OIL GEL

GEL DE PETROLE

English Abstract

A fracturing fluid for use in fracturing subterranean
formations comprises a hydrocarbon base. Neutralized alkyl
phosphate esters are complexed with metallic cations, to form
a gel, in the hydrocarbon base.

French Abstract

Un fluide de fracturation utilisé pour la fracturation de formations souterraines comprend une base d'hydrocarbure. Des phosphates d'alkyle neutralisés sont combinés à des cations métalliques afin de former un gel dans la base d'hydrocarbure.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A fracturing fluid for use in fracturing subterranean
formations comprising:
(a) a hydrocarbon base;
(b) neutralized alkyl phosphate esters complexed with
(c) metallic cations, to form a gel, in said hydrocarbon
base.
2. A fracturing fluid as claimed in claim 1, wherein said
alkyl phosphate esters include mono-alkyl phosphate ester, di-
alkyl phosphate esters and tri-alkyl phosphate esters.
3. A fracturing fluid as claimed in claim 2, wherein at least
50% of said alkyl phosphate esters are di-alkyl phosphate
esters.
4. A fracturing fluid as claimed in claim 2, wherein at least
65% of said alkyl. phosphate esters are di-alkyl phosphate
esters.
5. A fracturing fluid as claimed in claim 4, wherein no more
than 5% of said alkyl phosphate esters are tri-alkyl phosphate
esters.
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6. A fracturing fluid as claimed in claim 5, wherein the
alkyl component of said alkyl phosphate esters are formed from
primary monohydric alkyl alcohols of chain length C5-C16.
7. A fracturing fluid as claimed in claim 5, wherein the
alkyl component of said alkyl phosphate esters are formed from
primary monohydric alkyl alcohols of chain length C8-C12.
8. A fracturing fluid as claimed in any one of claims 1 to 7
wherein said alkyl phosphate esters are neutralized with a
primary amine.
9. A fracturing fluid as claimed in claim 8, wherein said
alkyl phosphate esters are neutralized to a pH of about 1.4 to
1.8.
10. A fracturing fluid as claimed in claim 8 or 9, wherein
said alkyl phosphate esters are neutralized with mono-ethanol
amine (MEA).
11. A fracturing fluid as claimed in any one of claims 1 to 10
wherein said metal cation is a trivalent ion.
12. A fracturing fluid as claimed in claim 11, wherein said
metal cation is a trivalent aluminum ion.
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13. A fracturing fluid as claimed in claim 12, wherein the
source of said metal cation is selected from the group
including aluminum isopropoxide, aluminum sec-butoxide, oxy-
aluminum octoate and other chelated aluminum compounds.
14. A fracturing fluid as claimed in any one of claims 1 to 13
further including a surfactant.
15. A fracturing fluid as claimed in claim 14, wherein said
surfactant is selected from the group including sulphates and
sulfonates.
16. A fracturing fluid as claimed in claim 14 wherein said
surfactant is ammonium alkyl (C6-C20) sulfate.
17. A fracturing fluid as claimed in claim 14, including 0.5 -
1.5% of said alkyl phosphate ester, 0.025 - 0.2% of said
surfactant and 0.1 - 1.5% of said metal cation.
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Description

CA 0221632~ 1997-09-19
OIL GEL
Field of the Invention
The present invention relates to methods and compositions
for fracturing subterranean formations. In particular, the
present invention provides a novel fracturing fluid for
fracturing such formations.
Backqround of the Invention
In order to increase the productivity of oil and gas
wells, or to bring back into production wells that have
essentially gone dry, it is common practice to conduct a
procedure known as fracturing the well. In such a procedure,
fluids known as fracturing fluids, are injected into the well
at very high hydrostatic pressures. The fluids are typically
viscous gels, and act under pressure to open pores and cracks
in the formation, thereby to increase the overall permeability
of the well. Typically, the fluids also are used to transport
proppants, such as sand or ground walnut shells into the cracks
and pores, thereby to ensure that the pores and cracks formed
during fracturing remain open under the lower pressures that
will exist after fracturing, when the well is producing.
The fluid that has been used to fracture the formation is
removed by the introduction of viscosity lowering agents into
same, which permit the fluid to be more easily pumped from the

CA 0221632~ 1997-09-19
formation. These agents are known as breakers because they
tend to break down the fracturing gel. Breakers can act on a
gel in a number of ways, such as by the random oxidation of
polymers to shorten the chain length thereof. In the present
invention, a breaker is utilized to adjust the pH of a gel, to
break same by hydrolysis.
The fracturing fluid of the present invention is a
phosphate alkyl ester gel. It is known that a gel can be
produced by mixing trivalent cations, such as aluminum, with
a phosphate alkyl ester. However, gels obtained with known
phosphate alkyl esters have not been commercially acceptable,
because the viscosity developed with same has been insufficient
or slow to develop. Moreover, to obtain such gels, tank mixing
has been needed.
Phosphate alkyl esters may be mono-esters, di-esters or
tri-esters. In the mono-ester, one primary mono-hydric
alcohol, of C5-Cl6 length is ester linked to a phosphate. A
di-ester has two such alkyl alcohols linked to a phosphate.
A tri-ester has three alkyl alcohols linked to it. The
applicants have discovered that commercially valuable gels are
feasible with a di-ester content above 50~, preferably above
65~, and a tri-ester content below 5~. The remainder may be
mono-ester.

CA 0221632~ 1997-09-19
The applicants enhance gel development in two important
ways. First, the phosphal alkyl esters are neutralized with
primary amines. Secondly, the applicants utilize a surfactant
to enhance gel development. An appropriate surfactant is
ammonium alkyl (C6-C20) sulfate.
In order to break the gel of the present invention the
preferred breaker is a pH adjusting breaker, such as calcium
oxide and sodium carbonate. It is preferred that the breaker
be encapsulated in a porous inert substance, such as nylon.
In a broad aspect, then, the present invention relates to
a fracturing fluid for use in fracturing subterranean
formations comprising: a hydrocarbon base; and neutralized
alkyl phosphate esters complexed with metallic cations, to form
a gel, in said hydrocarbon base.
Brief DescriPtion of the Drawings
In drawings that illustrate the present invention by way
of example:
Figure 1 is a graph illustrating the effect of pH and
surfactant on the phosphate alkyl ester gels of the present
invention.
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CA 0221632~ 1997-09-19
Detailed Description
The phosphate ester gels of the present invention are made
from primary mono-hydric alcohols of C5-C16 chain length. It
is preferred to utilize alcohols of chain length C8-Cl2, but it
will be understood that since the gels of the present invention
are intended to act on hydrocarbon fluids, such as diesel fuel,
kerosene, or other common hydrocarbon fluids, the selection of
an optimal chain length alcohol will be a matter of choice for
one skilled in the art. The alkyl alcohols are combined with
phosphates, by known techniques to produce mono-di and tri-
alkyl esters which may generally be represented as follows:
HO-P-OR
OH
mono-alkyl phosphate ester
o
RO-P-OR
OH
di-alkyl phosphate ester
o
ll
RO-P-OR
OR
tri-alkyl phosphate ester
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: CA 0221632~ 1997-09-19
. : . .
The relative proportions of mono-alkyl, di-alkyl and tri-
alkyl esters are important. The applicant has discovered that
the di-alkyl phosphate ester content must be at least 50% for
any gel development to occur, and must be at least 65% for
5 commercially feasible gel to form. Moreover, tri-alkyl
phosphate ester content must be lower than 5%, or poor gel
rheological characteristics will result. There must be at
least a small percentage of mono-alkyl phosphate ester present
to initiate gelling.
Primary amines are used to neutralize esters. The
quantity of amine used will, of course depend on the amine.
However, a suitable amine is mono-ethanol amine (MEA). From
1% to 5% of a concentrated (99%) MEA solution (relative to the
weight of the phosphate alkyl esters) is used to produce a
desired level of neutralization (pH) in the esters. A
preferred pH is about 1.4-1.8.
The partially neutralized phosphate ester is then mixed
in the hydrocarbon fluid to be gelled, with a trivalent cation,
either aluminum or ferric. It is, moreover, preferred to
utilize chelated ions, since these readily disperse in organic
solvents, thereby enhancing the rate of gelation. Examples of
suitable compounds are aluminum isopropoxide, aluminum sec-
butoxide, oxy-aluminum octoate, and so on.

CA 0221632~ 1997-09-19
The phosphate-alkyl esters and cations form a mesh-like
network in the hydrocarbon fluid, resulti~ng in a gel. The
hypothesized reaction is as follows:
11
O RO-P-OR
11 3+
RO-P-OR (excess) + Al ~ O
o OH Al
0~ ~ ~0 0\ ~0
RO OR RO \ OR
It will be understood that such a network will transform
the essentially two dimensional phosphate-alkyl ester to a
three dimensional, very viscous gel.
The gelling rate can moreover be greatly enhanced using
a surfactant. Sulphates or sulphonate groups provide the
necessary alteration of surface chemistry to result in
enhancement of reaction rates. Aluminum alkyl (C6-C20) sulfate
has been found to be an appropriate surfactant.
The effects of partial neutralization and the use of
surfactants are illustrated in Figure l. In the examples shown
in Figure l, it can be seen that the lowest viscosity is
achieved with an unneutralized gel, and no surfactant. It will
be understood that in each case 8 litres per cubic metre diesel

- CA 0221632~ 1997-09-19
oil, of a 70% dialkyl phosphate ester are utilized, with
81/cubic metre of aluminum tri-isopropoxide.
In the example neutralized to a pH of 1.57, resulting
viscosity is about doubled, with the use of a surfactant
resulting in a further tripling of the achieved viscosity. As
shown, if the pH is permitted to rise, viscosity will be
lowered somewhat. This information can be utilized, however,
to advantage, to design gels with viscosity characteristics to
particularly match those desired for fracturing a given
formation.
To form a gel in a hydrocarbon solvent according to the
present invention, 0.5 - 1.5% (by weight of hydrocarbon)
partially neutralized phosphate alkyl ester are utilized with
0.025 - 0.2% surfactant and 0.1 - 1.5% trivalent cation. About
80% of the maximum gel viscosity is attained, typically, in the
first minute of mixing. The constituents can and
advantageously are, therefore, blended "on the fly" as they are
pumped into a formation.
The gel may be broken by the use of pH adjusting breakers,
such as soda ash, caustic, lime, amines, and acids.
It will be appreciated, therefore, that the present
invention provides a reliable viscous gel for use with

CA 02216325 1997-09-19
hydrocarbon solvents, which can be broken on a consistent and
effective basis.
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