GB2157279A

GB2157279A - Well treating composition

Info

Publication number: GB2157279A
Application number: GB08409517A
Authority: GB
Inventors: Erik Bentley Nelson
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1984-04-12
Filing date: 1984-04-12
Publication date: 1985-10-23
Also published as: GB8409517D0; FR2563568A1

Abstract

A subterranean zone is treated by emplacing therein a composition of a hardenable aqueous slurry where the slurry comprises water, a hydraulic cement and as a set retarding agent, an amino-N-((alkylidine) phosphonic acid)-bearing compound or salt thereof.

Description

SPECIFICATION Well treating process and composition The invention 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 retarder, which composition is useful in this method.

Hydrophobic-substituted phosphonic or phosphinic acids and their alkali metal salts have been used in cements, primarily soil/cement mixtures, to improve the freeze-thaw properties and salt-resistance. Sixto eighteen-carbon alkyl phosphonic acids or their alkali metal salts are so described in U. S. Patent 3,794,506 (1974). A plugging mixture for high temperature oil and gas wells comprising Portland cement and 1-hydroxy ethylidene-phosphonic acid trisodium or tripotassium salts as set time extenders is described in Derwent abstract 71376B/39 (1979) of USSR Patent 640,019. The use of these phosphonate salts at temperatures of 75 to 1500C in amounts of 0.1-0.3% by weight is described in the abstract.

A phosphonobutane tricarboxylic acid, specifically 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 2,964,921 (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 turbulence 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 reducing agents for gypsum plasters. Lignins which have been phosphonoalkylated through an ether linkage or corresponding sulfonates, sulfides, hydroxyl or amine derivatives are taught to be useful primarily as dispersants of 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 characteristically powerful retarding property peculiar to them.

The present invention particularly resides in a process for treating a subterranean zone by emplacing therein a hardenable, aqueous slurry where the slurry comprises water, a hydraulic cement and a set retarder selected from organic compounds bearing one or more amino-N-((alkylidene) phosphonic acid) group or salts thereof, and thereafter permitting said slurry to harden.

The invention also resides in a composition useful 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 a hydraulic cement and from about 0.01 to about 5 parts by weight per 100 parts of hydraulic cement, of a set retarding composition comprising a compound represented by the formula R+N(X)-W), 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.

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 significant 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 temperatures.

One particularly useful and unexpected property 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 temperatures have been pursued by cement manufacturers. This change in Portland cement manufacturing can cause the tricalcium silicate (represented by 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 composition 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 potassium 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, aqueous slurry where the slurry comprises water, a hydraulic cement and a set retarder selected from organic compounds bearing one or more amino-N-((alkylidene)phosphonic acid) group of salts thereof, and then 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 communicating between that zone and the earth's surface. Communication may be accomplished through a borehole to a well drilled into the subterranean zone orthrough a string of steel tubing communicating from the surface of the subterranean zone or the annulus formed between such tubing and the borehole.The equipment for achieving emplacement of this slurry may be standard equipment commonly employed in the oil field 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 employing an aqueous cement slurry containing a set retarder comprising a compound represented by the formula RtN(X)-W)y (Formula I) 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)f OH)2 (Formula II) or salts thereof wherein -R'- represents an alkylidene group comprising one to three carbon atoms; and y represents an integer of one or more. R' preferably represents a methylene group but may suitably be selected from ethylidene; 1,1-propylene; and 2,2-propylene.

The set retarding composition utilized in the present invention is in many cases available as a commercial product. It may be prepared by causing a suitably substituted amine represented by the formula R+N(X)-H), 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- (alkylidene)dialkylphosphonate). Alternatively, in a Mannich reaction, an equivalent amount of phosphorous acid (H2PO3) and suitable aldehyde may be reacted with such an amine to yeild the phosphonomethylene derivatives of the amine.Representative of the amine reactants which may be utilized to prepare the set retarders of the instant invention are those such as ammonia, methylamine, ethylamine, dimethylamine, diethylamine, monoethanolamine, diethanolamine, piperidine, piperazine, dicyclopentadienediamine, ethylenediamine, diethylenetriamine, higher polyalkylene polyamines, (2-aminoethyl)-N-piperazine, and other miscellaneous crude amines produced as by-products in the preparation of polyalkylene polyamines by the reaction of ethylene dichloride and ammonia. Preferred in the invention, are set retarders prepared by phosphonomethylation of ammonia, of 1,6-hexylenediamine, of ehtylenediamine, of diethylenetriamine and the sodium, potassium and calcium salts derived from such phosphonomethylated amines.

Particularly preferred embodiments of the set retarder utilized in the present invention are those selected from compounds represented by Formula I wherein R' represents a methylene group and wherein X independently each occurrence represents hydrogen, hydroxyalkyl, hydrocarbyl or W; especially preferred are those where X, independently each occurrence, represents hydrogen or W; and even more preferred are those wherein X, each occurrence represents W. Those set retarders of Formula I are also preferred wherein y represents an integer from one to six; and more preferably an integer from two to five.

Another preferred group of set retarders useful in the invention are those represented by Formula I where 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. Especially preferred as set retarding compositions are those which comprise a compound represented by the formula: X'kN(X')-R"+,N(X')-W (Formula lli) where z is an integer from zero up to about ten, preferably from 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 compounds of Formula lil are those derived from ammonia wherein z represents zero. In another preferred embodiment where the set retarder is represented by Formula Ill, W represents up to (2z +5) equivalents of a sodium,, potassium or calcium salt. In another preferred embodiment, 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 selected from the compounds where z represents one or two and R" represents ethylene, i.e., those derived from ethylenediamine and diethylenetriamine.

Specific compounds especially preferred for use in the invention are the compounds ot Formula III prepared by phosphonomethylation of 1,6-hexylenediamine; of ethylenediamine; diethylenetriamine; ammonia; and mono- and di-lower alkylamines; and the sodium, potassium and calcium salts of such phosphonic acids prepared by replacingup to (2z+5) of the phosphonic acidic protons with the respective metallic cations.

The aqueous slurries employed in the invention are prepared by mixing a hydraulic cement with a sufficient quantity of water to give the desired slurry weight, thickness and other physical properties. The hydraulic cement is a cementitious material which typically is a mineral material that 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.

Portland 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 handling properties and the final strength of the cement desired when set. Commonly, for standard well cementing compositions, 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 lightweight cement slurries extended with fillers, gases, or added amounts of water may also be employed in the instant invention.Other suitable additives, e.g. dispersants and turbulence inducers such as sulfonated aromatic compounds, fluidloss control additives such as cellulose derivates. sodium chloride, thixotropic agents, polymers, extenders such as, bentonite, gilsonite, silica and the like, weighting agents such as barite, hematite and the like, and retarding agents commonly employed in aqueous cement slurries such as carboxylic acids and their salts, may be added to the cement slurry in the appropriate amounts, as is well within the knowledge of the cement artisan.

The amount of set retarder compound represented 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 substantially complete. This quantity 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 retarder, for thickening time in accordance with procedures set forth in the API Specification 10 for materials and testing for well cements.Then selecting that amount of retarder which gives the appropriate thickening time for the slurry under the conditions in which it will be used. Generally, from about 0.01 to about 5 parts by weight of the set retarder may be employed per 100 parts by weight of the hydraulic cement component. This amount is not a fixed number and as noted above, may vary depending on the nature of the cement and other components of the slurry and 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 set retarders, and combinations thereof with the cement are described hereafter.

Unless otherwise 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 salt of Retarder C and is available from commercial sources.

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 available 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 addition of the calcium carbonate solution until effervescence from evolution of carbcn dioxide ceases upon addition of further calcium carbonate. The resulting salt precipitates and is reccvered by filtering 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 commercial sources.

Retarder J - is the calcium salt of Retarder H with nine equivalents (4.5 mols) of calcium. It is prepared in the same manner as Retarder G.

Retarder K - is the phosphonomethylated derivative of monoethanolamine. It is prepared by adding phosphorous acid (H3PO2) to an aqueous solution of the amine which has been acidified with dilute hy drochloric acid to a pH of about 1. Two mols of H3PO3 are added per mol of the amine and the resulting mixture is refluxed for approximately one hour. Thereafter, two mols of aqueous formaldehyde are gradually added, with a reflux, over a one or two hour period. The resulting reaction mixture is then refluxed for an additional period of 1 1/2 to 3 hours. Such process yields a crude product which is about twentyfive percent active retarder represented by the formula: HOC2H4NfCH2P(O) +OH)212.

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 H3PO3 and formaldehyde. The reaction product is about twenty-five percent active retarder represented by the formula; HOC2H4-N(W)-C2H4-N(W)2 where W represents a -CH2P(O)tOH)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 condensation product of 4 mols of H2NCH2CH2 N+CH2CH2+2N-H, i.e., 2-aminoethyl-h-piperazine (''AEP") with about one mol ethylene dichloride ("EDC"). The crude reaction product is about twenty percent of the active phosphonomethylated 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 phosphonomethylated is the condensation product of one mol urea with two mols AEP. One mol of the condensation product is reacted wtih 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 represented by the formula

where W is as in Retarder L, above.

Retarder O is prepared in the fashion of Retarder K except that the organic amine to be phosphonomethylated is a crude AEP containing EDA and EDA oligomeric impurities. It is reacted with a stoichiometric excess of formaldehyde and H3PO3. The reaction product is about thirty percent active phosphonomethylated retarder.

Retarder P - is prepared in the fashion of Retarder 0 except that the organic amine to be phosphonomethylated is a crude piperazine reaction mixture prepared from EDA and containing EDA and EDA oligomeric impurities. The crude piperazine is reacted with a stoichiometric excess of formaldehyde and H3PO3. The reaction product is about twelve percent active phosphonomethylated retarder.

Retarder O - is prepared using the same phosphonomethylation 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 caustic is neutralized, and the pH of the reaction mixture is adjusted to about 1, with hydrochloric acid and the phosphonomethylation reaction 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)tOH)2 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 incorporated in standard slurries commonly used for oil field cementing operations. Some are evaluated for thickening times using API Specification 10 testing methods, specifically Section 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 percent fresh water, based on weight of 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 prevent excessive strength retrogression, in slurries to be tested above 225"F, 35 percent (BWOC) silica flour is included. Representative thickening times are described in Table I.

"Thickening Time", as employed in Table I means the time until a thickness of 100 Bearden consistency unit (Bc) is attained. For ultimate temperature of 150"F - casing schedule 5g/gradient 1.7"F per 100 ft is employed; for 200"F - casing schedule 7g/gradient 1.5"F per 100 ft is employed; for 250"F - casing schedule 9g/gradient 1.3"F per 100 ft is employed; for 300"F - casing schedule 9g/gradient 1.7"F per 100 ft is employed; for 350"F -casing schedule 10g/1.7"F per 100 ft is employed; and for 400"F - casing schedule 11g/1.7"F per 100 ft is employed.

TABLE I Percent Retarder Approximate Thickening Time (Mins.) (Active) To Attain 700 Bc Retarder (B WO C) 150"F 200 250"F NONE (Base slurry) < 100 < 60 < 60 A 0.5 +360 +360 260 A 1 --- +360 250 B 0.5 --- +360 250 90 (300"F) B 1 - - +360 120 (300 F) C 0.5 --- +360 120 C 1 --- +360 170 D 0.15 --- --- +360 +360 (4000F) D 0.3 --- +360 +360 +360 (400"F) 0.5 --- +360 +360 115 (3000F) E 1 --- --- --- +300 (300"F) G 0.5 - +360 170 90 (300 F) G 1 --- --- +360 140 (300OF) 0.6 --- --- +360 100 (300 F) 1 1.2 --- --- --- 220 (300 F) J 0.5 --- +360 +360 90 (300'F) J 1 --- --- +360 +360 (300 F) Example 2 A base slurry of Class H oilfield cement, 50 percent fresh water (BWOC) and 35 percent silica flour is prepared and to samples of the slurry are added Retarders 0 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 1, but at 400"F ultimate temperature. 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 Retarder Approx. Thickening (Active) Time (Mins.) Retarder (B WO C) 30 Bc 70 Bc None (base slurry) --- --- < 60 0 1 70 90 P 1 65 75 Q 1 70 75 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 1, but at 400"F (ultimate temperature) to determine the times required to attain a thickness of 70 Bc.

Representative thickening times are described in Table Ill.

TABLE Ill Percent Retarder (Active) Approximate Thickening Time Retarder (B WO C) 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 Example 4 As in Example 3, Retarder N is tested for time to attain thickness of 70 Bc, using schedule 11g/1.9"F per 100 ft, to about 450at ultimate temperature. To the base slurry of Example 3 is added 0.3 percent (BWOC) borax. To samples of this slurry containing 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 fins.) Borax only 0.3 180 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 weight 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. Into this cementiretarder 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 percent water, BWOC.

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 270"F. The slurry is easily placed within the 265 minute working time calculated for the slurry, without difficulty from undue thickening.

Within about 16 hours after emplacement of the slurry, it attains sufficient strength so that a pressure test may be applied to determin that a satisfactory cement job is accomplished and then excess cement and the casing shoe may be drilled out.

Claims

1. A hardenable aqueous slurry for emplacement in subterranean zones comprising a hydraulic cement and, as a set retarder, an organic compound bearing one or more amino-N-((alkylidene) phosphonic acid) groups or a salt thereof.

2. A slurry as claimed in Claim 1, wherein the said retarder is present in an amount of 0.01 to 5 parts by weight per 100 parts of weight cement.

3. A slurry as claimed in any one of the preceding Claims, wherein the cement comprises Portland cement.

4. A slurry as claimed in Claim 3, wherein the Portland cement has a C2S:C3S ratio exceeding 0.4:1.

5. A slurry as claimed in Claim 4, wherein said ratio exceeds 0.45:1.

6. A slurry as claimed in any one of the preceding Claims comprising 30 to 50 percent water based in the weight of cement.

7. A slurry as claimed in any one of the preceding Claims, wherein the slurry comprises sodium borate in an amount of 0.1 parts to ten parts by weight per part of the said set retarder.

8. A slurry as claimed in any one of the preceding Claims, 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, 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.

9. A slurry as claimed in Claim 8, wherein -R'- represents a methylene group.

10. A slurry as claimed in Claim 9, wherein X, independently each occurrence, represents hydrogen, hydroxyalkyl, hydrocarbyl or W.

11. A slurry as claimed in Claim 10, wherein X, independently each occurrence, represents hydrogen or W.

12. A slurry as claimed in Claim 11, wherein X, each occurrence, represents W.

13. A slurry as claimed in one of Claims 8 to 12, wherein y represent an integer from 1 to 6.

14. A slurry as claimed in 13, wherein y represents an integer from 2 to 5.

15. A slurry as claimed in any one of Claims 8 to 14, 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.

16. A slurry as claimed in any one of the preceding Claim 8, wherein said retarder comprises a compound represented by the formula X'(N(X')-R"),N(X')-W wherein: z is zero or an integer from one to ten; 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.

17. A slurry as claimed in Claim 16, wherein z is zero or an integer from one to three.

18. A slurry as claimed in Claim 17, wherein z is zero, one or two.

19. A slurry as claimed in Claim 16, wherein z is zero.

20. A slurry as claimed in Claim 16, wherein z is zero and X' is W, each occurrence, and W represents up to five equivalents of a sodium, potassium or calcium salt.

21. A slurry as claimed in Claim 16, 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 or a sodium, potassium or calcium salt.

22. A slurry as claimed in Claim 16, wherein X is W each occurrence; R" is 1,6-hexylene; and W represents up to seven equivalents of a calcium salt.

23. A slurry as claimed in Claim 16, 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.

24. A slurry as claimed in Claim 8, which does not include any compound represented by the formula WI | (N-cH2cH(oH)cH2-N)nw W' W' where n is an integer from 1 to 5 and W' represents the moiety -CH2P(O)(OH)2 or its sodium salts.

25. A slurry as claimed in any one of Claims 1 to 7, wherein the said retarder is a partial calcium salt of the amino-N ((alkylidene)phosphonic acid) organic compound.

26. A slurry as claimed in any one of Claims 1 to 7, wherein said retarder is a compound 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.

27. A slurry as claimed in any one of Claims 1 to 8, wherein the slurry comprises a Portland cement and a set retarder selected from 1,6-hexylenediamine- N,N,N',N'-tetrakis ((methylene)phosphonic acid; ethylene- diamine-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.

28. A slurry as claimed in any one of Claims 1 to 8, wherein the slurry comprises a Portland cement and a set retarder composition comprising a compound selected from nitrilo-N,N,Ntris((methylene)phosphonic acid) and its sodium, potassium and calcium salts.

29. A slurry as claimed in any one of Claims 1 to 8, wherein te said retarder is any one of those hereinbefore identified as Retarders A to Q.

30. A slurry as claimed in Claim 1 and substantially as hereinbefore described in any one of the Examples.

31. A process for treating a subterranean zone by emplacing therein a hardenable, aqueous slurry as claimed in any one of the preceding Claims and thereafter permitting the slurry to harden.

32. A composition for plugging a subterrarean 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 and from 0.01 to 5 parts by weight per 100 parts of hydraulic cement, of a set retarding composition comprising a compound represented by the formula R(N(X)- W)y wherein: R represent W or an organic substrate capable or 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, 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.

33. A composition for slurrying with water to form an aqueous slurry as claimed in Claim 1, comprising a hydraulic cement and a set retardant amount of an organic compound bearing one or more amino N- ((alkylidene)phosphonic acid) groups or a salt thereof.

34. A composition as claimed in Claim 33 having components as defined in any one of Claims 1 to 5 and 7 to 30.