Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
This invention relates to a method for treating
aqueous solutions of partially hydrolyzed polyacrylamides
prior to their use in enhanced oil recovery operations.
DESCRIPTION OF THE PRIOR ART
Among the more widely practiced methods for the
recovery of oil from an oil-bearing reservoir is water-
flooding. In this method, flood water is injected into the
reservoir via one or more injection wells, which water
displaces the oil in the formation toward one or more
production wells. More recently, improvements in water-
flooding methods have included the use of water-soluble
polymers whereby the viscosity of the flood water is
increased. The "thickened" water results in a more
favorable mobility ratio and leads to improved oil recovery.
Only materials with very high molecular weights of
one to ten million and rod-shaped molecules are suitable for
this purpose, and only such materials are effective in
increasing the viscosity of the water in the desired manner
at very small concentrations of less than 1 kg/m3. Two types
20 of such polymers that have been found suitable as thickeners
are the polyacrylamides, which may be partially hydrolyzed,
and polysaccharides which are mainly produced by means of
the bacteria Xanthomonas Campestris.
The polyacrylamides are long chain polymers of the
acrylamide with the general formula:
ECH2-CH ~CONH2~ ]n
with n = about 50,000 or more. The molecular weight is
3 to 6 million. With partially hydrolyzed polyacrylamides, a
part of the amide group - CONH2 ~ is converted into the
carboxylate group - COONa - by saponification reaction.
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Polyacrylamides that are hydrolyzed to 10 - 60 percent or
preferably 20 - 35 percent are especially well-suited for
polymer flooding.
The molecules of partially hydrolyzed polyacryl-
amides possess their elongated rod-like shape only in
practically non-conductive, i.e. salt-free water, because
of the repulsive energy of the negatively loaded carboxyl
group and behave like very long stretched elastic fibers.
The length is about 10,000 times the thickness and is about
10~ m.
Solutions of partially hydrolyzed polyacrylamides
in fresh water are considerably more viscous than solutions
of polysaccharides at the same concentration. Apart from
this, the price per kilo is only half as much and polymer
solutions of suitable viscosity with partially hydrolyzed
polyacrylamides can be produced at a quarter of the cost
per cubic meter ~m3~ of flooding liquid compared to that of
the polysaccharide. In reservoirs that contain salt water,
a polyacrylamide solution using fresh water can be
favorably applied after preconditioning the reservoir
by flooding with fresh water. Other advantages of the
partially hydrolyzed polyacrylamide include, better stability
against bacteria and against high temperatures of up to
100C compared to 70C for a polysaccharide.
Solutions of such high molecular materials are not
genuine liquids in the rheological sense, The viscosity
depends on the velocity gradient. The flow characteristics
of this pseudo plastic solution follows the power law:
~ = shear rate ~s 1~
~ ~ ~ = shear strength (dyn~cm2)
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3~
n and ~ are constants, ~ is the apparent viscosity at a
shear rate of 1 s 1, The exponent, n, is smaller than 1.
For genuine liquids n = 1.
Pseudo plastic liquids are less suitable than
genuine liquids of constant viscosity for displacing oil
from reservoirs with heterogeneous permeability which is
the case for more or less all natural reservoirs. The
Darcy law generalized to incorporate pseudo plastic liquids
is:
lo 2) ~ 4n . 1 ~ ~ ( p )/n
3~+1 71 ~/n ~ ~
= average velocity
= permeability (cm2)
= porosity
~/~ = pressure gradient
The flow velocity depends, therefore, more strongly
on permeability than that of genuine liquids (n = 1).
It is therefore desirable to achieve constant
viscosity within the range of the velocity gradient that
occurs in an oil reservoir. The velocity gradient along
20 the walls of the pore channel is:
3) ~= ~n l 1 . nr
~n
When 1 m of polymer solution per hour, per meter of
reservoir thickness is injected into the injection well, a
radial flow yields, at a distance of 100 m, a velocity of
= 1 cm/h = 2 .8 x 10 4 cm/s. For~ = 10 8 cm2 (1 Darcy),
~ = 0.25 and n = 0.5 the shear rate becomes ~= 7 s 1. In
this case the flow curve in the shear rate range below 7 s 1
is decisive. With lower permeabilities, greater distances,
lower injection rates the area of interest is still lower,
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in other cases also higher.
In practice there occur considerable deviations
from the theoretically correct relationships shown above.
As a result of the more difficult passage of the very large
molecules through the narrow parts of the pore channels the
pressure losses are greater than predicted by theory. At
low flow velocities they are a multiple of 1.2 to 1.5
with polysaccharide solutions and a multiple of 2 to 5 with
hydrolyzed polyacrylamide solutions. The pore cross section
is reduced by adsorption of polymer molecules on the surface
of the rock. Therefore, during two phase flow of oil and
water or oil and polymer solution only the effective
permeability for water should be applied. The lower the
absolute permeability, the stronger is the permeability
reduction by polymer adsorption.
The usual form of commercial hydrolyzed polyacryl-
amides is a fine grained, solid product. Even though it is
a water-soluble polymer it is practically impossible to
produce a completely molecular dispersed solution. In water,
the polymer grains swell and tend to form lumps which require
long periods of stirring or special dissolution methods
before an apparently homogeneous solution is obtained. Even
optically clear solutions contain micro gel particles or
aggregates of 10 or 30 molecules which cling to the narrow
parts of the pore channeI and lead to a partial blockage.
Multivalent cations ~Fe, Ca, Mg~ especially, can produce
larger particles or even lead to coagulation through the
cross linking of several molecules C'Mobility of Polymer
Solutions in Porous Media", SPE 3683, I. Ershachi and0 L. L. Handy, Los Angeles, November 1971).
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111;3Z~3~
Even though the removal of the undesirable micro
particles by means of filtration through a packing of very
fine grained material such as diatomacious earth or silica
flour is possible, it is only suited for laboratory
quantities, not for field applications. The following method
for the production of aqueous polysaccharide solutions from
polysaccharide is known ~"Improved Injectability of
Biopolymer Solution" SPE 5099, D. Lipton, Denver, April 1976).
The fine grained polymer trickles into a blender through a
10 water spray whereby practically every grain is individually ~-
wetted. The highly concentrated solution produced (stock
solution), e.g. 1 percent, in fresh water is stirred
vigorously for several minutes. The solution is subsequently
homogenized in a simple manner, whereby it is pumped through
perforated plates under high pressure, e.g. with a pressure
loss of 10 bar per plate. In order to destroy the insoluble
remnants of the bacteria, an enzyme treatment is necessary
which lasts for several hours at an increased temperature
of 50C. After this treatment has been completed, the stock
2a solution can be diluted with brine, process water or
reservoir water available. The injectability in reservoirs
with low permeability is not really good in spite of the
extensive treatment, as the solution still contains
aggregates from a large number of molecules which cling to
the narrow parts of the pore channels and cause a partial
blockage.
The production of solutions from hydrolyzed
polyacrylamides, which is in fine grained form, also takes
place with the aid of a blender. As the polyacrylamides areQ very sensitive, they are not forced through a perforated plate.
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An enzyme treatment is not necessary. A great number ofcommercially available polyacrylamides are either too slowly
or incompletely soluble as a result of drying and/or grinding
procedure during manufacturing of the product. Some
manufacturers therefore supply the product as a concentrated
aqueous gel which is a suspension of fine globules in
mineral oil. Such liquid polymers are therefore pre-swollen
and dissolve much quicker in water. But even with these
products, an efficient injectability is not assured. The
molecule aggregates that are formed from the polyacrylamides,
and partly also from cross-linking by bivalent cations, are
trapped in the pore space. In order to obtain the best
possible molecular dispersed solution the manufacturers
recommend that a stock solution, e.g. 1 percent, be mixed
and then be diluted to the required concentration after
24 hours. It is known that solutions of partially hydrolyzed
polyacrylamide of high molecular weight are subject to a
viscosity reduction caused by shearing when a high shear rate
is applied. The longest molecules rupture as a result of
the high tensile strain. In using these solutions, it has
been recommended that such solutions not be pumped through
narrow jets, and that only stirring apparatus with a low
revolution ~ate should be used, i.e. centrifugal pumps
should not be used. Also the injection rate in flood wells
should ~e kept within limits in order to keep the loss of
viscosity as low as possible during entry of the polymer
solution into the formation. ~See: "The Oil & Gas Journal",
July 12, 1976, p. 541. In another publication, it is
pointed out that hydrolyzed polyacrylamide is
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disadvantageously influenced by shearing. (See "Fundamentalsof Tertiary Oil Recovery", Petroleum Engineer, July 1976,
E. F. Herbeck et al, p. 48-59, 54).
It is an object of the present invention to overcome
the problems associated with the pretreatment of polyacrylamide
solutions prior to their employment in oil recovery operations
by subjecting the polyacrylamide solutions to a controlled,
limited shearing treatment.
The Figure shows the result of the change in shear
rate on the apparent viscosity of polyacrylamide solutions.
According to the invention there is provided a process
for the treatment of aqueous solutions of partially hydrolyzed
polyacrylamides prior to the employment of said solutions in
enhanced oil recovery operations wherein said solutions contain
said polyacrylamide at a concentration less than 1 kg/m3 and
said solutions are sheared at shear rates in the range of about
20,000 to about 50,000 s 1.
In a preferred embodiment said shearing is achieved
by forcing said aqueous polyacrylamide solutions through
perforated plates having holes 1 to 4 mm diameter and said shear
rate is selected so as to maintain a pressure loss of between
1 and 3 bar per perforated plate.
Thus there is provided a method for the pretreatment
of polyacrylamide solutions by a controlled, limited shearing
action wherein solutions of partially hydrolyzed polyacrylamides
are sheared at a shear gradient of 20,000 to 50,000 s 1
corresponding to a shearing stress of 500 to 2,000 dynes/cm2.
By the treatment thus described it has been found that solutions
of partially hydrolyzed polyacrylamides that are to be used in
~ ,
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enhanced recovery operations were improved and trouble-free
injectability was achieved.
The improvement has been demonstrated in the
following laboratory tests. A core from a reservoir, or a
sandpack with comparable pore space is flooded at a constant
rate with the polymer solution. With the sheared solution,
; in accordance with the invention, the flow pressure, except
for the first few minutes, (a certain increase in pressure
is produced due to adsorption) remains constant even after
.
-
,
~ .
1~,
,
;~ 20
- 7(a) -
'
,
ll~Z~3~
the throughput of large volumes. With unsheared solutions
good injecta~ility was achieved only after filtration
through fine pored filter media or after a very long
stirring period.
Referring to the accompanying Figure, it can be
seen that the flow curve of the sheared solution of partially
hydrolyzed polyacrylamide is more suited for polymer
flooding, as with small velocity gradients, such as prevail
in the reservoir, practically constant viscosity is
produced ~curve 2~, while the unsheared solution (curve 1)
is strongly pseudo plastic. This means that the apparent
viscosity at decreasing shear rates increases to values
which are too high. A sheared solution at a given pressure
gradient is forced more quickly into the reservoir and
sweeps a larger part of its lower permeability region. This
leads to a higher degree of oil recovery than with an
unsheared solution.
Further, according to the method of invention, the
injection pressure is considerably reduced by the shearing
of the solution of the partially hydrolyzed polyacrylamide.
During flooding through a sandpack with a grain size
of 60 - 90Jwm and at a shear rate of 100 s 1 the injection
pressure for an unsheared solution of partially hydrolyzed
polyacrylamide was Q.38 bar. If the same solution is
sheared ~n accordance with the invention the injection
pressure is reduced to Q.16 bar. The apparent viscosities
at a shear rate of lOQ s shows practically no difference
~7.5 and 7,3 cPl, This effect of the shear treatment on
the injection pressure is of special importance for0 reservoirs with relatively low permeabilities. As the
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injection pressure must not exceed the fracture pressureof the rock, the sheared solution of partially hydrolyzed
polyacrylamide, - according to the invention, - allows
higher injection rates to be realized and the duration of
the flooding project to be reduced.
The shearing equipment should be selected in
such a way as to permit the determination of the shear rate.
Stirrers or whirling sections built into pipelines are
unsuitable. During pumping through jets, perforated plates
or slits, the velocity gradient and the shearing stress are
not constant, but do not exceed in the entire flow region a
maximum that depends on flow rate or on pressure loss. As
during a single passage through a jet not all particles
are subjected to the same shearing stress, a multiple
throughput is recommended in order to destroy all particles
above the critical molecular size.
The pressure loss ~ in jets at high rates of flow
is in accordance with the Bernoulli law:
_
4) ~ _ ~ . C )
p = Density, g/cm3
~V = Average velocity, cm/s
~ = Pressure, dyne/cm2
C is a ~et factor, which, however, is also dependent on the
viscosity at high viscosities. In perforated plates of 2 mm
thickness with holes from 1 to 3 mm diameter C is 0.85 for
water, 0.65 for an aqueous solution of 0,5 g~l of partially
hydrolyzed polyacrylamide and 0.55 for a water solution of
5 g/l.
_g_
~lV2~3~
In practice the use of perforated plates proved
successful for the homogenization of the stock solution and
also of the diluted solution. Best results were obtained with
4 perforated plates with a pressure loss of 1.5 to 2 bar.
Improved in;ectability was attained as compared to using
homogenization equipment such as whirling sections, static
mixers or colloid mills and is also simpler and cheaper.
The concentration of the stock solution is chosen
as the highest possible multiple ~5 to 20) of the diluted
solution. Concentrations between 100 and 1000 ppm of
polyacrylamide are suitable for flooding. Thus, the range of
the concentration of the stock solution lies between 2 and
lQ g/l ~6 - 30 g/l liquid polymer) as long as the viscosity
of the stock solution is not too high which depends princi-
pally on the molecular weight of the polymer and the salinity
of the water.
As an example, for the continuous production of
injectable polymer solutions of about 30 percent partially
hydrolyzed polyacrylamide produced from the commercial liquid
polymer NALCO Q 41 F ~a product of NALCO Co.), an apparatus
was constructed that is capable of a throughput of 5 m3
polymer solution per hour and concentrations of 1.5 g/l
liquid polymer = 0.5 g/l polymer in fresh water ~based on
dry polymerl.
The liquid polymer is an emulsion of the water-in-
oil type. To ensure that the geI pellets are wetted as
quickly as possible and can hydrate during the injection of
the emulsion into the water an activator must be added.
This process takes place more quickly and completely when the
3Q activator concentration is higher. Therefore, a stock
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solution with a high concentration ~5 g/l polymer) is first
produced, which is diluted with water to the required end
concentration ~0.5 g/l polymer) only after it is inverted.
The throughput rate in the bypass of the aforementioned
apparatus has been set at 500 liters per hour.
On the suction side of the transportation centrifugal
pump the activator and a biocide is added, followed by the
dosage of liquid polymer. Three perforated plates each with
3 holes of 2.5 mm diameter support the homogenizing effect
of the centrifugal pump; a destruction of the molecule does
not take place at this stage, as the gel pellets have not
yet dissolved in the water. This takes place after the
solution has been diluted and requires a certain amount of
time. One perforated plate with 18 holes of 3.0 mm diameter
which is situated just behind the stock solution inlet point
into the main flowline primarily serves for the homogeniza-
tion of the solution. The actual shearing treatment takes
place after dilution and after this diluted polymer solution
has flowed through the field flowline to the injection well
where it has had about 15 minutes time to hydrate. This
shearing treatment is performed with 2 perforated plates
with 19 holes each of 2.5 mm diameter, each with a pressure
loss of 1.8 bar, and a perforated plate with 22 holes of
2.5 mm diameter with a pressure loss of 1.4 bar. A
trouble-free injectability was achieved and the flow
characteristics of the polymer solution were as desired.
