PREVENTION OF DEVIATION

The pendulum effect is the tendency of the drillstring to hang in a vertical position because of the force of gravity. If the hole deviates from the vertical, the bit and drill collars lie on the low side of the hole and seek to return to vertical unless an opposing force prevents them from doing so.

Three forces are at work on the bottom of the drillstring to restore the pendulum to a vertical position:

 The pendulum force supplied by the weight of the drill collars between the bit and the first point of contact with the borehole wall, called the point of tangency (Figure 55). The higher the point of tangency, the longer the pendulum and the greater the tendency of the drillstring to return to vertical.

 The axial load supplied by the weight of the drill collars, which affects the pendulum force. A greater load causes the bottom of the string to bend closer to the bit. The point of tangency is lower, and the pendulum force is reduced.

 The formation resistance to the pendulum force and the axial load. The formation resistance is a combination of two forces – one parallel to the hole axis and another perpendicular to the hole axis.

Figure 55: Pendulum Force and Formation Resistance

When equilibrium exists (that is, the pendulum force equals the formation resistance), the hole drills straight, though inclined. If the pendulum force is greater, the hole angle decreases. If the formation resistance is greater, the hole angle increases.

7.3.2 Pendulum Assembly

Working on the pendulum effect principal, the pendulum bottomhole assembly is typically used for drilling soft, unconsolidated formations in the surface hole when fast penetration rates can be maintained while running a lighter bit weight. A pendulum assembly can also be used as a corrective measure to reduce angle when deviation exceeds the set maximum.

When the pendulum assembly is composed of just bit and drill collars, it is typically known as a slick assembly (Figure 56).

Figure 56: Pendulum Assemblies—Slick and with Stabilizers

The assembly may also include one or more stabilizers installed in the drill string. For maximum pendulum force, one stabilizer is positioned as high as possible above the bit without allowing the drill collars between the stabilizer and the bit to touch the borehole wall. This stabilizer controls deviation.

A second stabilizer can be added higher up the drill string to reduce lateral force on the first stabilizer and prevent it from digging into the borehole wall.

Use of a pendulum assembly does not guarantee prevention of doglegs. Even when there is equilibrium, the pendulum assembly is free to move from side to side in a washed-out, soft formation until lateral movement is stopped by the drill collars when they come in contact with the borehole wall.

7.3.3 Packed-Hole Assembly

Wells are commonly drilled with some type of packed-hole assembly, because it allows maximum weight to be applied to the bit for faster penetration.

A properly designed packed-hole assembly has several benefits:

 Reduces the rate of hole angle change, thereby preventing doglegs.

 Improves bit performance and bit life by forcing it to rotate on its true axis.

 Improves hole conditions for drilling, logging and running casing.

 Allows more drilling weight to be applied when drilling crooked-hole formations (that is, formations known to cause deviation problems).

Figure 57: Packed-hole Assemblies

Characteristics of the packed-hole assembly include:

 Three-point stabilizer placement to ensure a straight course is maintained by the bit.

 Stiffness in the assembly is provided by using drill collars of maximum possible diameter.

 Sufficient stabilizer blade contact with the wellbore wall, to ensure the bit and collars are centralized, yet preventing wall erosion.

The number of stabilizers used, and their placement in the bottomhole assembly, depends on the severity of the crooked-hole tendencies of the formation. As crooked-hole tendencies become more severe, additional stabilizers are required directly above the bit to prevent bit deviation. Typical packed-hole assemblies, for varying severity in crooked-hole condition, are illustrated in the Figure 57.

7.3.4 Packed Pendulum Assembly

Packed-hole assemblies are used to minimize the rate of hole angle change, although it is always likely to have some amount of deviation. Pendulum assemblies are used to reduce total hole angle. If total hole deviation must be reduced and a packed-hole assembly is required after reducing hole angle, the packed pendulum assembly should be used.

In the packed pendulum assembly, pendulum-length collars are placed below the regular packed-hole assembly. When hole deviation has dropped to the required angle, the pendulum collars can be replaced by the original packed-hole assembly again.

Only the length of the pendulum collars must be reamed before resuming normal drilling.

If a vibration-dampening device is used in the packed pendulum assembly, it should remain in its original packed-hole assembly position, typically above the middle stabilizer point.

Figure 58: Packed Pendulum Assembly

7.3.5 Stabilizers and Reamers

Stabilizers are used to stabilize the bit and the drill collars in the hole. When properly stabilized, optimum-drilling weight can be applied to the bit, thereby forcing it to rotate on its true axis and drill straight ahead without sudden angle changes. Fewer bits are used and the rate of penetration increases.

Stabilizer blades should be as close to bit size as possible. Drilling hard formations requires more durable stabilizers, and only a small wall contact area is necessary. A larger wall contact area is required for softer formations and for severe crooked-hole formations.

The tungsten carbide blades may be short or long (that is, small or larger contact area) and either straight or spiral. Blades are typically welded or integral, rotating with the drill string. A non-rotating rubber sleeve stabilizer is often used for its advantage of not cutting into and damaging formations, but it has a short life and no reaming capability.

Roller reamers are used primarily for maintaining hole gauge in very hard formations. Set behind the bit, reamers effectively re-drill the formation to maintain hole diameter, extend bit life and prevent problems with sticking.

They are also used for additional stabilization in hard formations. However, their limited wall contact area prohibits them from being highly effective stabilizers in most instances and, in soft formations, reamer cutters penetrate the borehole wall which reduces stabilization and can increase bit deviation.

Reamers are run between the bit and the drill collars, and should be as close to bit size as possible.

Reamer cutters must be selected to match the formation being drilled.

7.3.6 Drilling Procedures

If deviation is a problem, or if a deviated well is being drilled, further practices in addition to the make-up of the bottom hole assembly can be adopted:

 Perform regular wiper trips, with keyseat wipers to open up developing keyseats.

 Ream back to bottom on bit trips to eliminate or minimize the severity of doglegs and developing keyseats.

 Avoid sharp changes in bit weight, which, because of the variable bend in the drill pipe, can result in doglegs. If weight must be reduced to straighten a hole, the reduction should be gradual to prevent a sudden change in direction.

 Conduct regular surveys to monitor the rate of change in angle and occurrence of doglegs.