Cooling and Lubricating Drilling Tools
Drill bits generate a significant amount of heat while crushing rock; the circulating drilling fluid effectively absorbs and carries away this heat, thereby lowering the bit's temperature. Simultaneously, it reduces friction between the drilling tools and the borehole wall, acting as a lubricant and consequently extending the service life of the drilling tools.
Transporting and Suspending Cuttings
Drilling fluid transports the rock cuttings (rock fragments and dust)-generated by the drill bit-from the bottom of the wellbore to the surface. This keeps the wellbore bottom clean and prevents the cuttings from being re-crushed. Furthermore, the fluid's appropriate viscosity allows it to suspend these cuttings when circulation is halted, preventing them from settling and clogging the wellbore.
Stabilizing and Protecting the Borehole Wall
The drilling fluid forms a dense "mud cake" on the surface of the borehole wall, which reinforces loose or fractured geological formations and prevents the wall from collapsing or narrowing. Additionally, the hydrostatic pressure exerted by the fluid column balances the earth pressure surrounding the borehole, thereby maintaining wellbore stability.
Balancing Formation Pressure
By adjusting the density of the drilling fluid to ensure the resulting hydrostatic pressure balances the formation pressure, it is possible to effectively prevent well blowouts (inflows of formation fluids) or fluid losses (seepage of drilling fluid into the formation). This constitutes a critical method for controlling downhole pressures.
Forming a Mud Cake to Protect Reservoirs
In oil and gas exploration, the filter cake formed by the drilling fluid minimizes contamination of and damage to hydrocarbon-bearing zones, preventing pore blockage. This helps preserve the permeability of the reservoir, thereby providing accurate data for subsequent exploration and development activities.
Auxiliary Functions
In specific engineering processes (such as caisson sinking), drilling fluid also serves to lubricate and reduce resistance by lowering the friction between the wellbore wall and the surrounding soil, thereby facilitating the sinking of the structure. Furthermore, the drilling fluid can act as a medium for transmitting downhole information (such as data from Measurement While Drilling systems).
