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Mud Removal

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Mud Removal

Master Table of Contents 1 Objectives 2 Introduction 3 The Three Step Process of Mud Removal 3.1 Hole Cleaning 3.2 Conditioning Mud 3.3 Displacing Mud from the nnulus 3.4 !"ercise 4 Criteria for !ffective Mud Removal 4.1 Overvie# 4.2 The Ideal $ellbore Casing 4.2.1 nnular %ap Standards 4.2.2 Sloughing 4.2.3 &lo# from the &ormation Into the $ellbore 4.2.4 &lo# 4.2.5 !ffects of Temperature 4.2.6 Mud &ilterca'e in the $ellbore 4.2.7 Mud Circulation 4.3 The Caliper (og and Mud Circulation 4.4 Mud Circulation !fficienc) 4.5 !"ercise 5 Influence of Casing Standoff  5.1 Overvie# 5.2 *e#tonian &luid+ !ffect of Standoff  5.2.1 (aminar &lo# in an !ccentric nnulus 5.2.2 Turbulent &lo# in an !ccentric nnulus 5.3 Casing Centrali,ation 5.3.1 T)pes of Centrali,ers 5.3.1.1 T)pes of Centrali,ers 5.3.1.2 T)pes of Centrali,ers 5.4 !"ercise 6 Casing Movement 6.1 Overvie# 6.2 Reciprocation 6.3 Rotation 6.4 !"ercise 7 Incompatibilit) bet#een &luids 7.1 Overvie# 7.2 $iper Plugs 7.2.1 -ottom Plugs 7.2.2 Top Plugs 7.3 &lo# Regimes 7.3.1 Turbulent &lo# Displacement 7.3.2 !ffective (aminar &lo# 7.3.2.1 Rules for the $!((C(!*. techni/ue in effective laminar flo#+ 7.3.2.2 0iscous Spacer for !(&pumping process1 7.4 Chemical $ashes 7.5 Spacers 7.5.1 The Schlumberger &amil) of Spacers 7.5.2 Re/uired Properties of Spacers 7.5.3 M2DP2SH. 3( Concentration Calculation 7.5.4 Composition and &ield Mi"ing Order of Spacers 7.6 !"ercise 8 Record 4eeping

8.1 Record 4eeping 8.2 !"ercise 9 Summar) 10 Test 1

Obe!t"ves In this training module5 )ou #ill learn to do the follo#ing+ # #

# # # #

#

Identif) the three steps in the mud removal process1 Demonstrate a 'no#ledge of the processes re/uired in mud removal to accomplish support and protection of the casing string1 Identif) and demonstrate a 'no#ledge of the 6 criteria for effective mud removal1 Demonstrate 'no#ledge of casing standoff and its importance in mud removal1 Demonstrate 'no#ledge of the application of casing movement in mud removal1 Demonstrate 'no#ledge of fluid incompatibilit) considerations and their effects on mud removal1 Demonstrate 'no#ledge of appropriate record 'eeping considerations in the achievement of successful mud removal and displacement1

2

$ntrodu!t"on Mud removal is the most important activit) to accomplish the main goal of primar) cementing 77 complete %onal "solat"on in "solat"on in the #ellbore1 The cement that replaces the drilling mud provides support and protection to the casing string1 Contamination and channeling of the cement occur if the drilling mud is not effectivel) removed1 *o slurr) can achieve ,onal isolation under these conditions1 &or these reasons5 effective mud removal falls mainl) under the cementing operation1

3

T&e T&ree 'te( )ro!ess of Mud Removal The three7step process of mud removal includes the follo#ing+ 1. 2. 3.

Clean the hole 8remove cuttings9 Condition the drilling fluid 8to achieve the lo#est possible rheolog)9 Displace the drilling fluid 8flush9 from the annulus

3.1

*ole Clean"n+ Hole cleaning occurs during drilling1 The p rocess includes controlling and optimi,ing mud properties to manage the hole and 'eep it in gauge 8uniform in si,e and shape91 More than :6; of the hole volume must be in circulation to 'eep a hole clean1 $hen less than :6; of the hole volume is in circulation+ # #

mud has gelled in #ashed out areas5 or there is a build up of cuttings some#here in the hole1

Running a !al"(er lo+ is an important step1 The caliper log is a record of the diameter of the #ellbore that indicates # #

undue enlargement due to cave7in5 #ashout5 or o ther causes5 and corrosion5 scaling5 or pitting inside tubular goods1

 caliper log of the hole is run to # # #

determine hole volume to compare #ith the volume of fluid in circulation5 identif) possible problem #ashed7out ,ones from the collected data5 and calculate the correct hole volume for slurr) re/uirements from the caliper measurements1

 ,"(er tr"( is the process of running the drillstring in and out of the hole o r to the previous casing shoe #ithout changing the drill bit1 $iper trips are performed at regular intervals to ensure that+ # #

the hole is being completel) emptied of the drilled cuttings5 and the formations are being controlled1

3.2

Cond"t"on"n+ Mud The second step in effective mud removal is conditioning the drilling fluid on the last #iper trip  just before runn"n+ t&e !as"n+1 In some cases5 conditioning is done #hen the casing is on the bottom1 Drilling fluid is conditioned to+ # #

reduce gel strength5 reduce or optimi,e the -"eld (o"nt 8the resistance to initial fluid flo#9 and (last"! v"s!os"t- 8the tangential shearing force in e"cess of the )ield point value re/uired to

induce a unit rate of shear95 # #

reduce solids in the fluid 8cuttings9 to less than <; of the total volume5 and achieve mud flo# all around the casing1 CemCD!. soft#are is used to calculate the M"n"mum )ressure rad"ent to determine the minimum rate to achieve this flo#1

3.3

/"s(la!"n+ Mud from t&e nnulus

The third and final step in the process of mud removal is to displace the mud from the annulus and replace it b) the injection of cement slurr)1 CemCD!. soft#are is used to ma'e the necessar) calculations to optimi,e slurr) placement1 Cas"n+ standoff  is the ratio of the smallest annular gap to the average annular gap bet#een t#o pipes #ith different diameters if one is completel) centered #ithin the other1 Casing standoff should be over =6; before cement is pumped1 -oth re!"(ro!at"on 8up and do#n motion9 and rotat"on 8circular motion around an a"is9 of the casing should be ta'en into account and allo#ed for5 if possible1 If reciprocation occurs5 as the casing moves up5 the relative pump rate in the annulus decreases 8s#ab pressure9> as the casing moves do#n5 the relative pump rate increases 8surge pressure91 This effect can create pressures different from those calculated for stationar) casing1 CemCD!. soft#are calculates the ma"imum allo#able up and do#n stro'e time1 Ta'e these stro'e times into account #hen ma'ing recommendations to the customer1 3.4

er!"se The Three Step Process of Mud Removal !"ercise

4

Cr"ter"a for ffe!t"ve Mud Removal Drilling the hole correctl) #ith good drilling fluid properties and mud removal is of primar) importance to complete an "n +au+e and stable hole1 4.1

Overv"e, The criteria for effective mud removal are+ #

#

a #ell7centrali,ed casing1  standoff of [email protected]@; is ideal1  minimum standoff of no less than =6; is acceptable1 the casing in motion5 if possible5 throughout the job5 from the start of circulation to the end of displacement1 This motion can be reciprocation or rotation1 If motion is possible5 use s!rat!&ers to help scrape filter ca'e off the #all and to help move an) thic'5 gelled mud1

# #

#

use of both top and bottom #iper plugs1 More than one bottom plug is recommended1 use of (reflus&es 8chemical #ashes and spacers9 to separate the slurries from the drilling fluid and to perform efficient hole cleaning1 The cement 8heavier than mud9 has a tendenc) to channel in the mud in the casing1 The same applies to the mud 7spacer interface1 Cement contamination #hile traveling do#n a casing can be ver) dramatic and can completel) ruin the cement job1 achievement of turbulent flo, for the most effective mud removal1 If turbulent flo# cannot be achieved5 ffe!t"ve am"nar lo, 89 is the second choice1

4.2

T&e $deal ellbore Cas"n+ !ffective mud removal is greatl) influenced b) the geometr) of the #ellbore in #hich the casing is run1 !ffective mud removal is influenced b)+ # # # # # # # # #

angular gap standards sloughing flo# from the formation into the #ellbore flo# effects of temperature mud filterca'e in the #ellbore mud circulation caliper log and mud circulation mud circulation efficienc)

4.2.1

nnular a( 'tandards

The industr) standard for the total gap bet#een the casing and the #ellbore is ?16 in1 The minimum standard annular gap is @1=6 in1 n annular gap of less than @1=6 in1 results in a cement sheath that is too thin and5 therefore5 ver) fragile and that could be shattered b) drilling7induced vibrations or the impact of perforating1 &or e"ample5 the total gap bet#een a : 6AB in1 casing and its standard hole si,e of ?16 in1 is ?1? in1 8?16 in1 7 :1<6 in1 E 1< in1 The annular gap E 1< in1A E ?1?in19 This gap ensures a good sheath of cement around the casing1 The gap bet#een a = in1 casing and its natural hole si,e of B16 in1 is @1=6 in1 8Total gap E B16 in1 7 [email protected] in1 E ?16 in1 nnular gap E ?16 in1A E @1=6 in15 the minimum acceptable si,e19 4.2.2

'lou+&"n+

The open hole should be stable #ith no slou+&"n+1 If the formation is caving in5 roc's from the underground formations could bloc' the flo# of fluids in the annulus #ith disastrous conse/uence for the cement job1 The hole should be as uniform as possible even if it is greater than the drilled hole1  uniform hole can be effectivel) cleaned out5 but caves in the #ellbore contain gelled mud that is not removed b) the spacers5 preventing a good cement job1 4.2.3

lo, from t&e ormat"on $nto t&e ellbore

There should be no flo# from the formation into the #ellbore1 The #ell must be perfectl) under control before cementing is initiated #ith no fluid losses or inta'e1 If there are losses5 part or all of the cement slurr) could be lost1 If formation fluids 8#ater5 oil5 or gas9 are allo#ed to enter the #ellbore during a cement job5 the # # #

cement slurr) #ill be contaminated b) the formation fluids5 the cement #ill not set properl)5 and a blo#7out situation ma) develop1

4.2.4

lo,

The casing is centered in the hole1 The ideal standoff is [email protected]@;1 Schlumberger re/uires that it is not less than =6;1 n ideal standoff ensures that fluids flo# e/uall) all around the casing1 &luid velocit) 8ftAsec or ftAmin9 is al#a)s higher on the #ide side of an eccentric annulus than on the narro# side1 The fluid flo#ing in an eccentric annulus ta'es the path of least resistance and flo#s through the #ide side5 #here friction pressure is reduced1 $ithout careful attention to flo# regime5 a channel of drilling mud can remain in an eccentric annulus after the fluid displacement process is completed1 Such a channel can ma'e the entire cementing job ineffective1 4.2.5

ffe!ts of Tem(erature

ccurate bottom hole static temperature 8 *'T9 and bottom hole circulating temperature 8*CT9 are necessar) to determine accurate placement time of both lead and ta"l slurries1 !rrors in placement time can result in premature setting or over7retardation of the slurr)1 The static temperature at the top of the cement should be higher than the bottom hole circulation temperature 8-HCT9 at total depth to ensure that the cement sets up as /uic'l) at the top as at the bottom1 The result is a more effective cement job1 4.2.6

Mud "lter!ae "n t&e ellbore

The mud f"lter !ae in the #ellbore should be thin and impermeable1 It should not be gelled o r unconsolidated1  thin filter ca'e is not moved b) the fluids passing b) it5 nor does it contaminate the slurr) or greatl) affect the results of the cement job1 %elled or unconsolidated filter ca'e5 ho#ever5 can contaminate the slurr) and result in a poor cement job1 4.2.7

Mud C"r!ulat"on

Ta'e care to ensure that there is no lost circulation into the #ellbore1 (ost circulation can result in the slurr) entering the formation instead of filling the annular gap1 The ideal #ellbore has an in gauge diameter hole1 The closer the hole is to in gauge5 the easier turbulent flo# is to achieve1 The more uniform the hole5 the less volume of fluid is re/uired to complete each phase of mud removal1 The mud is conditioned to ensure that all mud is movable and5 therefore5 can be removed1

4.3

T&e Cal"(er o+ and Mud C"r!ulat"on The entire volume of the hole must be in circulation to effectivel) displace the mud from the casing and the #ellbore1 2se flu"d !al"(ers #ith the caliper log to determine ho# much of the mud in the ho le is in circulation1 If parts of the hole are not in circulation5 then mud is not removed and the cement slurr) b)7passes those sections of the #ellbore1 The caliper log procedure is simple and is performed as often as possible1 The most important step is to run a T lo+ 8bore&ole +eometr- tool or mult"arm !al"(er9 to determine the difference bet#een the actual open hole volume and the total hole volume1 lthough t,oarmed !al"(ers are often used5 fourarmed !al"(ers give a more accurate reading of the hole and should be used #henever possible1

4.4

Mud C"r!ulat"on ff"!"en!fter the caliper log is run and the casing is on bottom5 the #ell is circulated at the e"pected ma"imum cementing rate1 t this time5 mud circulation efficienc) is determined1

The steps in determination of mud circulation efficienc) are #

# #

#

Drop marers or tra!ers at staged intervals during circulation1 The mar'ers can be colored fluids or objects 8li'e rice9 that can be easil) seen #hen the) return to the surface1 Monitor the returning fluids for mar'ers as the #ell is circulated1 Determine the volume of fluid circulated to return each mar'er from the pump rate and time1 This volume should be reasonabl) close to the total hole volume determined b) the caliper log1 Increase the pump rate and repeat the calculations if the circulated volume and caliper log volume are ver) different1 s the pump rate increases5 the volume of fluid should increase1

5

$nfluen!e of Cas"n+ 'tandoff  This section #ill address # #

*e#tonian fluid+ effect of standoff  casing centrali,ation1

5.1

Overv"e,

Casing standoff is an important consideration in mud removal1 In an eccentric annulus5 velocit) differences in fluid flo# bet#een the #ide side and the narro# side occur1 &lo# velocit) is highest in the #ide side of the annulus #here friction pressure is lo#est and is lo#est in the narro# part of the annulus #here friction pressure is higher1

5.2

:e,ton"an lu"d; ffe!t of 'tandoff 

$hen t#o pipes of different diameters are hoo'ed in parallel on a flo# line and fluid is pumped into each from the same source5 the (ressure at both the entrance and e"it of each pipe is the same1 The velo!"t- of the fluid in the smaller pipe is lo#er than that in the larger pipe at a certain pump rate because friction pressure in the smaller pipe is greater than the friction pressure of the larger pipe1 Re)nolds number is the ratio of a fluidFs inertial forces to its viscous forces1 The Re)nolds number has no d imension5 if consistent units are used in its calculation1 Re)nolds number is defined b) the formula+

$here+ # #

D E Dimensions of the flo# channel 0 E verage flo# velocit)

#

E &luid densit) #

E &luid viscosit)

In the ne"t series of visuals5 observe the steps in the calculation of Re)nolds number1 &or e"ample5 #e can use the Re)nolds number to compare t#o different sets of data1

The !onstants and v"s!os"t- are the same for both pipes and cancel out5 leaving a relationship bet#een the diameters and the velocities1 &or t#o pipe si,es 8D? and D95 #here D is t#ice the si,e of D? 8#hich is a close appro"imation of <=; standoff95 the velocit) of the fluid in the larger pipe is four times the velocit) in the smaller one1 The Re)nolds number calculated for the larger pipe is eight times the Re)nolds number calculated for the small pipe1 5.2.1

am"nar lo, "n an !!entr"! nnulus

am"nar flo, is generall) associated #ith lo# rates and orderl) patterns of flo#5 such as found in the annular regions of a #ell bore and #ith fluid movement in uniform la)ers1 In laminar flo#5 the flo# rateAflo# pressure relationship is a function of the viscous properties of the fluid 8ref1 Rheolog) Module91 This graph plots the ratio of the velocit) in the large pipe over the velocit) in the small pipe versus the standoff1  fluid #ith an * value of ? and standoff of [email protected]; has a velocit) in the larger pipe [email protected] times higher than the velocit) in the smaller pipe5 illustrating the importance of having as high a standoff as possible in laminar flo#1 The graph also illustrates that as the fluid deviates from a *e#tonian fluid 8*E?95 the effects of standoff decrease5 but are still ver) important1 &or e"ample5 for a fluid #ith an * value of @1 and a standoff of <@;5 the velocit) ratio bet#een the large diameter pipe and the small diameter pipe is five5 i1e15 the fluid flo#s five times faster in the larger pipe1

5.2.2

Turbulent lo, "n an !!entr"! nnulus Turbulent flo, occurs at high flo# rates and results in erratic5 random flo# patterns1 The flo# rateAflo# pressure relationship is governed b) the inertial forces of the fluid 8ref1 Rheolog) Module91 &or the same fluid in turbulent flo# through the t#o pipes5 the velocit) in the larger pipe is ?1<G times the velocit) in the smaller one5 and the Re)nolds number is 1B times greater1

This graph illustrates the same relationship for turbulent flo# as previousl) illustrated for laminar flo#1 *otice that there is ver) little difference bet#een the fluid characteristics of *e#tonian and non7*e#tonian fluids1 &or an) fluid in turbulent flo#5 the effect of standoff becomes ver) significant #hen standoff is belo# @;1 This is the primar) reason turbulent flo# is the p referred flo# regime1

5.3

Cas"n+ Central"%at"on

s previousl) discussed5 standoff  is the ratio of the smallest annular gap to the average annular gap bet#een t#o pipes #ith different diameters5 if one is completel) centered #ithin the other1 The graph illustrates the ratio of flo# rate necessar) to achieve turbulent flo# in an eccentric annulus compared to the flo# rate necessar) to achieve turbulent flo# in a concentric annulus1 bove =6;5 there is little difference in the flo# rate ratio1 Do#n to 6;5 the flo# rate ratio changes almost linearl) to about five times faster on the #ide side than on the narro# side1 -elo# 6; standoff5 the flo# rate ratio starts to increase e"ponentiall)1 There are t#o #a)s to increase standoff from casing+ # #

Drill perfectl) straight holes1 2se centrali,ers on the casing1

Perfectl) straight holes are rarel) possible or desired5 so centrali,ers on the casing are used e"tensivel)1 5.3.1

T-(es of Central"%ers The three most common t)pes of centrali,ers are 1. 2. 3.

bo# spring 8spiral or straight9 rigid bo# 8or positive9 rigid solid 8slip7on9

o, s(r"n+ !entral"%ers have fle"ible bo#s li'e the leaf springs on old truc's1 The) usuall) have an outside diameter slightl) larger than the diameter of the hole1 5.3.1.1

T-(es of Central"%ers

R"+"d bo, !entral"%ers 8also 'no#n as positive centrali,ers9 have non7fle"ible bo#s and have an outside diameter slightl) smaller than the smallest diameter in the #ell1 Since these rigid bo#s #ill not collapse5 it is important to chec' diameters in the #ell5 or the centrali,er can stic' uphole and prevent the casing from reaching the bottom1 These centrali,ers are t)picall) used # #

inside previous casings5 or in open hole sections that are in gauge 8uniform in si,e and shape91

Rigid bo# centrali,ers are also / uite common in hori,ontal cementing1 The) are used #hen do#n#ard #eight on the pipe #ould create an eccentric annulus if the bo# spring t)pe #ere used1 5.3.1.2

T-(es of Central"%ers

R"+"d sol"d !entral"%ers <also no, as turbol"%ers= are made of a solid material 8usuall) aluminum9 and have outside diameters smaller than the smallest diameter in the hole1 (i'e other rigid centrali,ers5 these centrali,ers are used in+ # # #

previous casings5 in7gauge holes5 or hori,ontal cementing1

The) have the added advantage of causing the fluids to s#irl as the) pass the !entral"%er and ma'e achievement of turbulent flo# easier1

6

Cas"n+ Movement This section #ill address # #

reciprocation and rotation1

6.1

Overv"e, Movement of the casing5 either reciprocating 8up and do#n9 or rotating5 is an effective aid in mud removal1 Satisfactor) results are achieved using movement5 but onl) in combination #ith other good removal practices1 6.2

Re!"(ro!at"on

-ecause it is easiest to perform5 reciprocation 8moving the casing up and do#n9 is the most common t)pe of casing movement1 Reciprocation is measured in c)cles per minute1  c)cle is one up#ard and one do#n#ard stro'e of the pipe or casing1 The length of each stro'e is usuall) bet#een @ feet and [email protected] feet and the duration of the t)pical c)cle is from one to five minutes1 &or reciprocation to be effective5 scratchers are fitted to the casing so that mud filter ca'e is scraped off and gelled mud moved1 To free articles in the ho le before cement is pumped into the annulus5 movement begins #hen circulation starts and continues until the end of displacement1 There are problems associated #ith reciprocation of a casing+ #

#

The casing ma) become stuc' during movement5 usuall) caused b) the casing ending up in the #rong location do#n hole1 Other limiting factors are the surge and s#ab pressures generated during casing movement1

'ur+e (ressure is created during a do#n stro'e1 Surge pressure increases pressure #ithin the hole and can cause formations to fracture1 ',ab (ressure is created b) the up#ard stro'e1 S#ab pressure reduces pressure in the #ellbore and can cause a drop in pressure resulting in a 'ic' or blo#out1 In addition5 the #eight of the casing during movement can result in e"cessive pull on or buc'ling of the casing5 either of #hich could lead to casing failure1 6.3

Rotat"on

Rotation is another casing movement1 In rotation the casing is turned5 #hich causes the gel of the mud to b rea' and increases mud mobilit) around the casing1 s in reciprocation5 rotation starts at the beginning of circulation and continues until displacement is complete1 This ensures that an) filter ca'e5 gelled mud5 or other debris is removed before cement is placed in the casing or annulus1 Rotation is measured in revolutions per minute 8 rpm91 T)picall)5 the casing is rotated bet#een [email protected] and [email protected] rpm1 Close monitoring of tor/ue is ver) important5 since the rotational motion can cause the casing to t#ist off and to be lost do#n hole1 Scratchers improve the efficienc) of rotation but are less necessar) in rotation than in reciprocation1 Some centrali,ers are fitted to aid the s#irling movement of the mud5 ma'ing scratchers even less necessar)1 Rotation re/uires special surface e/uipment5 including cement head s#ivels5 po#er s#ivels to turn the casing5 and a drill collar5 among others1 6.4

er!"se Casing Movement !"ercise

7

$n!om(at"b"l"t- bet,een lu"ds This section on incompatibilit) bet#een fluids #ill address+ # # # #

#iper plugs flo# regimes chemical #ashes spacers

7.1

Overv"e, Mi"ing a displacing fluid #ith a displaced fluid 8e1g15 cement slurr) and a drilling fluid9 can lead to a complete failure in mud removal and ,onal isolation1 This mi"ture potentiall) results in detrimental reactions of the t#o fluids5 causing a change in their rheolog)1 These rheolog) changes can result in high viscosit) or gel strength1 Slurr) propert) variations include+ #

thic'ening time5

# #

fluid loss5 and Compressive strength1

ll of these potentiall) result in the loss of a h)draulic bond and failure of the cement job1 Contamination of the different fluids is prevented b) using ,"(er (lu+s 8both top and bottom9 to separate them in the casing1 In addition5 !&em"!al ,as&es and s(a!ers are used to separate fluids in the casing and the annulus1 &inall)5 !om(at"b"l"t- tests are run to evaluate the impact of contamination of the cement b) the mud1

7.2

"(er )lu+s $iper plugs are used to 'eep the fluids separated #hile the) are inside the casing1 $iper plugs are of t#o t)pes+ # #

bottom plug top plugs

7.2.1

ottom )lu+s

-ottom plugs are used to remove the mud that is ahead of the preflushes and cement1 The) prevent cement and spacers from falling through the lighter fluids ahead of them1 This occurrence is particularl) serious in large casings1 dditionall)5 bottom plugs #ipe the casing #all clean from mud ca'e5 scale5 rust5 and other debris1 Debris is pushed ahead of the bottom plug and out of the casing1 If a bottom plug is not run5 this debris is pushed ahead of the top plug1 s a result5 the debris is pushed into the casing bet#een the float collar and the float shoe 8i1e15 the shoe trac'9 and possibl) out into the formation1 s the bottom plug moves into these areas5 it displaces the cement ahead of it and ma'es a good cement job ver) difficult5 if not impossible1

7.2.2

To( )lu+s

Top plugs are used to separate the cement from the displacing fluid5 #hich is usuall) drilling fluid1 Top plugs also provide a positive indication of the end of displacement1 $hen the top plug stops do#n hole5 an increase in pressure is recorded on the surface and displacement is complete1 This pressure increase is called bumping the plug1

7.3

lo, Re+"mes Turbulent flo# is the best flo# regime to remove drilling fluid5 based on both e"periments and statistics1 $hen formation features and other considerations ma'e it impossible to reach turbulent flo#5 effective laminar flo# is used1 7.3.1

Turbulent lo, /"s(la!ement

&or a fluid to be in turbulent flo#5 it must be pumped above a minimum flo# rate5 i1e15 the !r"t"!al flo, rate. The fluid particles move in a s#irling5 randomi,ed motion5 #hich aids in the removal of a cuttings bed in highl) deviated holes1 The critical flo# rate of a given fluid depends on four factors+ 1.

2.

3.

4.

The rheolog) of the fluid 8i1e15 the thinner the fluid5 the easier it goes into turbulent flo#95 The centrali,ation of the casing or casing standoff 8i1e15 the better centrali,ed the casing5 the easier the fluid goes into turbulence all around the casing95 The annular gap or clearance bet#een the casing and the hole si,e 8i1e15 the smaller the gap5 the easier the fluid #ill go into turbulence95 and The fracture gradient of the formation1

The fracture gradient of the formation is an indirect factor5 but it must be ta'en into account1 If the pressure re/uired to achieve turbulent flo# is higher than the fracture gradient of the formation5 the formation ma) rupture5 and losses of fluid #ill result1

&luids appropriate for use as preflushes are chemical #ashes and M2DP2SH. 3TA3S spacers1 $ater5 diesel oil5 or base oil can also be used1 There are minimum re/uirements for the use of #ashes and spacers1 The re/uirements for the use of #ashes and spacers are+ #

#

#

# #

Contact time in turbulent flo# must be at least [email protected] minutes1 $hen #ell conditions are favorable 8e1g15 the casing can be rotated5 good mud properties5 etc19 contact time can be dropped but never belo# 6 minutes1 ll fluids pumped must be compatible #ith both the drilling fluid in the #ell and the slurries to be pumped1 The cement slurr) properties must be optimi,ed1 In other #ords5 the -"eld (o"nt 8the resistance to initial fluid flo#95 and (last"! v"s!os"t- 8the tangential shearing force in e"cess of the )ield point value re/uired to induce a unit rate of shear95 must be reduced #ithout causing sedimentation or free #ater1 &luid loss should be controlled1 ll fluids must be designed to #ater7#et the casing and formation1

7.3.2

ffe!t"ve am"nar lo,

!ffective laminar flo# 8!(&95 not to be confused #ith normal laminar flo#5 is the alternative flo# regime #hen turbulent flo# is not possible1 (aminar flo# tends to o ccur in the lo# flo# regimes in the annulus> therefore5 most drilling fluids e"hibit laminar flo#1 7.3.2.1

Rules for t&e C:> te!&n"?ue "n effe!t"ve lam"nar flo,; &or a displacement to b e considered in effective laminar flo#5 four conditions must be met to prevent channeling andAor fluid contamination during cement placement1 The four conditions for a displacement to be considered in !(& are+ 1. 2.

3.

4.

The displacing fluid must have a densit) [email protected]; higher than the fluid being displaced1 The Minimum Pressure %radient must be satisfied for all fluid involved 8i1e15 there must be flo# all around the casing91 The displacing fluid must e"ert a friction pressure gradient @; higher than the fluid being displaced1 The d"fferent"al velo!"t- !r"ter"a 8also called the "nterfa!e stab"l"t- !r"ter"a9 should al#a)s be used during job design1 The differential velocit) criterion is easier to achieve #hen standoff is good to ver) good1

Conditions for achieving the differential velocit) criteria include+ #

#

Minimi,ing the velocit) difference bet#een the displaced and displacing fluids to establish a flat interface1 The sum of the gravitational force and the friction force of the displacing fluid in the #ide side of the annulus must be greater than that of the fluid being displaced in the narro# side of the annulus1 Maintaining the annular flo# rate belo# a certain value1 $hen the displacement rate is too high5 the displacing fluid tends to b)pass the fluid to be displaced rather than displacing it uniforml) around the annulus1

This standard establishes a ma"imum flo# rate for the placement of a slurr)1 It ensures that no displacing fluid b)passes a displaced fluid through the #ide side of an annulus1 7.3.2.2

@"s!ous '(a!er for  (um("n+ (ro!ess.  viscous spacer5 the M2DP2SH. 3(A3(O5 has been developed to meet the criteria of the !(& regime1 This spacer has an adjustable viscosit) based on changing the concentration of D?G: in the spacer1 $hen pumping 3(A3(O.5 a minimum of [email protected]@ feet in the annulus or <@ bbl must be used> @ to [email protected] bbl of chemical #ash should be used ahead of the spacer to start to disperse the drilling fluid1 The drilling fluid must be conditioned to reduce gel strength and rheolog) and remove solids1 The viscosit) of cement slurries ma) need to be adjusted for the slurr) to follo# the pressure gradient hierarch)1 D?6 is used for this purpose1

7.4

C&em"!al as&es Chemical #ashes are #ater7based fluids #ith lo# viscosities and densities 8much li'e #ater9 and are ver) eas) to get into turbulent flo#1 The) are the preferred fluid for turbulent flo# #ashes1 The chemical #ash is pre7mi"ed in a tan' but can also be mi"ed on short notice b) adding the products to the displacement tan's in the field as the #ater is being added1 There is too much product for C$B or [email protected]? to be added to the displacement tan's as the #ater is being added5 and this procedure is not recommended for these t#o products1 Mi"ing regimen for chemical #ashes )rodu!t C7 C100 C8 C101 C8' C101' $ater

G?16

D?

@16

=r

G?16 G?16 @16

@16

@16

&[email protected]

G?167G?

G?7:1=6

@16

@16

@16

@16 @16

D<@=

G?

@16

@16 @1[email protected]

The mi"ing order for chemical #ashes is+ 1. 2.

begin #ith #ater5 add D?5

@1[email protected]

3. 4.

add = if re/uired and agitate the mi"ture #ell5 and add &[email protected] or D<@= last5 just before pumping fluid into the #ell1

Mi"ing regimen for chemical #ashes )rodu!t

C7 C100 C8 C101 C8' C101'

&luid loss

no

)es

$aterAOil base muds #ater #ater

no

)es

no

)es

oil

oil

oil

oil

$ellbore conditions and mud t)pe can affect the choice of chemical #ash for a given #ellbore environment1

7.5

'(a!ers This section on spacers #ill address # # # #

common Schlumberger spacers re/uired properties of spacers M2DP2SH. 3( concentration calculation composition and field mi"ing order of spacers

7.5.1

T&e '!&lumber+er am"l- of '(a!ers The three most common spacers used b) Schlumberger are+ # # #

M2DP2SH 3T spacer M2DP2SH 3S spacer M2DP2SH 3( spacer

3T is used for turbulent flo# in a #ater7based environment1 3S is used for turbulent flo# in sea #ater or salt#ater environments1 3( is used for effective laminar flo# in either fresh or salt#ater environments1 '(a!er

lo, T-(e

nv"ronment

M2DP2SH. 3T Turbulent

$ater7based

M2DP2SH. 3S Turbulent

Sea or salt#ater

M2DP2SH. 3( !ffective (aminar &resh or salt #ater1

dding surfa!tants to these spacers ma'es them compatible #ith oil7based muds1 $hen surfactants are used5 the spacers are designated M2DP2SH. 3TO5 3SO5 and 3(O spacers1 Schlumberger uses t#o spacers for high7temperature environments1 '(a!er

lo, T-(e

M2DP2SH $HT. Turbulent

nv"ronment High7temperature5 fresh or salt #ater

M2DP2SH 3!O. !ffective (aminar High7temperature5 oil7based

7.5.2

Re?u"red )ro(ert"es of '(a!ers The re/uired properties of spacers are+ # #

#

# #

compatibilit) #ith the drilling fluid and the cement slurries in the #ell5 stabilit) and suspending properties even at high temperatures5 to avoid allo#ing the #eighting agent to drop out of suspension5 controllabilit)5 so the densit) and rheolog) properties are the same in the lab and in the field5 provision of good fluid loss control5 since it #ill be used across permeable pa) ,ones5 and environmentall) safe and eas) to handle in the field1

7.5.3

MA/)A'*> B Con!entrat"on Cal!ulat"on

%raphs from the Cementing Materials Manual similar to the one at left are used to calculate the concentration of D?G: needed to achieve a specific rheolog)1 &or e"ample5 use the [email protected]@7rpm reading on the &** 6 and read the plot to determine the concentration of D?G: needed if the &** 6 measures the mud at <@ and the cement at [email protected] and the desired rheolog) is ?6 ppg 3(1 Since the spacer 8M2DP2SH. 3(9 must be more dense than the mud5 but less dense than the cement5 calculate the average of the t#o &** 6 readings 8<@ and [email protected] and loo' for a &** 6 reading for [email protected]@ on the ?6 ppg plot1 The graph indicates that @ 'g of D?G: to one cubic meter of #ater [email protected] lbAbbl9 is needed1 This is the concentration of D?G: needed to achieve the ?6 ppg rheolog)1 Once the concentration is mi"ed5 the rheolog) of the fluid can be chec'ed both in the lab and in the field1 7.5.4

Com(os"t"on and "eld M""n+ Order of '(a!ers

&or spacers to #or' properl)5 the) must be both blended in the correct concentrations and mi"ed in the correct order1 The field mi"ing order for spacers is 1.

2. 3.

4.

5.

Clean tan's and lines before mi"ing1 Dirt) lines and tan's can contaminate the fluid5 ma'ing it less effective1 dd fresh or brac'ish #ater once the lines and tan's are clean1 Ta'e a one7gallon sample of the #ater before an) other chemicals or additives are mi"ed #ith it1 dd the antifoam agent 8DG= for fresh #ater and D?GG for salt or sea#ater9 at a concentration of @1? to @1 galAbbl after the sample is ta'en1 dd the spacer blend through the hopper and allo# pre7h)drating for @ minutes to one hour1

'(a!er

'(a!er lend

@"s!os"t- C&e! fter

M2DP2SH 3T =16 lbAbbl of D?G=  seconds M2DP2SH 3S 616 lbAbbl of D?GB 6 seconds M2DP2SH 3( 16 lbAbbl of D?G: 6. 7.

8.

9.

Determined b) lab

Chec' the viscosit) of the mi" using a MRSH funnel1 lter the rheolog) of 3( b) adjusting the concentration of the gelling agent5 if necessar)1 dd salt through the ho pper5 if necessar)1 Circulate salt5 if added5 in the mi" #ater for about @ minutes to dissolve cr)stals completel)1 dd necessar) #eighting agents through the hopper after an) added salt has completel) dissolved1 Chec' the densit) of the fluid because the specific gravities of some #eighting agents var)1 8Different #eighting agents are used depending upon the densit) re/uired19

/ens"t- <((+=

e"+&t"n+ +ent

J ??16 ppg

CaCO 8D?6?9

-et#een ??16 and ?< ppg -arite 8D?9 K ?< ppg

  10.

  11.

Hematite 8D=<9

dd surfactants to ma'e the mi" compatible #ith oil7based environments just before pumping1 The t)pe and concentration of the surfactants 8&=65 D<@=5 &=B5 and 2<<9 depend on lab tests and the t)pe of drilling fluid used1 Ta'e a sample of the spacer and chec' for densit) and rheolog) #hen ever)thing has been mi"ed1

8

Re!ord ee("n+ Comprehensive record 'eeping and informed field personnel are important to successful completion of the pre7cementing process1 The mud removal procedures performed for each job are recorded to confirm proper mud removal prior to cementing and to communicate #ith appropriate personnel1 The follo#ing records are 'ept+ # # #

# # #

mud conditioning5 including the rate and time5 the number5 location and t)pe of centrali,ers run on the casing5 motion of the casing during the procedure5 including t)pe 8reciprocating or rotating9 and duration5 times plugs are dropped5 densit) and rheolog) of the spacers5 and the correct volume of preflushes pumped1

8.1

Re!ord ee("n+ s much data as possible must be recorded on the PRISM. laptop1 The minimum data re/uired includes+ # #

#

#

the densities of the pre7flushes5 slurries5 and5 if possible5 the displacing fluid5 all the flo# rates+ 8the displacement pump rate is the most important and usuall) the hardest to record since the displacement is often done #ith the rig pumps1 *evertheless5 ever) effort should be made to get and record this data95 the surface pressure throughout the job1 The surface pressure during displacement can give valuable information if losses occurred do#nhole5 and all notable events during the job5 recorded on the PRISM5 such as changes in pump rate or densit)5 shutdo#ns5 and pea's in pressure5 etc1

This information ma) be helpful in e"plaining post7job occurrences1 ll up7to7date #ell data should be included on the service report5 including the actual casing depth5 etc1 The service technician or field engineer on the rig is in direct contact #ith the client1 He or she needs access to an) information that could ma'e a vital difference in the evaluation or design of a job1 Record all significant information and contact the appropriate service technician or field engineer #hen post7job anal)sis is re/uired1 8.2

er!"se

Record 4eeping !"ercise

9

'ummarTo ensure that drilling fluid has been ade/uatel) conditioned prior to cementing5 the ma"imum pumping rate for the job 8usuall) the displacement rate9 is chosen and at least one hole volume is circulated before the cement job is begun1 The casing is centrali,ed to ensure an optimum standoff1 In no case should the standoff be less than =6;1 $henever possible5 reciprocation or rotation of the casing is performed during mud removal1 Scratchers are used to improve the efficienc) of casing movement in mud removal1 -ottom plugs are used5 if possible5 bet#een each fluid interface to reduce contamination of fluids inside the casing1 Slurr) placement is optimi,ed b) using a turbulent flo# regime5 if possible5 or an effective laminar flo# regime if turbulent flo# is not possible1 Chemical #ashes are used #henever possible1 These fluids are easier to get into turbulent flo# than spacers1 Chemical #ashes are more cost effective and are eas) to prepare and handle1 $hen M2DP2SH. spacers are used5 their properties are controlled5 especiall) densit) and rheolog)1 The cement slurr) properties must also be controlled1 $henever possible5 slurries and spacers are batch7mi"ed to ensure homogenous properties for each fluid1 ll fluids should be lab tested and field chec'ed for compatibilit) to avoid adverse mudAspacerAcement reactions1  /uic' field test can be performed b) slo#l) pouring one fluid into the other1 If5 after slo#l) stirring the mi"5 the fluid thic'ens and cannot be stirred5 the DT! or lab should be consulted1 &inall)5 all #ell data must be correct on arrival on location1 If conditions in the #ell have changed5 the design ma) have to be rerun at the district1 In an) case5 if the #ell has changed5 the field service manager or DT! should be contacted regardless of the time1

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