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Overview/Definition of Drilling Mud

Drilling mud is a mixture of water, clay, and other additives such as lubricants. While water-based drilling mud is often used as it is less expensive and more environmentally friendly, there are situations where oil-based drilling mud can be a better solution, particularly when water-based fluids might cause unfavorable chemical reactions with formations and when lubrication and friction reduction are paramount. The main additive that makes drilling fluid into “mud” is bentonite clay, a special clay with the important attributes of a high density and the ability to create an impermeable filter cake on the wellbore wall. 

Drilling mud has many purposes which we will explore on the next page, such as cooling, cleaning, hydraulic power transmission, and wellbore stability. All of these components are vital for successful drilling operations! 

During drilling, mud travels down the drill pipe, through the drill bit, and then back up the annulus (yellow), carrying crushed rock back to the rig.

Image Credits

  • drilling mud image: U.S. Chemical Safety and Hazard Investigation Board
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