Page 30 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 19

of drilling through the earth, e.g., drill cuttings, and some come from
materials used to drill the well, e.g., drilling fluid and its associated
additives. This section reviews the drilling process, the drilling fluid
composition, methods to separate cuttings from the drilling fluid, the
use of reserves pits, and site preparation,

2.1.1 Overview of the Drilling Process

Most oil and gas wells are drilled by pushing a drill bit against the
rock and rotating it until the rock wears away. A drilling rig and
system is designed to control how the drill bit pushes against the rock,
how the resulting cuttings are removed from the well by the drilling
fluid, and how the cuttings are then removed from the drilling fluid
so the fluid can be reused.
The major way in which drilling activities can impact the environ-
ment is through the drill cuttings and the drill fluid used to lift the
cuttings from the well. Secondary impacts can occur due to air emis-
sions from the internal combustion engines used to power the drill-
ing rig.
During drilling, fluid is injected down the drill string and though
small holes in the drill bit. The drill bit and holes are designed to allow
the fluid to clean the cuttings away from the bit. The fluid, with
suspended cuttings, then flows back to the surface in the annulus
between the drill string and formation. At the surface, the cuttings are
separated from the fluid; the cuttings, with some retained fluid, are
then placed in pits for later treatment and disposal. The separated fluid
is then reinjected down the drill string to lift more cuttings.
The base fluid most commonly used in the drilling process is water,
followed by oil, air, natural gas, and foam. When a liquid is used as
the base fluid, either oil-based or water-based, it is called "mud."
Water-based drilling fluids are used in about 85% of the wells drilled
worldwide. Oil-based fluids are used for virtually all of the remain-
ing wells.
During the drilling process, some mud can be lost to permeable
underground formations. To ensure that mud is always available to
keep the well full, extra mud is always mixed at the surface and
kept in reserves or mud pits for immediate use. Reserves pits vary in
size, depending on the depth of the well. The pits can be up to an
acre in area and be 5-10 feet deep. Steel tanks are also used for mud