Page 20 - Drilling Technology in Nontechnical Language

Chapter 1 – DRILLING GEOLOGY 11

be drilled with special techniques, such as using foam as a circulating
medium rather than liquid mud.
Limestones often contain chunks of chert (flint), which is an amorphous

(without crystals) form of quartz. It results from the percolation of silica-

rich pore fluids. The rock breaks along curved surfaces, forming knife
edges and sharp points. Cherts can break teeth on drill bits while drilling.
Carbonates comprise about 13% of sedimentary rocks.

Evaporites (salts)

Evaporite sequences occur as a result of sea water evaporating, leaving
the soluble salts behind. There is a definite order of precipitation, as the

least soluble salts come out of solution and are deposited first. The most

soluble salts, which come out of solution last, are various rare potassium
and magnesium salts. These latter salts are very soluble and would only
precipitate out if dehydration was almost complete.

The complex sequences present in a mixed salt formation lead to
several problems that cannot be solved by using a conventional (NaCl)

salt-saturated, water-based drilling mud as a circulating fluid. The
highly soluble magnesium and potassium salts will dissolve in a sodium
chloride–saturated solution. This can give greatly enlarged holes with
many attendant problems.
Some types of salt can fl ow just as ice in a glacier fl ows. It can create
tremendous forces that act on any obstacle in its path, such as a well. It is
possible for flowing salt to break a well in half or to crush the steel casing

that lines the well. Salt can flow so fast that a hole can close around the
drill bit as it drills, stopping the bit from turning. Often when that happens,
fresh water has to be pumped down around the bit to dissolve some of the
salt in order to free the bit.

As salt is lighter than most other rocks, bubbles of the salt can try
to rise up through the rock above it, like a bubble of oil rising through
water. Of course, it takes millions of years for this to happen, rather than
a few seconds. These salt domes can be huge and can create traps for
hydrocarbons. Salt domes create suitable conditions for many reservoirs in
the Gulf of Mexico.

_Devereux_Book.indb 11 1/16/12 2:06 PM