Function of Drilling Fluid

Remove cuttings from well:

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 another 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 (they become a gel under static conditions). This characteristic keeps the cuttings suspended when the mud is not moving 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 hole adequately even with lower annular velocities (by increasing the buoyancy force acting on cuttings). But may have a negative impact if mud weight is in excess of that needed to balance the pressure of surrounding rock (formation pressure), so mud weight is not usually increased for  hole cleaning purposes.
• 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 are the best methods in high angle and horizontal beds.

Suspend and release cuttings:

• Must suspend drill cuttings, weight materials and additives under a wide range of conditions.

·         Drill cuttings that settle can causes bridges and fill, which can cause stuck-pipe and lost circulation.

• Weight material that settles is referred to as sag, this 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 (it causes increased mud weight and viscosity, which in turn increases maintenance costs and increased dilution)
• Rate of Penetration (ROP) (increases horsepower required to circulate)
• Mud properties that suspended must 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.
• Conduct a test to compare the sand content of mud at flow line and suction pit (to determine whether cuttings are being removed).

Seal permeable formations:

• When mud column pressure exceeds formation pressure, 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 pore 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).

Cool, lubricate, and support the bit and drilling assembly:

·   Heat is generated from mechanical and hydraulic forces at the bit and when the drill string rotates and rubs against casing and wellbore.

·   Cool and transfer heat away from source and lower to temperature than bottom hole.

·   If not, the bit, drill string and mud motors would fail more rapidly.

·   Lubrication based on the coefficient of friction. Oil- and synthetic-based mud generally  lubricate better than water-based mud (but the latter can be improved by the addition of lubricants).

·   Amount of lubrication provided by drilling fluid depends on type & quantity of drill solids and weight materials + chemical composition of system.

·   Poor lubrication causes high torque and drag, heat checking of the drill string, but these problems are also caused by key seating, poor hole cleaning and incorrect bottom hole assemblies design