CA1097555A - Polymeric scale inhibitors - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Copolymers of acrylic acid and methacrylic acid are disclosed which are capable of stabilizing calcium and barium salts under conditions of high pH and elevated temperatures. They are particu-larly useful for treating waters used in the secondary recovery of petroleum.

Description

It ix well known that natural haters and those found in underground fol~ations contain hardness scale forming ions such as ca]cium which tend to precipitate and form dcposits or scale. Thus, when such waters are used as cooling waters in cooling towers in many industrial processes, there is a tendency for scale to be deposited and eventually it is necessary to remove such scale which, in turn, makes it necessary to discontinue operation of the particular process for a period of time until the scale can be removed.
The formation of insoluble calcium salts is also a problem in the secondary recovery of oil from subterranean formations by processes in which water is introduced into injection wells and forced through the underground formations to cause oil to be produced in a producing well. This type of process is usually referred to as a waterflood system.
Many of the waters used in secondary reooYery operations, in addition to containing large quantities of calcium salts, also contain quantities of barium sulfate. Many scale inhibitors or suppressants are capable of complexing with either calcium salts and/or barium salts, but they are subject to several drawbacks. In certain cases they are selectiYe with respect to calci~ ions only. Further, under conditions of elevated temperatures and when the pH of the water being treated is highly alkaline, e.g. 9 or higher, many inhibitors decompose.
For instance, in our Uni'ed States Patent No. 49008,164, for acrylic acid methacrylate copolymers as scale inhibitors, it is shown that certain acrylic acid-methylacrylate copolymers are capable of inhibiting calcium-type scales at elevated temperatures and high pH's which are commonly found in subterranean formations containing oil. While these materials haYe shown themselves to possess great utility, they are relatively ineffective in treating waters which contain soluble barium salts such as barium sulfate.
~oth barium and calcium salts are readily inhibited from the standpoint of pTeventing or mitigating scale formation by using the now well-known phosphonate inhibitors such as, for instance, ~C~2)6N2fCH2PO~OM)2~4. ~hile inhibitors of this type are capable of inhibiting both calcium and bari~
salts, they are relatively unstable at elevated temperatures whereby they ~7SSS

decomposc and lose their aetivity.
It, therefore, would be of benefit to the art if a scale inhibitor were provided t.hich was stable at elevated tempeTatures, would be effective at highly alkaline pH's, and could stabilize or inhibit scale formation caused by both calcium and barium ions.
Uni~ed States Patent 3,756,257 shows that polyacrylic acid polymers prepared by specific polymerization techniques have utility to remove and inhibit scale formation in water-treating apparatus or in oil or gas wells.
United States Patent 3,904,685 is similar in its disclosure to that of United States Patent 3,756,257 discussed above. These particular acrylic acid polymers are used as chelants in such applicational areas as dishwashing detergents.
United States Patent 3,766,077 shows that polyacrylic acid, when combined with ligno sulfonates, provides a scale inhibition composition. The patentee further indicates that under the scaling tests used by him poly-acrylic acid alone did not satisfactorily control scaling.
United States Patent 2,783,200 relates to the use of polyacrylic acid or polymethacrylic acid for treating scaling in boileTs.
United States Patent 3,663,448 relates to acrylamide acrylic acid copolymers in combination with certain amino alkaline phosphonates as scale inhibitors.
United States Patent 3,514,376 deals with certain low molecular weight polyacrylic acid within certain molecular weight ranges as being effective in controlling scaling in saline water evaporators.
United States Patent 2,71~,497 shows a variety of acrylic acid polymers and copolymers as being effective as driIling mud additives to control water loss. Scale prevention is not discussed by the patentee.
In accordance with the present invention, the~e is provided a process for preventing scale deposits from the class consisting of calcium 3Q sulfate, calcium carbonate and barium sulfate and mixtures thereof ~rom water con~aining said scale forming chemicals which comprises adding to said wa~er an effective scale inhibiting amount of a copolymer of acrylic ~3975SS

acid and metllacTylic acid havillg a molecular weight within the range from 1,000 to 25,000 and obtained by copolymerizing acrylic acid and methacrylic acid using a mol~r excess of acrylic acid oveT methylacTylate of at least 3:1. The molar excess of acrylic acid over methacrylic acid is at least 3:1 and, preferably~ ~ithin the range of 4:1 to 5:1. These copolymers have a relatively low molecular weight as determined by gel permeation. Generally, those polymers ha~ing a molecular weight range of l,OOQ - 25,000 are useful in the practice of the invention. Preferably, those ha~ing molecular weight range of 3,000 - 20,000 and, most preferably, 6,000 - 8,000 may be employed to good advantage.
A typical preparation of these copolymers would utilize ammonium persulfate as the catalyst and thioglycolic acid would be used as the re-ducing agent for the catalyst and as the chain transfer agent. The poly-merizations are preferably conducted in water or a water miscible organic solvent such as isopropanol. A specific preparation would be as fGllows:
The water, acrylic acid, methacrylic acid and thioglycolic acid are added to the reaction chamber and full cooling is turned on. With the reaction mixture at 15Co to 25C. and with full cooling, the catalyst solution is added. After a short initiation period of from 1 to 4 minutes, a strong exotherm indicates the polymerization is in progress. After about 3 to 8 minutes, the polymerization is comple~e and the sodium hydroxide is added to neutralize the solution. Then the methanol is added as an antifreeze.

., ~ ( lV~7555 Us.ing the above generzlized procedure, the following ingreai-ents listed below would constitute a typical charge to a commercial reactor:
In~redients % by Weight Water 36.48~
Acrylic Acid 14.9g%
Methacrylic Acid 5.83~
Thioglycolic Acid 1.67%
Ammonium Persulfate 1.17%
Water 20 33 ~50~ sodium hydroxide - in water) 11.70~
Me~hanol 9.16%
E~hylene Glycol 16.6?% .
Total 100.0Q%
Properties of ~his material after reaction are set ~orth below:
Color - Light yellow - .
. Odor Sharp Pour point -15F. ~
Flash point Greater than 175F t.o~c.
p~ 5.3 ;
Specific gravity -- 1~14 - -Density ~5 pounds/gallon . -Solubility Soluble in fresh water and brines. Insoluble in hydro~
carbons~ -The above composition ~ill be hereinafter reerred to a~
Compssition 1.

~ ~ ~0975S5 Composition 1 as well as certain other prior art scale inhib-itors were subjected to scale inhibition tests, thermal stability tests, calcium stability tests, and their ability to inhi~it barium sulfate precipitation from various brines.
The other compositions which were used as comparisons to s~o~
the efficiency of the copolymers used in the practice of this inven-tion are listed ~elow: - `
Composition 2 Ingredients -% by Weight Acrylic acid 20.77%
Water 43.61 - Methyl acrylate 5.54~
Thioglycolic Acid - -2.08%
~NH~,)2 S2O8 1.39 Water ~,77~
NaOH Solution (5Q%) 13.84%
Methanol 10.00~
Total 100.00%
Composition 3 50~ solution of a phosphated triethanolamine whose mode of preparatior. is described in U.S. 3~620,974.

Composition 4 Composition 4 is an ethoxylated glycerine which has been phosphated and prepared as a 58% ~y weight solution. This materia-¦is described along with its method of manufacture in U.S. 3r728,42Q.

Composition 5 Composition 5 is a 25% aqueous solution of sodium polyacrylate ~a~ing a molecular weight believed to be about 8rO00.

~; ~ iO47555 Col,~position 6 Composition 6 is a 49% aqueous solution of:
(CH2)6N2[CH2PO(OM) 2] 4 Example l Lahoratorv Barium Sulfate Inhi~ition Tests In these tests ~rine X, Y, and the other letter designate~
brines are synthetic brines having compositions similar to those often found in Connate Waterflood waters. -. Barium Sulfate Test Stanaard Test, BO.ml Brine X, 20 ~1 Brine Y

Chemical Concen~ration . ~ . ..

2 3 5 1~ ~5 20 sa Chemical ~pm ppm ppm ppm ~ ppm ppm Comp. 1 90% lG0~
Comp. 2 -. :- 11% 13% 13% 16~ 13% -. Comp. 3 50% lOG~ . .
Comp. 5 81% 89% 89~ 100% .

Blank, before precipitation ~ 21 mg Ba/l, calc. as baFium.

Blank, after precipitation - 0 mg Ba/l, calc~ as barium.
. , . - .
Exam~le 2 - :

. This example illustrates the abilit~ o~ the composition~ of .
the invention to inhibit calcium carbonate scale even af~er the composition has been exposed to temperatures o~ 350~F~ for four ¦¦ day5 I _7_ ~ (~ ( `` ~97~55 Test #1 - Standard l~est, 50% Brine C, 50% Brine D
Chemical Concentration 1 2 3 4 5 ~ - 10 15 Chemical ppm ppm ppm ppm ~ ppm ppm Comp. 1 - - 43% 88% 94%lQ0%
Comp. 1* - 43~ 88% 94%100%
Co~p. 2 ~ 7~ ~3%100%
Comp. 3 23% 61% 81% laO%
Comp. 5 - - ~69% 94% 100%

Blank, beore precipitation - 4,200 mg Ca & Mg/l, calc. as CaC03 4,200 mg Co3~1, calc. as CaC03 . . 72,425 mg Cl~l, calc. as NaCl ~lank, after precipitation - ~,6QQ mg Ca & Mg~l, calc. as CaC~3 *A one-percent (1%) solution of Comp. 1 in distilled water was exposed to a temperature of 35.0F. for 4 days after which time the one-percent tl%) solution was used to run the standard calcium carbonate scale inhibition test.
Example 3 . Thi~ example illustrates the ability of the compositions of the invention to inhibit calcium sulfate.
Test $1 - Standard Test, 50% Brine A, 50% ~rine B
.
- Chemical Concentration 0.2 0.3 0.4 0.5 0.7 0~8 0.9 1 D Q 1.25 2.0 3.0 Chemical ppm ppm ppm ppm ppm ppm ppm` ppm:~ppm- ppm ppm .
Comp. 1 - - - 54% - - - 91~ 100~ - _ . C~mp. 2 - - 0% - ~ 50% 75% 100%
Comp. 3 33% 92% 100%
Comp. 5 - - - . 66% 83~ 91~ 100~
Blank, before precipitation - 5,1~/0 mg Ca~l, calc. as CaS04 5,170 mg S~-4/l, calc. as CaSO
11,950 mg Cl/l, calc. as NaCl Blank, after precipitation - 3,540 mg Ca/l, calc. as CaSO4 ~`
~ 7S~S

Example 4 Example 4 shows the effectiveness of the compounds when used in a standard calcium stability test.
Calcium Stability Tests 160F., 5% NaCl Brine,-l~ Chemical Chemical Calcium (mg/l) ~ at Cloud Point Comp. 2 2000 10.7 Comp. 2 3000 5.6 . .
Comp. 1 1400 . 10.5 :
Comp. 1 1600 5.1 . Room Temperature, CaCO3 Brine C, 1~ Chemical ChemicalpH at Cloud Point Comp. 1 7,5 - .
Comp, 3 Comp. 4 6.4 Comp. 2 10.3 Comp. 5 . 3 5 Comp. 6 4,9 . .

Composition of Brine C
Brine C - -. ' - . .
12,160 mg CaCl2 2 H2O

3,680 mg MgCl2 2 H20 per li~er of ` 66,00Q mg NaCl distilled water.
.' . '`.
` ' ' Having thus de~cribed our invention, it is claimed as : follows:

Claims (3)

1. A process for preventing scale deposits from the class consisting of calcium sulfate, calcium carbonate and barium sulfate and mixtures thereof from water containing said scale forming chemicals which comprises adding to said water an effective scale inhibiting amount of a copolymer of acrylic acid and methacrylic acid having a molecular weight within the range from 1,000 to 25,000 and obtained by copolymerizing acrylic acid and methacrylic acid using a molar excess of acrylic acid over methylacrylate of at least 3:1.

2. A process as claimed in Claim 1 in which the molar ratio of acrylic acid to methylacrylate is within the range of 4:1 to 5:1.

3. A process as claimed in Claim 1 in which said copolymer is added to water in an injection well used to force oil from under-ground formations to one or more producing wells in a waterflood system.