CA1216742A - Well treating process and composition - Google Patents

Abstract

Abstract of the Disclosure A process for treating a subterranean zone by emplacing therein a composition of a hardenable aqueous slurry where the slurry comprises water, a hydraulic cement and a set retarding agent and permitting the slurry to harden, wherein the set retarding agent is selected from amino-N-((alkylidene)phosphonic acids)--bearing compounds or salts thereof.

The composition and process of employing the same is particularly useful in the treatment of oil and gas wells at elevated temperatures.

Description

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 -10-7~
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-~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 -12-7~

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 -13-7~
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-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~

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 28,034A-F -17-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-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-

Claims (31)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. 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 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.

2. The slurry of Claim 1, wherein the cement comprises Portland cement.

3. The slurry of Claim 2, wherein the Portland cement has a C2S:C3S ratio exceeding 0.4:1.

4. The slurry of Claim 3, wherein said ratio exceeds 0.45:1.

5. The slurry of Claim 1, including from 30 to 50 percent water based in the weight of cement.

6. The slurry of Claim 1, wherein the sodium borate is present in an amount of from 0.1 to 10 parts by weight per part of the said set retarder.

7. The slurry of Claim 1, wherein said retarder comprising a compound represented by the formula R(N(X)-W)y wherein:
R represents 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 -R'-P(O)(OH)2, or salts thereof wherein -R'- represents an alkylidene group comprising one to three carbon atoms;
and y represents an integer of one or more, or a salt thereof.

8. The slurry of Claim 7, wherein -R'- represents a methylene group.

9. The slurry of Claim 8, wherein X, indepen-dently each occurrence, represents hydrogen, hydroxyalkyl, hydrocarbyl or W.

10. The slurry of Claim 9, wherein X, indepen-dently each occurrence, represents hydrogen or W.

11. The slurry of Claim 10, wherein X, each occurrence, represents W.

12. The slurry of Claim 7, wherein y represents an integer from 1 to 6.

13. The slurry of Claim 12, wherein y represents an integer from 2 to 5.

14. The slurry of Claim 7, wherein up to (4y-1) equivalents of the acidic protons of the phosphonic acid groups have been replaced by their respective sodium, potassium or calcium salt.

15. The slurry of Claim 7, wherein said retarder comprises a compound represented by the formula X'(N(X')-R")z-N(X')-W wherein:-z is zero or an integer from one to ten;
X' is 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.

16. The slurry of Claim 15, wherein z is zero or an integer from one to three.

17. The slurry of Claim 16, wherein z is zero, one or two.

18. The slurry of Claim 15, wherein z is zero.

19. The slurry as claimed in Claim 15, wherein z is zero and X' is W, each occurrence, and W represents up to five equivalents of a sodium, potassium or calcium salt.

20. The slurry of Claim 15, wherein z is an integer from one to three; X' independently each occurrence, represents hydrogen or W; R" represents ethylene or 1,2-propylene, and W represents up to 9 equivalents of a sodium, potassium or calcium salt.

21. The slurry of Claim 15, wherein X; is W
each occurrence; R" is 1,6-hexylene; and W represents up to seven equivalents of a calcium salt.

22. The slurry of Claim 15, wherein z is one;
X' is W each occurrence; R" is ethylene; and W represents up to seven equivalents of a sodium or calcium salt.

23. The slurry of Claim 7, which does not include any compound represented by the formula where n is an integer from 1 to 5 and W' represents the moiety -CH2P(O)(OH)2 or its sodium salts.

24. The slurry of Claim 1 wherein the said retarder is a partial calcium salt of the amino-N ((alkylidene)-phosphonic acid) organic compound.

25. The slurry of Claim 1 wherein said retarder is a compound prepared by phosphonomethylation of ammonia, 1,6-hexylenediamine ethylenediamine or diethylenetriamine or a sodium, potassium or calcium salt derived from said phos- phonomethylated amines.

26. The slurry of Claim 1 wherein the slurry comprises a Portland cement and a set retarder selected from 1,6-hexylenediamine-N,N,N'~N'-tetrakis ((methylene)-phosphonic acid); ethylenediamine-N,N,N',N'-tetrakis ((methylene) phosphonic acid); diethylenetriamine-N,N,-N',N"N"-penta ((methylene)-phosphonic acid); and their sodium, potassium and calcium salts.

27. The slurry of Claim 1 wherein the slurry comprises a Portland cement and a set retarder composition comprising a compound selected from nitrilo-N,N,N-tris((methylene) phosphonic acid) and its sodium, potassium and calcium salts.

28. The slurry of claim 1, wherein the said retarder is any one of those hereinbefore identified as Retarders A to Q.

29. A process for treating a subterranean zone by emplacing therein a hardenable, aqueous slurry as claimed in claim 1 and thereafter permitting the slurry to harden.

30. 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, from 0.01 to 5 parts by weight per 100 parts of hydraulic cement, of a set retarding composition and sodium borate in an amount of from 0.1 to 10 parts by weight per part of said 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 moeity -R'-P(O)-(OH)2, wherein -R'- represents an alkylidene group comprising one to three carbon atoms; and y represents an integer of one or more, or a salt thereof.

31. A composition for slurrying with water to form an aqueous slurry as claimed in claim 1, wherein the set retarder is an organic compound bearing one or more amino-N-((alkylidene) phosphonic acid) groups or a salt thereof.