HPHT Cementing Guidelines

7/30/2019 HPHT Cementing Guidelines

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HPHT Cementing Guidelines

H O W T O U S E T H I S D O C U M E N T

These Guidelines are an aid to assuring best practice in planning and execution of cementingjobs on HPHT wells. The aim is to provide the Drilling Engineer with a tool to promptactions by others. This will usually be the Service Company Engineer, his laboratorytechnician (who has a substantial and key role), their back-up from other locations andspecialists within their company, and the all-important Company Representative and theservice engineers on the rig.

The Guidelines are not intended to prescribe how the job must be done, but they shouldenable the Drilling Engineer to ask for evidence that a proper, technically assured process is inplace and being followed. It is intended that these Guidelines are passed to the servicecompany and that they ensure the issues raised are addressed and actions documented. Thefact that some aspect or other is not addressed here should not be construed as meaning it is

not important. Experience is very important in minimising risk.

There are a number of key questions and these are indicated in italics. A listing of these KeyQuestions is also given in an attached Table.

Areas covered are:

The Planning Process - what you need to be aware of from the start Slurry Design - some guidance on what to ask and what to look for Spacer Design Integrated Job Design - mixing, pumping, mud properties, making sure it all

comes together

On the rig - execution, what to look out for. It is often simple things which causethe major problems


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1. IntroductionHPHT cementing requires considerable additional vigilance on the part of all involved in theplanning and execution because the consequences of a job failure are more severe thanexperienced in more normal wells. It has probably cost much more to get to where you areand, if it all goes wrong, it will cost a disproportionate amount to put it right.

Investing time and effort in the detailed planning is vital to minimise risk.

The following areas need careful attention:

Slurry design Spacer design Basic materials quality and suitability Equipment selection Liner Systems, hangers and running tools Cement Mixing Slurry Placement/mud displacement Contingency planning:

in case of problems during the job in case the job fails to secure the objectives, eg shoe leaks

Train-wrecks in HPHT cementing operations are usually caused by simple things beingoverlooked or a complete, total, lack of knowledge about some aspect - (doesnt know whathe doesnt know!).

You cannot spend too much time looking at seemingly trivial details and asking simplequestions about issues which seem minor. (As an example; it may seem impossible on such acritical job that cement will be moved from one silo to another without the rig Cementerbeing aware of the move, but it isnt, and it has happened, and it will happen again and theconsequence can be catastrophic).


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2. The Planning ProcessInvolve the Service Company early, 3 months prior to spud is reasonable:

2.1 find out what experience the Service Company has locally with HPHT wells?

What experience do the individual engineers have with HPHT?

2.2 if the HPHT experience of the service company engineers is limited, establish howexperience will be brought in. One way this might be done is to have an experiencedHPHT service company engineer from another region come and review both theplanning procedures and the cementing programme. Also, consider getting a secondlab to verify the pilot slurry design(s), e.g. a lab of the same service company in adifferent region which has routine HPHT work.

What support is going to be brought in?

2.3 the experience, commitment and competence of the lab staff will be crucial. Gettingvalid lab test data for HPHT is very time consuming and far in excess of that requiredfor less critical wells. If the lab is weak, your exposure to risk is increased substantially.

How can the lab demonstrate that it is more than adequate?

2.4 the cementing service company must interact with the mud company and with theequipment suppliers. Darts, Plugs, liner hangers, running tools, sealing surfaces,packers, elastomers, etc, all require extra attention. Make sure everything will work at

the conditions you have and with the fluids you will be using. Dont address theseaspects in isolation from each other.

What commitment is there to provide more than a slurry and a pump?

2.5 ensure roles and responsibilities are clear and are understood. There many differentways of running the operation, with more or with less responsibility devolved by theBP DE to the service companies and drilling contractor. Just make sure everyoneknows what they are responsible for.

What are the roles and responsibilities?

2.6 Is the rig good enough?

2.6.1 the rig compressed air supply for bulk cement handling - check that the air driersare working properly. Is there a history of bulk supply problems? Moistureintroduced into the bulk cement can seriously compromise the slurry design andlead to very confused lab results because of differences between rig samples andlab held samples which have not been through the same exposure. This isparticularly true for blends, ie G+35%.

How and when will these issues be addressed?


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2.6.2 is the cement unit up to the job? Do you need to up-grade the HP fluid ends?What is its record of failure on ordinary jobs?

What assurance can be given?

2.6.3 are critical slurries to be batch mixed? Can the volumes be accomodated? Do you

need more than one batch tank? Can it be fitted on the rig?

What are the limitations imposed by the rig, logistics and weather?

2.7 Temperature:

this is the parameter which has the most influence on the slurry design and it is vital thatthe maximum amount of information about expected temperatures is laid out andmade available to all involved at an early stage. Lab testing must be bracketedaround the most likely temperature. The starting point for cementing is theundisturbed geothermal temperature - not log temperature, not MWD

temperature. These temperatures obviously have a role to play in the estimationof undisturbed geothermal temperature but do not themselves form the basis forestimating cementing temperatures. Both software modelling and API CementingTemperatures (BHCT) need to be viewed with caution. A coherent picture of themost likely cementing temperature environment needs to be built up from asmany inputs as possible.

What are the most likely temperatures, how are (were) they obtained, whatconfidence can be placed on them, what can be done while drillingto enhance confidence and reduce uncertainty? Who owns theissue and understands it?

2.8 Hydraulics:

Deatiled, thorough planning is absolutely critcal for wells with a narrow pore-fracturewindow. Rheologies of fluids will require special attention including special labmeasurement at HPHT conditions. It is frequently the case that computersimulations of the job create considerable, indigestable quantities of results and donot clarify the available options but obscure them. To avoid this the cementingservice company engineer should summarise outputs onto a spreadsheet toindicate trends. An example would be to produce a single XL Chart plot ofmaximum downhole pressure against displacement flow rate for three differentmud rheologies. This is easily understood as a chart with three lines but will

normally be presented as 30 sheets of A4 print-out.Is there a clear understanding of the risks?

Surge pressures and pressures to break circulation must be addressed - there must be agood integrated approach agreed by the mud company and the cementingcompany.

How will the mud company and the cementing company work together tounderstand what can be done to optimise the cementing process?

2.9 Contingency Planning:


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Again, much more attention is required than for non HPHT wells. Lab testing to obtain asuitable KO plug (say) will take several days. Make sure that contingency slurriesare developed as the well progresses. Do not wait until unscheduled events arise.For example, setting 9 5/8 casing deeper can have a profound effect on theslurry design and may take a week of lab testing if pilot studies are not alreadyunderway.

Have plug designs ready for shoe squeezes and Kick-offs

Have contingencies been identified, is there a documented and agreedplan of what is needed, by whom and when?

2.10 Assurance Process:

Is everyone clear what the process is and how it relates to them?Are the interfaces satisfactory?


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3.3.3.3. Slurry DesignSlurry DesignSlurry DesignSlurry DesignThe key areas to be considered in designing an HPHT cement slurry are listed below and theneach aspect is addressed:

Bulk cement sampling Temperature Cement selection Additive selection:

Retarder Fluid Loss Aid Weighting Agent Silica Other additives

Slurry SensitivityThe main aim should be to keep it simple. Avoid secondary additives or additives tocounteract the action of others, eg anti-settling aids, retarder aids.

3.1. Bulk Cement Sampling:HT wells will require silica, most commonly dry blended silica flour. The lab will work withsamples from three sources:

samples of cement and silica which they weigh out and mix themselves samples taken from the bulk blending plant samples brought back from the rig

For various reasons, these can all give different results when tested under the same conditions.

To minimise risk it is vital that correct sampling procedures are rigorously followed. Again,this is a point which cannot be overstressed. Imagine a busy lab, under pressure to get results.It has only two rigs but many samples. The samples are all in the correct type of tin can. The


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lids are all marked with the rig name, the date, the silo number, etc. Under pressure to get thismixed and that mixed, this report done, that file found. The lids get switched. You think itdosent happen? It does.

Containers must be moisture proof and airtight, full and uncontaminated by anything.It sounds basic but many times compliance is less than satifactory. The consequence can bethousands of feet of 9 5/8 casing to drill out. Details should be on the container side - noton the lid.

No job should proceed without a test on rig samples which is consistent with tests onother lab samples.

3.2. Additive Sampling and QC:For HT, the level of control on additives should be increased. Early in the lab design stagespecific batches of additives should be earmarked for the project and set aside in sufficient

quantity that there is no need to mix batches (manufacturing runs) of additives. This minimisevariability due to additive QC. Rigorous tracking of Batch Numbers needs to be in place andspecific products at the rig site should all have identical Batch Numbers

3.3. TemperatureIn high temperature wells, the standard API temperature equation for determining cementcirculating temperature (BHCT - the lab testing temperature) from the bottom hole statictemperature (BHST) should be viewed with caution. The data set on which this equation isbased is very limited for HPHT wells and the slurry design is usually sensitive to small changes

in temperature. Also, the API equation is not appropriate when the thermal gradient exceeds1.9oF/100 ft (35oC /km). Therefore it is recommended that for HT wells (BHST>300 oF,(149 oC)) and/or the thermal gradient >1.9oF/100 ft (35oC /km) Welltemp, or anequivalent heat transfer model, is used to predict cementing test temperatures. In explorationwells where thermal gradients are less certain, computer modelling should also be used tointerpolate logged temperatures back to BHST. The more data and modelling, the better themost likely temperatures can be bracketed and the appropriate lab designs chosen.

3.4. Cement Selection:Not only do different cement manufacturers products behave differently at HT, cementfrom the same manufacturer, manufactured at different times, will exhibit different behaviour.This is due to the complex chemical nature of the product and inherent variability in the rawmaterials being used.

For normal well cementing, an Well Cement which meets API Specification 10 should beacceptable. However, the API Specification does not address suitability for HT conditions.When the temperature exceeds 300 oF (149 oC) BHST and/or slurry density exceeds 2.1 SG(17.5 ppg), the choice of bulk cement will have an impact on slurry performance and the risksassociated with slurry design. The reactivity of some cements can make them very difficult toretard at high temperature, and the use of so-called difficult to disperse cements will impactslurry rheology and amount of weighting agent required to create a pumpable slurry.

Wherever possible in HPHT cementing, a recognised high quality well cement with a track-


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record at such conditions should be used. This aspect needs to be addressed early in theplanning process. Changes at a later stage can be problematic. Cements with non-linearresponse to retarder concentration should be avoided.

3.5. Additive Selection:The following is specific to HPHT productive intervals.

3.5.1. RetardersIn general, normal lignosulphonate retarders are not appropriate (e.g. D13/D800/D801/ R-12L/R-10L/ HR-4L/HR-6L) for wells where the BHCT exceeds 230 oF (110 oC) and the useof highly refined HR-12/HR-12L/HR-15/R15L/R-15/R-23L/D28) or synthetic or chemicalretarders e.g. D109/D110/ R-14L/SR-30/ SCR100/HR-25 is preferred.

3.5.2. Fluid Loss ControlFluid Loss Control is necssary when the slurry is used in a narrow annular clearance (


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presents a further difficulty. The density difference and coarseness of the haematite requirescareful slurry design to prevent haematite differentially settling from the slurry.

Manganese tetroxide appears to be the most suitable weighting agent and has the advantagethat it can be added to the water before the cement, its fineness prevents rapid settlementsimplifying the mixing operation. There has been recent concern about the use of thematerial because of variability in the particle size and impact on slurry sensitivity (particularlyretarder response). There has also been concern about resistance to acids, particularly itsability to act as an oxidising agent. Currently it is not believed these issues should prevent useof the material. The issue of variable particle sizes should be managed by particle size analysison materials supplied.

A further system in use is Dowells product DenseCrete. This is a blend ofcement/haematite and other dry bulks to give a carefully designed range of particle sizes.These blends appear to be more stable during transport and have better rheologies then theequivalent cement haematite blends.

3.5.4. SilicaWhere the BHST exceeds 230 oF (110oC) cements exposed over periods (months - years) willexhibit the phenomenon of strength retrogression. This is caused by a solid state phasetransformation at high temperature due to the calcium - silica ratio producing a weaker, morepermeable, material. To prevent this, silica is added to the slurry design to produce a morethermally stable product. The permeability deterioration is particularly relevant to HPHT GasWells.

It is also important for producer wells to consider flowing temperatures during peakproduction. Strings far above and close to surface may require silica

Two forms of silica are normally available:

silica flour (D66/SSA-1/S-8)

silica sand (D30/SSA-2/S-8C)

The minimum concentration of silica added to cement should be 35% however studies haveshown advantages when using silica sand of raising this to 50%.

When using silica flour, the material must be preblended with the cement. This requirementadds a significant risk for the following reasons:

when the initial blending is done, the bulk is often not fully homogeneous andsampling errors can occur

with subsequent transfers, the blend becomes more homogeneous, but may alsoundergo some changes (aeration) due to moisture in the compressed air transfersystem

Therefore,samples taken from a blend at the yard may not match those taken at the rig. A 5%variation in the flour content of a blend can have a substantial impact on the retarderresponse and requirement.


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Where large slurry volumes are to be pumped, preblended silica is only practical option.Where slurry volume permits batch mixing, the use of sand in tote bins is preferred tominimise the risk of sampling and ensuring and accurate silica content in the final slurrypumped.

3.5.5. Other AdditivesOther additives used in HPHT slurry designs include:

Dispersants (for mixability and rheology) (D80/D65/CFR-3L/ CFR-3/CD-31/CD-31L)

Anti-foam (to reduce air entrainment during mixing) (D144/NF-5/FP-14L) Anti-settlement aids(to prevent solid and/or liquid separation (Silicalite-

97L/FDP-C-533/D153/LW8-L)

Synergistic additives (which may support other additives e.g. retarder aids)(D121/D94/Component R)

3.6. Slurry SensitivityHPHT slurries can be very complex, and in most cases additives are being used at the upperend of their design limits. As a consequence the slurry properties are much more likely to beinfluenced by changes in:

temperature mixing/shear additive concentrations density additive order of addition contamination

The impact of each of these on the slurry design should be assessed.

3.6.1. Temperature: Sensitivity of the slurry toOnce the lab design temperature has been determined, the thickening time should beobtained for 15oF (8.3oC) hotter than BHCT. Should the thickening time reduce below jobtime the thickening time of the proposed slurry should be increased.


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The compressive strength should also be determined for a test temperature 15oF (8.3oC)lower, or at liner top BHST whichever is cooler. If no set is seen in 36 hours retarderselection should be re-addressed.

3.6.2. Mixing/ShearParticularly in the case of batch mixing, cement slurries can be affected by both the time theyare held on surface after mixing and the mixing energy they receive. The standard laboratorymixing procedure for cement slurry testing is defined in API Specification 10. When a slurryis designed for an HPHT application, it should be mixed and then held at room temperatureand pressure simulating the holding time expected on surface. Some cements have beenshown to have 6 hour pump times at high temperature but to exhibit significant viscosity risewhen held at low temperature and pressure. Any rise in consistency above 30Bc during thissurface conditioning should be sufficient to eliminate that slurry design.

In an extended mixing period, the slurry may be continuously circulated through arecirculating mixer where it will see intermittent period of high shear. For extended mixingtimes, where large batches are being mixed, the slurry may see more mixing energy than

during laboratory design. The intermittent nature of the shear has been demonstrated to havemore impact than when all the mixing energy is applied in a single continuous mixing interval.The final slurry design should be tested either with a simulation of the shear the slurry willexperience, by estimating the energy input during batch mixing, or using the following simplemixing programme:

Mix for 1 minute at 1,000 rpm

Mix for 20 seconds 12,000 rpm

Mix 1 minute 1,000 rpm

Mix 20 seconds 1,000 rpm





This is equivalent to about twice the mixing energy received in a standard test, some slurries

can reduce thickening time by >30% using this procedure; behaviour which needs to beavoided.

3.6.3. Additive ConcentrationThe thickening time should be determined for retarder concentrations +/- 5% of that in theexpected design. Acceptable slurries have:

pump time > job with 5% less retarder

pump time < 2 x base slurry with 5% more retarder


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pump times which decrease with less retarder and increase with more retarder.

In the field, when using a batch mixer, the additives should be gauged from calibrated pailsweighed on an appropriate scale. If the slurry is to be mixed continuously, it is advisable to usea cement batch tank to prepare the mix water, unless assurance can be obtained on cleaningthe pit and lines to prepare mix water. In most cases the number of additives excludes use ofan LAS, but if it were to be used a calibration check should be completed on volumesindicated before use.

3.6.4. DensityIt is critical that the slurry is pumped at the designed density for both well control and slurryperformance. Pumping the slurry light may increase the susceptibly to settlement of theweighting agent, pumping it heavy may reduce the thickening time. The slurry should bepumped after checking with a pressurised mud balance (calibrated at the appropriate range).

3.6.5. Additive Order of AdditionSlurry sensitivity has been shown to be affected by different orders of addition of additives.To prevent this affecting performance, the laboratory should document the order used duringslurry design and this should be included on the slurry recommendation, and followed whenpreparing the mix water in the field.

3.6.6. ContaminationThe impact of contamination by mud and/or spacer can have a substantial impact on slurrypumping time and rheology. The thickening time of the slurry should be confirmed with a10% contamination of both mud and spacer. The high solids content of high density spacerscan reduce pump time of the slurry even at low levels of contamination. If this occurs in the

lab, it is recommended that a similar concentration of retarder is included in the spacer.Should the mud have a dramatic effect on pumping time, spacer should be pumped ahead ofand behind the plug to prevent mixing. Additionally, care must be exercised when pulling outfrom the top of liner to minimise fluid mixing.

4. Spacer DesignOn HPHT wells, both water and emulsion spacers have been used (the later favoured byDowell with OBM). The main areas of concern with spacer design are:

spacer stability selection of weighting agent rheology compatibility


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4.1. Spacer StabilityThe ability of the spacer to support the weighting agent on surface and under downholeconditions should be confirmed. This is particularly critical where pore and fracture gradientmargins result in spacer density being very close to mud weight. The ability to support theweighting agent at surface should be confirmed in the laboratory by leaving a volume of

spacer static for 2 hours in a measuring cylinder using the Free Water procedure used fortesting Well Cements (API Specification 10). Prior to pumping, the density in the pit shouldbe confirmed with a pressurised mud balance. The stability downhole should be assessedusing the BP settlement tube and determining any free fluid after leaving for a minimum of 4hours under downhole conditions.

4.2. Selection of Weighting AgentThe options of weighting agent are:

Barite Manganese Tetroxide Haematite Brine

4.2.1. BariteBarite is the most common material used and is recommended where the required spacerrheology can be achieved. However, the stability of the spacer must be checked.

4.2.2. Manganese TetroxideWill produce the most stable spacers with the best rheology but at more expense. ManganeseTetroxide has been used in conjunction with Barite to produce better spacer rheology andstability. The fineness of the material results in spacers that are almost stable without anypolymer viscosifier.

4.2.3. HaematiteNot recommended unless spacer density precludes use of other weighting agents asformulating a stable spacer is more difficult.

4.2.4. BrinesWith the advent of the formate brines it has become possible to formulate low solids or solidsfree high weight spacers. The advantage of turbulent flow at low flow rates and good stabilitywould make these an option where fracture gradient and pore pressures are close. Whilstthese have not been pumped in HP/HT wells to date they should be considered for futureapplications.

Compatibility testing should be thorough and rigorous.

4.3. Spacer RheologyIn many cases the ability to achieve turbulent flow of the spacer, or a density hierarchybetween the spacer and the drilling fluid is compromised, due to the small difference between

pore and fracture pressures. Where one of the above cannot be achieved, rheology of the


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spacer and pipe movement will be the only available methods for achieving good muddisplacement.

To optimise spacer rheology it should be determined at simulated downhole temperatures(along with the mud) to ensure effective mud displacement. Surface rheology measurementshould also be taken to provide a quality control step on the rig.

4.4. CompatibilityIt is critical that the compatibility of the spacer is confirmed with both the mud and thecement. Using a range of mixtures (as a minimum 72/25-50/50-25/75), the rheology shouldbe determined at BHCT (or 180oF if BHCT not practical). The also 10 minute gels shouldalso be included. If the 100 rpm is 25% greater than the highest 100 rpm reading of theuncontaminated fluids, the spacer design should be reviewed. If plastic viscosity/yield pointor 10 minute gels suggest the slurry may become unpumpable, or separate into one or morephases, the spacer design should be changed.

In addition, the Thickening Time of the cement with spacer contamination as described in

above must be determined.

5.5.5.5. On the Rig:On the Rig:On the Rig:On the Rig:The attention and vigilence of all those on the rig is vital. Again, small, seemingly trivialmistakes can adversely impact the job. Experienced personnel who know what to look forand can work around unexpected difficulties are vital if risk is to be minimised. Someparticular aspects to look for are:

5.1. before delivery of any bulk cement blends, ensure that the receiving silo(s) isemptied and thoroughly cleaned. The system should be thoroughly checked,filters, vents, water traps, etc cleaned. Someone should specify just whatshould be done. This should be agreed by the Drilling Contractor, the ServiceCompany Engineer and the Cement Engineer on the rig. Someone shouldsign-off just what has been done.

5.2. there must be a rigorous, documented system maintained by the BargeCaptain, or other responsible person, which unambiguously records

movements to, from and between tanks

5.3. physically check all Batch Numbers on all materials to be used. This activityshould be signed-off

5.4. Mud Engineer(s) and Cementer(s) together check and record Chloridecontent of Drill Water prior to addition of additives. Is the Cl- contentconsistent with what the lab have used? Has there been any new delivery ofDrill Water?


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5.5. a brain-storming session should be held on the rig prior to runningcasing/liner to capture all possible ways the job may be jeopardised duringexecution. The points raised should addressed and responsibilities assigned

5.6. if there is a risk of having insufficient personnel to cover all aspects during jobexecution, bring in extra assistance, eg bring the lab guy who has worked onthe slurry

5.7. written contingency actions once cement pumping has started should be inplace and should have been discussed and agreed

5.8. if batch mixing is to be used, the lab testing will have been done to takeaccount of this. The lab simulated time for batch mixing should be stipulatedto the rig and if this time is exceeded, slurry should be dumped.

5.9. mix water should not be prepared before absolutely necessary. If mix waterpreparation starts but a delay occurs beyond a few hours, dump the mixwater.

5.10. software simulated pressures/rates should be compared with the actual duringthe job - not just afterwards

5.11. samples of blend, mixwater, additives, drill water, spacer(s), mud must betaken, fully labelled and held securely until the job is accepted as fullysatisfactory

5.12. for displacement volumes, remember the tolerances on id, uncertainties onpump efficiency (if using rig pumps) and mud compressibility

5.13. minimise the time to pull above the liner once the plug has bumped but donot swab the well. Keep pipe in motion, pulling, and monitoring weight at alltimes. It is better to keep moving slowly if at all possible unless weight is

increasing

HPHT Cementing Guidelines

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