Drilling muds are complex heterogeneous fluids,
consisting of several additives that were employed in drilling of oil and
natural gas wells since the early 1900. The original use of the drilling fluids
was to remove cuttings continuously. Progress in drilling engineering demanded
more sophistication from the drilling mud. In order to enhance the usage of
drilling fluids, numerous additives were introduced and a simple fluid became a
complicated mixture of liquids, solids and chemicals (Moore, 1974). As
the
drilling fluids evolved, their design changed to have common characteristic
features that aid in safe, economic and satisfactory completion of a well. In
addition, drilling fluids are also now required to perform following functions
(Growcock & Harvey, 2005; Darley & Gray, 1988)
·
Clean the rock formation beneath the bit
for rock cuttings;
·
Transport these rock cuttings to surface
through annulus;
·
Suspend cuttings in fluid if circulation is
stopped;
·
Cool and clean the bit;
·
Manage formation pressure to maintain
well-bore stability until the section of borehole has been cased;
·
Assist in cementing and completion of
well;
·
Seal the formation pores by forming
low-permeability filter cake to prevent inflow of
·
formation fluids into the well;
·
Provide necessary hydraulic power to
down-hole equipment;
·
Minimize reservoir damage;
·
Helps in collection and interpretation
of data available through drill cuttings, cores, and electrical logs;
·
Be favourable for freshly drilled bore
hole’s integrity and assessment;
·
Reduce any damaging effects on the
sub-surface equipment and piping;
·
Provide frictionless environment between
the drilling string and the sides of the hole; and
·
Have minimum negative impact to the
environment.
In order to ensure proper functionality of drilling
fluid appropriate drilling fluid is selected, an adequate understanding of the
key factors governing the selection of the fluid is critical.
1.1 SELECTION OF DRILLING MUD
Drilling engineers select specific drilling mud with
most favourable properties for the job. Most of the drilling fluid functions
are controlled by its rheological properties. A drilling fluid specialist or a
“Mud Engineer” is often on site to maintain and revaluate these properties as
drilling proceeds. The main factors governing the selection of drilling fluids
are;
Ø The
types of formation to be drilled;
Ø The
range of temperatures; and
Ø Strength,
permeability and pore fluids pressure exhibited by the formation.
While, in addition to the above, selection of the
drilling fluid can be informed through
consideration of other factors such as - production concerns,
environmental impact, safety and logistics, the most important factor that governs
selection of drilling fluid is the “overall well cost”.
1.2
DRILLING MUD RHEOLOGY
Rheology is the study of the deformation of fluids
and flow of matter. Its importance is recognised in the analysis of fluid flow
velocity profiles, fluid viscosity (marsh funnel viscosity, apparent viscosity
and plastic viscosity), friction pressure losses and annular borehole cleaning.
Rheological properties are basis for all analysis of well bore hydraulics and
to assess the functionality of the mud system. Rheological characteristics of
drilling mud also include yield
point and gel strength. Rheological properties (such
as density, viscosity, gel strength etc.) are tested throughout the drilling
operations. It is critical to control and maintain rheological properties as a
failure to do so can result in financial and loss of time, and in extreme
cases, it could result in the abandonment of the well (Darley & Gray,
1988). Besides rheological other
tests such as filtration tests, pH, chemical
analysis (alkalinity and lime content, chloride, calcium, etc.), resistivity
are conducted throughout drilling process.
1.3 DRILLING MUD ADDITIVES
To match the requirements of different depth
intervals, the properties for drilling fluids are modified using various
additives for the drilling process. Properties such as density, flow properties
or rheology, filtration and solid content as well as chemical properties must
be accurately measured,controlled and appropriately maintained at their
pre-selected level throughout drilling process.
Additives commonly used in drilling muds are broadly
classified into
·
Viscosifiers;
·
Viscosity reducers;
·
Weighting materials;
·
Fluid-loss reducers;
·
Emulsifiers;
·
Lost circulation materials;
·
Flocculants;
·
Corrosion control chemicals;
·
Defoamers; and
·
pH control additives.
A detailed description of each is presented in next
chapter.
1.4
CLASSIFICATION OF DRILLING MUDS
Physical and chemical properties of the drilling
fluids largely depend on the type of solids in the mud. These solids are
categorized as either active or inactive solids. The active solids are those that
react with water phase and the dissolved chemicals. On the other hand, the
inactive solids are those that do not react with the water and chemical to a
significant degree (Bourgoyne Jr.,
Millheim, Chenevert, & Young Jr., 1986; Azar
& Samuel, 2007). Some examples of the inactive solids include - Barite and
Hematite, these are added to drilling fluids as weighing agents. Examples of
inactive fluids include - clays, polymers and other chemicals, which are
viscosity enhancers.
Drilling fluids are classified into Pneumatic, or
Liquid, or Pneumatic-liquid mixtures. Broad classification of drilling fluids
is shown below:
Table 1.1 Classification of Drilling Fluids
(Bourgoyne Jr., Millheim, Chenevert, & Young Jr., 1986)
|
Pneumatic
Fluids
|
Liquid
Fluids
|
Pneumatic-Liquid
Mixtures
|
|
Air; Natural gas
|
Water
based muds; Oil based muds
|
Foam
(mostly gas); Aerated Water (mostly
water
|
Pneumatic
Drilling Fluids
Pneumatic drilling fluids are recommended for
formation where there is potential for circulation loss. Pneumatic drilling
fluids are used for underbalanced drilling. Pneumatic drilling is known to have
improved rate of penetration, better control of loss circulation zones and less
damage to formations. However, Pneumatic drilling fluids especially dry
air/natural gas, have been responsible for causing fire and corrosion to
down-hole equipment (Azar & Samuel, 2007).
Liquid
Fluids
Water based muds have water as continuous phase and
mixture of solids, liquids and chemicals, as additives. In general, liquid
phase drilling is more prominent as these are water based and are preferred
over the oil based muds due to their economical and a requirement for less
strict pollution control measure (Bourgoyne Jr., Millheim, Chenevert, &
Young Jr., 1986).
Oil
Based Drilling Muds
Oil based drilling muds have liquids phase as oil
(diesel, mineral or synthetics). All solids in oil based muds are considered
inactive as they do not react with oil. Oil based muds are highly
temperature-stable fluids. However, use of oil based muds requires strict
safeguards for environmental protection and safety. Oil based muds are preferred
for high temperature formations, water sensitive shale’s, thick salt sections
and low-pore-pressure formations.
1.5
CORROSION IN DRILLING
Corrosion is the degradation of metal due to
reaction with its environment. Metal degradation is implied by deterioration of
physical properties of metal, it can either be loss of weight, corrosion fatigue
or stress corrosion cracking.
Corrosion costs the oil industry billions of dollars
a year, a fact that makes the role of the corrosion engineer an increasingly
important field.
According to Oxford Jr. and Foss (1958), corrosion
in oil and gas industry can be classified into following types
1. Sweet corrosion;
2. Sour corrosion;
3. Oxygen corrosion; and
4. Electrochemical corrosion.
As earlier stated,Corrosion costs billions of
dollars to the industry every year, some of which is avoidable. Cron and Marsh
(1983) stated that cost of corrosion in the US was $70 billion (approx.), of
which $10 billion was considerably avoidable. According to the 2002 U.S.
corrosion study, the direct cost
of metallic corrosion is $276 billion on an annual
basis (Koch, Brongers, Virmani, & Payer, 2002
2.0 Literature Review
Drilling engineering is one of the challenging
disciplines in the petroleum industry. Significant advancements have been made
in past decades which have allowed the petroleum industry worldwide to
economically and successfully exploit underground reverses that were not been
possible before. Considerable research studies have been done in drilling fluid
technology to understand drilling fluid properties for successful and
economical completion of an oil well. The cost of the drilling fluid itself is
relatively small, but the maintenance of the right properties while drilling
profoundly influence total well costs. American Petroleum Institute has a
recommended practice for testing liquid drilling fluid properties; regular
interval testing of drilling fluid properties help mud engineers determine
proper functioning of drilling fluid. Bourgoyne Jr. (1986) mentioned that
drilling fluid is related directly or indirectly to most of drilling problems.
2.1 DRILLING MUD DIAGNOSTICS TEST
Diagnostic testing of drilling mud is not limited to
type of drilling mud for each hole interval but also to properties of such
muds; density, rheology (flow properties), filtration and solid content, as
well as chemical properties. Mud properties are field controlled and properly
maintained at their preselected values; to avoid drilling problems.
2.2 RHEOLOGY: VISCOSITY, YIELD POINT AND GEL STRENGTH
Rheology is the study of deformation of all forms of
matter but its greatest development has been in study of the flow behaviour of
fluids through conduits and pipes. Drilling fluids are classified into two
major groups: Newtonian fluids where
viscosity, µ, is independent of shear rate where
as non-Newtonian
fluids, viscosity is function of shear rate µ = µ (γ). Viscosity measures
the resistance to flow. Excessive viscosity is undesirable because of the
pressures that can be generated by higher viscosity in the borehole when
pumping horizontally. Newtonian fluids are those whose flow behaviour can be
fully described by a single term called the Newtonian viscosity, µ. For these
fluids, examples of which include water and light oil, the shear stress (τ;
force per unit area) is directly proportional to the shear rate (γ; in time−1),
see fig.
2.1. This implies if the shear stress is doubled, the
shear rate is also doubled and vice versa. The rheological equation is given as
τ =
τ0 + µ γ..................................... (2.1)
The equation shows that fluid begin to flow as soon
as a shearing force is applied. The fraction, is constant at constant
temperature and pressure and is called the dynamic viscosity of the fluid
expressed in poise or centipoise (cP); 1 cP = 0.001 kg/m∙s (Bourgoyne Jr.,
Millheim, Chenevert, & Young Jr., 1986).
Non-Newtonian fluids are those whose viscous
properties cannot be described by a single term. Rather the viscous properties
are approximated to behave in accordance with one of the following assumed
models; Bingham Plastic; Power law models or Herschel–Bulkley (HB) Model.
Moreover, Non-Newtonian fluids exhibit shear rate
dependency; if the apparent viscosity decreases with increasing shear rate they
are called pseudo-plastic fluids and if the apparent viscosity increases with
increasing shear rate they are referred to as dilatant fluids. Bourgoyne (1986)
mentioned that if fluid behaviour is shear-time dependent then they could be
classified into thixotropic, if apparent viscosity decrease with time after shear
rate is increased to new constant values; and rheopectic, if apparent viscosity
increases with time after shear rate is increased to a new constant value. Bingham
(1922) initially recognized plastic fluids; therefore they are referred to as
Bingham plastics fluids; which are distinguished from Newtonian fluids as they
require a finite stress to initiate flow. Bingham Plastic fluids, do not flow
until the applied shear stress τ, exceeds a certain minimum value (τy) known as
the yield point. Once the yield point has been exceeded, changes in shear
stress are proportional to changes in shear rate and the constant of proportionality
is called plastic viscosity (µp) (Clark, 1995). Bingham plastic fluids flow behaviour
for laminar flow is described by following equation:
τ =
τ y+ µp γ; τ ≥ τ y........................... (2.2)
2.3 CORROSION IN DRILLING
When it comes to
manufacturing projects within the oil and gas industry, one of the biggest
challenges to overcome is tackling the different types of corrosion that parts are
often exposed to.
Corrosion is the major cause of various forms of
drill string failures. Corrosion combined with mechanical cyclic loading causes
corrosion fatigue which results in major drill string premature failure. It is
an extreme cause of concern for those in drilling business. The issues of cost
and money in prevention of corrosion during drilling operation is very
important. It is important to prevent and understand fundamentals of corrosion
(Patton, 1974).It has also been shown that the presence of dissolved gasses
such as oxygen, Carbon (iv) oxide, Hdrogen sulphide and Chlorine, in drilling
muds aids the corrosion problem and negatively impact how reliable the down-hole
equipment could be (Asrar, 2010; Azar & Samuel, 2007). So many other factors
including high flow rates, pH, high temperature and mud composition also contribute
to drill pipe and down-hole equipment corrosion (Brondel, 1994; Tuttle, 1987).
Ranney (1979) stated that high rate of circulation
of the drilling fluid together with the fluids produced from the reservoir
result in corrosion and erosion problems. Down-hole tubing and equipment
subjected to corrosion problem faces issues such as casing/tubing burst
collapse and tension. Cyclic loading coupled with strength deterioration caused
by corrosion results in devastating consequences and loss of well (Samuel,
2007).
Patton (1971) stated that the endurance limit,
limiting stress below which material can withstand cyclic load indefinitely
without failure, of metal drastically reduces in environments that are
corrosive. It was also reported that fatigue life of carbon steel in drilling
mud with pH above 11 reduces remarkably. Asrar (2010) noted during its revision
of corrosion mechanism that erosion corrosion is a form of mechanical/corrosive
effect which occurs when erosion removes the protective film of the metal,
thereby inducing corrosion to proceed at faster rate. Particle size of the
fluid has been related to erosion corrosion.
Therefore, it is essential that the right materials
are chosen at the manufacturing stage to offer the best protection against
corrosion for the given environmental and product conditions.
2.4
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