The drilling fluid circulating system is like a close loop electric circuit through which drilling fluid (i.e. mud) can travel from surface to all the way down hole and back to its initial point (i.e. mud pit).
Weighted muds are the ones that contain weighting materials. These mud systems are usually used for drilling at deeper depths because of increasing formation pressures. The typical composition of weighted clay/water mud is active clay, and inert solids like barite are used for enhancing the mud weight. As mentioned earlier, the mud arriving at the surface from the bottom while drilling is in progress contains active and inactive drilled solids. Hence, the low gravity solids must be removed first, using a screen, because their disintegration reduces the particle size to less than that of barite or in the similar range. This may cause a loss of costly barite if the mud is subjected to solids control. Once the larger particles are removed the mud must be passed through mud cleaner, where the hydrocyclones are used in series with screens. This system works best for muds with density less than 15.0 lbm/gal.
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.
A mud/gas separator (poor boy degasser) sizing worksheet will assist drilling personnel with the sizing calculations. The worksheet provides a quick and easy evaluation of most mud/gas separators for a specific well application. A brief discussion of other mud/gas separator considerations is provided, including separator components, testing, materials, and oil-based-mud considerations. This paper reviews and analyzes existing mud/gas separator technology and recommends separator configuration, components, design considerations, and a sizing procedure. A simple method of evaluating mud/gas separation within the separator vessel has been developed as a basis for the sizing procedure.