Page 63 - Air and Gas Drilling Manual

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 will be given in
Chapter 9 and 10. Chapter 2: Surface Equipment 2-13
2.3.2 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 equipment
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 deposits were
created by millions of years of sediment burial and confinement by geologic
structures. Over time, increased sedimentary burial created high pressures and
temperatures in these deposits. Most newly discovered oil and natural gas deposits
have static pressures up to about 8,000 psi and temperatures of about 300˚F. There
are a few abnormally pressured natural gas deposits that have static pressures as high
as 16,000 psi. These pressures, although found in deposits at depths of 10,000 ft or
greater, are quite dangerous to drilling rig personnel and the environment. Blowout
prevention equipment (or the BOP stack) were developed to provide protection of
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 the
dangers of these high pressure deposits is weighted drilling mud. Water based and
oil based drilling muds can be designed so that their specific weights are sufficiently
high to provide bottomhole pressures that are slightly higher than the static pressure
of the deposits when the drill bit penetrates the host rock formation. When drilling
exploratory wells it is not possible to precisely know the static pressure in target oil
or natural gas deposits. Therefore, geologic and engineering judgment must be used
to estimate the static pressures that might be encountered. These estimates are used
to design the weighted drilling mud. But even after the first exploratory wells have
been successfully drilled and the oil or gas field is being developed with follow-on
development wells, surprises in deposit pressures can occur. When too light a
drilling mud is used and a high pressure deposit is drilled, the well will receive a
liquid or gas “kick.” A kick is a slug of formation liquid and/or gas that has flowed
from the formation into the annulus of the well bore. The kick is composed of
fluids that have lower specific weights than the heavily weighted drilling muds.
Therefore, the kick will “float” in the drilling mud and rise rapidly to the surface. If
the kick is mostly natural gas, the gas will expand as it moves up the drill string
annulus to the surface. The surface wellhead equipment is the second line of defense
against a blowout. The wellhead equipment in the form of the BOP stack must be
engineered so that it is capable of containing the high pressure of this gas when it
reaches the top of the annulus. This BOP stack must contain this gas pressure while
the slug is circulated under control to the surface and expelled from the annulus via a
flow line to a remote burn area where the slug can be safely burned off.
The BOP stack can be composed of two types of preventers; 1) the ram-type
blowout preventer and, 2) the annular-type preventer. The ram-type preventer can be
a blind (shear) ram and or a pipe ram. The blind ram is capable of sealing the well
completely by compressing the drill pipe from two sides and failing the pipe steel