Page 57 - Air and gas Drilling Field Guide 3rd Edition

48 CHAPTER 3 Surface Equipment

Air and gas drilling operations using small single drilling rigs drill only shallow
(usually less than 1000 ft in depth) water wells, monitoring wells, mining bore-
holes, and geotechnical boreholes (see Figure 2-3). Some air drilling operations
on these small rigs use direct circulation, but can be converted for reverse circu-
lation operations. These small single drilling rigs usually have hydraulic top head
rotary drives. The rig “floor,” or breakout platform, of these small rigs is protected
from cutting returns by a rubber seal around the drill string and a flexible skirt
around the edge of the floor (skirt not shown in figure). When direct circulating,
the air returning up the annulus (with the entrained rock cuttings) is kept from
coming through the rig floor by the rubber seal around the drill string. The dril-
ling cuttings are allowed to accumulate on the surface of the ground around
the top of the borehole where the skirt slows the air flow and allows the cuttings
to be dropped out.
Reverse circulation provides a useful way for dealing with the return flow of
compressed air and entrained rock cuttings from the borehole. The compressed
air is injected into the annulus of the borehole via a sealed fitting at the top of
the annulus, or a dual drill pipe annulus fitting. After circulating through the
bit, the air with entrained rock cuttings exits the borehole through the inside
of the drill string, then flows through the top head rotary drive, and then through
the rotary hose. The air with the cuttings can be diverted to a pit away from the
rig with a hose extension, or a hose extension run to a cyclone separator where
cutting samples can be obtained for analysis. Air drilling operations using small
single drilling rigs are generally safer for the operators and more environmentally
clean when utilizing reverse circulation. More details on reverse circulation
operations are given in Chapter 7, 9, and 10.

3.3.2 Blowout Prevention Stack
Blowout prevention equipment was developed for use in drilling deep wells for
the recovery of oil and natural gas. Later this unique oil and gas industry equip-
ment was adapted for use in drilling deep geothermal wells. Natural deposits of
oil and gas exist in porous rock formations deep in the Earth’s crust. These depos-
its were created by millions of years of sediment burial and confinement by geo-
logic structures. Over time, increased sedimentary burial created high pressures
and temperatures in these deposits. Most newly discovered oil and natural gas
2
deposits have static pressures up to about 8000 psi (5518 N/cm ) and tempera-

tures of about 300 F (149 F). A few abnormally pressured natural gas deposits

2
have static pressures as high as 16,000 psi (10,346 N/cm ). These pressures,
although found in deposits at depths of 10,000 ft (3048 m) or greater, are quite
dangerous to drilling rig personnel and the environment. Blowout prevention
equipment (or the BOP stack) was developed to provide protection of the surface
from these high pressured deposits.
A blowout occurs when oil and/or natural gas deposits are allowed to flow
uncontrollably to the atmosphere at the surface. The first line of defense against