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© 2006 PetroSkills LLC, All Rights Reserved

DRILLING FLUIDDRILLING FLUID

The drilling fluid is a large The drilling fluid is a large and important part of the and important part of the

drilling operationdrilling operation

2 © 2006 PetroSkills LLC, All Rights Reserved

Drilling FluidDrilling Fluid

Historical perspectiveHistorical perspective1901 – The Spindletop well introduced

rotary drilling methods and drilling fluid1920’s – Barite was first used for

densityOther additives and systems followed:

Bentonite Polymers Oil-based systems



Air, natural gas or nitrogen-based systems

1970’s – Safety and environmental issues in addition to performance and cost

1990’s – Synthetic based muds now called non-aqueous drilling fluids or NADF



Major Major factors in factors in drilling drilling fluid fluid selectionselection

EconomicsEconomics

PerformancePerformance

Health, Safety, EnvironmentHealth, Safety, Environment

Risk AssessmentRisk Assessment



Function of drilling fluid or mudFunction of drilling fluid or mudRemove cuttings from the holeClean bottom of the hole (below bit)Prevent flow of formation fluidsControl torque (reduce friction)Retard corrosionDeposit a thin filter cakeTransmit MWD information



Economically deliver a wellbore suitable for formation evaluation and completion

Prevent change in hole sizeAid in cement placementPrevent lost circulationMinimize formation damageAssist in formation evaluation



Drilling fluids cannot do Drilling fluids cannot do everythingeverything

Must decide on prioritiesMust decide on prioritiesNumber one function is to clean holeIf you do not clean the hole, you

cannot drill



Symptoms of poor hole cleaning Excessive torque Drag on connections Tight hole and bridges on trips Fill on bottom

Adjust annular velocity and mud viscosity to increase lifting capacity

Will cover lifting capacity after drilling fluids



Number two is to prevent blowouts

Blowouts can be catastrophic with loss of life, equipment and environmental damage

The most common additive for density is barite


Drilling Fluid

Weight Specific Average Average ChemicalMaterial Gravity Density, ppg lbs/bbl Formula

Barite 4.2 35.0 1470 BaSO4

Hematite 5.2 43.3 1819 FeO.Fe2O2

Ilmenite 4.8 40.0 1680 TiO2.FeO

Hematite and Ilmenite are not used very often in drilling fluids

Only for very high density drilling fluids



Bottomhole cleaning is primarily performed by the hydraulics and not the mud Mud additives generally reduce bottomhole

cleaning Bit balling is less of a problem with

dispersed mud systems



In most instances, hole lubrication is not a requirement It does become critical in extended reach

wells, some horizontal wells and some directional wells

A good bentonite content and low drill solids content will yield lower friction coefficients

Oil muds will usually yield the lowest friction coefficients but not always



Adding oil to the mud can reduce the friction coefficient

Specialty fluids containing fatty acids, complex alcohols and mineral oils are used to reduce torque and drag

All these things will influence other mud properties and must be considered



Protect formation productivity and assist in formation evaluation is a noble aim for the mud system All drilling mud is damaging to some extent,

if not, it is called lost circulation The question is whether the damage can

be reversed or limited to near the wellbore



Formations that will be cased, cemented, perforated and fracture stimulated may not present a problem with formation damage

Formations that will be completed with screens, pre-packed screens or gravel pack may have more problems with near wellbore damage



Retard corrosion Corrosion is reduced by maintaining a pH

above 8.5 to 9.0 A common additive to increase the pH is

caustic (NaOH) Potassium hydroxide can also be used

(KOH) Corrosion inhibitors or oxygen scavengers

can be used to improve corrosion resistance



H2S and CO2 are acid gases When mixed with water based mud, they

lower the pH and promote corrosion H2S is deadly in small quantities and should

be treated immediately H2S can cause hydrogen embrittlement in

high strength carbon steel CO2 will flocculate the clays and cause

severe viscosity problems



Assist in cementation Very few drilling muds assist in

cementation Typically mud hinders the displacement of

the cement and a lot of the additives will retard the cement

Thin filter cakes help reduce the potential for differential pressure sticking and can help reduce torque and drag


Types of Drilling FluidsTypes of Drilling Fluids

AirAir

WaterWater

Water based Water based mudmud

Oil based mudOil based mud

Synthetic based Synthetic based mudmud

Types of Drilling Fluids

Air Water Oil Synthetics

Fresh Saline Diesel Mineral

Sea Water Saturated Salt KCl



AirAirDrills faster than any other drilling

fluidCan only be used in harder rock with

lower production ratesNo control of formation pressure



WaterWaterWater drills better than drilling fluids

with solidsLow density, no solids, low viscosityNot all formations can be drilled with

water



Water based mudsWater based mudsClay for viscosity Barite for densityChemicals and polymers to get

desired propertiesDrill solids as a contaminant



Water based muds can be broken Water based muds can be broken down into fresh water or brine down into fresh water or brine systemssystems

Fresh water muds are by far the Fresh water muds are by far the most common drilling fluids in the most common drilling fluids in the worldworld

They can be anything from water They can be anything from water and drill solids to complex mudsand drill solids to complex muds



Brine systems or salt water Brine systems or salt water muds are specialty muds used to muds are specialty muds used to obtain a specific objectiveobtain a specific objective

The book presents some of the The book presents some of the advantages and disadvantages advantages and disadvantages of salt mudsof salt muds



ADVANTAGESADVANTAGES DISADVANTAGESDISADVANTAGESMay be cheapest alternative in off-shore locations

May create disposal problem in some areas

Can result in better hole stability than fresh water counterpart

Usually requires more chemical treatment than fresh water mud

Salt saturated systems offer potential of highest possible densities

Often more expensive than equivalent fresh systems

Resistant to contamination by gypsum bearing zones

Poor filter cake properties

High solids tolerance (sometimes) Corrosive

Reduces or prevents hole enlargements in salt zones

Difficult to maintain pH

May offer superior penetration rate due to low solids content at modest density

May interfere with open-hole logs

May be more dense than needed



Most salt muds are sodium Most salt muds are sodium chloride, potassium chloride or chloride, potassium chloride or calcium chloridecalcium chlorideBentonite will not yield or Bentonite will not yield or hydrate very well in salt waterhydrate very well in salt waterIf bentonite is used, it is If bentonite is used, it is premixed in fresh water to a high premixed in fresh water to a high viscosity and then bled into the viscosity and then bled into the salt water systemsalt water system



Oil based muds can be formulated to Oil based muds can be formulated to weigh as little as 7.5 ppg (900 kg/mweigh as little as 7.5 ppg (900 kg/m33) ) with mostly oilwith mostly oil

Oil based muds are used to drillOil based muds are used to drillHighly reactive shale and evaporite

formationsExtended reach drilling for low friction

coefficientDeep, high pressure, high temperature H2S

wells



Oil base muds are much more Oil base muds are much more expensive per barrelexpensive per barrelEconomics are a function of Economics are a function of preventing hole problems, preventing hole problems, overall, it must save time on the overall, it must save time on the rig to be cost effectiverig to be cost effectiveOil base muds are highly toxic Oil base muds are highly toxic and disposal costs must be and disposal costs must be considered in the economicsconsidered in the economics



Initially, crude oil was used but Initially, crude oil was used but was later replaced by dieselwas later replaced by dieselLow-toxicity mineral oils can be Low-toxicity mineral oils can be usedusedOil/water ratios can range from Oil/water ratios can range from 100:0 to 40:60100:0 to 40:60Low water ratios (60:40 to 40:60) Low water ratios (60:40 to 40:60) can have very high viscosities can have very high viscosities but they help minimize oil but they help minimize oil retained on cuttingsretained on cuttings



Oil based muds are termed invert Oil based muds are termed invert emulsion because the water emulsion because the water phase is emulsified in the oil phase is emulsified in the oil which means that oil is the which means that oil is the continuous phasecontinuous phaseThe emulsified water droplets The emulsified water droplets contribute to viscosity which contribute to viscosity which helps suspend weight material helps suspend weight material and reduce fluid lossand reduce fluid loss



High salinity levels in the water High salinity levels in the water phase improve wellbore stability phase improve wellbore stability by creating osmotic pressures by creating osmotic pressures that dehydrate and harden that dehydrate and harden reactive shalesreactive shalesUsually try to balance the activity Usually try to balance the activity so that the water does not move so that the water does not move from the mud to the formation or from the mud to the formation or from the formation to the mudfrom the formation to the mud



Components of an oil based mudComponents of an oil based mudOilBrine (usually calcium chloride)Primary and supplementary

emulsifiers (fatty acids are one)Oil-wetting agentOil dispersible bentoniteFiltration control additivesSlaked lime



Toxicity is the most serious Toxicity is the most serious drawback to oil based mudsdrawback to oil based muds

The characteristics that result in The characteristics that result in superior performance of oil superior performance of oil based muds can also based muds can also contaminate the environmentcontaminate the environment



A wide variety of refined mineral A wide variety of refined mineral oils have been developed for use oils have been developed for use in low-toxicity oil based mudsin low-toxicity oil based muds

However, mineral oils are still However, mineral oils are still considered toxicconsidered toxic



Synthetic based muds (SBM’s) Synthetic based muds (SBM’s) are an attempt to provide the are an attempt to provide the performance of oil based muds performance of oil based muds without the toxicity and without the toxicity and environmental problemsenvironmental problems

Unit cost per barrel is higher Unit cost per barrel is higher than oil based mudsthan oil based muds



Synthetics are highly refine oil or Synthetics are highly refine oil or man made, hydrocarbon liquidsman made, hydrocarbon liquids

The continuous phase of an SBM The continuous phase of an SBM can be as much as 99% synthetic can be as much as 99% synthetic materialmaterial

First generation synthetics were First generation synthetics were esters, ethers, polyalphaolefins esters, ethers, polyalphaolefins (PAO’s), and acetals(PAO’s), and acetals



Second generation SBM’s are Second generation SBM’s are isomerised olefins (IO’s), linear isomerised olefins (IO’s), linear alphaolefins (LAO’s), linear alphaolefins (LAO’s), linear alkylbenzenes (LAB’s) and alkylbenzenes (LAB’s) and paraffinparaffin

Second generation SBM’s are Second generation SBM’s are characterized by lower costs and characterized by lower costs and lower kinematic viscosities but lower kinematic viscosities but slightly higher toxicity levelsslightly higher toxicity levels


Solid PhaseSolid Phase

The solid phase of a drilling fluid The solid phase of a drilling fluid consists of clays, drill solids, consists of clays, drill solids, barite and LCMbarite and LCM

There are many different types of There are many different types of clays in the world but only three clays in the world but only three major groups are used in drilling major groups are used in drilling fluidsfluids



ClaysClaysBentonite (gel)Attapulgite (salt

gel)Sepiolite (high

temperature)



Bentonite particles are usually stacked plates like the pages of a book

The space between the plates for non-hydrated bentonite is less than 10 angstroms

When fully hydrated, the space between plates can be in excess of 40 angstroms



Non-hydrated Hydrated



The chemical binding of water between plates and solid to solid friction increase the viscosity of water

Bentonite has good filter cake properties and can reduce fluid loss to 10 to 18 cc’s



AttapulgiteAttapulgiteDoes not yield or

absorb water like bentonite

Viscosity comes from interference between particles

Has poor filter cake qualities



Sepiolite is similar to attipulgiteSepiolite is similar to attipulgiteIt is used mostly in high temperature

wells such as geothermal wells


Drilling Fluid PropertiesDrilling Fluid Properties

Common drilling fluid propertiesCommon drilling fluid propertiesViscosity DensityGel strengthWater lossChemical content (chlorides, calcium,

pH, etc.)



Rheology measurementsRheology measurementsViscosity

Funnel viscosity PV & YP n & k

Gel Strengths



Viscosity is the proportionality Viscosity is the proportionality constant between the shear constant between the shear stress and shear ratestress and shear rate

Simply stated, it is the slope of Simply stated, it is the slope of the line on a shear stress – shear the line on a shear stress – shear rate diagramrate diagram

dv

dr



Viscosity is a laminar flow Viscosity is a laminar flow concept and has little effect on concept and has little effect on turbulent flowturbulent flowNewtonian fluidNon-Newtonian fluid



Newtonian fluidsNewtonian fluidsViscosity is

constant with shear rate (pump rate or velocity)

Water, gasoline, glycerin



Non-Newtonian Non-Newtonian fluidsfluidsViscosity changes

with shear rate (pump rate)

Drilling mud



Mathematical Mathematical models used to models used to describe muddescribe mudBingham Plastic

PV and YP The YP is the

shear stress required to initiate flow

Power-law n and k

YPPV



Power-law modelCurved line on

Cartesian paperStraight line on

log-log paperUsed when

making any meaningful calculations with drilling mud



Viscosity Viscosity measurementmeasurementMarsh funnelThe funnel

viscosity of water is 26 sec per quart or 28 sec per 1000 cc’s



Funnel viscosity has nothing to do with shear stress versus shear rate

It cannot be used for any type of calculation

The roughneck uses funnel viscosity to determine which additives need to be mixed in the mud



ViscometerViscometerShear stress is

measured at a shear rate

PV = 600 rpm –300 rpm

YP = 300 rpm - PV



The shear rate is related to the viscometer speed by the following equation

Two speed viscometers run at 600 and 300 rpm to measure PV and YP

Multi-speed viscometers can provide viscosity data at additional shear rates

dv

drW1 7.



Plastic viscosity and yield point from the viscometer

300600 PV PVYp 300



n and k from a viscometer

Where θ2 is the viscometer reading at twice the viscometer speed as θ1

θi is the viscometer reading at the shear rate

1

2log32.3

n n

i

i

drdv

k

idr

dv



To get the n and k from a viscometer, we usually use the 600 and 300 rpm reading

For n

or

1

2log32.3

n

300

600log32.3

n



For k you need the shear rate for the viscometer reading

We usually use the 300 rpm reading

nk

511300

ii

Wdr

dv 703.1

1-

300

sec 511300703.1 dr

dv



PV is an indication of the total and size of the solids in the mud

Increasing solids percent and smaller solids gives higher PV

The lower the PV the better



YPYP is an indication of the is an indication of the viscosity of the mudviscosity of the mud

High High YPYP’s compared to ’s compared to PVPV indicates high viscosityindicates high viscosity

The The YPYP is a measure of the is a measure of the interaction of the solids in the interaction of the solids in the mudmud



The viscosity at a specific shear The viscosity at a specific shear rate (pump rate) can be rate (pump rate) can be calculated using the power law calculated using the power law modelmodel

English UnitsEnglish Units

SI UnitsSI Unitsv

DDk

n

n

DD

v ph

n

phe

)(200

3

124.2

v

DDk

n

n

DD

v ph

n

phe

)(5

3

12200



Drilling fluids are shear thinning, Drilling fluids are shear thinning, which means the faster you which means the faster you pump it the thinner it getspump it the thinner it gets



For annular flow, the annular For annular flow, the annular velocity can be calculated with velocity can be calculated with the following equationthe following equation

English UnitsEnglish Units

SI UnitsSI Units22

610273.1

ph DD

Qv

22

5.24

ph DD

Qv



Some fluids Some fluids match the power match the power law model very law model very wellwell

Figure 4-13 – Figure 4-13 – Sea water, gel Sea water, gel and polymer and polymer mud mud



Some fluids do not Some fluids do not match the power match the power law modellaw model

It may be It may be necessary to necessary to measure shear measure shear rates with a multi-rates with a multi-speed viscometerspeed viscometer

Figure 4-18 – Figure 4-18 – Flocculated Flocculated bentonitebentonite



EQUIVALENT THICKNESS, EQUIVALENT THICKNESS, μμee RHEOLOGY CONSTANTSRHEOLOGY CONSTANTS

FIGURENUMBER

DENSITY(ppg)

FUNNELVIS

(sec/qt) BPPL

(600,300)PL

(100,50) DSS (rpm) PV Yp

n(600,300) kk

n(100,50) kk

4-12 8.8 44 494 295 304 94 95 5 34 0.17 13.0 0.13 16.0

4-13 10.0 50 149 90 5 66 108 16 25 0.48 2.1 0.48 2.1

4-14 13.6 53 403 125 175 126 31 15 18 0.54 1.1 0.37 28.0

4-15 10.6 160 1288 174 174 108 43 21 114 0.74 1.3 0.74 1.3

4-16 15.0 62 82 46 102 70 81 29 7 0.85 0.2 0.43 2.45

4-17 9.0 43 226 66 66 62 25 8 10 0.54 0.6 0.54 0.6

4-18 9.0 48 1578 584 893 203 18 3 15 0.22 4.6 0.09 9.4



Temperature will also influence Temperature will also influence the viscosity downholethe viscosity downhole

The viscosity at the surface may The viscosity at the surface may not be the same as the viscosity not be the same as the viscosity in the annulus at elevated in the annulus at elevated temperaturestemperatures



The viscosity of The viscosity of bentonite and bentonite and water can water can change with change with temperaturetemperature



Increasing viscosityIncreasing viscosityAdd bentoniteDrill solids will add viscosity but drill

solid concentration should be minimized

Adding barite will thicken the mud due to an increase in solids content

In heavy muds, the bentonite content should be limited



Bentonite can be flocculated in order to increase viscosity by adding lime (Ca(OH)2)

Often called spud mudsPolymers can also be used to

increase the viscosity of drilling fluids



Polymers are added to water Polymers are added to water based muds to change viscosity based muds to change viscosity and other propertiesand other properties

Some polymers are:Some polymers are:SPAPolyacrylamides (PHPA)X-C Polymer



Sodium polyacrylate (SPA)Sodium polyacrylate (SPA)High molecular weight, long chain

acts as a bentonite extenderMedium molecular weights are used

to control fluid lossLow molecular weights are used as

deflocculants



Polyacrylamides (PHPA)Polyacrylamides (PHPA)Very long chain polymerAdds viscosityHelps to inhibit shaleStabilizes shale cuttings



XC polymersXC polymersVery long chain polymerAdds viscosity especially at low shear

ratesDegrades above 250°F (120°C)



Viscosity reductionViscosity reductionAdd water in unweighted mudMechanical solids controlChemical thinners reduce viscosity by

limiting the interaction of solids in the mud

Many chemical thinners are dispersants because they disperse the clays



Phosphates are the oldest thinners but are limited to about 175o F (80o C)

Lignosulfonates Byproduct of the lumber industry Decompose above 200o F (90o C) When complexed with heavy metals

they are stable to 300o F (150o C) (toxic) Insoluble at pH less than 9.5 Does not disperse clays but well keep

them separated if introduced after the lignosulfonate is in the mud



Lignite Sub-bituminous coal Reduces gel strengths Good to 250o F (120o C) When complexed with heavy metals it is

good to 350o F (175o C) (toxic) Not very soluble



Tannins Made from the bark of the Quebracho

tree Used in high pH lime muds Good to 200o F (95o C) Complexed with chrome for stability to

400o F (200o C) – toxic

Today – high temperature thinners are special polymers and are more expensive



ContaminantsContaminantsCement contamination

Show high pH and high viscosity Best to throw it away Pretreat with bicarbonate (NaHCO3)

Carbonate contamination The drilling fluid will flocculate Lime will precipitate the carbonates



CO2 contamination Lowers the pH Mud will be very thick Raise the pH with caustic Add lime Raise the mud weight to prevent influx of

carbon dioxide Lime muds are resistant to CO2

contamination by maintaining excess lime in the mud



Muds that exhibit erratic Muds that exhibit erratic behavior and are expensive to behavior and are expensive to maintain should be discardedmaintain should be discarded

It will be cheaper to build new It will be cheaper to build new mud rather than chemically treat mud rather than chemically treat an old mudan old mud



Gel strengthGel strengthMeasured at 10 seconds and 10

minutes with the viscometerMud is stirred at high speed for 10

seconds and then stoppedThe gel strength is the maximum dial

reading when starting the rheometer after stopped for 10 seconds and repeated for 10 minutes



Gel strengthGel strengthIndication of how well the drilling fluid

will suspend cuttings and bariteMay cause problems with lost

circulation if too high



DensityDensityThe density of the drilling fluid is

measured with a mud balanceDensity can be increased by adding

solids or saltsCommon solids

Gel Drill solids Barite



Fluid loss or Fluid loss or water losswater loss

Measured in a Measured in a filter press with filter press with a pressure a pressure differential of differential of 100 psi100 psi



Fluid loss is measured in cc’s Fluid loss is measured in cc’s per 30 minutesper 30 minutes

Can also measure high Can also measure high temperature, high pressure fluid temperature, high pressure fluid loss (HTHP), which is 500 psi loss (HTHP), which is 500 psi (3,447 KPa) pressure differential (3,447 KPa) pressure differential at a selected temperatureat a selected temperature



Why reduce the fluid lossWhy reduce the fluid lossTo minimize formation damageThin the filter cakeReduce differential pressure stickingReduce shale sloughing?



The fluid loss is measured through a The fluid loss is measured through a piece of filter paper with a piece of filter paper with a permeability of 30 to 40 mdpermeability of 30 to 40 mdFluid loss can be reduce by having a Fluid loss can be reduce by having a dispersed bentonitedispersed bentonitePolymers thicken the fluid phase and Polymers thicken the fluid phase and reduce fluid lossreduce fluid lossSome polymers attach themselves to Some polymers attach themselves to the clay particles to help reduce fluid the clay particles to help reduce fluid losslossSome polymers, such as starch, can Some polymers, such as starch, can actually form a filter cakeactually form a filter cake



There is a There is a difference difference between static between static and dynamic and dynamic filtrationfiltration

Device to Device to measure measure dynamic fluid dynamic fluid loss through loss through corescores



Depending upon Depending upon the additive, the the additive, the dynamic fluid dynamic fluid loss can actually loss can actually increase when increase when static fluid loss static fluid loss decreases below decreases below a certain pointa certain point



The concept The concept of reducing of reducing fluid loss to fluid loss to reduce shale reduce shale sloughing is sloughing is controversialcontroversial



The permeability of shale is very low and generally in the range of 0.001 md or less

The permeability of the filter cake is higher than the shale

Shales do absorb water but it is not due to filtration



Fluid up-take may be due to any number of effects such as imbibition, capillary pressure, osmosis, etc.


Specialized FluidsSpecialized Fluids

The search for special purpose The search for special purpose mud systems began almost mud systems began almost simultaneously with the simultaneously with the introduction of rotary drilling to introduction of rotary drilling to the petroleum industrythe petroleum industry

No one mud system will work No one mud system will work everywhereeverywhere



Calcium Treated Muds - Lime Calcium Treated Muds - Lime Treated MudsTreated MudsLime muds were introduced in the

1930’sThey are characterized by high pH

(pH=12+) water base fluids containing dispersant concentrations of 6 to 20 pounds/barrel and excess lime Ca(OH)2 concentrations of 2 to 8 pounds/barrel



Theoretically, the Ca++ ion helps to inhibit the mud

Gyp muds or muds treated with gypsum (CaSO4) where introduced because it has higher concentrations of the calcium ion

Calcium Chloride muds were introduced in the 1950’s



Regardless of all the attention given the calcium treated muds at the time of their introduction, such systems are not frequently used today

Lime muds are used where CO2 contamination is a problem



Potassium MudsPotassium MudsThe beneficial effects of K+ in the

prevention or reduction of the hydration of montmorillonite has been known to the industry for many years

In some areas, potassium muds work fairly well to inhibit shales

Shales still hydrate, the presence of potassium slows it down



Additionally, the activity of the KCl mud must be balanced with the formation as in oil muds

Phosphate MudsPhosphate MudsMud systems containing large

concentrations of the common additives TSP (Tri-sodium phosphate), SMP (Sodium Metaphosphate), SAPP (Sodium acid pyrophosphate) and STP (Sodium tetraphosphate) have been used with some success



The phosphate muds are usually used at low temperature when calcium contamination is a problem

Magnesium MudsMagnesium MudsSystems comprised of bentonite treated

with magnesium oxide (MgO) have enjoyed some following.

"MAG-OX" or "MAGOO" muds offered inexpensive thickness and the inhibited qualities of Mg++ simultaneously



Because of the high fluid loss, they are not used very often

The "perfect" mud system does The "perfect" mud system does not exist but the search not exist but the search continuescontinues

No system can reasonably be No system can reasonably be expected to be more than expected to be more than adequateadequate



Remember, the mud must be Remember, the mud must be cost effectivecost effective

If more is spent on the mud, the If more is spent on the mud, the total cost of the well must be total cost of the well must be reducedreduced

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