Page 116 - Drilling Technology in Nontechnical Language

Chapter 4 – PLANNING AND DRILLING A DEVELOPMENT WELL OFFSHORE 107

the reservoir section underbalanced; that is, the formation pore pressure

exceeds the mud hydrostatic. The reservoir will continually flow while the
well is drilled. If the problems of hydrocarbon production while drilling

can be handled, this almost guarantees that there will be no significant
damage to the formation face. There are horizontal wells drilled in Texas
that produce enough oil during drilling the reservoir that the well is paid
for by the time the well reaches the target depth!
On this rig, there is no facility to handle produced hydrocarbons, and
so the option to drill underbalanced is not available. The fluid used for

drilling the reservoir will be a special mud with no clay solids added. Clays
are especially damaging because they plug pore spaces and cannot be
removed with acid. Instead, acid-soluble solids such as calcium carbonate
can be used. These can later be removed by pumping acid down the well.
Once the well is drilled to the target depth, the drillstring can be pulled
out. A sand screen is run into the bottom of the well, over the full horizontal
section, to prevent sand from being produced with the oil. This is fixed in

place using a hanger (somewhat similar to a liner hanger).

Suspending the well

The well needs to be left in a safe and stable condition while further
wells are drilled and later while the platform is put in place on the template.
The practice of leaving a well for later reentry is called suspending
the well.
After placing the sand screen in the reservoir, the well has special plugs
(bridge plugs) set in the production casing. Cement will be placed on top
of at least one of these plugs, as extra insurance that any gas coming into
the well cannot migrate up to the surface. These can be drilled out later on.
Most operators and government regulations require at least three
physical barriers to prevent hydrocarbons reaching the surface. In this

well, one barrier is provided by the hydrostatic pressure of the fluid in the
well, one by the bridge plug, and one by the cement plug.
Finally, a suspension cap is placed on top of the wellhead. This is a

special cover that latches onto the wellhead. Corrosion inhibitor fluid is
pumped into the cap to help resist corrosion during the time that the well
is suspended.

_Devereux_Book.indb 107 1/16/12 2:08 PM