Drilling Fluids – 3 – Main Functions of Mud
Hydraulic and well cleaning Functions: mud jetting action out of bit nozzles, cleans drilled formation cuttings out from between bit teeth. That gives bit clear access to drill into uncut formation and helps in balancing the optimum ROP. Mud removes drilled formation cuttings; the effective cleaning achieved by mud circulation down through working string to bit and then up the annulus carrying cuttings of a cut formation from bottomhole to surface. In addition to function of mud in preventing mud balling and maximizing ROP, mud hydraulic energy provides power to Mud Motor, MWD and LDW. Hydraulic energy depends on bit nozzles size and mud pump horsepower to optimized jet impact at bottom hole.
Wellbore Stability Function: one of mud function is to maintain stable in gauge whellbore, hinder shale zones integration, and prevent formation damage. While drilling with mud, part of?fluid filters into pore voids of formation due to force of?the exerted pressure, and leaving behind mud filter-cake on face of wellbore walls. Thus, mud filter-cake containing bentonite limits hole enlargement (washout) especially sandstones formation. Moreover, mud containing inhibitors to control clay/shale interactions by hindering sloughing/tight hole that leads to hole cleaning problems and can cause stuck pipe. Thus, mud should have a good chemical compositions and rheological properties to provide a stable wellbore.
Cooling Function: Temperature is encountered downhole as a result of heat of deep formations and friction heat from rotating of string. That will lead to working string wear. Thus, function of drilling fluids to reduce heat and lubricate the string.
Well Integrity Function: Formation has two pressures: Pore Pressure PP and Fracture Pressure FP. During drilling well is full with mud and the hydrostatic pressure of mud column depends on mud weight in which to control the well while drilling. In another words, HP shall fall in between FP and PP so that HP neither fracture the weakest formation or let formation fluids entering into wellbore that can lead to blowout.
Providing Information Function: information about drilled formation and well can be achieved be testing the cutting on surface that was circulated out from bottomhole to surface by mud. Besides, information can be collected about formations and well conditions from MWD/LWD that is transferred to surface as pressure pluses through Mud.