STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drill String: Composition and Design 717
Basically, the purpose of drill collars is to furnish weight on bit. However,
both size and length of drill collars have an effect on bit performance, hole
deviation, and drill pipe service life. Drill collars may be classified according
to the shape of their cross-sections as round drill collars (conventional drill
collars), square drill collars, or spiral drill collars (drill collars with spiral grooves).
Square drill collars are used to increase the stiffness of the drill string and
are recommended for drilling in crooked hole areas. The spiral type of drill
collar is used for drilling formations in which the differential pressure can cause
sticking of drill collars. The spiral grooves on the drill collar side reduce the
area of contact between drill collar and wall, which considerably reduces the
sticking force.
Conventional drill collars are made with uniform outside diameter and with
slip and elevator recesses. Slip and elevator recesses are designed to reduce drill
collar handling time while tripping by eliminating lift subs and safety clamps.
However, the risk of drill collar failure for such a design is increased. The slip
and elevator recesses may be used together or separately.
Dimensions, physical properties, and unit weight of new, conventional drill
collars are specified in Tables 4-71, 4-72, and 4-73, respectively. Technical data
on square and spiral drill collars are available from manufacturers.
Selecting Drill Collar Size
Selection of the proper outside and inside diameter of drill collars is usually
a difficult task. Perhaps the best way to select drill collar size is to study results
obtained from offset wells previously drilled under similar conditions.
The most important factors in selecting drill collar size are:
1. bit size
2. coupling diameter of the casing to be set in a hole
3. formation’s tendency to produce sharp changes in hole deviation and
direction
4. hydraulic program
5. possibility of washing over if the drill collar fails and is lost in the hole
To avoid an abrupt change in hole deviation (which may make it difficult or
even impossible to run casing) when drilling in crooked hole areas with an
unstabilized bit and drill collars, the required outside diameter of the drill collar
placed right above the bit can be found from the following formula [38]:
Ddc = 2(casing coupling OD) - bit OD (4-49)
Example
The casing string for a certain well is to consist of 13 +-in. casing with coupling
outside diameter of 14.375 in. Determine the required outside diameter of the
drill collar in order to avoid possible problems with running casing if the
borehole diameter is assumed to be 17+ in.
Ddc = 2(14.375) - 17.5 = 11.15 in.
Being aware of standardized drill collar sizes, an 11 or 12-in. drill collar should
be selected. To avoid such large drill collar OD, a stabilizer or a proper-sized
square drill collar (or a combination of the two) should be placed above the