Chapter One


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. (Ikeh, 2014)

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 drilling.

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  re-circulating.  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 hydrocarbon  deposits.         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 conditions.

        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.

1.1.3    Contaminants

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 filtration rate.

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

                      V.            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.



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