Drilling fluids are generally categorized as “water-based” or “oil-based”, and as “weighted” or “unweighted” muds.

Water-based Muds contain water as the liquid phase and are used to drill most of the wells in the world because they are relatively simple, the expense is usually reasonable, and water is commonly available in most places.

Oil-base Mud contains either a natural oil or synthetic oil as the continuous liquid phase and is used for maximum hole protection. Oil-based mud and synthetic oil mud are usually much more expensive than water-base mud and therefore are only used when there is a specific need, such as to keep the hole from swelling or caving in or to reduce friction and prevent stuck pipe in very crooked or high angle holes. Either water-based or oil-base mud can be used as “weighted” mud.

Weighted Mud refers to any mud which has barite or barite substitutes added to increase density. These muds normally have a density greater than 10.0 lbs/gal. The solids in weighted mud consist of drilled solids from the hole, plus barite, plus commercial clays added to control fluid loss and viscosity.

Unweighted Mud refers to any mud which has not had barite added. This mud type normally has a density of fewer than 10.0 lbs/gal. The solids in unweighted mud consist of drilled solids from the hole, plus commercial clays.

Solids control techniques will vary considerably depending on the type of mud being used. For example, with many unweighted water-base muds, the loss of fluids along with the drilled solids may be economically insignificant, allowing simple solids control techniques. In the case of mud that contains expensive chemical additives and/or barite, especially oil-based mud, sophisticated solids control techniques must be utilized to minimize overall costs. In addition, the environmental costs of haul-off and disposal may require sophisticated solids control techniques.

Here is a list of the most common mud types, followed by a brief description of each type:

Water-Based Mud (WBM)

A. Spud Mud

B. Natural mud

C. Chemically-Treated Mud

  • Lightly Treated Chemical Mud
  • Highly Treated Chemical Mud
  • Low Solids Mud
  • Polymer Mud
  • Calcium Treated Mud
Water-Based Mud (WBM)

D. Saltwater Mud

  1. Sea Water Mud
  2. Saturated Salt Mud

Oil-Based Mud (OBM)

A. “True” Oil Base

B. Invert Emulsion

C. Synthetic (SBM)

Oil-Based Mud