THE EFFECTS OF CONTAMINANTS ON THE RHEOLOGICAL PROPERTIES OF OIL BASED MUD
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.
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)
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.