What are the function of mud during drilling? And why its so important?

Controlling formation pressures 

Removing cuttings from the borehole

 Cooling and lubricating the bit

Transmitting hydraulic energy to the bit and downhole tools 

It avoid hydrocarbon from reservoir entering the well and goes to atmospheric pressure

another function : to keep hole stability.  Mud is very important because mud gives us a lot of data of well bore

lubricate tools

help speed up drilling

clean the well

keep positive pressure for well control

It's the most important parameter when drilling because of the fact when it looses any of it's propreties (density, viscosity,...) you risk to have a kick or to get stuck, and also slow drilling and bad cleaning..

Controlling formation pressures 

Removing cuttings from the borehole

 Cooling and lubricating the bit

Transmitting hydraulic energy to the bit and downhole tools 

It avoid hydrocarbon from reservoir entering the well and goes to atmospheric pressure

lubricate tools

help speed up drilling

clean the well

keep positive pressure for well control

It's the most important parameter when drilling because of the fact when it looses any of it's propreties (density, viscosity,...) you risk to have a kick or to get stuck, and also slow drilling and bad cleaning..

another function : to keep hole stability.  Mud is very important because mud gives us a lot of data of well bore

measurement while drilling;swinging formation pressure;lubrication;stabilizes wellbore;cleaning hole;cooling bit

I consider that mud is the heart of drilling process. Mud have many functions such as control formation pressure to avoid kick, hole cleaning to avoid stuck of drilling tools, cooling and lubricity and hole stability.

Also, some types of mud make encapsulation of shale to avoid interact mud with shale or cuttings (e.g. PHPA mud). 

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measurement while drilling;swinging formation pressure;lubrication;stabilizes wellbore;cleaning hole;cooling bit

Remove cuttings from well[edit]

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 (viscosity increase during static conditions). 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 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 is the one of the best methods for increasing hole cleaning in high angle and horizontal wells.

Suspend and release cuttings[edit]

• 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 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.
• Conduct a test to compare the sand content of mud at flow line and suction pit (to determine whether cuttings are being removed).

Control formation pressures[edit]

• If formation pressure increases, mud density should also be increased to balance pressure and keep the wellbore stable. The most common weighting material is barite. Unbalanced formation pressures will cause an unexpected influx (also known as a kick) of formation fluids in the wellbore possibly leading to a blowout from pressured formation fluids.
• 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 stresses caused by tectonic forces, these may make 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[edit]

• 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).

Maintain wellbore stability[edit]

• Chemical composition and mud properties must combine to provide a stable wellbore. Weight of the mud must be within the necessary range to balance the mechanical forces.
• Wellbore instability = sloughing formations, which can cause tight hole conditions, bridges and fill on trips (same symptoms indicate hole cleaning problems).
• Wellbore stability = hole maintains size and cylindrical shape.
• If the hole is enlarged, it becomes weak and difficult to stabilize, resulting in problems such as low annular velocities, poor hole cleaning, solids loading and poor formation evaluation
• In sand and sandstones formations, hole enlargement can be accomplished by mechanical actions (hydraulic forces & nozzles velocities). Formation damage is reduced by conservative hydraulics system. A good quality filter cake containing bentonite is known to limit bore hole enlargement.
• In shales, mud weight is usually sufficient to balance formation stress, as these wells are usually stable. With water base mud, chemical differences can cause interactions between mud & shale that lead to softening of the native rock. Highly fractured, dry, brittle shales can be extremely unstable (leading to mechanical problems).
• Various chemical inhibitors can control mud / shale interactions (calcium, potassium, salt, polymers, asphalt, glycols and oil – best for water sensitive formations)
• Oil (and synthetic oil) based drilling fluids are used to drill most water sensitive Shales in areas with difficult drilling conditions.
• To add inhibition, emulsified brine phase (calcium chloride) drilling fluids are used to reduce water activity and creates osmotic forces to prevent adsorption of water by Shales.

Minimizing formation damage[edit]

• Skin damage or any reduction in natural formation porosity and permeability (washout) constitutes formation damage
• skin damage is the accumulation of residuals on the perforations and that causes a pressure drop through them .
• Most common damage;
  • Mud or drill solids invade the formation matrix, reducing porosity and causing skin effect
  • Swelling of formation clays within the reservoir, reduced permeability
  • Precipitation of solids due to mixing of mud filtrate and formations fluids resulting in the precipitation of insoluble salts
  • Mud filtrate and formation fluids form an emulsion, reducing reservoir porosity
• Specially designed drill-in fluids or workover and completion fluids, minimize formation damage.

Cool, lubricate, and support the bit and drilling assembly[edit]

• 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.("Coefficient of friction" is how much friction on side of wellbore and collar size or drill pipe size to pull stuck pipe) 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.
• Drilling fluids also support portion of drill-string or casing through buoyancy. Suspend in drilling fluid, buoyed by force equal to weight (or density) of mud, so reducing hook load at derrick.
• Weight that derrick can support limited by mechanical capacity, increase depth so weight of drill-string and casing increase.
• When running long, heavy string or casing, buoyancy possible to run casing strings whose weight exceed a rig's hook load capacity.

Transmit hydraulic energy to tools and bit[edit]

• Hydraulic energy provides power to mud motor for bit rotation and for MWD (measurement while drilling) and LWD (logging while drilling) tools. Hydraulic programs base on bit nozzles sizing for available mud pump horsepower to optimize jet impact at bottom well.
• Limited to:
  • Pump horsepower
  • Pressure loss inside drillstring
  • Maximum allowable surface pressure
  • Optimum flow rate
  • Drill string pressure loses higher in fluids higher densities, plastic viscosities and solids.
• Low solids, shear thinning drilling fluids such as polymer fluids, more efficient in transmit hydraulic energy.
• Depth can be extended by controlling mud properties.
• Transfer information from MWD & LWD to surface by pressure pulse.

Ensure adequate formation evaluation[edit]

• Chemical and physical mud properties and wellbore conditions after drilling affect formation evaluation.
• Mud loggers examine cuttings for mineral composition, visual sign of hydrocarbons and recorded mud logs of lithology, ROP, gas detection or geological parameters.
• Wireline logging measure – electrical, sonic, nuclear and magnetic resonance.
• Potential productive zone are isolated and performed formation testing and drill stem testing.
• Mud helps not to disperse of cuttings and also improve cutting transport for mud loggers determine the depth of the cuttings originated.
• Oil-based mud, lubricants, asphalts will mask hydrocarbon indications.
• So mud for drilling core selected base on type of evaluation to be performed (many coring operations specify a blend mud with minimum of additives).

Control corrosion (in acceptable level)[edit]

• Drill-string and casing in continuous contact with drilling fluid may cause a form of corrosion.
• Dissolved gases (oxygen, carbon dioxide, hydrogen sulfide) cause serious corrosion problems;
  • Cause rapid, catastrophic failure
  • May be deadly to humans after a short period of time
• Low pH (acidic) aggravates corrosion, so use corrosion coupons[clarification needed] to monitor corrosion type, rates and to tell correct chemical inhibitor is used in correct amount.
• Mud aeration, foaming and other O2 trapped conditions cause corrosion damage in short period time.
• When drilling in high H2S, elevated the pH fluids + sulfide scavenging chemical (zinc).

Facilitate cementing and completion[edit]

• Cementing is critical to effective zone and well completion.
• During casing run, mud must remain fluid and minimize pressure surges so fracture induced lost circulation does not occur.
• Temperature of water used for cement must be within tolerance of cementers performing task, usually 70 degrees, most notably in winter conditions.
• Mud should have thin, slick filter cake, with minimal solids in filter cake, wellbore with minimal cuttings, caving or bridges will prevent a good casing run to bottom. Circulate well bore until clean.
• To cement and completion operation properly, mud displace by flushes and cement. For effectiveness;
  • Hole near gauges, use proper hole cleaning techniques, pumping sweeps at TD, and perform wiper trip to shoe.
  • Mud low viscosity, mud parameters should be tolerant of formations being drilled, and drilling fluid composition, turbulent flow - low viscosity high pump rate, laminar flow - high viscosity, high pump rate.
  • Mud non progressive gel strength[clarification needed]

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Minimize impact on environment[edit]

Mud is, in varying degrees, toxic. It is also difficult and expensive to dispose of it in an environmentally friendly manner. A Vanity Fair article described the conditions at Lago Agrio, a large oil field in Ecuador where drillers were effectively unregulated.[citation needed]

Water based drilling fluid has very little toxicity, made from water, bentonite and barite, all clay from mining operations, usually found in Wyoming. There are specific chemicals that can be used in water based drilling fluids that alone can be corrosive and toxic, such as hydrochloric acid. However, when mixed into water based drilling fluids, hydrochloric acid only decreases the pH of the water to a more manageable level. Caustic (sodium hydroxide), anhydrous lime, soda ash, bentonite, barite and polymers are the most common chemicals used in water based drilling fluids. Oil Base Mud and synthetic drilling fluids can contain high levels of benzene, and other chemicals

Most common chemicals added to OBM Muds Barite Bentonite Diesel Water Emulsifiers

Indeed mud has many functions such as keep the stability at drill pipes and make a balance between bit pressure and different kind of informations . also to keep HWT ( heavy weight tools ) to be in cooling .  

The main functions of drilling fluids include providing hydrostatic pressure to prevent formation fluids from entering into the well bore, keeping the drill bit cool and clean during drilling, carrying out drill cuttings, and suspending the drill cuttings while drilling is paused and when the drilling assembly is brought in and out of the hole. The drilling fluid used for a particular job is selected to avoid formation damage and to limit corrosion.