Heavy wall drill pipe is an intermediate drill string member — heavier, stronger and stiffer than regular drill pipe, but at the same time more flexible than drill collars. Because it has the same external dimensions as regular drill pipe, it is much easier to handle than drill collars. Figure 1 and Figure 2 illustrate some of its important features.
Figure 2
The most important drill string application for heavy wall drill pipe is in the so-called zone of destruction — the area above the topmost drill collars where drill pipe fatigue failure is most likely to occur. To reduce fatigue failures in this area of the borehole, 18 to 21 joints of heavy wall drill pipe should be run above the drill collars. This provides a gradual change in stiffness between drill collars and drill pipe. Also, the ability of the heavy wall drill pipe to bend (unlike drill collars) serves to relieve high stresses at the connections.
To prevent too great a change in stiffness between the drill collars and the heavy wall drill pipe, the guidelines in Table 1., below, can be used.
Heavy Wall Drill Pipe OD ( inches ) Maximum Drill Collar OD ( inches )
3.50 6.00
4.00 6.50
4.50 7.25
5.00 8.25
Table 1. Sizing guidelines for heavy wall drill pipe and drill collars
Heavy wall drill pipe was first used in directional drilling, which generally requires flexibility in the drill string. It is now widely used in vertical and horizontal drilling as well. With less wall contact than would be experienced with drill collars, its usage reduces torque and wall- sticking tendencies. Its smaller degree of wall contact, together with its greater stiffness relative to regular drill pipe, results in increased stability and better directional control.
Heavy wall drill pipe is also useful in reducing hook loads, making it ideal for smaller rigs drilling deeper holes.
In certain special applications (most notably horizontal drilling) heavy wall drill pipe may be run in compression. It is important to keep in mind, however, that a joint of heavy wall drill pipe is not as strong as a drill collar, and that it is still susceptible to buckling and fatigue failure. In a vertical hole, heavy wall drill pipe must be maintained in tension.
Stabilizers
Stabilizers, by centralizing the drill string at selected points in the borehole, can be used to: • ensure that the weight of the drill collars is concentrated on the bit;
• reduce torque and bending stresses in the drill string; • prevent wall-sticking or key-seating of the drill collars; • build, drop or maintain hole angle in directional drilling; • maintain constant bit direction in straight-hole drilling.
The type and placement of stabilizers in the drill string depends on local drilling conditions and well objectives.
In areas with a tendency toward crooked drilling, the stabilizer(s) immediately above the bit must have adequate wall contact to provide bit stability and guidance. The stabilizer(s) must also have wear properties which will permit them to stay in gauge for the life of the bit, and must be able to centralize the drill string without digging into the borehole wall. In an angle-holding, or packed-hole assembly, which requires maximum stiffness to "lock in" the direction of the drill string, the stabilizers should have the largest possible cross- sectional area that will permit adequate circulation passage.
For packed-hole drilling techniques, stabilizers act like drill bushings to minimize drift and hole angle change by centering the drill string. When utilizing a pendulum, or angle- dropping assembly, stabilizers are frequently used to increase the effectiveness of the pendulum. They similarly serve to increase the effectiveness of an angle-building assembly. Stabilizers are also designed to prevent wall-sticking of the drill collars and to guide them out of key seats.
There are a variety of stabilizer types available for different drilling environments, the most common of which are:
• integral blade stabilizers;
• replaceable sleeve stabilizers; • replaceable wear pad stabilizers; • non-rotating sleeve stabilizers; • welded blade stabilizers.
These stabilizer types are designed either for bottomhole configuration (to be run just above the bit), or string configuration (to be run at various points in the drill string).
The integral blade stabilizer ( Figure 1 ) is designed for use in all degrees of formation hardness.
Figure 1
Its long ribs are designed to provide the wall contact necessary for proper stabilization, and are contoured to minimize torque while drilling. The ribs are milled directly on the stabilizer body.
For larger hole sizes, the integral blade stabilizer can be manufactured with a shop- replaceable sleeve. Its long ribs are milled directly on the sleeve to provide unitized
construction, thus preventing loss of ribs in the hole as sometimes happens with welded rib stabilizers.
These stabilizers can be provided with pressed-in tungsten-carbide compacts or welded-on hardfacing on the wear surfaces to increase gauge life. Integral blade stabilizers are available in both string and bottom hole types.
The replaceable steel sleeve stabilizer ( Figure 2 ) has the benefit of being replaceable at the wellsite.
Figure 2
Sleeves can be changed in the rotary table, making this stabilizer type economical for remote locations. Replaceable sleeve stabilizers are available both in bottomhole and string configuration. They can be used for more than one hole size by changing sleeve size. Deep- grooved passages between the sleeve ribs assure adequate circulation past the tool. The replaceable wear pad stabilizer, with rig-replaceable wear pads ( Figure 3 (string) and Figure 4 (bottomhole)),
Figure 3
Figure 4
The wear pads, which can easily be changed out in the rotary table with hand tools, provide long wall contact area, and pressed-in tungsten carbide and carburized wear surfaces provide maximum downhole life. Within limits, changing wear pads permits body use in more than one hole size ( Figure 5 ).
Figure 5
Built-in wear indicators are available to indicate the need for replacement before body damage occurs. The stabilizer body has a fluted configuration to provide for adequate circulation. This stabilizer type is available in either bottomhole or string configuration, so that two or three tools can be run in tandem hookups to provide additional wall contact as required.
Figure 6
A true "drill bushing," it can be used in hard to medium-hard non-abrasive formations, since the sleeve does not rotate or dig into the borehole wall. The one-piece, rig-replaceable sleeve is molded over a mild steel inner bushing to prevent it from being lost downhole, and has fluted marine-type inner bearings, designed to allow continuous cleaning by the
circulating fluid. Drilling fluid acts as a lubricant and coolant to avoid friction between the mandrel and the non-rotating sleeve. The non-rotating sleeve stabilizer is always a string- type tool.
The welded blade stabilizer ( Figure 7 ) is designed to centralize drill collars and provide better borehole alignment.
Figure 7
Its rotating blade is especially effective in soft formations, where balling-up of mud and cuttings on the drill string can cause problems.
Jars
Jars are used to provide quick, sharp, upward or downward motion to free stuck pipe. Although we most commonly associate them with fishing operations, they can also be run on drilling bottomhole assemblies as a precautionary measure. They are especially
appropriate for drilling in sticky, heaving, sloughing or crooked holes. Jars also have
applications in controlled-weight directional drilling, as well as in coring operations. Figure 1 shows a mechanical rotary drilling jar.
Figure 1
Jars are usually identified according to type, inside and outside diameter, thread connection and stroke length. There are a number of jar types available, and the type of jarring action utilized will depend on the jar being used and the specific operating conditions. For typical mechanical or hydraulic jars, upward or downward motion is initiated by pulling up or slacking off on the drill string until a pre-set triggering load is reached, initiating a sharp blow. A few such blows may be sufficient to free a stuck drill string without the expense and risk of a fishing job.
In drilling operations, jars are generally run high in the drill collar string, with some drill collars above them. Optimum jar placement and triggering load, along with the need to enhance the jarring impact by running a companion tool known as ajar accelerator, is best determined individually for each bottomhole assembly, with input from the service company providing the tools.
Reamers
In very hard (or hard and abrasive) formations, the outside cutting structure of a bit gradually wears away if it is not protected. The bit becomes "pinched" or undergauge, resulting in a hole diameter that becomes smaller with increasing depth. Figure 1 shows an extreme example of an under-gauge bit — this 6 " bit,
Figure 1
with no stabilization, wore down to 4 " OD after making 428 ft in 59 hours.
When a hole is severely undergauge, it is necessary to ream each new bit back to bottom before drilling can resume. This not only costs rig time and reduces bit life, but it increases the possibility of sticking the drill string.
Roller-cutter rotary reamers, with either three points or six points of wall contact ( Figure 2 and Figure 3 ), have proven to be effective for keeping holes in gauge.
Figure 2
This not only prolongs bit life, but it also minimizes wear on other tools used in packed-hole assemblies in crooked hole drilling areas.
Figure 3
Three-point bottomhole reamers are designed to be run between the bit and the drill collars, with the spacing from the bit to the reamer kept to a minimum. This set-up keeps the hole in gauge to ensure minimum time spent reaming back to bottom, protects the gauge of tools above the reamer and provides some stabilization to the bit.
Three-point string rotary reamers are sometimes run in the drill collar or drill pipe strings to centralize the strings in crooked hole areas. When run above the drill collars, they can be effective in reaming out dog-legs, key-seats and ledges. Both types of three-point reamers contain wall contact points spaced 120° apart.
Six-point bottomhole reamers are run between the bit and drill collars, and are used when greater reaming capacity is required than can be provided by the three-point design. They are especially effective in hard, abrasive formations where extremely hard cutters can be run in the lower points and regular hard cutters in the upper points. The six points of contact, spaced 60 °C apart, provide better stabilization than a three-point reamer in crooked hole areas.
A variety of roller reamer cutters are available for different formation types. For hard formations that require a scraping action, such as dolomite, hard lime and chert, the "Q" cutter ( Figure 3 , top) may be used. For extremely hard formations, the "K," or knobby cutter ( Figure 3 , bottom) may be used. Its tungsten carbide compacts act as teeth to fracture the formation.
The reamer bodies shown here utilize drive-fit hardened reamer pins and bearing blocks, which can be easily replaced at the wellsite without the need for special tools or welding.