, setting casing and avoiding costly future interventions.
There are three basic techniques for controlling wellbore deviation:
- Use a pendulum assembly, consisting of the bit, drill collars and strategically positioned stabilizers, to decrease hole angle.
- Use packed hole assemblies, consisting of reamers, short collars and stabilizers, or square drill collars, to “lock in” the bit and maintain a constant hole angle.
- Use directional drilling tools (e.g., downhole motors with bent housings).
The use of directional drilling tools more properly falls under the category of controlled directional drilling. We mention it here, however, to emphasize its use as a “straight hole” drilling method, and to point out that it is the most effective of these three techniques for controlling hole angle and direction. In fact, in many drilling applications, it has superseded the other techniques. Still, there are situations where it may be more cost-effective to use pendulum or packed hole assemblies (e.g., land locations where drilling characteristics are well established).
Pendulum Assembly
Strictly speaking, a pendulum is that part of any bottomhole assembly between the bit and the lowest point of contact between the drill collars and the borehole wall. In practice, the term pendulum assembly refers to a drill collar arrangement in which a stabilizer centers the drill collars in the wellbore at some point above the bit. When the stabilizer is correctly placed, the pendulum length will be substantially greater than it would be in an unstabilized BHA, thus increasing the restoring or straightening force (Figure 1(a)).
Because the pendulum assembly’s weight and angle of inclination determine the amount of straightening force, and because the weight is directly proportional to the length of the collars forming the pendulum, it is extremely important to properly place the stabilizer.
- Placing the stabilizer too high causes the drill collars to contact the hole wall between the bit and the stabilizer, near the point where they would have contacted if no stabilizer was used (Figure 1 (b)). In other words, the stabilizer has no effect on deviation.
- Placing the stabilizer below the optimal point (but above the point where the unstabilized collars contact the hole wall) results in some less-than-maximum restoring force.
When the optimal placement point is unknown, it’s best to lean towards a location closer to the bit.
A second stabilizer (or a reamer), inserted at any point between the bit and pendulum stabilizer will reduce the pendulum force, because it reduces the length and, consequently, the weight of the pendulum. Additional stabilizers can be located above the pendulum stabilizer, however, without affecting the pendulum’s restoring force.
Along with using stabilizers, we can increase the restoring force of a pendulum assembly by lowering the weight-on-bit. This works to reduce buckling and raise the point at which the drill collars contact the borehole wall. The problems with reducing bit weight, however, are that it tends to reduce the penetration rate, and that it increases the tendency for doglegs to form (Figure 2, Effect of bit weight reduction on hole deviation tendencies).
Another way to increase the restoring force is to use larger diameter drill collars. This increases the stiffness of the bottomhole assembly and provides more weight-per-foot to the pendulum. All of these techniques can be used singly or in combination.
Packed Hole Assembly
Figure 3 shows several adaptations of the packed-hole assembly for formations with varying crooked hole tendencies.
In each case, the assembly employs large-diameter drill collars and multiple stabilizers to provide added stiffness to the bottomhole assembly and thus reduce deviation tendencies. Obviously, designing such an assembly requires a good knowledge of local drilling characteristics.
problems that can accompany the use of large-diameter tools in packed-hole assemblies include increased torque, a reduction of the available weight-on-bit, accelerated hole erosion, and small annular clearances, which could present problems in case fishing becomes necessary.
Directional Drilling Tools
It is normal to think of directional drilling in the context of reaching out some horizontal distance from a well’s surface coordinates. But the same tools and procedures used for drilling extended-reach wells may be employed in “crooked hole country” to control wellbore deviation.
