High temperature stable aqueous brine fluids

An aqueous brine fluid, particularly useful as a drilling fluid, and containing a high salt concentration, a vinyl sulfonate amide copolymer such as a water soluble copolymer of acrylamidomethylpropanesulfonic acid salts, and a thermal stability enhancing additive, is provided. The aqueous brine fluid is particularly useful at elevated temperatures in the range of about 150.degree.-220.degree. C.

Co-pending Application Ser. Nos. 413,571, filed Aug. 31, 1982, and 502,054 
filed June 7, 1983, now U.S. Pat. No. 4,514,310 in the name of Israel J. 
Heilweil relates, inter alia, to drilling fluids containing a 
polyvinylpyrrolidone thickening agent. 
Co-pending Application Ser. No. 413,575, filed Aug. 31, 1982, now U.S. Pat. 
No. 4,498,994 in the name of Israel J. Heilweil relates, inter alia, to 
drilling fluids containing non-aqueous solvents such as an 
N-methyl-2-pyrrolidone. 
Co-pending Application Ser. No. 454,204, filed Dec. 28, 1982, now U.S. Pat. 
No. 4,490,261 in the name of Israel J. Heilweil relates, inter alia, to 
drilling fluids viscosified with basic N-heterocyclic polymers, such as 
polyvinylpyridine. 
Co-pending Application Ser. No. 490,623, filed May 2, 1983, now abandoned 
in the name of Israel J. Heilweil relates, inter alia, to drilling fluids 
viscosified with water-soluble copolymers of 
acrylamidomethylpropanesulfonic acid salts. 
The entire disclosures of these above-mentioned U.S. patent applications 
are expressly incorporated herein by reference. 
FIELD OF THE INVENTION 
The present invention relates generally to polymers and copolymers in 
aqueous and polar-organic mixtures, and specifically to high temperature 
stable viscosified brine drilling fluid systems for use in oil and gas 
recovery and other industrial applications. These drilling fluids have a 
high salt concentration and are enhanced in viscosity and thermal 
stability by the use of additives. 
BACKGROUND OF THE INVENTION 
Substantial future oil well drilling will be at depths between 15,000 and 
30,000 feet where temperatures encountered can be 220.degree. C. Drilling 
fluids must be tailored to accommodate the nature of the formation stratum 
being encountered at the time. When the drilling reaches the producing 
formations, special concern is exercised. Preferentially, low solids 
content fluids are used to minimize possible productivity loss by solids 
plugging, enhance drilling rate and reduce sedimentation in inclined 
wells. Proper fluid density for balancing formation pressure may be 
obtained by using high salt concentration aqueous brines, while viscosity 
and filter loss control may be obtained by polymer addition. 
The high temperatures, coupled with the desire for low solids content and 
preferably minimum added solids, require brine tolerant and high 
temperature stable polymers for viscosity and filtration control. 
Current high density, clear brine systems utilize hydroxyethyl cellulose 
polymers and related materials as viscosifiers, but these are normally 
unstable at about 135.degree. C., and tend to cross-link and gel with time 
and temperature, thus causing various drilling operational problems. 
In drilling for oil and gas, such as the Mobile Bay and Arun fields, the 
use of clear brine completion, workover, and packer fluids containing 
divalent calcium and zinc, can lead to precipitation of carbonates, 
plugging of wells, and subsequent loss of hydrocarbon productivity. Sodium 
bromide brines provide a possible alternative. Until the present, however, 
the use of clear brine fluids has not been fully developed because of the 
unavailability of viscosifying agents and additives to provide suitable 
rheological properties for removal of drilled solids and to minimize 
filtration losses. In addition, it is generally difficult to predict what 
effect the brine, particularly high concentrations thereof, will have on 
the thermal properties of the polymer. 
Viscosifier/brine systems, which are stable in a 150.degree.-220.degree. C. 
range, and are suitable for high temperature, deep-drilling operations for 
oil and gas, oil recovery and other possible applications are known to the 
art. For example, U.S. patent application Ser. Nos. 490,901 and 490,623 
disclose the addition of N-methylpyrrolidone in order to enhance the 
thermal stability of a vinyl sulfonate amide copolymer in a dense sodium 
bromide brine at 220.degree. C. However, a relatively high dosage of the 
added N-methylpyrrolidone is required. Additionally, U.S. Pat. No. 
4,309,523 to Engelhardt et al, the entire disclosure of which is expressly 
incorporated herein by reference, describes drilling muds, i.e., drilling 
fluids which contain clay, water-soluble copolymers of 
acrylamidomethylpropanesulfonic acid salts and other components, such as 
salts, but does not refer to clear brine. 
Thus, although some form of thermal stability of brine fluids has been 
found in the 150.degree.-220.degree. C. range, there is room for 
improvement. It is therefore an object of the present invention to provide 
an improved aqueous brine fluid particularly for use as a completion, 
workover, packer and drilling fluid. 
Further, it is an object of this invention to overcome the deficiencies of 
the prior art. 
It is another object of this invention to increase the thermal stability of 
polymeric viscosifiers in brine drilling fluids at high temperatures, 
e.g., in the range of 220.degree. C. 
It is still another object of this invention to provide an aqueous brine 
fluid with enhanced thickening characteristics. 
It is a further object of this invention to provide an aqueous brine fluid 
which is economically more desirable. 
Additionally, it is an object of this invention to provide new and superior 
additive systems for brine fluids containing vinyl sulfonate amide 
copolymers. 
Still further, it is an object of this invention to produce a new and 
improved aqueous brine fluid by the use of additives which increase the 
thermal stability of the brine fluid without having to chemically alter 
the brine fluid system. 
These and other objects will become apparent from the specification which 
follows. 
SUMMARY OF THE INVENTION 
According to one aspect of this invention, there is provided an aqueous 
brine fluid comprising: 
(a) a densifying amount of a densifying salt sufficient to densify the 
aqueous brine fluid, said densifying salt constituting at least about 30% 
by weight of said fluid; 
(b) a vinyl sulfonate amide copolymer in an amount from about 3% to about 
5% by weight of said fluid; and 
(c) at least one thermally stable enhancing additive, said additive 
comprising at least one unit of the following formula 
##STR1## 
wherein R.sup.1 is N or P; 
R.sup.2 is H, alkyl, aryl, alkyl amine or their derivatives; and 
R.sup.3 is O or S; wherein said additive constitutes from about 1% by 
weight to about 5% by weight of the fluid. 
According to another aspect of the invention, there is provided a method 
for enhancing the thermal stability of an aqueous brine fluid, said method 
comprising combining with said brine fluid a thermally stable enhancing 
additive as described above in (c). 
According to another aspect of the invention, there is provided a high 
temperature stable viscosified brine drilling fluid comprising the 
components disclosed above in (a), (b) and (c). 
According to still another aspect of this invention, there is provided an 
aqueous brine fluid comprising: 
(i) a viscosity increasing amount of one or more water-soluble copolymes of 
acrylamidomethylpropane-sulfonic acid salts, consisting essentially of a 
random distribution of 
(a) 5 to 95% by weight of units of the formula 
##STR2## 
(b) 5 to 95% by weight of units of the formula 
##STR3## 
and 
(c) 0 to 80% by weight of units of the formula 
##STR4## 
wherein R.sup.1 and R.sup.2 are the same or different and each is 
hydrogen, methyl or ethyl; and X.sup..sym. is a cation; 
(ii) a densifying amount of a densifying salt sufficient to densify said 
fluid, said densifying salt constituting at least about 30% by weight of 
said fluid; and 
(iii) a thermal stability enhancing amount of N-methyl-2-pyrrolidone. 
Still further, there is provided a method for increasing the viscosity and 
thermal stability of an aqueous brine fluid, said method comprising 
combining with said brine fluid a viscosity increasing amount of one or 
more water-soluble copolymers of acrylamidomethylpropanesulfonic acid 
salts, consisting essentially of a random distribution of 
(a) 5 to 95% by weight of units of the formula 
##STR5## 
(b) 5 to 95% by weight of units of the formula 
##STR6## 
and 
(c) 0 to 80% by weight of units of the formula 
##STR7## 
wherein R.sup.1 and R.sup.2 are the same or different and each is 
hydrogen, methyl or ethyl; and X.sup..sym. is a cation; and a thermal 
stability enhancing amount of N-methyl-2-pyrrolidone. 
There is also provided a method for flushing potentially formation-damaging 
materials comprising cuttings and clays from a wellbore prior to 
perforation of a well, said method comprising injecting a completion fluid 
through the drill pipe of said wellbore and recirculating said completion 
fluid to the surface in the annular area between the wellbore wall and the 
drill string, wherein said completion fluid is heated to a maximum 
temperature of between 135.degree. to 170.degree. C. during said method, 
and wherein said completion fluid comprises an aqueous brine viscosified 
with a viscosity increasing amount of one or more watersoluble copolymers 
of acrylamidomethylpropanesulfonic acid salts, consisting essentially of a 
random distribution of 
(a) 5 to 95% by weight of units of the formula 
##STR8## 
(b) 5 to 95% by weight of units of the formula 
##STR9## 
and 
(c) 0 to 80% by weight of units of the formula 
##STR10## 
wherein R.sup.1 and R.sup.2 are the same or different and each is 
hydrogen, methyl or ethyl; and x.sup..sym. is a cation; and a thermal 
stability enhancing amount of N-methyl-2-pyrrolidone.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is designed to be used in the exploration and the 
production of new oil and gas, especially in hot/deep formations, e.g. 
Mobile Bay. Additionally, it is believed that the present invention is 
applicable to other industrial processes and operations, e.g., oil 
recovery, desalinization, etc., where high temperatures in brines are 
encountered. Further, the present invention can be extended to a wide 
range of polymers and copolymers in aqueous and polar-organic solvents, as 
well as to solid and composite polymer and copolymer systems containing 
polar groups. 
The present invention applies to thermal stability enhancing additives 
containing one or more units of the following group: 
##STR11## 
in a single or multi-cyclic structure in which R.sup.1 may be either N or 
P; R.sup.2 may be H, alkyl, aryl, alkyl amines or their derivatives, and 
R.sup.3 may be either O or S. Significantly, the amount of additive 
necessary for producing the novel aqueous brine fluid described herein is 
comparatively low, i.e., from about 0.1% to about 5% by weight of the 
fluid. Examples of such additives are identified by the following table: 
TABLE 1 
______________________________________ 
Thermal Stability Enhancing Additive 
NAME STRUCTURE 
______________________________________ 
Nmethyl-2-pyrrolidone 
##STR12## 
1-methyl-2-pyridone 
##STR13## 
N,N'dimethyl-propylene area (Aldrich 25, 156-9) 
##STR14## 
##STR15## 
N(N,Ndimethylamino)- propyl-2-pyrrolidone (GAF) 
##STR16## 
Nmethyl caprolactam 
##STR17## 
Hydantoin 
##STR18## 
______________________________________ 
Other additives are exemplified by derivatives of thiourea, caffeine and 
thymine which may contain polar solubilizing groups, such as SO.sub.3 -, 
NO.sub.3 -, OH, N, O, COOH, esters, and halogens. 
Regarding the brine fluid, high density, aqueous brine completion, 
workover, and packer fluids have become recognized in the last few years 
for their effectiveness in minimizing formation damage and providing 
wellbore stability, as well as in establishing and maintaining high 
productivity of oil and gas wells, G. Poole, Oil and Gas. J., July 13, 
1981, p. 151; D. Acosta, Ibid., Mar. 2, 1981, p. 83; R. J. Spies et al, 
SPE 9425, September 1980. 
Specifically, high density brine fluids are solutions containing 10 to 60% 
by weight of salts, such as CaCl.sub.2, NaBr, NaCl, KSCN, CaNO.sub.3, 
ZnCl.sub.2, ZnBr.sub.2, CaBr.sub.2, Kl, LiCl, CaI.sub.2, Ca(SCN).sub.2 and 
their mixtures, having densities up to about 2.4 g/cc (about 20 lbs/gal). 
Their high salt content prevents swelling and dispersion of formation 
clays and shales by favorable ion exchange and osmotic pressure reduction. 
Their high densities provide sufficient hydrostatic head-to-balance 
formation pressures and thus prevent the influx of undesirable fluids into 
wells during drilling. Because they are free of dispersed solids, the high 
density fluids are particularly noted and have potential for prevention of 
formation plugging, high hydrocarbon recovery, efficient bottomhole 
cleaning, enhanced drilling rate and prevention of sediment formation in 
inclined wells. 
With current accelerated efforts to discover new oil in gas deposits by 
deep drilling, e.g., 15,000 feet, it has become recognized that future 
developments in drilling fluid technology must make use of viscosifiers, 
fluid loss control agents, and other additives that are capable of 
satisfactory performance at high temperatures (greater than 135.degree. 
C.) and high pressures (greater than 5,000 psi). 
With regard to the vinyl sulfonate amide copolymer viscosifiers, it is 
preferred to use a water-soluble copolymer of 
acrylamidomethylpropanesulfonic acid salt, which consists essentially of a 
random distribution of 
(a) 5 to 95% by weight of units of the formula 
##STR19## 
(b) 5 to 95% by weight of units of the formula 
##STR20## 
and 
(c) 0 to 80% by weight of units of the formula 
##STR21## 
wherein R.sup.1 and R.sup.2 are the same or different and each is 
hydrogen, methyl or ethyl; and X.sup..sym. is a cation. 
Such copolymers are described in the previously-mentioned Englehardt et al 
U.S. Pat. No. 4,309,523 and preferably have a molecular weight of at least 
10,000. 
One or more water-soluble copolymers of acrylamidomethylpropanesulfonic 
acid salts are added to aqueous high brine content solutions (e.g., 30-60 
wt % salt) providing a liquid of substantially enhanced viscosity which is 
particularly suitable for use in oil well drilling operations. The use of 
such polymers results in improved resistance to viscosity loss at elevated 
temperatures (e.g., more than about 300.degree. F.) compared to 
conventional viscosity building water/brine soluble polymers and thus is 
useful in drilling operations involving deep wells. They may also provide 
some filtration control properties, reducing fluid loss into, and possible 
damage to, the oil bearing formation. 
The effectiveness of -N-methyl-2-pyrrolidone is believed to be due to its 
multi-functional properties as a buffering agent, as a molecule capable of 
complexing metal ions, e.g., Fe, Cr, Cu, Mn, etc., that otherwise could 
lead to catalytic decomposition of copolymers, and as a peroxy radical 
terminator. In this connection, it has been reported in U.S. Pat. No. 
4,317,759 to Kanda et al that the thermal stability of amide-containing 
polymers is enhanced by incorporation of synergistic combinations of 
mercaptobenzoimidazole and phenyl derivatives during dissolution or 
synthesis. 
Quite surprisingly, it has now been discovered that the thermal stability 
enhancing additives of the present invention enhances the thermal 
stability of a vinyl sulfonate-amide copolymer, e.g., HOE 3118, in a dense 
brine fluid at low dosages and without necessitating complex chemical 
reactions. The additives are particularly effective for CaCl.sub.2 (11.2 
lbs/gal) and NaBr (12.5 lbs/gal) brines. The effective dosage for the 
thermal stability enhancing additive ranges from about 1 to about 5% of 
the weight of the brine fluid, and preferably is about 3% of the weight of 
the brine fluid. 
The present invention will be explained further by means of the following 
examples, which are not meant to limit the invention. 
EXAMPLES 
Vinyl sulfonate amide copolymers (HOE 3118 and 2825, Hoechst Corp.) will 
viscosify NaBr and CaBr.sub.2 dense, clear brine drilling fluids (about 
50% salt, 12.5 and 14.5 lbs/gal, respectively) and retain their 
viscosifying properties, especially with the case of HOE 3118, in about 
175.degree.-200.degree. C. range. By comparison, currently employed 
cellulosic viscosifiers become ineffective below 135.degree. C. C.sup.13 
NMR analysis of HOE 3118 showed this copolymer to be composed of 62% 
2-acrylamido-2-methylpropane-3-sulfonate and 38% N-vinyl-N-methylacetamide 
monomers. The 38% by weight CaCl.sub.2 brine (11.2 lbs/gal, 5.9 cp at 
25.degree. C.) used in the Table 5 was a laboratory sample prepared from 
anhydrous CaCl.sub.2 (J. T. Baker) in distilled water, and filtered 
through Whatman #42 filter paper. The final brine solution was made up by 
adding the polymer particles at a rate of 0.1 g/min/100 g solution into 
the surface of the liquid vortex generated by a magnetic stirrer. 
Occasionally, additional shaking of the solution for 1-4 hours was 
required to complete the process. Dissolved air was not removed, and the 
initial solution pH and the pH after heating was about 6-7. 
In static aging tests, about 3 ml of 1.5% by weight polymer solution were 
placed in an open pyrex cup and inserted into the interior of a T-type 
Swagelok filter, Model Nupro TF, made of 316 stainless steel and capable 
of withstanding 6,000 psi. The securely tightened assembly was then placed 
in an oven at a given temperature for up to 16 hours. Before and after 
placing the assembly in the oven, the cup and its contents and the total 
assembly were weighed so as to detect any leakage. 
After cooling the assembly at room temperature, 1 ml of solution was 
removed, and its viscosity determined with a Brookfield LVD cone and plate 
viscometer maintained at 25.degree. C., covering the general shear rate 
region between 2 and 50 sec.sup.-1. Results were compared with initial 
values at 25.degree. C. prior to aging. (Note: The ability of a fluid to 
suspend particles, which is essential to drilling fluid performance, is 
believed to be related to viscosity at 7 sec.sup.-1.) 
Table 2 summarizes 16 hour static aging experiments with 5.0 wt % HOE 3118 
and 2825 in NaBr brine (12.5 lbs/gal) up to 176.degree. C. and 
demonstrates little or no degradation for HOE 3118; HOE 2825 is stable up 
to about 150.degree. C. At 200.degree. C. (see Table 3) HOE 3118 retains 
its viscosifying properties for at least 4 hours. (The deviations are 
within experimental error.) After 16 hours heating at 200.degree. C., the 
viscosity of HOE 3118 is decreased by 23%, but the inclusion of 0.6 wt % 
of N-Methyl Pyrrolidine as an additive improves the performance. Table 4 
describes 16 hour static aging experiments of HOE copolymers in CaBr.sub.2 
(14.5 lbs/gal) and West Burkburnette (WBB) brines. The WBB brine contains 
13 wt % NaCl, 3.45 wt % CaCl.sub.2.H.sub.2 O and 1.5 wt % 
MgCl.sub.2.6H.sub. 2 O. The WBB experiment relates to the ability of HOE 
3118 to function as a mobility control agent. In CaBr.sub.2 brine HOE 3118 
is stable up to about 175.degree. C. 
TABLE 2 
______________________________________ 
NaBr BRINE (12.5 lbs/gal) 
5% VINYL AMIDE - VINYL SULFONATE POLYMERS 
STATIC AGING FOR 16 HOURS 
APENT VISCOSITY, CP 
HEATING AT 25.degree. AND 11.5 SEC.sup.-1 
TEMP., .degree.C. 
BEFORE AFTER % CHANGE 
______________________________________ 
HOE 3118 
90 38.0 37.5 -1.3 
150 38.0 37.0 -2.6 
176 35.5 36.5 +2.8 
HOE 2825 
90 99.5 97.5 -2.0 
150 99.5 96.0 -3.5 
169 92.0 55.5 -40.0 
______________________________________ 
TABLE 3 
______________________________________ 
AGING EXPERIMENTS OF 5.0 wt % HOE 3118 
IN NaBr (12.5 lbs/gal) at 200.degree. C. 
Apparent Viscosity, cp, at 25.degree. C. and 23 sec.sup.-1 
BEFORE HEATING AFTER % 
HEATING DURATION HEATING CHANGE 
______________________________________ 
35.0 (cp) 3 (hrs) 32.5 (cp) -7.1 
35.0 4.3 31.0 -11.0 
35.0 16 27.0 -23.0 
With 0.6 wt % N--Methyl Pyrrolidone 
35.5 (cp) 3 37.0 +4.2 
4 39.0* +9.9 
16 30.0 -15.5 
______________________________________ 
*1 hour at 206.degree. C. 
TABLE 4 
______________________________________ 
Viscosities of 2% by wt HOE 2825 and 3118 
Before and After 16 Hours Heating 
Apparent viscosity 
at 25.degree. C., cp.sup.(a) 
Medium Temp. .degree.C. 
Before After % Change 
______________________________________ 
HOE 3118 
CaBr.sub.2 
175 29.3* 34.8 +16 
WBB 125 4.9 4.2 -14 
10.7.sup.(b) 
-9.7.sup.(b) 
-9.sup.(b) 
HOE 2825 
CaBr.sub.2 
175 33.3* Gel 
WBB 125 5.5 3.2 -42 
______________________________________ 
.sup.(a) All data at 115 sec.sup.-1, except at 46 sec.sup.-1 when 
indicated by *. 
.sup.(b) 4% HOE in WBB. 
Table 5 summarizes 16 hours static aging experiments with 3.2% by weight 
HOE 3118 viscosifier in CaCl.sub.2 (11.2 lbs/gal) at 220.degree. C., and 
shows that additives 2-5 significantly enhance the thermal stability of 
the copolymer solutions when compared to the control system (Item 1). From 
Table 5, it can be seen that additive 2 at 3% by weight completely 
stabilizes the copolymer for 5 hours. Additive 3, at 1% by weight, retains 
the viscosity to within 17% after heating up to 16 hours. Additive 4, at 
3% by weight, retains the viscosity to within 6% after 8 hours heating, 
which is close to the error of measurements (about 5-10%); and after 16 
hours of heating, the lowering of viscosity is only about half that which 
occurs without the additive. It should be apparent that additives 2, 3 and 
4 enhance the viscosity above that of the control system 1 by 9-20%. 
Additive 5 shows good thermal stabilization for 16 hours but does not 
enhance viscosity. The slight turbidity is indicative of a borderline 
solubility at room temperature. However, it is believed that this additive 
is soluble and quite active at elevated temperature and that turbidity 
would be eliminated and/or decreased with derivatives containing shorter 
chain amines. 
It should be pointed out that adding increasing amounts of the thermal 
stability enhancing additive to the brine solution will increase the 
viscosity, which could lead to cost/performance advantages. The drawing is 
a plot showing the viscosity versus concentration for 
N-methyl-2-pyrrolidone (additive 2) in a calcium chloride-HOE 3118 brine 
solution. The drawing shows that viscosity increases as the concentration 
of additive 2 increases and the concentration of calcium chloride and HOE 
3118 subsequently decreases. The same may be true for additives 3 and 4. 
TABLE 5 
__________________________________________________________________________ 
Aging Experiments of 3.2 wt % of HOE 3118 
Viscosifier In CaCl.sub.2 (11.2 lbs/gal) at 220.degree. C. 
Apparent viscosity, cp, at 25.degree. C. and 
46 sec.sup.-1 after heating at 220.degree. C. 
Additive wt % 
Original 
5 hrs 8 hrs 6 hrs 
__________________________________________________________________________ 
(1) 
No additive 
0.0 48.0 35.0 (27).sup.(a) 
31.0 (35) 
20 (58) 
(2) 
N--methyl-2- 
3.0 58.0 58.0 (0) 
25.0 (56) 
20 (65) 
pyrrolidone 
(3) 
1-methyl-2- 
1.0 53.0 --.sup.(b) 
44.9 (15) 
43.0 (19) 
pyridone 
(4) 
DMPV 3.0 55.0 -- 51.4 (6.5) 
40.0 (27) 
(5) 
N--(N--N--dimethyl 
3.0 42.0.sup.(c) 
-- --.sup.(d) 
35.5.sup.(c) (16) 
amino)-propyl- 
2-pyrrolidone 
(6) 
N--methyl 3.0 42.3 -- 35.sup.(e) 
--.sup.(f) 
caprolactam incipient 
gelation 
(7) 
Hydantoin 3.0 42.3 -- 33.sup.(e) ppt.sup.(g) 
27.5 ppt 
__________________________________________________________________________ 
.sup.(a) % change after heating from original 
.sup.(b) Viscosity not measured at 5 hours 
.sup.(c) Slightly turbid 
.sup.(d) Viscosity not measured at 8 hours 
.sup.(e) 12 hours at 220.degree. C. 
.sup.(f) Viscosity not measured at 16 hours 
.sup.(g) ppt = Precipitates out 
The experimental procedure leading to the results in Table 5 was used for 
the following experiments. Table 6 demonstrates that hydantoin (additive 
7) at 3% by weight is a more effective additive than 
N-methyl-2-pyrrolidone at 9% by weight, when added to 5% by weight HOE 
3118 in NaBr brine (12.5 lbs/gal), and heated at 220.degree. C. for 
periods up to 16 hours. Hydantoin, as noted in Table 5, precipitates out 
in CaCl.sub.2 brine upon heating, but retains its solubility in NaBr. 
TABLE 6 
______________________________________ 
Aging Experiments of 5 wt % HOE 3118 
in NaBr (12.5 lb/gal) at 220.degree. C. 
Apparent Viscosity, cp, 
at 25.degree. C. and 46 sec.sup.-1 
after heating at 220.degree. C. 
Additive wt % Original 8 hrs 16 hrs 
______________________________________ 
No additive.sup.(a) 
0.0 55.0 29.0 (47).sup.(c) 
20 (64) 
N--methyl-2- 
9.0 68.0 50.0 (27) 
41.0 (40) 
pyrrolidone (MP) 
No additive.sup.(b) 
0.0 63.0 39.0 (38) 
15.6 (75) 
Hydantoin.sup.(b) 
3.0 64.0 60.0 (6) 42.0 (34) 
______________________________________ 
.sup.(a) and .sup.(b) refer to different brine preparations 
.sup.(c) % change after heating from original 
The aqueous brine fluids of the present invention are felt to be useful in 
formations having temperatures in the range of 150.degree.-220.degree. C. 
The embodiments of the Examples herein provide information and directions 
for other clear brines, polymers, and additives for high density, 
multifunctional fluids for deep well drilling. Here are some examples: 
increasing M.W. to reduce the polymer content and hence cost; extending 
the system to include other highly temperature stable polymers; selecting 
other high density brines and brine mixtures, e.g., brines containing 
LiCl, CaI.sub.2, Ca(SCN).sub.2, etc., with and without admixture of 
suitable solubilizing surfactants; selecting polar and hydrocarbon-derived 
solvents instead of water (note applicant's copending U.S. application 
Ser. No. 413,575, filed Aug. 31, 1982); in situ polymerization of monomers 
in wells and in gas and oil formations containing highly concentrated 
brines, etc. 
The viscosity increasing amount of polymer used in accordance with aspects 
of the present invention is that amount which is sufficient to achieve the 
desired viscosifying functions. In drilling, these functions involve 
transportation of cuttings to the surface and suspension of solids when 
the drilling fluid is not being circulated. The use of a viscosity 
increasing amount of polymer may result in an increase in viscosity at 
room temperature of a factor of, e.g., at least 4. In other terms, the 
amount of water-soluble copolymer of acrylamidomethylpropanesulfonic acid 
salts employed may be, e.g., about 0.5-10% by weight or as exemplified in 
the foregoing Examples from about 2 to about 5% by weight of the fluid. 
The salinity of the aqueous brine drilling fluid in accordance with aspects 
of the invention may be from at least 30% by weight up to the salt 
saturation point of the fluid, which is generally about 60-65% by weight. 
The brine fluids of the present invention may have a density of, e.g., 
from about 12 to about 16 pounds per gallon. 
Although the viscosifying effect of water-soluble copolymers of 
acrylamidomethylpropanesulfonic acid salts in accordance with aspects of 
the present invention is particularly useful in fluids for drilling in the 
vicinity of oil or gas producing formations, this effect may also be used 
when drilling in areas other than in the vicinity of oil or gas 
formations. Thus, viscosified drilling fluids containing water-soluble 
copolymers of acrylamidomethylpropanesulfonic acid salts may also contain 
constituents other than water and brine, such as filter loss control 
solids in an amount sufficient to prevent loss of fluid to the formation. 
Viscosified aqueous brine fluids containing water-soluble copolymers of 
acrylamidomethylpropanesulfonic acid salts are felt to be useful in 
formations having a temperature of at least 300.degree. F. (e.g., 
300.degree.-450.degree. F.). Such temperatures may occur at drilling 
depths of at least 15,000 feet (e.g., 15,000-30,000 feet). 
The drilling fluids of the present invention preferably do not contain 
thermally unstable polymeric thickeners, such as starch, carboxymethyl 
cellulose, and modified polyacrylates. 
Although fluids of the present invention may be used while drilling is 
occurring, they may also be used in other aspects of drilling operations. 
For example, these fluids may be used as completion, packer and workover 
fluids. Completion fluids are those used to flush potentially 
formation-damaging materials, e.g., cuttings and clays, from the wellbore 
prior to perforation. Packer fluids are left in the annulus, between 
casing and tubing, when the well is placed on production. Workover fluids, 
frequently clear brines, are used in cleaning and repairing old wells to 
increase productivity. 
Particularly in the above-mentioned drilling operations, the viscosified 
fluids described herein may be used at elevated temperatures. The HOE 3118 
brine solutions with the additives of the present invention are felt to be 
thermally stable for long periods of time at temperatures up to and 
including 220.degree. C. By way of contrast, conventionally employed 
cellulosic polymer viscosifiers decompose below 135.degree. C. 
Accordingly, brines as described herein are particularly useful in 
operations wherein the fluid is heated to temperatures above about 
135.degree. C. 
Although the aqueous brine fluids of the present invention have been 
described herein primarily with respect to their use as fluids in drilling 
operations, it will be understood that these fluids may also be quite 
useful in other fields. More particularly, these fluids should be 
applicable to a wide range of industrial applications, e.g., in processing 
minerals from salty solutions, in the production of speciality coatings, 
polymers, fibers, and membranes, in the formulation of "synthetic" 
water-based pneumatic fluids and novel lubricants, and in light energy 
conversion based on heat transport between concentrated and less 
concentrated brine layers in ponds. 
Although the invention has been described in conjunction with specific 
embodiments thereof, it is evident that many alternatives and variations 
will be apparent to those skilled in the art in light of the foregoing 
description. Accordingly, the invention is intended to embrace all of the 
alternatives and variations that fall within the spirit and scope of the 
appended claims.