Stabilized fracturing fluid and method of stabilizing fracturing fluid

A stabilized fracturing fluid and a method of stabilizing a fracturing fluid containing at least one of guar gum and its derivatives is disclosed, wherein at least one stabilizing compound selected from the thiol derivatives of heterocyclic compounds is added to the fracturing fluid. This method of stabilization prevents degradation of the fracturing fluid, i.e., reduction in its rheological properties at high temperatures and permits smooth practice of the fracturing operation.

FIELD OF THE INVENTION 
The present invention relates to a stabilized fracturing fluid and to a 
method of stabilizing a fracturing fluid. More particularly, the present 
invention relates to a stabilized fracturing fluid and a method of 
stabilizing a fracturing fluid whereby a reduction in rheological 
properties of the fracturing fluid at high temperatures is prevented, 
thereby permitting smoothly carrying out a fracturing operation. 
BACKGROUND OF THE INVENTION 
In recent years, in order to increase the production of oil and gas, a 
fracturing technique has been widely used in which a fluid is injected 
under high pressure in rock forming oil and gas layers so as to break the 
rock and form cracks therein and, at the same time, a proppant (propping 
agent), such as coarse sand, is introduced to hold or maintain the cracks, 
thereby accelerating the production of oil and gas. 
As a fluid for use in this fracturing, that is, a fracturing fluid, water 
with a polymer dissolved therein or with a polymer dissolved and 
cross-linked therein is mainly used. One of major functions required for 
the fluid is to have such rheological properties as to make it possible to 
form cracks in the reservoir and to convey a proppant thereinto. Another 
feature is that the fluid has only a small leak-off. As polymers for use 
in the fluid, guar gum and/or derivatives thereof, such as 
hydroxypropylguar, hydroxyethylguar and carboxymethylguar are mainly used. 
Recently, commercially successful exploration of oil and gas is difficult 
unless the oil and gas are prospected in a deeper stratum. For this 
reason, the temperature of the stratum tends to rise and, in many cases, 
it reaches 90.degree. C. or more. 
At such high temperatures, the guar gum and/or derivatives thereof used in 
the fracturing fluid are deteriorated or decomposed. This leads to a 
reduction in rheological properties of the fracturing operation and 
creates a serious obstacle to the fracturing operation. Accordingly, the 
inherent functions of the fracturing fluid can be satisfactorily obtained 
only if degradation (e.g., caused by decomposition) of its rheological 
properties can be prevented even at elevated temperatures. 
SUMMARY OF THE INVENTION 
The present invention is intended to overcome the above problems and an 
object of the present invention is to provide a stabilized fracturing 
fluid and a method whereby a fracturing fluid containing guar gum and/or 
derivatives thereof is prevented from degradation at elevated 
temperatures. 
It has been found that the above described object can be attained by adding 
thiol derivatives of heterocyclic compounds to the fracturing fluid. 
The present invention is directed to a stabilized fracturing fluid 
containing at least one of guar gum and its derivatives and a stabilizing 
amount of at least one stabilizing compound selected from the thiol 
derivatives of heterocyclic compounds, and a method of stabilizing a 
fracturing fluid containing at least one of guar gum and its derivatives, 
which comprises adding at least one stabilizing compound selected from the 
thiol derivatives of heterocyclic compounds to the fracturing fluids. 
DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention, at least one compound selected 
from the thiol derivatives of heterocyclic compounds is added to the 
fracturing fluid containing at least one of guar gum and its derivatives. 
Addition of such compounds provides a fracturing fluid having rheological 
properties much superior to those of the conventional fracturing fluids. 
Suitable examples of the thiol derivatives of heterocyclic compounds for 
use in the method of stabilization of the present invention are 
2-thioimidazolidone (formula (1)), 2-mercaptothiazoline (formula (2)), 
benzooxazole-2-thiol (formula (3)), N-pyridineoxide-2-thiol (formula (4)), 
1,3,4-thiadiazole-2,5-dithiol (formula (5)), and 
4-ketothiazolidine-2-thiol (formula (6)). 
##STR1## 
In the above formulae, X is a hydrogen atom, an alkali metal (e.g., Na, K, 
etc.) or ammonium. 
In accordance with the present invention, at least one stabilizing compound 
selected from the thiol derivatives of heterocyclic compounds is added to 
the fracturing fluid. The amount of the stabilizing compound added is 
preferably 0.001 to 0.1 part by weight, more preferably 0.005 to 0.05 part 
by weight, per 100 parts by weight of the fracturing fluid. If the amount 
of the stabilizing compound added is less than 0.001 part by weight, the 
stabilization effect is poor. On the other hand, even if the stabilizing 
compound is added in an amount in excess of 0.1 part by weight, no 
additional effect can be obtained, that is, the effect obtained by 
addition of more than 0.1 part by weight of the stabilizing compound is 
almost the same as that obtained by addition of 0.1 part by weight of the 
stabilizing compound. Thus, addition of the stabilizing compound in such 
an amount as to exceed 0.1 part by weight is not preferred from an 
economic standpoint. 
The stabilizing compound of the present invention can be added to the 
fracturing fluid in various manners. For example, the stabilizing compound 
may be added directly to the fracturing fluid, or it may be previously 
added to guar gum and/or its derivatives, or it may be added during the 
production of guar gum and/or its derivatives. 
Guar gum as used herein is obtained from seeds of a bean plant called 
Cyamopsis Tetragonoloba and is widely used. Its derivatives such as 
hydroxypropylguar, hydroxyethylguar and carboxymethylguar are also widely 
used. 
The total content of guar gum and its derivatives in the fracturing fluid 
is generally from 0.01 to 30 wt%, preferably 0.02 to 15 wt%, based on the 
total weight of the fracturing fluid. 
Additives which are commonly used in conventional fracturing fluids, such 
as a tackifier (e.g., hydroxyethyl cellulose and carboxymethylhydroxyethyl 
cellulose), hydrocarbons, methanol, a cross-linking agent, a surfactant, a 
pH adjusting agent, a fluid loss-adjusting agent, a proppant, and a clay 
swell-inhibiting agent can be added to the fracturing fluid. 
The fracturing fluid with the stabilizing compound and the method of 
stabilizing a fracturing fluid according to the present invention improve 
the rheological characteristics of the fracturing fluid after being stored 
at a temperature as high as about 90.degree. to 200.degree. C., that is, 
its apparent viscosity (AV), plastic viscosity (PV) and yield point (YP) 
are high as compared with those of the conventional fracturing fluids. 
Thus, when the fracturing fluid stabilized by the method of the present 
invention is used, the fracturing operation can be carried out smoothly. 
The present invention is described below in greater detail with reference 
to the following non-limiting examples. 
In the field, the reproducibility of fracturing fluid degradation 
determinations is a problem because of the difficulty of controlling the 
conditions of temperature and pressure. Thus, in testing the following 
examples, the fracturing fluids were aged at a high temperature in a 
roller oven. Then, its rheological properties were measured at 25.degree. 
C. with a Fann VG meter Model 35, and the method of stabilization of the 
present invention was compared with the conventional method using 
fracturing fluids not containing the stabilizing compounds of the present 
invention.

EXAMPLES 1 TO 4 AND COMATIVE EXAMPLE 1 
Guar gum (1 wt% viscosity as measured by the use of a Brookfield viscometer 
(No. 3 spindle, 6 rpm, 25.degree. C.), 7,800 cp) was dissolved in 
synthetic sea water (according to ASTM D-1141; hereinafter all references 
to synthetic sea waters are according to ASTM D-1141) in such a manner 
that the concentration was 1 wt%, thereby preparing a fracturing fluid. To 
100 parts by weight of the fracturing fluid thus prepared was added 
2-thioimidazolidone in an amount of 0.003, 0.01, 0.03 or 0.1 part by 
weight. These fracturing fluids of the present invention and a comparative 
fracturing fluid not containing 2-thioimidazolidone were each aged at 
110.degree. C. for 6 hours. Then, their rheological properties were 
measured. The results are shown in Table 1. 
TABLE 1 
______________________________________ 
Amount of 2- 
Thioimidazolidone 
AV PV YP 
Run No. (parts by weight) 
(cp) (cp) (lb/100 ft.sup.2) 
______________________________________ 
Comparative 
None 10 8 2 
Example 1 
Example 1 0.003 35 21 30 
Example 2 0.01 60 23 73 
Example 3 0.03 64 24 78 
Example 4 0.1 70 25 90 
______________________________________ 
EXAMPLES 5 TO 8 AND COMATIVE EXAMPLE 2 
The same guar gum as used in Example 1 was dissolved in synthetic sea water 
in such a manner that the concentration was 0.8 wt%, thereby preparing a 
fracturing fluid. To 100 parts by weight of the fracturing fluid thus 
prepared was added 2-mercaptothiazoline in an amount of 0.003, 0.01, 0.03 
or 0.1 part by weight. These fracturing fluids of the present invention 
and a comparative fracturing fluid not containing 2-mercaptothiazoline 
were each aged at 120.degree. C. for 8 hours. Then, their rheological 
properties were measured. The results are shown in Table 2. 
TABLE 2 
______________________________________ 
Amount of 2- 
Mercaptothiazoline 
AV PV YP 
Run No. (parts by weight) 
(cp) (cp) (lb/100 ft.sup.2) 
______________________________________ 
Comparative 
None 6 5 0 
Example 2 
Example 5 
0.003 20 15 13 
Example 6 
0.01 50 21 43 
Example 7 
0.03 53 22 52 
Example 8 
0.1 57 22 61 
______________________________________ 
EXAMPLES 9 TO 12 AND COMATIVE EXAMPLE 3 
The same guar gum as used in Example 1 was dissolved in synthetic sea water 
in such a manner that the concentration was 1 wt%, thereby preparing a 
fracturing fluid. To 100 parts by weight of the fracturing fluid thus 
prepared was added benzooxazole-2-thiol in an amount of 0.002, 0.006, 0.02 
or 0.06 part by weight. These fracturing fluids of the present invention 
and a comparative fracturing fluid not containing benzooxazole-2-thiol 
were each aged at 100.degree. C. for 3 hours. Then, their rheological 
properties were measured. The results are shown in Table 3. 
TABLE 3 
______________________________________ 
Amount of Benzo- 
oxazole-2-thiol 
AV PV YP 
Run No. (parts by weight) 
(cp) (cp) (lb/100 ft.sup.2) 
______________________________________ 
Comparative 
None 18 17 6 
Example 3 
Example 9 0.002 29 18 16 
Example 10 
0.006 34 20 29 
Example 11 
0.02 56 22 59 
Example 12 
0.06 59 23 68 
______________________________________ 
EXAMPLES 13 TO 16 AND COMATIVE EXAMPLE 4 
The same guar gum as used in Example 1 was dissolved in synthetic sea water 
in such a manner that the concentration was 0.7 wt%, thereby preparing a 
fracturing fluid. To 100 parts by weight of the fracturing fluid thus 
prepared was added 0.003, 0.007, 0.02 or 0.1 part by weight of 
N-pyridineoxide-2-thiol. These fracturing fluids of the present invention 
and a comparative fracturing fluid not containing N-pyridineoxide-2-thiol 
were each aged at 130.degree. C. for 3 hours. Then, their rheological 
properties were measured. The results are shown in Table 4. 
TABLE 4 
______________________________________ 
Amount of N-- 
Pyridineoxide-2-thiol 
AV PV YP 
Run No. (parts by weight) 
(cp) (cp) (lb/100 ft.sup.2) 
______________________________________ 
Comparative 
None 5 4 0 
Example 4 
Example 13 
0.003 20 13 14 
Example 14 
0.007 30 16 31 
Example 15 
0.02 39 20 53 
Example 16 
0.1 40 20 56 
______________________________________ 
EXAMPLES 17 TO 20 AND COMATIVE EXAMPLE 5 
The same guar gum as used in Example 1 was dissolved in synthetic sea water 
in such a manner that the concentration was 1 wt%, thereby preparing a 
fracturing fluid. To 100 parts by weight of the fracturing fluid thus 
prepared was added 1,3,4-thiadiazole-2,5-dithiol in an amount of 0.005, 
0.01, 0.05 or 0.1 part by weight. These fracturing fluids of the present 
invention and a comparative fracturing fluid not containing 
1,3,4-thiadiazole-2,5-dithiol were each aged at 140.degree. C. for 6 
hours. Then, their rheological properties were measured. The results are 
shown in Table 5. 
TABLE 5 
______________________________________ 
Amount of 1,3,4- 
Thiadiazole-2,5- 
dithiol AV PV YP 
Run No. (parts by weight) 
(cp) (cp) (lb/100 ft.sup.2) 
______________________________________ 
Comparative 
None 6 7 0 
Example 5 
Example 17 
0.005 21 12 16 
Example 18 
0.01 40 20 33 
Example 19 
0.05 50 21 66 
Example 20 
0.1 53 22 84 
______________________________________ 
EXAMPLES 21 TO 24 AND COMATIVE EXAMPLE 6 
The same guar gum as used in Example 1 was dissolved in synthetic sea water 
in such a manner that the concentration was 1 wt%, thereby preparing a 
fracturing fluid. To 100 parts by weight of the fracturing fluid thus 
prepared was added 4-ketothiazolidine-2-thiol in an amount of 0.005, 0.01, 
0.05 or 0.1 part by weight. These fracturing fluids of the present 
invention and a comparative fracturing fluid not containing 
4-ketothiazolidine-2-thiol were each aged at 120.degree. C. for 3 hours. 
Then, their rheological properties were measured. The results are shown in 
Table 6. 
TABLE 6 
______________________________________ 
Amount of 4-Keto- 
thiazolidine-2-thiol 
AV PV YP 
Run No. (parts by weight) 
(cp) (cp) (lb/100 ft.sup.2) 
______________________________________ 
Comparative 
None 9 7 0 
Example 6 
Example 21 
0.005 35 21 26 
Example 22 
0.01 60 23 73 
Example 23 
0.05 63 24 79 
Example 24 
0.1 67 24 83 
______________________________________ 
EXAMPLES 25 TO 28 AND COMATIVE EXAMPLE 7 
Hydroxypropylguar (1 wt% viscosity as measured by the use of a Brookfield 
viscometer (No. 3 spindle, 6 rpm, 25.degree. C.), 6,800 cp) was dissolved 
in synthetic sea water in such a manner that the concentration was 0.6 
wt%, thereby preparing a fracturing fluid. To 100 parts by weight of the 
fracturing fluid thus prepared was added 2-thioimidazolidone in an amount 
of 0.002, 0.006, 0.02 or 0.06 part by weight. These fracturing fluids of 
the present invention and a comparative fracturing fluid not containing 
2-thioimidazolidone were each aged at 120.degree. C. for 6 hours. Then, 
their rheological properties were measured. The results are shown in Table 
7. 
TABLE 7 
______________________________________ 
Amount of 2- 
Thioimidazolidone 
AV PV YP 
Run No. (parts by weight) 
(cp) (cp) (lb/100 ft.sup.2) 
______________________________________ 
Comparative 
None 15 9 3 
Example 7 
Example 25 
0.002 20 14 26 
Example 26 
0.006 25 15 33 
Example 27 
0.02 33 17 43 
Example 28 
0.06 36 17 50 
______________________________________ 
EXAMPLES 29 TO 32 AND COMATIVE EXAMPLE 8 
The hydroxypropylguar as used in Example 25 was dissolved in synthetic sea 
water in such a manner that the concentration was 1 wt%, thereby preparing 
a fracturing fluid. To 100 parts by weight of the fracturing fluid thus 
prepared was added a 1,3,4-thiadiazole-2,5-dithiol sodium salt in an 
amount of 0.003, 0.01, 0.03 or 0.1 part by weight. These fracturing fluids 
of the present invention and a comparative fracturing fluid not containing 
1,3,4-thiadiazole-2,5-dithiol sodium salt were each aged at 130.degree. C. 
for 12 hours. Then, their rheological properties were measured. The 
results are shown in Table 8. 
TABLE 8 
______________________________________ 
Amount of 
1,3,4-Thiadiazole- 
2,5-dithiol 
Sodium Salt AV PV YP 
Run No. (parts by weight) 
(cp) (cp) (lb/100 ft.sup.2) 
______________________________________ 
Comparative 
None 20 14 30 
Example 8 
Example 29 
0.003 50 29 30 
Example 30 
0.01 70 32 55 
Example 31 
0.03 74 32 65 
Example 32 
0.1 79 33 76 
______________________________________ 
From the above results, it can be seen that the method of stabilizing a 
fracturing fluid according to the present invention is quite excellent. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.