Page 36 - Air and Gas Drilling Manual

later formation fluid flows to the borehole, thus, reducing the productivity of the
well.

Figure 1-11: Formation damage avoidance. Chapter 1: Introduction 1-13
Figure 1-12 shows a schematic of the various drilling fluids and their respective
potential for avoiding loss of circulation. Loss of circulation occurs when drilling
with drilling muds or treated water through rock formations that have fractures or
large interconnected pores or vugs. If these features are sufficiently large and are not
already filled with formation fluids, then as drilling progresses the drilling fluid that
had been flowing to the surface in the annulus can be diverted into these fractures or
pore structures. This diversion can result in no drilling fluid (with entrained rock
cuttings) returning to the surface. The rock cuttings are left in the borehole and
consolidate around the lower portion of the drill string and the drill bit. If this
situation is not identified quickly, the drill string will begin to torque-up in the
borehole and mechanical damage to the drill string will occur. Such damage can
sever the drill string and result in a fishing job to retrieve the portion of the drill
string remaining in the borehole.
For deep oil and natural gas recovery wells, loss of circulation can result in even
more catastrophic situations. If drilling fluids are lost to thief formations, the fluid
column in the annulus can be reduced resulting in a lower bottomhole pressure.
This low bottomhole pressure can cause a high pressure oil and/or natural gas kick,
or geothermal fluid kick (a slug of formation fluid) to enter the annulus. Such kicks
must be immediately and carefully circulated out of the annulus (to the surface)
otherwise 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 1-13 shows a schematic of the various drilling fluids and their respective
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