PgDip/MSc Energy Programme/Wells Drilling Fluid – Composition
Drilling Fluid – Composition
Preview
This topic introduces the physical phases of muds and the basic
chemistry of drilling fluids.
Content
Composition of Muds
The first discussion looked at what a mud is required do, and at what
type of properties are needed in a mud. The next step is to examine the
basic chemistry involved, and to deal with specific mud additives. To get
a good grasp of these things, it is important to understand the
composition of muds.
Muds consist of a mixture of liquids, solids, and in some cases gases.
These are normally referred to as the liquid phase, solids phase and gas
phase. In most muds at least two phases are present. Exceptions to this
are gas alone in air drilling, or liquid alone in a clear brine completion
fluid.
Where we have two liquids which do not mix (eg, oil and water), the fluid
is described as having two liquid phases. Normally when there are two
liquid phases present, one liquid is emulsified in the other (eg, in Oil
Based Mud water is emulsified in the oil). To distinguish between the
liquid phases one is called the continuous phase and the other is called
the discontinuous or internal phase,, because it is present only as
isolated, emulsified drops. In OBM the continuous phase is the oil, while
the discontinuous phase is water.
Some examples of the different phases follow:-
1. Gaseous Phase
This can be a continuous phase as in air drilling, or a discontinuous
phase, as in foam drilling. Here the gas, normally air, is contained in
separate bubbles in the foam.
2. Liquid Phase
As with the gases, the liquid phase can be either continuous or
discontinuous. In either case the liquid phase is made up as follows :-
a) The Base Fluid for example:
• Water
• Crude Oil
• Diesel
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PgDip/MSc Energy Programme/Wells Drilling Fluid – Composition
• Paraffin Oil
• Low Toxicity Oil
• Synthetic Oil
b) Products which dissolve in the Base Fluid
These may be Mud Products which have been added to the Base Fluid to
modify its properties. Examples are shown below:
• Salts for density
• Salts for inhibition
• Chemicals for alkalinity
• Asphalts to alter viscosity
There may also be Soluble Formation Products introduced into the mud
from reaction between the mud and the formation, such as:
• Salts from salt sections
• Ions from clay reactions
All solids which dissolve will to some extent change the properties of the
Base Fluid.
3. Solid Phase
Unlike the other two phases, the Solid Phase is always discontinuous. It is
made up of:
a) Mud Products which have been added to the mud.
These may be Active Solids which react with the Base Fluid to provide
particular mud properties, such as prehydrated Bentonite for viscosity.
They may also include Inert Solids which do not react with the fluids, for
example Barite added to increase mud density.
b) Drill Solids from the formation.
As with the Mud Products above. Drill Solids can include both Active and
Inert Solids, because some formations react with the mud, and some do
not.
Mud Control
It is important to understand the difference between the added products,
and those from the formation. Mud Tests are carried out by the Mud
Engineer to establish the composition of the mud. Obviously the Mud
Engineer has control of any products added to the mud. One purpose of
the tests is to establish what additions are necessary. With formation
products the ideal situation is that nothing from the formation dissolves
in the Base Fluid, and that all Drill Solids are removed from the mud. The
mud tests tell the Mud Engineer how close he is to achieving this ideal. At
the start of each interval, the mud is made up to the properties specified
in the mud programme. As soon as drilling starts, formation solids
become mixed with the mud. These solids may dissolve in the Base Fluid,
react with components of the mud, or simply remain suspended.
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PgDip/MSc Energy Programme/Wells Drilling Fluid – Composition
Mud control is directed at preventing reactions between the mud and the
formation. The intention is to try to keep the Drill Solids in large, easily
removed pieces. This is a major difference between oil-based and water-
based muds. In oil base muds reaction between fluid and drill solids is
minimal.
Water Base Muds and Oil Base Muds
Water is cheap and readily available. However it is very good at
dissolving things, and allows reactions to take place quickly.
Maintenance is directed at making the water a less effective solvent, and
at slowing down the reactions that may occur.
Oils by contrast are not very reactive. However they are expensive, and
they are pollutants. Maintenance is directed at reducing waste. This is
both for cost and for environmental reasons.
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