Page 90 - Drilling Technology in Nontechnical Language

Chapter 3 – DRILLING A LAND EXPLORATION WELL 81

the liner is perforated with this fluid in the well, those solid particles will

enter the perforation tunnel and can plug off pore spaces in the reservoir.
This is one possible mechanism that will reduce the ability of the well to
produce oil. This damage must be minimized.
First, a drill bit is made up on the BHA, followed by a tool called a
scraper. A scraper has sets of spring-loaded blades that push against the
casing, scraping off loose rust, grease, etc. When this is at the bottom of

the liner, a clean, filtered brine is pumped around the well that contains
very little solids. This pushes the drilling mud out of the hole. With the

mud out, brine is circulated around until it is as clean as is specified by
the test program. When the well is sufficiently clean, the bit and scraper

are removed.

Running the test string

Now the well is clean and contains a clean, specially formulated, solids-
free brine. A string of tubing is run in through which the hydrocarbons
will be produced. In the “old days,” drill pipe was often used, and hence
the production test became known as a drillstem test. Drill pipe is not used
any more for well testing, because there is no guarantee that it will seal
with high-pressure hydrocarbons inside it.

The well should not produce hydrocarbons from inside the casing except
through a separate tubing string. If tubing is in place, the well integrity is

much better (safer), and the well may be controlled by circulating fluids
around, which cannot be done without tubing in the well.
The tubing string will have various tools incorporated. One will be
a subsurface safety valve (SSSV), as previously discussed. The SSSV is
positioned some hundreds of feet or meters below the surface (or below the
seabed on an offshore well). It is a valve that would normally be closed but
is held open by pressure on a special line that is run to the surface outside
of the tubing. If something goes wrong, the control line to the SSSV is
depressured, and this closes the valve. Also if something disastrous
happens, such as the wellhead gets blown off by marauding Iraqi troops,
the well will shut itself in automatically. Had the wells in the Kuwaiti
desert incorporated SSSVs, the result of Saddam Hussein’s deliberate
sabotage would have been minor oil spillage around the wellheads and not

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