Note: Descriptions are shown in the official language in which they were submitted.
~;~ r~ ~08~ 3
K 5715
PROOESS FOR THE PREP~ATION OF HYDR0CARBONS AND/OR
O~YGEN-CCNTAINING HYDROC~R~ON DERIU~TIVES
AND FOR CRUDE OIL PRODUCTION
The inventlon relates to a process for the preparation of
hydrocarbons and/or oxygen-containiny hydrocarbon derivatives and
for the production of crude oil fram an underground formation,
wherein
a) a mixture of carbon mono~ide and hydrogen at elevated
temperature and pressure is partly converted to hydrocarbons
and/or oxygen~containing hydrocarbon derivatives, carbon
dio~ide belng formed;
b) the hydrocarbons and/or oxygen-containing hydrocarbon
derivatives are separated at least partly frcm the product of
step a) and a carboll dioxide-containing off-gas ls obtained;
c) at least the carbon dioxide of the off-gas is injected at
elevated pressure into the llnderground formation and crude oil
is pro*uced from the formation.
Such a process is kncwn frcm US paten-t specification No.
4,098,339. This specification discloses a process in which natural
gas with a substantial c æbon dioxide content is converted by steam
into a mixture of carbon ~onoxide, hydrogen and carbon dioxide. me
carbon dioxide content is raised by means of the water gas shift
reaction in which carbon monoxide and steam react to form carbon
dioxide and hydrogen. Carbon monoxide and hydrogen in the product
are subsequently convert~d into methanol or liquid hydrocarbons.
The methanol or liquid hydrocarbons are ~eparated from the product
and an off-gas with a substantial carbon dioxide content is obtained.
In addition to carbon dioxide, tha off-gas still contains scme
carbon monoxide. This carbon monoxide is converted in a methanator
into methane, and the mixture of carbon dloxide and ~.ethane is
pumpad into an oil-bearing underground formation in order to
promote the production of crude oil.
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It is known that cæbon dioxide under pressure dissolves
readily in crude oil. This lowers the oil's viscosity and increases
the volume of the oil/carbon dioxide mixture so that the now less
viscous oil flcws more easily out of the pores of the formation.
This effect is intensified by the surface tension reduction which
is caused by the presence of cæbon dioxide and which reduces the
cohesion be-tween the oil and the formation.
Oil production techniques using carbon dioxide are known in
the art. For example, cæbon dioxide injection can be realized via
an injection well, p æt of the cæbon dioxide dissolving in the oil
while another part, functioning as displacing medi~n, drives the
oil to a production well. Other examples of a displacing medium are
water and nitrogen. It is also possible to lower the pressure in
the well after the carbon dioxide injection so that the now less
viscous oil can be produced through the same well.
It will ~e clear from the above that the solubility of cæbon
dioxide in oil is very important. According to the abcve-mentioned
US patent specification, however, the solubility of carbon dio~ide
in oil is decreased by the presence of methane. The viscosity
reduction of the crude oil is therefore not ideal in the known
process. In the process according to the present invention the
presence of methane is avoided.
The invention therefore relates to a process for the prep æ -
ation of hydrocarbons and/or o~ygen-containing hydrocarbon deriva-
tives and for the production of crude oil fron an underground
formation, the process ccmprising the above-mentioned steps a) to
c), chæ acterized in that the off-gas is subjected to oxidation to
increase the carbon dioxide content before at least the cæbon
dioxide of the off-gas is injected into the formation.
me mixture of carbon monoxide and hydrogen can be obtained in
many ways, for example as described in the above-mentioned US
patent specification, with the aid of steam re-forming of methane.
The mixture is, however, preferably obtained from the gasification
of a carbon-containing fuel with the aid of an oxygen-containing
gas and/or steam. Suitable fuels include many liquid hydrocarbon
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fractions, as well as tar, oil from tar sands, shale oil, etc.
Solid fuels, such as coal, lignite, peat, etc., can also be
employed. Possibilities for the oxygen-containing gas include air,
oxygen-enriched air, or almost pure oxygen. The p~lre oxygen
required is usually prepared in an air separation plant.
Gasification is generally carried out at a pressure of 5-100 bar
and at a temperature of 900-1900 C.
Depending on the sulphur content of the fuel emplcyed, it is
sG~etimes desirable to subject the gasification product to desul-
phurization. q~lis is to prevent poisoning of the catalyst in thesynthesis stepO In addition, a carbon dio.xidP-rich gas is obtained
which contains no hydrogen sulphideO This is desirable, since the
transport of H2S-containing gases by pipeline to the oil field is
subject to stringent safety requirements, or even prohibited.
The molar ratio of carbon monoxide to hydrogen in the miYture
depænds on the gasification process emplcyed. In general, the
H2 : CO ratio is betwe2n 0.25 and 2. If it is desirable to raise
the hydrogen content in the mixture in order to obtain good con-
version of the mixture to hydrocarbons and/or oxygen-containing
hydrocarbon deriva-tives, the mixture can be subjected to a water
gas shift reaction in which carbon moncxide and steam react to form
carbon dioxide and hydrogen.
In the synthesis step the mixture of carbon monoxide and
hydrogen is converted to hydroc æbons and/or oxygen-containing
hydrocar~on derivatives. Methanol is the most important exponent of
the latter group. A suitable catalyst will be necessary if it is
intended to synthesize methanol. qhe st preferred catalysts
contain either copper and zinc, or zinc ~xide and chrc~ium oxide.
Custcm~y pressures and temperatures for methanol synthesis are in
the ran~e of 50 to 300 bar and 230 to 350 C, respectively.
Preferably, hcwever, hydrocarbons are prepared. Since hydrocarbons
contain no oxygen, in contrast to methanol, an oxygen-containing
byproduct is obtained from the synthesis which contains the oxygen
originally present in the carbon monoxide. m is is water or carbon
dioxide, depending on the catalyst chosen. Preferably, catalysts
i3
are chosen which form hydrocarbons and carbon dioxide as almost the
only oxygen-containing products, sueh as the catalysts mention d
below. Arcmatic or paraffinic hydrocarbons can be prepared, depend-
ing on the catalyst used.
If it is wished to prepare arcmatic hydrocarbons, it is
advantageous to use particular crystalline metal silicates.
Particularly advantageous for the preparation of arcmatic hydro-
carbons together with carbon dioxide is the use of speeial iron
silicates or iron aluminium silicates which are described in
British patent specification No 1,555,928 in cambinatlon with a
catalyst for the synthesis of methanol or dimethyl ether. It is
advantageous to employ a mixture of such a silicate and a catalyst
for methanol synthesis, such as a ZnO/Cr2O3 ccmposition, as is
disclosed in British patent specification No. 2,009,778. The
reaction conditions under whieh the conversion is carried aut are
usually a pressure of 5 to 100 b æ and a temperature of 200 to
500 C and a space velocity of 300 to 3000 Nl gas/l catalyst/hour.
If it is wished to prepare predcminantly paraffmic hydro-
carbons, Fiseher-Tropsch catalysts are often used. Particularly
suitable catalysts for the preparation of paraffinic hydrocarbons
together with carbon dio~ide are the iron/magnesiumlalumina catalysts
described in British patent specification No. 2,053,713~ and the
iron/chroMium/silicate catalysts described in British patent
specification No. 2,053,016. The conversion is usually conduct~d at
a temperature of 200 to 450 C, a pressure of 10 to 7Q bar and a
space velocity of 500 to 5000 Nl gas/l catalyst/hour.
Th~ conversion is preferably performed such that more than 90%
of the hydrogen present is converted in a single step. In this way
a considerable quantity of hydrocarbon products are obtained from
the available mixture of carbon monoxide and hydrogen. Moreover,
after the separation of the hydrocarbons, the off-gas has a fairly
high carbon dioxide cont~nt. The carbon dioxide content is then
usually above 70 vol.%.
The desired hydrocarbons and/or oxygen-containing hydrocarbon
derivatives are separated in a separating plant. The hydrocarbvns
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in question will often be -the liquid ones; these are hydrw arbons
with 5 or more carbon atGms per molecule (C5 ). Any C3 and
C4 hydrocarbons formed can also be separated. mey can be used
as, for example, LEG. Methane and the C2 hydrocarbons are usually
discharged in the off~gas. In addition to C02, H20 and auantities
of unconvexted hydrogen and carbon monoxide, the off-gas can
therefore also contain cG~bustible constituents, such as ~ethane
and C2 hydrocarbons. In the process according to the invention
these ccn~ustible hydrocarbons, together with the unconverted
lo quantities of carbon monoxide and hydrogen, are oxidized to form
carbon dioxide and water vapour.
The oxidation can be performed with or without a catalyst. If
there æe only small amounts of ca~bustible constituPnts present in
the off-gas, the oxidation is preferably perormed with the aid of
a catalyst. A variety of after-burnLng catalysts are suitable.
Preferred catalysts are copper oxide, whether or not on a vanadium
pentoxide carrier, or platinum on a carrier such as asbestosO The
oxidation is preferably performed at a pressure of 10 to 100 bar
and at a temperature in the ra~ge of 300 to 1000 C. After the
oxidation, the temperature of the gas can be lowered in a known way
to any desired level, recovering usable heat at the same time.
Part of the oxidation products, e.g. water vapour, can be
seFarated frGm the rest after the oxidation. This can be simply
done by cooling until condensation takes place. The rest of the
axidation products, consisting mainly of carbon dioxide, is then
injected into the undergroNnd formation.
Preferably, hcwever, the entire oxidation product of the
off-gas is injected into the formation. me presence of water can
be advantageous. The carbon dio~ide dissolves in the water that is
also injected into the formation. This solution has a weakly acidic
effect and can thereby pramote the opening of oil-containing pores
in the formation. This eases the flow of the oil, rendered less
viscous by the carbon dioxide dissolved in it, and thereby also its
production. Moreover, the oxidation products do not need to be
cooled until the water vapour condenses. me gas can therefore be
injected into the formation at a high temperature, which helps in
reducing the viscosity of the crude oil in the formation and
thereby eases its production.
I'he entire oxidation product, or at least the carbon dioxide
of the off-gas, is injected into the formation preferably at a
pressure of 50 to 400 bar and at a temperature of 20 to 500 C. In
the process according to the invention, carbon dioxide is therefore
obtained that has a favourable temperature and has already been
brough-t to an attractively high pressure, which, if desired, can be
raised further by compression. me o~timum conditions are determined
in accordance with the natuîe of the formation and o~ the crude
oil. The location of the field where the crude oil is produced
determines the distance over which the car~on dioxide-containing
gas has to be transpor~ed. This is a contributory factor in deter-
mining the tempel^ature and pressure at which the gas is transportedto the field.
In addition to water vapour, the oxidation prcduct of the
off^^gas can also contain some nitrogen. This occurs if air is used
for the oxidation. The solubility of nitrogen in crude oil is,
however, much less than that of carb~n dioxide. Nitrogen therefore
only lo~ers the viscosity of the oil to a small extent. It can,
however, be used as a displacing gas. Preferably, the presence of
nitrogen is avoided and the off-gas is oxidized with almost pure
oxygen. If the concentration of combustible materials in the
off-gas is low, it is highly desirable to use pure oxygen for the
oxidation, since o~idation with air proceeds far less readily, and
in the absence of a catalyst sc~etimes dcies not proceed at all. The
o~ygen preferably originates from the alr separating plant which
also supplies the oxygen for the gasification of a carbon-containing
fuel with the formation of a mixture of carbon monoxide and hydrogen,
said mixture being partially converted into hydrocarbons and/or
oxygen-containing hydrocarbon derivatives in step a) of the process.
The oxidation is preferably carried out wi-th a stoichiometric
quantity of oxygen. It is, hcwever, not disadvantageous to use a
small excess of oxygen. The oxygen is then injected with the carbon
dioxide into the formation. A cc~bustion reaction can there take
place between oxygen and oil in which heat is released. This heat
lcwers the viscosity of the oil.
me gas injected into the formation can therefore con-tain
relatively small amounts of water vapour, nitrogen and oxygen.
Preferably, hcwever, it contains no more than 10 vol.% impurities,
so that a carbon dioxide-rich gas with at least 90 vol.% carbon
dioxide is injected into the formation. The sas contains no harmful
impurities, such as methane or hydrogen sulphide.
From an econcmical point of view, it is desirable to separate
the carbon dioxide frc~ the produced oil and to re-use it. me
carbon dioxide recovered from the oil already produced is preferably
brcught to an elevated pressure and added to the off-gas. This
addition to the off-gas can ~e done either before or after the
oxidation. It is advantageous to add the carbon dioxide frc;m the
oil after the oxidation, since in this case the oxidation does not
need to be carried out with an off-gas having an extra high carbon
dioxide content.
