Page 35 - Air and Gas Drilling Manual

1-12 Air and Gas Drilling Manual
1.3 Comparison of Mud and Air Drilling
The direct circulation model is used to make some important comparisons
between mud drilling and air and gas drilling operations.
1.3.1 Advantages and Disadvantages
There are some very basic advantages and disadvantages to mud drilling and air
drilling operations. The earliest recognized advantage of air and gas drilling
technology was the increase in drilling penetration rate relative to mud drilling
operations. Figure 1-10 shows a schematic of the various drilling fluids (the top
four comprise air and gas drilling technology) and how these drilling fluids affect
drilling penetration rate. The drilling fluids in Figure 1-10 are arranged with the
lightest at the top of the list and the heaviest at the bottom. In general, the lighter
the drilling fluid the greater the drilling penetration rate (the arrow points upward for
increasing penetration rate). The lighter the fluid column in the annulus (with
entrained rock cuttings) the lower the confining pressure on the rock bit cutting face.
This lower confining pressure allows the rock bit to be more easily advanced into the
rock (see Chapter 3 for more details).

Figure 1-10: Improved penetration rate.
Figure 1-11 shows a schematic of the various drilling fluids and their respective
potential for avoiding formation damage. Formation damage is an important issue
in fluid resource recovery (e.g., water well, environmental monitoring, well drilling
operations, oil and natural gas, and geothermal fluids). The lighter the fluid column
in the annulus (with entrained rock cuttings), the lower the potential for formation
damage (arrow points upward to increasing avoidance of formation damage).
Formation damage occurs when the fluid column pressure at the bottom of borehole
is higher than the pore pressure of the resource fluid (oil, gas, or water) in the rock
formations. This higher bottomhole pressure forces the drilling fluid (with entrained
rock cutting fines) into the exposed fractures and pore passages in the drilled rock
formations. These fines plug these features in the immediate region around the
borehole. This damage is called a “skin effect”. This skin effect damage restricts