The drilling fluid called mud only looks like mud. Actually, it is a complex mixture of water or oil, clays, and chemicals. It's composition and properties have been carefully studies and tested. The study is closely associated with chemistry, math, and physics. The term mud refers technically to a suspension of solids in water or oil, while drilling fluid is a broader term including air, gas, water, and mud.
Drilling fluid is the more appropriate term for including all types of fluid used, but term mud is preferred the field for in naming the most common type. The drilling mud basically perform the following functions:-
1. Removal of Cuttings
2. Control Formation Pressure
3 Prevent Caving
4. Caking off Per. Formations
5. Suspension of Cuttings
6. Release of Cuttings
7. Cooling & Lubrication
8. Formation Damage
9. Formation Evaluation
Common types of mud used are :
1. Polymer Muds - incorporating generally long-chain, high-molecular-weight polymers are utilized to either encapsulate drill solids to prevent dispersion and coat shales for inhibition increasing reducing loss inhibition, or for viscosity and fluid loss.
Various polymers are available for these purposes, including acrylamide,cellulose and natural gum-based products. Frequently, inhibiting salts, such as KCl or NaCl, are used to provide greater shale stability. These systems normally contain a minimum amount of bentonite. Most polymers have temperature limits below 300°F, but under certain conditions, may be used in wells with appreciably higher BHTs.
2. Oil-based muds. Oil-based systems are used for a variety of applications, where fluid stability and inhibition are necessary, such as high-temperature wells, deep holes, and where sticking and hole stabilization are problems. They consist of two types of systems:
a. Invert emulsion muds are water-in-oil emulsions, typically with calcium chloride brine as the emulsified phase and oil as the continuous phase. They may contain as much as 50% brine in the liquid phase. Relaxed, invert emulsion muds are a “relaxed” emulsion, and have lower electrical stabilities and higher fluid-loss values. Concentration of additives and brine content/salinity are rheological, filtration and varied to control emulsion stability.
b. Oil-based muds are formulated with only oil as the liquid phase and are often used as coring fluids. Although these systems pick up water from the formation, no additional water or brine is added. All oil systems require higher additional gelling agents for viscosity. Specialized oil-based mud additives include: emulsifiers and wetting agents (commonly fatty acids and amine derivatives) for high molecular weight viscosity; high-molecular-soaps; surfactants; amine treated organic materials; organo clays and lime for alkalinity.
3. Synthetic muds. Synthetic fluids are designed to mirror oil-based mud performance, without the environmental hazards. Primary types of synthetic fluids are esters, ethers poly alpha olefins and isomerized alpha olefins They are esters, ethers, olefins. environmentally friendly, can be discharged offshore, and are non-sheening and biodegradable.
Mud weight, or density, is the weight per unit volume of the mud. With simple water base mud a mud, density can be regarded as measure of the suspended solids.
Excessive solids can:
cause wear on pumps bits drill strings; and retard penetration rates;
cause a thick filter cake to be deposited on permeable formations;
cause fluids loss to the formation;
causes unnecessary work for the pump, having to push unwanted weight in the circulating fluids.