Drilling Fluid: Cooling the system Another of the fluids many tasks if to keep the string and bit cool and well lubricated. Working with small surface volumes of drilling fluid can save money. How ever there must be sufficient volume to keep the string and hole cool. Long bit runs and the constant circulation add to the problems. It is well understood the all need to make a profit but at what cast and to hume  Some years ago while drilling an 18000 foot hole in the Middle East the rig was using a base oil mud. Due to the fact there was a lot of H2S in the area the volume was kept as low as possible.  The constant friction created by the pipe and bit using a PDC soon bought the mud temperature up. Having such a low surface volume never gave it a chance to cool. Pumps that had never given problems before started to go down and it became impossible to work on them. Cooling water was spray over the pump and fluid ends started to crack Piping began to warp and bend under their own weight. This was finally capped of with an accident,  when the de-sander piping now so hot came loose spraying scolding fluid into the face of one of the men had it not been for the fact he was wearing classes he would have been blinded but even this never stopped him spending a lot of time in hospital. Considerable heat is generated by friction at the bit and where the drill string is in contact with the formation. This heat must be absorbed by the fluid so that it can be transmitted to the surface and dissipated. Small volumes are quickly re-circulated and in the case of an oil base drilling fluid are extremely hard to keep cool. It must also be pointed out that oil mud's get far hotter than water base mud's This in turn can have a devastating effect on the other mud properties. In the case mention we found the fluid density could not be maintained. Thus allowing the salt  zone to close in and a constant need to short trip. The fluid also works to lubricate the bit and drill string. If additional lubricity is needed, there are several lubricating products which can be added to the system. The lubricity of the fluid helps to decrease torque, increase bit life, reduce pump pressure and reduce bit balling. Liquid soap is a good lubricant and is also handy for washing down the rig,  hint.