Introduction
There’s a lot of references refer to drilling fluid system, which
sometimes confusing the reader about how many system that’s really
exist in drilling fluids, on this writing I would try to share my
knowledge in drilling fluids system.
Some of the text below was adopt from several references, to increase
the reader knowledge.
Drilling Fluids
According to API drilling fluids are fluids that circulate through drilling
process to support the drilling operation, and have significant
influences to the successful of the drilling operation.
These fluids were classified one after another according to the solvent
material, later it will call as mud system, see figure below to
understand each of them
Figure 1 Drilling Fluids System
These are the explanation for each of the system
Pneumatic Fluids
Pneumatic (air/gas based) fluids are used for drilling depleted zones or areas
where abnormally low formation pressures may be encountered. An
advantage of pneumatic fluids over liquid mud systems can be seen in
increased penetration rates. Cuttings are literally blown off the cutting surface
ahead of the bit as a result of the considerable pressure differential. The high
pressure differential also allows formation fluids from permeable zones to flow
into the wellbore. Air/gas based fluids are ineffective in areas where large
volumes of formation fluids are encountered. A large influx of formation fluids
requires converting the pneumatic fluid to a liquid-based system. As a result,
the chances of losing circulation or damaging a productive zone are greatly
increased. Another consideration when selecting pneumatic fluids is well
depth. They are not recommended for wells below about 10,000 ft because
the volume of air required to lift cuttings from the bottom of the hole can
become greater than the surface equipment can deliver.
Oil-Based Fluids
A primary use of oil-based fluids is to drill troublesome shales and to improve
hole stability. They are also applicable in drilling highly deviated holes
because of their high degree of lubricity and ability to prevent hydration of
clays. They may also be selected for special applications such as high
temperature/high pressure wells, minimizing formation damage, and native-
state coring. Another reason for choosing oil-based fluids is that they are
resistant to contaminants such as anhydrite, salt, and CO 2 and H2S acid gases.
Cost is a major concern when selecting oil-based muds. Initially, the cost per
barrel of an oil-based mud is very high compared to a conventional water-
based mud system. However, because oil muds can be reconditioned and
reused, the costs on a multi-well program may be comparable to using water-
based fluids. Also, buy-back policies for used oil-based muds can make them
an attractive alternative in situations where the use of water-based muds
prohibits the successful drilling and/or completion of a well. Today, with
increasing environmental concerns, the use of oil-based muds is either
prohibited or severely restricted in many areas. In some areas, drilling with
oil-based fluids requires mud and cuttings to be contained and hauled to an
approved disposal site. The costs of containment, hauling, and disposal can
greatly increase the cost of using oil-based fluids.
Water-Based Fluids
Water based fluids are the most extensively used drilling fluids. They are
generally easy to build, inexpensive to maintain, and can be formulated to
overcome most drilling problems. In order to better understand the broad
spectrum of water-based fluids, they are divided into three major sub
classifications:
1. Inhibitive
2. Non-inhibitive
3. Polymer
Figure 2 Water Based Fluids
1. Non-Inhibitive Fluids
Those which do not significantly suppress clay swelling, are generally
comprised of native clays or commercial bentonites with some caustic
soda or lime. They may also contain deflocculants and/or dispersants
such as: lignites, lignosulfonates, or phosphates. Non-inhibitive fluids
are generally used as spud muds. Native solids are allowed to disperse
into the system until rheological properties can no longer be controlled
by water dilution.
2. Inhibitive Fluids
Those which appreciably retard clay swelling and, achieve inhibition
through the presence of cations; typically, Sodium (Na+), Calcium (Ca++)
and Potassium (K+). Generally, K+ or Ca++, or a combination of the two,
provide the greatest inhibition to clay dispersion.These systems are
generally used for drilling hydratable clays and sands containing
hydratable clays. Because the source of the cation is generally a salt,
disposal can become a major portion of the cost of using an inhibitive
fluid.
3. Polymer Fluids
Those which rely on macromolecules, either with or without clay
interactions to provide mud properties, and are very diversified in their
application. These fluids can be inhibitive or non-inhibitive depending
upon whether an inhibitive cation is used. Polymers can be used to
viscosify fluids, control filtration properties, deflocculate solids, or
encapsulate solids. The thermal stability of polymer systems can range
upwards to 400°F. In spite of their diversity, polymer fluids have
limitations. Solids are a major threat to successfully running a cost-
effective polymer mud system.
