Drilling fluid is any fluid that is circulated in the borehole to help in carrying out a cost-effective and efficient drilling operation resulting in stable and gauged borehole to targeted depth with minimum possible damage to prospective formations. A major component in drilling operation success is drilling fluid performance.
The successful completion of borehole and its cost depend, to a great extent, on the properties of the drilling fluid. The functions of drilling fluid, which are critical to the drilling process, are: (1) aid formation stability and productivity; (2) clean the bottom of the hole; (3) lift formation cuttings to the surface; (4) suspend cuttings while circulation is stopped; (5) cool the bit; (6) control subsurface pressure, and (7) lubricate the drill string and assist in its corrosion control.
Selection of drilling fluid mainly depends on the type of formation and the borehole depth. The various kinds of drilling fluid normally used are water, bentonite mud, cutting oil, and polymers (both water-based and mud-based).
Water: It is freely available fluid and is primarily used in core drilling operation. It has very good cooling properties and acts as moderate lubricant and vibration dampener.
Bentonite Mud: This increases the viscosity and gives better cleaning of the hole. It acts as very good lubricant and also reduces the vibrations in the string considerably. It forms cake on the borehole walls and thus protects the caving in the borehole. Bentonite mud solution has gelling properties and thus it keeps the cutting in suspension even when the circulation is stopped.
Polymer: The polymer has similar properties as that of bentonite mud except gel strength. It has excellent flushing capacity. The only drawback is that circulation is to be maintained all the time else the cuttings will quickly settle at the bottom and jam the string.
