1.1 Background of Study
Drilling for oil and gas with a drilling mud began many years ago in the 18th century.
The first reported use of a drilling fluid was noted in France in 1845,
when water was pumped down a hollow boring rod, while drilling water
wells to bring the cuttings from the bottom of the well to the surface.
Through the 1920's Iron oxide and Barium
Sulphate (Barite) was used to increase the density of the drilling mud,
thus preventing entry of the formation fluid into the borehole. The use
of bentonite in 1930's to suspend Barite formed the basis for today's
large commercial drilling mud industry. (Evabeta, 2004)
The term "drilling fluid" includes air,
gas, water and mud or could either be a combination of two or all the
above. The common type of fluid most often used in mud suspension of
solid clay is a liquid and emulsion mud (suspension of solid and
droplets of liquid). The drilling fluid is a term that comprises all the
components of clay and additives suspensions used to effect the removal
of rock cuttings from the subsurface (bottom hole) to the surface while
Thus, in other to enhance the drilling
operation, the selected drilling mud must perform certain functions to
avoid delay in operation and occurrence of associated drilling problems.
Some of these functions are highlight below.
1.1.1 Functions of drilling fluids
- Lubricate the drill bit, string, bearings, mud pump and
drill pipe, particularly as it wears against the sides of the
well when drilling deviated wells around corners.
- Clean and cool the drill bit as it cuts into the rock.
- Lift rock cuttings to the surface and allow cuttings to
drop out in the mud pit or shakers to prevent them
- IV. Suspend cuttings and weighting material when circulation is interrupted
- Protect the formation from caving
- Provide information to the drillers about what is happening
down hole by monitoring the behavior, flow rate, pressure and
composition of the drilling fluid.
- Prevent well blow-out by including very heavy minerals such
as barite (weighing agent) to counteract the pressure in the
hole (reservoir pressure).
- Drilling mud helps in suspension of drilling assemble and
casing, delivery of hydraulic energy, being a suitable medium
for logging and to being environmentally acceptable.
1.1.2 Properties of the fluid
The satisfactory performance of the
functions of a drilling fluid require that the composition of the mud be
more varied and it's properties subjected to a greater control, hence
the success of any drilling operation depends largely on the mud
properties. These properties are;
- I. Rheology: The success of
the overall drilling operation is determined by the rheological
properties of the drilling mud. The rheology of the mud
indicates the flow behavior of the mud and is characterized
by viscosity (which affects the ability to carry cuttings),
plastic viscosity, gel strength and yield value. (Max, Annis,
Martin &Smith, 1974)
- II. Density: The weight of mud
affects the ability of formation fluid blow out. Additives
increase the density of drilling fluid. Therefore, additives to add
depend on the reservoir pressure. The hydrostatic pressure should be
higher than the reservoir pressure to avoid blow out of
- III. Fluid loss control: This is a
fundamental property of the drilling fluid and becomes important when
porous formations are being drilled, particularly when those formations
may contain gas or oil. Special consideration may have to be given to
the high temperature and high pressure fluid loss in particular
- IV. Filtration Rate: This affects the ability of the mud to build an effective wall cake to prevent fluid loss.
- V. Solid Content: This affects
the rate of penetration of the drill bit. For any type of
drilling fluid, these properties may be manipulated using
various additives. A type of mud additive used for lowering
rotary and axial friction in the well bore as well as
lubricate bit bearings in oil well drilling is referred to
as drilling mud lubricants.
- VI. The other related properties: The determination pH value and alkalinity filtrate analysis, liquids and solids content, methylene blue test for Cation Exchange Capacity and bentonite content, sand content, electrical conductivity, lubricity, electrical stability of emulsions, corrosiveness.
Contaminants are foreign bodies which
alter the properties of a good drilling mud, preventing the mud from
performing it's functions adequately during circulation. Circulation is
defined as the movement of the mud from the suction tank (at the
surface), down the hole and back to the surface through the annulus
between the drilling pipe and the bore wall.
In general, a contaminant is any
material that causes undesirable changes in drilling fluid properties.
Solids are by far the most prevalent contaminant. Excessive solids,
whether Commercial or from the formation, lead to high rheological
properties and slow the drilling rate. Most other contaminants are
chemical in nature and require chemical treatment to restore fluid
properties. While there are specific treatments for each contaminant, it
is not always possible to remove the contaminant from the system.
Some contaminants can be predicted and a
treatment started in advance. The Predictable contaminants are: cement,
make-up water, and sometimes salt, gypsum, and acid gases such as,
hydrogen sulfide and carbon dioxide. Pretreatment can be advantageous as
long as it is not excessive and does not adversely affect mud
properties. Some of the contaminants encountered are;
- Sodium chloride (NaCl)
- Cement or lime (Caco3)
- Hydrogen sulphide (H2S)
- Carbon dioxide (Co2)
- Carbonate and Bicarbonate
- Gypsum anhydrite (CaSo4.H2O)
- Salt water
- temperature and
- Drilled solids
1.2 Statement of Problem
Drilling mud without the inclusion of
contaminants is a key to the success of every drilling operation. The
contamination of drilling fluid is a continuous process while drilling
and the problems that emanate from drilling operations such as
kick/blowout, pipe sticking, lost circulation, poor hole cleaning, hole
instability, formation damage etc. as a result of contaminants on the
drilling mud alters its properties which can as well result to
inadequate performance of the drilling mud.Also, addition of solids in
drilling fluid can increase the viscosity, fluid loss, filter cake and
gel strength which as a result cause circulation of mud cake. Also, High
Concentration of sodium chloride in bentonite based mud generates an
energy barrier and result to several flocculations. Thus, in small
amounts, sodium chloride thickens fresh water mud and also increases the
1.3 Aims and Objectives
The aim of this project is to
investigate the effects of contaminants on the flow or rheological
properties of oil based mud experimentally. To achieve this aim, the
following objectives will be looked at;
v Formulation of oil based mud (without and with contaminants)
v Determine which of the contaminants will have significant effect on the drilling fluid properties.
v Determine which of the contaminants significantly alter the mud rheological properties
1.4 Significance of Study
The significance of this project is to
show how contaminants will reduce the quality of the drilling fluid, and
thus reduce its functions while drilling a hole. Knowing when a
contaminant enters the mud system, the type of contaminants in the mud
system and treatment techniques will help to;
I. Reduce drilling cost
II. Increase personnel safety
III. Minimize downtime
IV. Increase productivity
Minimize drilling problems for example plastic viscosity and yield point
are used to analyze:
1.5 Scope and Limitation
This study is basically on how to
determine the effect of contaminants on oil based drilling fluid
properties and also, how it will indirectly affect it performance. Thus,
the contaminants analyzed in this study are;cement, sodium chloride
(NaCl), Sodium bicarbonate (NaHco3) and cement.
1.6 Project Methodology
This project was performed in the
laboratory where an Oil-based mud was impregnated with contaminants such
as cement, sodium bicarbonate and sodium chloride and a careful
monitoring of how they affect the quality preventing the effective
function of the drilling mud in drilling operations and also a
demonstration of how such muds could be treated, in other to reverse the
effects of these contaminants.