Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to the field of oil and gas
well fracturing.
In the area of oil and gas field exploration and
development, a technique that is utilized to increase production
from a well, or to make a non-productive well productive again
is hydraulic fracturing. Generally speaking, this involves
injection of fluid under high pressure down a well bore. This
causes an increase in pressure in the subterranean formation with
which the well bore communicates, and fracturing of such
formation thereby creating access to the well bore for untapped
oil or gas in the formation. The fluid that is introduced into
the formation may include various propping agents, such as sand,
or walnut shell fragments, to increase the flow of oil or gas
through a newly fractured formation.
It is known, moreover, to utilize a gel in fracturing
techniques, in order to increase the viscosity of the fracturing
fluid, and enhance propant transport. For instance, in Canadian
Patent No. 1,174,841, which has been assigned to the present
applicant, a fracturing fluid is described that comprises at
least one substantially anhydrous aliphatic alcohol, a nonionic
homopolymer to form a gel with the alcohol that has a molecular
weight from about 4 million to about 5 million. Such a
fracturing fluid also includes, as a gel activating agent, an
alkali metal halide or an alkaline metal earth halide. The
aliphatic alcohol utilized in such a fluid is generally a one to
four carbon alcohol, including up to five percent water. The non
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ionic homopolymer utilized is a polyalkaline oxide such as
poly(ethylene oxide) in an amount of from 0.1% to 3% by volume,
and a suitable activating agent is potassium, sodium, calcium or
magnesium halide, in amounts generally under 1%. This prior art
fracturing fluid of the applicant was developed ta meet a need
that was clear from the existing prior art, for a fracturing gel
having a low water content, because of the fact that water may
be considered as a subterranean formation contaminant.
There have been other attempts to utilize substantially
anhydrous alcohol gels, such as is taught in U.S. Patent No.
4,012,327 which relates to the use of alcohols that are
thickened, but not necessarily gelled, by aminomethylated
polyacrylamide. The gel in such a case, though, is formed by a
substantial linear polymer, and so will exhibit limited
viscosity, shear strength, and stability.
The object of the present invention is to provide an
improved anhydrous alcohol gel, that will exhibit a high degree
of shear stability, and high viscosity at elevated temperatures.
A further object of the present invention is to provide an
improved fracturing fluid that will form a shear stable gel in
an anhydrous alcohol medium at elevated temperature, and exhibit
a predictable breakdown rate in view of measured downhole
conditions. In this latter regard, it will be understood that
injection of a fracturing fluid in the form of a gel is of little
use if the gel remains in its gelled state in the formation. It
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is essential that the gel break down under known physical or
chemical conditions.
The fracturing fluid of the present invention therefore
includes, as well as a gel complexor, a gel breaker. It will be
understood that the gel breaker chosen for use will be relatively
slow acting a compared to the complexor or cross linker, so that
the fluid gel may perform its fracturing function before the gel
is broken. The use of a slow acting gel breaker, that will act
in a predictable manner, in combination with a relatively faster
acting gelling agent permits the addition of the gel breaker to
the fracturing fluid as it is being pumped down a well.
In accordance with the present invention, a fracturing fluid
is provided that utilizes a modified guar gum polymer, in
anhydrous alcohol, preferably methyl alcohol, to form a gel. As
a gel complexor, a sodium borate salt is used.
Guar gum is a known, naturally occurring polymer, the ground
endosperms of cyamopsis tetragonalobus. It is a linear polymer
of (1 ~ 4)-B-D mannopyranosyl units with ~ - D - galactopyranosyl
units attached by (1 ~ 6) linkages. In its uncomplexed linear
state it exhibits a molecular weight of about 220,000.
Particularly useful modification of guar gum according to the
present invention include hydroxy, carboxy and alkyl guar gums.
The applicant has found that about 6 kg of guar gum, or modified
guar gum, per cubic metre of anhydrous alcohol, will form an
appropriate gel, when complexed with 14 - 18 litres of sodium
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borate or sodium tetraborate. The borate solution is prepared
on the basis of 0.5 to 4.0 kg per cubic metre of fracturing
fluid.
Prior to addition of the cross linking agent, fumaric acid
(or another suitable acid) may be added into methanol with guar
gum to assist in formation of the base gel.
The gel breaker that is added to the fracturing fluid is
chosen for its ability to degrade the cross linked guar gum in
a desired time frame, and suitable breakers include ammonium
persulfate, sodium persulfate and sodium perborate, in quantities
of 0.1 to 10.0 Kg per cubic metre of fracturing fluid. The large
degree of variation is a result of combined facts that: (a)
depending on the concentration of guar gum polymer in the
solution, a more or less thick gel will result; (b) varying
amounts of cross linking agent (borate) will have been used,
depending on the extent of cross linking, and corresponding
viscosity of the gel desired; (c) the desired down hole residence
time (d) the downhole depth and temperature, and other factors
that will be obvious to one skilled in the art.
Furthermore, the breaker utilized may be coated, in a
conventional manner, if desired. The use of a coating on a
granular breaker provides a further degree of control, for
situations wherein it is desired to have substantially no gel
breakdown for a given period of time. Coating a granular breaker
achieves such a degree of control by selection of coatings having
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known rates of dissolution in the alcohol chosen as the basis
for the fracturing fluid.
It is to be understood that any desirable proppant may be
combined on the fly with the fracturing fluid of the present
S invention. Suitable proppants for use in conjunction with the
fracturing fluid of the present invention include sand,
synthetic sand, or synthetic coated sand.
In a broad aspect, then, the present invention relates to
a fracturing fluid composition for use in fracturing
underground formations, such as oil or gas wells, comprising:
at least one anhydrous aliphatic alcohol selected from the
group including methanol, ethanol and isopropanol; from 0.1 to
2.0 wt %, relative to said alcohol, of a modified guar gum
polymer selected from the group including hydroxy carboxy and
alkyl guar gums; from 0.5 to 4.0 Kg per m3 of alcohol of a
sodium borate salt complexor selected from the group including
sodium borate and sodium tetraborate, in solid or liquid form;
and from 0.1 to 10.0 Kg per m3 of alcohol of a gel breaker
which acts over a time period to degrade the guar gum cross
linked polymer, and thereby reduce the viscosity of the gel,
to aid in the removal thereof from a well, said breaker being
selected from the group including ammonium persulfate, sodium
persulfate, sodium perborate, or mixtures of one or more of the
foregoing.
_, . .. . ..
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Also preferably, the complexor is sodium tetraborate in
a solid or liquid form equivalent to from 0.5 to 4.0 Kg per
cubic metre of alcohol.
To obtain an effective fracturing fluid, said modified
guar gum polymer is mixed with the alcohol in a concentration
of from 0.1 to 2.0% by weight per volume of alcohol, and a
sufficient quantity of acidifying agent such as fumaric acid
is added. Furthermore, the quantity of gel breaker is selected
to break down the polymer within a predetermined time period,
usually 1 to 24 hours.
The gel breaker is, in a preferred embodiment, ammonium
persulfate, sodium persulfate, sodium perborate or a mixture
of one or more of the foregoing and wherein said gel breaker
may be coated or covered. The concentration of the breaker may
be 0.1 to 10.0 Kg/m3 of alcohol.
The composition, in its preferred form, is admixed with
a proppant selected from sand, synthetic sand or synthetic
coated sand.
In another broad aspect, the present invention relates to
a method of fracturing an underground formation such as an oil
or gas well comprising: injecting the formation with an
effective amount of a fracturing fluid gel composition
,,--
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comprising: at least one anhydrous aliphatic alcohol selected
from the group including methanol, ethanol and isopropanol;
from 0.1 to 2.0 wt %, relative to said alcohol, of a modified
guar gum polymer selected from the group including hydroxy
carboxy and alkyl guar gums; from 0.5 to 4.0 Kg per m3 of
alcohol of a sodium borate salt complexor selected from the
group including sodium borate and sodium tetraborate, in solid
or liquid form; and from 0.1 to 10.0 Kg per m3 of alcohol of
a gel breaker which acts over a time period to degrade the guar
gum cross linked polymer, and thereby reduce the viscosity of
the gel, to aid in the removal thereof from a well, said
breaker being selected from the group including ammonium
persulfate, sodium persulfate, sodium perborate, or mixtures
of one or more of the foregoing; and applying pressure to the
composition in the formation to induce fracture of the
formation.
In the method as aforesaid the complexor is sodium
tetraborate in a solid or liquid form equivalent to from 0.5
to 4.0 Kg per cubic meter of alcohol, and the modified guar gum
polymer is mixed with the alcohol in a concentration of from
0.1 to 2.0% by weight per volume of alcohol. A sufficient
quantity of acidifying agent such as fumaric acid is added.
The quantity of gel breaker is selected to break down the
polymer within a predetermined time period, such as 1 - 24
hours.
A
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The order of addition is as follows: the alcohol and guar
gum are mixed first, with the fumaric acid, to form a base gel,
that is permitted to gel for several minutes. The borate
complexor is then added, followed by the gel breaker.
In the method of the present invention, the gel breaker
is ammonium persulfate, sodium persulfate, sodium perborate or
a mixture of one or more of the foregoing, and said gel breaker
may be coated or covered. The concentration of the breaker is
0.1 to 10.0 Kg/m3 of alcohol.
7(a)
210~
Moreover, said alcohol, said guar gum, said complexor and
said gel breaker are mixed together as they are being injected
in a said well, in a most preferred embodiment.
In the method of the present invention, the order of
addition of complexor and breaker is not considered crucial. Of
more importance will be the selection of an appropriate quantity
of breaker, and in this regard, one must consider the temperature
at the bottom of the well. As an example, utilizing a fracturing
fluid prepared with 6 Kg of guar gum per cubic metre of 100%
methanol, with about 16 litres of a 3.5% sodium borate solution
added as a complexor, and 0.08 Kg of fumaric acid per cubic metre
as an acidifying agent, the following results are obtained with
sodium perborate breaker:
TABLE I
BREAK TIMES OF METHANOL GEL AT VARIOUS
CONCENTRATIONS AND TEMPERATURES
TEMPERATURE ( C)
Breaker
Concentration
(Kg/m3) 60 70 80 90
O . 1
0.2 4 hrs 3 hrs 2 hrs
0.3 3 hrs 2 hrs
0.4 7 hrs
0.5 6 hrs
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It will be understood, then, that depending on the extent
of fracturing desired at a given location, and the measured down
hole temperature, different amounts of breaker will be added to
the fracturing fluid as it is pumped down a well.
It a preferred method according to the present invention,
after the base fel is formed, the complexor and breaker of the
fracturing fluid are mixed together "on the fly", as the fluid
is being pumped down a well. In this way, exposure of the guar
gum to the complexor and breaker is not effected until just prior
to injection of the fluid into a formation. This results in
better pumpability into a well, and more accurate determination
of when the gel will have broken. Moreover, should there be any
delay due to equipment failure or the like, there is no wasted
gel/complexor/breaker fluid.
It is to be understood that the examples described above are
not meant to limit the scope of the present invention. It is
expected that numerous variants will be obvious to the person
s~illed in the oil and gas well fracturing fluid design field
without any departure from the spirit of the invention. The
appended claims, properly construed, form the only limitation
upon the scope of the invention.
