Note: Descriptions are shown in the official language in which they were submitted.
WELL TREATING PROCESS AND COMPOSITIO~
The i.nvention pertains to a process for
treating subterranean zones, particularly oil and gas
wells where elevated temperatures are encountered, with
aqueous hydraulic cement slurries and a composition
comprising hydraulic cement and a particular set retara-
er, which composition is useful in this method.
Hydrophobic-substituted phosphonic or phos-
phinic acids and their alkali metal salts have been
used in cements, primarily soil/cement mixtures, to
improve the freeze-thaw properties and salt-resistance.
Six- to.eighteen-carbon alkyl phosphonic acids or their
alkali metal salts are so described in U. S. Patent
3,794,50~ (1974). A plugging mixture for high~tempera-
ture oil and gas wells comprising Portland cement and
l-hydroxy ethylidene-phosphonic acid tri~odium or tri-
po.tassium salts as set time extenders is described in
Derwent abstract 71376B/39 (1979) of USSR Patent 640,019.
The use of tnese phosphonate salts at -temperatures of
75to 150C in amounts of 0.1-0.~% by weight is describ-
ed in the abstract.
28,034A-F -1-
-2~
.
A phosphonobutane tricarboxylic acid, speci-
fically 2-phosphonobutane tricarboxylic acid-1,2,4, is
described as a turbulence~inducing additive and flow-
-property improver for cementing compositions useful in
well cementing operations in U.S. Patents 3,964,921
(1976) and 4,040,854 (1977). A pyrolysis product of a
bis(alkylene pyrophosphate) and urea is described in
U.S. Patent 3,409,080 (1968) to be useful as a turbu-
lence inducer for cementing compositions employed in
oil field cementing operations.
Alkylene diphosphonic acids and their water
soluble salts are described in U.S. Patent 4,225,361
(1980) as set time extenders and water reduclng agents
for g~psum plasters. Lignins which have been
phosphonoalkylated through an ether linkage or corres-
ponding sulfonates, sulfides, hydroxyl or amine deriva-
tives are taught to be useful primarily as dispersants
or surfactants in U.S. Patent 3,865,803 (1975). They
are also said to be useful as "cement additives" without
further explanation.
In U.S. Patent 4,066,469 (1978) ultra-rapid
hardening Portland cement compositions are described
which contain various acid salt additives. It states
that use of acid phosphates as the acid salt additives
is excluded since the phosphates have a characteristic-
ally powerful retarding property peculiar to them.
The present invention resides in a hardenable
aqueous slurry for emplacement in subterranean zones
comprising a hydraulic cement, sodium borate, and,
as a set retarder, an organic compound bearing one or
more amino~N-((alkylidene) phosphonic acid) groups or
28,034A-F -2-
-3~
a salt thereof, wherein the said retarder is present
in an amount of 0.01 to 5 parts by weight per 100
parts by weight cement.
The present invention also resides in a
process for treating a subterranean zone by emplacing
therein the hardenable, aqueous slurry set forth
hereinabove and thereafter permitting said slurry to
harden.
The invention further resides in a composition
for plugging a subterranean zone at an elevated temperature
when slurried with water to form an aqueous, pumpable
slurry thereof, said composition comprising 100 parts
by weight of hydraulic cement, sodium borate in an
amount of from 0.1 to 10 parts by weight per part of
said set retarder, and from 0.01 to 5 parts by weight
per 100 parts of hydraulic cement, of a set retarding
composition and comprising a compound represented by
the formula R(N(X)-W)y wherein:
R represent W or an organic substrate capable
of bearing or comprising up to at least y amine
moieties;
X, independently each occurrence, represents
hydrogen, hydroxyalkyl, hydrocarbyl, W or a bond
to R;
W, independently each occurrence, represents
the moiety ~ P(O)-(OH)2, wherein
-R'- represents an alkylidene group
comprising one to thr~e carbon atoms;
and
y represents an integer of one or more, or
a salt thereof.
28,034A-F -3-
-3a~
The process is especially useful in cementing
of subterranean oil or gas wells where a quantity of
the set retarder is present in the claimed composition
sufficient to delay sign.ificant hardening of the slurry,
at an elevated temperature, until emplacement of the
slurry is substantially complete. The process and
composition are particularly useful because the set
retarder is compatible with other cement slurry
additives such as fluid loss agents and other retarders
and particularly because the retarder is effective
at elevated temperaturesO
28,034A F -3a-
`\ -4~
One particularly useful and unexpected prop-
erty of the retarder is that it may be employed in
hydraulic cement slurries which comprise Portland
cement which has a relatively high content of dicalcium
silicate ~represented by C2S). Such Portland cement
has become more prevalent as attempts to save energy by
burning the cement clinker at lower temperature-s have
been pursued by cement manuEacturers. This change in
Portland cement manufactur1ng can cause the -tricalcium
silicat~ ~represented as C3S) content of such cements
to be reduced and the C2S content to be elevated. The
composition of the invention is particularly useful in
aqueous slurries of Portland cement having a C2S:C3S
ratio which exceeds about 0.4:1 and preferably where
the ratio exceeds about 0.45:1.
Especially effective for use in the composi-
tion and process of the present invention is a set
retarder which is a partial calcium salt of the amino
-N-((alkylidene)phosphonic acid) organic compound. When
such calcium salts are employed as the set retarder in
the instant invention compounds, the resultant aqueous
cement slurry exhibits lower fluid loss properties when
tested by standard American Petroleum Institute (hereafter
API) methods than the corresponding acid or sodium or
po-tassium salt analogues. For -this reason, the calcium
salts are particularly preferred in the instant invention.
The process of the invention is carried out
by emplacing in a subterranean zone a hardenable,
a~ueous slurry where the slurry comprises water, a
hydraulic cement and a set re-tarder selected from
organic compounds bearing one or more amino-N-((alkyli-
dene)phosphonic acid) group or salts thereof, and -then
28,034A-F -4-
-5~ f~
permitting said slurry to harden in the subterranean
zone. This emplacement is suitably carried out by
pumping the aqueous slurry into the subterranean zone
through conduits communica-ting between that zone and
the earth's surface. Communication may be accomplished
through a borehole of a well drilled into the subter-
ranean zone or through a string of steel tubing communi-
cating from the surface -to -the subterranean zone or the
annulus formed between such tubing and the borehole.
The e~uipment for achieving placement of this slurry
may be standard equipment commonly employed in the oil
~ield and comprises a positive displacement pump of
three cylinders, often refered to as a -triplex pump,
and downhole tools and hardware commonly associated
with a cementing operation such as stage collars, float
collars, cement shoes and the like.
The process is suitably carried out by employ-
ing an aqueous cement slurry containing a set retarder
comprising a compound represented by -the ~ormula
R~N(X)-W)y (Formula Ij
wherein R represents W or an organic substrate capable
of bearing or comprising up to at least y amine moie-ties;
X, independently each occurrence represents hydrogen,
hydroxyalkyl, hydrocarbyl, ~ or a bond to R; W, indepen-
dently each occurrence, represents the moiety
-R'-P(O)~OH)2 (Formula II)
or salts thereof wherein -R'-represents an alkylidene
group comprising one to -thxee carbon atoms; and y
represents an integer of one or more. R' preferably
28,034A-F -5
represents a methylene group but may suitably be select-
ed from ethylidene; 1,1-propylene; and 2,2-propylene.
The set re-tarding composition utilized in the
present invention is in many cases available as a
commerc.ial product. It may be prepared by causing a
suitably substituted amine represented by thé formula
R~N(X~-H)y
to react with up to a stoichiometric amount of a dialkyl
phosphite and an aldehyde or a ketone of suitable
structure (to yield the corresponding alkylidene group),
in the manner described in U.S. Patent 3,346,488 and
thereafter hydrolyzing the resultant amino-N-(alkyli-
dene)dialkylphosphonate). Alternatively, in a Mannich
reaction, an equivalent amount of phosphorous acid
(H3PO3) and suitable aldehyde may be reacted with such
an amine to yeild the phosphonomethylene derivatives of
the amine. Representative of the amine reac-tants which
may be utilized to prepare the set retarders of the
instant invention are those such as arnmonia, methylamine,
ethylamine, dime-thylamine, diethylamine, monoethanol-
amine, diethanolamine, piperidine, piperazine, dicyclo-
pentadienediamine, ethylenediamine, diethylenetriamine,
higher polyalkylene polyamines, (2-aminoethyl)-N~piper-
azine, and other miscellaneous crude amines produced as
by-products in the preparation of polyalkylene polya-
mines by the reaction of ethylene dichloride and ammonia.
Preferred in the invention, are set re-tarders prepared
by phosphonomethylation of ammonia, of 1,6-hexylenedi-
amine, of ethylenediamine, of diethylenetriamine and
the sodium, potassium and calcium salts derived from
such phosphonomethylated amines~
28,034A-F -6-
7~
Particularly preferred embodiments of the set
retarder utilized in the present invention are those
selected from compounds represented by Formula I where-
in R' represents a methylene group and wherein X inde-
pendently each occurrence represents hydrogen, hydroxy-
alkyl, hydrocarbyl or W; especially preferred are those
where X, independently each occurrence, represents
hydrogen or W; and even more preferred are -those where-
in X, each occurrence represents W. Those set re-tarders
of Formula I are also preferred wherein y represents an
integer from one to si~; and more pre~erably an in-teger
from two to five.
Another preferred group of set retarders
useful in the invention are those represented by Form-
lS u~la I where up to (4y-1) equivalents of the acidic
protons of the phosphonic acid groups have been replac-
ed by their respective sodium, potassium or calcium
salt. Especially preferred as set retarding composi-
tions are those which comprise a compound represented
by the formula:
X'4N(X')-R''~zN(X')-W (Formula III)
where z is an integer from zero up to about ten, prefer-
ably ~rom zero to about four, more preferably from one
to three and most preferably is zero, one or two; where
X' independently each occurrence, represents hydrogen, -
hydroxyalkyl or hydrocarbyl comprising up -to four
carbon atoms, or W; and R" represents an alkylene group
comprising up to six carbon atoms. One preferred group
of such compGunds of E~ormula III are those derived from
ammonia wherein z represents zero. In another prefer-
red embodiment where the set retarder is represented by
28,034A F -7-
Formula III, W represents up to (2z ~5) equivalents of
a sodium, potassium or calcium salt. In another prefer-
red embodlment, the set retarder is selected from
compounds represented by Formula III, derived from the
polyalkylene polyamines, where z is an integer from one
to three particularly those were R" represents ethylene
or 1,2-propylene. Even more preferred are those select-
ed from the compounds ~here z represents one or two and
R" represents ethylene, i.e., those derived from ethy-
lenediamine and diethylenetriamine.
Specific compounds especially preferred foruse in -the invention are the compounds of Formula III
prepared by phosphonomethylation of l,6-hexylenediamine;
of ethylenediamine; diethylenetriamine; ammonia; and
mono- a~d di-lower alkylamines; and the sodium, potas-
sium and calcium salts of such phosphonic acids prepared
by replacing up to (2z~5) of the phosphonic acidic
protons with the respective metallic cations.
The aqueous slurries employed in the invention
are prepaxed by mixing a hydraulic cement with a suffi-
cient guantity of water to give the desired slurry
weight, thickness and other physical properties. The
o hydraulic cement is a cementitious material which
typically is a mineral material tha-t when wetted with
water changes from a finely or coarsely divided material
into a continuous hardened material. Portland cement
is representative as are burned or slaked lime, plaster
of paris, alumina or pozzolan cements. ~ortland cement
is preferred for utilization in the present invention.
The amount of water to be added to the hydraulic cement
component may vary in accordance with the handlin~
properties and the final strength of the cement desired
28,034A-F -8-
when set. Commonly, for standard well cementing compo-
sitions, a slurry comprising about 30 to about 50%
water, based on the weight of cement, will be utilized.
However, low water slurries of increased density and
greater ultimate strength or ligh-tweight cement slur-
ries extended with fillers, gases, or added amounts of
water may also be employed in the instant invention.
Other suitable additives, e.g. dispersants and turbu-
lence inducers such as sulfonated aromatic compounds,
fluid-loss control additives such as cellulose deri-
vates, sodium chloride, thixo-tropic agents, polymers,
extenders such as, bentonite, gilsonite, silica and the
like, weighting agents such as barite, hematite and the
like, and retaxding agents commonly employed in aqueous
cement slurries such as carboxylic acids and their
salts, may be added to the cement slurry in the appro-
priate amounts, as is well within the knowledge of the
cement axtisan.
The amount of set retarder compound represent-
ed by the Formula I is suitably a quantity sufficient
to delay significant hardening of the aqueous slurry at
an elevated temperature until the emplacement of the
aqueous slurry is subs-tantially complete. This quan
tity will vary widely depending upon the nature of the
aqueous cement slurry employed and the conditions of
temperature and pressure to which the slurry will be
subjected once placed in the subterranean zone. The
amount to be employed in the process of the invention
may easily be determined by testing a series of aqueous
cement slurries, containing various quantities of the
set re-tarder, for thickening time in accordance with
procedures set forth in the API Specification lO for
materials and testing for well cements. Then selecting
28,034A-F -9-
-10~
7~ .
that amount of retarder which gives the appropriate
thickening time for the slurry under the conditions in
which i-t will be used. Generally, from about 0.01 to
about 5 parts by weigh-t of the set retarder may be
employed per 100 parts by weigh-t of -the hydraulic
cemen-t component. This amount is no-t a fixed number
and as noted above, may vary depending on the na-ture of
the cement and other components of the slurry an~d on
the physical properties desired of the slurry.
Various cementitious compositions of the
invention are prepared by combining a selected quantity
of an amino-N~((methylene)phosphonic acid) set retarder
or salts thereof with a commercial API Portland cement.
The various s-et retarders, and combinations thereof
with the cement are described hereafter. Unless other-
wise noted, all parts and percentages are expressed on
a weight basis.
. .
Retarder A - is nitrilo N,N,N,-tris((methylene)
phosphonic acid) which is available from commercial
sources.
Retarder B - is the pentasodium salt of
Retarder A and is also available from commercial sources.
Retarder_C - is ethylenediamine-N,N,N',N'-te-
trakis((methylene)phosphonic acid) which is available
from commercial sources.
Retarder D - is the heptasodium sal-t of
Retarder C and is available from commercial sollrces.
28,034A-F -10-
11-
7~
Retarder E - is the calcium salt of Retarder
C with seven equivalents (3.5 mols) of calcium, and is
available from commercial sources.
Retarder F - is 1,6-hexylene diamine-N,N,N',
N'-tetrakis((methylene)phosphonic acid) which is avail-
able from commercial sources.
Retarder F' - is the hexapotassium salt of
Retarder F and is available from commercial sources.
Retarder G - is the calcium salt of Retarder
F with seven equivalents (3.5 mols) of calcium and is
prepared by slowly reacting a stirred aqueous solution
of Retarder F with incremental amounts of a dilute
aqueous calcium carbonate solution, at room temperature,
by dropwise additon of the calcium carbonate solution
until effervescence from evolution of carbon dioxide
ceases upon addition of further calcium carbonate. The
resulting salt precipita-tes and is recovered by filter-
ing it from the aqueous phase and then drying it.
Retarder H - is diethylenetriamine-N,N,N',N",
N"-penta((methylene)phosphonic acid) which is available
from commercial sources.
Retarder I - is the sodium salt of Retarder H
_ .
with nine equivalents of sodium and is available from
comm~rcial sources.
Retarder J - is the calcium salt of Re-tarder
H with nine equivalents (4.5 mols) of calcium. It is
prepared in the same manner as Retarder G.
28,034A-F -11-
-12-
~Z~
Retarder K - is the phosphonomethylated
derivative of monoethanolamine. It is prepared by
adding phosphorous acid (H3P03) to an aqueous solution
of the amine which has been acidified with dilute
hydrochloric acid to a pH of about l. Two mols of
H3P03 are added per mol of the amine and the resulting
mixture is refiuxed fox approximately one hour.
Thereafter, two mols of aqueous formaldehyde are
gradually added, with a reflux, over a one to two
hour period. The resulting reac-tion mixture is
then refluxed for an additional period of 11~ to 3
hours. Such process yields a crude product which
is about twenty-five percent ac-tive retarder
represented by the formula:
H0C2H4N~cH2P(o)toH)2]2
Retarder L - is a phosphonomethylated
derivative of aminoethylethanol amine (AEEA).
The three active amino hydrogens of the AEEA are
phosphonomethylated in -the manner described above
for Retarder K utilizing three mols each of
H3PG3 and formaldehyde. The reaction product
is about twenty-five pexcent active retarder
represented by the formula:
HOC2H4-N(W)-C2H4-N(W)2 where W represents a
-CH2p(o)~o~)2 moiety.
Retarder M - is a product phosphonomethylated
in the manner described for Retarder K except that the
organic amine to be phosphonomethylated is a condensa-
tion product of 4 mols of H2NCH2CH2-N~CX2CH2~2N-H,
28,034A-F -12-
-13-
7~
i.e., 2-aminoethyl-N-piperazine ("AEP") with about one
mol ethylene dichloride ("EDC"). The crude reaction
product is about twenty percent of the active phosphono-
methylated amine retarder, and is prepared using an
excess of the H3PO3 and formaldehyde.
Retarder N - is prepared in the fashion of
Retarder K except that the organic amine to be phospho-
nomethylated is the condensation product of one mol
urea with two mols AEP. One mol of the condensation
product is reacted with slightly more than four
mols of the phosphonomethylating reagents employed in
preparing Retarder K. The crude product, about twenty-
five percent active, is primarily a retarder represent
ed by the formula
[W ~ ~ N-cH2cH2-N(w)~2
\CH2 H2
where W is as in Retarder L, above.
Retarder_O is prepared in the fashlon of
Retarder K except that the organic amine to be phospho-
- nomethylated is a crude AEP containing ED~ and EDA
oligomeric impurities. It is reacted with a stoichio-
metric excess of formaldehyde and H3PO3. The reaction
product is about thirty percent active phosphonomethy-
lated retarder.
Retarder P - is prepared in the fashion of
Retarder O except that the organic amine -to be phospho-
nomethyla-ted is a crude piperazine reaction mixture
prepared from EDA and containing EDA and EDA oligomeric
impurities. The crude piperazine is reacted with a
28,034A-F -13-
-14-
7~
stoichiometric excess of formaldehyde and H3PO3. The
reaction product is about twelve percent active phospho-
nomethylated retarder.
Retarder ~ - is prepared using the same phosphono-
methylation process as Retarder K except that -the
organic amine employed is a reaction product prepared
by initially reacting the bis (methylamine) of
dicyclopentadiene (DCPD) with glycolonitrile in
the presence of excess caustic. Following this
initial reaction, the caus-tic is neutralized,
and the pH of the reac-tion mixture is adjusted
to about 1, with hydrochloric acid and the
phosphonomethylation reac-tion is -then conducted
as described above for Retarder K using two mols
each of H3PO3 and formaldehyde. The resulting
reaction product contains two acetic acid groups
and two -CH2-P(O)~O~)~ groups on the nitrogens
of the DCPD bis (methylamine) starting material
and the yield of such product is approximately
twenty percent.
Various of the foregoing Retarders are incor-
porated in standard slurries commonly used for oilfield
cementing operations. Some are evaluated for thi~kening
times using API Specification 10 testing methods,
25 . specifically Sec-tion 8 and casing schedules of Appendix
E, or minor variations thereof.
Example 1
Various of the Retarders A through J are
added to a base slurry prepared from a Class H oilfield
cement and 38 percen-t fresh water, based on weight of
28,034A-F -14-
-15-
cement (BWOC). Varying amounts of the active Retarders
are added to samples of the slurry and thickening time
tests are run at various temperatures, according to
Section 8 of API Specification 10. To preven-t excessive
strength retrogression, in slurries to be tested above
225F, 35 percent (BWOC) silica flour is included. Rep
resentative thickening times are described in Table I.
"Thickening Time", as employed in Table I
means the time until a thickness of 100 Bearden consis-
tency unit (Bc) is attained. For ultimate temperatureof 150F - casing schedule 5g/gradient 1.7F per 100 ft
is employed; for 200F - casing schedule 7g/gradient
1.5F per 100 ft is employed; for 250F - casing sched-
ule 9g/gradient 1.3F per 100 ft is employed; for
300F ~ casing schedule 9g/gradient 1.7F per 100 ft is
employed; for 350F ~casing schedule lOg/1.7F per 100
ft is employed; and for 400F - casing schedule
llg/1.7F per 100 ft is employed.
28,034A-F ~15~
-16-
TABLE I
Percent
Retarder Approximate Thickening Time (~lins.)
(Active) To Attain lOO Bc
Retarder _BWOC) 150F 200F 250FOther
NONE (base slurry) <100 <60 <60
A . 0.5 +360 +360 260 ---
A 1 --- -~360 250 ---
B 0.5 --- +360 250 90
(300F)
B 1 ~ - +360 120
'(,300F)
C 0.5 --- -~360 120 ---
C 1 --- +360 170 ~--
D 0.15 ---- --- +360 +360
(400F)
D 0.3 -- +360 +360 +360
(400F)
E 0.5 --- -~360 +360 115
(300F)
E 1 - - --- --- +300
~300F)
G 0.5 --- +360 170 90
(300F)
G 1 --- --- +360 l40
. (300F)
I 0.6 --- --- +360 100
(300F)
I 1.2 --- - --- 220
(300F)
J 0.5 --- +360 +360 90
(300F)
J 1 --- --- +360 +360
(300F)
28,035A-F -16-
Example _
A base slurry of Class H oilfield cement, 50
percent fresh wa-ter (BWOC) and 35 percent silIca flour
is prepared and to samples of the slurry are added
Retarders O through Q, one percent (BWOC) of the active
retarder. These "retarded" slurries are examined for
thickening time in essentially the same manner as the
slurries in Example I, but at 400F ultimate tempera-
ture. The times required to attain a thickness of 30
Bc and 70 Bc are observed. Representative thickening
times are described in Table II.
TABLE II
Percent
RetarderApprox. Thlckening
(Active)Time (Mins.)
Retarder (BWOC) 30 Bc 70 Bc
None (base slurry) ~ <60
O 1 70 90
P 1 65 75
Q 1 70 75
28,034A-F -17-
-18-
Example~3
In the fashion of Example 3, a slurry of
Class H cement, 35 percent silica flour and 50 percent
fresh water (BWOC) is prepared and to samples of the
slurry are added varying amount of Retarders K through
N. The "Retarder" slurry samples are examined for
thickening -times in the same manner as in Example l,
bu-t at 400F (ultimate tempera-ture) to determine the
times required -to attain a thickness of 70 Bc. Repre-
sentative thickening -times are described in Table III.
TABLE III
Percent
Retarder
(Active) Approximate Thickenin~ Time
Retarder (BWOC) To Attain 70 Bc (Mins).
None ~Base Slurry) --- <50
K .4 75
K .8 105
L .3 150
L .4 . 255
L .5 ~360
M .25 130
M .4 320
M 1.0 -~360
N . .2 60
N .5 180
N .7 - 300
28,035A-F -18-
--19--
Example 4
As in Example 3, Retarder N is -tested for
time to attain thickness of 70 Bc, using schedule
llg/1.9F per 100 ft, to abou-t 450F ultimate tempera-
ture. To the base slurry of Example 3 is added 0.3percent (BWOC) borax. To samples of this slurry con-
taining 0.3% borax are added 0.5 percent and 0.7 percent
(BWOC) of Retarder N. Representative 70 Bc thickening
times are described in Table IV.
TABLE IV
Percent
Retarder Approximate Thickening Time
Retarder (Active BWOC~ to Attain 70 Bc (Mins.)
Borax only 0.3 180
15 Borax + N 0.3 + 0.5 330
Borax + N 0.3 + 0.7 ~360
Example 5
Retarder E is combined with sodium borate
decahydrate ("borax") in a weigh-t ratio of about 2:3,
respectively. To the mixture is added about two percent
magnesium stearate. This retarder blend is dry added
to a Class H oilfield cement in -the amount of about 0.3
percent, BWOC. In-to this cement/retarder blend is also
blended about 35 percent fine sand (about 100 to 200
mesh), about 7.5 percent weighting agent (ilmenite), a
minor amount (<1%) of a cellulosic fluid loss additive
and an antifoam agent, and about 9 percent sodium
chloride, all BWOC. A slurry is prepared from this
cement/dry additive blend with 50 percen-t water, BWOC.
28,034A-F -19-
~20-
The slurry is pumped down a 7 inch casing
liner positioned in an 8 inch borehole of a well to a
depth of about 11,000 feet and circulated back up the
annulus between the liner and the borehole to the top
of the liner at about 8000 feet. The bottom hole
static temperature is about 270F. The slurry is
easily placed within the 265 minute working time calcu-
lated for t.he slurry, without difficulty from undue
thickening. Within about 16 hours after emplac~ment of
the slurry, it attains su~ficient strength so that a
pressure test may be applied to determine that a satis-
factory cement job is accomplished and then excess
cement and the casing shoe may be drilled out.
28,034A-F -20-
