Page 31 - Air and gas Drilling Field Guide 3rd Edition

22 CHAPTER 2 Air and Gas Versus Mud

fluid resource recovery (e.g., oil and natural gas, and geothermal fluids). The lighter
the fluid column in the annulus (with entrained rock cuttings), the lower the poten-
tial for formation damage (arrow points upward to increasing avoidance of forma-
tion damage). Formation damage occurs when the fluid column pressure at the
bottom of the borehole is higher than the pore pressure of the resource fluid (oil,
gas, or water) in the potential producing rock formations. This higher bottom hole
pressure forces the drilling fluid (with entrained rock cutting fines) into the
exposed fractures and pore passages in the producing rock formations. These fines
plug these features in the immediate region around the borehole. This damage is
often called a “skin effect.” This skin effect damage restricts later formation fluid
flows to the borehole, thus reducing the productivity of the well.
Figure 2-12 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 bottom hole
pressure. This low bottom hole pressure can cause a high pressure oil and/or nat-
ural gas “kick” or geothermal fluid “kick” (a slug of formation fluid) to enter the

Ability to Drill in Loss of Circulation Zones
Increasing
Air and Gas

Unstable Foam
Stable Foam

Aerated Mud

Mud

FIGURE 2-12. Loss of circulation avoidance.