Drilling Fluids Functions

–They  

 

1-Cool the drill bit and lubricate its teeth
2-Lubricate and cool drill string
3-Control formation the pressure
4-Carry cutting to the surface
5-Stabilize the well bore
6-Help in the well logs
7-limitting corrosion

1-Cool bit and lubricate its teeth

  

•Drilling action requires mechanical energy in form of weight on bit, rotation and hydraulic energy

•Large part of energy dissipated as heat

•A Heat must be removed to allow drilling

•Mud helps remove heat

•Mud helps remove cutting between the teeth and prevent bit balling

2-Cool and lubricate drill string

•Rotating drill string generates heat

•Mud help dissipate heat from hole

•Mud absorbs heat by convection and release it by radiation

•Mud lubricate drill string and reduce friction

 

 3-Control formation pressure

•For safe drilling, high formation pressures must be contained within the hole to 

prevent damage and injury

•This achieved by hydrostatic pressure of mud

•Mud pressure should be higher than formation pressure

•An overbalance of 100-200 psi is used

•Pressure overbalance id referred as chip hold down pressure

    (CHDP)

•Penetration rate decreases as CHDP increases

•For abnormal pressure CHDP becomes negative and kick will happen

 

4-Carry cutting to surface

 

•Cutting generated by the bit must be removed

•Drilling mud carries cutting form bottom to surface

•A Carrying capacity depend on annular velocity, plastic viscosity and yield 

point of mud and slip velocity of generated cutting For power law fluid

 

 5- Stabilize the wellbore  

•Good mud cake stabilize the hole

•Differential pressure between mud and formation keep hole stable

•Reduce drilling time also help keep hole stable

 

 6-Help in the evaluation 

 
and interpretation of the well logs

•During logging mud fill the hole

•Logs used to detect hydrocarbon, measure porosity, formation ,pressure

•Mud should posses properties that help evaluate these properties

•Water base -mud  is  better  for  electric  logging  

7-Limiting corrosion

•The drilling  fluid in most cases  will have water which contain dissolved salts

•This serves as a medium for corrosion

•Precautions should be taken  to prevent  damage  of the  equipment

•Mud containing oil as the continuous phase may decrease

   or  prevent corrosion  completely