The un-weighted muds are the ones that do not contain any weighting materials such as barite or lead sulfide. This is the type of mud that is usually used for drilling shallow formations. The cuttings comprise active solids like clay that hydrate and inert solids like sand, silt, limestone, feldspar and small quantity ofAmerican Petroleum Institute (API) barite. Apart from the API barite which may be used for density control, the other solids are abrasive, and tend to increase frictional pressure loss during fluid flow, increase viscosity and form thick permeable cakes. This may lead to stuck-pipe, excessive torque and drag, lost circulation, and poor cementation. The inert solids are removed from the drilling fluids by using solids control equipment like shale shaker, desanders, desilters (hydrocyclones and decanting centrifuges) in that order. The active solids, like clays, are removed by using chemical flocculant or by diluting the fluid with water.
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