Functions of Drilling Fluid

Drilling fluid serves several essential functions in drilling:

  1. Transporting cuttings to the surface: As the drill bit cuts the rock downhole, small fragments of rocks accumulate at the bottom. Drilling fluid is continuously pumped down the hole through a drill string conduit. It lifts the drilled cuttings to the surface through the annular space between the drill string and the open hole to continuously clean the hole.

  2. Cooling and lubricating: The drilling process generates high frictional forces and heat. As the depth increases, the wellbore temperature also increases. The continuous circulation of drilling fluid helps reduce friction and cool down the wellbore. It also reduces torque and drag while rotating and tripping the drill string. This also helps extend the life of various components in the drill string by preventing overheating and wear.

  3. Preventing influx of formation fluid: Reservoir formations in a well have fluid stored under pressure. As the drill bit penetrates, the formation fluid will enter the wellbore and flow to the surface due to the pressure differential. The drilling fluid exerts hydrostatic pressure in the wellbore. In static conditions, the hydrostatic pressure of the fluid column is equivalent to 0.052xρh, where ‘ρ’ is the mud weight in pounds per gallon (ppg) and ‘h’ is the height of the fluid column in feet. The mud weight is kept such that the total hydrostatic head of the fluid column in the well exceeds the expected formation pressure with a safety margin, thus preventing the influx of formation fluid at any point while drilling the well.

  4. Providing Well Stability: The open hole formations have built-in stresses due to the overburden pressure of formation layers above it and tectonic forces. While drilling, the open hole could collapse under these stresses, resulting in a stuck pipe or lost circulation. The hydrostatic head of the drilling fluid column balances out these formation stresses and provides well stability, avoiding hole collapse.

  5. Minimizing formation damage: During drilling a well, the solid particles in the drilling fluid tend to invade the near wellbore region, causing a change in the permeability of the near wellbore zone. This relative reduction in permeability is called formation damage and can adversely affect the well's productivity. A well-designed drilling fluid system with low solids, compatible salinity, and pH with a thin filter cake helps minimize formation damage.

  6. Providing downhole information: Drilling fluid lifts the drilled cuttings to the surface, providing vital downhole details on temperature, formation, lithology, porosity, and fluid content. It also helps in well logging, which measures the physical parameters of the formations downhole using specialized tools and sensors.