conditions. Drill cuttings that settle can cause bridges and fill, which can cause stuck-pipe and lost circulation. Heavy material that settles is referred to as sag, which causes a wide variation in the density of well fluid. This more frequently occurs in high angle and hot wells. High concentrations of drill solids are detrimental to: Drilling efficiency because they increase mud weight and viscosity, which in turn increases maintenance costs and increased dilution. Rate of Penetration (ROP) because they increase the horsepower required to circulate. Drill cuttings that are suspended must be balanced with properties in cutting removal by solids control equipment. For effective solids controls, drill solids must be removed from mud on the 1st circulation from the well. If re-circulated, cuttings break into smaller pieces and are more difficult to remove. A test must be conducted to compare the sand content of mud at the flow line and suction pit (to determine whether cuttings are being removed). Control formation pressures Source: If formation pressure increases, mud density should be increased to balance pressure and keep the wellbore stable. The most common weighting material is baryte. Unbalanced formation pressure will cause an unexpected influx (also known as a kick) of formation fluids into the wellbore possibly leading to a blowout from pressurized formation fluid. Hydrostatic pressure = density of drilling fluid * true vertical depth * acceleration of gravity. If hydrostatic pressure is greater than or equal to formation pressure, formation fluid will not flow into the wellbore. Well control means no uncontrollable flow of formation fluids into the wellbore. Hydrostatic pressure also controls the stress from tectonic forces, which can render wellbores unstable even when formation fluid pressure is balanced. If formation pressure is subnormal, air, gas, mist, stiff foam, or low density mud (oil base) can be used. In practice, mud density should be limited to the minimum necessary for well control and wellbore stability. If too great it may fracture the formation. Seal permeable formations Source: Mud column pressure must exceed formation pressure, in this condition mud filtrate invades the formation, and a filter cake of mud is deposited on the wellbore wall. Mud is designed to deposit thin, low permeability filter cake to limit the invasion. Problems occur if a thick filter cake is formed; tight hole conditions, poor log quality, stuck pipe, lost circulation and formation damage. In highly permeable formations with large bore