Fluid loss control in oil field cements

An aqueous slurry which comprises (a) a cement; (b) about 0.05 to 5.0 wt. % of a copolymer of N-vinylpyrrolidone and a salt of styrenesulfonic acid based on the dry weight of said cement; (c) about 0.05 to 5.0 wt. % of a formaldehyde condensed naphthalene-sulfonic acid salt based on the dry weight of said cement; and (d) water, said cement, said copolymer and said dispersant being dispersed in said water to form said aqueous slurry.

FIELD OF THE INVENTION 
The present invention relates to materials which reduce the filtration of 
fluid into permeable earth formations during cementing processes in the 
drilling and completing of subterranean wells, particularly wells for the 
recovery of petroleum resources. 
BACKGROUND OF THE INVENTION 
Petroleum well cementing is the process of mixing a slurry of cement, 
water, and other additives and pumping it down through steel casing to 
critical points in the oil well annulus around the casing or in the open 
hole below the casing string. The primary functions of the cementing 
process are to restrict fluid movement between geological formations and 
to bond and support the casing. In addition, the cement aids in protecting 
the casing from corrosion, preventing blowouts by quickly sealing 
formations, protecting the casing from shock loads in drilling deeper 
wells, and sealing off lost circulation or thief zones. 
A common problem in petroleum well cementing is the flow of liquid from the 
cement slurry into porous earth formations in contact with the cement. 
This fluid loss is undesirable since it can result in dehydration of the 
cement slurry, and it causes thick filter cakes of cement solids which can 
plug the well bore. The fluid loss can damage sensitive formations. Cement 
fluid loss is particularly a problem in the process known as squeeze 
cementing. 
There is a requirement, therefore, for materials which, when added to the 
cement formulation, reduce the loss of fluid from the slurry to porous 
formations. 
A type of fluid loss agent used in oil well cementing consists of a medium 
molecular weight grade of hydroxyethylcellulose (HEC) which may be 
employed with or without a dispersant such as condensed 
naphthalenesulfonic acid salts. There are several disadvantages to the use 
of hydroxyethylcellulose as a cement fluid loss control agent. Among these 
disadvantages are its properties of causing undesirable viscosification 
and retardation of the cement and losing effectiveness in the presence of 
soluble calcium salts and at elevated temperatures. 
Another cement additive employed for control of fluid loss is a copolymer 
of acrylamide and acrylic acid (L. F. McKenzie et al, SPE 1-623,279 
[1982]). Although this material performs well at high temperatures and in 
the presence of soluble calcium salts, it has the undesirable property of 
strongly retarding cement. This retarding effect increases with increasing 
temperature, most likely due to the hydrolysis of the unstable amide 
groups contained in the polymer which hydrolysis accelerates at elevated 
temperature. Such hydrolysis of the amide moieties produces additional 
carboxylic acid residues which are credited with cement retarding 
activity. 
Still other materials utilized for the control of fluid loss during well 
cementing are combinations of polyamines and either lignosulfonate or 
condensed naphthalenesulfonic acid salts (L. F. McKenzie et al, Oil and 
Gas J. 80 [13] 146 [1982]). These additive combinations are valuable since 
they cause little viscosification of the cement, and have little 
sensitivity to dissolved calcium and moderately high temperatures. The 
utility of the polyamine sulfonate combination is somewhat limited, 
however, by two side effects they produce: retardation and settlement 
(free water) of the cement. 
Mixtures of HEC, poly(vinylpyrrolidone) and sodium naphthalenesulfonate 
have been reported as cement fluid loss additives (Wersent, U.S. Pat. No. 
3,132,693) as have mixtures of poly(vinylpyrrolidone) and sodium 
naphthalenesulfonate-formaldehyde condensation products (U.S. Pat. No. 
3,359,225). 
A fluid loss control agent in oil field cements is disclosed to be a 
mixture of a copolymer of N-vinylpyrrolidone and a salt of styrenesulfonic 
acid in U.S. Pat. No. 4,480,693. 
Also complex mixtures of maleic anhydride N-vinylpyrrolidone copolymers 
with polymers of poly(aryl-vinylbenzyl) alkyl and hydroxyalkyl substituted 
quaternary ammonium bases and salts have been used (Wahl, U.S. Pat. No. 
3,140,269). 
An improved fluid loss additive mixture for well cements consists of 30 to 
70 weight percent of N-vinylpyrrolidone homopolymer, from about 5 to 19 
weight percent of carboxylmethylhydroxyethylcellulose, and the sodium salt 
of condensed naphthalenesulfonic acid. (B. W. Hale, U.S. Pat. No. 
4,258,790). 
Copolymers of N-vinylpyrrolidone and metal salts of styrenesulfonic acid 
are known, but only as components in oil containing microcapsules used in 
films (K. Saeki et al, U.S. Pat. No. 3,855,146). 
Despite the art for the control of cement fluid loss which is already 
known, there exists a need for novel agents capable of improving fluid 
loss control which are not reduced in utility by the limitations described 
above. 
SUMMARY OF THE INVENTION 
The present invention discloses cementing formulations which comprise: (a) 
water; (b) a hydraulic cement; (c) from 0.05 to 5.0 wt% based on the 
weight of the dry cement of a copolymer of N-vinylpyrrolidone and a metal, 
preferably sodium, salt of styrenesulfonic acid, said copolymer ranging in 
composition from 5 to 95 mole % vinylpyrrolidone and in reduced viscosity 
in 2% NaCl at 0.5 wt% from 0.5 to 15 dl/g; and (d) from 0.05 to 5.0 wt% 
based on the weight of dry cement of a cement dispersant composed of the 
free acid form and metal salts therefrom of formaldehyde condensed 
naphthalenesulfonic acid. 
This provides a method for the control of fluid loss to permeable earth 
formations from formulations for the cementing of subterranean wells, 
particularly wells from which are produced natural petroleum. 
GENERAL DESCRIPTION OF THE INVENTION 
The present invention relates to fluid loss control mixtures of copolymers 
of N-vinylpyrrolidone and metal salts of styrenesulfonic acid and 
dispersants such as condensed naphthalenesulfonic acid salts and to the 
use of the mixture as agents for the control of fluid loss to permeable 
earth formations from formulations used for cementing subterranean wells, 
particularly those wells from which natural petroleum resources are 
produced. 
The polymers useful in this invention are copolymers of N-vinylpyrrolidone 
and metal salts of styrenesulfonic acid. These copolymers are prepared by 
free radical copolymerization in aqueous solution at 40.degree.-60.degree. 
C. Typical free radical initiators for the polymerization are 
azoisobutyronitrile (AIBN), potassium peroxydisulfate, and the like. The 
copolymers consist of from 5-95 mole % of N-vinylpyrrolidone and 95-5 mole 
% of a salt of styrenesulfonic acid. Preferred for their better 
performance as cement fluid loss control agents are the copolymers 
consisting of 90-35 mole % of N-vinylpyrrolidone and 10-65 mole % of 
sodium styrenesulfonate. 
The cation of the styrenesulfonic acid salt may be any cation which 
provides a water soluble salt in combination with the styrenesulfonate 
anion and which causes no undesirable effects in the cementing formulation 
such as excessive cement acceleration, retardation, free water, thickening 
or thinning, etc. Some cations which provide styrenesulfonic acid salts 
suitable for use in these copolymers are lithium, potassium, sodium, 
triethylammonium, tri(hydroxyethyl)ammonium and the like. These cations 
represent typical examples of appropriate species, but a wide range of 
cations may be properly used, and the listing of typical cations is in no 
way intended to limit the scope of the invention. 
The copolymers found useful according to this invention for the control of 
cement fluid loss are further characterized by the so-called reduced 
viscosity parameter which describes the viscosity which the polymers 
impart to aqueous solutions. As used herein, this parameter is based on 
the viscosity of solutions consisting of 2% by weight sodium chloride, 
0.5% by weight polymer, and the remainder water. The copolymers of 
N-vinylpyrrolidone and styrenesulfonic acid salts which are disclosed as 
part of the subject invention are those having a reduced viscosity of from 
0.5 to 15 dl/g. 
This invention also discloses a means for controlling the fluid loss to 
permeable earth formations and from formulations used in the cementing of 
subterranean wells, particularly those wells drilled for the production of 
oil and gas, by use of the copolymers discussed above and subsequently 
detailed as the highly useful specific polymer in Example 1. The amount of 
the copolymer mixture utilized in a cement formulation will vary with the 
particular borehole environment at hand. Geological formation 
characteristics, borehole properties, borehole depth, contaminants, 
temperatures and pressures encountered, cement type and other additives, 
and purpose and method of application of the cementing operation will 
influence the determination of quantities of the copolymers in the 
mixtures of this invention to be used in a particular cement formulation 
to achieve the desired effect. In addition the particular properties of 
the copolymer mixture will influence the determination of quantities to be 
used. Because of these factors, it is impossible to specifically state 
nominal usage levels under all environments or conditions. Those skilled 
in the art of subterranean well cementing will be able to easily determine 
needed quantities of the copolymers and dispersant(s) in the mixtures for 
cement fluid loss control by testing formation characteristics, formation 
temperatures and pressures, and cement formulation characteristics, and by 
otherwise assessing the fluid loss characteristics required of the cement 
formulation. Nevertheless, it can be stated that a treatment rate of 0.2% 
to 5% (by weight based on dry cement weight) of the mixture should be 
appropriate. It is possible that under some circumstances as little as 
0.05% or as much as 10% of the mixture would be required. 
The dispersants useful in the copolymer mixture are low to medium molecular 
weight sulfonated polymers such as condensed formaldehyde lignosulfonate, 
formaldehyde sulfonate, and condensed naphthalenesulfonates. The preferred 
salt is calcium, although any metal salt can be used. The dispersant and 
copolymer can be used in any ratio in order for beneficial effects to be 
observed in the cement formulation although preferred ratios range from 
10:1 to 1:10 with about a ratio of 3:1 to 1:3 being optimal. Blending of 
the dispersant-copolymer mixture prior to introduction into the slurry or 
to the dry cement is easily carried out by known mixing procedures for 
handling dry powders or if the copolymer is in an aqueous solution, the 
dispersant is readily stirred into the solution. 
The oil well cements employed in the instant invention are API 
classification cements--A, B, C, D, E, F, G, and H, pozzalanic cements, 
pozzolan-lime cements, resin or plastic cements, gypsum cements, diesel 
oil cements, expanding cements such as API classification cements K and M, 
calcium aluminate cements, and latex cements. 
The copolymer-dispersant mixture prepared and used according to this 
invention may be added to the cement slurry in a variety of ways. It may 
be applied in a solid form, including being premixed with the cement 
before it is added to water or the dry polymer-dispersant mixture may be 
added to the cement water slurry or individually admixed with the dry 
cement or slurry. Alternatively, the copolymers and dispersants may be 
applied as aqueous solutions to the dry cement or cement slurry. In the 
latter cases the water included in the copolymer solution replaces an 
equal amount of water normally included in the cement slurry. The dry 
mixture usually contains from 0.01 to 100, preferably 0.1 to 10, optimally 
0.3 to 3.0 parts by weight of the copolymer of N-vinylpyrrolidone per part 
by weight of the sale of condensed formaldehyde naphthalene sulfonic acid 
(usually the calcium salt). 
The amounts of water and cement contained in the subject cementing 
compositions which include the copolymers of N-vinylpyrrolidone and 
styrenesulfonate salts are highly dependent on the type of cement being 
used, the other additives being used in the cementing formulation, the 
borehole conditions and the techniques being used to apply the cement 
formulation to the borehole. Those skilled in the art of borehole 
cementing will by consideration of these factors be able to determine the 
proper cement slurry formulation which will maximize benefits from 
inclusion of the mixtures of this invention. 
A variety of other additives may be used in a cement formulation treated 
with the copolymers of this invention. For example, cement hardening 
retarders, hardening accelerators, materials to lower or raise the density 
of the slurry, lost circulation-control agents, friction reducers, 
stabilizers for high temperature strength, etc. may all be used together 
with this polymer. This list of formulation ingredients which might 
accompany the polymer of this invention in a cement slurry is meant to be 
exemplary, but in no way are the accompanying materials limited to those 
mentioned herein. These and other cement additives may be combined with 
the subject mixtures as it is determined appropriate by workers skilled in 
the well cementing art. 
The copolymer cement formulations and processes described herein as 
embodiments of the subject invention represent substantial improvements to 
the art of subterranean well cementing. As compared to the materials and 
methods known previously for the control of fluid loss in well cementing 
formulations, use of the subject copolymer-dispersant mixtures are 
advantageous in several respects. Unlike many of the previously known 
fluid loss agents, the copolymer described herein is shown to possess a 
synergistic relationship with the dispersants accompanying it in the 
subject mixtures, above and beyond the additive effects displayed by 
either copolymer or dispersant alone in filtration control activity. The 
copolymer-dispersant mixtures have the remarkable thermal stability of 
their components and provide excellent fluid loss control at temperatures 
greater than 95.degree. C. without many of the undesirable side effects 
common to other cement fluid loss control agents. The copolymer-dispersant 
mixtures cause little thickening of the cement slurry, little retardation 
of the cement thickening, and no increase in free water. Thus, these 
mixtures represent a broadly applicable, general purpose cement fluid loss 
control treatment, of the type needed for the difficult cementing 
environments more and more commonly encountered in today's well drilling 
situations. 
The following examples will further illustrate the novel qualities of the 
additive compositions and processes of the present invention without in 
any way limiting the scope of this invention.

EXAMPLE 1 
Synthesis of copolymers of sodium styrenesulfonate (SSS) and 
N-vinylpyrrolidone (NVP) 
Copolymer (PD860-29.2) 
In a flask, to 300 ml of distilled water, purged with N.sub.2 for 1 hour at 
60.degree. C., freshly distilled N-vinylpyrrolidone (NVP) 7.748 g and 
sodium styrenesulfonate (SSS) 42.26 g were added and dissolved. After 
sparging the solution with N.sub.2 for an additional 1 hour, 0.10 g of a 
azoisobutyronitrile (AZBN) was added. 
The solution became thick during 20 hours at 60.degree. C. The resulting 
copolymer was purified by precipitation from a large volume of acetone. 
Yield: 72.1% (based on weight of monomers). Copolymer composition: 
NVP/SSS=15.3/84.7 mol/mol. Reduced viscosity: 3.6 dl/g at 0.42% in 2% NaCl 
aqueous solution. 
EXAMPLES 2-9 
These examples demonstrate the fluid loss control activity of the subject 
copolymer-dispersant mixture as a cement filtration control agent at 
temperatures ranging from from 30.degree. to 79.degree. C. 
For comparison, the results of the use individually of the copolymer 
(PD860-29.2) and the preferred dispersant (the calcium salt of condensed 
naphthalenesulfonate) are set forth to demonstrate the synergistic results 
of the mixture as a cement fluid loss control additive at 30.degree. C. 
For each test sample, 860 g of API Class H cement (Lone Star Industries; 
Pasadena, Tex.), and 327 g of tap water and 0.3 g of silicone defoamer 
were mixed at high speed (12,000 rpm) in a 1 qt capacity Waring Blender 
for 35 seconds. Then 7.14 g (0.6% based on total slurry weight and 100% 
active polymer) of candidate fluid loss control material was added, and 
the slurry was aged at 30.degree. C. in an atmospheric cement 
consistometer at which time a consistency value (Bauden units) was 
recorded with a value ranging from 9-15 desired. 
For measurement of fluid loss, the slurry was transferred to a Baroid High 
Pressure Filter Press (Model 311, NL Baroid/NL Industries, Inc., Houston, 
Tex.). The fluid loss was measured according to API Specification 10, 
Appendix F (1982) at 1,000 psi differential pressure and at temperatures 
ranging from 30.degree. to 79.degree. C. 
Recorded results were corrected to represent filtration through the 
standard 7.1 sq in filtration area by multiplying the reading obtained by 
2.0. 
The results obtained were as follows: 
TABLE 1 
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Use rate of Use Rate of 
Copolymer Dispersant* 
Filtrate 
Consistency 
PD860 29.2 % of Dry ml. in 30 
(Bauden 
Example 
% of Dry Cement 
Cement wt. 
Min. Units) 
______________________________________ 
Tested at 30.degree. C. 
2 0.6 -- 180 15 
3 0.6 0.3 64 10 
4 0.6 0.6 31 9 
5 -- 0.6 348 9 
6 -- -- 3000+ 11 
Tested at 60.degree. C. 
7 0.6 -- 568 13 
8 0.6 0.6 60 9 
Tested at 79.degree. C. 
9 0.6 0.6 120 6 
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*Commercially sold as Lomar D by DiamondShamrock, Morristown, NJ and 
believed to be the sodium salt of formaldehyde condensed 
naphthalenesulfonic acid. 
It is clear from the results data that a particular NVP/SSS copolymer in 
which the molar ratio of NVP to SSS was about 7:3 in admixture with a 
condensed naphthalenesulfonate calcium salt optimally at a weight ratio 
range of about 1:1 provides an exceptional decrease of fluid loss (at 
30.degree. C., 1.2 wt.% of the mixture reduced fluid loss from the cement 
to 31 ml/30 minutes as from Example 3 of the Table).