Petroleum Engineering Summer SchoolDubrovnik, Croatia.Workshop #26 June 9 – 13, 08Hole. H MJune 2008DRILLING FLUIDS FOR DRILLING OF GEOTHERMAL WELLSHagen HoleGeothermal Consultants NZ Ltd., Birkenhead, Auckland, New Zealand. ABSTRACTDrilling fluids are required to remove cuttings from thewell during drilling, to cool and lubricate the drill bitand drill string, to apply pressure to formation fluids tocontrol flow into or out of the well, and to cool theformation, particularly prior to cementing casings.Various drilling fluids are selected according toreservoir pressures and temperatures and to the drillingtechniques to be utilised. Drilling fluids normally usedinclude water, water based bentonitic (or other) muds,aerated water, and stiff foam.Because many geothermal reservoirs are set ininterlayered volcanic and sedimentary rock and arenormally associated with local and regional faulting,highly permeable features are common and causemajor and frequent losses of drilling fluid circulation. Reduces losses of drilling fluid by forming animpermeable ‘wall cake’ or lining to the holewall.Reduces the rate of breakdown of watersensitive formations.These functions are those desirable in drilling fluidsutilised in petroleum wells, some water wells, and inthe upper parts of a geothermal well. However, not allof these properties are necessarily desirable in allsections of a geothermal well.DRILLING FLUID PROPERTIESThe primary function of a drilling fluid is to removethe drilling cuttings from the bottom of the hole andcarry them to the surface.The utilisation of aerated fluids and the concept of‘balanced’ downhole pressure conditions allows forfull circulation of drilling fluids and drilling cuttingsback to the surface while drilling through permeableformations, thus significantly reducing the risk of thedrill string becoming stuck, of formation and wellboreskin damage, and for full geological control.Slip VelocityThe ability of a drilling fluid to entrap and carrygranular particles from the drill bit face to the surface isdependent upon the annular velocity of the drillingfluid exceeding the ‘slip velocity’ of the cuttingsparticles in that drilling fluid.In the context of drilling, this ‘slip velocity’ may bedescribed as the upwards annular drilling fluid velocityrequired to impose an upwards drag force on a cuttingsparticle equal to the downward gravitational force onthat particle. If the upwards drag force does not exceedthe downwards gravitational force the cuttings particlewill not be lifted.The drag force on a cuttings particle is dependent uponthe size, shape and density (or wetted surface area andmass) of the particles, the viscosity of the fluid, and theupwards vertical velocity component of the fluid.The size, shape and density of the particles beingdrilled are related to the rock type, the drill bit type andhow well cuttings are being cleared away from the bit(and not being reground). The rock density, and thesize and shape of the cuttings being produced areparameters which are not easily controlled or changed,however the drilling fluid viscosity and the drillingfluid flow rate and therefore annular flow velocity canbe controlled within certain limits.Keywords: geothermal, drilling fluids, aerated drillingINTRODUCTIONThe circulation of ‘drilling fluid’ is an integral part ofrotary and percussion drilling, and depending on thefluid type can fulfil all or some of the followingfunctions: Removes cuttings from the bottom of the hole– at the bit face. Returns cuttings to the surface (circulatingconditions). Holds cuttings in suspension when circulationis stopped. Releases cuttings from the drilling fluid at thesurface. Cools and lubricates the drill bit. Lubricates the drill string. Cools the hole and prevent liquid in the wellfrom boiling. Controls downhole pressure preventing thewell from flowing. Carry weighting material to increase fluiddensity to prevent the well from flowing andpossibly blowing out.Fluid Viscosity and Flow VelocityFluid viscosity and fluid flow velocity are inverselyproportional with respect to ‘Slip Velocity’ of aparticular particle – in other words, if the fluidviscosity is increased, a reduced fluid flow velocitywill be required to maintain the same slip velocity for aparticular particle.1

Hole. H MPetroleum Engineering Summer SchoolDubrovnik, Croatia.Workshop #26 June 9 – 13, 08June 2008 However, there are practical limitations to the range offluid viscosities and fluid annular velocities that can beutilised.Higher viscosity drilling fluids impose higher dragforces upon entrained cuttings particles and thereforeproduce better hole cleaning – but, higher viscosityfluids also impose higher pressure losses and thereforerequire higher pumping pressures.Higher annular velocities ensure the particle – fluid slipvelocity is exceeded, but increase the risk of scouringunconsolidated formation from walls of the hole andalso impose higher pressure losses and thereforerequire higher pumping pressures.Corrosion control additives may also be addedto the mud.The solid content of the mud is derived from bentonite,non-clay materials contained in the bentonite,weighting materials if utilised, and drilled cuttingsparticles which may include sand and clay minerals.Solids other than bentonite or weighting materialsgenerally have adverse effects on the drillingoperations. Increased mud density can reducepenetration rates and cause circulation losses. Sandscan increase wear on pumping equipment anddownhole tools (stabilisers, reamers, bits), drill stringand casing. Drilled clays can cause excessive viscositybuild-up and, together with other drilled solids, canbuild up thick wall cakes in the hole and aroundstabilisers. It is therefore desirable to remove as manyof the drilled solids from the drilling fluid as ispossible.Thixotrophy and Gel StrengthThe ability of a drilling fluid to hold cuttings insuspension during periods of no circulation, and ofreleasing the cuttings from suspension at the surfacerequire a special property – Thixotrophy.A Newtonian fluid such as water, oil, and glycerine,maintains a constant viscosity while stationary or whileflowing – the fluid viscosity is independent of anyapplied sheers stress.The viscosities of NonNewtonian fluids such as water based bentonite mud,some polymers and some cement slurries varies as afunction of the applied sheer stress – this property isThixotrophy. When the fluid is stationary the fluidbuilds gel strength and the viscosity increases; if thefluid is pumped and forced to flow, the viscosityreduces.This thixotropic property is ideal for holding cuttingsin suspension during period of no circulation, and forreleasing cuttings when the fluid is subjected to highcheer stress, such as passing over a linear motion shaleshaker.In addition to this process of holding cuttings insuspension and releasing them at the shale shaker, thisthixotropic property also allows a layer of gelled fluidto build on the hole wall, creating a protective andsomewhat impermeable lining or ‘wall cake’ on thehole wall.As drilling proceeds and the formation temperaturesincrease with depth, the drilling fluid is inevitablyheated. At elevated temperatures the gelling propertiesand viscosity of bentonite muds increase, and the mudbegins to flocculate. Dispersant and deflocculatingadditives, and cooling the circulating fluid can assist incontrolling this problem.Over the past 10 years polymeric fluids have beendeveloped and introduced into the drilling industry.Synthetic drilling polymers exhibit many of the sameproperties as water based bentonite and are now beingutilised more frequently in geothermal drilling –however, polymeric drilling fluids are extremelyexpensive and are therefore used sparingly.UNDERBALANCE AND OVERBALANCEIn a typical ‘under-pressured’ geothermal system, thepressure of the drilling fluid in the well exceeds thepressure of the fluids in the formation at the samedepth. This is an “overbalanced” condition – theopposite condition or “underbalanced” conditions mayoccur when a total loss of circulation allows the liquidlevel in the annulus to move down the well, or whenintentionally established using aerated drillingmethods. Drilling in an underbalanced conditionencourages inflow of formation fluids (gas, steam orhot water) and sloughing of formations. Unlesscontrolled, licks and stuck drill string can result.However, drilling with excessive overbalancedpressures can cause slow penetration rates, high loss ofmud filtrate resulting in thick soft wall cakedevelopment and breakdown of the formation andsubsequent loss of circulation.Water Based Bentonite MudThe most commonly used geothermal drilling fluid thatexhibits the properties described above is water basedbentonite mud, which typically comprises bentonite,water and caustic soda. Other chemicals may be addedto control the physical properties of the fluid asrequired by the downhole conditions, and these willinclude: Thinners to control viscosity and gel strengthsFluid loss control agents to control the loss ofwater from the mud which in turn controlsexcessive build-up of wall cake.Weighting materials such as barite to increasemud density (rare in geothermal)Loss of Circulation Materials (LCM) to aid inreducing the loss of drilling fluid to theformation.Where conditions of a large overbalance pressure and athick soft wall cake are present adjacent to the drillstring (particularly non-stabilised and slick drillcollars), the drilling tubulars can be forced into the wallcake by the overbalance pressure and cause the drillstring to become securely stuck in the wall cake. Thisaction, referred to as “differential sticking”, is afrequent cause of stuck drill strings and is best avoided2

Petroleum Engineering Summer SchoolDubrovnik, Croatia.Workshop #26 June 9 – 13, 08Hole. H MJune 2008fluid flows into the formation rather than returning tothe surface.The traditional method of dealing with this situationwas to continue drilling ‘blind’ with water – thepumped water being totally lost to the formation withthe drilling cuttings being washed into the formation aswell. The major problem with this method of drillingis that the cuttings rarely totally disappear into theformation. Stuck drill string due to a build up ofcuttings in the hole, and well-bore skin damage beingcommon using mud weights which give minimum water loss(to reduce the build up of wall cake) and low inactivesolids content (to reduce the strength of the wall cake).LOSS OF CIRCULATIONThe common denominator of all convectivehydrothermal systems – the majority of all developedgeothermal fields, is the highly permeable, fracturedand faulted nature of the formations in which thegeothermal reservoirs reside. This high permeabilitybeing one of the fundamental and requisite componentsfor any geothermal system to exist.Typically, the permeable nature of the formations is notlimited to the geothermal reservoir structure alone, butoften occurs in much of the shallower and overlyingformations as well. This, coupled with the underpressured nature of most geothermal systems, results inthe partial or total loss of circulation of drilling fluid atsome stage during the drilling of the well - in factultimately if circulation is not lost in an underpressured system this is an indication that there is nopermeability and therefore the well a ‘dry well’.The thixotropic and gelling nature of water basedbentonite mud assists in the sealing of minor losszones, and with the addition of loss circulationmaterials (LCM) many minor loss zones can becompletely sealed. However, if major or total losses ofcirculation are encountered, and can’t be sealed withLCM added to the mud, then it becomes impracticaland uneconomic to continue drilling with mud. If highpermeability and therefore significant or total losses ofcirculation are encountered within the upper casedsections of the well the use of water based bentonitemud and additives is normally ceased, and drilling iscontinued with water or with aerated water.The advantages using water as drilling fluid are: As the water is not recirculated but is lost tothe formation, the downhole temperaturesignificantly lower, extending drill bit life andreducing the likelihood of a kick developing. As lower bottom hole circulating pressures aredeveloped, penetration rates are higher. Because mud and thick wall cake ae notsqueezed into permeable zones, reducedformation sealing and increased wellproductivity are achieved. Because a wall cake is not developed,differential sticking does not occur. Where awall cake is present from earlier drilling, thelower downhole circulating pressuressignificantly reduces or eliminates the risk ofdifferential sticking.The disadvantages of using water as drilling fluid are: A continuous large volume ( 3500 lpm)supply of water to the drilling rig is required. As waterhas a low viscosity, is notthixotropic and cannot develop gel strength,slip velocities are higher requiring increasedannular fluid velocities, and as soon aspumping to the drill string is stopped (e.g. tomake a connection), any cuttings suspended inthe annulus will start settling immediately,which increases the risk of stuck drill string. Cutting are not returned to the surface, butwashed into the permeable zones. No geological data, as no return of cuttings tothe surface. The loss of cuttings into the permeable zonesmay reduce permeability (not as much asmud). When pumping is stopped cuttingsaccumulated in permeable zones may flowback into the well increasing the risk of stuckdrill string. Loss of large volumes of cold water to theformation can cause long recovery periodsafter drilling is completed before the well canbe discharged.When drilling the production section of the well withinthe reservoir structure, the elevated temperatures andthe targeted permeability render the properties ofbentonite muds undesirable. The drilling of ageothermal well has as it’s primary objective, drillinginto, and preserving permeable formations within thereservoir structure, which will, after completion ofdrilling become the production zone of the well.If bentonite mud is forced into the permeable structureof the reservoir, the gelling and sealing properties cancause permanent damage to the productivity of thezone. The high temperatures dehydrates and bakes thebentonite clay into a relatively inert and impermeablematerial. A process similar to baking clay into pottery.It is therefore usual and accepted practice that thissection of the well is drilled with water or aeratedwater.DRILLING WITH WATERWater as a drilling fluid was, in the past, used tocontinue drilling past an unsealable loss zone and forthe final production section of a geothermal well.When drilling into a permeable ‘under pressured’ zonesthe drilling fluid circulation is lost, and the drillingGreat care must be exercised when drilling with waterto avoid becoming stuck with cuttings settling downthe annulus.3

Hole. H MPetroleum Engineering Summer SchoolDubrovnik, Croatia.Workshop #26 June 9 – 13, 08June 2008Wells drilled with aerated fluids, and thus with fullcirculation and removal of drill cuttings show less skindamage than those drilled ‘blind’ with water.In general terms, wells with the production zone drilledwith aerated fluids demonstrate better productivity thanthose drilled blind with water, and significantly betterproductivity than those drilled with bentonite mud inthe production zone.A previous drilling campaign in Kenya allows for adirect comparison between a number wells drilled asimmediate offsets, to similar depths in similarlocations; the original set of wells were drilled blindwith water(and in one case mud) and a more recent setdrilled with aerated water. The productivity of thewells drilled with aerated fluids, on average is morethan double that of the wells drilled without air.AERATED DRILLING‘Aerated Drilling’ may be defined as the addition ofcompressed air to the drilling fluid circulating systemto reduce the density of the fluid column in thewellbore annulus such that the hydrodynamic pressurewithin the wellbore annulus is ‘balanced’ with theformation pressure in the permeable ‘loss zones’ of ageothermal well.Drilling ProcessesThe primary objective of utilising aerated drillingfluids is the ability to maintain drilling fluid circulationand therefore the clearance of cuttings from the hole asdrilling proceeds into permeable and ‘under pressured’zones. This continuous clearance of cuttings from thehole significantly reduces the risk of the drill stringgetting stuck in the hole.Wells Drilled Blind withwaterWell No.Output(MWt)143.31212.75422.155(drilled with mud) 14.76621.38Aeration of the drilling fluid reduces the density of thefluid column and thus the hydraulic pressure exerted onthe hole walls and the formation. As the introduced airis a compressible medium, the density of the columnvaries with depth – at the bottom of the hole where thehydrostatic pressure is greatest, the air component ishighly compressed and therefore the density of thefluid is greatest; at the top of the hole, where thehydrostatic pressure is least, the air component ishighly expanded and therefore the density of the fluidthe least. The ratio of air to water pumped into the hole,and the back pressure applied to ‘exhaust’ or flowlinefrom the well, allows the down-hole pressures in thehole to be ‘balanced’ with the formation pressure in thepermeable zones, thus allowing for the return of thedrilling fluids to the surface and therefore maintainingdrilling fluid circulation. (In fact the term ‘underbalanced’ drilling as applied to this form of geothermaldrilling is a misnomer).Wells Drilled withAerated FluidWell WtTable 1: Comparison of Thermal Outputs of wells drilledwith and without Aerated Fluids at Olkaria – Kenya.Cuttings ReturnInitially the technique was utilised only in the smallerdiameter production hole section of a well, however, insome fields permeability is prevalent in the formationslocated above the production zone, and significantamounts of lost time can be incurred in attempting toplug and re-drill such zones. Utilising aerated fluids todrill these zones has proven to be a highly successfulsolution.As indicated above, the primary objective of utilisingaerated drilling fluids is the maintenance of drillingfluid circulation, the obvious corollary to this is thecontinued return of drilling cuttings back to the surface,and thus the ability to collect and analyse cuttings fromthe total drilled depth. While this is not alwaysachieved for the entire drilled depth of wells drilledwith aerated fluids, it is usual for circulation to bemaintained for a significant proportion of the drilleddepth.Formation and The ResourcePerhaps the most important feature of aerated drillingis its effect on the productivity of the well. Theremoval of the drill cuttings from the well bore, ratherthan washing the cuttings into the permeable zones,reduces the potential of blocking up and in some casessealing the permeability close to the wellbore – theeffect called well-bore skin damage. A relatively smallamount of interference to the flow from the formationinto the well-bore, or skin damage, can have asignificant effect on the productivity of the well.Drilling MaterialsA significant reduction in the consumption of bentonitedrilling mud and treating chemicals, cement pluggingmaterials, and bentonite and polymer ‘sweep’ materialscan result from the use aerated water or mud.In addition a major reduction in the quantities of waterconsumed occurs. Typically, approximately 2000 litresper minute will be ‘lost to the formation’ while drillingan 8½” hole ‘blind with water’. Aeration of the fluidallows almost complete circulation and re-use ofdrilling water.4

Petroleum Engineering Summer SchoolDubrovnik, Croatia.Workshop #26 June 9 – 13, 08Hole. H MJune 2008Typically this additional cost will be in the order ofUS 150,000 to 250,000 per well, or if we assume atypical geothermal cost of US 3.5 million, the aerateddrilling component of this cost will be in the order of 6.0%.A Fishing ToolPerhaps the most common reason for stuck drill-stringis inadequate hole cleaning – the failure to removecuttings from the annulus between the hole and the drillstring. Often, the hole wall in the region of the losszone acts as a filter, allowing fine cutting particles tobe washed into the formation while larger particlesaccumulate in the annulus. Under these circumstances,if a new loss zone is encountered and all of the drillingfluid flows out of the bottom of the hole, theseaccumulated cuttings fall down around the bottom holeassembly and can result in stuck and lost drill strings.Aerated drilling prevents the accumulation of cuttingsin the annulus and allows for circulation to bemaintained even when new loss zones are encountered.In the event that a significant loss zone is encounteredand the pressure balance disrupted, circulation may belost and in severe cases the drill string may becomestuck; with adjustment of the air / water ratio it isusually possible to regain circulation, clear the annulusof cuttings and continue drilling with full returns ofdrill water cuttings to the surface.Non-Productive Time ActivitiesAerated drilling requires the utilisation of a number ofnon-return valves or ‘string floats’ to be placed in thedrill string. Prior to any directional survey these floatsmust be removed from the drill string – thisrequirement imposes additional tripping time ofapproximately half an hour each time a survey iscarried out.However, when comparing ‘non-productive’ betweenaerated drilling and ‘blind’ drilling with water, the timelost when washing the hole to ensure cuttings arecleared when ‘blind’ drilling is comparable if not morethan that lost retrieving float valves when aerateddrilling.Potential DangersDrilling with aerated fluids requires the drilling crew todeal with compressed air and with pressurised hightemperature returned fluids at times, neither of whichare a feature of ‘blind’ drilling with water. Thesefactors are potentially dangerous to the drilling crewand require additional training, awareness andalertness. The author is not aware of any notifiable‘Lost Time Injuries’ that have occurred as a directresult of using aerated drilling fluids since thetechnique was introduced in the early 1980’s.The air compression equipment has on numerousoccasions been utilised to pressurise the annulusaround a stuck drill-string, such that the water level inthe annulus is significantly depressed. If the pressure inthe annulus is then suddenly released the water in theannulus surges back up the hole, often washing cuttingsor caved material packed around the drill string up thehole and thus freeing the stuck drill string.While drilling within a geothermal reservoir systemunder aerated ‘balanced’ conditions, the potential forthe well to ‘kick’ is significantly higher than if beingdrilled with large volumes of cold water being ‘lost’ tothe formation’. Well ‘kicks’ are a relatively commonoccurrence when drilling with aerated fluids, howeverthe use of a throttle valve in the blooie line causes anincrease in back-pressure when an increase in flowoccurs, which tends to automatically control andsubdue a ‘kick’. The author is not aware of anyuncontrolled blow-outs of geothermal wells that haveresults from the use of aerated fluids.Well RecoveryWells drilled ‘blind with water’ usually experience asignificant recovery heating period after completion ofthe well. The large volumes of water lost to thereservoir can take a long period to heat up. Aeration ofthe drilling fluid limits the loss of fluids to theformation and the cooling of the reservoir around thewell. The temperature recovery of wells drilled withaerated fluids is significantly faster. Typically a welldrilled with water ‘blind’ can take from 2 weeks to 3months for full thermal recovery. Wells drilled withaerated fluids tend to recover in periods of 2 days to 2weeks.Drill Bit LifeAerated drilling prevents the loss of drilling fluid to theformation and thus reduces the cooling of theformation and near well bore formation fluids. The drillbits and bottom hole assemblies used are thereforeexposed to higher temperature fluids especially whentripping in, reducing bearing and seal life, and thus thebit life.This reduced life is however, usually a time dependantfactor, which, when drilling some formations iscompensated by significantly increased rates ofpenetration. For example – the current aerated drillingDISADVANTAGESWhilst the aerated drilling technique provides manybenefits, it also introduces some negative aspects.CostThe rental of aerated drilling equipment, the additionalfuel consumed plus two operators imposes anadditional operational daily cost against the well.5

Hole. H MPetroleum Engineering Summer SchoolDubrovnik, Croatia.Workshop #26 June 9 – 13, 08June 2008operations in Iceland have seen average penetrationrates of up to two time (2x) that previously achieved.0THE PROCESSAs stated above, to maintain drilling fluid circulationwhile drilling permeable formations, the hydraulic(hydrostatic and hydrodynamic) pressure in the holemust be ‘balanced’ with the formation pressure. Tobalance the pressure in the hole with the formationpressure, the density of the fluid in the hole must bereduced. Figure 2. depicts some typical geothermalformation pressure regimes with respect to a coldhydrostatic column of water from the surface. A staticwater level of 400 metres has been assumed.1000Drilling mud S.G. 1.1DEPTH(m)Cold water S.G. 1.0FoamS.G. 0.2Aerated waterS.G. 1.02000Water at boilingtemperature fordepth30000102030PRESSURE (MPa g)Figure 3. Typical Downhole Pressures0Effective water levelat 400mTo ‘balance’ the downhole circulating fluid pressurewith under-pressured formation conditions the densityof the circulating fluid is reduced with the addition ofair. The ratio of liquid to air, and the throttling of thecirculating fluid outlet to produce a backpressure in theannulus are the variables which can be altered toprovide the required pressure balance.Shallow hot liquidCold Hydrostatic Column1000DEPTHPressure Differential(m)VapourDominatedZoneHowever, the addition of air into the drilling circulationsystem introduces a compressible component. Thevolume occupied by a unit mass of air at a particulardepth in the hole is dependant on the fluid pressure atthat depth. In other words the volume of a bubble of airat the bottom of the hole will be a small fraction of thevolume occupied by the same bubble of air at the top ofthe hole. The density of the fluid column varies withdepth and for simplicity purposes is described as a‘liquid volume fraction’ (LVF).2000Liquid phaseover full depthLiquid phasebelow steamcap30000201030PRESSURE (MPa g)Figure. 2: Typical Formation PressuresA liquid volume fraction (LVF) of 1.0 100% liquidFigure 3. depicts typical pressures within a well with arange of drilling fluids with respect to a column ofboiling water. The effective drilling fluid density canbe varied in the approximate specific gravity range of1.1 for un-aerated mud to 0.1 for air, by varying theratio of air to liquid.FluidEffective Specific GravityWater based bentonite MudWaterOil Based mudsAerated bentonite mudAerated waterMistFoamAir1.11.00.820.4 – 1.10.3 – 1.00.05 – 0.40.05 - 0.250.03 – 0.05A liquid volume fraction (LVF) of 0.0 - 100% airSo not only is the pressure regime within the holealtered, but circulating fluid volume, (the LVF) andtherefore the fluid velocity varies with depth of thehole.Table 2. indicates an output from the GENZL AeratedDrilling Computer Simulation Package, of a typicalaerated downhole annular pressure profile withdownhole pressure, differential pressure (the differencebetween the downhole pressure and the formationpressure with a nominal static water level at 300 mdepth), the flow velocity, and the Liquid volumefraction (LVF) indicated as a function of depth.The simulation is of a well with production casing setat 700 m depth, and a 100 m bottom hole drillingassembly (drill collars) – hence the parameter changesat these depths.6

Petroleum Engineering Summer SchoolDubrovnik, Croatia.Workshop #26 June 9 – 13, 458.758.7June 1-0.4- 0400.0Depth (m)Meas.Depth(m)Blooie 00.0900.01000.0Bottom HoleHole. H 0600.0700.0800.0Annular Velociy (m/min.)Figure 6. Annular Velocity V’s DepthTable 2. Simulation of Aerated Downhole ConditionsLiquid Volume Fraction0.0Plots of the various parameters are indicated in Figures4, 5, 6, and 7.200.0Depth (m)400.0600.0Annular Pressure800.00.01000.0Annular Pressure200.0Formation 00.800.901.00Depth (m)Liquid Volume Fraction600.0Figure 7. Liquid Volume Fraction V’s depth800.0Perhaps the most critical point displayed by this data isthat the fluid velocities around the drill bit and bottomhole assembly are very similar to the velocities thatwould occur without the addition of air. The volume ofliquid to be pumped must be sufficient to provide lift tocuttings over the top of the bottom hole assembly,where the diameter of the drill string reduces from thedrill collar diameter to the heavy weight drill pipe ordrill pipe. Typically for water drilling, a minimumvelocity of 45 to 55 metres per minute is required. Thevolume of air to be added to this

Drilling fluids are required to remove cuttings from the well during drilling, to cool and lubricate the drill bit and drill string, to apply pressure to formation fluids to control flow into or out of the well, and to cool the formation, particularly prior to cementing casings. Various drilling fluids are selected according to