Page 36 - Environmental Control in Petroleum Engineering

24 Environmental Control in Petroleum Engineering

shear thinning, which lowers the viscosity and required pumping power
during high pumping rates, when a high viscosity is not needed,

Density Control

Another important function of a drilling fluid is to control the fluid
pressure in the wellbore. Because many formations are hydrostatically
pressured or overpressured and the pressure in the wellbore must be
kept higher than that in the formation, the pressure in the wellbore
must normally be higher than the hydrostatic pressure for pure water
to prevent the well from blowing out. The fluid pressure in the
wellbore is controlled by varying the density of the drilling fluid. The
density is varied by adding heavy solids to the fluid.
Although the clays added to control the fluid viscosity also increase
the fluid density, their specific gravity of 2.6 and low concentration
in the mud is insufficient to provide the needed density for many
applications. Materials having a higher specific gravity are normally
required to obtain the desired mud density.
The most common material used to increase the density of drilling
mud is barite (barium sulfate, BaSO 4). Barite has a high specific
gravity of 4.2. In some wells requiring a very high density, barite can
constitute as much as 35% of the drilling fluid by volume. Because
of the high specific gravity of barite, viscosity control additives (clays)
are normally used to keep the barite suspended in the fluid.
Other materials that can be used to control drilling fluid density
include calcium carbonate, iron carbonate, ilmenite (FeO–TiO 2) and
hematite (Fe 2O 3). These materials are harder than barite and are less
susceptible to particle size reduction during drilling. Although these
materials have a lower specific gravity than barite, they have the added
benefit of lowering the barium concentration in the drilling rnud,
Galena (PbS) can also be used, but will result in lead being added to
the drilling mud. Rarely, barium carbonate has been used.

Lost Circulation Control

During drilling, fluid is lost to the formation as drilling fluid leaks
into permeable strata. To minimize this loss, small particles are added
to drilling fluids that will filter out on the formation face as fluid is
lost. These solids then form a low permeability mudcake that limits