Drilling Fluid Functions |
Fig. 1 Drilling fluid (mud)
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Drilling fluid is an important component in the drilling process.
A fluid is required in the wellbore to:
- Cool and lubricate the drill bit,
- Remove the rock fragments, or drill cuttings, from the
drilling area and transport them to the surface,
- Counterbalance formation pressure to prevent formation
fluids (such as oil, gas, and water) from entering the well
prematurely (which can lead to a blowout), and
- Prevent the open (uncased) wellbore from caving in.
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Drilling Fluid Types |
There are several types of drilling
fluids used depending on the drilling conditions encountered:
- Water-based muds are used most frequently. The base may be either:
- fresh water, or
- salt water.
- Oil-based muds.
- Synthetic materials. The oil and gas
extraction industry has developed many new oleaginous (oil-like)
base materials from which to formulate high-performance drilling
fluids.
A general class of these fluids is called synthetic materials,
such as
- The vegetable esters,
- Poly alpha olefins,
- Internal olefins,
- Linear alpha olefins,
- Synthetic paraffins,
- Ethers, and
- Linear alkylbenzenes, among others.
- Air and foam fluids may be used in drilling wells.
- These fluids are less dense than drilling muds.
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Drilling Fluid Additives |
Fig. 2 Additive mixing hopper
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Drilling muds typically have several additives. (Air and foam
fluids typically do not contain many additives because the additives
are either liquid or solid, and will not mix with air and foam
drilling fluids.) The following is a list of the more significant
additives:
- Weighting materials, primarily barite (barium sulfate), may be
used to increase the density of the mud in order to equilibrate
the pressure between the wellbore and formation when drilling
through particularly pressurized zones. Hematite (Fe2O3 )
sometimes is used as a weighting agent in oil-based muds (Souders,
1998).
- Corrosion inhibitors such as iron oxide, aluminum bisulfate,
zinc carbonate, and zinc chromate protect pipes and other
metallic components from acidic compounds encountered in the
formation.
- Dispersants, including iron lignosulfonates, break up solid
clusters into small particles so they can be carried by the
fluid.
- Flocculants, primarily acrylic polymers, cause suspended
particles to group together so they can be removed from the
fluid at the surface.
- Surfactants, like fatty acids and soaps, defoam and emulsify
the mud.
- Biocides, typically organic amines, chlorophenols, or
formaldehydes, kill bacteria and help
reduce the souring of drilling mud.
- Fluid loss reducers include starch and organic polymers and
limit the loss of drilling mud to under-pressurized or
high-permeability formations.
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