Marsh Funnel Viscosity

•Funnel viscosity is an indication of the overall viscosity of a drilling mud.
•It is affected by the concentration, type, size, and size distribution of the solids present, and the electrochemical nature of the drilling mud’s solid and liquid phase
•Consequently, funnel viscosity should only be used to provide an indication of change or consistency of viscosity from time to time. •Since Gel Strength can have a great effect on the magnitude of the funnel viscosity, the measurement should be taken quickly as possible Test Procedure
•With the funnel in an upright position, cover the orifice with a finger and rapidly pour a freshly collected mud sample through the screen, and into the funnel until the mud just touches the base of the screen (1500 ml).
•Immediately remove the finger from the orifice and measure the time required for the mud to fill the viscosity cup to the one (1) litrelevel for sec/l and (1) quart level for sec/q
•Report the result to the nearest second as the marsh funnel viscosity, at the temperature of measurement in degrees Celsius..

Mud Density (Pressurized mud balance)

Pressurized mud balance
•When a drilling mud contains entrapped air, or it is experiencing a foaming problem, the mud density may be accurately determined with a pressurized mud balance.
•This a mud balance is similar in operation to the instrument described above the difference being that the sample is pressurized to expel air or gas. Test procedure (pressure mud balance):
•Fill the sample cup with drilling mud to a level, which is approximately 10 mm below the upper edge of the cup. •Place the lid on the cup with the attached check valve in the down (open) position. Push the lid downward into the mouth of the cup until surface contact is made between the outer skirt of the lid and the upper edge of the cup allowing any excess mud to be expelled through the open check valve.
•Pull the check valve up into the closed position, rinse off the cup and threads, and the screw the threaded cap onto the cup.
•With the plunger in hand, push its handle into place in the inner piston to its lower most position.
Fill the plunger by immersing its nose in the mud to be tested and pulling out the handle until the inner piston is in its upper most position. (The plunger’s operation is similar to a syringe or bicycle pump).
•Place the nose of the plunger onto the mating o-ring surface of the valve on the cap. The sample cup is pressurized by maintaining a downward force on the cylinder in order to hold the check valve down (open), and at the same time forcing the piston inward. Approximately 220 Newton’s of force is required on the plunger handle in order to pressure the cup.
•The check valve in the lid is pressure actuated and will close (move up) under the influence of pressure within the sample cup.
Therefore the valve is closed by gradually easing up on the plunger cylinder while maintaining pressure on the piston. When the check valve closes, disconnect the plunger from the lid, rinse the cup in water and wipe it dry.
•Place the pressurized balance with the knife edge on the fulcrum of the balance stand. Adjust the sliding weight on the balance beam until the bubble oscillates equally to the left and right of the centering mark above the bubble vial. Note the value of the specific gravity at this point.
•The pressure in the mud balance is now released by reconnecting the empty plunger to the lid an pushing to the plunger cylinder while permitting the handle to move freely. To complete the procedure all components should be washed and rinsed thoroughly.

Mud testing

Mud testing•Necessary for a successful drilling operation•The tests are also used as a tool to aid in diagnosing mud related problems.•The mud properties are routinely checked at the well site, and recorded on a daily drilling mud report





Mud Density
•Drilling mud density is required to calculate the hydrostatic pressure that is being exerted by a column of drilling mud at any given depth. Density is also used to provide an indication of the solids content of a drilling mud.
•When the test is performed using a standard mud balance, care must be taken to ensure the cup is full and free of entrapped air.
Test procedure:
•Remove the lid from the cup and completely fill the cup with the mud to be tested. It may be necessary to tap or vibrate the cup lightly to bring the entrapped air to the surface for high viscosity muds.
•Replace the lid and seat it firmly on the cup in a rotating manner. Allow the excess drilling mud to be expelled through the centrally located hole in the lid.
•Wash the mud from the outside of the cup, and dry the mud balance.
.•Place the balance arm on the base with the knife edge resting on the fulcrum.•Adjust the rider until the bubble oscillates equally to the left and right of the centering mark above the level vial.•Read the mud density as shown by the indicator on the rider.•Report the result to the nearest scale division in kg/m3or lb/ga

l.

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 •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

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 •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
•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.
•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

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
–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
–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

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.

•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
•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
•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

.

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 •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 •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 overbalenceand good quality filter cake containing bentonite •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 •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

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 lubrictesthe BHA further reducing frictional heat.
When required lubricaingadditives are put into the fluid to further mitigate the problem •Without the cooling and lubricating action of the drilling fluid many sensitvemotors 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
•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

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 •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 •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)

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.

•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. •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
•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.
•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.

Transport Cuttings

Transport 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

•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
•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

•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

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

•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

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 Ten functions of a drilling fluid:
1.Transport cuttings
2.Release cuttings at the surface
3.Control bottom hole pressure
4.Cool and lubricate the bit and drillstring
5.Provide borehole stability
6.Provide buoyancy for the drillstring
7.Suspend cuttings when circulation is interrupted
8.Facilitate the retrieval of information from the wellbore
9.Protect formation productivity
10.Transmit hydraulic energy to the tools and bit

Completion and Workover Fluids

completion and workover fluids are specialized fluids used during well completion operations and remedial workover procedures.
•These fluids are designed to cause the least amount of damage to the production zone
–The potential for damage to the zone is greater during the completion of workover operation
•Completion fluids are placed across the chosen pay zone after the well has been drilled but prior to putting it on production.
•Workover fluids are used during remedial work in producing wells, usually as an attempt to enhance or prolong the economic life of the well
•Functions of a completions or workover fluid:
–Control subsurface pressures.
–Minimize formation damage.
–Maintain wellbore stability.
–Control fluid losses to the formation.
–Transport solids.
–Maintain stable fluid properties
.•Types of completions and workover fluids:
–Clear, solids-free brines.
•These are the most common
–Polymer-viscosifiedbrines with bridging/weighting agents.
–Other fluids: oil-base, water-base, converted muds, foam


..•Brines are usually selected based on the appropriate density required to control the well

.

Well Completion

•After the well is drilled, logged, tested and cased it is time to proceed to completing the well •Completion simply means making the well ready to produce oil and gas under controlled pressures and flow rates •The most common types of completions are open hole, gun-perforated, liner, and gravel-packed liner •Open hole completion –If the formation is competent and can hold together by iteself(such as a limestone or dolmiteformation) a simple open hole completion is adequate.

•Gun-perforated completion –In this case the casing or liner is ran and cemented right through the zone of interest –Perforations are shot through the casing or liner and cement in the formation •Slotted liner completion –a pre-perforated or slotted liner is hung from the bottom of the last string of casing. –If the producing formation is weak or poorly consolidated, sand and other solids will be carried into the well as the oil or gas is produced. –To prevent this “sand production,”the slotted or perforated liner may contain a wire-wrapped or a prepacked-gravel protective layer to keep the sand from entering the wellbore.

Gravel-packed liner –A gravel-packing operation consists of circulating and placing carefully sized gravel into the annular space between the liner and the wellbore wall –Used when there is a weak formation such as loose sand

Directional Drilling

•Sometimes wells must be drilled at high angles or even horizontally•Economic, environmental and technical reasons sometimes require that the well be dilled directionally•Deviated wells can sometimes access more of the reservoir than vertical ones •Large expensive offshore platform act as a central point for several deviated wells and can make these wells economical to drill •Specialized BHA’sare required to drill these wells –Include MWD (Measurements will drilling) tools –Mud motors –Stabilizers •Wells can be drilled in almost any direction now: S or U shaped, Horizontal