Note: Descriptions are shown in the official language in which they were submitted.
53~
--1--
LI~TWEIGHT CEMENT SLURRY AND ME~FE[OD OF USE
~he invention is a lightweight aqueous foamed
cemen~ slurry and a method of using same to fill a sub-
terranean void. The slurry is particularly us~ful in
the art of oil and gas well cementing.
.
Cement slurries have ~een foamed as a method
of lightening such slurries fo~ use in oil and gas
wells or~for-subterranean grouting; U.K. Patent
819,229; ~S~E Paper No~ 75 PET-10 (1975). U.S. Patent
3,926,25't suggest~ adding a foaming agent to a cement
slurEy which ~aming agent later collaborates with gas
escapi~g from a gas-containing formation ~o form a foam
barriex to prevent ~ubse~uent migra~ion of gas during
the setting o ~he cement. It is stated ~hat the
foami~g agent may be a nonio~ic, anionic, ox cationic.
The cationic surfact~nt is broadly sta~ed to be "a
guat rnar~ ammonium saltl'. The only embodiment
s~ecifically me~io~ed is a mi~tu~e of an o~ye~hylene
aliphatic compou~d and at least one sulfate of a
poly~thoxy fatty alcohol~. In U.S. Patent 4,300,633 it
28, 850Pa ~F
.
~53~6
--2--
is stated that the foaming agents to be used must be
active in a highly alkaline environment and show
resistance to multivalent organic cations if sea water
is used. Anionic surfactants, specifically organic
alkali metal sulfates or sulfonates are said to ~e
particularly suitable for this purpose.
The present invention basically resides in a
foamed hydraulic cement slurry comprising an aqueous
slurry of hydraulic cement, entrained gas bubbles and a
foaming agent selected from certain specific quaternary
ammonium compound~. The cement slurry is superior to
foamed cemen~ slurries prepared from other quartexnary
ammonium compound~ and from o~her nonionic and anionic
surfactants. When prepared with the specified foamin~
agents, a stable, uniform foam of relatively small
bubble size is rapidly formed in which there is rela-
tively little segregation of ~he liquid, solid and
gaseous compo~ents. Hardened cement prepared from such
slurries in many cases exhibit~-superior streng~h and
low permeability when compared with hardened foamed
ceme~ts prepared utilizing o~her foaming agents. The
invention further compxises a process of cementing
subterranean voids by emplacement of ~his slurry to
give a strong, impermeable seal in applica~ions such as
the grouting of underground bore holes and plugging the
voids created or encountered during ~he drilling of
oil, gas or geothexmal wells. Becaus~ of the light
waight o ~he slurry, the breakdown of weak formation~
duri~g cementing o ~uch sub~erranean voids is avoided.
Foame~ slurries of ~h~ invention exhibit a~
additional ad~antage of maintaini~g their fo~med nature
at temperatures in excess of about 65.6C ~150F), and
28,~50A-F -2~
--3--
even in excess of about 93.3C (200 F). At such
temperatures, foamed cement slurries prepared with
other than the foaming agent of this i.nvention may
begin -to collapse be~ore the slurry has hardened.
The ~oamin~ agent utilized in the instant
invention which permits attainment of these desirable
benefits is a co~position represented by the formula
R (R'-)3 N X (I)
wherein X is chloride, bromide, iod.ide ox hydroxyl, R
is a aliphatic hydrocarbyl radical comprising from 8 to
16 carbon atoms and R' is, independently each occurrence,
an alkyl radical of one to three carbon atoms, or
hydroxyethyl, and the s~m of the carbon atoms in R and
all R' is not ~reater than about 20.
More particularly, ~he invention resides in a
foamed hydraulic cement sluxry comprising a gas-contalning
agu~ous-slurry hydraulic cement and a foaming a~ent
represented by Formula I. The in~ention further resides
i~ a method of plugging a subterranean void by emplacing
the foamed ceme~ slurry in the subterranean void and
allowing the slurry to harden. The foamed slurry is
preexab1y emplaced in a borehole drilled for an oil or
gas well or in the a~nulus created between the borahole
and the casing of such a well. Except for ~he eguipment
adapt~d for ~he generation of the foam, as described
more ~ully heraater, sta~dard oilfield equipment an~
emplacemen~ methods may be utili7!ed to carry out ~he
prscess of the i~vention.
28,850A F -3-
S3~
--4~
The necessary elements of the foamed cement
slurry of the invention are an aqueous slurry of hydraulic
cement, prepared by adding water to a hydraulic cemen-t
component, which preferably comprises Portland cement,
a gas which is entrained as discrete bubbles in said
aqueous cement slurry and a foaming agent represe~ted
by Formula I. The aqueous cement slurry, as described
more fully below is commonly mixed in ~he usual fashion
and thereafter the foaming agent and gas are added to
the slurry in an enclosed conduit.
The most i~portant element of the foamed
sl~rry of the invention is the foaming agent which is a
quaternary ammonium compound represented by Eormula I,
above. In Formula I, R represents a hydrocarbyl group
having from 8 to 16 carbon atoms and preferably from lO
to 14 carbon atoms. R may be saturated ox un~aturated
a~d;is suitably selected rom fatty alkyl groups having
from 8 to 16 carbon atoms. Preferably, R is a s-traight
chain hydrocarbyl radical. More ~refera~ly, R represents
a mixture of hydrocarbyl radicals derived from a vegetable
oil `such as cocoa oil. In the so called "coco" alkyl
group~, the carbon chain length may range from 8 to 16
carbon atoms ~ith chains of lO, 12 and 14 carbon atoms
predominating. Accvrdingly, R may represent a mixture
of such hydrocarbyl groups ra~ging from 8 -to 16 carbon
atoms or may be selected strictly fxom hydrocarbyl
groups of one uniform chain l~ngth, i. 8 . the dodecyl
group~
Rl represents the o~her ~hree hydrocaxbyl
radicals associated with ~he guaternary nitrogen atom.
Suitably, R' is a one ~o ~hree carbon al~yl radical or
a hydro~yethyl radical, independently in eac:h occurrence.
28, ~50~ 4 ~
3~
--5--
Preferably, it is an ethyl or methyl group, or is a
hydroxyethyl group in up to two occurrences. More
preferably, R' is hydroxyethyl in up two occurrences or
is methyl. The sum of all the carbon atoms in the R
and R' groups associated with one quaternary nitrogen
atom is suitably at least 12, preferably a-t least 13,
and more preferably at least 14 carbon atoms and ~he
sum is 20 or less, preferably l9 or les~ and more
preferably 18 or less total carbon atoms.
In Formula I, ~ represents a chlorid~, bromide,
iodide or hydroxyl radical; preferably it represents a
chloride or hydroxyl radical and more pre~era~ly a
chloride radical. Quaternary ammonium compounds of
Formula I are widely known or may readily be prepared
by known methods, for example by the reaction of an
alkyl halide o~ the Formula RX with a ~ertiary amine of
the formula (R'-)3 N under known conditions. In som
instances, where the foaming agent of Formula I exhibits
limited solubility in water, a cosolvent such as
2 0 iso~propanol may be added to I to permit full solubil -
i~ation in the aqueous portion of the hydraulic cement
slurxy~ The addition of other nonionic surfactants in
conjunction wi~h the foaming agent of Formula I may
serve to enhance ~he stability of ~he foam in the
aqueous cement slurry. Matexials which are known foam
stabilizers, such as low molecular weight polypropylene
glycols of from 200 to 6CQ molecular weight, may suitably
be used in addition to ~he agen~ of ~ormula I. Certai~
fa~y amine o~ides also ser~e in thi~ fa h.ion. For
e~ample, bis(2-hydroxyethyl) cocoamine oxide combined
wi~h ~he foaming agenk o~ I serves to fo~m a uniorm,
skable foam i~ ~he agueous hyd~aulic cement slurry.
Likewise, e~Ao~ylated derivatives of the quaternary
28,850A-F -5~
9S3~
-6- ~
compound of Formula I formed by the addition of several
moles of ethyle~e oxide to a compound of Forumla I
where R' is hydroxyethyl in one or two occurrences, is
a suitable adjunct to be used in combination with the
compound of Formula I. For example, a compound prepared
by the addition of about 15 moles of ethylene oxide to
cocoamine and 6ubsequent reaction with methyl chloride
serves to enhance the foam st~bility of aqueous cement
slurries containing the foaming agent of Formula I.
The quantity of the foaming agent I to be
added to the aqueou~ hydraulic cement is sufficient to
permit the major portion of the gas to r~main entrained
as discrete bubbles in ~he slurry until the slurry has
hardened suficiently so that coalescence or migration
of bubbles is no longer possible. This ~uantl-ty will
vary accordin~ to the other component~ and properties
af this slurry and may be determined for a slurry by
first measuring the API Standard ~PlOB "thickening
time" of the slur.ry a~d then empirically determining
~h~ quan~ity of foaming age~t needed to maintain the
desired stable foam~d slurry for that time under the
cond.itions of temperature and pressure wh~ch will be
encountered by the cemen-t slurry after it is emplaced.
To be certain ~hat the stability of the foamed slurry
is maintained, it is preferable to add excess foaming
agenk rather than too lit.tle. Commonly, by volume, at
least ~bout 0.1 part foaming agent per 100 parts of
water, preferably at least about 0.3 part and more
preferably at lea~t about 0.5 part foaming agent will
be ade~uate. Normally, about 3 parts, more preferably
~o mare ~ha~ about 2 part~- a~d more preferably no more
~han about 1.5 part fo~ming agent per 100 parts water
will b~ adequate to serve the i~ended puxpose. ~owever
28,850A-F ~6-
--7--
as noted above, excess may be added without adversely
affecting the resul-ting foamed slurry.
As is readily apparent, ~he foamed cement
slur~y will compxise solids, liquids (primarily water~,
c~nd the entrained gas. The solid components of the
cement slurry will primarily compri~e the hydraulic
cement component. Preferably this comprises a Portland
cement and more preferably is selected from the categories
of cements of Class A, C or G, set forth in Standards
of ~he American Petroleum Institute tAPI) for use in
oil and gas well cementing. In addition ~o Portland
cements, cements known as high alumina cements are also
useful for ~his purpose.
- Powdered anhydrous sodium silicate is another
solid component which may be added to the aqueous
ceme~t slurry pre~ercibly in an amount of from 0.5 to 3
parts, by weight, per 100 parts o ~he hydraulic cement
component. Other solid additives commonly inco.rporated
i~ oil well ceme~t slurries may likewise be added so
long as they do not ad~ersely affect the guality and
stability of the foc~m. Such additives include fluid
loss control agents, retarders, accelerators, extenders
or fillers such a~ fly ash or po7~zolans ~ finely divided
~ilica as silica flour, sodium chloride, calcium chloride
or sulfate, and the like. Thickeners such as bentonite
a~d attapulgite, HEC, and the like are gener211y not
employad in ~he inva~tion slurries. Similarly, si~ce a
lightwe.ight slurry is desired, hematite, barite or
other such weighting age~ts are ge~erally no~ added.
3~ Also, dispersing age~ts generally ~end to degrade or
destabilize ~he foam or cause separatio~ of the liquid
in solid phases, ~d for ~his reason will ordi~arily
no~ be employed.
28,850A-F ~7
53~j
--8--
Overall, the solids included in the foamed
hydraulic cement slurry ~omprise, by volume, typically
about 10 preferably at least about 15 up to about 35,
preferably up to about 30 percent of the total o~med
- 5 slurry.
The second major portion of the foamed cemen~
slurry is the portion made up of liquid components.
Naturally, the major portion of such liquids is water
but may fur~her comprise liquid versions of the additives
lQ previously mentioned above such as fluid los~ control
age~ts, retarders, accelerators and sodium silicate
solutions. The water may be relatively fresh water or
may be an aqueous ~rine containing calcium chloride or
sodium chloride, commonly produced from underground
lS formation in oil and gas production or seawater.
Preerab1y, the water to be employed is relatively
fresh and lacking in dissolved mineral components.
Typically, the liguid components will make up about 25,
preferably at least about 30 percent up to about 60,
20 pre~erably up to about 55 percent of the total volume
of ~he foamed slurry~
~ he gaseous portion of the foamed cement
slurry is preferably added aæ a gas or mixture of gases
to the preformed aqueous hydraulic ceme~t slurry which
already contains the foaming agent. Alte.rnatively, the
gas may be generated 1~ situ by ~he chemical reaction
of an active metal such as aluminum or mag~esiwTI with
the ~trongly basic cement slurry to ge~erate hydrogen
gas. However, because o~ the explo~ive na~ure of hydrogen,
such age~ts ar~- moxe ~uitably replacad by synthetic gas
blowing a~ents such as are employed in ~he pla~tîc ~oam
genex~ting arts. ~uitably, orga~ic nitrogen containing
28,850A~F ~8-
~ 3~
compounds which generate gaseous nitrogen when decomposed
are preferred as ln situ gas-generating agents. However,
because of the general availabillty of compressed air,
nitrogen and carbon dioxide in the oilfield, the addition
of one or more of these gases to ~n aqueous hydraulic
slurry containing the foaming agent is the method of
choice in generating the foamed cement slurry of the
invention. More prefexably, nitrogen and/or air are so
employed because of ~heir lower solubility in aqueous
solutions than carbon dioxide.
Gas suitably comprises, by volume, abou-t 20
percent, preferably at least abou~ 25 percent up to
about 55 percent, preferably up to about 50 percent of
the foamed slurry. When referred to herein, the volume
1~ o~ gas or of foamed slurry means that volume of ga~ or
foam under the conditions at which the slurry will be
u~ed, e. g. pressure and temperature encountered in a
subterr~nean v~id at which ~he foamed slurry is emplaced.
. This generally can be de~ermined with sufficient accuracy
from ~he ideal gas equation, i.e.
V2 - PlVlT2
P 2Tl
It is commonly available from tables used by those who
pump nitrogen or carbon dioxide in oil and gasfield
opera~ions. V2 represents the volume o~ ~he gas under
downhole temperature and pr ssure condi~ions, T2 and
P~, and ~lVlT~ represent the pressure, volume and
tem~erature of ~he gas in ~ e foamed slurry upon
preparation a~ the surfac~.
2~,850A-F 9-
~ ~53~
--10--
An "average diameter" of bubbles entrained in
the hardened foamed slurry may be roughly approximated
by cutting a statistically significant number of vertical
cross~sectio~s through the hardened slurry, measuring
the visible bubbles' diameters, averaging the sum of
these measured diameters and then doubling that number.
A more accurate calculation of the average diameter may
be obtained by multiplying the average of the measured
diameters by 4/~4; i.e., by 1.27. The average diameter
of the major portion of the entrained gas bubbles is
preferably less ~han about 1.0 mm, more preferably less
than aboutØ5 mm and most preferably less than about
O.3: mm When employing the preferred foaming agents
o the inve~tion, the foam produced by strongly shearing
- 15 the gas-containing hydraulic cement slurry will be a
highly uniform foam with bubbles of a described average
diameter and with only small amounts of bubbles having
a.diameter greater ~han l mm.
The foamed slurxies of the invention are
suitably prepared by first mixing the base aqueous
: hydraulic cement slurry with any standard ceme~t blending~
equipment, such as a paddle mixing tank or a venturi type
cement slurry mixer. The means for preparing the
slurxy is not a critical eleme~t of tha instant inve~tion.
Once the slurry is prepared, it is suit~bly
moved by a transfer mea~s into an enclosed conduit
sultable for tra~sporting fluids. Oilfield ~reati~g
pipe can easily serve as suc~ a con~uit. The slurry
~ransfer means can be a co~mon hydraulic pump such as a
trip}e cyli~der positive displac~ment pump commo~ly
known as a "~riplexll pump. This pump i~ widely u~ed in
~he oilfield. The transfer means is ~o~ critical as
28,~50A F 10-
.
long as it has the abili-ty to transport a liguid/solid
slurry with suitable velocity and a centrifugal pump
may likewise be employed for this purpose. To the
slurry i~ the conduit is added the foaming agent which
may be injected into the slurry-carrying conduit by
means of a small liquid blending pump connected to the
conduit by a "tee" connection or a "Y~bend" connection
in a suitable fashion. It is not advisable to add the
foaming agent to the slurry upstre~m from the slurry
transfer means, e.g. in the mixing apparatus, since
addition at that point may cause air to be entraine~ in
the sl~rry making it difficult to accurately measure
the ~mount of gas entrained and difficult for all but
especially designed pumps to handle such a foamed
mi~ture. Such pro~lems in handling may be avoided by
adding the foaming agent directly at the suction o~ a
pump utilized a~ a t~an~fer means or immediately down-
stream from the transfer means to the conduit containing
the aqueous slurry and foaming agent. The gas is then
20 suitably added at a given rate to obtain the desirPd
gas.liquid:solids proportio~s for the intended application.
Ordinarily, for oilfield applications, sufficent gas is
added to obtain a resulting foamed slurry of the density
o~ from 600 to 1560 kg/m3 (5 to 13 pounds per gallon).
Pumping rates o~ either the slurry or of ~he gas may be
adjusted so that:the desired ratio is obtained.
The foamed slurry will be genexated at the
point o injectio~ of the gas and injection may be
accompli~hed by a "tee" connection to generate suitable
turbulence at ~he poi~t o~ mixing of ~he ga~ and ~he
aqueous slurry. ~owever, it is prefer~ble to ~ur~h~r
shear ~he gas~containing slurry to ob~ain a smooth,
u~iform, small bubble size foam by dividing the condu1-t
28,850A-F ~11
-12-
into two separate streams and then rejoining the thus
divided streams in a mixing chamber by impacting the
streams against one another at a common in-line focal
point. This may be accomplished by forcing the two
streams throush orifices in a generally opposed fashion.
Such a dividing o the slurry stream may be accomplished
by a tee in the conduit, piping from the tee through
two separate conduits which are then reco~nected at
ano~her tee to cause the two opposing streams to mix at
the junction of the tee. The resulting fo~med slurry
is then taken off as a single combined stream again.
Thi~ combined stream then is ~onducted through the well
bore tubing or the annulus into the subterranean void
in ~he fashion well known in th~ oilfield for cementing
subterranean voids and boreholes. After being pexmitted
to harden in the subterranean void, the hardened foamed
cemen~ slurry often has low permeability and high
strength xelative to other eguivalent density cements
prepaxed from slurries other than that of the ins~ant
- 20 invention. While this is the preferred me~hod of
preparing the foamed slurry, o~her high shear mixing
devices may be used for this purpose.
Example 1
By way of 0~ample, the following embodiments
of foamed slurries of the i~vention are prepared. The
cement component is an ~PI Class H cement having a
surface area of about 2500 cm2/g (b~ SediGraph 5000).
The "extender" in hal of the embodiments is a fly ash
(LaDue) having a surface area (also by SediGxaph) of
about 3500 ~m2/g, A~h~drous sodium metasilica~e i~
pre~ent in each slurry at a level o~ about one percen~,
based on weight of the c~ment component ("BWOC~).
Foaming agent of ~ormula ~I) is present at about 0.5-3
28,850~ 12-
5~
-13-
percent, based on volume of water. Gas-is entrained in
the foaming agent-contairling slurry prepared from the
dry components and water. Resultant foamed slurry
densities are described in ~he ~ollowing Table I.
T~ LE I
Percent Percent D~nsity
BY Volume of Slurry Extender of Foamed Slurry
Gas Water Solids BWOC in k~_3 (lb~ga]~
A 56 24.6 19.4 --- 852 (7.1)
~ 56 24.6 19.4 50 768 (6.4)
B 56 32.5 11.5 --- 684 (5.7~
BB 56 32.5 11.5 50 636 (5~3)
C 50 28 22 -~- 972 (8.1)
CC 50 28 . ~2 50 8~4 (7.2)
D 50 37 13 --- 780 (6.5)
DD 50 37 13 50 720 (6.0)
E 40 33.6 26.4 --- 1164 (9.7)
EE 40 33.6 26.4 50 1044 (8.7)
E 4Q 44.4 ~ 15.6 - 936 (7.8)
FF 40 44.4 15.6 50 864 (7.23
G 30 39.2 30.8 --- 135~ (11.3~
GG 30 39.2 30.B 50 1224 (10.2)
51.8 18.2 ~-- 10~2 (9.1~
~ 30 51.8 1~.2 50 1008 (~.4)
I 20 44.8 35.2 ___ 1560 (13.0)
II 20 44.8 35.2 50 1392 (11.6)
J 20 59.2 20.8 ~-- 12~8 (10.4)
JJ 20 59.2 20.8 50 115~ ~9.6)
Example 2
Utilizi~g the basic slurxy of Example lAA,
various surfactants are sub~tituted as the foaming
ag0nt. The base ~lurry is mixed in a Waring blender at
hi~gh speed, with a special screw-on lid for th~ ble~der,
~he lid having a s~all 1.9 cm (3/4 i~ch) hole in its
cent~r~ After ~h~ ~olids/w~ter blend is mi~ed or about
30 second~ to form a homoge~eous slurry, about 1~2
perce~t, b~sed on volume o~ water~ ~"BVOW") of a solution
of foaming agent is injected ~hrough ~he hole in ~he
13
28,850A-F
.~L ~ 3 5 3L'l~
-14-
lid. The most pre~erred oaming agents of the invention
generate an immediate foam which fills the blender
chamber in about 2-5 seconds with a small bubble,
uniform foam, having an averaye bubble diameter of 0.5
mm or less. The fo~m wh.en permitted to stand for 15-20
minutes remains unifoLm and stable without appreciable
collapse or defoaming. When allowed to cure, the
resultant hardened foamed cement often has permeability
of about 0.1 millidarcy or less.
When less preferred foaming agents of the
i~vention are substituted, from 5 20 seconds may be
required to fill the chamber a~ter i~troduction of the
foaming agent. The resultant foam has relatively
uniform, small bubbles of an average diameter of 1 mm
or less. There may be a small number of noticeably
larger bubbles. The foam is fairly stable without
si~nificant collapse in 15-~0 minutes. When allowed to
harden, th~ resultant oamed cement often has a per-
me~bility on ~he order of from 0.2 to 1 millidarcy and
a cro~section of the hardened foamed cement will show
only minor gravity segr~ga~ion of solids and gas bubbles.
~ hen other surfactants outside the limits o~
Fonmula I are employed as the foaming agent, the chamber
may not fully fill even after l 2 mi~utes of shearing,
25 may reguire significantly mor than 3 percent ~BVOW)
fc)~ni~ ager~t to fully fill the chamber wi~ foam, if
at all, aIld ma~ exhibit substantial foam collapse after
standing 15-20 minutes~ ~he foam ma~ contain a signi
ant portion of b~bles having average diame~er
30 qreater than o~e mm and the hardened resultant cement,
if remain~ng foamed at all, may exhibit sevexe segreg~-tlor
and stratiie::ation of bubbles and solids resulting in
28, 850P~F ~14;
~553~
highly permeable portions in the upper part and dense,
impermeable portions in the lower part.
Example 3
A cement slurry is prepared from a 35:65
(weight) pozzolan:Class G cement blend with sufficient
water to make about a 1680 kg/m3 (14 pound per gallon)
slurry. About 59.2 m3 (370 barrels) of the slurry is
prepared with the addition of about 1 percent of
anhydrous sodium metasilicate, about 18 percent sodium
chlsride and about 0.7 percent of a calcium lignosulfonat~
re~arder (all BWOC). To ~he slurry so prepared is
added abou~ 1.5 percent (BVOW) of a foaming agPnt
comprising ~by vol-ume) about 3 parts trimethylcocoammonium
chloride and about 1 part bis(2-hydro~yethyl) cocoamine
o~ide, in about an equal volume o iso-propanol. To
this 59.2 m3 (370 barxels) of slurry co~taining -the
~oamin~ agent is added about 3500 m3 (125,000 standard
cubic f~e~) of nitrogen by iniecting through a "tee" to
the line containing the slurry. The lin~ is thereafter
~0 split into 2 lines at the "tee" and ~he 2 line~ are
later reco~binad at another "tee" through a choke
device in each of khe lines causing the 2 slurry streams
to impact with co~siderable kurbulence and good mi~ing,
as described ~arlier. The rate of nitrogen addition to
the slurry i5 varied, in a staged fashion, throughout
wi~h more being added to slurry in~ended for the bottQm
o the hole a~d less to slurry added to ~he top. In
~his a~hion, a slurry of relatively uniform densi~y o
about 1260 kgJm3 (10.5 ppg~ rom top to bottom of the
hole i5 obtained. The cem~nt slurries are used to
ceme~t a fi~e and o~e-half inch ca~ing to a depth of
abou~ 2490 m (~300 feet) in a previously ~rilled borehole.
~bout 8 m3 (50 barrels3 of a gelled æpacer ~lui~ is
28,250A~F -15
~ 6-
first pumped into -the casing followed by about 4.8 m3
(30 barrels) of an aqueous surfactant wash solution.
Then about 3.84 m3 (24 barrels) of the 1680 kg/m3 (14
ppg) base slurry follows ater which the remaining
59.2 m3 (370 barrels) of slurry to which the nitrogen
has been added are pumped, followed by about 14.4 m3
(90 barrels) of a 36:65 pozzolan:Class G tail-in slurry
made up to about 1800 kg/m3 (15 ppg). About 30.4 m3
(190 barrels) of salt water is then pumped until returns
of the spacer fluid and chemical wash are seen. The
well bore is then shut in and the cement slurries are
permitted to set. In ~his manner, a "long-string"
casing job is completed in one single pumping operation
without ha~ing to "stage" the different cement slurries
over the desired intervals o the borehole.
..
Instead of varyin~ the gas addition, the yas
may be metered into ~he slurry at abou~ a constan-t
rate. The resulting slurry will be of greater density
near -the bottom of ~he wellbore when in place and of
lesser density near ~he tQp. The density of the foamed
slurr~ being pumped into the wellbore can be monitored
by an instrument commonly used in oilfield service
called a densiometer.
28,850A~F 16
