part ii functions of a drilling fluid

Advanced Mud SchoolPart II The Ten Functions of a

Drilling Fluid

Presented By: Jeff Imrie

August 2006

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Functions of a Drilling Fluid

Drilling fluid is a very important part of the drilling operation.

Drilling fluid has many functions and is very complex

The understanding of the uses of drilling fluid can make a drilling operation successful

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Functions of a Drilling FluidTen functions of a drilling fluid:1. Transport cuttings2. Release cuttings at the surface3. Control bottom hole pressure4. Cool and lubricate the bit and drillstring5. Provide borehole stability6. Provide buoyancy for the drillstring7. Suspend cuttings when circulation is interrupted8. Facilitate the retrieval of information from the wellbore9. Protect formation productivity10. Transmit hydraulic energy to the tools and bit

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Transport CuttingsTransport cuttings The well is drilled and

cuttings are produced the must be removed from the well

The drilling fluid is circulated down through the pipe and bit nozzles entraining the cuttings and carrying them up the annulus to surface

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Transport Cuttings Cuttings removal is a function of cuttings size, shape

and density; rotation of the drillstring; and mud properties such as viscosity, density and annular velocity

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Transport Cuttings Viscosity describes the

rheological properties of the drilling fluid Cuttings settle faster

in low viscosity fluids Higher viscosity fluids

improve cuttings transport

Most drilling fluids are thixotropic meaning that they gel under static conditions

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Transport Cuttings Velocity refers to the annular velocity

of the fluid Generally the higher the annular

velocity the better cuttings removal If velocity is too high then turbulent

flow may occur resulting in less efficient cuttings removal and possible wellbore erosion

The net velocity is the difference in the slip velocity of the cuttings and the annular velocity

Transport velocity=Annular velocity - slip velocity

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Transport Cuttings Cuttings transport in high angle wells is more

challenging than vertical ones Cuttings tend to accumulate at the low side of the hole

creating cuttings beds The use of thixotropic fluids with high Low-Shear-Rate

Viscosity run in Laminar flow can help clean out these cuttings beds

High flow rate and thin fluid to try and achieve a turbulent flow can keep these wells clean

Generally a mixture of high LSRV fluids and thin turbulent fluids are required to keep the hole clean

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Transport Cuttings

High density fluid sweeps aid in hole cleaning The higher density fluid tends to get into the

smaller cuttings beds and push them into the higher section of the hole to be cleaned off

Pipe rotation This helps stir up the cuttings and lets the

fluids take them away

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Release Cuttings at the Surface High concentrations of drill

solids are detrimental to almost every aspect of the drilling operation, primarily drilling efficiency and ROP

Drill cuttings increase the mud weight and viscosity, which in turn increases maintenance costs and the need for dilution.

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Release Cuttings at the Surface

Drill cuttings also increase the horsepower required to circulate, the thickness of the filter cake, the torque and drag, and the likelihood of differential sticking.

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Drilling fluid properties that suspend cuttings must be balanced with those properties that aid in cuttings removal by solids-control equipment.

Cuttings suspension requires high-viscosity, shear thinning thixotropic properties, while solids-removal equipment usually works more efficiently with fluids of lower viscosity

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Solids-control equipment is not as effective on non-shear-thinning drilling fluids, which have high solids content and a high plastic viscosity.

For effective solids control, drill solids must be removed from the drilling fluid on the first circulation from the well.

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If cuttings are re-circulated, they break down into smaller particles that are more difficult to remove.

One easy way to determine whether drill solids are being removed is to compare the sand content of the mud at the flow line and at the suction pit.

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Controlling Formation Pressures Controlling the pressure of the

formations drilled is a very important function of the drilling fluid

As the formation pressure increases, the density of the drilling fluid is increased to balance or slightly overbalance the well and keep it in control

Pressure exerted by the drilling fluid while static is called hydrostatic pressure

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Controlling Formation Pressures Drilling fluids are typically weighted up with heavy

material such as barite, calcium carbonate, hematite and in extreme situations galena

The well is considered under control when no formation fluids or gasses are allowed into the well bore In some situations small amounts of background gas

are allowed into the well bore the well bore may be considered in control if the flow is controllable

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Controlling Formation Pressures

Hydrostatic pressure is also used to control unstable wellbores Formations may be tectonically stressed

especially in deviated wells Drilling fluid density can be increased to

balance the tectonic stress and help provide a stable wellbore

Density of drilling fluid ranges from air (0 psi/ft) to 20 lb/gal (2400 kg/m3)

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Cool and Lubricate the Bit and Drillstring Considerable friction and heat by rotational

and hydraulic forces of the bit and drillstring Circulation of the fluid cools the drillstring

and bit distributing it throughout the wellbore.

The drilling fluid also helps to cool down the bottom hole temperature.

Drilling fluid also lubrictes the BHA further reducing frictional heat. When required lubricaing additives are put into the fluid to further mitigate the problem

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Cool and Lubricate the Bit and Drillstring

Without the cooling and lubricating action of the drilling fluid many sensitve motors and components could not function or would fail under the heat.

Indications of poor lubrication are increased torque and drag, abnormal wear and heat checking of the drillstring compenents

Altering the lubricity of the drilling fluid is far from an exact science.

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Cool and Lubricate the Bit and Drillstring

Many different drilling fluids exist from oil-based to silicate water based.

Different methods and additives are available to reduce torque and drag from actual lubricating oils to graphite material to lubricating polymeric beads.

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Provide Borehole Stability Borehole instability is most often identified by a

sloughing formation, which causes tight hole conditions, bridges and fill on trips.

This means the well must be reamed and cleaned and in extreme cases re-drilled

Borehole stability is greatest when the hole maintains its original size and cylindrical shape.

Once the hole is eroded or enlarged in any way, it becomes weaker and more difficult to stabilize

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Provide Borehole Stability Hole enlargement leads many problems

low annular velocity poor hole cleaning increased solids loading fill increased treating costs poor formation evaluation

higher cementing costs and inadequate cement bonding

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Provide Borehole Stability

Hole enlargement through sand and sandstone formations mechanical actions: erosion most often being caused by hydraulic

forces and excessive bit nozzle velocities Need to reduce impact force and nozzle

velocity Weaker sands require a slight overbalence and

good quality filter cake containing bentonite

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Hole enlargement through shale Water based muds can penetrate shale making

it swell and soften over time and slough in Higher mud weights and chemical/polymer

inhibitors can reduce or eliminate slough Highly fractured shales are very unstable

Usually require mechanical methods to clean, they require higher mud weights to control or oil based muds

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Extremely water sensitive shales require an oil based or synthetic based fluid to drill successfully These fluids provide better shale inhibition

than water based fluids Clays and shales do not hydrate or swell in the

presence of oil Osmotic forces created by the emulsified brine

phase prevent adsorption of water by the shales

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Provide Buoyancy for the Drillstring Basic Buoyancy

When you place a block of wood in a pail of water, the block displaces some of the water, and the water level goes up.

If you could weigh the water that the wood displaces, you would find that its weight equals the weight of the wood.

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This doesn't mean that if you had a few blocks of wood that were exactly the same size and shape, they would each displace the same amount of water.

A block of wood made of oak, for example, sits deeper in the water (and therefore displaces more of the water) than does a block of pine.

The reason is that it's heavier for its size, or denser -- in this case, the molecules that make it up are more closely packed together than the molecules that make up the pine.

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If you could somehow keep increasing the density of the block, it would sink lower and lower into the water.

When its density increased enough to displace an amount of water whose weight was equal to the weight of the block, it would, in a sense, become weightless in the water.

Making the block just slightly denser would cause it to sink to the bottom.

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Provide Buoyancy for the Drillstring

The drilling fluid helps to support a portion of the drillstring or casing string weight through buoyancy.

If a drillstring, liner or casing string is suspended in drilling fluid, it is buoyed by a force equal to the weight of the mud displaced, thereby reducing hook load on the derrick.

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Buoyancy is directly related to the mud density, so an 18-lb/gal fluid will provide twice the buoyancy of a 9-lb/gal fluid.

The weight that the derrick can support is limited by its mechanical capacity, a consideration that becomes increasingly important with increased depth as the weight of the drillstring and casing becomes tremendous

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While most rigs have sufficient capacity to handle the drillstring weight without buoyancy, it is an important consideration when evaluating the neutral point (where the drillstring is in neither tension nor compression).

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When running long, heavy strings of casing, buoyancy can be used to provide a significant benefit.

Using buoyancy, it is possible to run casing strings whose weight exceeds a rigs hook load capacity

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If the casing is not completely filled with mud as it is lowered into the hole, the void volume inside the casing increases buoyancy, allowing a significant reduction in hook load to be used. This process is referred to as floating in the

casing

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Suspend Cuttings When Circulation is Interrupted

Drilling muds must suspend drill cuttings, weight materials and additives under a wide range of conditions, yet allow the cuttings to be removed by the solids-control equipment.

Drill cuttings that settle during static conditions can cause bridges and fill, which in turn can cause stuck pipe or lost circulation.

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Weight material which settles is referred to as sag and causes a wide variation in the density of the well fluid.

Sag occurs most often under dynamic conditions in high-angle wells, where the fluid is being circulated at low annular velocities.

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High viscosity shear thinning fluids (thixotropic) and required to suspend cuttings during connections and other interruptions in circulation.

Thixotropic fluids have properties that thin when stress is applied (such as circulation of the fluid) and thicken up or gel when static

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Most drilling fluids are thixotropic, polymers added to the fluid increase the low end rheology of the fluid and help to suspend cuttings and barite.

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Facilitate the Retrieval of Information from the Wellbore

Accurate information retrieval is essential to the success of the drilling operation, particularly during exploration drilling

The chemical and physical properties of the mud affect evaluation During drilling mud loggers retrieve samples

for evaluation After drill electric logs must be run in the hole

to further evaluate the wells economics

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Logging may be performed while drilling using LWD (Logging While Drilling) tools

Drill stem test may need to be completed Core may have to be cut

All these techniques and tools may be affected by the mud properties both chemical and physical If the cuttings are dispersed mud loggers will

have difficulty evaluating cuttings

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Additives (such as lubricants and asphalts) may mask oil shows

Certain electric logs require conductivity through the drilling fluid

Washouts will affect DST packer seats

Poor hole cleaning will make coring difficult

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Protect Formation Productivity Protecting the formations

productivity is a big concern. After all the well was drilled to produce hydrocarbons, not as a science project.

Formation damage can happen as a result of solids plugging up the porosity or permeability or through chemical or mechanical interactions with the formation

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Protect Formation Productivity

The type of the completion will determine the level of protect required For example an open hole completion will

require much more protection than a cement and perforation completion

Consideration should be given to the type of fluid chosen to protect the formation

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Protect Formation Productivity Common mechanisms for formation damage are:

Mud or drill solids invading the formation matrix, plugging pores.

Swelling of formation clays within the reservoir, reducing permeability.

Precipitation of solids as a result of mud filtrate and formation fluids being incompatible.

Precipitation of solids from the mud filtrate with other fluids, such as brines or acids, during completion or stimulation procedures.

Mud filtrate and formation fluids forming an emulsion, restricting permeability.

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Protect Formation Productivity

Offset well information can help to predict formation damage

Return permeability tests run with different fluids on cores will help to determine the best non-damaging fluids

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Transmit Hydraulic Energy to the Tools and Bit

Hydraulic energy if becoming very important in modern day drilling Proper hydraulics program can increase ROP,

help minimize hole enlargement, help to clean the hole

Special tools like MWD, LWD and mud motors require an available pressure to function properly

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Transmit Hydraulic Energy to the Tools and Bit

Hydraulic forces are limited to the available pump horsepower.

All the pressure losses (pipe, bit, annular, tools etc) should be calculated beforehand to ensure adequate pressure is available for tools and hole cleaning.

Density, plastic viscosity, BHA design all affect hydraulics

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Functions of a Drilling Fluid

END

Advanced Mud SchoolPart II The Ten Functions of a Drilling FluidFunctions of a Drilling FluidFunctions of a Drilling FluidTransport CuttingsTransport CuttingsTransport CuttingsTransport CuttingsTransport CuttingsTransport CuttingsRelease Cuttings at the SurfaceRelease Cuttings at the SurfaceRelease Cuttings at the SurfaceRelease Cuttings at the SurfaceRelease Cuttings at the SurfaceControlling Formation PressuresControlling Formation PressuresControlling Formation PressuresCool and Lubricate the Bit and DrillstringCool and Lubricate the Bit and DrillstringCool and Lubricate the Bit and DrillstringProvide Borehole StabilityProvide Borehole StabilityProvide Borehole StabilityProvide Borehole StabilityProvide Borehole StabilityProvide Buoyancy for the DrillstringProvide Buoyancy for the DrillstringProvide Buoyancy for the DrillstringProvide Buoyancy for the DrillstringProvide Buoyancy for the DrillstringProvide Buoyancy for the DrillstringProvide Buoyancy for the DrillstringProvide Buoyancy for the DrillstringSuspend Cuttings When Circulation is InterruptedSuspend Cuttings When Circulation is InterruptedSuspend Cuttings When Circulation is InterruptedSuspend Cuttings When Circulation is InterruptedFacilitate the Retrieval of Information from the WellboreFacilitate the Retrieval of Information from the WellboreFacilitate the Retrieval of Information from the WellboreProtect Formation ProductivityProtect Formation ProductivityProtect Formation ProductivityProtect Formation ProductivityTransmit Hydraulic Energy to the Tools and BitTransmit Hydraulic Energy to the Tools and BitFunctions of a Drilling Fluid

part ii functions of a drilling fluid

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