Drilling Fluid Functions

The following is the function of Drilling Fluid: 

1. Cool the drill bit and lubricates its teeth: one of the prime functions of the drilling fluid or mud is to cool the drill bit and lubricate its teeth. The drilling action requires a considerable amount of mechanical energy in the form of weight on bit, rotation, and hydraulic energy. A large proportion of this energy is dissipated as heat, which must be remove to allow the drill bit to function properly, the drilling mud also helps the removing of the rock cuttings from the space between the bit teeth, thereby preventing bit balling which is one of the common problems in drilling process. 
2. Lubricates and cool the drillstring: a rotary drillstring generates a considerable amount of heat which must be dissipated outside the hole. The drilling mud helps to cool the drillstring by absorbing the heat and releasing it, by convection and radiation, to air surrounding the surface mud tanks (pits). The mud also, provides lubrication by reducing friction between drillstring and borehole walls. Lubrication is usually achieved by the addition of bentonite, oil, graphite, etc.
3. Control formation pressure: for safe drilling, high formation pressure must be contained within the hole to prevent damage to equipment and injury to personnel. The drilling mud achieves this by providing a hydrostatic pressure just greater than the formation pressure. For effective drilling, the difference between the hydrostatic pressure and formation pressure should be zero. the hydrostatic pressure depends on the mud weight which, in turn, depends on the type of solids added to the fluid making up the mud and the density of the continuous phase. In practice, an overbalance,(Where the pressure in the wellbore in higher than the pressure in the formation), 100 to 200 psi (trip margin) is normally used to provide an adequate safe guard against well kick. The pressure overbalance sometimes referred to as chip hold down pressure (CHDP), and its value directly influences penetration rate. In general, penetration rate decreases as (CHDP) increases. When an abnormally pressured formation is encountered, the (CHDP) becomes negative and sudden increase in penetration rate is observed. This is normally taken as an indication of a well kick.
4. Carry cuttings out of the hole: for effective drilling, cuttings generated by the bit must be removed immediately. The drilling mud carries these cuttings up the hole and to the surface, to be separated from the mud. The removal of cuttings depends on the viscous properties called "Yield Point" which influences the carrying capacity of the flowing mud and "gels" which help to keep the cuttings in suspension when the mud is static to prevent them from accumulating on the bottom of the hole and causing pipe sticking. The flow rate of mud is also critical in cleaning the hole.
5. Stabilize the wellbore and prevent it from caving in: the formation of a good mud cake helps to stabilize the walls of plaster to interior walls (like plastering a room walls to keep them from flaking). The pressure differential between hydrostatic pressure of mud and that of the wellbore stable. Shale stability is largely dependent on the type of mud used To minimise the swelling stresses caused by the reaction of the mud with the shale formations. This reaction can cause hole erosion or cavings resulting in an unstable wellbore. Minimisation of wellbore instability is provided by the "inhibition" character of the drilling mud.. At last it should be noted that the best way to keep a hole stable is to reduce time during which the hole is kept open.
6. Helps in the evaluation and interpretation of well logs: wire line logs are run in mud-fills holes in order to ascertain the existence and size of hydrocarbons zones. Open hole logs are also run to determine porosity, boundaries between formations, location of geopressured (or abnormally pressured) formations and the site for the next well. Hence, the drilling mud must possess such properties that it will aid the production of good logs (Log response may be enhanced through selection of specific fluids and conversely, use of a given fluid may eliminate a log from use. Drilling fluids must be evaluated to assure compatibility with the logging program).
7. Limiting the corrosion of drilling equipment: the drilling mud in most cases will have water that contains dissolved salts as its base liquid. This serves as a medium in which corrosion takes place. If corrosion is suspected, then the cause should be determined and steps taken to prevent damage of the equipment. It has been found that in muds containing oil as the continuous phase, little or no corrosion occurs.
8. Transmit Hydraulic Horsepower to Bit: Hydraulic horsepower generated at the bit is the result of flow volume and pressure drop through the bit nozzles. This energy is converted into mechanical energy which removes cuttings from the bottom of the hole and improves the rate of penetration.