HISTORY OF DRILLING MUD
In order to have a good understanding of drilling fluid and before going into detail it is interesting to know the sequence of events which lead gradually to the various developments and ultimately resulted in the present stage when success of drilling a hole is attributed to be significantly depends on the better performance of a circulating fluid.
The history of drilling period can be separated into three distinct periods. First there is early period ranging from an acient times up to the completion for the spindle top well in 1901, admittedly the first commercially importance oil well produced by rotary process. Second there is the period from 1901 up to 1928, when engineers were conducting test on the drilling mud to meet specific drilling needs the third period extend from 1928 to the present. The first era was an experiment. The second was of practice. The third is of science.
In ancient time most wells are dug or drilled to produce water for drinking. It was then learned that there were other fluids worth seeking specially brine. As garly as 256BC. The chine’s had drilled drip wells for bridles. In later times they start throwing a few pails of water time to time into the holes to soften the rock and reduced it into pulps. At intervals they used to lower a tubular container to bail out the cuttings. The well drilled by David and Joseph Ruffner in west virgina in 1806-1808 for producing salt brine is recognized to the first rock-bored well in United State. In 1829 one of the first wells in America, which produced a consistent flow of oil was drilled near Burkerille, Kentucky. This oil was bottled and sold under the name of “American medicinal oil”.
In 1845 Fauvelle successfully drilled a well in France by using water flushed set of tools. Then he drilled more water wells using this equipment. Many years/later, same equipment was developed for drilling oil wells and plain water was he only fluid used at the time. However, no one thought of mixing day in water to make the muddy fluid till 1880’s when Chapman came up with the idea of using stream of water and quantity of plastic material to form an impervious wall along the well bore and this was beginning of science of mud engineering. Buckingham also mentioned possible use of oil base mud in 1885. In October 1900, Hamill Brothers Started drilling a well and soon hit quicksand. Curt Hamill mixed a muddy fluid and pumped into the hole which promptly sealed of the quicksands and solved the problem.
In 1901, a well drilled by rotary process in spindle top field. Texas using the mud started producing approximately 100,000 barrels a day. The rotary system and flowing mud has become a definite part of the drilling technique by the time, with the above mentioned developments in the drilling fluids in the both century presently this technology has attained the status in the petroleum industry that better performance of a drilling fluid is considered to be a major factor in the efficient, economical and safe completion of the drilling operation.
MAJOR DRILLING FLUID COMPONENTS
Drilling fluid (mud) is made up of several components that made it possible to successfully effect if functions in drilling operations. There are over forty components that can be identified as constituents or drilling fluids same like barite and bentonite play major roles in the preparation and us of drilling fluid, while others play minor roles but are very important to achieve the required objectives.
Barite: Barite is a Mineral Composed of Barium Sulphate.
It receives its name from the Greek word “barys” which means “heavy” this name is in response to barites high specific gravity of 4.5, which is exceptional for a non manufacture mineral. The high specific gravity of barite makes it suitable for a wide range of industrial and manufacturing uses. This part of the mud component is known as the inert fraction or non reactive part, it does not swell or change the chemical composition of the mud. Usually this component is added when the need arises to increase the mud density.
Bentonite is used in trilling fluids to lubricate an cool the cutting tools to remove cuttings and to help prevent blowouts. Much of bentonites usefulness in the drilling industry coming from its unique rheological properties. Relatively small quantities of bentonite suspended in water from a viscous, shear thinning material. Most often, bentonite suspensions are also thixotropic, although rare cases of rheopectic behavior have also been reported. At high enough Concentrations bentonite suspensions begin to take on the characteristics of gelso, it is a common component of drilling mud used to curtail drilling fluid invasion by its propensity for aiding in the formation of mud cake.
The liquid phase used to prepare mud may be fresh or saturator. Usually fresh water is a bester mixing phase because it gives a homogenous mixture. Bentonite disposes more richly in fresh water than hard water. However in deep offshore where fresh water may not be easily accessible, scale salt water can be used but with a lot of chemicals to condition the mud.
TYPES OF DRILLING FLUID (MUD)
Many types of drilling fluids are used in industry major categories include air, water and oil base fluids each has many subcategories based on purpose, additives, or day states.
Water Based Mud
Water based mud’s consist of four basic phase water active based consist solids, inert solids, chemicals. Water is the continuous phase is t provide the initial viscosity which can be modified to obtain any desirable rheological properties. The second function of the continuous phase is to suspend the reactive colloidal solids such as bentonite, inert solids, such as barite water also act as a medium for transforming the surface available hydraulic horse power to the bit on the bottom of the hole. Water is also a solution medium for out conditioning chemicals which are added to the frilling fluid. In water based mud’s clay is added to increase density viscosity, gel strength and yield point and to decreased fluids are mainly in three groups montinorillonites (bentonite), haolinito, illitus chemicals used in water based mud’s can be grouped according to their functions as thinners, dispersants and defloculants.
TYPES OF WATER BASED MUDS
The following are the types of water based drilling mud
These types of mud are used to drill the holo for conductor pipe. They consist of viscosity series of bentonite that provide sufficient carrying capacity of clean larger holes, the spud mud is usually discarded after drilling out of the conductor pipe because of cement contamination and unsuitable drilling properties.
Usually used to treat most fresh water mud to develop satisfactory rheological properties and to control fluid loss. Salt greatly affects bentonite ability to produce desirable properties.
They are inorganic chemical used to treat disposed fresh water mud whose viscosity have been increased by contamination with cement.
Unsaturated salt water mud! Here, the water sources from the water source from the open seas or bay. There are characterized by high filtrated loss, medium to high gels and poor suspending properties unless treated with organic colloid or with a thinner.
The saturated salt water mud liquid phase is saturated with sodium chloride. These type of mud are used to drill salt formation where salt water flows are on countered the sodium silicate mud consists of 35% saturated salt solution and 65% Sodium Silicate solution (by volume). They are used in drilling heavy shale.
Oil Based Drilling Fluid (Mud):
Oil mud’s are sometimes employed when a well is about to enter a producing zone or when special drilling problems such as high temperatures, sloughing shale, or stuck pipe are encountered. The two types of oil mud’s are oil must be handled with special care on the job.
Oil-base mud’s: basically, an oil-base mud consists of diesel oil, emulsifier, stabilizing agent, salt and less than 5 percent water. The exact composition depends on the supplier. Although oil-base mud has a small amount of water, any additional water is a contaminant that must be avoided.
Even a very small amount may cause undesirable thickening of the fluid.
Gas Raised Drilling Fluid (Mud):
Gas base drilling mud is the ideal medium of faster’s penetration rate, cutting carrying capacity is dependent upon annular velocity (v). The gas or aid pumped down into the annulus which results in faster penetration of the bit that limits the application of this method.
The gaseous drilling fluids are mostly used in areas of dry and hard formation and air are commonly used of about 95% the gas based drilling fluid can be classified as dry gas must (in which drop lot of water containing a foam stabling substance and the glean (in which the foam contain film strengthening materials, such as organic polymers or bentonite). The composition of the foam at ay temperature and pressure depends upon the liquid volume fraction of the foam at that particular temperature and pressure.
Functions for Drilling Fluid (Mud):
Drilling fluid (mud) plays important roles in the process of drilling a well and it performs various functions which includes.
The combined effects of weight and speed of rotation generate at the bit unless the bit is cooled it will overheat and quickly wear out I mud contains a lubricate such as oil the friction in the bearing will be reduced. This will also reduce the friction between the drill string and the borehole as the hole is being drills not only will this prolong the service life of the down hole equipment but it will also help to prevent drilling problems such as torque, drag differential sticking.
For efficient drilling most of the power delivered by the pumps should be used at the bit. The hydraulic horsepower is transmitted by the drilling fluid and so mud properties such as plastic viscosity, yield point and mud weight are important. The high velocity created by pumping the mud though the bit nozzles provides the turbulence which clean the bottom of the holo.
As a bit penetrates the formation the rock cuttings drilled must be removed, otherwise the drilling efficiency will decrease separate operations which are lifting the cuttings to surface while circulating suspension of cutting while net circulating and dropping out of cuttings on surface. The mud properties must be carefully engineer to fulfill these requirements. Te lifting capacity depends on a getting thixotropic effect, which develops after circulation is stopped.
The hydrostatic mud pressure must be sufficient to prevent an influx of formation fluid into the will bore. The mud density is the controlling factor.
HP = 0.052 X MW X TVD
Where HP = Hydrostatic pressure (psi)
MW = mud weight (ppg)
TVD = true vertical depth (ft)
by increasing the mud weight however the risk of for motion breakdown and lost circulation increases and te rate of penetration will decrease. The mud weight must be selected very carefully to meet specific requirement. Barite is added as a weighting material due to its high specific gravity (S.G = 4.2)
The drilling fluid should deposit filter each on the wall of the borehole to consolidate the formation and to prevent formation damage. The filter cake is formed by the hydrostatic pressure forcing the liquid part of the mud filtrate into the formation, leaning source solid material on the side of the borehole.
The mud loggers depend on the analysis of drilling mud and its cutting to describe the various formations drilling through. The will site geologist make use of the rock cutting that are carried to the surface to determine the various rock type drilled through add the possible rock among other methods sometimes the telemetry pulse of the mud is used in the survey technique to obtain real time data sc as MWD and LWD.
Caring of formation does result only from hydration of clay or shale but from other factors. Vertically bodied formation, over hangs etc may tend to break off and fall into the hole if a high pressure of high density differential exists between the formation and the mud fluid. Therefore, the gel strength has to be increased to develop a strong plastering effect on the crumbling section.
DRILLING FLUID PROPERTIES AND FLOW CHARACTERISTICS
Density or Mud Weight
It is the weight per unit of various volumes: control of density is essential for prevention of blowouts and also sometimes used for maintenance of adequate borehole stability.
The instrument in field used for the determination of the mud weigh or density is calibrated in several scales such as: pounds per gallon, pounds per cubic feet, pounds per square inch per 100ft dept, specific gravity. Frequent density test help in preserving a safety factor by disclosing and changes taking place in the unit weight of the mud.
Viscosity is a measure of internal resistance of fluid to flow, the greater the resistance, the greater the viscosity of drilling operations. Viscosity of mud must be controlled and a standard means of measurement should be provided. Factors influencing the specification of viscosity of any given mud is, hole size, hole condition, pumping rate, drilling rate, cutting size, caving, presence or absence of shale separators, mud weight design of the pit system and gel characteristics. Instrument used in determining mud viscosity are. Thermometer, the rotary viscometer and the marsh funnel viscometer.
The filtration properties of drilling mud are a measure of the ability of the solid component of the mud to form a thin, low permeability filter cake on the wellbore during drilling. The lower the permeability, the thinner the filter cake and the lower the volume of filtrate from mud of comparable solid concentration. The properties are dependent upon the amount and physical state of the colloidal materials in the mud. A mud low in colloidal and high in merits solid deposits a thick filter cake on the walls of the holes and these restricts the passage of an excessive amount of filtrate to pass into the formation, thereby preventing a potential cause of caring. When the walling properties are not met, it may result to further trouble such as difficulty in running casing, creating a swabbing effect and also problem in securing water shut off.
The sand content determination is necessary because excessive sand result in the deposition of a thick filter cake on the walls of the holes, or may settle in the hole, when circulation is in process, thus inferring with the successful operation of drilling tools or setting of casing.
High Sand contents may also cause excessive abrasion of pump parts and pipe connection sand content is determined by sieve-analysis.
The degree of acidity or alkalinity of drilling mud is indicated by the hydrogen ion concentration which is commonly expressed in terms of PH. A perfect neutral solution has a PH of 7. Alkaline solutions have PH reading ranging from just above 7 for slight alkalinity, 14 for the strongest alkalinity. The PH measurement is used as an aid in determining the need for chemical control of mud as well as indicating the presence of contaminants such as cement, gypsum etc. the optimum PH for any drilling fluid (mud). They are: the calorimeter method such as phydrion dispenser, an electrometer such as the analytic PH meter.
2.5.6 Gel Strength
This is the ability of the mud to thicken immediately the mud stops flowing. This is when the mud is at rest. Gel strength measurement is made by using a direct-indicating viscometer.
2.5.7 Chloride Content
The amount of chloride in the mud is a measure of the salt contamination from the formation. The procedure involves taking a small sample of filtrate, adding phenolphthalein and titrating with acid until the colour changes. Adding 25-50 ml of distilled water and a small amount of potassium chromate solution. Stir continuously while silver nitrate is added drop by drop. The end point is reached when the colour changes. The chloride content is calculated from:
Cl Content (PPM) = Ml of Silver Nitrate x 100
Ml of Filtrate Sample
The activity of a substance is its affinity for water, which is directly related to relative humidity. An electro-hygrometer is an instrument used to measure relative humidity. A set of known activity solutions should be available to calibrate the instrument. The probe is very sensitive and must not come into contact with the mud or shale sample.
The sample is put into a jar, and the probe screwed on the top like a lid. Having adjusted the scale to give the correct treading for a known substance. The probe can then be used to give a direct reading for the sample. This test is often used for oil-base muds to balance the activity of the mud and the shale’s.
Drilling Mud Additives
Typically a particular compound or drilling fluid or drilling mud would have myriads of additives in them. This quite unlike foam or air based drilling fluid that may not be containing too many drilling fluids because most of these additives are either available in solid or in liquid form some of the significant compounds that work well as additives have been detailed out below.
Refers to those additives or materials added to the drilling fluid as a miscellaneous part to increase mud weight or density. The main weighting material is barite others are:
Barite (BaSO4) with an average density = +3
Hematite (Fe2SO3) with an average density = 4.9 <d<3.9 soluble in HCL and mainly used in completion fluids Galena (PBS) with average density 6.7 <d<7, it is used as weighing material for special cases.
Calcium Carbonate (CaCO3) with average density = 2.6<d<2.8 used mainly for low density fluids.
Since drilling would involve the installation and usage of several metallic components, it would be essential to introduce corrosion inhibitors through the drilling fluids that are being used for the process considering that the metallic parts would encounter a flow of acidic compounds during the course of the drilling process, corrosion maybe rampant popular anticorrosive that are used as additives would include aluminum bisulfate, iron oxides, protect pipes of zinc-chromate, zinc-carbonate and so on.
During the drilling process, it is essential to introduce agents that can help in breaking up solid by the drilling fluid from one place to the other, without causing any unnecessary obstructions. This is specifically what dispersants do. Examples include iron lignosulfonates.
Flocculants are nothing but acrylic polymer compounds that help in the cluster formation of suspended particles, so that they can be grouped together and removed from the resultant fluid when they reach the surface.
Surfactants are nothing but compounds like soaps ad fatty acids that would emulsify and do foam the drilling mud or fluid.
The drilling mud or fluid compound would be a fertile ground of breeding bacterial that could lead to complete souring of the compound. In order to reduce the sourness and thwart the growth of bacteria, biocides would have o be introduced in the form of chlorophenols, formaldehydes or organic amines.
Reducers or Fluid Loss:
Drilling may often involve working with highly permeable formations that might also be under pressurized. Typically, one has to introduce fluid loss reducing compounds like organic polymers and starches.
Are used to increase mud viscosity and in many cases, bentonite is use. They are Bentonite, Actapulgatite, Ben-Ex, Cinc, Aquagel, Drispac etc.
Control of Drilling Mud with Additive:
Since drilling mud properties may be at an undesirable value and requires selective adjustments, chemical additive commonly used are PH Control, viscosity control and filtrate control agents.
Caustic (NaOH): This is used to alter the mud PH. A high mud PH is desirable to suppress corrosion rate, Hydrogen embrittlement and the solubility of Ca2+ and Mg2+ with the aid of NaOH, the PH of most mud’s are maintained between 9.5 and 10.5. KOH is another PH control agent.
CASSAVA (MANIHOT ESCULENTA)
Cassava is a perennial woody shrub with an edible root which grows in tropical and subtropical areas of the world. Cassava originated from tropical America and was first introduced into Africa in Congo basin by the Portuguese around 1558. Today, it is a dietary staple in much tropical Africa.
It is rich in carbohydrates, calcium, vitamins B and C and essential minerals. However, nutrient composition differs according to variety and age of the harvested crop, and soil conditions, climate and other environmental factors during cultivation.
Sweet potatoes have a creamy texture, they are of two types. One with bright orange flesh, the other with pale cream flesh, sweet potatoes are native to the tropical Americas and are sometimes referred to as “Yams” in the USA. These tubers are rich in fibre, vitamins A, C and B6 and an excellent source of carbohydrates. The orange fleshed variety are also rich in betacarotene.
By Ashekamen Andrew ‘ Petroleum Engineering And Geo-Science Department . Petroleum Training Institute , Nigeria
photo credits; www.zealenvironmental.com