12- reverse circulation cementing

7/22/2019 12- Reverse Circulation Cementing

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Beirute Consulting, L.L.C. 1

Reverse Circulation

CementingThe Complete Picture


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We will Review

1. Differences between conventional and

reverse circulation cementing (RCC)

2. The main advantages of the use of RCC

3. The main objections to the use of RCC

4. Literature report applications of RCC from

very shallow wells to very deep applications

5. The theory and physics governing thedifferences between conventional and RCC

applications


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We will Review (cont.)

6. Computer simulations of RCC jobs

compared to jobs performed using

conventional circulation

7. Several possible modifications to the float

equipment to allow reverse circulation, and a

tool design specific to RCC

8. General guidelines to design RCC jobs


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What is Reverse Circulation

Cementing?

During conventional cementing, pumping is

done down the casing and returns are taking

from the annulus.

During Reverser Circulating Cementing

(RCC),pumping is done down the annulus and

returns are taken from the casing.

RCC makes sense! It is the most direct,

shortest wayto do the job!


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Conventional Cementing


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Reverse Circulating Cementing


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Introduction Reverse Circulation Cementing is a viable alternative

to conventional cementing practices, particularly insituations were weak formations may be brokendown during normal cementing because ofexcessive pressures in the annulus.

This situation is not uncommon.

For example, excessive pressures in the annulus islikely in wells with narrow annuli like in slim holeapplications or when long columns of cement need to

be used,

Reverse circulation cementing generates much lowerdownhole and job pressures.


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Introduction (cont.)

Reverse circulation cementing was used initiallyalmost exclusively in relatively shallow wells.

However, successful applications of the technology indeep applications are being reported by the industry.

In shallow applications, cementing was performed bytaking returns through an inner string run inside thecasing after getting the casing to bottom.

The inner string stings into a tool (for example aretainer) at the bottom of the casing.

The valve in the tool closes after the inner string isun-stung from the tool after the end of the cement

job.


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Some Potential Advantages of RCC

Much lower placement pressuresacross weak zones during

hole conditioning and during cementing. This is the mainadvantageof reverse circulation.

Generally lower surface pumping pressures (lower horsepowerrequirements) than for conventional cementing.

Because of the lower placement pressures, the technique mayproduce good cement jobs in situations where the conventionalmethod would fail due to lost returns, for example.

The lower placement pressures allow faster placement rateswhen needed for better displacement, without breaking down

weak formations. Much shorter cement jobsbecause the cement slurry is

pumped down the annulus directly, instead of beingpumped down the casing and up the annulus.


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Some Potential Advantages of RCC

(cont.) Because of the way the jobs are pumped, not all

of the cement slurry sees the high well

temperatures located toward the bottom of the

well. Additives can be staged

Since placement times are shorter, this can lead to

cheaper cement slurry designs: less additives

(retarders, fluid loss, expensive gas migration

materials, etc.)

Lower slurry densities may be used.


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Some Objections to the Use of

Reverse Circulation Cementing

Following is a list of possible concerns and/orobjections to the use of reverse circulation cementing.

These objections are addressed later in this

presentation. As will be seen, the bulk of the concerns are

unfounded, several have been resolved, and the fewleft can be relatively easily addressed.

Clearly, RCC is not for all cementing jobs but it isapplicable, with potentially very good results, in quitea bit more cases than it is being tried.


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Some Objections to the Use of Reverse

Circulation Cementing (cont.)

Unconventional approach.

This is perhaps the main reason reverse

circulation cementing is not widely used.

Hopefully this presentation will help change

this objection.


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Conventional float equipment cannot be used.

This problem has been solved in shallow wellapplications by installing a retainer or other device at

the bottom of the casing, in conjunction with the useof an inner string. Returns are taken during the jobthrough the inner string.

At the end of the job, the inner string is pulled from

the tool to close the valve at the bottom of the casing,allowing the cement to set without having to applypressure to the casing.


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Circulation Cementing (cont.) For deeper applications, running an inner string is not

operationally easy, and in some cases undesirable.

Therefore, a newor modified float equipment is neededto be able to use reverse circulation cementing without

the application of pressure to the casing after the end ofthe cement job.

Several ways to modify float equipment are discussedduring this presentation.

New tool designs for RCC are not currently available inthe industry. One was designed between Amoco andWeatherford.

The RCC tool will be discussed during thispresentation.


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During reverse circulation cementing, it is not possibleto clearly tell when the job is done (since there is no topplug, there is no clear pressure increase when the pluglands on the float collar).

This is a reasonable objection. In shallow well applications, it is possible to tag (die) the

spacer fluid ahead of the cement slurry to help determinethe fluids location at the end of the job, by observing the

spacer when it gets to the surface through the inner string. Others have used viscous pills to observe a pressure

increase at the surface when the pill gets to the small I.D. ofthe inner string.


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For deeper well applications without the use of inner

string, fluid volumes have to be well measured,and

good open hole calipershave to be used to help

prevent pumping too littleor too muchcement insidethe casing.

Think about this: drilling a little more cement inside

the casing is not too big of a price to pay if the well

can be cemented properly in cases where it could

not have been done using the conventional

approach.


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Circulation Cementing (cont.) Extra cement to drill inside the casing. As said before, this

may not be too big of a price to pay for being able to cementthe well and obtain good zonal isolation across weak zonesand pay horizons.

Concerns about the quality of the cement around the shoe.This is a valid concern,but again, displacing enough cementinside the casing at the end of the job allows a good job aroundthe shoe. This is the reverse problem of getting a good linertop cement job.

Concerns regarding proper hole conditioning in the

reverse circulation mode. Experiments conducted in largescale models to be discussed later suggest that the efficiency ofhole conditioning prior to cementing is independent of thedirection of flow.


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Circulation Cementing (cont.) Concerns related to displacement efficiency during the

cement job.

Field applications of the technique suggest that goodcement jobs can be obtained by reverse circulation

cementing. However, large scale displacement efficiency experiments

with cement slurries being pumped down the annulus havenot been conducted to properly address this concern.

Some guidelines are provided here based on what is knownabout optimization of the displacement of one fluid byanother.

These guidelines should maximize the efficiency of the jobswhile the industry waits for the test results.


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Circulation Cementing (cont.) Since the casing is open to flow in the upward

direction, there may be safety concerns in highpressure situations, mainly while running thecasing.

This may indeed be a problem in some highpressure applications.

Therefore, these situations may not be goodcandidates for reverse circulation.

For the same reasons, automatic fill-up floatequipment is not always applicable in highpressure situations.


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If during a reverse circulation cement job you losecirculation above the shoe, chances are you willnot have a good cement job around the bottom ofthe casing, requiring a casing shoe squeeze job.

This is possible, but considering that reversecirculation cementing generates much lower pressuresin the annulus than conventional cementing,the

possibility of this problem occurring is minimized.

Actually using RCC may be the correct way toreduce this risk


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Circulation Cementing (cont.) Casing collapse considerations if bridging occurs in the annulus

during the job due to sloughing clays, cuttings, etc.

Again this is possible, but by the most part, unlikely.

The possibility of this occurring can be minimized by properlyconditioning the hole prior to the cement job, if possible, using theconventional circulation method (moving cuttings, etc. up theannulus), to fully clean the hole.

Prior to the cement job, the well should again be circulated, this timein the reverse circulation mode.

In all cases, the condition of the hole (problem shales, etc.) should be

closely examined before deciding that reverse circulation cementingis a proper option.

As for conventional cementing, in RCC application, the use of KCl inthe cement slurry and/or properly designed spacer fluid ahead of thecement needs to be considered to help with troublesome formations.


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Other hole stability concerns: sloughing shales,

cuttings, etc. could bridge the bottom tool (or

float).

The possibility of this occurring can be minimized byproperly conditioning the hole prior to the cement

job, if possible, using the conventional circulation

method (moving cuttings, etc. up the annulus), to

fully clean the hole.

Prior to the cement job, the well should again be

circulated, this time in the reverse circulation mode.


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Circulation Cementing (cont.) When using an inner string, it could get cemented up in the

hole if the cement starts to set before un-stinging from thetool.

The inner string could also get cemented up in the inside.

This is possible, but should not happen if good quality control

practices are followed with the slurry design, mixing procedures,etc., and with the job execution.

Inner string cementing of surface casings using the conventionalmethod has been around for years with minimum problems.

In addition, liner cementing presents similar possibilities for

trouble if the drillpipe was to become cemented-up in place. However, hundreds of liner cementing operations are performed

every month throughout the world without this situationmaterializing very frequently.


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Literature Review

The first recorded usage of reverse circulation cementingis found in a paper by Marquaire and Brisac.

These French investigators tried reverse cementing afterconventional cementing techniques failed to give

satisfactory results in the North Hassi-Messaoud field inAlgeria.

Conventional cementing failed in this field because of acombination of exposed high pressure zones requiring

high density muds,and weak formations that tended tobreakdown during the jobs,due to excessive ECDsgenerated when attempting to cement in turbulent flow.


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Literature Review (cont.)

The following figure describes the drilling programused in the Hassi-Messaoud field.

The cementing difficulties were centered around the 7

in. production casing set in the 8-3/4 in, hole. At 8,500 ft in the lower Jurassic, a high pressure,

8,500 psi CaCl2-saturated formation wasencountered.

To minimize severe contamination of the salt-saturated mud weighted with barite, they used a muddensity of 17 to 18.5 lb/gal.


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Drilling

Program

Hassi-

Messaoud

Field


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A clay zone about 15 ft thick was encounteredat 9,500 ft.

These clays were abnormally plastic and

tended to extrude into the wellbore, againrequiring the use of 17 to 18.5 lb/gal mud totry to keep them under control.

Other methods used to control the claysincluding clay control additives and oil basemuds had failed.


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Between 10,500 ft and 11,000 ft, they encounteredred sandy shales.

These shales had a lower mechanical strength thanthe overlying sediments, and could not alwayssupport the hydrostatic head from the highdensity mud needed to control the high pressurezones located above.

Thus, formation breakdown frequently occurredduring drilling with total or partial loses of mud,requiring squeezing of the weak zones with cement.


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Literature Review (cont.) For successful cementation of the 7 in. casing, it was required

to obtain a good seal of the high pressure CaCl2 zone, and agood seal of the casing shoe.

Their experience in this field indicated that to achieve thisgoal, they wanted to be able to pump the salt-saturated cementslurry in turbulent flow.

However, when using conventional cementing, turbulent flowcould not be achieved without breakdown of the weakformations due to the excessive ECDs generated during

pumping.

Faced with this problem, these investigators decided to try

RCC with the goal of reducing the circulating pressures(equivalent ECDs) across the weak zones duringcementing, to prevent breakdown, while still being able topump the jobs in turbulent flow.


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The application of reverse circulation cementing generatedseveral difficulties that they had to overcome.

The differential fill-up equipmenthad to be modified toallow reverse circulation.

The next figure shows the changes they made to thisequipment.

The modified fill-up tool allowed normal use of the equipmentwhile running the casing in the hole, but once on bottom,shearing of pins with pressure allowed removal of the bronze

plug, leaving the tool fully open for reverse circulation.

It is not clear from their paper what procedure they used toprevent the cement from U-tubing into the casing after the endof the job (possibly held pressure in the casing).


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From the schematic of the equipment layout given in the paper(next figure), it is assumed that they held pressure in the casingafter the cement job.

They also realized the problem of not being able to detect the

end of the job by a pressure increase at the surface, and theneed to place good quality, uncontaminated cement around theshoe without leaving excessive amounts of cement inside thecasing.

To overcome these concerns, they used good caliper logsto

calculate the cement and displacement volumes, and theydesigned the excess cement slurry volume to give them about300 ft of cement inside the casing after the job.


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Equipment

Layout


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Literature Review (cont.) Leturno et. al. described the use of RCC to cement a scab-liner

in an old well to repair the leaky production string.

The flush joint, 4 in., 11 lb/ft liner was run to 8,560 ft insidethe 44 year old 5-1/2 in., 17 lb/ft, J-55 production string.

The pipe-in-pipe annulus was successfully cemented to thesurface using two slurries.

They used a lead slurry at a density of 11 lb/gal, and a tailslurry, at a density of 14.8 lb/gal. (the heavy tail was pumped,according to the authors, to reduce the build-up of annularsurface pressure.)

The job was pumped at about 1 BPM.

The authors indicated in their paper that the RCC techniqueresulted in much lower circulating pressures when comparedto the conventional approach.


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Conventional cementing would had exceeded the burststrength of the old 5-1/2 in. casing, they said.

They used no float equipment, and apparently they were takingreturns through the inside of the 4 in. liner during the job.

The paper does not give details concerning the method used tohold the liquid cement in place in the annulus after the job(prevention of U-tubing into the liner before setting).

A 41 feet rat hole was present below the 4 in. liner, above acast iron bridge plug.

After WOC, they tagged cement at 8,430 ft (about 149 ft ofcement inside the casing).


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Griffith et al. also wrote about the advantages of reversecirculation cementing to reduce ECDs across weak formationsto prevent fracturing.

Their paper illustrates the use of this technique in cementingshallow coal bed methane wells in Trinidad CO, USA to

obtain higher cement tops. The coal beds exhibit severe lost circulation characteristics.

Conventional cementing of previous wells had not beensuccessful even with the use of lightweight cements, foamcements and/or bridging materials.

When the paper was written, six successful reverse circulationcementing job had been performed, achieving higher cementcolumns than with the conventional cementing method.


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The paper by Griffin includes results of a few large scale

tests performed to evaluate the effect of reverse circulationon circulatable hole (amount of hole circulating during holeconditioning prior to the cementing job).

The removal of partially dehydrated-gelled (PDG) mud andfiltercake studies in Griffins paper were conducted in large

scale models comprising 5 in. x 6-1/2 in. annuli. The casing had an average standoff of 23%. The

circulatable hole was measured during the experiments bythe resistivity probe method.

Their experiments suggested that the circulatable hole

efficiency is independent of the direction of flow. No actual displacement studies with cement being

pumped down the annulus were conducted during theirinvestigation.


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Literature Review (cont.) The jobs in Trinidad CO. were performed using a 2-7/8 in.

inner string inside the 5-1/2 in. casing landed at about 2,000 ft. The casing was cemented in a 7-7/8 in. hole.

The wells were cemented using a viscous pill ahead of thecement slurry to indicate when the cement got to the casingshoe.

This approach was possible according to the paper, becausemost of the friction in the system was generated by the innerstring.

A cast iron cement retainer (CICR) was used at the bottom ofthe casing.

Once the spacer/cement was detected at the shoe by theincrease in surface pressure, the valve at the CICR was closed

by un-stinging the inner string.


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Literature Review (cont.) A few Amoco Experiences with RCC:

In September 1994, Dennis High et.al. with Amoco Canadareported a very good description of a different application ofreverse circulation in their area.

Their situation dealt with the difficulty of establishing

circulation after running casing to bottomin wells drilledwith relatively high mud densities and gels, in the presenceof weak zones uphole.

In these cases, if fracturing of the weak upper zones tookplace while attempting to re-gain circulation after runningthe pipe to bottom, lost circulation would prevent thecement column from obtaining the desired height duringthe cement job.


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The intermediate 8-3/4 in. hole was drilled to 2,634 m (8642ft) with a fresh water Gel/PHPA mud with a density of 1,860kg/m3 (15.5 lg/gal).

This mud density was needed to control the sloughing Fernieshale.

The casing was tapered 177.8 mm (7) by 219.1 mm (8-5/8). After running the casing, proper conventional circulation

could not be established due to high mud gelsgeneratedduring the static period. Discuss: is this good?

While attempting to re-gain circulation, large mud loses tookplace, possibly near the pay zone.

Finally, it was decide to attempt reverse circulation withthe heavy density, gelled mud on the back side.


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To be able to reverse-circulate, a 88.9 mm (3-1/2)

drill string was run inside the casing to drill out thefloat equipment.

Next, a cement retainer was run into the casing andset at 2,582 m (8474 ft).

Fresh water was circulated down the drillpipe, withthe pipe un-stung from the retainer.

The drillpipe was then stung back into the retainer.

With the heavy mud on the back side, this causeda U-tubing tendency (large pressure dropgenerated at the bottom in the reverse direction)for the well to flow into the drillpipe.


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They also pumped on the back side to help the heavymud flow into the drill string.

After a few trials using this procedure, the wellwas successfully circulated in the conventional

direction (mud gels were broken). The well was then successfully cemented to surface

pumping down the drillpipe (conventional method).

Full returns were seen during the job.

After the job, it was reported that the mud losesdid not go into the pay zone as initially feared.How do you know this?

Lit t R i ( t )


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Literature Review (cont.) Bob Ovitz reported the successful use of RCC of coal

wells in the Raton Basin.

This basin in located near the East corner of theColorado-New Mexico border.

In these wells, the 5-1/2 in. casing needed to be cementedto 2,400 ft across highly fractured coal beds in an 7-7/8

in. hole. The use of blocking agents (LCMs) had not been fully

successful in allowing complete circulation.

In addition, the use of LCMs brought concerns regarding

production from those wells (production from fracturedcoal seams).

Proper cementation of the wells was needed due toproduction and legal ramifications.

Lit t R i ( t )


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Literature Review (cont.) The wells were air-drilled to TD.

After landing the casing, 2-3/8 in., 4.7 lb/ft tubing was runinside the casing and stung into the float valve at the bottom ofthe casing.

The tubing/casing annulus was filled with water.

The cement job was pump down the casing/hole annulus at

rates of around 2 to 3 BPM. Cement mixing was stopped when spacer returns were

observed at the surface.

Since they used spacer volumes about equal to the tubingvolume, when the spacer was seen at the surface, the cement

was at the casing shoe. The tubing was next sting-out of the float valve and reverse-

out.

They allow a maximum of 400 psi at the surface to reverse outto clean the tubing.


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Literature Review (cont.) A recent (about 4 years ago) deep applicationof the

technology: They needed an effective method to cement a casing string

to 23,700 ft.

The well was cased to 20,700 ft.

Open hole was from 20,700 ft to 23,700 ft. Casing was tapered: 14,000 ft of 10-3/4and 9,700 ft of 7-3/4.

Because of well control issues and extremely high sour gasproduction required that the casing be cemented to surface.

Conventional cementing was ruled out because of retarder

issues (cement had to be pumped down to TD at ~ 410Fand back to surface) and the difficulty of wiping the

tapered string.

Two stage job was also ruled out due to tight clearancesbetween casing strings restricting stage tool design.


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Literature Review (cont.) Finally, RCC was considered.

Open hole fracture gradient was 0.9 psi/ft The short placement time provided by RCC was

considered a plus.

In addition, the retarder loading of the cement slurry

could be staged since not all the cement would beexposed to the high temperatures at the bottom of thehole.

If the casing was run open-ended to allow returns to be

taking from the casing, this presented a safety issue whilerunning the casing if the well was to kick. So, a specialshoe allowing the insert valve to be pumped out bydropping a ball was designed as a contingency.


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Literature Review (cont.) By pumping the valve out of the shoe after landing the casing, the

RCC procedure could be executed. Because during drilling the open hole the formation showed very

little gas, the casing was run open ended with a guide shoe(reduction of surge pressures).

Centralizers were used to try to centralize the casing string.

The annulus was closed and the cement slurry pumped down theannulus via valves below the mandrel hanger.

Returns were taken from the casing.

Total job displacement was one (1) bbl of the OMB.

The cement slurry density was only slightly higher than the

density of the mud. This allowed manageable returns from thecasing.

Returns were monitored so that for every bbls pumped, a bblof returns was taken to the pits.


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The cementing operation lasted only about 3 hrs.

About 3,300 sks of cement were pumped.

Formation breakdown was prevented (essentially no losses).

The job was planed to bring 1,500 ft of cement inside the 7-

3/4 casing. After the cement set, cement was tagged at 2,200 ft inside the

casing shoe.

After drilled out, the shoe tested fine.

A savings of about $100,00 was reported from reduced rigtime and waiting on cement time.

Running a liner was avoided.

Th f R Ci l i d


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Theory of Reverse Circulationand

Reverse Cementing

It is known that use of reverse circulation can

substantially reduce the annular pressures

during pumping of a cement job.

To understand the reasons for this, lets

investigate the pressures in a well for the

conventional (normal) cementing case and the

reverse circulation cementing case.

B H l P


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Bottom Hole Pressures Conventional Circulation Cementing:

In a conventional (normal) circulation caseof a cement job inprogress, the fluids are pumped down the casing and up theannulus. In this situation, the pressure at the bottom of theannulusis given by:

PBCC = PHA + PFA + PSA (1)

where:

PBCC = Pressure at the bottom of the annulus in theconventional case

PHA = Hydrostatic pressure in the annulus

PFA = Friction pressure in the annulus

PSA = Surface pressure applied to the annulus (if any)

B tt H l P C ti l


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Bottom Hole Pressures Conventional

Circulation Cementing (cont). The casing surface pressure needed to circulate in the case of

conventional cementing is calculated by:

PSC = PBCC - PHC + PFC (2)

where: PSC = Surface pressure in the casing at the surface

PHC = Hydrostatic pressure in the casing

PFC = Friction pressure in the casing

Substituting (1) into (2) we get:

PSC = (PHA - PHC) + (PFA + PFC) + PSA (3)

In normal cementing operations, pressure is not applied at the surfaceto the annulus; thus, PSA in equations (1) and (3) is usually zero.

B tt H l P C ti l


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Circulation Cementing (cont).

B tt H l P C ti l


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Circulation Cementing (cont). Stages of Conventional Cementing: Studying equation (3), early during the cementing

operation (stage A) when the heavy fluids (spacers,cement slurry) are being pumped into the casing, theterm (PHA - PHC) becomes increasingly negativebecause the hydrostatic head in the casing iscontinuously growing over the hydrostatic head in theannulus.

This in turn causes the casing surface pressure to declinewith time during this stage of the cementing job.

Eventually the casing surface pressure can become equalto zero and even negative (vacuum).

However, physically this pressure cannot become lessthan absolute vacuum.


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Bottom Hole

Pressures

Conventional

Circulation

Cementing

(cont).

Surface Pressure

Behavior

B tt H l P R


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Bottom Hole Pressures Reverse

Circulation Cementing (cont). Reverse Circulation Cementing:

In a reverse circulation situation, the fluids are pumped down theannulus and up the casing. The pressure at the bottom of theannulusis calculated in this case by:

PBRC = PHC + PFC + PSC (4)

where:

PBRC = Pressure at the bottom of the annulus in the reversecirculation case

The required surface pressure in the annulus side is given by:

PSA = PBRC - PHA + PFA (5)

B tt H l P R


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Substituting (4) into (5):

PSA = (PFA + PFC) - (PHA - PHC) + PSC (6)

PSC in equations (4) and (6) is usually zero since

normally no surface pressure is applied to the

casing at the surface.

B tt H l P R


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SurfacePressures Comparing Equations (3) and (6), it is concluded that

reverse circulation in general requires lower surfacepumping pressures(lower horsepower requirements)than conventional cementing:



In the reverse circulation case, the term (PHA - PHC)ispositive most of the time and contributes to reducing thesurface pressure needed during the entire job (noticethant in Eq. (6) the term has a negative sign).

In fact, looking at equation (6), it is easy to see that in

general, during reverse circulation cementing, the wellmay tend to be on free-fallduring a large portion of thejob, particularly at low pump rates (low frictionpressures).


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SurfacePressures

Further indication that the surface pressureswill tend to be lower in the RCC case can beseen by subtracting Eq. (6) from Eq.(3):

PSC PSA = 2(PHA PHC)

In the above equation, we set the terms PSAand PSC equal to zero.

Notice that in the case that only one fluid (for

exaple mud) is circulated, surface pressuresinthe conventional or the reverse case are thesame since in this case (PHA PHC) = 0


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Bottom HolePressures

Back to looking at the bottom hole pressures: By studying Equations (1) and (4) it is possible to

conclude that in general, during pumping of cementjobs, the reverse circulation approach will yield lower

annular pressures toward the bottom of the hole thanthe conventional circulation method.

This can be further illustrated by comparing aconventional and a reverse cementing operation both

being conducted with the same fluids (same densityand rheology), being pumped at the same rate, at a

point in time toward the end of the job, and with theinterface of the fluids at the same elevation.


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Bottom HolePressures (cont.)

With those assumptions, subtracting Equations (4) from (1):

(PBCC - PBRC) = (PHA - PHC) + (PFA - PFC) (7)

where, for simplicity, we have made PSA~0 for the conventional

case, and PSC~0 for the reverse circulation case.

Equation (7) can be used to conclude that reverse

circulation becomes even more attractive from the

point of view of reducing annular placement

pressures during cement jobs, when using high

cement slurry densities (high PHA)and withincreasing annular friction pressures like in

narrow annuli (high PFA).


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Bottom HolePressures (cont.)

Large friction pressures are likely in narrow

annuli.

Thus, reverse circulation is quite attractive in

slimhole applications. So, from theory, we conclude that reverse

circulation can allow execution of some cement

jobs across weak zones located near the bottom of

the hole, without breaking down those formations,while the conventional approach may not.

P U H l


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Pressures Up Hole It is also important to understand the pressures across upper

weak formations, or at a previous casing shoe.

Conventional Circulation Cementing Case:

In this situation, the pressure in the annulus across an upperlocation at a distance Z from the bottom of the holeis givenby:

PCC Z = PHA + PFA + PSA - PHAZ - PFAZ (8)

where:

PCCZ = Pressure at a distance Z from the bottomin the annulusin the conventional case

PHAZ = Hydrostatic pressure in the annulus along the distance Z

PFAZ = Friction pressure in the annulus along the distance Z


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Pressures Up Hole (cont.)

Reverse Circulation Cementing Case: In a reverse circulation situation, the fluids are pumped

down the annulus and up the casing. The pressure acrossan upper location at a distance Z from the bottom of

the holeis given by:

PRCZ = PHC + PFC + PSC - PHAZ + PFAZ (9)

where: PRCZ = Pressure at a distance Z from the bottom in the

annulus in the reverse circulation case.


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Pressures Up Hole (cont.)

Again lets compare a conventional and areverse cementing operation both beingconducted with the same fluids (same density

and rheology), being pumped at the same rate,at a point in time toward the end of the job,and with the interface of the fluids at the sameelevation.

With those assumptions we can write,subtracting Equations (9) from (8):

P U H l ( t )


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Pressures Up Hole (cont.) (PCCZ - PRCZ) = (PHA - PHC) + (PFA - PFC) - 2 x PFAZ 10)

where again for simplicity, we made PSA~0 for the conventionalcase, and PSC~0 for the reverse circulation case.

Equation (10) suggests that it is possiblefor the pressure up holefor the reverse circulation mode to be higher than for theconventional mode, at large distances up hole from the bottom of

the hole, if the cement slurry density is close to the muds and ifthe annulus is tight (high annular friction pressures).

Therefore, it is possible that if weak formations are found uphole in the open hole, the reverse circulation approach may notbe the best choice. Thus, pressures need to be check across all

weak zones in the open hole and the previous shoe. In general, the best application for RCC is for situation were the

weak zones are located close to the bottom of the hole,and theformations up hole in the open hole are competent or cased off.

Important Note


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Important Note As shown above, the main use of reverse

cir culation is to protect weak formations in theopenhole section of the well by reducing theplacement pressures in the annulusversus theconventional circulation method dur ing pumpingof the cement job.

PBRC = PHC + PFC + PSC (4)

Although Equation (4) suggests that dur ingreverse circulation cementing the annulus maynot exper ience the full annularhydrostatic head,

the exposed formations in the well wil l eventuallysee the full hydrostatic head generated by thedensity of the fluids in the annulus after the end ofthe cement job.

I t t N t ( t )


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Important Note (cont.) This can be understood by realizing that Equation (4) gives

the bottomhole pressure dur ing the cement job,from the

start to the end of the pumping operation.

As soon as the pumping stops, the pressure profi le in the

annulus increases and becomes the annular hydrostatic

head by either the action of the closing of a valve at the

bottom of the casing, or by application of surface pressureto the casing,needed to control the imbalance of the

annulus-casing hydrostatic heads (the U-tubing effect).

Therefore, for a successful application of reverse

cir culation cementing, the weak formations in the annulus

must be able to support the ful l hydrostatic head of the

f luids at the end of the job, even if they do not see it dur ing

the job,or they may exper ience breakdown after the cement

job.


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Important Note (cont.) Therefore, one of the first calculations that needs to be

made is the annular hydrostatic head with the full columnof cement in the annulus and the displacement fluid.

This hydrostatic needs to be compared to the pressure theweak zones are capable of supporting.

For the RCC operation to be successful, the supportingpressures of the zones need to be higher than thecalculated annular hydrostatic.

If needed, adjustments to the densities of the slurry,

spacer and displacement fluids need to be made at thispoint to try to prevent breakdown of the weak zones ifthey cannot support the full annular hydrostatic at theend of the job.


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Simulating RCC Jobs

To be able to determine if RCC is an

appropriate way to perform the cement job, a

simulator capable of simulating the job in the

reverse mode is needed. The simulator should be able to simulate the

pressures profile along the entire annulus.

Cem-Job Simulates RCC


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Conventional vs RCC - Comparison


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Conventional vs. RCC - Comparison

of Job Simulations

Example 1: A high pressure situation (requiring heavy

fluids) similar to the one described in the paperby Marquaire et al. was used for this example.

Cementing of a 7 in. casing inside an 8-3/4 in.hole was simulated at a depth of 11,000 ft.

The operation was studied using theconventionaland the reverse circulationmethods.

The same job rate and the same fluids andvolumes were used in both cases.

Conventional Cementing Option at TD


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Conventional Cementing Option at TD


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Reverse Circulation Option at TD


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Both Options Together at 10,500 ft


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Conclusions from Example 1

Clearly the reverse circulation method

generated the lower circulating pressures in the

annulus.

In addition, as can be seen, the reversecirculation job would be performed in about

60% of the time needed for the conventional

job.



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p

of Job Simulations Example 2:

This example simulates a situation similar to the one discussedby Leturno et al. in their paper.

The simulator was run to look at cementing of a 4 in. scabliner inside a 5-1/2 in. casing to 8,560 ft.

The following figure shows ECDs at the bottom of the liner vs.cumulative volume pumped for the conventional and thereverse circulation cement job simulations.

If we assume that the exposed formations at the bottomwere barely able to support the full hydrostatic of thecement after the job, then the normal cementing job wouldbreak the zones way before the end of the job (see thefigure).

On the other hand, the reverse circulation job would notgenerate lost circulation, and the job would be completed inabout half the time.


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Both Options Together at 8.560 ft



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Conventional vs. RCC Comparison

of Job Simulations Example 3: A case like the ones illustrated by Griffith et al. and Ovitz

was simulated.

The well depth was 2,400 ft, with a 5-1/2 in. casingcemented inside a 7-7/8 in. hole.

An inner string (2-7/8 in., 6.4 lb/ft) was used to take thereturns inside the casing.

A viscous pill ahead of the cement slurry was used to see ifthe pill would give a good indication of the end of the jobwhen the pill got to the inner string.

The first simulation looked at cementing the well in theconventional way, by pumping the cement down the casing(no inner string). The second run simulated reversecirculation cementing, with the tubing inside the casing.

E l 3 ( )


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Example 3 (cont.) The following figure compares the data from the two methods.

Notice that the reverse circulation cementing approach againgenerated much lower pressures in the annulus.

During the reverse circulation case, the pressure at the bottomof the annulus increased rapidly toward the end of the job.

This was due to the viscous pill flowing inside the narrowinner string (increase in friction pressure).

This bottom pressure increase caused a surface pressure jumpat around the same time,suggesting that the use of a viscous

pill may indeed help indicate when the cement slurry is at thebottom of the hole.

Off course, care must be exercised to make sure that the pilldoes not generate prohibitively high bottom hole pressures.

B h O i T h 2 400 f


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Both Options Together at 2,400 ft



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Conventional vs. RCC Comparison

of Job Simulations Example 4: This example illustrates the use of reverse circulation of mud

to reduce ECDs across weak formations.

It is similarto the application illustrated by the engineers fromAmoco Canada.

An 8 in. casing is set at 8,740 ft in an 8-3/4 in. hole. Densityof the mud is 15.5 lb/gal.

Circulation of this well with only mud in the hole wassimulated in the conventional and the reverse circulationmode.

The simulations showed that the surface pressure was thesame for the normal and the reverse circulation modes sincethere was only one fluid in the well, BUT, the bottom holepressures were drastically different!

E l 4 ( t )


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Example 4 (cont.) The conventional circulation mode generated for this

narrow annulus, an ECD of over 19 lb/gal at 1 BPM,while the reverse circulation approach generated onlyabout 15.6 lb/gal!

This can be explained by looking at the equations givenbefore.



For the case of PSA and PSC = 0 then:

Equations (3) and (6) show that for only one fluid in thehole, the surface pressure generated by circulationshould be the same regardless of the direction of flow(no hydrostatic difference between casing and annulus).

E l 4 ( t )


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Example 4 (cont.)

On the other hand, Equations (1) and (4) showthat in the conventional circulation mode,the

bottom hole pressurehas to overcome the high

friction pressure in the narrow annulus, while in

the reverse circulation mode,the bottom holepressure sees instead the much lower, casing

friction pressure.

PBCC = PHA + PFA + PSA (1) PBRC = PHC + PFC + PSC (4)



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of Job Simulations Example 4B:

Lets now examined another case similar to the Canadian field case(Example 4), but lets looked at the situation after they ran the innerstring and tried to circulate with water in the hole.

We simulated a 4 in. drill pipe inside the 8 in. casing, with the drillpipe full of water.

Again we looked at the conventional and reverse circulation cases. The relevant part of the simulation is for both situation, obviously,

when the annulus is full of mud (water filling the entire drill pipe).

For the conventional circulation case, the ECD at the bottom of thehole is again over 19 lb/gal.

For the reverse circulation case, the simulation is on free-fall, so it isdifficult to tell the ECD at the bottom when the annulus is full ofmud, but it is low, at a value ranging from the density of water toless than 11.6 lb/gal, which is the ECD at the bottom of the holewhen the simulation finally comes out of free-fall.

E l 4B ( t )


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Example 4B (cont.)

As illustrated by the Canadian field

experiences and supported by the last two

simulations, reverse circulation of mud has

tremendous potential for applications innarrow annuli to reduce breakdown of

weak formations when attempting to

establish circulation after running casing inthe hole.

Float Equipment for Use With RCC


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Float Equipment for Use With RCC New float equipment to use for RCC can be designed or

current equipment can be modified as we already saw.

Here is a valve designed by Beirute while with Amoco.Design was shared with Weatherford.

A PDC drillable float equipment valve was designed thatwould permit the application of reverse circulation

cementing in deeper wells, without having to use an innerstring or holding pressure in the casing after the job.

The new tool would permit circulation at any rate in theconventional and reverse circulation modes.

After the reverse circulation cement job, by the action of a

ball, it would allow closing of the bottom of the casing toprevent the cement slurry from U-tubing into the casing.

This would facilitate having the casing in radialcompression during WOC time to minimize the formationof a micro-annulus during cement curing.



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q p

(cont.)

The equipment would consist of two sections: anupper and a lower tool seat.

The lower seat would contain the valve designed

to close at the end of the reverse circulationcement job.

The space between the two seats was named the

ball chamber. The two seats would be located at a reasonable

distance from each other, for example 20 to 40 ft.



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q p

(cont.)

Figure 11 gives a sketch of the upper seat, and Figure13 a sketch of the lower seat with the valve in its open

position.

In these two figures, the drillable ball is shown but

while circulating and cleaning the hole in theconventional mode (down the casing), or the reversemode (down the annulus), the ball would not bedropped, and therefore, the bottom of the casing would

be open to circulation in either direction. Notice in Figure 13 that since the valve is pinned in its

open position, the casing could be circulated in eitherdirection at any rate without concerns of closing the

valve.


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Upper

Seat


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Lower

Seat

Design could include Auto FillCapability



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q p

(cont.)

The only purpose of the upper seat (Figure 11) is to trap thedrillable ball in the ball chamber, so that the ball will be in close

proximity to the valve after the reverse circulation cement job.

This is the way the ball is utilized:once the hole has been fullycirculated-clean and conditioned in the conventional and/or

reverse circulation mode, the ball is dropped and circulated to theupper seat (Figure 11).

The ball then enters the upper seat throat and seals the flowopening. Application of a preset pressure (detectable at thesurface), shears the pins holding the two shear-arms, and allows

the ball to enter the ball chamber ( Figure 12). The spring-loaded shafts located on the side of the shear-arms are

locked in place and prevent the arms from returning to their closeposition, to prevent the ball from seating on the lower opening ofthe upper seat flow channel of Figure 11 blocking the flow in thereverse mode.


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Ball in the

BallChamber



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q p

(cont.)

The reason for the length of the ball chamber (20to 40 feet) is to make sure that when shearing theshear-arm pins, the ball does not continue downwith force and also shears the valve pins located in

the lower seat. Once the ball is trapped in the ball chamber,

circulation can only be performed in the reversecirculation mode.

At this point circulation would again beestablished in the reverse direction, followed bythe reverse circulation cementing job.



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q p

(cont.)

At the end of the reverse circulation cement job, the ballis near the valve since it is trapped in the ball chamber.

After stopping the pumps and switching to pressurize thecasing, the ball is forced, by applying a preset pressure (in

addition to the hydrostatic differential) and after pumpinga small volume of fluid, to shear the lower flow cylinder

pins that hold the valve open.

After shearing this other set of pins, releasing of the

pressure in the casing causes the spring activated valve toclose.

The hydrostatic differential helps hold the valve closeafter the job (Figure 14).


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Bottom Valve

Closed at endof Job

Guidelines for Reverse Circulation


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Cementing Job Design

The main question still remaining regarding the application ofreverse circulation cementing is the level of displacementefficiency achievable during the cement jobs, when comparedto conventional primary cementing.

Literature documented field applications of the technique

suggest that good cement jobs can be obtained by reversecirculation cementing.

However, large scale displacement efficiency experimentswith cement slurries being pumped down the annulus have not

been conducted to properly address this concern.

Thus, some guidelines for reverse circulation cementing areprovided below based on what is known about optimization ofthe displacement of one fluid by another.



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Cementing Job Design (cont.) 1. Design reverse circulation cementing jobs using a simulator

that can simulate RCC. Be aware of fracture pressures across allthe weak formations and previous shoe, and high pore pressuresacross pressurized zones (wells window) to make sure youmaintain well control during the job.

2. The use of good caliper logs to properly estimate the volumesof cement needed is highly recommended.

3. Remember that weak formations in the well will see the fullhydrostatic of the fluids in the annulus after the reversecirculation cementing job(same as with a conventional primarycementing operations.). Make sure the weak zones can support

the full annular hydrostatic after the job. As needed, design thedensity of the spacer, cement slurry and displacement fluid toprevent breakdown due to the hydrostatic after the job.



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Cementing Job Design (cont.)

4. Since gravity (density differences among the fluids) tends toenhance mixing of the fluids when pumping in the reverse mode, thedensity differences among the mud, spacer and cement slurry should

be minimized. Thus, spacer and cement slurry densities close to thedensity of the mud are recommended in this case whenever possible(this is different than when cementing in the conventional mode).

5. To maximize displacement efficiency, the yield point and plasticviscosity of the fluids should be higher as you move up the hole. Inother words, whenever possible, you want to have a cement slurrywith more consistency than the spacer fluid, and the spacer fluidwith higher consistency than the muds. This is the same as withnormal primary cementing. Use Pressure Drop Hierarchy.

6. Flushes (densities and viscosity similar to water) may beconsidered. However, there is the possibility of contaminating moreof the cement slurry at the flush-slurry interface than withconventional cementing (cement channeling down into the flush dueto the density and rheology differences.



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Cementing Job Design (cont.) 7. Thick spacers (pills) ahead of the cement slurry may help

detect when the job is near completion when using an inner string(pressure increase at the surface when the pill enters the innerstring), but there is the possibility of contamination of the cementslurry at the cement-pill interface (cement slurry may tend topenetrate the less mobile pill during displacement due toviscosity differences.) This contamination tendency may bereduced by making the density of the pill equal or slightly higherthan the density of the cement slurry. A thick cement slurrywould also help in this case.

8. As with conventional cementing, spacers and flushes must betested for compatibility with the mud and cement slurry to

minimize contamination and channeling of the fluids duringplacement. Compatibility tests with the fluids (mud, spacer,cement slurry) must be conducted in the laboratory beforedeciding that a given system can be used in the reversecirculation cementing operation.



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9. Whenever possible, clean the hole by pumping in theconventional (normal) mode after getting the casing to bottom, toclear the hole of cuttings, etc., before pumping in the reversecirculation mode. This will minimize the chances of plugging thevalves used at the bottom of the casing during reverse circulation.Off course, if the idea is to minimize pressures across weak zones

whilebreaking circulation, pumping in the conventional directionwould defeat this purpose. However, as indicated by theCanadian experience, after getting the well circulating in thereverse mode, it may be possible to establish circulation in theconventional direction to complete the cleaning of the hole in thenormal way.

10. Prior to the reverse circulation cement job, pump in thereverse circulation direction and condition the hole to remove asmuch as possible of the PDG mud and filtercake in the open holeannulus.



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11. It is desirable to pump the jobs as fast as possible, withinthe window allowed by the wells fracture gradient across theweakest zones. Since as seen, reverse circulation cementinggenerates much lower friction pressures in the annulus, fasterrates than with the conventional cementing method are easilyachievable during reverse circulation cementing. UseErodibility technology.

12. Other proven, good cementing practices such ascentralization of the casing, pipe movement, etc. should also

be used with reverse circulation cementing.

13. Remember that during reverse circulation cementing, wellsmay often tend to go on vacuum or free-fall, as seen underthe Theory of Reverse Circulation. During free-fall, the fluidsmove at rates that are different from the surface pump rates.



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14. When using an inner string, consider using a spacervolume close to the capacity of the inner tubing, to helpyou detect when the cement gets to the bottom of thecasing. The cement slurry is at about the bottom in this

case, when the spacer is seen at the surface (tagging of thespacer with dies is suggested in this application.) Leaveseveral bbls of cement in the casing to assure a good jobaround the shoe.

15. Plan to leave about 500+ ftof cement inside the pipe

to make sure good cement is placed around the shoe.This volume may be reduced later if experience indicatesthat too much hard cement is being drilled-out after the

12- reverse circulation cementing

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