Page 37 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 25

further fluid loss. In most cases, the clay particles added to control
the viscosity of a drilling fluid are successful in controlling fluid loss
to the formation.
In some formations, however, the pore sizes may be so large that
the clay particles are unable to bridge the pores and build a filter cake.
Such formations may include those having natural or induced fractures,
very high permeability sands, or vugs. To limit fluid loss in such
formations, larger solids can be added to the drilling fluid, A mudcake
of clay particles is then built on the bridge created by those solids,
Solids that are commonly used for this application include mica, cane
fibers, ground nutshells, plastic, sulfur, perlite, cellophane, cottonseed
hulls, and sawdust.
If solids cannot be used to build a filter cake, the viscosity of the
drilling fluid can be increased to limit fluid loss. Water-soluble poly-
mers like starch, sodium polyacrylate, and sodium carboxymethyl-
cellulose can be used.

A high mud pH between 9.5 and 10.5 is almost always desired in
drilling operations. A high pH suppresses the corrosion rate of drilling
equipment, minimizes hydrogen embrittlement of steel if hydrogen
sulfide enters the mud, lowers the solubility of calcium and magnesium
to minimize their dissolution, and increases the solubility of ligno-
sulfonate and lignite additives. A high pH is also beneficial for many
new organic viscosity control additives. To keep the pH in the desired
range, caustic (sodium hydroxide) is normally added to the mud. Some
of the new polymer muds, however, have better shale stabilization
properties -at a lower pH (Clark, 1994).

Lubricants

During drilling, a considerable amount of friction can be generated
between the drill bit and formation and between the drill string and
wellbore walls, particularly for deviated and horizontal wells. To
reduce this friction, lubricants are sometimes added to drilling fluids.
These lubricants speed drilling and help maintain the integrity of the
well. Common lubricants include diesel oil, mineral/vegetable oils,
glass beads, plastic beads, wool grease, graphite, esthers, and glycerols.