Page 90 - Air and Gas Drilling Manual

3-18 Air and Gas Drilling Manual
The assembly in the above figure is composed of a drill bit at the bottom, drill
collar tubulars, a near bit stabilizer directly above the bit, a stabilizer at the middle
of the assembly, and a stabilizer at the top of the assembly. The addition of
stabilizers to the drill collar string generally improves the straight drilling capability
of the drill string. Highly stabilized drill strings are necessary when drilling in
“crooked hole country”. Crooked hole country usually refers to rock formations that
tend to deflect the bit and thus the drill string as the drill bit is advanced.
Hard to medium hard rock formations that are tilted to a high angle from
horizontal is one of the main causes of severe borehole deviations from vertical. All
deep rotary drilled boreholes will tend to have some deviation and tend to have a
cork-screw three dimensional shape (usually to the right). The deviation from
vertical can usually be kept below 3˚ to 5˚ with good drilling practices. In general,
air drilled boreholes can have more deviation than a mud drilled boreholes (assuming
same rock formations). But most of increased deviation from vertical is due to the
fact that air drilling penetration rates are significantly higher than a mud drilling
operation and drillers tend to take advantage of that increased drilling rate and let the
deviation get away from them. To correct this tendency, it is good practice to utilize
a more stabilized BHA when drilling an air drilled borehole than would be used in a
comparable mud drilled borehole. For more detail regarding the design of stabilized
BHA and where and how to apply such assemblies the readers are referred to
References 1 and 4 or service company literature.
3.3.1 Drill Collars
Drill collars are thick walled tubulars that are used at the bottom of the drill
string (see Figure 3-1). Their principal purpose in the drill string is to provide the
axial force needed to advance the drill bit (see Figure 1-2). When drilling a vertical
borehole, the axial force is the weight of the drill collars. Drill collars are available
in API range lengths given in Table 1-1. Figure 3-16 shows a BHA with Range 2
(~30 ft long) drill collars. Range 2 lengths are typical for double and triple land
rotary drilling rigs. Also shown in Figure 3-16 are short drill collar lengths used to
adjust positions of the stabilizers in the BHA. These shorter drill collar lengths are
selected from Range 1 stock of drill collars.
Appendix B gives the dimensions and mechanical properties for API drill
collars. These drill collars vary in outside diameter from 2 7/8 inches to 12 inches.
Table B-1 shows that a drill collar having an outside diameter of 6 3/4 inches can be
obtained with an inside diameter of 1 1/2 inches, 1 3/4 inches, 2 inches, 2 1/4
inches, 2 1/2 inches, 2 13/16 inches, 3 inches, 3 1/4 inches, and 3 1/2 inches. From
this same table a 6 3/4 inch Column (1), by 2 13/16 inch Column (9), drill collar
will have a weight per unit length of 100 lb/ft and a standard 30 ft length of this
drill collar would weigh 3,000 lb. Table B-2 shows that the 6 3/4 inch Column (3),
by 2 13/16 inch Column (11), drill collar is available with either, the 5 inch Double
Streamline, or the 4 1/2 inch H-90 connections, Columns (1) and (2). The
recommended make-up torque for each connection type is 22,426 ft-lb, 23,159 ft-lb,
respectively, Column (11). These are the recommended make-up torques required to
insure that the shoulder connections will not leak circulation fluid and will remain
tight when used in a rotary drill operation.