The Bottomhole Assembly

The bottomhole assembly (BHA) is the name applied to drill collars and tools incorporated with them, including the bit.

The drillstring is made up of the drillpipe (heavyweight drillpipe is normally distinguished as well) and the BHA.

2.4.8.1 Stabilizers

These are short lengths of pipe that are positioned between drill collars to centralize them and maintain a straight hole. Through a scraping action, they maintain a full sized or gauged hole.

The full gauge is provided by ribs or blades mounted on a mandrel (Figure 24). These can be made from solid rubber or aluminum. More typically, they are made from steel with tungsten carbide inserts on the facing edges.

Stabilizers can be categorized into rotating or non-rotating blades, with the ribs or blades being generally spiral or straight.

Figure 24: Stabilizers 2.4.8.2 Reamers

Roller reamers ream the hole just behind the bit and perform a similar function to stabilizers because they stabilize the assembly and help maintain a full gauge hole. Reamers are typically used when problems are experienced in maintaining a full gauged hole, particularly in abrasive formations, when the bit is worn undergauge. Similarly, they may be used if key seats or ledges are known to exist in the borehole.

The number and position of reaming blades categorize the type of reamer. For example, with three blades, it is a 3-point reamer. If blades are positioned toward the base of the sub (Figure 25), it is a 3-point near-bit reamer. A stabilizer reamer has the blades positioned centrally in the sub.

Figure 25: 3-Point Near-Bit Reamer

Under-reamers are also placed directly behind the bit to ream the hole and maintain full gauge or enlarge the hole. The reaming or cutting action occurs through rotating cones located on collapsible arms. These are opened and held out during drilling by the pressure of the mud passing through the tool. This enables the tool to pass through a narrow diameter hole section, then open up and drill a wider hole.

2.4.8.3 Hole Opener

This is a similar tool to the under-reamer in that a cutting action is provided by rotating cones to enlarge a hole. However, unlike the under-reamer, the cones are in a fixed position, so the hole opener must be

able to pass through the previous hole diameter. Therefore, they are generally used on a surface hole section to widen the hole where large-hole diameters are required.

2.4.8.4 Cross Over Sub

This is a small length of pipe enabling drillpipe and / or collars of different diameters and threads to be connected together.

2.4.8.5 Rotary Drilling Jar

This is a mechanically or hydraulically operated device that provides a high impact hammer blow to the drillstring downhole if it becomes stuck. Jars are designed for drilling or fishing (retrieval of part of the drillstring left downhole) operations.

If the drill string becomes stuck and incapable of being freed with normal working of the pipe (such as upward and downward movement) or by pulling on the pipe without exceeding drill string and surface equipment limitations, a rotary drilling jar is used. A jar strikes heavy-impact hammer blows, in an upward or downward direction, to the drillstring. The direction in which the jar is activated depends on the pipe movement when it became stuck. A downward blow is struck if the pipe was stationary or moving upwards. An upward blow is struck if the drill string was moving downwards. Most stuck pipe situations result from an upward moving or stationary pipe. Typically, downward jarring is required.

To free the pipe the jar must be situated above the stuck point, so a jar is usually placed in the upper part of the bottomhole assembly, above stabilizers and other tools prone to sticking.

A jar can be hydraulically or mechanically triggered, the design is different but triggering works on the same principle. That is, the jar consists of an outer barrel attached to the drillstring below the stuck pipe and an inner mandrel which, attached to the free string above, can slide, delivering rapid upward or downward acceleration and force.

Hydraulic jars operate on a time delay produced by the release of hydraulic fluid. As the mandrel is extended, the hydraulic fluid is released slowly through a small opening. Over several minutes, opening continues but is restricted by the hydraulic metering. The fluid channel then increases in diameter, allowing rapid flow and unrestricted rapid opening of the jar, known as its stroke. At the end of the stroke, typically 8 inches, a tremendous blow is delivered by the rapid deceleration of the drill string above the jars which were accelerating through the stroke.

Mechanical jars deliver the hammer blow by the same acceleration / deceleration of the jars, but the triggering mechanism is by a preset tension with no time delay when the jar is cocked (Figure 26).

Figure 26: Mechanical Jar Operation

A jar accelerator can be set above rotary jars, typically within the heavyweight drill pipe, to intensify the blow delivered by the jars. Upward strain compresses a charge of fluid or gas (commonly nitrogen) and, when the rotary jar trips, the expansion of fluid or gas in the accelerator amplifies the jarring effect.

A jar accelerator confines movement to the drill collar—or close to the stuck point—and minimizes shock on the drill string and surface equipment by cushioning rebounds through the compression of fluid or gas.

If jarring cannot free the stuck pipe, the only recourse is to back-off the pipe that is still free. This may be achieved by simply twisting off, or unscrewing, the free pipe or by determining the free point with a wireline tool, then running an explosive charge on a wireline to blow the string apart. The remaining stuck pipe must be retrieved, removed, or avoided before drilling can continue.

2.4.8.6 Shock Sub

This is positioned close behind the bit. Hard formations cause the bit to bounce on the bottom of the hole.

The shock sub will absorb the impact from this bouncing to prevent damaging the remaining part of the drillstring. This can be done by springs or rubber packing.