Figure 3-16 shows a typical bottomhole assembly (BHA) for a direct circulation rotary drilling operation. The BHA is the section of the drill string below the drill pipe (see Figure 3-1). This section of the drill string is the most rigid length of the string. It is this section of the drill string that determines how much weight can be placed on the drill bit and how "straight" a vertical borehole will be drilled with the drill string.
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.
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