Page 31 - Environmental Control in Petroleum Engineering

20 Environmental Control in Petroleum Engineering

pits, especially in offshore operations. Pits are also used to store
supplies of water, waste fluids, formation cuttings, rigwash, and
rainwater runoff.

2.1.2 Drilling Fluids

Drilling fluids serve a number of purposes in drilling a well. In most
cases, however, the base fluid does not have the proper physical or
chemical properties to fulfull those purposes, and additives are required
to alter its properties. The primary purpose of drilling fluid is to
remove the cuttings from the hole as they are generated by the bit
and carry them to the surface. Because solids are more dense than the
fluid, they will tend to settle downward as they are carried up the
annulus. Additives to increase the fluid viscosity are commonly used
to lower the settling velocity.
Drilling fluids also help control the well and prevent blowouts.
Blowouts occur when the fluid pressure in the wellbore is lower than
the fluid pressure in the formation. Fluid in the formation then flows
into the wellbore and up to the surface. If surface facilities are unable
to handle this flow, uncontrolled production can occur. The primary
fluid property required to control the well is the fluid's density,
Additives to increase fluid density are commonly used.
Drilling fluids also keep the newly drilled well from collapsing
before steel casing can be installed and cemented in the hole. The
pressure of the fluid against the side of the formation inhibits the walls
of the formation from caving in and filling the hole. Additives are
often used to prevent the formation from reacting with the base fluid.
One common type of reaction is shale swelling.
A final function of drilling fluids is to cool and lubricate the drill
bit as it cuts the rock and lubricate the drill string as it spins against
the formation. This extends the life of the drill bit and reduces the
torque required at the rotary table to rotate the bit. Additives to
increase the lubricity oT the drilling fluid are commonly used, particu-
larly in highly deviated or horizontal wells.
Many of the additives used in drilling fluids can be toxic and are
now regulated. To comply with new regulations, many new additives
have been formulated (Clark, 1994), These new additives have a lower
toxicity than those traditionally used, thus lowering the potential for
environmental impact.