other substances. The returning mud may contain natural gases or other flammable materials which will collect in and around the shale shaker / conveyor area or in other work areas. Because of the risk of a fire or an explosion if they ignite, special monitoring sensors and explosion-proof certified equipment is commonly installed, and workers are trained in safety precautions. The mud is then pumped back down the hole and further re-circulated. After testing, the mud is treated periodically in the mud pits to ensure it has desired properties that optimize and improve drilling efficiency and borehole stability. Function The functions of a drilling mud can be summarized as: Remove well cuttings Drilling fluid carries the rock excavated by the drill bit up to the surface. Its ability to do so depends on cutting size, shape, and density, and speed of fluid traveling up the well (annular velocity). These considerations are analogous to the ability of a stream to carry sediment. Large sand grains in a slow-moving stream settle to the stream bed, while small sand grains in a fast-moving stream are carried along with the water. The mud viscosity is an important property, as cuttings will settle to the bottom of the well if the viscosity is too low. Other properties include: Most drilling muds are thixotropic (viscosity increases when static). This characteristic keeps the cuttings suspended when the mud is not flowing during, for example, maintenance. Fluids that have shear thinning and elevated viscosities are efficient for hole cleaning. Higher annular velocity improves cutting transport. Transport ratio (transport velocity / lowest annular velocity) should be at least 50%. High density fluids may clean holes adequately even with lower annular velocities (by increasing the buoyancy force acting on cuttings) but may have a negative impact if mud weight exceeds that needed to balance the pressure of surrounding rock (formation pressure), so mud weight is not usually increased for hole cleaning. Higher rotary drill-string speeds introduce a circular component to annular flow path. This helical flow around the drill-string causes drill cuttings near the wall, where poor hole cleaning conditions occur, to move into higher transport regions of the annulus. Increased rotation speed is the one of the best methods for increasing hole cleaning in high angle and horizontal wells. Suspend and release cuttings Source: Drilling mud must suspend drill cuttings, weight materials and additives under a wide range of