Drilling mud, a type of drilling fluid, also known as spud mud, is a fluid used in operations to drill boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs but can also be used for much simpler holes, such as water wells. The main classification scheme used broadly separates the mud into 3 categories based on the main component that makes up the mud:

1. ‘Water Based Mud’ (WBM). This can be sub divided into Dispersed and Non-Dispersed
2. ‘Non Aqueous’ or more commonly ‘Oil Based Mud’ (OBM) this also includes synthetic oils (SBM).
3. Gaseous or Pneumatic mud.