High temperature gel stabilizer for fracturing fluids

The present invention relates to the use of oximes as thermal decomposition reduction additive for gels used in oil field drilling fluids, fluids used as proppant carriers and fluids used during well completion and workover.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the use of oximes, especially 
methylethylketoxime, as thermal oxidative free radical decomposition 
prevention additives for gels used in oil field drilling fluids, fluids 
used as proppant carriers and fluids used during well completion and 
workover. 
2. Description of the Prior Art 
Sols formed when a soluble dispersible gum is mixed with water find use in 
a wide variety of industrial applications. In a significant number of 
these applications it is necessary that the sol be exposed to elevated 
temperatures for extended periods of time. 
In oil field applications the sols may become hot in use, e.g. the use of 
brine sols as agents to control the fluid loss in gas or oil well drilling 
operations or as proppant carriers and as fluid loss control agents in 
well completion and workover. 
Unfortunately when sols of water soluble gums are exposed to elevated 
temperatures for any extended length of time, they lose their viscosity in 
part or in whole and therefore become less effective or completely 
ineffective. Dissolved oxygen is the major cause of an oxidative free 
radical polymer breakdown leading to a deterioration of the fracturing gel 
at elevated temperatures. Therefore, to prevent a premature viscosity 
degradation, oxidation inhibitors or free radical scavengers are a 
necessary component of fracturing gels being used in hot wells. 
The normal concentration of diluted oxygen in air saturated fresh water at 
20.degree. C. is about 35 g/1000 gallons which is approximately the 
equivalent of 1.5 lbs/1000 gal. of a typical oxidizing gel breaker. Even 
more oxygen is entrained in the gel and subsequently dissolved at high 
pressure when dry proppant is introduced into a blender and mixed with a 
gel. This diatomic oxygen is not very reactive at ambient pressure and 
temperature. However, its reactivity increases exponentially with 
temperature and pressure increase and becomes significant at high 
temperature or in deep wells. 
Corrosion prevention in steam boilers makes a useful model for high 
temperature and high pressure fluids being pumped in oil field conditions. 
The strategy to control oxidation in both environments is to remove all of 
the molecular oxygen. The chemicals which are used to control oxygen are 
referred to as "oxygen scavengers". In boilers, the norm is to preheat 
water and to deaerate mechanically then add oxygen scavengers. In oil 
fields, oxygen scavengers are normally used without prior mechanical 
deaeration. 
When first introduced, oxygen scavengers were seen as agents which "remove" 
dissolved oxygen. However, the "removal" of dissolved oxygen from water is 
actually a chemical reduction of zerovalent molecular oxygen to compounds 
in which this element appears in the lower -2 oxidation state. The reduced 
oxygen combines with an acceptor atom, molecule or ion to form an 
oxygen-containing compound. Hence, all oxygen scavengers are reducing 
agents, although not all reducing agents are necessarily oxygen 
scavengers. 
To be suitable as oxygen scavenger, a reducing agent must satisfy the 
thermodynamic requirement of having an exothermic heat of reaction with 
O.sub.2, a condition satisfied by most reducing agents, and the kinetic 
requirement of a reasonable reactivity at lower temperatures, a condition 
not satisfied by many. 
A variety of new oxygen scavengers were patented in recent years for boiler 
applications and several are now in commercial use. These include 
hydroxylamine (H.sub.2 NOH) in the form of its salts and alkyl derivatives 
(U.S. Pat. No. 4,067,690), Hydroquinone (U.S. Pat. Nos. 4,278,635 and 
4,282,111) and hydroquinone formulated with amines (U.S. Pat. No. 
4,279,767), carbohydrazide (H.sub.2 N-NH-CO-NH-NH.sub.2) as a substitute 
for toxic hydrazine (U.S. Pat. No. 4,269,717), ammonium erythroborate 
(U.S. Pat. No. 4,419,327), and methylethylketoxime (2-butanoneoxime) (U.S. 
Pat. No. 4,487,745). 
In oil field fracturing operations, the kinetic requirements are very 
important. The oxygen scavenger has to remove available oxygen at low 
temperatures before it can damage vulnerable polysaccharides at higher 
temperatures. Oil field applicable oxygen scavengers must have a limited 
lifetime. They should be consumed in the course of a treatment so that 
they do not interfere with the after treatment gel breaking process. Most 
catalytic type, preventive antioxidants have a long life time and 
consequently do not qualify for oil field application. 
Chemical incompatibility between reducing agents and crosslinkers creates 
another product choice limitation. Since all oxygen scavengers are 
reducing compounds, they are electron donors. Electron donors are Lewis 
bases capable of chelating metals which makes them unsuitable for 
application in metal crosslinked gels. Based on the above limitations, 
most common antioxidants have been rejected from oil field applications. 
The most common gel stabilizer currently used in oil fields, sodium 
thiosulfate, can be oxidized to two products by two reactions. The first 
reaction, oxidation to tetrathionate, is fastest, least efficient and is 
usually the dominant reaction. 
EQU 2Na.sub.2 S.sub.2 O.sub.3 +1/2O.sub.2 +H.sub.2 O .fwdarw.Na.sub.2 S.sub.4 
O.sub.6+ 2NaOH 
The second reaction, the oxidation to sulfate ion, requires high 
temperatures to occur and is usually not significant. 
EQU Na.sub.2 S.sub.2 O.sub.3 +2O.sub.2 .fwdarw.Na.sub.2 SO.sub.4 +H.sub.2 
SO.sub.4 
Twenty parts of sodium thiosulfate per part of oxygen are required for 
oxygen scavenging according to stoichiometry in the first reaction. 
The oxidation reaction of methylethylketoxime is as follows: 
EQU 2MeEtC.dbd.NOH+O.sub.2 .fwdarw.2MeEtC.dbd.O+N.sub.2 O+H.sub.2 O 
Only 5.5 parts of methylethylketoxime per part of oxygen are necessary and 
therefore methylethylketoxime is almost 4 times more efficient than sodium 
thiosulfate for oxygen scavenging. 
SUMMARY OF THE INVENTION 
The present invention relates to a composition and a process for the 
substantial reduction of thermal degradation of aqueous gels by the 
addition of an oxime preferably methylethylketoxime.

DETAILED DESCRIPTION OF THE INVENTION 
It has been discovered that oximes including ketoximes and aldoximes, 
particularly methylethylketoxime (2-butanoneoxime) are useful for 
extending the high temperature effectiveness of aqueous gels commonly used 
in oil field operations. It has been found that oximes prevent the thermal 
degradation of such gels at temperatures above 100.degree. C. and as high 
as 200.degree. C. or higher. The reaction kenetics are fast enough even at 
low temperatures to stabilize such gels. The reaction of 
methylethylketoxime with oxygen yields methylethylketone, nitrous oxide 
and water which indicates a theoretical requirement of only 5.5 parts of 
methylethylketoxime per part of oxygen. Other oximes will be effective in 
the method of the present invention including ketoximes and aldoximes 
especially, dimethylketoxime, diethylketoxime, ethylpropylketoxime, 
acetaldoxime, butyraldoxime and the like. 
The amount of an oxime needed to stabilize a gel will be an effective 
amount depending on the amount of oxygen present in the gel and the gels 
subsequent exposure to atmospheric oxygen, for example, during blending 
with a proppant. The amount of an oxime employed according to the present 
invention will therefore be that necessary to scavenge the oxygen existing 
in any particular situation, plus some excess so as to maintain a small 
residual amount of additive while stabilization is needed. Normally, about 
five to six parts by weight of methylethylketoxime will be required to 
remove one part by weight of dissolved oxygen. In practice, however, an 
excess of an oxime will be employed to assure a fast and complete oxygen 
removal. The amount of an oxime will usually be measured in terms of the 
amount of gel being treated. Typical loadings of methylethylketoxime may 
vary from 0.01 parts per thousand by weight to 100 parts per thousand, 
preferably, 0.1 part per thousand to 20 parts per thousand, and most 
preferably, from 0.5 parts per thousand to 10 parts per thousand. 
Available literature information indicate that the reaction of oximes 
including methylethylketoxime with oxygen is temperature dependent but 
unusual in comparison to other oxygen scavengers in that their reactivity 
is relatively independent of pH. This characteristic makes the oximes very 
universal oxygen scavengers/gel stabilizers for oil field applications at 
basic or acidic conditions. 
The gels which can be used with the thermal decomposition reduction 
additive of the present invention include all gels formed when a soluble 
dispersible gum is mixed with water. These gels can include galactomannan 
gums and their derivatives, glucomannan gums and their derivatives, guar 
gum, locust bean gum, cara gum, carboxymethyl guar, hydroxyethyl guar, 
hydroxypropyl guar carboxymethylhydroxyethyl guar, 
carboxymethylhydroxypropyl guar, cellulose and its derivatives, 
hydroxypropyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose 
and carboxymethyl cellulose. 
An oxime according to present invention can be added to the mix water prior 
to or at the same time as other additives are added to a water based oil 
field gel. An oxime could be added on-the-fly if necessary for continuous 
process operation. 
EXAMPLES 
The following examples further illustrate the invention and are for 
illustrative purposes only. The examples may obviously be modified without 
departing from the scope of this invention and the invention is not 
limited to the specific embodiments given. 
In each of the examples given in Table 1, the gel, as described, was mixed 
with an antioxidant at a loading calculated to give the required oxygen 
scavenging. Samples of gel containing each of the antioxidants were tested 
for room temperature low pH crosslink time, room temperature high pH 
crosslink time, elevated temperature crosslink time and high temperature 
gel viscosity decrease with time. 
The antioxidants were tested at room temperature to determine whether they 
would interfere with the metal crosslinker at low temperatures. 
The elevated and high temperature tests were performed to demonstrate that 
the antioxidant would allow crosslinking to occur and actually stabilize 
the gel at high temperature. 
The room temperature, low pH crosslink time was determined by adding about 
1 gram of NH.sub.2 SO.sub.3 H (sulfamic acid) buffer to 1 liter of 0.48% 
carboxymethylhydroxypropyl guar gel solution along with the antioxidant as 
given in the table. The pH was adjusted to about 3.5. 200 ml of the 
buffered stabilized gel was placed in a waring blender 500 ml cup and 
mixed using a 1-7/8 inch blade at about 25% of the maximum blender speed 
so that a vortex was created. 0.4 ml of TIC.TM. crosslinker containing 
Ti.sup.+4 cations available from The Western Company of North America 
under the trade name TIC was added by syringe into the vortex and the time 
required to crosslink the gel was measured. The gel was considered 
crosslinked when the vortex in the blender closed completely and the 
surface of the gel stopped turning. The test was repeated two to four 
times and an average was calculated. The solutions were tested at 
24.degree. C. 
The room temperature, high pH crosslink time was determined by adding 1 ml 
of 35% carbonate buffer solution to 1 liter of 0.48% hydroxypropyl guar 
gel solution along with the antioxidant as given in the table. The pH was 
adjusted to about 9.5. 200 ml of the buffered stabilized gel was placed in 
a waring blender 500 ml cup and mixed using a 1-7/8 inch blade at about 
25% of the maximum blender speed so that a vortex was created. 0.2 ml of 
CL-14.TM. crosslinker containing Zr.sup.+4 cations available from The 
Western Company of North America under the trade name CL-14 was added by 
syringe into the vortex and the time required to crosslink the gel was 
measured. The gel was considered crosslinked when the vortex in the 
blender closed completely and the surface of the gel stopped turning. The 
test was repeated two to four times and an average was calculated. The 
solutions were tested at 24.degree. C. 
To determine the elevated temperature crosslink time the blender test and 
gels as in the room temperature, high pH crosslink time test were used. 
However, the gel solutions were preheated to between 49.degree. C. and 
55.degree. C. prior to the test. Antioxidants which allowed crosslinking 
to occur in about four (4) minutes or less at room temperature were 
considered successful candidates and were not tested for elevated 
temperature crosslink time. Crosslink times at 55 .degree. C. for these 
antioxidants would be very fast occurring in about ten (10) seconds or 
less. 
The high temperature gel viscosity test examined how rapidly the gel 
viscosity decreased at high temperatures. The test was performed as 
follows. A Fann model 50 C. viscometer with metal parts machined from 
histalloy was used. The R1/B1 rotor/bob combination was used. Gel samples 
prepared as in the room temperature high pH crosslink time procedure were 
loaded into the viscometer. The samples and viscometer were heated to 
150.degree. C. The viscosity was measured at 10.0 RPM (170 sec.sup.-1) and 
monitored for rate of decrease. The gels containing an antioxidant passed 
the high temperature viscosity test if they retained the viscosity for 
about two hours longer than an unstabilized gel. Some antioxidants were 
not tested for high temperature gel viscosity since the high cost of the 
antioxidant made their use not economical. 
TABLE I 
__________________________________________________________________________ 
Room Temp 
Room Temp High 
Low pH High pH Elevated Temperature 
Exp. Crosslink 
Crosslink 
Temperature 
Gel Viscosity 
No. 
Antioxidant Loadings* 
Time Time Crosslink Time 
Test** 
__________________________________________________________________________ 
1 Na.sub.2 SO.sub.3 
2.5 
ppt 0:50 
min 4:10 
min fail 
2 NaSCN 2.5 
ppt 0:40 
min 3:00 
min fail 
3 Na.sub.2 So.sub.3 /catalyst 
2.5 
ppt 1:00 
min &gt;11 min 10 sec @ 55.degree. C. 
fail 
4 HONH.sub.2 .times. HCl 
0.5 
ppt &gt;10 min &gt;10 min &gt;10 min @ 49.degree. C. 
fail 
5 N.sub.2 H.sub.4 .times. HCl 
2.5 
ppt &gt;10 min &gt;13 min &gt;10 min @ 49.degree. C. 
fail 
6 Methylethylketoxime 
2 ppt 0:40 
min 3:20 
min pass 
7 Hydroquinone 
0.5 
ppt 1:00 
min &gt;10 min 5 sec @ 49.degree. C. 
pass 
8 t-Bu-hydroquinone 
0.5 
ppt 1:00 
min &gt;14 min 3 min @ 55.degree. C. 
pass 
9 Di-t-Bu-hydroquinone 
0.5 
ppt 0:30 
min 2:55 
min 7 sec @ 52.degree. C. 
fail 
10 Benzoquinone 
0.5 
ppt &gt;10 min &gt;14 min &gt;10 min @ 52.degree. C. 
fail 
11 Methyl Niclate 
0.5 
ppt 0:35 
min 3:00 
min fail 
12 NACAP 0.6 
ppt 1:40 
min 3:00 
min 
13 Potassium ethyl xanthate 
1.2 
ppt 1:20 
min 3:00 
min 
14 Butyl Zimate 
1.2 
ppt 0:40 
min 3:00 
min 
15 AMA-331 5 ppt 4:00 
min 4:00 
min pass 
16 Vancide 51 5 ppt 4:00 
min 4:00 
min 
__________________________________________________________________________ 
*ppt = parts per thousand 
**Fail = Gel's viscosity decrease rapidly Pass = Gel's viscosity decrease 
slowly 
11 nickel dimethyldithiocarbamate R.T. Vanderbilt Co., Inc., 30 Winfiel 
St. Norwalk, CN 06855 
12 2-mercaptobenzothiazole R.T. Vanderbilt Co., Inc. 
14 zinc din-butyldithiocarbamate R.T. Vanderbilt Co., Inc. 
15 AMA-331 mixture of disodiumethylene bisdithiocarbamate and sodium 
dimethyldithiocarbamate Vinings Industries Inc. 3950 Cumberland Parkway 
Atlanta, GA 30339 
16 mixture of sodium dimethyldithiocarbamate and sodium 
2mercaptobenzothiazolale R.T. Vanderbilt Co., Inc. 
Methylethylketoxime did not effect crosslinker performance at low 
temperature and was found to be an effective high temperature gel 
stabilizer applicable for oil field use. 
While particular embodiments of the invention have been described, it is to 
be understood that such descriptions are presented for purposes of 
illustration only and that the invention is not limited thereto and that 
reasonable variations and modifications, which will be apparent to those 
skilled in the art can be made without departing from the spirit or scope 
of the invention.