Page 32 - Air and gas Drilling Field Guide 3rd Edition

2.3 Comparison of Mud and Air Drilling 23

annulus. Such kicks must be immediately and carefully circulated out of the annu-
lus (to the surface), as an uncontrolled blowout of the well could occur. Here
again heavier drilling fluids are generally more prone to loss of circulation (arrow
points upward to increasing loss of circulation avoidance).
Figure 2-13 shows a schematic of the various drilling fluids and their respec-
tive potential for use in geologic provinces with high pore pressures. High pore
pressures are encountered in oil, natural gas, and geothermal drilling operations.
New discoveries of oil, natural gas, or geothermal fluid deposits are usually highly
pressured. In order to safely drill boreholes to these deposits, heavily weighted
drilling muds are utilized. The heavy fluid column in the annulus provides the
high bottom hole pressure needed to balance (or overbalance) the high pore
pressure of the deposit.
Figure 2-13 also shows that the heavier the drilling fluid column in the annulus,
the more useful the drilling fluid is for controlling high pore pressure (the arrow
points downward to increasing capability to control high pore pressure). There
are limits to how heavy a drilling mud can be. As discussed earlier, too heavy a dril-
ling mud results in overbalanced drilling, which can result in formation damage.
However, there is a greater risk to overbalanced drilling. If the drilling mud is too
heavy, the rock formations in the open hole section can fracture. These fractures
will result in a loss of the circulating mud in the annulus that could result in a
blowout.
In the past decade it has been observed that drilling with a circulation fluid
that has a bottom hole pressure slightly below that of the pore pressure of the
fluid deposit gives near optimum results. This type of drilling is denoted as under-
balanced drilling. Underbalanced drilling allows the formation to produce fluid as
the drilling progresses. This lowers or eliminates the risk of formation damage
and eliminates the possibility of formation fracture and loss of circulation. In gen-
eral, if the pore pressure of a deposit is high, an engineered adjustment to the

Ability to Drill in High Pore Pressure Zones

Air and Gas

Unstable Foam

Stable Foam

Aerated Mud

Mud
Increasing

FIGURE 2-13. Controlling high pore pressure.