Drilling Fluids: A drilling fluid is any fluid which is circulated through a well in order to remove cuttings from a wellbore. This section will discuss fluids which have water or oil as their continuous phase. Air, mist and foam, which can be used as drilling fluids, will not be discussed at this time. A drilling fluid must fulfill many functions in order for a well to be drilled successfully, safely, and economically. The most important functions are: Remove drilled cuttings from under the bit, Carry those cuttings out of the hole, Suspend cuttings in the fluid when circulation is stopped, Release cuttings when processed by surface equipment, Allow cuttings to settle out at the surface, Provide enough hydrostatic pressure to balance formation pore pressures, Prevent the bore hole from collapsing or caving in, Protect producing formations from damage which could impair production and Clean, cool, and lubricate the drill bit.
Drilling Fluid Properties:� For those working at wellsites, a basic knowledge of “fluid” properties is required, especially those properties that distinguish fluids from solids. Fluids can be either a gas or a liquid, where gases are highly compressible and its volume being dependent upon pressure and temperature. Liquids, on the other hand, are only slightly compressible, and their volume being only slightly dependent upon temperature. We shall be dealing with only liquids in this text. Since drilling muds are commonly referred to as drilling fluids, the term “fluid” will be used throughout the text. The effects of temperature and pressure on a volume of drilling fluid will be ignored. A cube of water measuring 1 foot along each edge weighs 62.4 lbs. The density or “specific weight” is then 62.4 lb/ft 3. Specific weight divided by the gravitational constant is known as “mass density” or just density. This same cube of water exerts a hydrostatic pressure of 62.4 lbs distributed evenly over its bottom surface of 1 ft2 or 0.433 psi (62.4lbs 144 in2).
Hydrostatic pressure of a column of fluid is thus determined by:
Hp = (Dv – Fl) x MD x g where: Hp = hydrostatic pressure.
Dv = vertical depth. Fl = flowline depth. MD = fluid density. g = gravitational constant.
Note that this is dependent upon vertical depth and fluid density. In oilfield units the fluid density will be the “mud density”, with a conversion factor 0.0519. The conversion factor is derived from: There are 7.48 gallons in 1 cu/ft and 144 sq inches in 1 sq/ft because: lb/gal x 7.48 gal/ft3 x 1/144 ft2/in2 = psi/ft and: 7.48/144 = psi/ft/lb/gal therefore: 0.0519 = psi/ft/lb/gal. A drilling fluid of 8.34 lb/gal exerts a pressure of; 8.34 x 0.0519 = 0.4328 psi/ft. In SI units the conversion factor is 0.0098, therefore: Hp (kPa) = MD (kg/m3) x Dv(m) x 0.0098
Drilling Fluid Additives:� Many substances, both reactive and inert, are added to drilling fluids to perform specialized functions. The most common functions are:
Alkalinity and pH Control:� Designed to control the degree of acidity or alkalinity of the drilling fluid. Most common are lime, caustic soda and bicarbonate of soda.
Bactericides:� Used to reduce the bacteria count. Paraformaldehyde, caustic soda, lime and starch preservatives are the most common.
Calcium Reducers: These are used to prevent, reduce and overcome the contamination effects of calcium sulfates (anhydrite and gypsum). The most common are caustic soda, soda ash, bicarbonate of soda and certain polyphosphates
Corrosion Inhibitors: Used to control the effects of oxygen and hydrogen sulfide corrosion. Hydrated lime and amine salts are often added to check this type of corrosion. Oil-based muds have excellent corrosion inhibition properties.
Defoamers: These are used to reduce the foaming action in salt and saturated saltwater mud systems, by reducing the surface tension.
Emulsifiers: Added to a mud system to create a homogeneous mixture of two liquids (oil and water). The most common are modified lignosulfonates, fatty acids and amine derivatives.
Filtrate Reducers: These are used to reduce the amount of water lost to the formations. The most common are bentonite clays, CMC (sodium carboxymethylcellulose) and pre-gelatinized starch.
Flocculants: These are used to cause the colloidal particles in suspension to form into bunches, causing solids to settle out. The most common are salt, hydrated lime, gypsum and sodium tetraphosphates.
Foaming Agents: Most commonly used in air drilling operations. They act as surfactants, to foam in the presence of water.
Lost Circulation Materials: These inert solids are used to plug large openings in the formations, to prevent the loss of whole drilling fluid. Nut plug (nut shells), and mica flakes are commonly used.
Lubricants: These are used to reduce torque at the bit by reducing the coefficient of friction. Certain oils and soaps are commonly used.
Pipe-Freeing Agents: Used as spotting fluids in areas of stuck pipe to reduce friction, increase lubricity and inhibit formation hydration. Commonly used are oils, detergents, surfactants and soaps.
Shale-Control Inhibitors: These are used to control the hydration, caving and disintegration of clay/ shale formations. Commonly used are gypsum, sodium silicate and calcium lignosulfonates.
Surfactants: These are used to reduce the interfacial tension between contacting surfaces (oil/water, water/solids, water/air, etc.).
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