Here, we will learn more about another  one of the functions of drilling fluid: Seal permeable formations

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 throats, whole mud may invade the formation, depending on mud solids size;

Use bridging agents to block large opening, then mud solids can form seal.

For effectiveness, bridging agents must be over the half size of pore spaces / fractures.

Bridging agents (e.g. calcium carbonate, ground cellulose).

Depending on the mud system in use, a number of additives can improve the filter cake (e.g. bentonite, natural & synthetic polymer, asphalt and gilsonite).

From the above, you will find the importance of drilling fluid. After coming out from the pit, drilling fluid will come into the drilling fluid solids control system, to complete the control of solids through a series of solids control equipment, then enter the pit again to assist the drilling. For example：

Shale shaker: The shale shaker is the first stage of solids control equipment in the fluids processing system,which removes cuttings with diameter above 75μm from the drilling fluids brought up from the wells.Afterwards,the treated fluids flow into the tank waiting to be processed again in the next stage of the system.

Decanter centrifuge: Decanter centrifuge is used in separating suspension of solid phase with particle diameter d≥2μm for drilling mud and fluids handling.