What are drilling fluids?

Drilling fluid, which is also referred to as drilling mud, is used to aid the drilling of boreholes into the earth. It is used in exploration drilling rigs, and also while drilling oil and natural gas wells. Drilling fluids can also be used for less complex drilling tasks such as the drilling of water wells. 

There are two major types of drilling fluids for exploration: Water-based drilling fluids and oil-based drilling fluids. 

Water-based drilling fluids

As the name implies, water-based drilling fluids use water as the continuous phase. They are also used to drill almost 80% of all wells. The base fluid may be freshwater, seawater, brine, etc. Water-based drilling fluids are cheap as the base material is easy to get. Additives are used to change the density and viscosity. It is also used to control filtrate losses. They are particularly good for curing mud losses. When coarse bridging materials are pumped, mud losses are often cured. However, water-based drilling fluids have a major disadvantage which is that they are reactive to clays and lead to time-dependent borehole problems. 

Oil-based drilling fluids

The oil-based drilling fluid was formulated to help address issues which the drilling sector faced. Some of such problems include:

• Stuck pipe and torque
• Increased downhole temperatures
• Formation clays that swell or reacted after exposure to water-based drilling fluids. 
• Contaminants

Oil-based fluids are formulated with mineral oil, diesel, and paraffin. One major advantage of oil-based drilling fluid is its lower well friction, therefore, they are used in long-reach wells where friction is critical. Capillary pressure prevents oil from invading a water-wet rock.

However, one main disadvantage of oil-based drilling fluids is that if circulation losses arise during drilling, it is difficult to stop the losses. A severe loss case can be time-consuming to cure.