Note: Descriptions are shown in the official language in which they were submitted.
1070492
The invention relates to an oil recovery process in which an aqueous
solution which contains enough water-soluble anionic polysaccharide polymer
to reduce its mobility is injected into a subterranean reservoir to displace
fluid toward a production well or production wells.
The chemical composition of water-soluble anionic poly-saccharide
polymers (or Xanthan gums) which are effective as water-thickening agents is
such that the polymers are susceptible to chemical degradation or depolymeriz-
ation. The degradation, which tends to increase with increasing temperature,
reduces the viscosity of a solution containing the polymers. Two paths by
which such a degradation can occur comprise hydrolysis and free-radical reac-
tions. The hydrolysis involves the reaction of molecules of water with the
ether-type lin~ages in the structure of the polysaccharide polymer. The free-
radical reactions are usually those initiated when the polymer solution is
mixed with air or oxygen. Such a mixture tends to form hydroperoxides and the
decomposition of the hydroperoxides produces reaction-initiating free-radicals
that propagate polymer-degrading radical reactions.
Numerous types of materials and techniques for treating aqueous
solutions to remove dissolved oxygen are known to those skilled in the art.
In general, such treatments are based on or completed by dissolving a strong
reducing agent or oxygen scavenger in the solution. In a solution in which
a radical degradable polymer is present, a combination of oxygen and oxygen
scavenger creates an oxidation reduction couple or redox system Since free-
radical reaction initiating radicals are intermediate products of the reaction
between the oxygen scavenger and the oxygen, the reactions are such that, al-
though the system will remove the oxygen, it will also degrade the polymer.
Ob3ect of the invention is a method of stabilizing the viscosity of
an aqueous solution of polysaccharide polymer which does not show the a~ove-
mentioned d~sadYantage.
According to the present invention there is provided oil recovery
process comprising the steps of:
treating an aqueous liquid solution to remove substantially all
1~70492
dissolved oxygen;
a~ding to the deoxygenated solution at least one water-soluble sul-
phur-containing antioxidant;
adding to the deoxygenated solution at least one water-soluble
readily oxidizable alcohol or glycol;
adding to the deoxygenated solution at least one water-soluble
anionic polysaccharide polymer; and
injecting the polymer-thickened aqueous solution into an oil-con-
taining subsurface reservoir via at least one in~ection well for displacing
fluids in the reservoir to at least one production well.
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10~70492
The aqueous li~uid used in the present procesS can be substantially
, any fresh or saline water but is preferably a relatively soft and not
! extensively saline water. Such a water preferably has a total dissolved
salt content of not more than about 5,000 ppm and a hardness (in terms of
parts per million of calcium ions) of not more than about 500 ppm. When
deoxygenated for use in the present process, such a water is preferably
substantially completely free of dissolved oxygen and contains from about
10 to 100 parts per million S03-group-containing oxygen sca~enger (in
terms of S03-group equivalent~. The aqueous liquid may be treated to
remove substantially all dissolved oxygen therefrom by adding to it a
reducing agent or an oxygen scavenger.
Water-soluble inorganic compounds that contain or form ions that
contain an S03-group are particularly suitable for use in the present
process as reducing agents or oxygen scavengers. Such compounds include
water-soluble al~ali metal sulphites, bisulphites, dithionites, etc. As
known to those skilled in the art, such a reducing agent or oxygen scavenger
is preferably used in a slight stoichiometric excess (relative to the amount
needed to remove substantially all of the dissolved oxygen in the solution
being treated). Such an excess is preferably from about 10 to 500% more
than stoichiometric.
The sulphur-containing antioxidant used in the present process can
comprise substantially any such water-soluble antioxidant composition
~transfer agent, terminator, peroxide decomposer) which is effective with
respect to decomposing peroxides in a~ueous solutions. Examples of such
compounds include relati~ely water-soluble mercaptans, thioethers, thio-
carbinols, and the li~e. Particularly suita~le examples are thiourea,
thiodiacetic acid (thiodiglycolic acid), 3,31-thiodiacetic acid ~dithio-
diglycolic acid) and their water-soluble homologues.
Suitable readily oxidizable alcohols or glycols for use in the
present process include substantially any water-soluble primary and
secondary alcohols or glycols that are easily oxidized. Examples of such
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7049Z
compounds include methanol, ethanol, alkyl alcohol, isopropyl alcohol,
isobutyl alcohol, ethylene glycol, and the like.
As indicated in laboratory tests, in a deoxygenated aqueous solution
in which an oxygen scavenger is present, a significant beneficial
synergistic antioxidant effect is exhibited where a readily oxidizable
water-soluble alcohol or glycol is added to the sulphur-containing
antioxidant.
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1070492
-- 4 --
The anionic polysaccharide polymers, or Xanthan gums suitable for
use in the present process, can be substantially any such materials pro-
duced by the fermentation of carbohydrates by bacteria of the genus
Xa~thomonas. In general, the anionic polysaccharide B-1~59 is preferred.
Examples of commercially available polymers comprise the Pfizer Xanthan
Biopolymers (trademark) available for Pfizer ~hemical Company, the
General Mills Xanthan Biopolymers (trademark) available from General
Mills Company, and the Kelzan (trademark) or Xanflood (trademark)
anionic polysaccharides available ~rom Kelco Company.
The anionic polysaccharides u::ed in the present process ~and/or the
fermentation broth in which they are made) are preferably treated with
enzymes such as a proteinase to ensure the removal of (or destruction of)
bacterial cells which may impede the flow of a solution into fine pores
within subterranean earth formations. Alternatively, such clarifications
may be accomplished by or supplemented by means of centrifugation, filtra-
tion, and the li~e.
As known to those skilled in the art, in an oil recovery process
in which fluids are displaced within a subterranean reservoir by injecting
~ viscosity enhanced aqueous solution, the effective viscosity (or recip-
rocal mobility within the reservoir) should be at least substantially
equal to and preferably greater than that of the fluid to be displaced.
In the present process, the concentration of anionic polysaccharide
in such a solution should be in the order of about 100 to 2,000 parts by
weight of pol~acchariae per million parts by weight of aqueous liquid.
Such concentrations gener~lly provide viscosities in the order of from
about 2 to 50 centipoises at room temperature, in a water containing about
400 parts per million total solids.
In t~e present process, the concentration of antioxidant can be
relatively low, in the order of about 50 parts per million (weight per
weight of a~ueous liquid) and preferably from about 200 to 800 parts per
million. The readily oxidizable alcohol or glycol concentration can be
from about 50 to 2,000 parts per million, snd preferably from about 500
to ~,000 parts per million. In general, the concentrations of the readily
oxidizable alcohol or glycol and the polysaccharide polymer are preferably
kept at least nearly equal (e.g., at least within about 10% of each other~.
- 1070492
-- 5 --
Substantially fresh w~ter solutions containing aQ0 parts per million
~elzan polysaccharidel from about 200 to 800 parts per milli`on thiourea,
and 500 to 1,000 parts per millior isoprGpyl alcohol have retained from
about 75-90% o~ their original viscosity after eight months storage at
97C. In such storage tests, the best and most consistant results
were obtaired uhen the isopropyl alcohol and Kelzan concentrations were
about equal and the thiourea concentration was about half ~he isopro~yl
alcohol concentratîon.
Stability in the presence of air was indicated by the follouing
laboratory tests. Enzyme clarified solutionswere stored at 97C, said
solutions containing 1,000 parts per million Kelzan MF polysaccharide
polymer (available from Kelco Company~, 3,000 parts per million
sodium chloride, 1,000 parts per mill-on of each of sodium sulphite,
isopropyl alcohol, 500 parts per million thiourea, and 20 parts per
million Dowicide G (trademark) available from Dow Chemical Company.
In order to simulate the contacts with air which are l;kely
to occur (due to leaks) in a water~lood oil recovery system in the
field, 35 cc samples of the polymer solution were retained in bottles
containing 1 cc of air above the liquid. The so stored sa~ples retained
more than 80% of their initial viscosity after storage for seven
months.
The drawing shows a graph of viscosity (in cps at 7.3 sec 1,
Brcokfield) vers~s time ~ lin months) at 97C. The cur~e labe~ed
A relates to a basic test solution of 80o ppm o~ each of Kelzan
MF polysaccharide polymer, sodium chloride and sodium sulphite in
distilled water; which also contained ~00 ppm thiourea and 80~ ppm
isopropyl alcohol (IPA~. The curves labe~ed B, C and D relate to
solutions in which the compositions were the same except for the
ommission~of IPA in curve B; thiou~ea in curve C; and IPA and
thiourea in curYe D.
The unobviously beneficial results pro~ided by the combination
of the sulphur-containing antioxidant and the readily oxidizable
alcohol are indicated by the retention of a Yiscosity of substantially
15 cps throughout the six-month test period.
1070492
(~
In prepsring an aqueous solution in accordance with the
present process, the water should be deoxygenated before the
other components are added. This avoids any chance that the anionic
polysacchi~ride polymer, oxygen, and oxygen scavenger will be
mixed together within the solution. The antioxidant (and any
antibacterial agent, or the like, to be used) can be added
before, with, or after the anionic polysaccharide polymer.
If the antioxidants are added before or simultaneously with the
oxygen scavenger they may be comsumed in the reactions that ensue.
Antibacterial agents suitable for use in the present process
can comprise sodium ss-ts of tri- and pentachlorophenols, form-
aldehyde, aliphatic diamine salts and alkyldimethyl-benzylsmmonium
chlorides.
