Note: Descriptions are shown in the official language in which they were submitted.
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~AC~GROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to a process for reducing
the influx of water into production wells by the injection
thereinto of emulsions of gels of polyacrylamides or
partially hydrolyzed polyacrylamides in mineral oil.
DESCRIPTION OF THE PRIOR ART
As the oil fields become older their flooding
increases constantly. Therefore, more than ever, a search
goes on in the petroleum industry for processes for an
effective reduction of the influx of water into production
wells. In many fields, particularly when the permeability
is high, the lighter petroleum is separated off from and above
the water-bearing layer. In such cases, by means of the
introduction of cement into the bottom portion of the well,
there is achieved a reduction of the influx of water into
the well. Also, the injection of waterglass solutions, with
which an electrolyte such as hydrochloric acid or ammonium
chloride has been mixed, so that the thinly-flowing solution
can stiffen into a stiff gel after a specific incubation
period, has been proposed. With this method, however,
there is a certain risk that the oil-bearing layer is also
sealed off, even if both layers are separated each from the
other by a packer placed at the dividing line and oil is
injected into the upper layer. Such a sealing is practically
impossible to avoid.
; For some years it has been recommended to use
water shut-off processes with solutions of polyacrylamides
or partly hydrolyzed polyacrylamides, in ~arious alternatives,
and they have in fact been used on several occasions.
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Polyacrylamides are long-chain polymers of
acrylamide corresponding to the general formula:
( ~ C~2 ~ CH (CoNH2) ~ )n
wherein n is about 50,000 or more. The molecular weight is
three to six million. With partly hydrolyzed polyacrylamides
a part of the amide groups - CONH2 is converted by a
saponification reaction into carboxylate groups - e.g. COONa.
Particularly advantageous are polyacrylamides that are
hydrolyzed to 10 - 60%, preferably in the range of 20 - 35%.
Aqueous solutions of polyacrylamides or partly hydrolyzed
polyacrylamides, starting from a concentration of about 5%,
behave as gels, i.e. they become plastic and possess a flow
limit that increases with concentration.
There exists a possibility of injecting poly-
acrylamide solutions in water or also salt water over the
entire face of the deposit. Thereby, the very large
molecules in the water-bearing pore area should be adsorbed
on the matrix, the pores become narrower as a result and thus
they impede the flow of water, while the oil-bearing pore
area is little affected. Unfortunately, it has not been
possible to verify this effect either in tests or in practice.
A modification of this method is represented by the fact
of cross-linking polyacrylamide solutions in the deposits
and thus obtain immobile gels. Either a solution of non-
hydrolyzed polyacrylamides suffers the admixture of a cross-
linking agent (e.g. aluminum salt), or solutions of partly
hydrolyzed polyacrylamides, the pH value of which is adjusted
to 3-4 to produce a minimum viscosity, are alternately
injected with solutions of the cross-linking agent.
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This method is extremely costly and has not proved effective
with sandstone deposits. F'or example, in field treatment,
it has bee~ ascertained that out of six wells treated only
in one was a partial success recorded, which possibly could
also be attributed to the bore hole clearance effected
before treatment. In laboratory tests it could be demon-
strated that the intended gel formation, or the adherence to
sandstone, does not take place, so that the flow conditions
in existence before the treatment are affected only for a
short time by the treatment.
SUMMARY OF THE INVENTION
This invention relates to a process for reducing
the influx of water from a subterranean petroleum-bearing
porous matrix formation into a producing well, that has
undergone a preliminary pumping of clean oil, comprising the
injection into the said porous matrix formation via said
producing well of a slug of an emulsion of gel globules of a
polyacrylamide polymer in mineral oil, said globules having
a maximum diameter of less than half of the pore width of
said porous matrix, wherein said slug is injected in amounts
sufficient to penetrate from 3 to 10 meters into said
formation adjacent said producing well.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1. Top view of plexiglass test model.
Figure 2. View of steel pipe test model.
DETAILED DESCRIPTION OF THE INVENTION
The object of the present invention is to produce
a process whereby the flow of water into production wells
can be reduced for a long period, without impending the pro-
duction of oil therefrom.
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It has IlOW been found that it is possible to usepolyacrylamide liquid polymers, as available in the trade,
for inhibiting the flow of water into production wells.
These polymer solutions are emulsions of highly concentrated
gels of polyacrylamides in the form of little balls in
mineral oil. The diameter of the balls should be 0.5 to
5.0~m. Preferred are emulsions of the gel globules which
have a diameter smaller than 2 ~ m. The concentration of the
emulsion in dry polyacrylamide is 25 to 35%, in oil 33 to
50%, and in water 25 to 35%. It is possible to dilute as
desired these liquid polymers with mineral oil, e.g. diesel
fuel, without the gel becoming modified; they simply sediment.
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If the emulsion in water or salt water is introduced while
stirring, the gel particles are cross-linked with water,
they swell, they partly agglomerate and after a relatively
long time, a few hours, or several days, depending upon the
temperature and the intensity of stirring, they go into
solution. As polyacrylamide solutions, because of their high
molecular weight - several million - already in a small
concentration, increase the viscosity of the water or salt
water considerably, it is possible to dilute the liquid
polymers to a very high multiple with mineral oil for
effective water inhibiting processes. Thus, the vis¢osity
of water of small hardness (3.5 dh) is already considerably
increased by 0.25 kg partly hydrolyzed polyacrylamide
per m3, particularly in the case of a small speed gradient;
highly-saline deposit water requires at any rate about 2 kg
polyacrylamide per cubic meter, the hydrolysis degree having
little effect in this connection. These polyacrylamide
emulsions thus remain unaltered on mixing with oil, i.e. with
a low degree of viscosity, but they become highly vlscous to
gel-like on mixing with water. Their colloid fine particles
are injectable into the pore spaces, as their diameter is
smaller than the pore diameters in the deposit. On cross-
linking with water or salt water they swell and already in
small concentration they considerably increase the viscosity
of the water.
The concentration of the solution of liquid polymer
to be injected into a particular deposit, as well as the
appropriate volume of the solution to be injected will be
determined empirically by means of tests on deposit cores
or comparable sand packs with deposit oil and water, as
well as at deposit temperature.
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The following characteristics must be made on the
liquid polymer: the emulsion must be emulsified finely enough,
the polyacrylamide being as far as possible high-molecular
and preferably partly hydrolyzed, as anionic polymers are
better absorbed on the matrix.
The tests described below illustrate the effective-
ness of the process according to the invention.
In the first test use was made of a deposit model
of plexiglass filled with sand, the top view and dimensions
of which are shown in Figure 1. The thickness of the sand
pack was 2 cm. The grain size of the sand was 0 to 0.15 mm,
the porosity of the pack was 48%, the permeability
approximately 6~m2 (Darcy), the average pore diameter
approximately 4 Jum. The pack was first filled with deposit
water with a salt content of 165 g/l. Thereafter the model
was flooded with a mineral oil of 15 mPa.s viscosity to
simulate an original deposit condition. The pore space
of 1760 ml was to the extent of 1388 ml filled with oil (79%)
and contained 21% connate water. For comparison, in the
first instance, a floodlng test was carrled out wlth deposit
water without any treatment for water shut-off. Deposit
water was in~ected into well A and well M acted as a production
well. (The other wells marked in Fig. 1 were used only for
charging the model). Three times the volume of the entire
pore volume had to be injected before the liquid produced,
the wet oil, was flooded to 96%. Until then 66% of the
originally available oil was produced. Thereafter, by
flooding with oil, the original state was again produced.
The second flooding test was started in the same
0 way as the first, deposit water injected in A and oil or
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oil and ~later was drawn out of M. For a water contentof the ~iet oil of 80% flooding was interrupted and the
process according to the invention used. Then, the following
liquids were iniected one after the other into the well M,
while wells A and C lay open for outflow: 19 ml diesel fuel,
130 ml of a 6% solution of a highly viscous polyacrylamide
liquid polymer in diesel fuel, 13 ml mineral oil with
15 mPa.s. The liquid polymer was partly hydrolyzed to 30%
and had a limit viscosity of about 28 cP as well as a gel
particle size of < 2 ~m. In the test, from C there flowed
only oil, from a rather smaller quantity consisting of 94%
salt water and 6% oil.
; The next day flooding was again undertaken as at
the beginning, injection in A, production from r~. In the
test, at first, 165 ml oil were produced free of water, a
mixture of the injected diesel fuel and 15 mPa.s mineral oil.
Then, flooding increased relatively quickly to about 75%.
To allow more time for the swelling out of the gel particles -
the test was conducted at room temperature - the packing was
closed for three days. After the resumption of the flooding
the water intrusion was only 20% approximately, and did not
increase again till a substantial amount of oil had been
obtained. Up to 96% water intrusion there were injected
in the flooding phases altogether only 1.35 pore volumes of
salt water and 76% of the original oil were obtained (the oil
: remaining in the packing in the treatment being deducted).
The two facts, the oil production increased by some 15%
and the shortening of the life of a field, associated with
the substantially smaller amount of wet oil to be clarified,
mean for the field application of the process considerable
profit or savings.
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A third test for the reduction of the water inflow
was better suited tO the deposit conditions in that crude
oil instead of mineral oil refined product was used and the
operations conducted at a temperature of 58 C, corresponding
to a formation temperature at a depth of about 1500 m. A
steel pipe of 3 cm diameter and 80 cm long was filled with
said of the same type as in the first test. It was possible
to flood and also to produce into the tube from both ends
and from a hole in the middle (Fig. 2). The pore space was
first of all filled with deposit water, then with crude
oil (9 mPa.s at 58C) flooded over the entire length. Only
one side, A after M, was flooded out with deposit water up
to 99% intrusion of water. An initial state was produced in
that in A deposit water (0.7 mPa.s at 58C) and in B crude
oil at the same pressure were injected. The wet oil
produced from M became constant at 86.7% water intrusion.
The flow rates were for oil 167 ml/h bar and for water
- 1090 ml/h bar.
The treatment for the reduction of the water inflow
consisted in the injection in M of 8 ml diesel fuel, 84 ml
of a 6% solution of polyacrylamide liquid polymer as available
in the trade in diesel fuel and 31 ml of a mineral oil
(6 mPa.s at 58C). Flooding was resumed after 16 hours,
injection of deposit water in A, of crude oil in B.
After almost all the injected oil quantity was flowed free
from water out of M, the oil flow stabilized at 48 ml/h
bar, while the water inflow increased at first, but after
: a maximum of 100 ml/h bar was reached, it decreased constantly,
particularly after a further interruption of flooding at
10 ml/h bar. ~ater intrusion amounted still to only 18%,
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the f`low rate for water was less than 1% of the original.The flow rate for oil was reduced to 29% of the original.
~or oil production the water/oil flow rates ratio is decisive.
The absolute values at adequately high initial permeability
are without effect upon the daily production rates, and when
conditions are favorable an increased daily oil production
is even possible.
In further tests it was ascertained that already
with 1% solutions of polyacrylamide liquid polymer in diesel
oil substantial reductions of the water inflow are obtained.
Furthermore, instead of diesel oil it is possible to use
other hydrocarbon oils for dilution to liquid polymers, such
as heavy petroleum or light machine oil refined products and
also crude petroleum. It is necessary that the oils should
be free from water and contain no substances which would lead
to the clotting of gel globules, which could be the case
with many crude oils. Low-viscous oils are given preference,
because, as they flow through water-bearing pore spaces,
there takes place a faster thorough mixing with water, the
gel particles are cross-linked more quickly and swell faster.
The viscosity of these oils should be about 2 to 10 cP.
When choosing suitable oils the suitability of the deposit
oil plays a part.
For a good effectiveness of the process according
to the invention various parameters are of importance. The
economic optimum shall be determined empirically. A
prerequisite is that the polyacrylamide liquid polymer
should be emulsified finely enough, and the largest gel
globules should be about half as big, as a maximum, as the
pore width of the matrix rock. Furthermore, it is important
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that the ~1O~J p3ths up to entry into the matrix be free
from water, ~}liS condition being achieved by the preliminary
pumping of clean oil. The concentration of the liquid
polymer dissolved in oil (partly hydrolyzed polyacrylamide)
may be small, e.g. 1%, if the deposit contains soft water. -~
With a high salt content of the deposit water higher concen-
trations are to be recommended, 5 to 10%. The volume of the
emulsion solution must be selected such that it penetrates
to several meters distance into the deposit formation,
- 10 adjacent the producing well. Preferably the penetration
into the formation should be about 3 to 10 meters. Also
- the volume of the more highly viscous oil subsequently
injected into the formation, which oil may also be crude oil,
is to be selected appropriately such that on the one hand the
liquid polymer solution mixes more intensively in the water-
bearing layer with the deposit water and, on the other hand,
in order to push back in the oil-bearing layer in the
' immediate vicinity of the produclng well, as a result of
which the oil inflow is prevented to a lesser degree. It
is preferred that the oil be in~ected in amounts such that
the emulsion solution is displaced into the formation to a
distance of 1 to 5 meters. In this system, the perforations
present in the zone originally supplying water also remain
; open and are useful for production. Filling these perfora-
tions with cement before or after treatment would therefore
be detrimental.
The technical carrying out of a treatment requires
no unusual or costly equipment. The underground installations
or the well are not modified, and only the incorporated
plunger pump only requires to be drawn. The making available
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ls requ1red of tanks with the dilution oil (diesel oil,
degassed conderlsate or the like), with the more viscous
oil (machine oil, crude oil), and a smaller tank for the
polyacrylamide liquid polymer, also an injection pump and
a metering pump, which in.jects the liquid polymer solution
into the sucf ion duct of the injection pump. Both pumps
should have provision for regulating and controlling flow
rates, so that the desired injection rate and the concentra-
tion may be adjusted.
The advantage of the present liquid polymer process
lies in that the whole thickness of the deposit can be
treated, independently of which part of the thickness bears
water or whether the water flows in a particular area over
:~ the whole thickness to the well, while the oil flows from
specific parts in a continuous stream to the drill hole,
which is afforded in most cases up to very high intrusion
of water. As emulsions can be ma.de finely dispersed almost
as desired, the size of the gel globules of the liquid
polymer - insofar as the gel globules are not bigger than
0.5 to 5.0JU m - hardly causes a restrlction of applicability,
as deposits with permeabilities of 0.1 ~ m2 (100 m Darcy) or
over can be treated with success.
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