Page 62 - Environmental Control in Petroleum Engineering

50 Environmental Control in Petroleum Engineering

methods include adding salts to the fluids to minimize exchange of
water molecules with the cations in the clays. These salts are then
returned when the well is placed on production. Salts used as tempo-
rary stabilizers include sodium chloride, potassium chloride, calcium
chloride, and ammonium chloride. Permanent stabilizers, such as
quaternary amines and inorganic polynuclear cations like zirconium
oxychloride or hydroxyaluminum, bond to the clay surfaces to stabilize
them. Permanent stabilizers remain in the formation and are not
removed with produced fluids.
When the viscosity of the fracture fluid is increased, the pressure
drop in the pipe is also increased from friction. This results in a higher
pressure at the pump, but a lesser increase in pressure at the formation
face where it is needed. To suppress the pressure drop in the pipe,
high molecular weight polymers can be added to the fracture fluid.
These polymers suppress turbulence, keeping the flow in the pipe
laminar and lowering the friction losses.
A related method for reducing the pressure drop in the pipe is to use
a cross-linking polymer that has a slow gelling time. The cross-linkers
are added to the polymer at the wellhead just prior to injection. The
mixing is timed so that the gel reaches its maximum strength when it
reaches the formation face. This causes the maximum fluid pressure
at the formation face and minimizes the pressure drop down the pipe.
The polymers used to alter the viscosity of fracturing fluids are
subject to bacterial degradation. Bactericides, such as glutaraldehyde,
chlorophenates, quaternary amines, and isothiazoline, are often added
to control the level of bacteria.
To control fluid loss into high permeability zones, fluid loss addi-
tives can be added to fracture fluids. These solids include silica flower,
granular salt, carbohydrates, and proteins for water-based fluids and
organic particulates such as wax, pellets, or naphthalene granules for
oil-based fluids. Another popular fluid loss method is to use an oil-
in-water emulsion. This causes two-phase flow through the filter cake
along the fracture wall, lowering the relative permeability of the water
through the filter cake.
After a fracture has been created, breakers are used to lower the gel
viscosity so the fracture fluids can be easily removed from the fracture
and not inhibit subsequent production. A common breaker for water-
based fracture fluids are peroxydisulfates. Altering the pH by adding
acids or bases is a common way to break oil-based fracture fluids.