Note: Descriptions are shown in the official language in which they were submitted.
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Process for obtaining viscous mineral oil from an underground deposit
Description
The present invention relates to a process for producing mineral oil,
especially viscous
mineral oil, from mineral oil deposits, wherein the mineral oil yield is
increased by injecting
aqueous urea solution having a temperature of at least 80 C into the deposit.
The present
process forms part of the group of mineral oil production technologies which
are typically
characterized as water-gas-steam flooding.
In natural mineral oil deposits, mineral oil occurs in cavities of porous
reservoir rocks which
are closed off from the surface of the earth by impervious overlying strata.
In addition to
mineral oil, including proportions of natural gas, a deposit further comprises
water with a
greater or lesser salt content. The cavities may be very fine cavities,
capillaries, pores or the
like, for example those having a diameter of only approx. 1 pm; the formation
may
additionally also have regions with pores of greater diameter and/or natural
fractures.
After the well has been sunk into the oil-bearing strata, the oil at first
flows to the production
wells due to the natural deposit pressure, and erupts from the surface of the
earth. This
phase of mineral oil production is referred to by the person skilled in the
art as primary
production. In the case of poor deposit conditions, for example a high oil
viscosity, rapidly
declining deposit pressure or high flow resistances in the oil-bearing strata,
eruptive
production rapidly ceases. With primary production, it is possible on average
to extract only
2 to 10% of the oil originally present in the deposit. In the case of higher-
viscosity mineral
oils, eruptive production is generally completely impossible.
In order to enhance the yield, what are called secondary production processes
are therefore
used.
The most commonly used process for secondary mineral oil production is water
flooding.
This involves injecting water into the oil-bearing strata through what are
called injection
wells. This artificially increases the deposit pressure and forces the oil
from the injection
wells out of the production wells. However, water flooding cannot
substantially increase the
yield level of viscous crude oils. In the ideal case of water flooding, a
water front proceeding
from the injection well should force the oil homogeneously over the entire
mineral oil
formation to the production well. In practice, a mineral oil formation,
however, has regions
with different levels of flow resistance. In addition to oil-saturated
reservoir rocks which are
of fine porosity and have a high flow resistance for water, there also exist
regions with low
flow resistance for water, for example natural or synthetic fractures or very
permeable
regions in the reservoir rock. Such permeable regions may also be regions from
which oil
has already been extracted. In the course of water flooding, the flooding
water injected
naturally flows principally through flow paths with low flow resistance from
the injection well
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to the production well. The results of this are that the oil-saturated deposit
regions which are
of fine porosity and have high flow resistance are not flooded, and that
increasingly more
water and less mineral oil is produced via the production well. In this
context, the person
skilled in the art refers to "watering out of production". The effects
mentioned are particularly
marked in the case of heavy or viscous mineral oils. The higher the mineral
oil viscosity, the
more likely it is that production will water out rapidly.
In order to reduce the oil viscosity and increase the oil extraction level,
different processes
based on steam flooding, CO2 flooding, gas-water flooding and aqueous
solutions of
io chemicals have been developed. These processes exhibit a distinctly
improved yield as
compared with conventional water flooding. Steam flooding involves injecting
steam into the
deposit to heat the mineral oil and thus reduce the oil viscosity. As in the
case of water
flooding, however, it is also possible for steam and steam condensate to
penetrate
undesirably rapidly through zones of high permeability from the injection
wells to the
is production wells, and hence reduce the efficiency of tertiary production.
It is additionally known that aqueous solutions of chemicals which generate
gases under
particular conditions can be used for flooding. For example, CO2 dissolves in
the mineral oil
and reduces the viscosity thereof. For example, RU 2007 113 251 A discloses a
process in
20 which an aqueous urea solution and steam are pressure-injected cyclically
into the deposit.
After the hydrolysis of the urea in the deposit, gases which bring about an
increase in oil
extraction are generated.
Bocksermann A., Kotscheschkov A. and Tarasov A. ("Vervollkommnung der
thermischen
25 Methoden zur Entolungssteigerung der Erdollagerstatten" [Enhancement of the
thermal
methods for increasing oil extraction from mineral oil deposits], Russian
Institute for
Scientific and Technical Information; "Development of oil and gas deposits"
series;
volume 24, Moscow 1993) describe a process for deposit development in which
water
flooding/steam flooding and cyclic pumping of aqueous urea solution are
conducted. Under
30 the action of the deposit temperature and the steam temperature, the
hydrolysis of the urea
commences to form ammonia and carbon dioxide, which promote and enhance oil
extraction. Disadvantages of this process are that it can be used only in
deposits with
temperatures of at least 80 C, and homogeneous hydrolysis of the urea in the
deposit is
difficult to achieve since the urea solution can also flow within
cooled/cooler zones of the
35 deposit. The hydrolysis of the urea in the deposit can be controlled only
with difficulty, if at
all.
Another problem for the use of aqueous urea solution is that the temperature
range within a
deposit which has already been developed for a few years is usually
inhomogeneous. Exact
40 prognoses for the temperature ranges are often very difficult or
impossible, and so the
prediction of the effect of the aqueous urea solution in the mineral oil
deposit can be
predicted only with difficulty, if at all.
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A difficulty which can arise in the case of prolonged water flooding is the
cooling of the zone
close to the injector. Typically, a particular amount of aqueous urea solution
is injected,
followed by flooding with water, in order to displace the urea solution to
sites in the deposit
which have the elevated temperatures required for the urea hydrolysis. If the
cooled zone
around the zone close to the injector is relatively large, the urea solution
has to be
transported over a relatively great distance to reach deposit zones with the
appropriate
temperatures. In the course of this, the urea concentration of the injected
urea solution is
reduced significantly as a result of the subsequent flooding.
For irregular deposits, it is barely possible to predict the change in
concentration of the
aqueous urea solution underground, the exact definition of the temperature
range of the
deposit, and the paths that the flooding media will take. Therefore, the use
of aqueous urea
solution by the processes known is also always associated with the risk that a
large portion
of the aqueous urea solution injected ends up in deposit sections in which the
urea
hydrolysis is impossible.
As described above, the marked thermal hydrolysis of aqueous urea solutions
begins only at
temperatures of 80 C. Below 80 C, the thermal hydrolysis of the urea is too
slow to form
ammonia and carbon dioxide to a sufficient extent within an economically
rationable
timespan.
In order to allow urea to be used additionally in those mineral oil deposits
whose temperature
is below 80 C, proposals have been made in the prior art to heat the
underground
formations. US 4,982,789 proposes for this purpose injecting hot water or
steam into the
deposit until the deposit has a temperature which is sufficient for the
thermal hydrolysis of
urea. This process has the disadvantage that the heating of a deposit entails
enormous
energy costs. Furthermore, the disadvantage exists that the heating of the
deposit may
likewise take place irregularly, and so, with this method as well, precise
forecasts of the
temperature fields and predictions of the effect of the subsequently injected
aqueous urea
solution in the mineral oil deposit are extremely difficult and complicated.
Processes in which water and gas are introduced alternately into the deposit
have the
disadvantage that the mixing of the gas phase with the liquid phase is
difficult to achieve
underground in the mineral oil deposit.
Described in the prior art, furthermore, are processes using urea solutions
which even prior
to injection into the borehole have temperatures which ensure thermal
hydrolysis of the urea.
US 5,209,295 discloses a process for lowering the viscosity of high-viscosity
mineral oil in
situ in a mineral oil deposit by injecting a mixture of steam and an aqueous
urea solution.
This mixture of steam and the aqueous urea solution may in this case be
prepared above
ground or at the borehole head.
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J
US 4,227,575 and US 4,594,609 disclose processes for treating mineral oil
deposits, again
injecting steam, admixed with urea, into mineral oil deposits.
According to the teaching of US 4,227,575, the mineral oil deposit is heated
by the injection
of steam to temperatures of at least 260 C. The principle objective here is to
modify the
matrix of the mineral oil deposit, which comprises montmorillonite clays.
The advantage of these processes is that the thermal hydrolysis of the urea is
ensured. A
disadvantage, however, is that the decomposition of the urea begins as early
as above
io ground or at the borehole head to a considerable extent, meaning that
considerable amounts
of carbon dioxide and ammonia are formed as early as above ground. Ammonia is
a toxic
and caustic gas, whose handling above ground necessitates special safety
precautions,
thereby giving rise to extra costs in plant construction. Carbon dioxide,
especially in
conjunction with steam, is extremely corrosive, and hence attacks the steel
components of
the mineral oil production plant. Particularly affected by this are the steel
pipe carriers of the
well, such as riser pipes or injection pipes, for example.
It was therefore an object of the invention to provide a process for producing
mineral oil from
mineral oil formations, which is also usable in deposits with low temperatures
and which
combines the advantages of gas flooding, gas-water flooding, steam flooding
and optionally
surfactant treatment. More particularly, the process is to be suitable for
production of viscous
mineral oil. A further object of the present invention was to provide a
process by means of
which the formation of poisonous ammonia and of corrosive carbon dioxide above
ground or
at the borehole head is substantially avoided.
This object is achieved by the following process for producing mineral oil
from an
underground mineral oil deposit into which at least one injection well which
is in contact with
the mineral oil deposit via at least one perforation zone and at least one
production well have
been sunk, comprising injecting at least one aqueous urea solution into the
injection well and
pressure-injecting the aqueous urea solution through the perforation zone into
the mineral oil
deposit, wherein the aqueous urea solution on pressure injection into the
mineral oil deposit
is at a temperature of at least 80 C.
In a preferred embodiment, this object is achieved by the following process
for producing
mineral oil from an underground mineral oil deposit into which at least one
injection well
which is in contact with the mineral oil deposit via at least one perforation
zone and at least
one production well have been sunk, comprising injecting at least one aqueous
urea solution
into the injection well and pressure-injecting the aqueous urea solution
through the
perforation zone into the mineral oil deposit, wherein the aqueous urea
solution on pressure
injection into the mineral oil deposit is at a temperature of at least 80 C,
wherein the at least
one aqueous urea solution is heated to at least 80 C in the injection well by
means of
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electrical and/or inductive heating elements arranged above the perforation
zone in the
injection well.
At temperatures of about 80 C, noticeable thermal hydrolysis of the aqueous
urea solution
s commences. This means that the thermal hydrolysis of the urea to ammonia and
carbon
dioxide proceeds sufficiently rapidly that a sufficient amount of ammonia and
carbon dioxide
is formed within an economically viable period of time to bring about the
desired increase in
mineral oil production, for example within one to three days. If the profile
is to be modified
more quickly, the temperature that the aqueous urea solution has on pressure
injection may
io be more than 80 C, for example at least 85 C or at least 90 C.
Some of the gases which evolve in the course of hydrolysis of the aqueous urea
solution are
dissolved in the water (preferably ammonia) and form an alkali bank, some of
the other
gases formed (predominantly CO2) dissolve partially in the oil, and the rest
of the gases
formed fill the liquid present with gas. The gas-filled flooding liquid has an
increased
viscosity and promotes profile modification. The aqueous bank enriched with
gases and
alkali is pressure-injected through the deposit. The formation of ammonium
hydroxide
reduces the interfacial tension between oil and water. Since the urea solution
is what is
called "a true solution" with a homogeneous urea concentration, it is
simultaneously enriched
zo with the gases formed in the hydrolysis. In the case of appropriate
selection of the urea
concentration in the aqueous solution, it is thus possible to generate water-
gas mixtures in
situ within the deposit.
The process according to the invention thus has the advantage that the
hydrolysis of the
urea is conducted under control, and the deposit is flooded with a mixture of
water, ammonia
and carbon dioxide. This avoids the use of costly technologies for the
preparation and
pumping of gas-water mixtures, which are typically produced above ground. The
process
according to the invention can be used in exploitation of deposits with
different temperatures,
different mineralizations of the formation water and different storage
properties.
The following specific details can be given for the invention:
The process according to the invention for production of mineral oil is a
process for
secondary or tertiary mineral oil production, which means that it is employed
after primary
mineral oil production due to the intrinsic pressure of the deposit has ceased
and the
pressure in the deposit has to be maintained by injecting flooding media. The
flooding
medium, especially water and/or steam, is injected into the deposit through at
least one
injection well, and crude oil is withdrawn from the deposit through at least
one production
well. The term "crude oil" in this context of course does not mean single-
phase oil; instead
90 what is meant is the customary emulsions which comprise oil and formation
water and are
produced from mineral oil deposits.
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Deposits:
The deposits may be deposits for all kinds of oil, for example those for light
or heavy oil. The
process is preferably used for production of viscous mineral oil having a
viscosity of at least
30 cP, preferably of at least 50 cP, measured at deposit temperature.
Preferably in
accordance with the invention, the deposits are heavy oil deposits, which
means deposits
comprising mineral oil with an API gravity of less than 22.3 API. Mineral oil
is generally
produced from such deposits by injecting steam.
io The initial deposit temperature - which means the temperature before
employment of the
process according to the invention - is typically within the range from 8 C to
120 C,
preferably 8 C to 100 C, more preferably 8 C to 90 C, even more preferably 8 C
to 80 C
and especially 8 C to 70 C.
The deposit temperature may, however, be altered by prior flooding with
flooding media
when the temperature of the flooding medium differs from the temperature of
the deposit.
For example, the temperature of an initially cooler deposit can increase in
the course of
prolonged flooding with steam; in the case of prolonged flooding with water,
the temperature
of an initially warmer deposit can fall. In this case, the temperature change
takes place
essentially in the region close to the injection well and in the region
between the injection
and production wells. The expression "region between the injection and
production wells"
relates to those volume elements of the deposit through which the flow paths
lead from the
injection wells to the production wells, though the flow paths of course need
not necessarily
proceed in a straight line between injection and production wells. The person
skilled in the
art is aware of methods by which such volume elements through which flow
proceeds can be
determined. The reference points for determination of the "region between the
injection and
production wells" are of course not the production well and the injection well
over their entire
length, but rather, in the case of the injection well, the point in the well
at which the
formulation (F) actually enters the formation from the injection well, and, in
the case of the
production well, that point at which crude oil actually enters the production
well from the
formation, or is to enter it in the future.
Preferably in accordance with the invention, the present process is used when
the
temperature close to the injection well and in the region between the
injection and production
wells is 8 C to 70 C, preferably 8 C to 40 C and more preferably 8 C to 20 C.
Process
To execute the process, at least one production well and at least one
injection well are sunk
into the mineral oil deposits. In general, a deposit is provided with several
injection wells and
with several production wells. These may be wells which have already been used
at an
earlier stage of mineral oil production, for example in the course of a prior
water flooding or
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steam flooding operation, but the wells may also have been sunk into the
deposit specially to
execute the process according to the invention.
The process according to the invention comprises the injecting of at least one
aqueous urea
solution into the at least one injection well and pressure-injecting of the at
least one aqueous
urea solution through the at least one perforation zone into the mineral oil
deposit, wherein
the at least one aqueous urea solution on pressure injection into the mineral
oil deposit is at
a temperature of at least 80 C, preferably of at least 85 C and more
preferably of at least
90 C. The aqueous urea solution is in this case heated to at least 80 C in the
injection well
by means of electrical and/or inductive heating elements arranged above the
perforation
zone in the injection well.
Urea is converted in the presence of water in the course of hydrolysis to
ammonia and
carbon dioxide according to the following equation:
H2N-CO-NH2 + H2O - 2NH3 + C02-
The carbon dioxide formed dissolves partially in the mineral oil and reduces
the viscosity
thereof. The ammonia formed dissolves in the water present in the deposit and
forms an
alkaline ammonia buffer system with a pH of 9 to 10. This buffer system has a
surfactant-like
effect in the deposit. As a result of the production of gas in the water and
the formation of
alkali, foam-like structures which cause an increase in viscosity of the water
used for
flooding are also formed. This serves to modify the profile of the flooding
and to balance out
the flood front. If surface-active substances are additionally used with the
alkaline bank, they
may bring about a further increase in the level of oil extraction. Surface-
active substances
and surface-active components are understood in the present case as materials
which can
reduce the surface tension of water. Preference is given here to surfactants
such as the
nonionic, anionic and cationic surfactants listed below.
From about 80 C, the thermal hydrolysis of the urea solution proceeds
sufficiently rapidly to
bring about an increase in mineral oil production within economically viable
periods.
Significant amounts of urea are converted above the melting temperature of the
urea
(133 C). In the case of an increase in the temperature to 200 C, the urea is
decomposed to
an extent of 75% within a relatively short time, and, in the case of very
rapid heating to
280 C, the reaction proceeds almost to completion. Above 400 C, urea is
converted
completely to gaseous components, which means that no solid residues remain.
The aqueous urea solutions are typically provided above ground by dissolving
the urea in
water. It is optionally possible to add further additives, such as surface-
active components
(surfactants) and ammonium salts.
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Typically, the aqueous urea solution comprises 3 to 79% by weight of urea and
21 to 97% by
weight of water, based on the total weight of the urea solution. The aqueous
urea solution
preferably comprises 20 to 40% by weight of urea and 80% by weight to 60% by
weight of
water, based on the total weight of the urea solution.
In a preferred embodiment, the aqueous urea solution comprises water and urea
in a ratio of
21 to 25% by weight of water to 75 to 79% by weight of urea, based on the
total weight of
water and urea, more preferably in a ratio of 23.1 % by weight of water to
76.9% by weight of
urea. The weight ratio of 23.1 % by weight of water to 76.9% by weight of urea
corresponds
io to a stoichiometric ratio of water to urea of 1:1, which means that the
urea present in the
aqueous urea solution reacts completely with the water present therein to give
ammonia and
carbon dioxide. If the aqueous urea solution comprises less urea, the water is
not converted
completely; if the aqueous solution comprises more urea, isocyanic acid is
also formed as
well as ammonia and carbon dioxide. For a maximum yield of ammonia and carbon
dioxide,
the ratio of water to urea should be within the aforementioned range.
In a further preferred embodiment, the aqueous urea solution comprises 30% by
weight to
34% by weight of urea and 70% by weight to 66% by weight of water, based on
the total
weight of the urea solution.
A urea solution with 32.5% by weight of urea is optimal, since the solution at
this
concentration forms a eutectic mixture which remains liquid down to -11 C.
This simplifies
the solution preparation, and the transport and storage of the solution. When
the urea
concentration is increased, the stability of the urea solution is guaranteed
only by the
constant supply of heat. This makes the process costly and unprofitable when
the outside
temperatures are lower.
One use of such a mixture is as a reducing agent for offgas aftertreatment of
nitrogen oxide
emissions, and is commercially available under the Ad Blue name. The urea
concentration
of the aqueous urea solution used as a reducing agent is fixed in ISO 22241-1
at 31.8 to
33.2% by weight of urea, based on the overall solution. The aqueous urea
solution used
most preferably has a content of 31.8 to 33.2% by weight of urea and 76.8 to
78.2% by
weight of water, based on the total weight of water and urea.
In addition, the aqueous urea solution may comprise at least one surface-
active component
(surfactant), preferably 0.1 to 5% by weight of at least one surfactant, more
preferably 0.5 to
1% by weight of at least one surfactant, based on the total weight of the urea
solution. The
surface-active components used may be anionic, cationic and nonionic
surfactants.
Commonly used nonionic surfactants are, for example, ethoxylated mono-, di-
and
trialkylphenols, ethoxylated fatty alcohols and polyalkylene oxides. In
addition to the unmixed
polyalkylene oxides, preferably C2-C4-alkylene oxides and phenyl-substituted
C2-C4-alkylene
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oxides, especially polyethylene oxides, polypropylene oxides and
poly(phenylethylene
oxides), particularly block copolymers, especially polymers having
polypropylene oxide and
polyethylene oxide blocks or poly(phenylethylene oxide) and polyethylene oxide
blocks, and
also random copolymers of these alkylene oxides. Such alkylene oxide block
copolymers are
known and are commercially available, for example, under the Tetronic and
Pluronic names
(BASF).
Typical anionic surfactants are, for example, alkali metal and ammonium salts
of alkyl
sulfates (alkyl radical: C8-C12), of sulfuric monoesters of ethoxylated
alkanols (alkyl radical:
C12-C18) and ethoxylated alkylphenols (alkyl radicals: C4-C12), and of
alkylsulfonic acids
(alkyl radical: C12-C18).
Suitable cationic surfactants are, for example, the following salts having C6-
C18-alkyl,
alkylaryl or heterocyclic radicals: primary, secondary, tertiary or quaternary
ammonium salts,
pyridinium salts, imidazolinium salts, oxazolinium salts, morpholinium salts,
propylium salts,
sulfonium salts and phosphonium salts. Examples include dodecylammonium
acetate or the
corresponding sulfate, disulfates or acetates of the various 2-(N,N,N-
trimethylammonium)ethylparaffin esters, N-cetylpyridinium sulfate and N-
laurylpyridinium
salts, cetyltrimethylammonium bromide and sodium laurylsulfate.
The presence of the surface-active component promotes oil extraction and the
formation of
foam-like structures. The surface-active component is already present where
the gases form
in the aqueous solution. In the known processes for foam formation involving
serial pumping
of water portions with surface-active components and pumping of gas portions,
a large
portion of the water comprising surface-active substances does not come into
contact with
the gases underground. The formation of a homogeneous bank of foam-like
structures in the
productive zone of the mineral oil deposit is barely possible in this case,
since the gases
escape principally into the upper edge zone of the reservoir of the mineral
oil deposit, and
the liquid phase (surfactant-containing water) into the zone close to the
brine. The mixing of
the gases with the liquid in the reservoir zones which are relatively far away
from the
injection well is barely possible in the case of serial pumping of liquid and
gas. As already
mentioned above, the addition of surfactants to the aqueous urea solution
increases oil
extraction and the formation of foam-like structures, especially in
combination with the alkali
which forms after hydrolysis of the urea in the deposit.
In a further embodiment, the aqueous urea solution comprises at least one
ammonium salt,
preferably 5 to 30% by weight of at least one ammonium salt.
The ammonium salts may be selected, for example, from ammonium chloride,
ammonium
bromide, ammonium formate and ammonium nitrate.
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The ammonium salts form an alkaline pH buffer system with the ammonia formed
in the
hydrolysis; this is advantageous especially in the presence of surface-active
components
since the alkaline pH buffer system ensures the good wash properties of the
surfactants.
5 The use of ammonium formate has the advantage that ammonium formate can
likewise
decompose to ammonia and formic acid, and the formic acid decomposes further
to carbon
monoxide and water. For example, it is possible to use a mixture of 20% by
weight of urea,
26% by weight of ammonium formate and 44% by weight of water, known
commercially as
Denoxium-30. This mixture is particularly preferred since it has a
comparatively low
10 crystallization point of about -30 C. It is particularly important in
development of the deposits
in northern regions where the outside temperatures are low.
In a further embodiment of the invention, the aqueous urea solution comprises,
as well as
urea, at least one surfactant and at least one of the aforementioned ammonium
salts.
Preference is given to the concentration ranges stated above in each case.
At least one of the above-described aqueous urea solutions is injected into
the injection well
connected to the mineral oil deposit via a perforation zone, and pressure-
injected into the
mineral oil deposit through the perforation zone. In the context of the
present invention,
zo "aqueous urea solution" is used synonymously with "at least one aqueous
urea solution".
Preferably, the at least one aqueous urea solution is injected into the at
least one injection
well in serial portions and pressure-injected into the deposit through the at
least one
perforation zone. This means that the aqueous urea solution is injected in
several portions in
succession, optionally alternately with other flood solutions or flooding
water, and pressure-
injected into the deposit. The portion volume typically used for the aqueous
urea solution
depends on the properties of the deposit and of the mineral oils and may vary,
for example,
between 100 and 1000 m3. Experience has shown that the amount of urea in one
portion
should be at least 15 to 20 tonnes, calculated as dry urea. The pumping of
such a portion of
the aqueous urea solution normally does not take longer than one to three
days. This is
followed by further flooding with water or steam. The flooding with aqueous
urea solution can
be repeated cyclically, for example at a time interval of one to three months.
It is also
possible to heat the flooding water in all flooding phases. In this case,
flooding is effected
with warm water/warm aqueous solution.
The at least one urea solution has a temperature of at least 80 C on pressure
injection into
the deposit.
In general, the temperature that the aqueous urea solution has on pressure
injection is
selected taking account of the following influencing factors:
temperature of the rocks in the zone close to the borehole,
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length of the injection borehole,
the injection rates,
the temperature gradient of the deposit.
s The aqueous urea solution may be provided already with the desired
temperature of at least
80 C, for example by using water of appropriate temperature to make up the
aqueous urea
solution.
The aqueous urea solution can, however, also be provided above ground with a
temperature
io below 80 C and then heated to at least 80 C in the injection well. This
embodiment is
preferred. According to the temperature of the aqueous urea solution provided,
there is a
greater or lesser temperature difference from the desired temperature on
pressure injection,
which means that more or less energy has to be supplied for heating.
15 Preferably in accordance with the invention, warm accompanying water which
has also been
produced in oil production is used to prepare the aqueous urea solution. This
water,
however, need not necessarily already have a temperature of at least 80 C. The
water
preferably has a temperature below 80 C.
20 In one embodiment of the present invention, the urea solution on pressure
injection into the
deposit has a temperature of at least 80 C, but below the boiling temperature
of the urea
solution. The boiling temperature of the urea solution depends on the
concentration and the
pressure existing in the deposit. The urea hydrolysis is preferably completed
here in the
region close to the injection well. This variant is used predominantly in
development of
25 deposits whose temperatures are below 80 C. However, the process can also
be used in
development of deposits with temperatures of 80 C or more. What is meant here
in each
case is the temperature of the deposit directly before use of the process
according to the
invention.
30 In a further embodiment of the present invention, the aqueous urea solution
is vaporized in
the well prior to pressure injection, and the resulting steam or the gas
mixture is pressure-
injected into the deposit. This means that the temperature Tv is above the
boiling
temperature of the aqueous urea solution. The vaporization of the aqueous urea
solution is
preferably performed directly in the injection well. The vaporization more
preferably takes
35 place above the perforation zone, and a mixture of steam, carbon dioxide,
ammonia and
possibly further products of the urea hydrolysis is pressure-injected into the
deposit.
The heating of the at least one aqueous urea solution to at least 80 C can
take place above
ground using conventional techniques for water heating. The hot solution is
injected into the
40 injection borehole and pressure-injected through the perforation zone into
the deposit. Since
the transport speed of the hot solution in the injection well is relatively
high, the aqueous
urea solution is cooled only insignificantly during the injection and pressure
injection. In order
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to minimize possible cooling effects, heat-insulating pipes can also be used
in the injection
well.
The at least one aqueous urea solution can also be heated to at least 80 C in
the injection
well. This embodiment is preferred. For this purpose, suitable apparatuses may
be arranged
in the injection well, preference being given to arranging electrical and/or
inductive heating
elements above the perforation zone within the injection well, with which the
aqueous urea
solution is heated in the injection well.
The aqueous urea solution is heated by the electrical and/or inductive heating
elements in
the injection well to preferably at least 80 C, more preferably to at least 85
C, and very
preferably to at least 90 C.
With particular preference the aqueous urea solution is at least partly
vaporized in the
injection well by the electrical and/or inductive heating elements. With
particular preference
the aqueous urea solution is vaporized completely.
In one preferred embodiment, the water of the aqueous urea solution may be
vaporized, by
virtue of the high pressure within the injection well, at temperatures in the
range from 200 to
300 C, preferably 200 to 250 C and more preferably 230 to 250 C. At these
temperatures,
the urea becomes substantially completely hydrolyzed within a very short time.
The full decomposition of the urea may also, however, take place below the
vaporization
temperature.
In one preferred embodiment, the at least one aqueous urea solution is heated
in the
injection well above the perforation zone, before being injected into the
mineral oil deposit, to
a temperature in the range from 230 C to 250 C.
In accordance with one preferred embodiment, an aqueous urea solution with a
weight ratio
of 21 to 25% by weight of water to 75 to 79% by weight of urea, preferably of
23.1% by
weight of water to 76.9% by weight of urea, based on the total weight of water
and urea, is
heated/vaporized by the heating elements in the injection well until the urea
has been
essentially fully hydrolyzed, and is then pressure-injected through the
perforation zone into
the mineral oil deposit together with steam or hot water. "Essentially fully
hydrolyzed"
preferably means that at least 90% of the urea, more preferably at least 95%
and very
preferably at least 99% of the urea is present in hydrolyzed form, based on
the total amount
of the urea present in the aqueous solution. For this purpose, the aqueous
urea solution can
be heated to temperatures of 150 to 300 C in the injection well, for example.
In accordance
with this embodiment, the aqueous urea solution preferably comprises
essentially water and
urea; more preferably, it consists of water and urea.
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In the vaporization of aqueous urea solutions of the composition described
above, the full
hydrolysis of the urea is accompanied by full consumption of the water for the
formation of
carbon dioxide and ammonia. In this case only gas is pressure-injected into
the mineral oil
deposit (pure gas flooding).
The heating/vaporization of the aqueous urea solution takes place preferably
above the
perforation zone in the lower region of the injection well, in other words in
the bottom 50%,
preferably in the bottom 30%, more preferably in the bottom 20% and more
particularly in the
bottom 10%, based in each case on the overall length of the injection well.
The overall length of the injection well here means the section (the length)
between borehole
head and perforation zone. Preferably, therefore, the heating elements as well
are arranged
above the perforation zone in the lower region of the injection well, in other
words in the
bottom 50%, preferably in the bottom 30%, more preferably in the bottom 20%
and more
particularly in the bottom 10%, based in each case on the overall length of
the injection well.
The at least one aqueous urea solution can also be heated to at least 80 C in
several steps,
the heating comprising the following steps:
(a) heating the at least one aqueous urea solution to a temperature below 80
C, and
(b} heating the at least one aqueous urea solution to at least 80 C,
preferably at least
85 C and more preferably at least 90 C.
In a preferred embodiment, the at least one aqueous urea solution is heated in
step b) to a
temperature in the range from 200 to 300 C, preferably 200 to 250 C and more
preferably
230 to 250 C.
Preferably, step (a) is performed outside the injection well and step (b)
within the injection
well. Likewise preferably, step (a) is performed at the borehole head of the
injection well.
More preferably, step (a) is performed at the borehole head of the injection
well and step (b)
within the injection well. If only the two steps (a) and (b) are performed for
heating, this is a
two-stage heating operation.
Depending on the temperature that the aqueous urea solution has on pressure
injection, the
noticeable hydrolysis of urea commences in the first few hours or the first
few minutes after
attainment of this temperature. This time is normally sufficient to avoid the
unwanted
commencement of intensive urea hydrolysis within the injection well, since the
flow rate of
the aqueous urea solution within the injection well is rapid compared to the
flow rate in the
mineral oil deposit, and the intensive urea hydrolysis therefore commences
essentially in the
zone close to the borehole, in which the flow rate of the aqueous urea
solution declines
significantly. This is also the case when the hydrolysis already begins in the
injection
borehole to such a significant degree that a mixture comprising water, urea
and gases
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already formed is pressure-injected into the deposit. Since the hydrolysis of
the urea starts
up rapidly after the attainment of at least 80 C, but a few minutes or even
hours are required
for the completion, the complete hydrolysis of the urea takes place in the
zone close to the
injection well.
Even in the case that the temperature of the rocks in the zone close to the
borehole is much
lower than the urea hydrolysis temperature, the urea hydrolysis takes place to
a sufficient
degree, since comparatively large masses of aqueous urea solution are pressure-
injected,
and hence the cooling rate thereof is low.
In addition to the decomposition of the urea, the thermal contact can bring
about controlled
vaporization of the water in the aqueous urea solution. This establishes non-
eutectic mixing
ratios between water and urea, which leads to a change in the properties of
the aqueous
urea solution.
In a further embodiment, an aqueous urea solution with a weight ratio of 21 to
25% by
weight of water to 75 to 79% by weight of urea, preferably of 23.1% by weight
of water to
76.9% by weight of urea, based on the total weight of water and urea, is
heated above
ground until the urea has been essentially fully hydrolyzed, and then injected
into the at least
one injection well together with steam or hot water and pressure-injected into
the deposit.
"Essentially fully hydrolyzed" preferably means that at least 90% of the urea,
more preferably
at least 95% and most preferably at least 99% of the urea is present in
hydrolyzed form. For
this purpose, the aqueous urea solution can be heated to temperatures of 150
to 300 C, for
example in a vessel directly at the drilling site. In this variant of the
process according to the
invention, heating elements arranged in the injection well are superfluous. In
this
embodiment, the aqueous urea solution preferably comprises essentially water
and urea;
more preferably, it consists of water and urea.
In the case of the vaporization of aqueous urea solutions of the above-
described
composition, in the complete hydrolysis of the urea, the water is completely
consumed for
the formation of carbon dioxide and ammonia. In this case, only gas is
pressure-injected into
the mineral oil deposit (pure gas flooding).
In principle, the higher the urea concentration of the aqueous urea solution,
the less
additional water, which is not required for the hydrolysis of urea, need also
be heated; this
saves costs since the volume of the aqueous urea solution is reduced. However,
the urea
concentration is limited by the possible crystallization of the urea solution
and depends on
the temperature.
In a further advantageous embodiment of the present invention, the process is
performed by
pressure-injecting at least one aqueous urea solution comprising only water
and urea as
components, after heating to the temperature Tv, into the deposit, it being
preferable that at
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least the last stage of the heating in the injection tube is effected by means
of a heater
arranged therein. Subsequently, with the heater switched off in the injection
tube, one or
more aqueous solutions comprising at least one additive selected from surface-
active
components and ammonium salts can be pressure-injected into the deposit. In
this case, the
5 aqueous additive solution mixes with the hydrolyzing urea solution directly
in the deposit.
The serial pumping of aqueous urea solution comprising only water and urea,
and aqueous
solution comprising additives, saves costs and, in cases where additives are
thermally
sensitive, protects the additive solution. The sequence of pumping of the
individual solutions
may be different, for example
- additive solution (heater passive) -- urea solution (heater active) -
additive solution
(heater passive) or
- additive solution (heater passive) - urea solution (heater active) or
- urea solution (heater active) - additive solution (heater passive) or
- urea solution (heater active) 4 additive solution (heater passive) - urea
solution
(heater active).
"Heater active" means that the heater is heating; "heater passive" means that
it is not
heating.
Between the individual portions, buffer water can also be pressure-injected.
The possible
sequences shown above can be repeated cyclically.
Figure 1 shows a typical injection well, as usable in the process according to
the invention. It
comprises a well with an injection tube (5) enclosed by the feed tube/casing
(14a), the
injection tube/riser tube (5) being sealed from the well by what is called a
packer (13). In the
perforation zone (4) the feed tube (14a) has perforation holes (6) through
which the urea
solution (7) injected into the injection tube is pressure-injected into the
deposit (2).
Figures 2 and 3 show two injection wells which can be used for preferred
embodiments of
the present process.
In figure 2, two heaters/heating elements (3) are arranged around the
injection tube (5). In
the present context, the terms "heaters" and "heating elements" are used
synonymously. In
addition, the borehole head (8), the vessel (9) for the flood solution/urea
solution and the
pump (12) are shown.
Figure 3 shows an injection well in which the aqueous urea solution is heated
in two stages.
For instance, the vessel for the flowing solution/aqueous urea solution (9)
may comprise a
heating element (11). The injection well head (8) may likewise comprise a
heating element
(10). In figure 3, a heating element (3) is arranged on the injection tube
(5).
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In a further embodiment, after the completion of the pressure injection of the
urea solution,
the heaters remain switched on and the flooding water pressure-injected
subsequently is
brought to at least 80 C, preferably to at least the temperature that the
aqueous urea
solution had on pressure injection. This prevents the rapid cooling of the
heated urea
solution by mixing with cold water, and enables very substantially complete
urea hydrolysis.
The volume of the subsequently pressure-injected flooding water with this
temperature may
be equal to the volume of the pressure-injected urea solution.
The same aim is pursued by a further embodiment, in which, before and after
the at least
1o one aqueous urea solution, at least one portion of flooding water is
injected through the at
least one injection well and pressure-injected into the deposit, the flooding
water on pressure
injection having a temperature of at least 80 C in each case. In this
embodiment, two heat
buffers are formed. The use of this embodiment is appropriate particularly in
development of
deposits with temperatures of 8 to 40 C.
The heating element(s) (3) arranged in the injection tube can be used to heat
the aqueous
urea solution to a temperature above the hydrolysis temperature of the urea,
the
temperature being below the boiling temperature of the aqueous urea solution,
but the
aqueous urea solution can also be heated in the injection tube to such an
extent that it
vaporizes in the injection well. The person skilled in the art is aware of
different heaters for
heating and/or vaporization of flooding water, which are installed in the
borehole itself; see,
for example, RU 2086759, RU 2198284, US 5,465,789 and US 5,323,855. It is
possible in
principle to mount one heating element, or else two or more heating elements,
along the
injection tube.
For example, an inductive heating element (3), which is configured, for
example, as a coil
with housing, may be mounted on the riser tube/injection tube (5), and the
riser tube/injection
tube (5) can be used as a ferromagnetic core. The heater may also be an
electrical heater. A
heater typically comprises one or more heating elements. In the process
according to the
invention, on commencement of flooding with aqueous urea solution, the heater
(3) which
heats the injection tube (5) and the aqueous urea solution flowing through is
switched on.
The aqueous urea solution heated to at least 80 C is pressure-injected through
the
perforation orifices (6) into the deposit. This aqueous urea solution
comprises, water, urea,
and hydrolysis products of the urea hydrolysis which has already at least
partly taken place,
comprising ammonia and carbon dioxide, and possibly further additives present,
such as
surfactants and ammonium salts. If the aqueous urea solution has a temperature
below the
boiling temperature of the aqueous urea solution, the urea hydrolysis in this
embodiment is
preferably completed in the region close to the injection well. This variant
is used
predominantly in development of deposits with temperatures below 80 C. The
process can,
however, also be used in development of deposits with temperatures above 80 C.
After the
flooding phase with aqueous urea solution has ended, the heater (3) is
switched off and the
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deposit is flooded with water. The pumping of the aqueous urea solution can be
repeated
cyclically. It is also possible to heat the flooding water in all flooding
phases. In this case,
flooding is effected with warm water/warm aqueous solution.
The aqueous urea solution and the flooding water are pumped, for example, with
a
conventional pump (12) situated above ground.
The structure shown in figure 3 is suitable, for example, for the inventive
multistage heating
of the aqueous urea solution. In this embodiment of the invention, the heating
comprises the
following two steps:
a) heating the aqueous urea solution to a temperature below 80 C, and
b) heating the aqueous urea solution to at least 80 C, preferably at least 85
C and more
preferably at least 90 C.
Preferably, step a) is performed outside the injection well, for example in
the tank (9) or the
borehole head of the injection well, and step b) within the injection well.
For this purpose,
additional heaters (10) and (11) are installed in the injection well head (8)
or in the vessel (9)
for the aqueous solution. The second heating stage is preferably executed with
the aid of the
heater (3) directly in the injection well before the solution is pressure-
injected into the deposit
(2). This embodiment has the advantage that the heater (3) can be operated
underground
with minimal electrical power since the necessary temperature rise (Td) of the
aqueous
solution to the desired temperature on pressure injection into the deposit can
be kept as
small as possible. This simplifies the application and increases the
reliability of the
apparatus, since only a small amount of corrosion-hazardous carbon dioxide, if
any, is
generated during the transport of the urea solution within the riser
tube/injection tube (5).
Carbon dioxide occurs to a significant degree in the zone close to the
borehole only. Thus,
the corrosion risk for the borehole equipment is minimal. When the urea
solution is heated
above ground (for example vessel (9) or at the borehole head (8)), the riser
tube/injection
tube (5) can be provided with heat insulation.
If the power of the heater (3) is insufficient to rapidly heat up the desired
amount of aqueous
urea solution, the flood rates in the flooding with the aqueous urea solution
are reduced.
In order to enhance the efficiency of the heaters (3), in the case of
employment of induction
heaters, spiral-like ribs can be installed in a fixed manner in the injection
line. In this case,
the ribs and the corresponding sections of the injection tube (5) are heated
inductively to
temperatures of 100 to 500 C. This allows the enhancement of the flow
turbulences in the
heated section of the injection tube (5) and rapid transfer of the thermal
energy to the
aqueous urea solution. In the case of an abrupt rise in the temperature of the
aqueous urea
solution from a temperature below the hydrolysis temperature to a temperature
above the
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hydrolysis temperature of the urea, it is possible in this way to induce the
commencement of
the urea hydrolysis.
Figures 4a and 4b show the temperature profile of the aqueous urea solution
along the
injection well from the borehole head to the perforation zone (4). The arrow
labeled TL
represents the temperature of the solution in the manner of an abscissa, with
a dotted line
drawn in for the temperature 80 C in each case. The arrow labeled "d"
vertically downward
represents a measure of the depth of the well. Td denotes the temperature jump
in the
aqueous urea solution caused by heater (3).
In figure 4a, the aqueous urea solution has not been heated beforehand, but
rather is
injected directly into the injection well, the ambient temperature in the
surrounding rock being
higher, such that the aqueous urea solution heats up gently until it reaches
the heater (3).
According to figure 4b, the aqueous urea solution was heated to a temperature
which is
below the hydrolysis temperature of the urea but is much higher than the
temperature
according to 4a. In addition, either the injection tube was provided with
thermal insulation, or
else the surrounding rock has a similar temperature to the urea solution
injected, such that
the temperature of the aqueous urea solution is unchanged from the injection
borehole head
until it reaches the heater (3). It becomes clear from the temperature
profiles that the heater
(3) in the process shown in figure 4a must bring about a much greater
temperature rise than
in the process according to figure 4b.
The invention has the following advantages:
- simple design,
- controlled and complete hydrolysis of the urea,
- no losses of the urea in the deposit,
- no deposit temperature limit for the use of the process,
- increase in oil extraction.
List of figures
Figure 1: Vertical section of an injection well in the lower region in the
injection well
Figure 2: Vertical section of the injection well in the case of one-stage
heating of the
flood solution
Figure 3: Vertical section of the injection well in the case of two-stage
heating of the
flood solution
Figure 4a: Temperature profile of the flood solution in the injection well in
a one-stage
heating process
Figure 4b: Temperature profile of the flood solution in the injection well in
a two-stage
heating process
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List of reference numerals used
1: Injection well
2: Deposit/reservoir/carrier
3: Heating elements (electrical or inductive)
4: Perforation zone
5: Riser tube/injection tube
6: Perforation orifices
7: Aqueous mixture comprising urea
8: Injection well head
9: Vessel
10: Heater at the borehole head
11: Heater in the vessel
12: Pump
13: Packer
14: Thermally resistant surround (for example cementation)
14a: Feed tube/casing
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