2.1.1 Functions of Drilling Fluids

A drilling fluid, or mud, is any fluid that is used in a drilling operation in which that fluid is circulated or pumped from the surface, down the drill string, through the bit, and back to the surface via the annulus. Drilling fluids satisfy many needs in their capacity to do the following [M-I llc]:

• Suspend cuttings (drilled solids), remove them from the bottom of the hole and the well bore, and release them at the surface

• Control formation pressure and maintain well-bore stability

• Seal permeable formations

• Cool, lubricate, and support the drilling assembly

• Transmit hydraulic energy to tools and bit

• Minimize reservoir damage

• Permit adequate formation evaluation

• Control corrosion

• Facilitate cementing and completion

• Minimize impact on the environment

• Inhibit gas hydrate formation

The most critical function that a drilling fluid performs is to minimize the concentration of cuttings around the drill bit and throughout the well bore. Of course, in so doing, the fluid itself assumes this cuttings burden, and if the cuttings are not removed from the fluid, it very quickly loses its ability to clean the hole and creates thick filter cakes. To enable on-site recycling and reuse of the drilling fluid, cuttings must be continually and efficiently removed.

2.1.2 Types of Drilling Fluids

Drilling fluids are classified according to the type of base fluid and other primary ingredients:

• Gaseous:...