Page 37 - Air and Gas Drilling Manual

1-14 Air and Gas Drilling Manual
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
bottomhole pressure needed to balance (or overbalance) the high pore pressure of the
deposit.

Figure 1-12: Loss of circulation avoidance.
Figure 1-13 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 was discussed above, too heavy a
drilling mud results in overbalanced drilling and this can result in formation
damage. But there is a greater risk to overbalanced drilling. If the drilling mud is
too heavy the rock formations in the openhole section can fracture. These fractures
could result in a loss of the circulating mud which could result in a blowout.

Figure 1-13: Controlling high pore pressure.