Page 289 - Drilling Technology in Nontechnical Language

280 Drilling Technology in Nontechnical Language Second Edition

steel strength and brittleness in the choke manifold. The normal working
pressure of the choke manifold might have to be derated by 50% or more.
When gas in the presence of water cools down and reduces pressure,
icelike compounds called hydrates may be formed. Hydrates can plug lines
during well killing, which presents a serious problem because it stops the
killing operation. Hydrate formation can be prevented by injecting glycol
into the choke manifold, so part of the preparation for HPHT drilling must
include provision of an injection facility for glycol.

Relief wells

When a blowout occurs, the rig is likely to be damaged or even
destroyed. It is also possible that the blowout will damage the wellhead
in a way that makes it impossible to enter the well from the top in order
to kill it. Under such circumstances, the only way to actually kill the well
is to drill another wellbore to intercept the blowing well. As explained
previously, the wellbore used to intercept the blowing well is called a
relief well.

The process of drilling a relief well requires fine directional drilling
control and an accurate knowledge of the path of the blowing well. The
relief well will be spudded from about 300 m (1,000 ft) away. It will deviate
so as to come alongside the blowing well, following it from a few meters
away and alongside the last casing that was set above the section of the
well that is blowing out. Wireline ranging tools run in the relief well will

detect the metal casing in the blowing well and confirm the azimuth and
direction to it. These tools work within a range of about 15 m. Casing will
be run and cemented at a similar depth to the casing on the blowing well,
so as to get the maximum strength below the casing shoe, ready to drill
the intercept.

With the two wellbores parallel and the casing shoes at a similar depth,

the final hole section is drilled. The drillstring in the blowing well provides
the means to use the ranging tools to find the well. (If there is no drillstring

in the well, the relief well will be drilled to intercept the casing and has
to drill or mill through it.) It is nudged towards the blowing well with an
intercept angle of around 10° and carefully guided until the interception is
made. Once the relief well intercepts the blowing well, heavy kill mud may
be pumped down the relief well and into the blowing well to kill it.

_Devereux_Book.indb 280 1/16/12 2:12 PM