Page 253 - Air and Gas Drilling Manual

Chapter 5: Shallow Well Drilling Applications 5-95
The diesel fuel consumption rate for this example is constant through the
drilling time of 30 hours. The approximate total diesel fuel needed to drill the
borehole is given by the integration of the area under the curve in Figure 5-22. This
is approximately 728 gallons. It is standard practice to assume a 20 percent
additional volume of fuel for blowing the hole between connections and other
operations on the drill rig. Therefore, the approximate total diesel fuel needed at the
drilling location is 873 gallons. For this example, this diesel fuel amount is for the
drilling rig prime mover that operates both the integrated rotary screw compressor
and the hydraulic pump that operates the rotary top drive system.
This illustrative example demonstrates the calculation procedure used to plan a
typical dual wall pipe reverse circulation drilling operation. This particular drilling
operation uses the concealed inner-tube pipe and the special skirted tri-cone drill bit
fabricated for this type of operations (see Figure 3-36). The injected air (at low
volumetric flow rate) moves slowly down the annulus inside the dual pipe. No air
moves down the annulus between the outside of the dual pipe and the openhole.
5.4 Conclusions
These illustrative examples given in this chapter the relative fuel efficiencies of
the reciprocating piston compressor and the rotary type compressor. In general, the
reciprocating piston compressor is the more fuel efficient machine for most shallow
drilling operations, but its bulk limits its applicability in small rig operations.
Reverse circulation techniques are uniquely applicable to shallow drilling
operations. The principal advantage to using reverse circulation techniques (vis-à-vis
direct circulation technique) is that the minimum volumetric flow rates are
significantly less than the corresponding minimum volumetric flow rates for direct
circulation drilling (for the same well geometry and drilling rates, etc). The
principal disadvantage to the reverse circulation technique is the high resistance to
the air flow up the inside of the drill string (due to pipe friction). This pipe friction
resistance ultimately restricts the reverse circulation technique to shallow drilling
operations since return flow pipe friction losses become excessive for the long pipe
lengths that accompany deep drilling operations (i.e., usually at depths greater than
approximately 3,000 ft).
The illustrative examples given in this shallow drilling chapter were developed
considering only the major friction flow losses in simple openhole drill pipe
geometry. Consideration of only these major losses is deemed appropriate for
shallow well analysis due to the length of the drill strings considered and the
simplicity of the surface flow equipment used in most shallow drilling operations.
Deep drilling operations require consideration of major and minor flow losses and
more complicated well geometry. Chapter 8 will consider these in detail.

References
1. Lapedes, D. H., McGraw-Hill Encyclopedia of the Geological Sciences, McGraw-
Hill, 1978.
2. Daugherty, R. L., Franzini, J. B., and Finnemore, E. J., Fluid Mechanics with
Engineering Applications, Eighth Edition, McGraw-Hill, 1985.