Page 58 - Air and gas Drilling Field Guide 3rd Edition

3.3 Wellhead Equipment 49

the dangers of these high pressure deposits is weighted drilling mud. Water- and
oil-based drilling muds can be designed so that their specific weights are suffi-
ciently high to provide bottom hole pressures that are slightly higher than the
static pressure of the deposits when the drill bit penetrates the host rock forma-
tion. When drilling exploratory wells it is not possible to precisely know the
static pressure in target oil or natural gas deposits. Therefore, geologic and engi-
neering judgment must be used to estimate the static pressures that might be
encountered. These estimates are used to design the weighted drilling mud.
However, even after the first exploratory wells have been drilled successfully
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 forma-
tion 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 naturalgas,the gas willexpand 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 burned off safely.
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 structure in a manner to prevent the well fluids from escaping to the
surface through either the inside of the drill pipe or around the outside of the drill
pipe. This vise-like action of the two rams essentially forces the pipe to deform
between the two rams. The pipe ram acts in a somewhat similar manner as the
blind ram, except that the pipe ram has a geometric shape on the end of the
ram that conforms to the outside surface of the drill pipe. Thus the pipe ram seals
against the outside of the drill pipe and prevents well fluids from escaping to the
surface around the outside of the drill pipe. The pipe ram does not fail the pipe
structure; therefore, drilling mud can be circulated down the inside of the drill
pipe to safely allow the kick to be circulated to the surface.
Figure 3-11 shows a cutaway view of a twin ram-type blowout preventer.
A typical twin preventer will have a pipe ram on the top and a blind ram on
the bottom. The cutaway shows the bottom blind ram. In the event of a blowout,
the pipe ram would be used to seal the well and allow the slug in the annulus to
be circulated to the surface safely. In the event that the pipe ram cannot seal the
well for the safe circulation of the slug, the blind ram can be actuated to seal