Functions Of Drilling Fluid.pdf

Functions of Drilling Fluids

Functions of a Drilling Fluid z

Hole Cleaning

z

Pressure Control

z

Suspend Solids

z

Minimize Formation Damage

z

Isolate Fluids from Formation

z

Cooling and Lubrication

Functions of a Drilling Fluid cont’ d z

Power Downhole Tools

z

Environment

z

Maximum Hole Information

z

Corrosion

z

Support Part of DS

z

Cost

Choice of Drilling Fluid z Cost

z Availability

z Environment

z Storage

z Safety z Suitability „ Inhibition „ Rheology „ Fluid Loss „ Temperature

Drilling Fluid Systems z Types of Systems: „ Spud Muds : used to start the well „ Drilling Fluids : used to drill the well • drill to top of the reservoir • drill through the reservoir „ Completion Fluids : used to complete the well

Top Hole Drilling Fluids z Requirements: Fast drilling - large hole size, 16” / 17 1/2” Young formation - reactive or unconsolidated Low annular velocities - need high pump rates (1000 gpm) SCE may have difficulty in handling the flow, so use dump & dilute method with WBM’s - cheap system. „ More problems with OBM’s

„ „ „ „

– – –

reduce pump rates to avoid losses. fit coarser screens. need good shakers and centrifuges

Water Base Mud Systems z

Cheap basic systems: „ Gel/Cmc Muds „ Lime Muds „ Gyp Muds „ Non-Dispersed Polymer muds

z

Expensive Inhibited systems: „KCl/Polymer Muds „High Temp Muds „Special Mud Sytems >Quadrill >Visplex >Silicates >Formates

Advantages of OBM z Good penetration rates and prolonged bit life. (fewer trips, less bit balling) z Excellent inhibition of reactive shales and clays. z Thermally stable. z Low damage factors to oil reservoir. z High lubricity, low torque. z Good corrosion protection, particularly for H2S. z Gauge hole. z Good fluid loss control . z Less chance of stuck pipe. z High solids tolerance. z Salt sections not dissolved. z Good coring fluid.

Disadvantages of OBM z High initial well Cost. z Some electric logs cannot be run. z Can damage gas reservoirs due to water or emulsion blocks. z Lost circulation is expensive. z Greater chance of a poor cement bond. z Detection of gas kicks difficult due to solubility of gas in oil. z Logistic problems (supply boats, storage tanks). z Messy working environment., carcinogenic fumes

potential

fire

hazard

can

produce

z Difficult to detect crude oil. z Hole cleaning may be a problem. At high temperatures the viscosity of OBM is low.. z Fire Hazard. z Environmental pollution - fumes

Oil Base Mud Systems z Invert Oil Emulsion muds: INTERDRILL NT, INTERDRILL LOR „ Realtively cheap, re-usable, better enviromental footprint

z 100% Oil Muds: TRUDRILL „ best inhibition, good for depleted gas reservoirs, poor environmental impact

z Synthetic Oil Base muds: ULTIDRILL „ Best environmental footprint, can be used exactly as INTERDRILL system.

Introduction to Drilling Fluids z

Hole Cleaning

z

Pressure Control

z

Suspend Solids

z

Minimize Formation Damage

z

Isolate Fluid From Formation

z

Cooling and Lubrication

Remove Cuttings From the Well Bore z The most important parameter is the Annular Velocity (A.V.) „ Where possible the annular velocity should be 100 ft/min, higher in deviated holes. „ In large hole sections the A.V. can be as low as 20 ft/min.

z If the A.V. is insufficient to clean the hole the viscosity must be increased

„ For top hole high viscosities must be used

z Cuttings removal is harder in deviated and horizontal holes as the vertical component of the mud is reduced.

A.V.(ft/min) = Pump rate (bbls/min) Annular vol (bbls/ft)

Slip Velocity (ft/min) =Cuttings velocity - A.V.

Introduction to Drilling Fluids z

Hole Cleaning

z

Pressure Control

z

Suspend Solids

z

Minimize Formation Damage

z

Isolate Fluid from Formation

z

Cooling and Lubrication

Balancing Sub-Surface Pressures z The pore pressure depends on: „ The density of the overlying rock „ The pressure of the interstitial fluid „ Whether the rock is self supporting or is supported by the fluid. „ Tectonic activity „ Surface terrain z If the fluid hydrostatic pressure does not balance the pore pressure the following may occur: „ Influxes of formation fluid into the wellbore „ Lost circulation „ Hole Instability „ Stuck pipe

Balancing Sub-Surface Pressures z

The pressure balancing the formation pressure is composed from the hydrostatic pressure under static conditions:

Hydrostatic Pressure (psi) = Height (ft) x Density(ppg) x 0.052

„ P = Depth (ft) x Density (ppg) x 0.052 z

Under circulating conditions the effective pressure is increased by the pumping pressure. This forms the Equivalent Circulating density (ECD): „ ECD = Density (ppg) + Ann Press Loss Depth x 0.052

Pore Pressure

(Not normally known)

Introduction to Drilling Fluids z

Hole Cleaning

z

Pressure Control

z

Suspend Solids

z

Minimize Formation Damage

z

Isolate Fluid from Formation

z

Cooling and Lubrication

Suspension of Solids z Whenever the pumps are switched off solids will start to settle. This can result in: „ „ „ „

Bridging off of the wellbore Stuck pipe Hole fill Loss of Hydrostatic

z A gel structure is required to suspend the cuttings under zero shear conditions: „ The gel structure is caused by time dependant attractive forces which develop in the fluid. „ The longer the fluid is static the stronger these forces become „ The gel structure should be easily broken „ The gel properties are especially important for deviated and horizontal wells as the distance solids have to settle is very small

Introduction to Drilling Fluids z

Hole Cleaning

z

Pressure Control

z

Suspension of Solids

z

Minimize Formation Damage

z

Isolate Fluid from Formation

z

Cooling and Lubrication

Minimize Formation Damage z Damage to the formation while drilling to the reservoir: „ Formation swelling (Normally clay and Salt formations) „ Washouts (Clay and Salt formations or any unconsolidated formation) This can result in: – – – – – –

Difficult directional control Poor zonal isolation Excess mud and cement costs Poor Hole Cleaning Stuck Pipe Difficult fishing jobs

The Need For Inhibition

Minimize Formation Damage z Damage to the reservoir will result in loss of production or the need for remedial treatment. This can result from: „ „ „ „

Solids blocking reservoir pores Emulsion droplets blocking reservoir pores Swelling clays Ions from the formation and drilling fluid forming insoluble salts

Damage by Drilling Muds Mud damage can occur by: z

Physical reduction of pore / pore throat size „ „ „ „ „

z

Mud solids invasion Formation fines migration Clay swelling Adsorption / precipitation of mud polymers Reaction and precipitation (scale)

Relative permeability reduction „ Wettability change „ Emulsion formation „ Fluid saturation change/fluid blocking

Formation Fines Migration Conglomeration of loose material around pore throat

Kaolinite stack almost completely disaggregated

Depth of Invasion z

Mud composition & reservoir characteristics influence the degree of damage

z

Depth of damage is influenced by „ Mud formulation „ Time in open hole „ Mud overbalance

Depth of damage is often less than the total depth of invasion due to depletion of damaging species

Introduction to Drilling Fluids z

Hole Cleaning

z

Pressure Control

z

Suspension of Solids

z

Minimize Formation Damage

z

Isolate Fluid from Formation

z

Cooling and Lubrication

Isolate the Fluid From the Formation z The differential pressure forces fluid into the wellbore, resulting in whole mud or filtrate entering the formation. Either, or both, of these is undesirable because:

„ The loss of whole mud into the wellbore is expensive

and damaging „ The loss of filtrate into the wellbore may cause

formation damage

Isolate the Fluid From the Formation z The flow of fluid is affected by the formation of a filter cake z The filter cake reduces the flow of fluid into the formation. Special additives are added to improve the cake quality: – Bridging material – Plate like material – Plugging material z The filter cake should be thin with a low permeability „ This avoids reducing the effective hole diameter „ It also reduces the chance of differential sticking „ Minimises filtrate penetration into formation

Introduction to Drilling Fluids z

Hole Cleaning

z

Pressure Control

z

Suspension of Solids

z

Minimize Formation Damage

z

Isolate Fluid from Formation

z

Cooling and Lubrication

Cooling and Lubrication z The drilling fluid removes heat from the bit which is then dispersed at the surface „ Fluid formulations are not changed to improve this function „ Very occasionally the temperature of the fluid exceeds the flash point. In this case it is necessary to improve surface cooling z Extra lubrication may be required between the drill string and the casing or wellbore, especially in directional wells „ Liquid additives are used, or Oil based mud „ Solid additives are sometimes used such as glass beads, plastic beads, graphite or nut plug „ Drill pipe rubbers are sometimes added to reduce wear between the casing and drill pipe

Other Considerations z

Power Downhole Tools

z

Environment

z

Maximum Hole Information

z

Corrosion

z

Support Part of the DS

z

Cost

Other Functions z Power Downhole motors „ Turbines to turn the bit or power MWD / LWD equipment

z Transfer information from measurement equipment to the surface „ This is done with a pressure pulse

Other Considerations z

Power Downhole Tools

z

Environment

z

Maximum Hole Information

z

Corrosion

z

Support Part of the DS

z

Cost

Environmental Impact - Offshore Man Discharge Discharge

Mussel (Mytilus sp.)

Bioaccumulation Bioaccumulation

Zooplankton (Acartia sp.)

Algae (Skeletonema sp.)

Shrimp (Mysidopsis sp.) Biodegradation Biodegradation

Taint Taint

Fish (Scophthalamus sp.) Sediment Reworker (Corophium sp.)

Environmental Impact - Land Discharge - Chlorides - Heavy metals - pH - TDS - BOD, COD - Clarity

Treat

Recycle - Solids content

Liquid Waste

Drilled Cuttings

Disposal - Toxicity - Chlorides - Heavy metals - Oil content - Solids content

Other Considerations z

Power Downhole Tools

z

Environment

z

Maximum Hole Information

z

Corrosion

z

Support Part of the DS

z

Cost

Secure Maximum Hole Information z The operator will always require the following information: z Rock type being drilled „ The cuttings should not dissolve or disintegrate z Analyses of gases „ The gases should separate easily from the mud z The fluid should have a defined resistivity „ Formation resistivity measurements need to be made

Other Considerations z

Power Downhole Tools

z

Environment

z

Maximum Hole Information

z

Corrosion

z

Support Part of the DS

z

Cost

Control Corrosion z The fluid should be non corrosive to the: „ Drill string „ Casing „ Surface equipment

Corrosion leads to loss of

z Corrosion can lead to: „ Wash outs „ Twist offs „ Pump failure „ Surface Leaks

&

Other Considerations z

Power Downhole Tools

z

Environment

z

Maximum Hole Information

z

Corrosion

z

Support Part of the DS

z

Cost

Support Part of the Tubular Weight z Aids in supporting part of the weight of the drill string and casing z The degree of buoyancy is directly proportional to the density of the fluid.

„ The fluid density is never changed to increase the buoyancy

Other Considerations z

Power Downhole Tools

z

Environment

z

Maximum Hole Information

z

Corrosion

z

Support Part of the DS

z

Cost

Maximize Penetration Rates z The fluid properties greatly influence penetration rates by: „ Removing cuttings from below the bit and wellbore „ Reducing the cushioning effect of solids between the bit teeth and the formation „ Reducing the hydrostatic differential „ Increasing the jet velocity

Bits

Rig days, Bits, Ft/Bit.

Rig days Ft/Bit 5

10

Solids Content (%volume)

Well Cost Well Cost

=

($)

DFS Cost ($)

Daily Cost

+

(days x $/day)

=

Fluids Engineering (days x $/day)

Footage Cost

+

(ft x $/ft)

+

Drilling Fluid (ft x $/ft)

Once off and Other Costs ($)

+

Completion Fluid ($)

– DFS direct cost is relatively small (5 to 10% of well cost) – Greatest savings achieved by improving Drilling Efficiency

Key Drilling Fluid Issues

Minimise loss of fluid to the formation Control formation pressure

Lubricate the drill string

Maintain borehole stability Suspend barite under static and dynamic conditions Remove drilling cuttings from the hole Provide hydraulic horse power to the bit

Building blocks z

There are two basic building units from which all the different clay minerals are constructed :

z

The Octahedral Layer

z

This consists of two sheets of closely packed oxygens or hydroxides in which aluminium (Gibbsite), iron or magnesium (brucite) ions are embedded.

Building blocks z

The tetrahedral Layer

(A)

(B)

z

In each tetrahedral unit, a silicon atom is located in the centre of the tetrahedron, equidistant from the four oxygen atoms.

z

Note that when viewd from above this gives a hexagonal opening.

Building blocks z

Tetrahedral and octrahedral sheets may combine in different combinations to form over 26 different clay minerals

z

The type of clay mineral will depend on „ the ratios of the silica to octahedral layer . „ The nature of ions inbeded in these structures. „ Other ions associated with the structures.

Shale Shaker z The Shale Shaker performance determines the total efficiency of the complete solids control package.

Poor performance here cannot be rectified later !

Overhead Manifold Design

Screen Blinding • Blinding can be due to solids jamming the openings. • The usual remedy is to remove the screen and jet wash it from the rear of the screen. • Fitting finer screens may allow the solids to pass over the openings, if not then the only answer is to fit coarser screens

Screen Plugging

Degasser z Gas cut mud is : „ Damaging to rig equipment, „ A potential well control problem, „ Lethal if Sour or Flammable. z Common with high viscosity / heavy muds. z Centrifugal pumps, hydrocyclones and the rig pumps loose efficiency if the mud is gas cut. z The degasser should be installed between the sand trap and the first hydrocyclones.

Degasser Operation of a degasser

Mud Input

Hydrocyclones A general view of a Desander and Desilter

Mud Cleaner z The purpose of the mud-cleaner is to screen the underflow of the hydrocyclones in order to:

„ „ „

Reclaim base liquid. Reclaim discarded barytes. Produce relatively dry cuttings.

Mudcleaner Note that a Mudcleaner is basically a combination of a hydrocyclone and a shale shaker. Hydrocyclone use has diminished rapidly with the introduction of better shakers and centrifuges.

Centrifuges z Centrifuges are usually the last piece of solids control equipment. They process about 10% of the flow. z The capacity for separation is approx. 95% for 20 micron and 50% of 10 micron particles. z Due to the high G forces, ultra-fine particles (as small as 3 microns) can be removed form the mud.

Centrifuges Solids

Solids discharge with absorbed liquid only

Colloidal liquid discharge

Feed inlet Colloidal liquid discharge

Gearbox

Pool level controlled by weir settings

Feed ports

Beach

Applications z The centrifuge is used for the separation of solids and for reclaiming fluid. It is extremely useful to remove ultra-fine solids especially from more expensive fluids such as KCl-polymer mud or OBM. z The removal of ultrafine drilled solids (LGS) will enhance viscosity and weight control without excessive dilutions being required. z In weighted muds the centrifuges can be useful to lower the mud weight by removing the weighting material (HGS) for the following reasons: „ During a pressure differential stuck pipe situation. „ During lost circulation „ When the next hole section is scheduled to be drilled with lighter mud. z Another application is the recovery of oil from oil-contaminated cuttings.