Water-Based Drilling Fluids - Essay Example

Drilling Fluid Drilling fluids are used in drilling boreholes, oil and gas wells. The fluid remains in contact with the walls of the well during the drilling operation for structural support and enhances the efficiency of the drilling operation. The drilling fluid is used for a number of functions in the drilling operation. The drilling fluid cools the bottom hole and lubricates the drilling bit to reduce friction and heat generated due to mechanical and hydraulic forces between the bit and the well casing. The drilling mud removes rock fragments and drills cuttings from the borehole to the surface. The mud also provides hydrostatic pressure to balance the formation pressure resulting from the formation fluids in the borehole that if uncontrolled might lead to blowout. The mud is also used in transmitting hydraulic horsepower from the surface to the drilling bit and power the measurement and logging while drilling tools. The choice of drilling mud is determined by the drilling operation to be done, and its properties should be such that it prevents back pressure and blow outs within the minimum costs as possible. (Committee, 2004, pp. 15-19) Drilling fluids are classified into three major types like water, oil, and pneumatic based fluids. The three major types are further classified depending on the purpose, chemicals added fluid phases, alkalinity and dispersion. 1. Water based drilling fluids Water based muds have four phases which include water, Active colloidal solids, Inert solids and Chemicals. The water phase or continuous provides the initial viscosity which is modified to achieve the required flow properties. The water phase also suspends the active colloidal solids and the inert solids. A sample of colloidal solid present is the Bentotite, and that of an inert solid is the Berite. Bentonite, which is a clay provides just sufficient viscosity for carrying the fragments to the surface. Baruim Sulfate increases the weight of the column for borehole stabilization. Caustic soda and other ingredients are added to increase the alkalinity and reduce corrosion. Salts such as Potassium chloride are also added to prevent water from infiltrating the rock formation. Clay is added to water based fluids to increase their density, yield point, gel strength, viscosity and to minimize fluid loss. The clays added are classified as Montmorillonites or bentonite, Kaolinites, and Illites. The chemicals are also added to improve the flow properties and are listed as Thinners, Dispersants and Deflocculants. (Caenn, 2011, pp. 538-542) Based on the dispersion, water fluids are classified as fresh water mud that is dispersed-non inhibiting fluid and water based drilling mud (dispersed-inhibiting fluids). Dispersed-inhibiting fluids prevent the hydration and dispersion of clay. The water based fluids are classified based on their alkalinity as high pH lime fluids(pH value is more than 9.5) and low pH gypsum (7-9.5).They are the most commonly used fluids since they are less expensive than the other drilling fluids and include fresh water and salt water fluids. a. Non dispersed-non inhibited They include spud mud, polymer or bentonite mud and extended bentonite mud. Non dispersed-non inhibited have no inhibiting ions such as calcium, potassium or chloride ions in their continuous phase and do not need the addition of chemical thinners or dispersants for flow control. Spud fluids are used in cleaning the hole, preventing sloughing of the hole surface, removing fragments from the hole. The mud has a high content of solids and high filter loss to form a thick filter cake which makes it suitable for drilling wells with hard, compacted and relatively pressurized formations. The bentonite fluids tolerate low calcium concentrations and are used in drilling operations where the formations contain low reactive solids. Extended bentonites have extended yield that improves its penetration rate. The extended yield is as a result of adding chemicals such as xanthum gum, potato or corn starch. b. Dispersed-inhibited systems They include lime-fluids, seawater-prehydrated bentonite fluids and gyp-lignosulphonate fluids. Chemical dispersants are added for dispersion of clays and drilled solids. They also include inhibiting ions that prevent hydration and dispersion of the formation materials. The presence of the inhibiting electrolytes reduces the effects of the drilled solids on the fluid rheological properties even when in under low treatment conditions. Calcium is added to a mixture of water and Bentonite to cause a reaction that replaces the sodium ions in the Bentonine with the Calcium ions and results in the reduction, in size of the shale formations. Calcium mud is best suited for drilling massive sections of gypsum and anhydrite and has superior qualities compared to fresh water mud. The mud can contain high concentrations of drilled solids without affecting its viscosity. They are classified according to the concentration levels of soluble calcium into Lime mud with 120parts per million of soluble calcium and Gyp mud with 1200 parts per million of soluble calcium. The composition of Lime mud is Bentonite, lime, caustic soda, thinner and a filtration control agent. Gyp mud has better temperature stability and is composed of bentonite, gypsum, caustic soda, thinner and a filtration agent. c. Dispersed-non inhibited fluids They include Lignosulphonate mud and phosphate-bentonite muds. Lignosulphonate mud is composed of salt or fresh water, caustic soda, stabilized starch and ferrochrome lignosulphate. The mud accumulates water making it not suitable for use in wells with shale formations. It forms a high density mud able to work under relatively high temperatures with great tolerance for drilled solids and requires low filter loss. d. Non dispersed-inhibited systems They include Salt fluids and Potassium chloride water based fluids. Potassium chloride fluid is composed of fresh or sea water, potassium chloride, inhibiting polymer, viscosity building polymer, caustic soda and stabilized starch. The mud has high yield strength, good bit hydraulics and reduces pressure losses to increase borehole stability. It has excellent slough inhibiting properties making it suitable for drilling wells with shale sections, but its use is limited due to its instability at temperatures above 77°C. Salt fluids prevent the occurrence of solution cavities in salt domes and stringers and minimize the washout of holes in salt and carbonate beds. 2. Oil based drilling fluids, classified as dispersed fluids. The continuous or external phase in oil based drilling fluids is a petroleum product which includes mineral oil, diesel, or paraffins. The external phase consists of Asphalt, a suitable oil, water, emulsifiers, calcium hydroxide, weight increasing materials, surfactants and chemical additives. Oil and asphalt increase its functionality while the other components are used to enhance and stabilize rheological and plastering properties. The oils used as the continuous phase is Lease crude oil and refined oils which have been accepted globally. Selection of oil for use in the drilling operation is guided by its specific weight which determines its viscosity. Aniline point shows the oil aromatic content. Flash point refers to the oil ignition temperature when a flame is passed, and fire point shows the temperature that can sustain a fire. Bodying agents and small amounts of water are added to oil mud to react with the chemical additives for enhancement of rheological and plastering characteristics. The bodying agents added are listed as Colloidal size materials and metal soaps with high molecular weight. Asphalts are used in colloidal size to modify the properties and reduce fluid loss. Emulsifiers are used to emulsify the water in the oil picked in the drilling operation and control fluid loss. (Caenn, 2011, pp. 538-542) 3. Pneumatic based drilling fluids Pneumatic based drilling fluids include gas, mist, forms and mixtures of air and liquid. Compressed air is circulated through the wells annular space or the drill string. Air is mixed with water to improve the viscosity, clean the hole, reduce dust while cooling the bottom hole. Specialized equipments are required for use with pneumatically based fluids to ensure safe management of the chips and formation fluids. They are best suited for hard rock formations and prevent formation damage and fluid loss. (Lyons, 2009, pp. 9-10) References Caenn, R., & Darley, H. C. (2011). Water-Based Muds. Composition and properties of drilling and completion fluids (6th ed., pp. 538-542). Waltham, MA: Gulf Professional Pub.. Committee, A. S. (2004). Drilling Fluids. Drilling Fluids Processing Handbook (pp. 15-19). Burlington: Elsevier. Lyons, W. C. (2009). Air and gas versus mud. Air and gas drilling manual applications for oil and gas recovery wells and geothermal fluids recovery wells (3rd ed., pp. 9-10). Amsterdam: Elsevier/Gulf Professional Pub Read More