2-Criteria for Successful Cementing

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TEACHING GUIDEDate: May 95Module No: CF01Page: 1Dowell

CRITERIA FOR

SUCCESSFUL CEMENTING

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• As a dowell engineer what do we need to consider to provide a sucessful cement job?

• Discuss each one in brief

– Dowell Concept - A method of continous improvement from gaining experiecne from a particular field and across the company.

– Job Objective - What are we trying to achieve with the cement job

– Mud removal - Given the well conditions / type of mud what is the best method

– Slurry Properties - What is the requirements for the job - 18 5/8” or linerect.

– Special Cement Systems - is there a requirement given the well conditions / possible contingency for losses ect

– CemCADE Design - Ensure we have all the correct imputs to produce a competent design.

– Job Execution - Carried out as per design. Pumped at the designed rates ect.

– Job Evaluation - Follow up and ensure any key learning is taken forward.

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CRITERIA FOR SUCCESSFUL CEMENTING

Job objectiveMud removal (ahead of the slurry)Temperature predictionSlurry propertiesSpecial slurriesPlacement design (CemCADE)Job executionJob evaluation

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DEE

Design - Execute - Evaluate

Job Planning& Slurry Design

Logs Well/Job DataWell Post-Job

History

Blending Slurry Mixing& Placement

• This is know as Dowell DEE loop

• Consider all relevant aspects of the design well parameters / slurry designs / logistical and operational constraints ect.

• Program is then passed to the field and the job is executed

• Then the job is analysed and any key learning is taken forward to the next. Bond logs, volumes, losses, plug bumps ect

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• What are the objectives of a cement job? Take a liner as an exaqmple.

• Zonal isolation

– Ensure no annular communication between zones

– Isolation of a hydrocarbon bearing / overpressured zone / water zone

– Protect the casing from corrosive brine flows / moving salt formations

– Casing support - mechanical properties of the cement must provide to the casing

– Mechanical properties reuire to provide a hydraulic seal

– Provide isolation around the shoe to provide a competent leak off to drill ahead

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Job Objective

• Isolation of productive zones

• Protection of water zones

• Isolation of problem interval

• Protection of casing

• Casing support

Cement bonded to formations

Complete cement sheath w/no mud or gas channel

Cement bonded to formations

Cement bonded to casing

Oil or Gas pay Zone

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Practices Affecting Primary Cementing

Poor Centralization

Wash out: Incorrect flow regime

Channeling: Incompatible

preflush or incomplete mud

removal

• Out of guage hole - As we drill through different formations such as sands or shales different well conditions will apply

• Gelled mud may be difficult to removed and may become trapped

• Flow regieme and preflush my not be applicable to the washed out sections

• Mud may mix in with the menet and will comprimise a hydraulic seal

• Poor centralisation will make it difficult to remove mud from the narrow side and will channel

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Slurry Design Factors Affecting Primary Cementing

High Free water

Water Intake

Gas IntakeShrinkage or

microannullus

• Slurry Design Aspects

– Slurry needs to be designed for its application

– Stable Competent Slurry

– Fluid loss across permeable or hydrocarbon formations

– Low shinkage propertries

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• Mud removal is the key to any cemening operation

• Well preparation

– Ensure the hole is clean / stable (no losses)

– Centralise the casing

– Rotate / Recipricate the casing

– Circulate to ensure >95% of mud in the hole is moving

• Improve Mud removal

– Circulate and condition mud to ensure it is in good shape

– Design appropriate spacers / washes

– Design pumping schedules

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Mud Removal

Well preparation

Mud removal efficiency during the cementing

operation

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• How do we condition the mud?

– Define the specification required for cementing

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Mud Conditioning

Lower Density – by removing cuttings and sand

Reduce ViscosityReduce Gel Strength by:– Circulation– Addition of Dispersants– Pipe Movement

Stabilize Well = no losses / no kicks

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• Centralisation is another key issue in cementing

– To ensure competent cement bond we must centralise over the section

– Design to acieve a good stand off

– Centralisation will help towards flow all round the pipe

– Centraliser design must be taylored the the well profile

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Casing Centralizers

Centralizer

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Casing Centralization

% Stand-off = Wn x 100/ (R1-R2)

R2

R1

Wn

• Calculation of Standoff Example :- 9 5/8” Casing in 12 1/4” OH

R1 = 9 5/8” / 2 = 4.82”

R2 = 12 1/4” / 2 = 6.125”

Wn = 1”

% Standoff = (1 * 100) / (4.82 * 6.13) = 76%

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• If we have an equal pressure drop:

– 1. 100% Standoff - flow will be uniform

– 2. 60% Standoff - Flow will always take the line of least resistance

– 3. 20% Standoff - Very little mud being removed from the narrow side

• To achieve isolation across a zone it must be centralised

• All fluid design is targeted at the narrow side

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Effects Of Standoff on Mud Displacement

Decreasing Stand-off

Mud

Cement

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• Rotation - The preferred Option :-

– Rotation will help achieve flow all round the pipe

– Move the mud from the narrow side of the annulus

– Place cement into the narrow side

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Casing Movement: Rotation

Rotation Started

Mud Almost Removed

Resisting drag forces can become

positive mud displacing force

Casing StationaryFlowing Cement

Gelled Mud

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• Reciprocation -

– If you have poor centralisationand trapped mud between centralisers you will be able to centralisers up and down over the areas

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Casing Movement: Reciprocation

Stat

ic M

ud

Cem

ent S

lurr

y

Poor Standoff

Cem

ent S

lurr

yCem

ent S

lurr

y

Improved Standoff

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Scratchers and Other Mud Removal Aids

Reciprocating sctatcher

Rotating Scratcher

Well Wiper

Hydro-Bonder

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Cement - Mud Contamination

Acceleration or Retardation

Reduction of Compressive Strength

Reduction of Hydraulic Bond

Increase of filtrate loss

Change of Rheological Properties

• Cement Contamination

– Cement and Mud are not compatable

– These are the effets to the slurry if there is intermixing of the mud

• What can we use st separate these?

– Spacers / Chemical washes which are compatable with both the mud and cement

– Plugs / Wiper darts ect

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Wiper Plugs

Prevent– Contamination of Spacer

– Contamination of Cement slurry

Wipe Casing clean

Indicate end of Displacement

Displacing Mud

Top Plug

Bottom Plug

Cement

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No Bottom PlugDisplacing

Mud Top Plug

Cement

Accumulated Mud from Film

Mud Film

MudCement Slurry

Mud

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Fill up - No Bottom Plug

Fillup per 1000 ft ofcasing *

Mud FilmThickness bbl/ 1000 ft ft m

1 / 64 in1 / 32 in1/ 16 in1 / 8 in

0.230.450.891.83

15.430.259.7

122.8

4.79.2

18.237.4

* Note : 4 1/2”, K-55, 13.5 lb/ft

Volume of mud wiped from casing wall

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Chemical Washes

Low Viscosity Fluids

Usually Water Based

Contain Surfactants and mud thinners

• What are Chemical Washes?

– Water + Surfactantnts + sometimes mud thinners (U066)

– Typically 1 SG - Viscosity - 5Cp

– Can be Weighted up with Salt - NaCl, Kcl - Not CaCl2

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Chemical Washes

Separate Mud and Cement– No incompatibility effect

Remove mud from annulus– Turbulence at low pump rate

– Erode, dilute and disperse particles

Leave casing and formation water wet– Function of the Surfactant

Provide less hydrostatic pressure– Water or oil-based

What They Do / How They Work

• Separate Mud and Cement

– Mud and Cement are not compatible!

– If high oil / water ratio OBM use mud thinners

• Remove mud from the annulus

– The preferred method of mud removal as they act in a turbulent flow regeim

– Act as a mud dilution

• Leave the casing and formation water wet

– Oil and water do not mix on there own

– Change from oil in water to water in oil emulsion

• Reduce the hydrostatic effect on the formation

– If pumped behind the cement, improve the positive pressure of the cement

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Spacers

Definition

Densified viscous fluid separating mud and slurryFunction

Thorough removal of mudProperties

Compatible with mud and cements– Mixtures less viscous than thicker fluid– No gel

Specified rheology– Low for Turbulent Flow– Adjustable for Effective Laminar Flow

• Viscous Spacer

– Gell used to control weight and viscosity

– Used when pore pressure / hole conditions do not allow the use of surfactants.

– Heavier than the mud 10% density increase

• Compatable with mud and cements

– Use surfactants for OBM

• Rheology

– Low rheology for Turbulent spacers

– Viscous for laminar flow spacers

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Cement Slurry Properties

Conventional Cement system:– Cement Slurry density

– Cement Slurry Rheology

– Free water

– Thickening Time

– Compressive Strength

– Fluid Loss Control

– >> UCA >> Z (Acoustic Impedance)

• Cement Systems

– Each of the above properties will be covered in more detail

– 15.8 - 16ppg is standard weight for neat cement to yield to give the required qualities - stable slurries

– Below 15.8ppg extenders are required to increase the water content and produce a stable slurry

– Rheology - specified rheology depending on the way the cement will be mixed in the field and the down hole requirements

– Free Water - if slurry is not stable the cement will settle out and water will be left at the surface - could allow a channel or micro annulus

– TT - Must be long enough + safety margin for the job

– Compressive strength - enough to support the axial force of the casing +hydraulic seal in the annulus

– Fluid loss control - used if pumping across production zones or pearmeable formations, and for squeeze.

– Maintain solid / water ration - a stable slurry

– Avoid bridging off while pumping

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Cement Slurry Density

HEAVY WEIGHT SLURRIES

Settled particles do not contribute to hydrostatic pressure

SettledParticles

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Cement Slurry Rheology

Friction Pressure

Flow RegimeLaminar ( sliding motion - zero flow on walls )

Turburlent ( swirling motion )

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Effects of Free Water

Channelling

Incomplete Fill-up

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Thickening time

If BHCT estimated lower than actual:

– OOPS > cement set prematurely in csg

If BHCT estimated higher than actual

– Slurry takes days to set

– Low set cement strength

– Poor bond

– Fluid migration thru gelled slurry

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Temperature Prediction

Two Basic influences on downhole performance of cement

– Temperature

– Pressure

Temperature has the biggest influence and affects– Thickening time

– Transition time

– Compressive Strength

– Fluid loss

– Rheology

– Free water

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Compressive StrenghPoor protection against lateral forces

Stable System

Overburden Pressure

Unstable System

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Fluid Loss Control

Hydrostatic pressure

Annular Bridging

Filtrate

Weak Zone

FiltrateDehydrated

Cement

FracturedWeak Zone

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Why use Fluid Loss Control

Maintain constant water-to-solid ratio– Constant Density

– Desired Yield

– Thickening Time

– Compressive strength

– Rheology

– Constant Properties

Avoid annular bridging or excessive pump pressureReduce formation damage

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CemCADE Job Design

Better zonal isolation– Optimum mud Romoval

Flow RegimePump rateMud removal effeciency vs stand-off

– Slurry design

Well security and control– No loss circulation– No fluid influx– No casing collapse– Anticipated surface pressure

Job evaluation– PRISM– CBL Adviser

• CemCADE takes into account all the factors discussed so far to predict the best placement design.

• Closses the DEE Loop.

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Job Execution

IS EXECUTION

AS PER DESIGN ?

Real Time Job Monitoring

PRISM

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On-site Data AcquisitionPRISM

– Pressure, Density, Flow rate sensors– Portable acquisition computer– Standard remote display

Quality of treatment execution through:– Record of treatment in real time– Accurate, reliable data acquisition regardless of

environmental conditions– Immediate post-job treatment reports

Treatment efficiency through:– Post-job analysis of data which

(1) ensures pressures and rates were as designed and

(2) helps to improve future designs

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• Job Procedures

• The first thing to do on arrival on location, is to meet the company man and check the calculations with him and to verify that no changes have been made to the programme.

• Rig up all the equipment and lines.

• STEM 1 on all the equipment including priming and testing the units and lines.

• Verify that all the products and materials arrived correctly.

• Perform the prejob safety and organization meeting with everyone that will be involved in the job - the company man, the Dowell personnel and the rig crew.

• Start preparing the different mix fluids, spacers and washes. The compnay man may want you to wait until the casing is on bottom and the rig has started circulating - this is to avoid lost products if the casing cannot get to bottom. If this is the case, make sure everything is ready to go including correct water volumes in the tanks.

• This is a good moment to start taking samples of the different fluids.

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PRISM Plot

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• The job itself starts when the company comes onto the unit:

– The lines are tested again but in the presence of the compnay man.

– The plugs are loaded into the cement head, in the correct order.

– The preflushes are then pumped also in the correct order.

– The bottom plug is dropped by the operator.

– The cement slurries can then be mixed and pumped at the correct density.

– The TOP plug should now be dropped, by the operator.

– Displacement is then done at the rate specified by the programme. The volume that can be pumped above the calculated displacement volume should be defined prior to the job and agreed to by the company man to avoid misunderstandings at this critical moment.

– The plug is bumped with the agreed pressure. Always check the returns to make sure that the float equipment is holding. If there is a constant return flow rate after about the first 4 or 5 barrels of returns, this can be considered as leaking float equipment. If this happens, then pump the fluid that has returned back into the well to push the plug back onto the float collar but without allowing the pressure to build up in the casing above the end of displacement pressure - then install a pressure sensor close to the cement head and close the line.

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Post Job Evaluation

Job Design (CemCADE) vs Job Execution (PRISM)

CBL Adviser

Cement Bond and Variable Density Logs (CBL-VDL)

Cement Evaluation Log

Pressure Test ( Shoe Bond Test )

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• The end of the job can be as important as the rest of the operation and can leave a good or bad impression on the client.

– The washup should be performed to the waste pit of the rig. No water should be thrown on the client’s location - even though he has no problem with this, his boss may come and visit the rig the next day and he may not be as happy with our leftovers. The location should be left in as good or better condition than we found it.

– Rig down our equipment completely - avoid leaving Dowell’s equipment on the rig, the rig crew may not take the same care of our equipment as we do.

– STEM 1 the equipment. This is important to prepare for the next job and to identify idf there are any potential problems that appeared during the job.

– All the paperwork should be completed and either signed by the client or left with him. Remember that the client has probably been awake during the whole running of the casing, so avoid trying to pressure him to sign your paperwork when he is tired.

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SummaryDefine Cementing ObjectivesDesign with CemCADEImprove Mud Displacement

– Condition mud prior to cementing– Use centralizers– Rotate and / or Reciprocate – Avoid adverse mud cement reations– Control displacement rate and sapcer and slurry rheology

Optimize cement slurry designExecute the job as per designPerform a Post-job Evaluation

2-Criteria for Successful Cementing

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