Page 61 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 49

enough so that the fluid pressure in the wellbore exceeds the tensile
strength of the formation, rupturing the rock.
The most commonly used base fluid for hydraulic fracturing
is water. Water is inexpensive and inflammable. Various hydro-
carbons can also be used as a base fluid, particularly where surface
freezing may occur. Acid is also occasionally used when a com-
bination of acidizing and hydraulic fracturing is desired. Liquefied
gases, such as carbon dioxide or liquefied petroleum gases, can also
be used, particularly to fracture gas wells. The use of a liquid base
fluid in gas wells can reduce the gas production rate by lowering the
gas relative permeability.
After fracturing, the fluid pressure in the fracture drops when the
well is placed back on production. This allows the fracture to close.
To keep the fracture open during production, solids are injected with
the base fluid to fill the fracture and prop it open. Materials used for
proppants include sand, aluminum pellets, glass beads, walnut shells,
and plastic beads.
To lower the pump size required to fracture the rock, additives are
used to increase the viscosity of the fracturing fluid to enhance its
proppant-carrying capability. To viscosify the water-based fracture
fluids, polymers such as guar or xanthan gum, cellulose, or acrylics
can be used. These polymers are frequently cross-linked with metal
ions like boron, aluminum, titanium, antimony, or zirconium to further
enhance their viscosity. To viscosify the oil-based fracture fluids,
aluminum phosphate esters are commonly used. Surfactants are also
occasionally used to create a liquid-air foam or oil-water emulsion to
be used as the fracture fluid. To prevent degradation of many gels at
high temperatures, stabilizers like methanol and sodium thiosulfate can
be added.
Most polymers and cross-linkers operate in a solution having an
optimum pH. For fluids needing a low pH, buffers of acetic, adipic,
formic, or fumeric acids can be used. For fluids needing a high pH,
sodium bicarbonate or sodium carbonate can be used.
Many formations have sensitive clays that swell during water
injection from the exchange of small cations inside the clays with
larger water molecules. Swelling clays plug the pores, limiting fluid
flow. Clay minerals can also break loose and migrate through the
pore network to lodge in pore throats and limit fluid flow. Clay stabil-
izers are often used to prevent such damage. Temporary stabilization