Drilling Tools & Deflection Methods
About this chapter
Before the arrival of the positive-displacement mud motor (PDM), whip-stocks, knuckle
joints and jetting (in soft formations) were used as deflection methods. DD tools and
technology have evolved tremendously in the past 20 years. Today, there is a broad range
of PDMs for different applications.
The various methods used to deflect a wellbore are described in this chapter. Orientation
is covered separately in Chapter 11. The DD must be familiar with all the DD tools at the
rig-site and in the workshop. EQ Jars and PDMs are covered separately in this manual.
The remaining DD tools are briefly described here. More detailed information is
available from the manufacturers. Most of the DD tools are straightforward to operate.
While a directional drilling simulator is a useful aid in the teaching of DD concepts, the
only way to fully understand how a wellbore is deflected and how the various DD tools
are used is to get some on-the-job training. This chapter should provide a lot of the
background knowledge required.
Objectives of this Chapter
On completing this chapter the directional driller should be able to do the following
exercises
1. Describe the use of an open-hole whip-stock.
2. Explain how deflection is achieved using the jetting kickoff technique.
3. Explain what determines the dogleg severity achieved with a non-steerable PDM
kickoff BHA.
4. Describe the uses and applications of:
DC, SDC, NMDC, SNMDC, I.B. stabilizer, Sleeve-type stabilizer(s), clamp-on
stabilizer, RWP, Bent Sub, UBHO, BOS, Underreamer, String Reamer, Bullnose,
Hole Opener, Key-seat Wiper, Section Mill, Shock Sub, Rebel Tool.
5.1 Drilling Tools
The major drilling tools likely to be used by the DD are discussed briefly here. For more
detailed information on a particular tool, it is necessary to refer to the "Composite
Catalog" or to the manufacturer’s data sheets.
5.1.1 Drill Collar (DC)
Drill collars are heavy, stiff steel tubulars. They are used at the bottom of a BHA to
provide weight on bit and rigidity. Flush or spiral drill collars are available. In directional
drilling, spiral drill collars are preferable (Figure 5-1). The spiral grooves machined in
the collar reduce the wall contact area by 40% for a reduction in weight of only 4%. The
chances of differential sticking are greatly reduced. Spiral drill collars usually have slip
and elevator recesses. Stress-relief groove pins and bore back boxes are optional. The
drill collars (various sizes) are normally owned by the drilling contractor.
5.1.1.1 Short Drill Collar (SDC)
Often called a pony collar, this is simply a shortened version of a steel drill collar. Short
drill collars may be manufactured or a steel drill collar may be cut to make two or more
short collars. For the DD, the SDC and the short non-magnetic drill collar (SNMDC)
have their widest application in the make-up of locked BHAs. SDCs of various lengths
(e.g. 5’, 10’, 15’) are normally provided by the DD company.
5.1.1.2 Non-Magnetic Drill Collar (NMDC)
Non-magnetic drill collars are usually flush (non-spiral). They are manufactured from
high-quality, corrosion-resistant, austenitic stainless steel. Magnetic survey instruments
run in the hole need to be located in a non-magnetic drill collar of sufficient length to
allow the measurement of the earth’s magnetic field without magnetic interference.
Survey instruments are isolated from magnetic disturbance caused by steel components
in the BHA and drillpipe. ANADRILL’s M1 MWD tool and its successors are fixed
inside their own special MWD non-magnetic drill collars. SLIM-1, however, is run
inside a standard NMDC. Stress-relief groove pins and bore back boxes are optional.
5.1.1.3 Short Non-Magnetic Drill Collar (SNMDC)
A short version of the NMDC, SNMDCs are often made by cutting a full-length NMDC.
The SNMDC may be used between a mud motor and an MWD collar to counteract
magnetic interference from below. It is also used in locked BHAs, particularly where the
borehole's inclination and direction give rise to high magnetic interference. Finally,
BHAs for horizontal wells often use a SNMDC.
5.1.2 Float Sub
This is a PIN x BOX sub which is bored out to take a float valve. It is often run above a
mud motor. In conventional rotary BHAs, a float valve is inserted either in the bit sub (in
the case of a pendulum BHA) or in the bored-out near-bit stabilizer. Poppet and flapper
designs of float valve are available. Note that some clients may not allow the use of a
float valve (because of kick-control problems). The DD should check the client's
regulations on arrival at the rig. The float sub is usually provided by the DD company.
The float valve is usually provided by the drilling contractor.
5.1.3 Bit Sub
This is a BOX x BOX sub which is run directly above the bit (hence its name) when no
near-bit stabilizer is used. It is bored out to take a float valve. Various sizes of bit sub are
normally provided by the drilling contractor.
5.1.4 Junk Sub
A junk sub is fabricated from a solid steel body with a necked-down mid-portion. A
"skirt" is fitted to the lower part of the body, around the necked-down portion, forming a
basket for junk to settle in (Figure 5-2).
The junk sub is run directly above the bit. It catches pieces of junk which are too heavy
to circulate out. Bleed holes in the skirt allow the mud to return to the system. The junk
sub is provided by the drilling contractor.
5.1.5 Extension Sub
This is a short sub which can be used to fine-tune a BHA. It is normally PIN x BOX. A
float sub can be used as an extension sub. The extension sub is usually provided by the
DD company.
5.1.6 Heavyweight Drill Pipe (HWDP)
This is an intermediate-weight drill string member with drill pipe dimensions for easier
handling. Its heavy wall tube is attached to special extra-length tool joints. These provide
ample space for recutting the connections and reduce the rate of wear on the OD. The
OD of the tube is also protected from abrasive wear by a centre wear pad (Figure 5-3).
Tool joints and wear pad are hard-banded. Some HWDP have two wear pads.
HWDP is less rigid than DCs and has much less wall contact. Chances of differential
sticking are reduced. Its three-point wall contact feature solves two serious problems in
directional drilling. It permits high-RPM drilling with reduced torque. HWDP can be run
through hole angle and direction changes with less connection and fatigue problems.
Today, the trend in BHA design is to minimize the number of DCs in the BHA and use
HWDP to comprise a major portion of available weight on bit
HWDP is normally provided by the drilling contractor. However, it is the DD’s
responsibility to ensure there are sufficient joints of HWDP on the rig. For normal
directional jobs, 30 joints of HWDP should be sufficient.
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