Drilling fluid

Drilling fluid

Overview
In geotechnical engineering
Geotechnical engineering
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering is important in civil engineering, but is also used by military, mining, petroleum, or any other engineering concerned with construction on or in the ground...

, drilling fluid is a fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 used to aid the drilling of boreholes into the earth. Often used while drilling oil
Oil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...

 and natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

 wells and on exploration drilling rig
Drilling rig
A drilling rig is a machine which creates holes or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person...

s, drilling fluids are also used for much simpler boreholes, such as water wells. Liquid drilling fluid is often called drilling mud. The three main categories of drilling fluids are water-based muds (which can be dispersed and non-dispersed), non-aqueous muds, usually called oil-based mud, and gaseous drilling fluid, in which a wide range of gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

es can be used.

The main functions of drilling fluids include providing hydrostatic pressure to prevent formation fluid
Formation fluid
Formation fluid refers to the naturally occurring liquids and gases contained in geologic formations. Fluids introduced during the drilling process are called drilling fluids. Fluids in an oil or gas reservoir are called reservoir fluids. The fluids flowing from the wellhead of an oil or gas well...

s from entering into the well bore, keeping the drill bit
Well drilling
Well drilling is the process of drilling a hole in the ground for the extraction of a natural resource such as ground water, natural gas, or petroleum...

 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.
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In geotechnical engineering
Geotechnical engineering
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering is important in civil engineering, but is also used by military, mining, petroleum, or any other engineering concerned with construction on or in the ground...

, drilling fluid is a fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 used to aid the drilling of boreholes into the earth. Often used while drilling oil
Oil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...

 and natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

 wells and on exploration drilling rig
Drilling rig
A drilling rig is a machine which creates holes or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person...

s, drilling fluids are also used for much simpler boreholes, such as water wells. Liquid drilling fluid is often called drilling mud. The three main categories of drilling fluids are water-based muds (which can be dispersed and non-dispersed), non-aqueous muds, usually called oil-based mud, and gaseous drilling fluid, in which a wide range of gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

es can be used.

The main functions of drilling fluids include providing hydrostatic pressure to prevent formation fluid
Formation fluid
Formation fluid refers to the naturally occurring liquids and gases contained in geologic formations. Fluids introduced during the drilling process are called drilling fluids. Fluids in an oil or gas reservoir are called reservoir fluids. The fluids flowing from the wellhead of an oil or gas well...

s from entering into the well bore, keeping the drill bit
Well drilling
Well drilling is the process of drilling a hole in the ground for the extraction of a natural resource such as ground water, natural gas, or petroleum...

 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.

Types of drilling fluid


Many types of drilling fluids are used on a day-to-day basis. Some wells require that different types be used at different parts in the hole, or that some types be used in combination with others. The various types of fluid generally fall into a few broad categories:
  • Air: Compressed air is pumped either down the bore hole's annular space or down the drill string
    Drill string
    A drill string on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit...

     itself.
  • Air/water: The same as above, with water added to increase viscosity, flush the hole, provide more cooling, and/or to control dust.
  • Air/polymer: A specially formulated chemical, most often referred to as a type of polymer, is added to the water & air mixture to create specific conditions. A foaming agent is a good example of a polymer
    Polymer
    A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

    .
  • Water: Water by itself is sometimes used.
  • Water-based mud (WBM): A most basic water-based mud system begins with water, then clays and other chemicals are incorporated into the water to create a homogenous blend resembling something between chocolate milk and a malt (depending on viscosity). The clay (called "shale
    Shale
    Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

    " in its rock form) is usually a combination of native clays that are suspended in the fluid while drilling, or specific types of clay that are processed and sold as additives for the WBM system. The most common of these is bentonite
    Bentonite
    Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...

    , frequently referred to in the oilfield as "gel". Gel likely makes reference to the fact that while the fluid is being pumped, it can be very thin and free-flowing (like chocolate milk), though when pumping is stopped, the static fluid builds a "gel" structure that resists flow. When an adequate pumping force is applied to "break the gel", flow resumes and the fluid returns to its previously free-flowing state. Many other chemicals (e.g. potassium formate
    Potassium formate
    Potassium formate is the potassium salt of formic acid. It is an intermediate in the formate potash process for the production of potassium. Potassium formate has also been studied as a potential environmentally-friendly deicing salt for use on roads....

    ) are added to a WBM system to achieve various effects, including: viscosity control, shale stability, enhance drilling rate of penetration, cooling and lubricating of equipment.
  • Oil-based mud (OBM): Oil-based mud can be a mud where the base fluid is a petroleum product such as diesel fuel. Oil-based muds are used for many reasons, some being increased lubricity, enhanced shale inhibition, and greater cleaning abilities with less viscosity. Oil-based muds also withstand greater heat without breaking down. The use of oil-based muds has special considerations. These include cost and environmental considerations.
  • Synthetic-based fluid (SBM) (Otherwise known as Low Toxicity Oil Based Mud or LTOBM): Synthetic-based fluid is a mud where the base fluid is a synthetic oil. This is most often used on offshore rigs because it has the properties of an oil-based mud, but the toxicity of the fluid fumes are much less than an oil-based fluid. This is important when men work with the fluid in an enclosed space such as an offshore drilling rig.


On a drilling rig, mud is pumped from the mud pits through the drill string where it sprays out of nozzles on the drill bit, cleaning and cooling the drill bit in the process. The mud then carries the crushed or cut rock ("cuttings") up the annular space ("annulus") between the drill string and the sides of the hole being drilled, up through the surface casing, where it emerges back at the surface. Cuttings are then filtered out with either a [shale shaker], or the newer shale conveyor technology, and the mud returns to the mud pits. The mud pits let the drilled "fines" settle; the pits are also where the fluid is treated by adding chemicals and other substances.

The returning mud can 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
Electrical Equipment in Hazardous Areas
In electrical engineering, a hazardous location is defined as a place where concentrations of flammable gases, vapors, or dusts occur. Electrical equipment that must be installed in such locations is especially designed and tested to ensure it does not initiate an explosion, due to arcing contacts...

 equipment is commonly installed, and workers are advised to take 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 properties which optimize and improve drilling efficiency, borehole stability, and other requirements listed below.

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
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 is too low.

Other properties include:
  • Most drilling muds are thixotropic (that is, 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
    Lost circulation
    Lost circulation can be one of the more serious problems that can arise during the drilling of an oil well or gas well. Circulation is said to be lost when the drilling fluid, known commonly as "mud", flows into one or more geological formations instead of returning up the...

    .
  • Weight material that settles is referred to as sag
    Sag
    Şag is a commune in Timiş County, Romania. It is composed of a single village, Şag; Parţa village broke off as a separate commune in 2004.-References:...

    , 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
      Solids control
      Solids control is a technique used for well drilling to provide prepared drilling fluids for drilling rigs. Drilling fluid maintenance cost, clean up, and disposal cost as well as the overall cost of boring can be reduced dramatically when proper solids control techniques are utilized...

  • 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

  • If formation pressure increases, mud density should also be increased, often with barite
    Barite
    Baryte, or barite, is a mineral consisting of barium sulfate. The baryte group consists of baryte, celestine, anglesite and anhydrite. Baryte itself is generally white or colorless, and is the main source of barium...

     (or other weighting materials) to balance pressure and keep the wellbore stable. Unbalanced formation pressures will cause an unexpected influx of pressure in the wellbore possibly leading to a blowout
    Blowout (well drilling)
    A blowout is the uncontrolled release of crude oil and/or natural gas from an oil well or gas well after pressure control systems have failed....

     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

  • 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
      Calcium carbonate
      Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

      , ground cellulose).
  • Depending on the mud system in use, a number of additives can improve the filter cake (e.g. bentonite
    Bentonite
    Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...

    , natural & synthetic polymer, asphalt
    Asphalt
    Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...

     and gilsonite
    Gilsonite
    Gilsonite is the registered trademark for a form of natural asphalt found only in the Uintah Basin of Utah; the non-trademarked mineral name is uintaite or uintahite. It is mined in underground shafts and resembles shiny black obsidian...

    ).

Maintain wellbore stability

  • 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
    Bentonite
    Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...

     is known to limit bore hole enlargement.
  • In shales
    Shalës
    Shalës is a municipality in the Elbasan District, Elbasan County, central Albania. The municipality consists of the villages Shalës, Licaj, Kurtalli, Xibrake, Xherie and Kodras....

    , 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
    Potassium
    Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

    , 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 Shale
    Shale
    Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

    s in areas with difficult drilling conditions.
  • To add inhibition, emulsified brine phase (calcium chloride
    Calcium chloride
    Calcium chloride, CaCl2, is a salt of calcium and chlorine. It behaves as a typical ionic halide, and is solid at room temperature. Common applications include brine for refrigeration plants, ice and dust control on roads, and desiccation...

    ) drilling fluids are used to reduce water activity and creates osmotic forces to prevent adsorption of water by Shale
    Shale
    Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

    s.

Minimizing formation damage

  • Skin damage or any reduction in natural formation porosity and permeability (washout) constitutes formation damage
  • 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
      Permeability (fluid)
      Permeability in fluid mechanics and the earth sciences is a measure of the ability of a porous material to allow fluids to pass through it.- Units :...

    • 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

  • 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 motor
    Mud motor
    Mud motor refers to a Progressive Cavity Positive Displacement Pump placed in the Drill string to provide additional power to the bit while drilling. The PCPD Pump uses Drilling Fluid to create eccentric motion in the power section of the motor which is transferred as concentric power to the...

    s 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.
  • 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
    Derrick
    A derrick is a lifting device composed of one tower, or guyed mast such as a pole which is hinged freely at the bottom. It is controlled by lines powered by some means such as man-hauling or motors, so that the pole can move in all four directions. A line runs up it and over its top with a hook on...

     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

  • Hydraulic energy provides power to mud motor
    Mud motor
    Mud motor refers to a Progressive Cavity Positive Displacement Pump placed in the Drill string to provide additional power to the bit while drilling. The PCPD Pump uses Drilling Fluid to create eccentric motion in the power section of the motor which is transferred as concentric power to the...

     for bit rotation and for MWD (measurement while drilling
    Measurement While Drilling
    MWD stands for Measurement While Drilling in the oil & gas industry. The simplest way to describe MWD is to relate it to the measurements a pilot takes. A pilot needs to know the direction they are flying , the angle they are fly at , and what type of skies they will be flying through...

    ) and LWD (logging while drilling
    Logging While Drilling
    Logging While Drilling is a technique of conveying well logging tools into the well borehole downhole as part of the bottom hole assembly ....

    ) 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

  • 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
    Lithology
    The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...

    , ROP, gas detection or geological parameters.
  • Wireline logging measure – electrical, sonic, nuclear and magnetic resonance
    Resonance
    In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

    .
  • 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)

  • Drill-string and casing in continuous contact with drilling fluid may cause a form of corrosion
    Corrosion
    Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

    .
  • Dissolved gases (oxygen, carbon dioxide, hydrogen sulfide
    Hydrogen sulfide
    Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

    ) cause serious corrosion problems;
    • Cause rapid, catastrophic failure
      Catastrophic failure
      A catastrophic failure is a sudden and total failure of some system from which recovery is impossible. Catastrophic failures often lead to cascading systems failure....

    • May be deadly to humans after a short period of time
  • Low pH
    PH
    In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

     (acidic) aggravates corrosion, so use corrosion coupons 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

  • 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.
  • Mud should have thin, slick filter cake, wellbore with no cuttings, cavings or bridges.
  • To cement and completion operation properly, mud displace by flushes and cement. For effectiveness;
    • Hole near gauges
    • Mud low viscosity
    • Mud non progressive gel strength

Minimize impact on environment


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
Lago Agrio oil field
The Lago Agrio oil field is an oil-rich area in the Ecuadorian province of Sucumbíos, discovered in the 1960s. The Lago Agrio field is known internationally for the serious ecological problems that oil development has created there, including water pollution, soil contamination, deforestation and...

, a large oil field in Ecuador where drillers were effectively unregulated.

Composition of drilling mud


Water-based drilling mud most commonly consists of bentonite
Bentonite
Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...

 clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 (gel) with additives such as barium sulfate
Barium sulfate
Barium sulfate is the inorganic compound with the chemical formula BaSO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and materials prepared from it...

 (barite), calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 (chalk) or hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

. Various thickeners are used to influence the viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 of the fluid, e.g. xanthan gum
Xanthan gum
Xanthan gum is a polysaccharide, derived from the bacterial coat of Xanthomonas campestris, used as a food additive and rheology modifier, commonly used as a food thickening agent and a stabilizer...

, guar gum
Guar gum
Guar gum, also called guaran, is a galactomannan. It is primarily the ground endosperm of guar beans. The guar seeds are dehusked, milled and screened to obtain the guar gum. It is typically produced as a free-flowing, pale, off-white-colored, coarse to fine ground powder.-Production:Guar gum is an...

, glycol, carboxymethylcellulose, polyanionic cellulose (PAC), or starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

. In turn, deflocculants are used to reduce viscosity of clay-based muds; anionic polyelectrolyte
Polyelectrolyte
Polyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous solutions , making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes and polymers , and are sometimes called polysalts. Like salts, their...

s (e.g. acrylate
Acrylate
The acrylate ion is the ion of acrylic acid.Acrylates are the salts and esters of acrylic acid. They are also known as propenoates ....

s, polyphosphate
Polyphosphate
Triphosphates are salts or esters of polymeric oxyanions formed from tetrahedral PO4 structural units linked together by sharing oxygen atoms. When two corners are shared the polyphosphate may have a linear chain structure or a cyclic ring structure. In biology the polyphosphate esters AMP, ADP...

s, lignosulfonates
Lignosulfonates
Lignosulfonates, or sulfonated lignin, are water-soluble anionic polyelectrolyte polymers: they are byproducts from the production of wood pulp using sulfite pulping....

 (Lig) or tannic acid
Tannic acid
Tannic acid is a specific commercial form of tannin, a type of polyphenol. Its weak acidity is due to the numerous phenol groups in the structure...

 derivates such as Quebracho) are frequently used. Red mud was the name for a Quebracho-based mixture, named after the color of the red tannic acid salts; it was commonly used in 1940s to 1950s, then was made obsolete when lignosulfonates became available. Other components are added to provide various specific functional characteristics as listed above. Some other common additives include lubricants , shale inhibitors, fluid loss additives (to control loss of drilling fluids into permeable formations). A weighting agent such as barite is added to increase the overall density of the drilling fluid so that sufficient bottom hole pressure can be maintained thereby preventing an unwanted (and often dangerous) influx of formation fluids.

Mud engineer



"Mud engineer" is the name given to an oil field service company individual who is charged with maintaining a drilling fluid or completion fluid system on an oil and/or gas drilling rig
Drilling rig
A drilling rig is a machine which creates holes or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person...

. This individual typically works for the company selling the chemicals for the job and is specifically trained with those products, though independent mud engineers are still common. The work schedule of the mud engineer
Mud engineer
A mud engineer works on an oil well or gas well drilling rig, and is responsible ensuring the properties of the drilling fluid, also known as drilling mud, are within designed specifications.-Use of mud:Mud is a vital part of drilling operations...

 or more properly Drilling Fluids Engineer, is arduous, often involving long shifts. Typical shifts range from twenty-one days on and seven off, to five weeks on and five weeks off. Consulting mud men can work up to one hundred days or longer, in a row. Nowadays, the internationally accepted shift pattern at off-shore drilling operations is personnel (including mud engineers) work on a 28 day shift pattern, where they work for 28 continuous days and rest the following 28 days. In Europe this is more commonly a 21 day shift pattern.

In offshore drilling, with new technology and high total day costs, wells are being drilled extremely fast. Having two mud engineers makes economic sense to prevent down time due to drilling fluid difficulties. Two mud engineers also reduce insurance costs to oil companies for environmental damage that oil companies are responsible for during drilling and production. A senior mud engineer typically works in the day, and a junior mud engineer at night.

The cost of the drilling fluid is typically about 10% (may vary greatly) of the total cost of drilling a well, and demands competent mud engineers. Large cost savings result when the mud engineer and fluid performs adequately.

The mud engineer is not to be confused with mudloggers, service personnel who monitor gas from the mud and collect well bore samples.

Compliance engineer


The compliance engineer is the most common name for a relatively new position in the oil field, emerging around 2002 due to new environmental regulations on synthetic mud in the United States. Previously, synthetic mud was regulated the same as water-based mud and could be disposed of in offshore waters due to low toxicity to marine organisms. New regulations restrict the amount of synthetic oil that can be discharged. These new regulations created a significant burden in the form of tests needed to determine the "ROC" or retention on cuttings, sampling to determine the percentage of crude oil in the drilling mud, and extensive documentation.
It should be noted that no type of oil/synthetic based mud (or drilled cuttings contaminated with OBM/SBM) may be dumped in the North Sea. Contaminated mud must either be shipped back to shore in skips or processed on the rigs.

A new monthly toxicity test is also now performed to determine sediment toxicity, using the amphipod Leptocheirus plumulosus. Various concentrations of the drilling mud are added to the environment of captive L. plumulosus to determine its effect on the animals. The test is controversial for two reasons:
  1. These animals are not native to many of the areas regulated by them, including the Gulf of Mexico
  2. The test has a very large standard deviation and samples that fail badly may pass easily upon retesting

See also

  • Directional drilling
    Directional drilling
    Directional drilling is the practice of drilling non-vertical wells. It can be broken down into three main groups: Oilfield Directional Drilling, Utility Installation Directional Drilling Directional drilling (or slant drilling) is the practice of drilling non-vertical wells. It can be broken down...

  • Driller (oil)
    Driller (oil)
    The driller is a team leader in charge during the process of well drilling. The term is commonly used in the context of an oil well drilling rig....

  • Drilling rig
    Drilling rig
    A drilling rig is a machine which creates holes or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person...

  • Formation evaluation
    Formation evaluation
    In petroleum exploration and development, formation evaluation is used to determine the ability of a borehole to produce petroleum. Essentially, it is the process of "recognizing a commercial well when you drill one"....

  • MWD (measurement while drilling)
  • Roughneck
    Roughneck
    Roughneck is a slang term for a person whose occupation is hard-manual labour, typically in a dangerous working environment. The term applies across a number of industries, but is most commonly associated with oil rigs...

  • Underbalanced drilling
    Underbalanced drilling
    Underbalanced drilling, or UBD, is a procedure used to drill oil and gas wells where the pressure in the wellbore is kept lower than the fluid pressure in the formation being drilled. As the well is being drilled, formation fluid flows into the wellbore and up to the surface...

  • Mud Gas Separator
    Mud Gas Separator
    Mud Gas Separator is commonly called a gas-buster or poor boy degasser. It captures and separates large volume of free gas within the drilling fluid. If there is a "KICK" situation, this vessel separates the mud and the gas by allowing it to flow over baffle plates. The gas then is forced to flow...

  • Mud systems
    Mud systems
    Effective solids control can be attributed to the overall performance of all the components of the mud systems. Conditioning the drilling fluid with the goal of dramatically lowering maintenance cost, avoiding excessive chemical treatment and maintaining mud systems volume will decrease the chance...

  • Oil well control
    Oil well control
    Oil well control is the management of the dangerous effects caused by unexpected high pressures upon surface equipment of oil or gas drilling rigs. Technically, oil well control involves preventing Formation fluid, usually referred to as kick, from entering into the Wellbore during drilling...


Further reading

  • ASME Shale Shaker Committee (2005) The Drilling Fluids Processing Handbook ISBN 0-7506-7775-9
  • Kate Van Dyke (1998) Drilling Fluids, Mud Pumps, and Conditioning Equipment
  • G. V. Chilingarian & P. Vorabutr (1983) Drilling and Drilling Fluids
  • G. R. Gray, H. C. H. Darley, & W. F. Rogers (1980) The Composition and Properties of Oil Well Drilling Fluids

External links