Note: Descriptions are shown in the official language in which they were submitted.
CA 02356081 2001-05-17
WO 00/36272 . PCT/CA99/Oi 180
FOAMED NITROGEN IN LIQUID C02 FOR FRACTURING
FIELD OF THE INVENTION
The present invention relates to the f eld of fracturing subterranean
formations around
oil and gas wells. In particular, the present invention relates to an
improvement in
fracturing using liquid COZ as a fracturing medium.
BACKGROUND OF THE INVENTION
Hydraulic fracturing has been widely used for stimulating the production of
crude oil
and natural gas from wells completed in reservoirs of low permeability.
Methods
employed normally require the injection of a fracturing fluid containing
suspended
propping agents into a well at a rate sufficient to open a fracture in the
.exposed
formation. Continued pumping of fluid into the well at a high rate extends the
fracture
and leads to the build up of a bed of propping agent particles between the
fracture
walls. These particles prevent complete closure of the fracture as the fluid
subsequently leaks off into the adjacent formations and results in a permeable
channel
I S extending from the well bore into the formations. The conductivity of this
channel
depends upon the fracture dimensions, the size of the propping agent
particles, the
particle spacing and the confining pressures.
The fluids used in hydraulic fracturing operations must have fluid loss values
Buff ciently low to permit build up and maintenance of the required pressures
at
reasonable injection rates. This normally requires that such fluids either
have adequate
viscosities or other fluid loss control properties which will reduce leak-off
from the
fracture into the pores of the formation.
Fracturing of low permeability reservoirs has always presented the problem of
fluid
compatibility with the formation core and formation fluids, particularly in
gas wells.
For example, many formations contain clays which swell when contacted by
aqueous
fluids causing restricted permeability, and it is not uncommon to see reduced
flow
through gas well cores tested with various oils.
Another problem encountered in fracturing operations is the difficulty of
total
recovery of the fracturing fluid. Fluids left in the reservoir rock as
immobile residual
SUBSTITUTE SIIEI; T ~R ULL~' ? 6)
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z
fluids impede the flow of reservoir gas or fluids to the extent that the
benefit of
fracturing is decreased or eliminated. Attempting the removal of the
fracturing fluid
may require a large amount of energy and time, sometimes not completely
recovering
all the products due to formation characteristics. Consequently the reductian
or
elimination of the problem of fluid recovery and residuE; removal is highly
desired.
In attempting to overcome fluid loss problems, gelled fluids prepared with
water,
diesel, methyl alcohol, solvents and similar low viscosity liquids have been
useful.
Such fluids have apparent viscosities high enough to support the proppant
materials
without settling and also high enough to prevent excessive leak-off during
injection.
The gelling agents also promote laminar flow under conditions where turbulent
flow
would otherwise take place and hence in some cases, the pressure losses due to
fluid
friction may be lower than those obtained with low viscosity-base fluids
containing no
additives. Certain water-soluble, poly-acrylamides, oil soluble poly-
isobutylene and
other polymers which have little effect on viscosity when used in low
concentration
can be added to the ungelled fluid to achieve good friction reduction.
In attempting to overcome the problem of fluid compatibility when aqueous
fracturing
fluids are used, chemical additives have been used such as salt or chemicals
for pH
control. Salts such as NaCI, KCl or CaCI2 have been widely used in aqueous
systems
to reduce potential damage when fracturing water sensitive formations. Where
hydrocarbons are used, light products such as gelled condensate have seen a
wide
degree of success, but are restricted in use due to the nature of certain low
permeability reservoirs.
Low density gases such as C02 or NZ have been used in attempting to overcome
the
problem of removing the fracturing (load) liquid. The :low density gases are
added to
the load fluid at a calculated ratio which promotes back. flow subsequent to
fracturing.
This back flow of load fluids is usually due to reservoir pressure alone
without
mechanical aid from the surface because of the reduction of hydrostatic head
caused
by gasifying the liquid.
Moreover, low density liquefied gases have themselves, been used as fracturing
fluids.
Reference is made to Canadian Patents 687,938 a:nd 745,453 to Peterson who
SUBSTITUTESHEET (RULE?6)
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3
discloses a method and apparatus for fracturing underground earth formations
using
liquid C02. Peterson recognized the advantages of liquid C02 as a means to
avoid
time consuming and expensive procedures involved in the recovery of more
conventional fracturing fluids. Peterson however does not disclose the use of
entrained proppants in conjunction with liquid CO2. The combination of a
liquid COZ
fracturing fluid and propping agents has been described lby Bullen in Canadian
Patent
932,655 wherein there is described a method of entraining proppants in a
gelled fluid,
typically a gelled methanol, which is mixed with liquid carbon dioxide and
injected
into low permeability formations. The liquid carbon dioxide is allowed to
volatilize
I O and bleed off and the residual liquid, primarily methyl alcohol, is in
part dissolved by
formation hydrocarbons and allowed to return to the surface as vapor. The
balance,
however, is recovered as a liquid using known recovery techniques. It has,
however,
been demonstrated that the need to use a gelled carriier fluid has resulted in
the
negation of some of the fluid recovery advantages attendant upon the sole use
of
liquefied gas fracturing fluids.
Subsequent disclosures have been concerned primarily v~rith the development of
more
advantageous gelled fluids to entrain prappants for subsequent or simultaneous
blending with the liquefed carbon dioxide fracturing fluid. Reference is made
to
Canadian Patents 1,000,483 (reissued as Canadian Patent 1,034,363), 1,043,091,
1,197,977, 1,241.826 and 1,242.389 in this regard. Each of these patents
teaches the
nature and composition of gelled or ungelled carrier fluids, typically
methanol or
water based, which, when blended with liquid C02 produce a two-phase liquid
system
which allegedly is useful in attempting to overcome the problems of leak-off
and fluid
compatibility with formation fluids while at the same time being capable of
transporting increased concentrations of proppant material into the fracture
zones.
Treatments have also been designed utilizing combinations of fluids with
nitrogen or
carbon dioxide and even binary foams where nitrogen a.nd liquid carbon dioxide
are
combined into an aqueous or water-based fracturing fluid. Reference is made in
this
regard to U.S. Patent No. 5,069,283 issued on December 3, 1991 to the Western
Company of North America. The addition of nitrogen .and/or liquid carbon
dioxide
provides a non-combustible gas that aids in the recovery ~of the treatment
fluids. These
gasified fluids also reduce the amount of potentially damaging aqueous fluid
pumped
SUBSTITUTESHEET (RULE?G)
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4
into the formation. Despite this, this method nevertheless requires the
incorporation of
a thickening agent into an aqueous fluid to provide sufficient viscosity to
entrain
adequate proppants and to prevent leak-off. Although these gasified fluids
reduce the
amount of potentially damaging gelled and/or cross-linked load fluid pumped
into the
formation, the risk of contamination by significant residual liquid fractions
remain
high.
From the foregoing, it will be readily appreciated that; the use of liquid COz
as a
fracturing agent is known. It is further known to use other liquids having
propping
agents entrained therein fox blending with the liquefied gas fracturing fluid.
The
propping agents are subsequently deposited in the liquid ar foam-formed
fractures for
the purpose of maintaining flow passages upon rebound of the fracture zone. It
is
further known that proppant materials can be introduced into a liquid carbon
dioxide
system if a chemically gelled or cross-linked liquid, usually alcohol or water-
based, is
mixed with the C02 to impart sufficient viscosity to the: mixture to support
proppant
particles and to control leak-off in the fracture. So-called "binary" systems
incorporating additional quantities of nitrogen in a thickened aqueous
substrate are
known. All of these practices lead to residual chemicals and gel precipitates
left in the
fracture proppant pack that can impair production of the well.
In Canadian Patent 1,134,258, it has been recognized drat proppant materials
can be
introduced directly into a liquid carbon dioxide stream using little or no
other
viscosifying liquid components while still transporting significant quantities
(up to
800 kg/m3 and more in some situations) of proppant material into the fracture
zones.
This has been achieved by pressurizing and cooling the ;proppants to
substantially the
storage pressure and temperature of the liquified COZ prior to blending of the
two for
injection down the well bore.
This method, based as it is on the injection of pure or virtually pure C02,
enjoys the
obvious advantage of lessening the impact of the treatment fluid on the
formation. A
gas as mentioned in this application describes any substance that at
atmospheric
conditions exists in the vapour phase of that substance. Liquid C02, and gases
such as
nitrogen, air, exhaust gas, natural gas and inert gases, axe all relatively
inert to the
formation being stimulated and therefore no damage is done to the formation
due to
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S
injection since it is believed that C02 and the other aforementioned gases do
not
change the relative permeability of the reservoir rock. The liquid C02
fracturing
medium converts to a gaseous state after being subjected to formation
temperatures
and pressures to eliminate associated fluid pore bloclkage in the formation
and to
S promote complete fluid recovery on flow back. Moreover, no residual
chemicals or
gel precipitates are Left behind to impair fracture conductivity.
Moreover, as demonstrated in U.S. Patent No. S,SS8,1i50, significant
advantages can
be obtained from combining gases, in particular N2, with liquid CO2. In
particular,
liquid C02/N2 treatment pressures at equivalent volumetric rates.
The applicant has now discovered that significant viscosity increases in a
liquid COz
fracturing fluid system can be obtained by introducing .a foamer or surfactant
into the
liquid C02, and then bubbling N2 into the C02. This results in a viscous foam
with
NZ~e~ as the internal phase, COZ~i~ as the external phase, and the foamer or
surfactant as
the interface between the phases.
1 S SUMMARY OF THE INVENTION
The object of the present invention is to provide a metlnod of fracturing
subterranean
formations surrounding oil and gas wells comprising injecting into said wells
a liquid
C02 fracturing fluid with higher viscosity than currently available.
A further object of the present invention is to provide a method of fracturing
subterranean formations surrounding oil and gas wells using a fracturing fluid
comprising foam of N2tb~ in COZ~~~ in which a foam forming substance has been
dissolved.
A further object of the present invention is to provide a method of fracturing
subterranean formations surrounding' oil and gas wells using a liquid C02
fracturing
2S fluid capable of creating a wide fracture.
A further object of the present invention is to provide method of fracturing
subterranean formations surrounding oil and gas wells using a liquid C02
fracturing
fluid capable of transporting a large proppant load.
SUBSTITUTE SHEET (RULE ?6)
21-02-2001 ~ 02356081 2001-05-17 CA 009901180
6
A further object of the present inven~on is 1:a provide a m$~d o f ~turing
subterranean formations surrounding oil and gas walls using a liquid COZ
fractuxing
fluid capable of controlled leak a~'inta a formation,
In a broad aspect, tlxen, t'he present invention relates to a method of
fract~g
subterranean formations surrounding oil and gas wells using a foam constituted
by a
liquid phase and a gaseous phase, said liquid phase heaving a non-functional,
nonionic
fluorochernical stabilizer dissolved th~e~,
DETAILED DESCRIPTIpN
~n furtherance of tire present invention, a foam worming substance that is
soluble
I S in liquid or supercritical CtO~ is added to CQ2 in modest volume (about Z-
30 refers
p bly
2-~0 mare preferably about 10 Llm3). Nitrogen is den lbubbled into the liquid,
to create
a foam useful in the method of the present invention. 7!'he proportion of
gaseous phase
in the liquefied C;OZ is from about I to 75 weight °/a.
The foam forrrling substatlce useful in the method of the present invention is
a
ZO ftuorochezztical ~stabili2er comprising a nonionic, fluorinated hydrocarbon
that may be
linear, branched, or cyclic, and optionally may contain one yr more additional
catenary
heteroatoms, such as nitrogen or oxygen. The stabilizer may be selected from
the group
consisting of fully- and partially.fluorinated alkanes. amines, ethers, and
aromatic
compounds. Preferably, the #tuorochemical stabilizer is non-functional, i.e.
lacking
25 functional groups that are polymerizabIe, reactive toward acids, bases,
oxidising agents,
reducing agents or nucleophiles. Preferably, the number of fluorine atoms
exceeds the
number of hy~agen atoms in. the fluorocheirfical stabili2:er. To be non-
flammable, the
relationship between the numbex of fluorine, hydrogen, and carboat atoms cart
preferably be related in that the ztumber of #luorine atoms its equal to or
exceeds the sttm
~0 of the number of number of hydrogen atoms and carbon-cfu~bon bonds:
# F atoms ~ (# H atoms + # C-C bands).
Orie class of compounds useful as fluorochemi:cal stabilizers comprises
perfluoroca,.~ons in which sIl carbon-bound hydrogen is replied by fluo~ne
stoma,
35 Such compounds are known to be inert and exhibit high thermal 5tabilit . Su
Y ch
AMENDED SHEET
CA 02356081 2001-05-17
WO 00/36272 PCT/CA99t01180
. 7
perfluorinated compounds may include perfluoroall~:anes, perfluoroamines and,
perfluoroethers, which may be linear or branched, and cyclic or acyclic.
Examples of
perfluorinated compounds include perfluoroalkanes :having the general formula
CnF2~+z~ perfluoroethers and polyethers having' the general formula C"F2"+ZOm
and
perfluoroamines having the general formula C~F2n+3N, where n is an integer of
3 to 20
and rn is 1 to 5.
Useful perfluorinated liquids typically contain from 3 to 20 carbon atoms and
may
optionally contain one or more catenary heteroatoms, such as divalent oxygen
or
trivalent nitrogen atoms. The term "perfluorinated liquid" as used herein
includes
organic compounds in which all (or essentially all) of the hydrogen atoms are
replaced with fluorine atoms. Representative perfluorinated liquids include
cyclic and
non-cyclic perfluoroalkanes, perfluoroamines, perfluoroethers,
perfluorocycloamines,
and a.ny mixtures thereaf. Specific representative perfluorinated liquids
include the
following: perfluoropentane, perfluorohexane, perfluoroheptane,
perfluorooctane,
perfluoromethylcyclohexane, perfluorotributyl amine, perfluorotriamyl amine.
perfluoro-N-methylmorpholine, perfluoro-N-ethylmorpholine, perfluoroisopropyl
morpholine, perfluoro-N-methyl pyrrolidine, perfluoro-1,2
bis(trifluoromethyl)hexafluorocyclobutane, periFluoro-2-butyltetrahydrofuran,
perfluorotriethylamine, perfluorodibutyl ether, and mixtures of these and
other
perfluorinated liquids.
Commercially available perfluorinated liquids that ca.i1 be used in this
invention
include: FLUORINERT FC-43TM- Electronic Fluid, FLUORTNERT FC-72TM
Electronic Fluid, FLUORINERT FC-77TM Electronic Fluid, FLUORINERT FC-84TM
Electronic Fluid, FLUORINERT FC-87TM Electronic :Fluid, Performance Fluid PF-
5060TM, Performance Fluid PF-5070TM, and Performance Fluid PF-SOS2TM. Some of
these liquids are described in FLUORINERTTM Electronic Fluids, product
bulletin 98-
0211-6086(2I2)NPI, issued 2/91, available from 3M Co., St. Paul, Minn. Other
commercially available perfluorinated liquids that are considered useful in
the present
invention include perfluorinated liquids sold as GALDE;NT'" LS fluids
available from
Montedison Inc., Italy, KRYTOXTM fluids available from DuPont and FLUTECTM PP
fluids available from BNFL Fluorochernicals Ltd.
SUBSTITUTE SHE.t: T (R ULE ?G)
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8
Perfluorinated compounds are known and can be made by techniques such as
direct
fluorination, electrochemical fluorination, addition polymerization of
fluorine-
containing monomers and the oxidative polymerization of fluorine containing
monomers. See, for example, Chemistry of Organic Fluorine Compounds II, M.
Hudlicky and A. Pavlath, Eds., ACS Monograph 187, American Chemical Society,
Washington, D.C., I995, pp. 95-120.
It is preferred that the fluorochemical stabilizer contains aliphatic hydrogen
atoms.
Perfluorinated compounds, since they lack chlorine atoms, are not ozone-
depleting
agents, but these compounds may exhibit a global warming potential (GWP) due
to
their long atmospheric lifetimes. It is preferred that the fluorochemical
stabilizer
contains at least one aliphatic hydrogen atom in the nnolecule. These
compounds
generally axe very thermally and chemically stable, yet are much more
environmentally acceptable in that they degrade in the atmosphere and thus
have a
low global warming potential, in addition to a zero ozone; depletion
potential.
Partially fluorinated liquids, containing one or more aliiphatic or aromatic
hydrogen
atoms, may be employed in the fluid compositions of the invention. Such
liquids, like
the above perfluorinated counterparts, typically contain from 3 to 20 carbon
atoms
and may optionally contain one or more catenary he;teroatoms, such as divalent
oxygen or trivalent nitrogen atoms. Useful partially fluorinated liquids
include cyclic
and non-cyclic fluorinated alkanes, amines, ethers, cycloamines, and any
mixture or
mixtures thereof. Preferably, the number of fluorine atoms exceeds the number
of
hydrogen atoms and more preferably the number of fluorine atoms is equal to or
exceeds the sum of the number of combined hydrogen atoms and carbon-carbon
bonds. Although not preferred, due to environmental concerns, the partially
fluorinated Liquids optionally rnay contain one or more chlorine atoms
provided that
where such chlorine atoms are present there are at least two hydrogen atoms on
the
geminal or adjacent carbon atom(s).
One class of partially fluorinated liquids useful as fl.uorochemical
stabilizers are
hydrofluorocarbans; i.e. compounds having only carbon, hydrogen and fluorine,
and
optionally catenary divalent oxygen and/or trivalent nitrogen. Such compounds
are
nonionic, may be linear or branched, cyclic or acyclic. Such compounds are of
the
SUBSTl'TUTE SNEL'T (RULE ?6)
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WO 00/36272 _ PCT/CA99101180
9
formula C~FmH2n+z-m, where n is from about 3 to 20 inclusive, m is at Ieast
one, and
where one or more non-adjacent -CF2- groups may be replaced with catenary
oxygen
or trivalent nitrogen atoms. Preferably the number of fluorine atoms is equal
to or
greater than the number of hydrogen atoms, and more preferably the number of
fluorine atoms is equal to or exceeds the sum of the combined number of
hydrogen
atoms and carbon-carbon bonds of fluorine atoms.
Another useful class of partially fluorinated liquids includes fluoroaikyl-
substituted
aromatic compounds such as hexafluoroxylene.
A preferred class of hydrofluorocarbon liquids particularly useful to form the
stable
fluid composition of the invention comprises fluorinated ethers of the general
formula:
(R~-~)r; Rz (I)
where, in reference to Formula I, n is a number from 1 to 3 inclusive and R~
and RZ
are the same or are different from one another and are selected from the group
1 S consisting of alkyl, aryl, and alkylaryl groups and their derivatives. At
least one of R~
and R2 contains at least one fluorine atom, and at Ieast one of R, and R2
contains at
least one hydrogen atom. Rr and R2 may also be linea~x, branched, cyclic or
acyclic
and optionally, one or both of Rl and R2 may contain one or more catenary
heteroatoms, such as trivalent nitrogen or divalent oxy~,en. Preferably the
number of
fluorine atoms is equal to or greater than the number of hydrogen atoms, and
more
preferably the number of fluorine atoms is equal to or exceeds the sum of the
number
of combined number of hydrogen atoms and carbon-~;.arbon bonds. Although not
preferred; due to environmental concerns, R, or R2 or both of them optionally
may
contain one or more chlorine atoms provided that w3aere such chlorine atoms
are
present there are at least two hydrogen atoms on the R, or RZ group on which
they are
present.
More preferably, the fluid compositions of the present invention are prepared
with
fluorinated ethers of the formula:
R~-O-R (II)
SUBSTITUTE SHEi~T (R ULE ?6)
CA 02356081 2001-05-17
WO 00/36272 PCT/CA99/01180
where, in reference to Formula II above, Rf and R are as defned for RE and Rz
of
Formula I, except that Rf contains at least one fluorine atom, and R contains
no
fluorine atoms. More preferably, R is an acyclic branched or straight chain
alkyl
group, such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, i-butyl, or t-
butyl, and Rf
is preferably a fluorinated derivative of a cyclic or acyclic, branched or
straight chain
alkyl group having from 3 to about 14 carbon atoms, such as n-C4F9-, i-C4F9-,
i-C3F~,
(n-C3F~)CF- or cyclo-C6F1 ~-. Rf may optionally contain one or more catenary
heteroatoms, such as trivalent nitrogen or divalent oxyl;en atoms.
In a preferred embodiment, R, and R~, or Rf and R, arc: chosen so that the
compound
10 has at least three carbon atoms, and the total number of hydrogen atoms in
the
compound is at most equal to the number of fluorine atoms. In the most
preferred
embodiment. R~ and R~ or Rf and R are chosen so that the compound has at least
three
carbon atoms. and more preferably number of fluorine; atoms is equal to or
exceeds
the sum of the number of combined hydrogen atoms and carbon-carbon bonds.
I S Representative hydrofluoroether compounds described by Formulas I and II
include
the following:
.~~P--CF~OCH; F ;~-CF~OC~H;
CFz F CF~OCH,
n- CaFyOCH3
CF3CFCF~OCH; CF3CFCF~OC~H;
l
CFz CFz
SUBSTITUTE SNE~: T (R ULE 16)
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OCFf~
F
n-C4FgOC2H~
GgF~70CH3 CH;O(CFZ~OCH3
CF3 ~ CF20CH3 C3F~OCH3
CF3
CSFI f OCZHS CSF~ ~ OC3H~ F
OCH;
GF30CZF.~OCZHS C3F~OCFCFZOCH; {CF3)2CFOCH3
I
CF3
(CF3);C-OCH3 C.,FQOC~F.~OCF~CF~O(:~H; C.~FQO(CF~);OCH;
C6F~30C,f-f: (C,FS)~NCF,CF,OCH, iC~F~)=~CaF~OCE~:
CF3CFCF~OC~H;
F
F (CF2)30CaH;
SUBSTITUTESHE~:T (RULE26)
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12
(CF3~N(CFz)30CH3
(CF3~N(CFz~OC2H5
C2FSNCF,CF2CFZOC2H5
CF3
(C~F~}ZNCF2CF2CF20CH3
(C3F~~NCFZCF2CFZOCZHS
(C3F7~NCF,CF,CF,OC=~-i~
O F NCFCF~CF20CH3
I
~"~ CF3
O~ (CFZ)"OCH3; n_-_ J -4
O~ (CFa)~OC~H; n=!--i
N(CF~)"OCHz n=1-~
N(CF~)"OC?H; n=1--t
%:
N(CFZ)nOCH3
n= l _-~
N(CF~)"OC~H;
J
(C,~F9)2N(CF2r;OCH~
SUBSTITUTE SHEL'T (R ULE ?6)
CA 02356081 2001-05-17
WO OOI36272 PCT/CA99/01180
13
(C2F5)2N(CF2)60CH3 .
CF3-~N(CF2}IOCH3
CzFs F CF20CZHs
CF3 CF30C3H~
F CF2OCxHs F F
CH30FZC
F CFzOCH;
F CF~OCH~
CFZOCH3
CFzOCH;
CF~OCH: F~CF=OOHS
F ; F j
CF;
~CFZOCH;
F
C3F~CF(OC~ H 5 }CF(CF; )~
C?FSCFIOC~H;)CF(CF~)~
C~F~CF(OCH; )CF(CF; )~
CF;CFfOCH3)CF(CF; )~
SUBSTITUTE SHEET (RULE?6j
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' 14
wherein cyclic structures designated with an interior "F" are perfluorinated.
Preferred segregated hydrofluoroethers include C3F~OCH3, (CF3)2CFOCH3,
C4F9OCH3, (CF3)2CFCF20CH3, (CF3)2CFCF20CZHs, (CF3)3COCH3,
CH30(CF2)4OCH3, and CH30(CFZ)6OCH3. C3F~OC2Hs, C4F9OC2Hs, c-C~F,30CH3,
c-C~Fi30CzHs, C~F,sOCH3, C~FisOC2H;, CnF2~OCH3, and C,oF2iOC2Hs. By
"segregated" it is meant that hydrogen atoms) and fluorine atom{s) are not
found on
adjacent carbon atoms. Blends of one or more fluorinated ethers are also
considered
useful in practice of the invention.
A number of synthetic routes to hydrofluoroethers are known. These methods may
be
broadly divided into two groups; methods of fluorinating an ether compound,
and
methods where the ether linkage is formed within a compound by reaction with a
fluorine-containing precursor. The former methods include: (I) direct
fluorination of
an ether compound; and {2) electrochemical fluorination of an ether compound.
The
latter methods include: {3} the addition reaction of an alcohol to a
fluorinated olefin;
l 5 (4) alkylation of a partially fluorinated alcohol; and (5) non-catalytic
alkylation of a
fluorinated carbonyl compound with a suitable alkylating agent. Japanese
Patent No.
JP 6-293686 provides a partial summary description of these varied methods.
The fluorinated ethers (alkoxy-substituted perfluorocom~pounds) suitable for
use in the
method of the invention can be prepared by alkylation of perfluorinated
alkoxides
prepared by the reaction of the corresponding perfluorinated acyl fluoride or
perfluorinated ketone with an anhydrous alkali metal fluoride (e.g., potassium
fluoride
or cesium fluoride) or anhydrous silver fluoride in an anhydrous polar,
aprotic
solvent. {See, e.g., the preparative methods described in French Patent
Publication
No. 2,287,432, German Patent Publication No. 1,294,949, and U.S. 5,750,797
(Flynn
et al.). Alternatively, a fluorinated tertiary alcohol can be allowed to react
with a base,
e.g., potassium hydroxide or sodium hydride, to produce a perfluorinated
tertiary
aIkoxide which can then be alkylated by reaction with aJlkylating agent.
Suitable alkylating agents for use in the preparation include dialkyl sulfates
{e.g.,
dimethyl sulfate), alkyl halides (e.g., methyl iodide), alkyl p-
toluenesulfonates {e.g.,
methyl p-toluenesulfonate), alkyl perfluoroalkanesulfonates (e.g.; methyl
SUBSTITUTE SNE1_ T ~R ULE 16J
CA 02356081 2005-12-02
perfluoromethanesulfonate), and the like. Suitable polar, aprotic solvents
include
acyclic ethers such as diethyl ether, ethylene glycol dimethyl ether, and
diethylene
glycol dimethyl ether; carboxylic acid esters such as methyl formate, ethyl
formate,
methyl acetate, diethyl carbonate, propylene carbonate, and ethylene
carbonate; alkyl
5 nittiles such as acetonitrile; alkyl amides such as N,N-dimethylformamide,
N,N-diethylformamide, and N-methylpyrrolidone; alkyl sulfoxides such as
dimethyl
sulfoxide; alkyl sulfones such as dimethylsulfone, tetramethylene sulfone, and
other
sulfolanes; oxazolidones such as N-methyl-2-oxazolidone; and mixtures thereof.
As yet another alternative, the fluorinated ethers may be prepared by reacting
a
10 fluorinated carbonyl compound, such as a ketone or acid fluoride, with an
alkylating
agent in the presence of a Lewis acid catalyst as described in U.S. Patent No.
6,046,368
filed 3/17/98 (Iamanna et al.). '
Other useful hydiofluoroethers are the omega-hydrofluoroalkyl ethers described
in
U.S. Patent No. 5,658,962 (Moore et aL), which can be described by the general
15 structure shown in Formula III:
X-Rf'-(O-R~ ~y-O-R"-H (Formula III)
wherein:
X is either F or H;
Rf' is a divalent perfluorinated organic radical having from 1 to about 12
carbon atoms;
Rf' is a divalent perfluorinated organic radical having from 1 to about 6
carbon atoms;
R" is a divalent organic radical having from 1 to 6 carbon atoms, and
preferably, R" is perfluorinated; and
y is an integer from 0 to 4.
Representative compounds described by Formula III which are suitable for use
in the
processes of the invention include the following compounds:
CA 02356081 2001-05-17
WO 00/36272 PCT/CA99/01180
16
CgF,~OCF20(CF2)SH HCF2CF2~OCFzC(CF3)2CF20C2F4H
C:3F~0[C(CF3)CFZOJpCFHCF3, whereinHC;F20(C2F40)"(CFzO)mCF2H,
p wherein rn = 0 to 2 and n =
= 0 to 5 0 to 3
CgFI~OCZF40C2F40C2F40CF~H C~FISOCF'HCF3
C4F9OCzF4H HC3F60C.3F6H
HC3F60CH3 CSF, I OC2lF4H
C6F,30CFZH C6FI3OCZJ"4OC2FaH
c-C6FIICF20CFZH C3F70CH;~F and
C4F90CF2C(CF3)ZCFzH
The omega-hydrofluoroalkyl ethers described by Fornnula III can be prepared by
decarboxylation of the corresponding precursor fluoroall';yl ether carboxylic
acids and
salts thereof or, preferably, the saponifable alkyl esters thereof, as
described in U.S.
Patent No. 5,658,962.
Alternatively, the omega-hydrofluoroalkyl ethers can be prepared by reduction
of the
corresponding omega-chlorofluoroalkyl ethers (for example, those omega-
chlorofluoroalkyl ethers described in U.S. 5,785,950 and U.S. 5,403,575 (Flynn
et
al.); which is also described in U.S. Patent No. 5,658,962.
I O The fluorochemical stabilizer should be soluble in the liquid or
supercritical COZ from
at least 0.01 weight percent to completely miscible. Preferably the
fluorochemical
stabilizer should be soluble in the liquid or supercritical C:02 from at least
0.05 weight
percent. The solubility of the stabilizer in COZ may be determined by charging
a
pressure vessel having a sight glass with liquid or supercritical COZ, and
adding a
I S known amount of stabilizer and known amount of carbon dioxide. Generally,
the
fluorinated stabilizer of the present invention produce clear solutions (or
microemulsions) and no interface between separate phasea is observed. Less
soluble
materials will form a hazy solution or two separate phases will develop, and
an
interface between phases may be observed.
20 The fluorochemical stabilizer is generally used at concentrations from
about 0.01
volume percent up to about I 0 volume percent. Preferably, -the stabilizer is
used at
SUBSTITUTE SHEET' (R ULE ?6)
21-02-2001 CA 02356081 2001-05-17 CA 00990118(
. . 1?
. concen bans from about O.OZ volume percent up to about 5 volume ercent.
p For
most applications due to cost vansiderations, the stabilizers -are used in the
hum
amounts necessary to pm,duce a stable fracturing Quid.
The preferred foam ~aisubstaaccs.are hydr~~fluorethors, such as 3M HF$-7100
tnethoxy nonafluoxobutane, or 3M HFEf7200 etboxy.aonafluorvbutane.
3M I~'E-?100 (C4Fg(~CH3) consists of two inseparable isomers with essentially
identical properties. These are (CF3~CFCgZpCy~3 and CF3CFZCF~CFZOCH3.
3M HF&7200 (CaF9~CaHs)corisists of two inseparable isomers with essentially
identical properties, These axe (CF3)CFCFZpCaHs ;end CF3CF2CFZCF2QCZH$.
Each of these hydrofluomethers ig soluble in Cpa~,~, but neither has
heretofore been
used in a well fracturirrlg fluid as a foam foraniag substance in CQZn>,
Efficacy of these hydrofluorethers as foamers r~aay be demonstrated by the
fc~tlowing
examples:
Example 1
I3 Ethoxy-nanofluorob~xtaa~e is added to a pressure cell, containing Cpz~a at
2°C, $40 psi
(5520 kPa), at a rate of 20LIM3 COs. A clear, onf; phase liquid results.
Additiane~I
CQzty is added, with no change, Pressure'is increased to 1300 psi (8960 spa)
~ylth no
change. NZig> bubblerd at 1740 psi {12000 lcPa) into the Cp,~ solution,
resulting in two
pies, v'i~ $ fuzzy interface The cell is then shakem fax five seconds,
resulting in a
2Q single foatM phase, Additional ZV'a will mix into the foam. Ifthe foam is
permitted to
rise in temperature, a n'~inimnal gas phase is noted.
'The conclusion fmm Example I is #hat ethoxy-nono~fluorobutarte is soluble itt
Cp~~~
under the conditions stated, and functions as an effectiive foamer for NZ.
E~2
25 Following the procedure of Example 1, rOLIM~ methoxy nonafluombutane were
added to C~«~ at -3°C, 915 psi (6310 kPa). A clear one phase liquid
resulted.
Addition of N2 at 1475 psi (10200 kPa) arid agitation resulted in a stable
foam, which
AMENDED SHEET
a 21-02-2001 - CA 02356081 2001-05-17 CA 009901180
18
remain stable upon chilling to -21 °C, and pre;sears adjustment to 1090
kpa). p~ t?520
It was concluded that metboxy nanafiuombutaine is soIubie in Cpzc her, ~$
conditions stated and gbactions as an effective foa~mer for Nz.
Examdle 3
SL~M~ methoxy fluorobutane was added at O°~C ?00 psi (4830 kPa) to
CO
zcn~
resulting irt a clear one phase liquid. pressuxe was grsd~ly reduced to 300
p$i (20?0
kPa), and the solution chilled to -24° C, h1o c~~ge in the single phase
dqwd was
rioted. 1V2~~ (?0%) was bubbled through the Iicluiid rapidly, raising the
pressure to
1100 pai ('75$0 kPa), and causing a cloudy foam. Pressure and temperature
increases
caused the foam to change to a clear one phase liquid.
The conclusion of Lxarnple 3 ~ that at SLfM3 a satisfactory foam was created
with
methoxy fluoz~c~butane.
Exatmple 4
I5 In Example 4, the ss,me process and Example 3 eras carried out, but with
SL/M3
ethoxy fluorobutane. The same results were obtained, ~d ~e she conclusion
reached.
In additional tests, the procedures of Examples 3 ,and ~~ urere followed, with
2 LIh~l3 of
each foamer, and 1 IJM3 was the minimum quantity of foatner required to give
satisf~toty results.
It was found, moreover, iz~ alI satisfactflry.foarns, that a mi ~imum volume
of 52% N
Z
was required, A minimum COa content of 5% is required. Thus COZIN~ volurnetzic
ratios at temperature acid pressure (Mitcheh QualitY~ are in the range of from
48:,5 to
5:95.
Utilising the NZ foamed GO2 of the present invention it .has been found that
fair
hr high
viscosity foams, with high prdpp~t loariia8 characteristics are created.
AMENDED SHEET
CA 02356081 2005-12-02
19
It will be appreciated, therefore, that a significant increase in viscosity is
obtained
with the present invention. This, in turn results in higher proppant loading
characteristics and improved fracturing capability.
SPE paper 40016 co-authored by one of the present inventors, Satyanarayana
Gupta,
~ shows that the viscosity of liquid C02 at pressures
and temperatures where it exists in liquid phase is between 0.02 to 0.16 cP,
depending
on temperature and pressure. The viscosity of a foam of nitrogen gas in liquid
C02 is
expected to be between 50 and 200 cP, depending on the quality of the foam
(volume
fraction of the nitrogen) based on SPE 18214, titled "Rheological and Physical
Differences Between Foam and Fracturing Fluids" by R.E. Blauer and A.M.
Phillips
and D.P. Craig. The viscosity of a foamed C02~~~/N2~~~/28/72 foam according to
the
present invention, will have a viscosity of 60 cP at 20°C.
In the fracturing method of the present invention, the proppant is cooled to a
low
temperature with liquid COz in a pressurized blender such as described in
Canadian
I S Patent No. 1,134,258. The system is pressurized with compressed nitrogen
to
maintain the C02 in the liquid state. The cooled proppant is added (metered)
to' a
stream of liquid C02 in the primary line under pressure. The surfactant (foam
stabilizer) is added to this stream. A proper amount of nitrogen is added
under
pressure to yield the required Mitchell Foam quality. The volumetric ratio of
COZ:N2
is based on the downhole temperature and pressure and care must be taken to
calculate the expansion factors of both liquid CO~ and nitrogen. The resulting
fracturing fluid is pumped using high pressure pumps into the formation to
create
fractures for stimulation.
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 skilled in the field of petroleum engineering without any
departure from
the spirit of the invention. The appended claims, properly construed, form the
only
limitation upon the scope of the invention.
