High temperature guar-based fracturing fluid

A fracturing fluid based on quar gum exhibiting good viscosity and stability at temperatures from about 80.degree. C. to at least about 120.degree. C. The fracturing fluid includes a guar gum, a zirconium or hafnium cross-linking agent, and a bicarbonate salt in an aqueous solution at a pH from about 8 to about 10.

FIELD OF THE INVENTION 
The invention relates to a composition and method of fracturing 
subterranean formations at high temperatures utilizing natural guar-based 
fluid. 
TECHNOLOGY REVIEW 
The treatment of subterranean formations penetrated by a well bore to 
stimulate the production of hydrocarbons therefrom or the ability of the 
formation to accept injected fluids has long been known in the art. One of 
the most common methods of increasing productivity of a 
hydrocarbon-bearing formation is to subject the formation to a fracturing 
treatment. This treatment is effected by injecting a liquid, gas or 
two-phase fluid which generally is referred to as a fracturing fluid down 
the well bore at sufficient pressure and flow rate to fracture the 
subterranean formation. A proppant material such as sand, fine gravel, 
sintered bauxite, glass beads or the like can be introduced into the 
fractures to keep them open. The propped fracture provides larger flow 
channels through which an increased quantity of a hydrocarbon can flow, 
thereby increasing the productive capability of a well. 
Certain hydrophilic materials, hereinafter referred to as "gelling agents", 
have been used to increase the viscosity of a liquid fracturing fluid. 
High viscosity aqueous fracturing fluids are useful in the development of 
wider fractures to improve productivity further into the formations, 
increase the proppant carrying capacity of the fracturing fluids, and 
permit better fluid loss control. 
High viscosity treating fluids are useful in carrying out subterranean well 
completions, for transporting sand in sand and gravel packing procedures 
and in various other well treating procedures. Also, high viscosity 
treating fluids have utility in cleaning applications such as in the 
cleaning of tubular goods, production equipment, and industrial equipment. 
Equipment typically cleaned includes oil well piping tubes, tanks and 
process equipment, boilers, heat exchangers, conventional and nuclear 
power plants and accessory equipment and the like. 
Hydrophilic gelling agents, such as partially hydrolyzed polyacrylamides, 
natural gums and modified natural gums, celluloses and xanthan polymers, 
have been utilized before to increase the viscosity of aqueous solutions. 
However, the gells produced with such gelling agents generally have 
limited stability at elevated temperatures, i.e., the viscosity of the 
gelled aqueous solutions decreases substantially after only a short period 
of time. Chemicals which cross-link or complex hydrated gelling agents 
have also been utilized heretofore for further increasing their viscosity. 
For example, U.S. Pat. Nos. 3,888,312; 4,021,355 and 4,033,415 describe 
and claim organotitanate, permanganate salts, and antimony cross-linking 
agents respectively. U.S. Pat. No. 3,959,003 teaches the use of a water 
soluble cellulose complexed with a polyvalent metal salt as a thixotropic 
agent for cementing compositions. U.S. Pat. No. 3,979,303 teaches an oil 
well drilling fluid containing complex polysaccharides, and U.S. Pat. Nos. 
4,313,834 and 4,324,668 disclose and claim acidic treating fluids of a 
hydratable gelling agent and a zirconium cross-linking agent which further 
increases the viscosity. 
U.S. Pat. No. 4,579,670 describes cross-linked fracturing fluids including 
a hydratable polysaccharide in aqueous solution, a transition metal 
chelate cross-linking initiator, and a cross-linking rate controller which 
is either a rate accelerator or a rate retarder. 
Among hydratable gelling agents, natural guar gum is relatively 
inexpensive, and requires little processing. However, crosslinked 
fracturing fluids prepared with a natural guar gum provide lower 
viscosities at high temperatures. It would be desirable to crosslink a 
natural guar gum fracturing fluid and obtain high temperature performance 
comparable to fluids prepared by crosslinking the more expensive polymers. 
SUMMARY OF THE INVENTION 
The present invention provides a fracturing fluid based on natural guar gum 
useful at high temperatures. The natural guar gum based fracturing fluid 
of the present invention exhibits good viscosity and is particularly 
stable at moderate and high temperatures. As used herein, moderate 
temperatures refer to temperatures of about 80.degree. C. and above, and 
high temperatures refer to temperatures of about 120.degree. C. and above. 
The present invention therefore provides a particularly inexpensive and 
convenient fracturing fluid. 
The composition of the present invention is a high temperature fracturing 
fluid comprising a guar gum, and a cross-linking agent, and a stabilizing 
agent, in an aqueous solution. 
The method of the present invention includes using the composition of the 
invention for well stimulation to increase well productivity by creating 
wider fractures through which hydrocarbons may flow. The method of the 
present invention provides improved transport and placement of proppant 
material in subterranean formations. 
DETAILED DESCRIPTION OF THE INVENTION 
The high temperature fracturing fluids of the present invention are 
prepared from natural guar gum, a cross-linking agent, and a stabilizing 
agent for use in the pH range of 8 to 10. By proper selection of the 
crosslinker composition and the stabilizing agent concentration, 
crosslinked natural guar gum fluids may be prepared which exhibit delayed 
crosslinking and improved high temperature performance. 
In Table 1, the viscosity of cross-linked hydroxypropyl guar (HPG) (fluid 
1) is compared to the viscosity of cross-linked guar (fluid 2). By 
comparing the apparent viscosity after 4 hours at 121.degree. C. 
(250.degree. F.) it is seen that the apparent viscosity of HPG at high 
temperature is clearly superior to that of cross-linked guar. Due to the 
poor high-temperature performance of cross-linked guar, other more 
expensive polymers have been used at high temperatures. 
TABLE 1 
______________________________________ 
Comparison of the Viscosity of Cross-linked HPG and Guar 
______________________________________ 
Fluid Composition: 
Additive Concentration 
Fluid 1 Hydroxypropyl Guar 
0.42% by weight 
KCl 2% by weight 
Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O 
0.12% by weight 
Na.sub.2 CO.sub.3 
to pH 8.5 
Zr Triethanolamine 
0.0025% Zr by weight 
Fluid 2 Guar 0.42% by weight 
KCl 2% by weight 
Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O 
0.12% by weight 
Na.sub.2 CO.sub.3 
to pH 8.5 
Zr Triethanolamine 
0.0025% Zr by weight 
______________________________________ 
Fluid Performance: 
Apparent Viscosity (centipoise) @ 170 sec.sup.-1 
@ 121.degree. C. after 
Fluid # 0 hours 1 hour 2 hours 
3 hours 
4 hours 
______________________________________ 
1 92 99 86 76 68 
2 23 23 25 28 25 
______________________________________ 
Table 2 compares the performance of two pH-control agents, sodium 
bicarbonate and sodium carbonate. Tests performed with cross-linked guar 
show that both sodium bicarbonate and sodium carbonate maintain the 
desired pH after 4 hours at 121.degree. C. (250.degree. F.). 
TABLE 2 
______________________________________ 
Performance of pH-control Agents 
______________________________________ 
Fluid Composition: 
Additive Concentration 
______________________________________ 
Guar 0.42% by weight 
KCl 2% by weight 
Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O 
0.12% by weight 
Zr Triethanolamine 0.0025% Zr by weight 
______________________________________ 
Fluid Performance: 
pH-control 
Conc. pH before pH after 4 hours 
Additive (g/l) test at 121.degree. C. 
______________________________________ 
NaHCO.sub.3 
0.5 9.0 8.83 
Na.sub.2 CO.sub.3 
* 9.0 8.75 
______________________________________ 
*sufficient to produce pH = 8.5 
Surprisingly, although both sodium bicarbonate and sodium carbonate were 
equally suited to maintain pH-control in pH range of 8 to 10 as 
illustrated in Table 2, crosslinked fluids containing sodium bicarbonate 
and sodium carbonate did not exhibit similar fluid performance. Using the 
same fluid composition described in Table 2, the apparent viscosity of a 
solution containing sodium bicarbonate was compared with the apparent 
viscosity of a solution containing sodium carbonate at 24.degree. C. 
(75.degree. F.), and at 121.degree. C. (250.degree. F.). As can be seen 
from Table 3, the solution containing bicarbonate provides a lower 
viscosity at ambient temperature (24.degree. C.), and a higher viscosity 
at 121.degree. C. (250.degree. F.). 
TABLE 3 
______________________________________ 
The Effect of pH-control Agents on Cross-linked Fluid 
Viscosity 
______________________________________ 
Fluid Composition: -From Table 2 
Fluid Performance: 
Apparent Viscosity (centipoise) @ 170 sec.sup.-1 
pH-control 
Conc. @ 24.degree. C. after 
@ 121.degree. C. 
Additive 
(g/l) 3 minutes 0 hours 
1 hour 2 hours 
______________________________________ 
NaHCO.sub.3 
0.5 92 117 113 89 
Na.sub.2 CO.sub.3 
* 308 23 23 25 
______________________________________ 
*sufficient to produce pH = 8.5 
Without limiting the invention, it is believed that the lower viscosity 
observed at ambient temperature indicates delayed cross linking. Delayed 
cross linking is an advantageous property of fracturing fluids because it 
avoids excessive frictional losses during introduction of the fracturing 
fluid into the wellbore. The discovery that only the bicarbonate 
containing fluid exhibits a low viscosity at ambient temperature and a 
higher viscosity at elevated temperature is both surprising and very 
desirable. 
Table 4 illustrates the effect of bicarbonate concentration on the rate of 
viscosity development at ambient temperature and the fluid viscosity at 
high temperature. Viscosity development of cross linked fluids was 
measured using the Vortex closure test. The Vortex closure test is 
described in U.S. Pat. Nos. 4,657,080 and 4,657,081, incorporated herein 
by reference. As described therein, longer Vortex closure times indicate 
slower cross linking rates. As illustrated in Table 4, increasing the 
bicarbonate concentration increased the vortex closure time, increased the 
fluid viscosity at 121.degree. C., and stabilized the fluid pH during the 
test. However, as illustrated by the data in Table 4, the bicarbonate 
concentration must lie within a certain range to obtain the desired 
performance with a given cross linking compound. For example with fluid #1 
(cross linker zirconium triethanolamine), the bicarbonate concentration 
had to be greater than or equal to about 363 ppm and less than about 3000 
ppm to obtain optimum high temperature performance. At bicarbonate 
concentrations below about 363 ppm, fluid #1 provided no improvement in 
viscosity at elevated temperature. At a bicarbonate concentration of about 
2179 ppm, the viscosity of fluid #1 at 121.degree. C. was diminished. For 
fluid #2, the minimum bicarbonate concentration required for optimum 
performance was about 1089 ppm. 
While the mechanism responsible for improved performance obtained with 
crosslinked guar and bicarbonate is not understood, it does not appear to 
be simply pH-control and/or simply delayed cross linking. If the improved 
performance was due simply to delayed cross linking, fluid compositions 1 
and 2 from Table 4 delayed with compounds other than bicarbonate should 
provide performance at elevated temperature similar to fluids 1 and 2 
containing the optimum concentration of bicarbonate. In Table 5, fluid 
compositions 1 and 2 are delayed with compounds reported in the 
literature. Fluid 1 was delayed with 2,4 pentanedione and the pH was 
adjusted with triethanolamine. Fluid 2 was delayed with triethanolamine 
and the pH was maintained with a non-delaying amount of NaHCO.sub.3 (see 
U.S. Pat. No. 4,579,670). The results contained in Table 5 show two fluids 
with delay times similar to the fluids in Table 4 which provided improved 
performance. The performance of these fluids (1C and 2E) at 121.degree. C. 
is compared to the performance of fluids 1D and 2A (fluid compositions 
containing no delay additive) in Table 6. Note the delayed fluid 
compositions 1C and 2E performed only slightly better than the non-delayed 
compositions 1D and 2A. Furthermore, neither 1C nor 2E matched the 
performance of the fluids reported in Table 4 which contained an optimum 
concentration of NaHCO.sub.3 only. 
TABLE 4 
__________________________________________________________________________ 
Vortex Closure Results 
__________________________________________________________________________ 
Fluid Composition: 
Additive Concentration 
__________________________________________________________________________ 
Fluid 1 
Guar 0.42% by weight 
KCl 2% by weight 
Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O 
0.12% by weight 
Zr Triethanolamine 
0.0022% Zr by weight 
Fluid 2 
Guar 0.42% by weight 
KCl 2% by weight 
Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O 
0.12% by weight 
Zr Lactate 0.0025% Zr by weight 
__________________________________________________________________________ 
Fluid Performance: 
Viscosity 
pH Before pH After 
cp @ 170 sec.sup.-1 
pH 
Fluid 
HCO.sub.3.sup.- Conc. 
X-linker 
Closure 
X-linker 
at 121.degree. C. after 
After 
# (ppm) Addition 
Time (s) 
Addition 
0 hr 
2 hr 
4 hr 
test 
__________________________________________________________________________ 
1 0 6.59 64 9.29 22 23 22 10.1 
1 182 7.92 38 9.19 17 11 11 8.93 
1 363 8.16 101 9.13 117 
89 67 8.83 
1 726 8.43 &gt;900 9.03 156 
91 62 8.83 
1 1089 8.35 &gt; 900 
8.88 147 
101 
52 8.85 
1 1452 8.47 &gt;900 8.88 149 
87 50 8.85 
1 2179 8.70 &gt;900 8.91 78 56 -- 8.75 
2 0 8.49 18 6.56 7 4 5 6.94 
2 182 8.05 52 7.80 17 11 15 7.22 
2 363 8.17 68 7.73 11 9 19 6.90 
2 726 8.40 103 8.16 17 30 29 7.83 
2 1089 8.50 &gt;900 8.40 70 95 96 7.40 
2 1452 8.53 &gt;900 8.40 143 
184 
149 
8.26 
2 2179 8.71 &gt;900 8.68 139 
107 
81 8.10 
__________________________________________________________________________ 
TABLE 5 
__________________________________________________________________________ 
Vortex Closure Results 
__________________________________________________________________________ 
Fluid Composition: 
Additive Concentration 
__________________________________________________________________________ 
Fluid 1 Guar 0.42% by weight 
KCl 2% by weight 
Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O 
0.12% by weight 
Zr Triethanolamine 
0.0022% Zr by weight 
Fluid 2 Guar 0.42% by weight 
KCl 2% by weight 
Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O 
0.12% by weight 
Zr Lactate 0.0025% Zr by weight 
__________________________________________________________________________ 
Fluid Performance: 
Fluid 
pH-Control Delay Vortex Time 
pH after 
# Additive 
Conc. 
pH 
Additive* 
(g/l) (min:sec) 
x-linking 
__________________________________________________________________________ 
1A TEA 0.48 
g/l 
8.5 
2,4 Pdione 
0.24 00:22 8.97 
1B TEA 0.86 
g/l 
8.5 
" 0.48 3:16 -- 
1C TEA 1.39 
g/l 
8.5 
" 0.96 &gt;20 minutes 
8.70 
1D NaCO.sub.3 
to pH 
8.5 
-- -- 00:44 9.05 
2A NaHCO.sub.3 
0.1 
g/l 
8.4 
None 0.00 00:22 -- 
2B NaHCO.sub.3 
0.1 
g/l 
8.5 
TEA 0.29 2:27 8.51 
2C NaHCO.sub. 3 
0.1 
g/l 
8.5 
" 0.43 4:37 -- 
2D NaHCO.sub.3 
0.1 
g/l 
8.5 
" 0.72 &gt;10 minutes 
-- 
2E NaHCO.sub.3 
0.1 
g/l 
8.5 
" 0.86 &gt;10 minutes 
8.90 
__________________________________________________________________________ 
*2,4 Pdione is 2,4pentanedione. 
TABLE 6 
______________________________________ 
The Effect of Delay Additives on Cross-linked Viscosity 
______________________________________ 
Fluid Composition: -From Table 5 
Fluid Performance: 
Apparent Viscosity (centipoise) @ 170 sec.sup.-1 
24.degree. C. after @ 121.degree. C. after 
Fluid # 
3 minutes 0 hours 1 hour 
2 hours 
3 hours 
4 hours 
______________________________________ 
1C 68 108 45 33 30 -- 
1D 308 23 23 25 28 25 
2E 55 32 27 23 29 36 
2A 171 16 14* 
______________________________________ 
*Viscosity after 0.5 hours at 121.degree. C. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In order that those skilled in the art may better understand how the 
present invention may be practiced, the following Examples are given by 
way of illustration and not by way of limitation. All parts and 
percentages are by weight unless otherwise noted. 
The compositions of the present invention may be prepared by mixing 
deionized water, 2% potassium chloride, and 0.025% (vol/vol) of 
polypropylene glycol (an antifoaming agent) to form a mixwater. The 
mixwater is placed in a blender, and mixed at approximately 2000 rpm, 
while the required quantity of guar gum is added. The guar gum is mixed 
for approximately 30 minutes to fully hydrate the guar. Thereafter the 
desired amount of sodium bicarbonate is added along with 0.12% of sodium 
thiosulfate (a high temperature gel stabilizer). The solution is mixed for 
about 30 minutes to effect solubilization. The resulting gel is aged for 
at least about one hour. 
The zirconium cross-linking agent may, if necessary, be diluted with 
deionized water before use. The diluted solution is allowed to age for at 
least about 30 minutes prior to use. 
The guar gel is mixed at about 2000 rpm, and the zirconium solution is 
added to the vortex. The viscosity of the solution thus prepared is 
measured in a Fann model 50C viscometer with a R1/B5 rotor/bob 
configuration. The sample is pressurized to 400 psi and sheared at 100 rpm 
(85 sec.sup.-1) for three minutes. To measure ambient viscosity, a shear 
rate ramp is used in 50 rpm increments from 250 rpm to 50 rpm. Upon 
completion of the ambient temperature measurement, the shear rate is 
returned to 100 rpm. The bath temperature is then increased at about 
5.5.degree. C. per minute to the test temperature. When the sample 
temperature is within 3.degree. C. of the set point, another shear rate 
ramp is performed, which is the test "T=0".

EXAMPLE #1 
Test temperature: 121.degree. C. 
Guar gum concentration: 0.42% 
Bicarbonate concentration: 1453 ppm 
Crosslinker: zirconium sodium trilactate 
Zirconium concentration: 29 ppm 
Crosslinked pH: 8.5 
______________________________________ 
Time RT 0 0.5 1 1.5.sup. 2 
2.5 3 3.5.sup. 4 
(hrs.) 
Visc. 62 143 156 157 191 184 
178 167 158 149 
(cps) 
______________________________________ 
EXAMPLE #2 
Test temperature: 121.degree. C. 
Guar gum concentration: 0.42% 
Bicarbonate concentration: 1453 ppm 
Crosslinker: zirconium diisopropylamine lactate 
Zirconium concentration: 29 ppm 
Crosslinked pH: 8.5 
______________________________________ 
Time (hrs.) 
RT 0 0.5 1 1.5 2 2.5 3 3.5 4 
Visc. (cps) 
68 149 95 76 64 54 -- -- -- -- 
______________________________________ 
EXAMPLE #3 
Test temperature: 121.degree. C. 
Guar gum concentration: 0.42% 
Bicarbonate concentration: 1453 ppm 
Crosslinker: zirconium triethanolamine lactate 
Zirconium concentration: 29 ppm 
Crosslinked pH: 8.5 
______________________________________ 
Time (hrs.) 
RT 0 0.5 1 1.5 2 2.5 3 3.5 4 
Visc. (cps) 
63 107 119 111 107 99 90 88 85 -- 
______________________________________ 
EXAMPLE #4 
Test temperature: 121.degree. C. 
Guar gum concentration: 0.42% 
Bicarbonate concentration: 756 ppm 
Crosslinker: zirconium triethanolamine 
Zirconium concentration: 22 ppm 
Crosslinked pH: 9.0 
______________________________________ 
Time (hrs.) 
RT 0 0.5 1 1.5 2 2.5 3 3.5 4 
Visc. (cps) 
55 94 108 120 111 103 96 87 -- -- 
______________________________________ 
EXAMPLE #5 
Test temperature: 135.degree. C. 
Guar gum concentration: 0.60% 
Bicarbonate concentration: 1453 ppm 
Crosslinker: zirconium triethanolamine 
Zirconium concentration: 26 ppm 
Crosslinked pH: 9.0 
______________________________________ 
Time RT 0 1 2 3 4 5 6 7 8 
(hrs.) 
Visc. 115 247 274 235 199 180 
155 137 133 109 
(cps) 
______________________________________ 
EXAMPLE #6 
Test temperature: 149.degree.C. 
Guar gum concentration: 0.72% 
Bicarbonate concentration: 1453 ppm 
Crosslinker: zirconium triethanolamine 
Zirconium concentration: 26 ppm 
Crosslinked pH: 9.0 
______________________________________ 
Time RT 0 0.5 1 1.5.sup. 2 
2.5 3 3.5.sup. 4 
(hrs.) 
Visc. 167 394 307 269 226 196 
157 138 115 100 
(cps) 
______________________________________ 
EXAMPLE #7 
Test temperature: 163.degree. C. 
Guar gum concentration: 0.72% 
Bicarbonate concentration: 1453 ppm 
Crosslinker: zirconium triethanolamine 
Zirconium concentration: 26 ppm 
Crosslinked pH: 9.0 
______________________________________ 
Time (hrs.) 
RT 0 0.5 1 1.5 2 2.5 3 3.5 4 
Visc. (cps) 
154 236 140 58 27 -- -- -- -- -- 
______________________________________ 
The cross-linking agent is preferably an organic zirconium or an organic 
hafnium compound. Suitable organic zirconium compounds include either 
zirconium lactate or a zirconium complex of lactic acid, also known as 
2-hydroxypropanoic acid. Suitable zirconium complex lactates include 
zirconium ammonium lactate, zirconium triethanolamine lactate, zirconium 
diisopropylamine lactate, and zirconium sodium trilactate salts. 
Corresponding hafnium lactate and hafnium complexes of lactic acid may be 
used as cross-linking agents. Titanium containing compounds such as 
titanium ammonium lactate and titanium triethanolamine may also be used as 
cross-linking agents in the practice of the present invention. 
Other organic zirconium or organic hafnium compounds useful as 
cross-linking agents include monoalkylammonium, dialkylammonium and 
trialkylammonium zirconium or hafnium compounds obtained by reacting an 
organozirconate or an organohafnate with monomethylamine, dimethylamine 
and trimethylamine, monoethylamine, diethylamine, and triethylamine, 
monoethanolamine, diethanolamine and triethanolamine, 
methyldiethanolamine, ethyldiethanolamine, dimethylethanolamine, 
diethylethanolamine, monoisopropanolamine, diisopropanolamine and 
triisopropanolamine, methyldiisopropanolamine, ethyldiisopropanolamine, 
dimethylisopropanolamine, diethylisopropanolamine, n-butylamine, sec. 
butylamine, dibutylamine and diisobutylamine. For example, a zirconium 
triethanolamine complex (Zr TEA) may be used as the cross-linking agent in 
the practice of the present invention. Zr TEA complexes are described in 
U.S. Pat. No. 4,534,870 and U.K. Patent Application 2,108,122. 
Other organozirconium compounds useful as cross-linking agents include 
citrates and tartrates such as zirconium sodium citrate and zirconium 
sodium tartrate. 
The compositions of the present invention include a cross-linking agent as 
described above, a guar gum gelling agent, and a bicarbonate salt. The 
gelling agent is present in the aqueous composition in a concentration in 
the range of from about 0.2 to 1.25%, preferably from about 0.2 to about 
1.0% and most preferably from about 0.3 to about 0.8% by weight of the 
aqueous fluid. A concentration of guar gum of less than 0.2% by weight of 
the aqueous fluid is not sufficient to permit effective cross-linking. 
The cross-linking agent is present in an amount from about 5 ppm to at 
least about 50 ppm of the aqueous fluid, and preferably in an amount from 
about 10 ppm to about 35 ppm. 
The pH in the aqueous fracturing fluid is preferably in the range from 
about 8 to about 10 depending on the cross-linking agent. Generally the 
bicarbonate salt stabilizing agent will be present in an amount from about 
250 ppm to about 3000 ppm, and preferably in an amount from about 350 ppm 
to about 2250 ppm. 
It is understood that various other modifications will be apparent to and 
can readily be made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description set 
forth above, but rather that the claims be construed as encompassing all 
the features which reside in the present invention, including all features 
which would be treated as equivalents thereof by those skilled in the art 
to which this invention pertains.