GB2521116A

GB2521116A - Method for enhanced hydrocarbon recovery using captured acidic gas

Info

Publication number: GB2521116A
Application number: GB1318740.6A
Authority: GB
Inventors: Henrik Solgaard Andersen
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2013-10-23
Filing date: 2013-10-23
Publication date: 2015-06-17
Also published as: GB201318740D0

Abstract

A method of hydrocarbon recovery, utilising captured acidic gas comprises contacting a gas comprising an acidic gas such as carbon dioxide (CO2) with brine to produce acidic gas-rich brine and acidic gas-depleted gas. The acidic gas-rich brine is de-pressurised preferably in a flash unit 114, to obtain pure acidic gas and brine and a hydrocarbon recovery is conducted using the pure acidic gas. The contacting may take place in an absorption column 101 or a horizontal tunnel comprising treatment sections. The hydrocarbon recovery is preferably an oil recovery technique such as carbon dioxide flooding where CO2, is injected into a hydrocarbon reservoir to increase reservoir pressure thereby increasing the drive of hydrocarbons out of the reservoir.

Description

Method for enhanced hydrocarbon recovery using captured acidic gas The present invention relates to a method and system of enhanced hydrocarbon recovery using captured CO2.

BACKGROUND

The continually increasing combustion of fossil fuel, such as coal, natural gas, and oil, has resulted in a dramatic increase in the concentration of CO2 in the atmosphere. There is overwhelming evidence that the greenhouse effect is at least partly caused by this increased CO2 concentration and that this has already contributed to the climate changes that have occurred over the last decades. According to simulation models, it is suspected to cause further and potentially more dramatic changes in the climate in the future.

As a result, scientists, environmentalists and politicians throughout the world are driving initiatives to reduce the amount of CO2 discharged into the atmosphere by combustion of fossil fuel and other industrial processes. Simultaneously there is also a drive to remove acidic gases such as CO2 and H2S from fuel gases, e.g. natural gas recovered from subterranean formations and syngas from processes such as steam reforming and the gasification of coal. One approach being adopted is to capture CO2 (i.e. prevent the release of CO2) from the gas before it is released to the atmosphere in the case of exhaust gases or before it is used in energy production in the case of fuel gases.

Historically the captured CO2 was released into the ocean. By releasing it at significant depths, e.g. greater than 100 m, it took a significant amount of time for the CO2 to circulate to the surface. There are numerous documents disclosing such methods including US 4,322,227, US 2003/055116, WOO3/00807, EP0429154, US 6,406,219 and US 4,235,607. In more recent times there has been a move towards carbon capture and storage wherein the CO2 is captured! compressed and then stored in subterranean formations. This is often referred to as carbon capture and storage or CCS.

Whilst these approaches are advantageous in that they reduce the amount of CO2 that is released directly into the atmosphere, they still utilise a certain amount of energy purely for disposal of CO2 in a safe way. A need therefore exists for new methods and systems that enables CO2 to be recovered from gases, such as exhaust gas and flue gas, and utilised, rather than stored. Such methods are sometimes referred to as carbon capture and utilisation techniques.

SUMMARY OF INVENTION

Viewed from a first aspect the present invention provides a method of enhanced hydrocarbon recovery, wherein said recovery utilises captured acidic gas, comprising: (i) contacting a gas comprising acidic gas with brine to produce an acidic gas-rich brine and an acidic gas-depleted gas; (U) depressurising said acidic gas-rich brine to obtain pure acidic gas and brine; and (iii) conducting hydrocarbon recovery using said pure acidic gas In a preferred embodiment the acidic gas is CO2.

Viewed from a further aspect the present invention provides a system for enhanced hydrocarbon recovery, wherein said recovery utilises captured acidic gas, comprising: (a) a means for supplying a gas comprising acidic gas to a contact vessel; (b) a means for supplying brine to said contact vessel; (c) a contact vessel for contacting said gas with said brine to produce an acidic gas-rich brine and an acidic gas-depleted gas having an inlet for said gas, an inlet for said brine, an outlet for said acidic gas-depleted gas and an outlet for said acidic gas-rich brine; (d) a means for depressurising said acidic gas-iich brine having an inlet fluidly connected to said outlet for acidic gas-rich brine of said contact vessel, an outlet for brine and an outlet for pure acidic gas; and (e) a means for transporting said pure acidic gas into a subterranean formation for hydrocarbon recovery fluidly connected to said outlet for pure acidic gas of said means for depressurising.

DEFINITIONS

By the term "acidic gas" is meant a gas which when dissolved in water produces a pH of less than 7. The acidic gas may be, for example, C02, H2S, SO2, CS2, HCN, COS, NO2 or mercaptans. Most often, however, the acidic gas will be H25 or GO2, especially CO2. The acidic gas may, for example, be present in an exhaust gas and/or in a fuel gas.

By the term "exhaust gas" is meant a flue gas from an industrial process. The process may be, for example, the combustion of coal, organic waste or oil, steam generation or the manufacture of cement. The gas may also be acidic gas, e.g. 002, produced during hydrocarbon recovery, especially when in situ combustion or enhanced recovering techniques using CO2 are employed.

By the term Tuel gas' is meant a gas that is combusted to generate energy.

Representative examples of fuel gas include natural gas and syngas.

By the term "brine" is meant water comprising dissolved salts such as sodium chloride and potassium chloride. A typical example of brine is sea water.

By the term "acidic gas-rich brine" is meant brine containing a higher concentration of acidic gas than the original brine used in the method. Similarly by the term "acidic gas-depleted gas" is meant a gas containing a lower concentration of acidic gas than the gas treated in the method. The production of an acidic gas-rich brine and an acidic gas-depleted gas indicates the process has been successful since it means the acidic gas has been stripped or sequestered from the gas into the brine.

By the term "untreated brine" is meant that no chemicals are added to the brine and no constituents of the brine are removed. Particularly preferably proton removing agents are not added to the brine.

By the term "absorption column" is meant any elongated structure having a body, at least one inlet and at least one outlet.

By the term "depressurising" is meant that a reduction in pressure, at a given temperature, occurs.

By the term "hydrocarbon" is meant a combination of different hydrocarbons, i.e. to a combination of various types of molecules that contain carbon atoms and, in many cases, attached hydrogen atoms. A "hydrocarbon" may comprise a large number of different molecules having a wide range of molecular weights. Generally at least 90 % by weight of the hydrocarbon mixture consists of carbon and hydrogen atoms. Up to 10% by weight may be present as sulfur, nitrogen and oxygen as well as metals such as iron, nickel and vanadium (i.e. as measured sulfur, nitrogen, oxygen or metals).

By the term "heavy hydrocarbon" is meant a hydrocarbon comprising a greater proportion of hydrocarbons having a higher molecular weight than a relatively lighter hydrocarbon mixture. Terms such as "light", "lighter", "heavier" etc. are to be interpreted herein relative to "heavy".

DESCRIPTION OF INVENTION

The method and system of the present invention are advantageous because they utilise brine, e.g. sea water, which is available in abundance to capture an acidic gas from gases to be treated. Preferably the acidic gas is CO2. The method and system of the invention are also advantageous because the captured acidic gas, e.g. 002, is subsequently purified and then utilised in hydrocarbon recovery. The methods and system of the invention therefore represent a carbon capture and utilisation strategy.

The gas treated in the methods and systems of the present invention may be any gas comprising an acidic gas. Typically the gas treated in the present invention is an exhaust gas or a fuel gas. Representative examples of exhaust gases are flue gases (e.g. produced during the generation of electricity in gas or coal power plants and steam generation) and gases from industrial manufacturing processes (e.g. cement production). Another type of exhaust gas comprising acidic gas, e.g. 002, is the gas produced during hydrocarbon recovery operations, especially when in situ combustion or enhanced recovering techniques using 002 are employed.

Representative examples of fuel gases include natural gas (e.g. sour gas or acidic gas) and syngas (e.g. from reforming processes or from goal gasification (e.g. underground coal gasification)). When the gas treated is an exhaust gas, the acidic gas-depleted gas is typically released to the atmosphere following treatment. When the gas treated is a fuel gas, the acidic gas-depleted gas is typically routed to a plant for further treatment or sent for storage prior to use.

The total concentration of acidic gas in the gas to be treated varies but is typically in the range 1-80 mol%, e.g. 5-50 mol %. The concentration of CO2 in the gas to be treated is typically in the range 1-80 mol %, e.g. 5-50 mol%. The concentration of acidic gas in the gas to be treated depends on, e.g. whether it is an exhaust gas or a fuel gas. Some preferred values for different types of gases are set out in the table below.

Type of Source [002] [H2S] Total [acidic gas] gas (mol%) mol% mol% Exhaust Flue gas from gas power 3-5 <1 3-5 gas plant Exhaust Flue gas from coal power 10-15 <1 10-15 gas plant Exhaust Gas from cement 18-25 <1 18-25 gas production Exhaust Gas from steam generator 6-10 <1 6-10 gas Fuel gas Sour gas 5-40 1-40 5-80 Fuel gas Acid gas 3-80 <1 3-80 Fuel gas Syngas from coal 20-50 <1 20-50 gasification Fuel gas Syngas from natural gas 20-30 <1 20-30 reforming The gas may be treated or untreated prior to carrying out the methods of the present invention. For instance the gas may be compressed prior to carrying out the method of the invention. Alternatively or additionally the gas may be cooled prior to carrying out the method of the invention. In preferred processes, an exhaust gas is compressed and/or cooled. Fuel gases are typically at higher pressures than exhaust gases so are less likely to require compression. Nevertheless in some cases a fuel gas may be compressed and/or cooled prior to carrying out the method of the invention.

In preferred methods of the invention the brine used in step (i) derives from the sea and/or a subterranean formation. If the brine derives from the sea, preferably the brine derives from a deep sea location, e.g. over 500 m or more preferably over 750 m in depth. More preferably, however, the brine derives from a subterranean formation.

In preferred methods of the invention the brine used in step (i) has a salinity of 3 to 20 % by weight, more preferably 3.2 to 15.0 % by weight. Preferably the brine used in step (i) has a composition as described in the table below.

Element Preferred % wt More preferred % wt Chlorine 1.5-12.0 1.6-10.0 Sodium 1.0-7.0 1.08-6.80 Magnesium 0.040-0.150 0.045-0.130 Calcium 0.030-0.700 0.035-0.650 Potassium 0.035-0.800 0.039-0.700 Preferably the brine used in step (i) has a density of 1.020 to 1.150kg/rn3, still more preferably 1.020 to 1.120 kg/m3, yet more preferably 1.020 to 1.100 kg/m3.

Brines from deep sea locations, and particularly from subterranean formations, tend to have relatively high salinities. This improves the removal of acidic gas, e.g. CO2 from the gas during the contacting step. Moreover in some preferred embodiments of the method and system of the invention a portion of acidic gas-rich brine is pumped into a subterranean formation and in this case there are a number of further advantages realised from the use of brine from a formation. This is discussed further below.

The brine may be removed from the formation by pumping using conventional equipment. The brine may be stored in tanks prior to use in the methods and systems of the invention or it may be used directly. More preferably it is stored. Depending on the gas to be treated or cleaned in the method of the invention, the brine may be cooled and/or pressurised prior to use. Cooling and pressurisation may be carried out using conventional equipment. In some cases it may be preferable to add corrosion inhibitors and/or scale inhibitors to the brine to protect the equipment used in the methods of the present invention. More preferably, however! the brine is untreated prior to contact with the gas. Particularly preferably cations (e.g. divalent cations) are not added to the brine. Particularly preferably no constituent of the brine is removed prior to contact with the gas.

In the methods of the present invention the gas comprising an acidic gas is contacted with brine to produce an acidic gas-rich brine and an acidic-gas depleted gas stream. In preferred methods of the invention, the acidic gas-rich brine produced in step (i) is a solution. In further preferred methods of the invention, the acidic gas-rich brine produced in step (i) does not comprise solids.

The contacting may be carried out in any mass transfer device known in the art.

When the gas enters the contacting device, the gas is preferably at a temperature of 5- °C and more preferably 5-15 °C. The pressure of the gas on entry to the contact vessel is preferably 50-200 barg, more preferably 80-1 50 barg. The brine will typically be supplied to the contact vessel at a temperature of 5-40 00 and more preferably 5-15 "0. The pressure of the brine on entry to the contact vessel is preferably 50-200 barg, more preferably 80-150 barg. In preferred methods of the invention no chemicals are added during the contacting step.

Preferably the contacting is carried out in an absorption tower, a horizontal tunnel comprising treatment sections or in a channel used to transport the gas to the acidic gas capture plant.

Preferably the contacting is in at least one absorption column. The column may be any shape, e.g. cylindrical or oblong. Absorption columns are well known in the alt and any conventional absorption column may be used. Absorption columns are generally used in a vertical orientation.

In some preferred methods and systems of the invention the contacting is in one absorption column. In this case the acidic gas-rich brine produced in the method is preferably pumped straight to the means for depressurising and the acidic gas-depleted gas is preferably sent to storage for future use as a fuel in the case of fuel gas or released to the atmosphere in the case of exhaust gas. Alternatively the acidic gas- rich brine may be transported to the means for depressurising and the acidic gas-depleted gas stream recycled to the inlet of the column wherein it is contacted with further brine. The latter may be done, for example, if the acidic gas-depleted gas still contains too high a concentration of acidic gas for sending to storage or release to the atmosphere.

In other preferred methods and systems, however, the contacting is in a plurality of absorption columns connected in series. In this case the acidic gas-rich brine is preferably transported to the means for depressurising but the acidic-gas depleted gas is supplied to the inlet of a second absorption column wherein it is contacted with further brine. The acidic gas-rich brine is again preferably transported to the means for depressurising. The acidic gas-depleted gas may be sent to storage for future use as a fuel, released to the atmosphere or supplied to the inlet of a third absorption column. There is no limit on the number of absorption columns. This may be, for example, 2, 3, 4 or 5. The columns may have the same or different structures.

The columns may be operated under the same or different conditions. The skilled man will readily be able to ascertain the optimal working arrangement.

In the methods and systems of the present invention, the gas and the brine may be in cocurrent or countercurrent flow in the column. Preferably, however, the gas and the brine are in countercurrent flow in the column.

The structure and operation of the absorption column used in the methods and systems of the present invention are conventional and as described in the prior art.

Thus the average temperature and pressure of the absorption column will be typical in the art. Usually the column will comprise collection trays and packing as is conventional in the art.

The method of the invention preferably involves introducing a gas to be treated into the bottom of an absorption column wherein the gas flows upwards and countercurrent to a flow of brine in one or more contact sections of the column. The contact sections may optionally comprise a structural or random packing to increase the contact area between the brine and the gas. After leaving the contact sections the acidic gas-depleted gas is preferably washed and is then withdrawn through a line at the top of the column. Alternatively, after leaving the contact sections, the acidic gas-depleted gas may be recycled to the bottom of the absorption column or to another absorption column wherein it is contacted with further brine.

The brine, rich in acidic gas, is typically collected (e.g. on collection trays) and withdrawn by a line at the bottom of the column. It is then fed to a storage tank for subsequent pumping to the means for depressursing or is pumped directly to the means for depressurising.

In other preferred methods and systems of the present invention the contacting is in a horizontal tunnel comprising treatment sections. A suitable horizontal tunnel for use in the methods and systems of the invention is described in W02008/1 56374, the entire contents of which are hereby incorporated by reference. The horizontal tunnel preferably comprises an absorption section and a cleaning section. The absorption section optionally comprises filler. The tunnel also preferably comprises an inlet for gas to be treated into the tunnel structure and downstream of the absorption and cleaning sections, an outlet for acidic gas-depleted gas. Preferably the outlet for acidic gas-depleted gas is connected to a heat exchanger. Preferably the horizontal tunnel further comprises a cooling section downstream and/or upstream of the absorption section. Preferably the tunnel comprises one or more outlets for acidic gas-rich brine.

The geometry of the horizontal tunnel is not restricted and its cross section may be any shape, e.g. circular, square, rectangular or oval. The tunnel may be linear or may comprise curves or bends. The cross-section may be any size but advantageously is large enough to provide relatively low gas velocities through the tunnel. Preferably the velocity of gas to be treated through the tunnel is in the range 1 to 10 mIs, more preferably 2 to 7 mIs and still more preferably 2 to 5 mIs.

In the method of the invention utilising a horizontal tunnel the gas to be treated is preferably fed into the essentially horizontal tunnel in a horizontal flow. The gas is optionally cooled and then contacted with brine. The brine absorbs acidic gas, e.g. C02, from the flow to produce an acidic gas-depleted gas and an acidic gas-rich brine.

The acidic gas-depleted gas is cleansed in the cleansing section. Optionally the acidic-gas-depleted gas is cooled. The acidic gas-rich brine is preferably collected at the bottom of the tunnel. The acidic gas-rich brine is preferably fed to a storage tank for subsequent pumping to the means for depressurising or is pumped directly to the means for depressurising.

In the cooling section, water with a temperature below the desired gas temperature is preferably sprayed as droplets onto the gas stream flowing horizontally therethrough. The water droplets absorb heat from the gas as they fall through the stream. The cooled gas flows horizontally from the cooling section into the absorbing section. Preferably means are provided for collecting the water at the bottom of the tunnel. The water used in the cooling section may be fresh water or may be brine.

When brine is used, preferably brine from the same source as the brine used in the contacting step described above is used.

In the absorbing section, the brine is preferably brought into contact with the gas flow. Preferably the brine is sprayed onto the gas flow using nozzles. The brine may be allowed to partly follow the horizontal gas stream as its droplets fall to the bottom of the tunnel. Alternatively the absorption section may include filler whereon the absorbent forms a liquid film. This increases the contact surface between the brine and the gas phases. Preferably means are provided for collecting the acidic gas-rich brine at the bottom of the tunnel. The brine is then fed to a storage tank for subsequent pumping to the means for depressurising or is pumped directly to the means for depressurising.

To remove brine droplets and prevent them from being transported with the gas into the next section, the gas preferably passes through a demister before leaving the absorption section. The demister collects the droplets of brine and directs them to the reservoir from where it is pumped to the means for depressurising.

The acidic gas-depleted gas flows horizontally into the next section of the tunnel structure where the gas is preferably washed or cleansed. The exact cleansing process used will depend on the source of the gas to be treated, whether the gas is to be used or released to the atmosphere and any restrictions associated therewith. The cleansing is preferably performed in the same manner as cooling. Thus the cleansing fluid is sprayed onto the gas flow and allowed to fall through the horizontal gas stream.

Preferably the acidic gas-depleted gas is then passed through a heat exchanger.

In the cooling, absorbing and cleaning sections, the liquid is preferably sprayed using spray nozzles that are arranged on any side of the tunnel wall, or within the tunnel, and the nozzles may direct droplets in any direction. The droplets may have a counter-current, co-current or orthogonal direction compared to the horizontal gas flow.

The nozzles are preferably selected to provide droplets of a size adapted to the velocity of the gas flow, e.g. to allow the droplets to follow the gas stream for a while before settling at the bottom of the tunnel.

In another preferred method and system of the invention the contacting is carried out in a part of the transport channel used for transporting gas to be treated to the acidic gas capture plant. Suitable systems for use in the method of the invention are described in W0201 1/005116, the entire contents of which are hereby incorporated by reference.

The transport channel preferably comprises an absorption zone wherein contact is achieved by spraying liquid droplets of brine into the transport channel itself. The channel also comprises an outlet for acidic gas-depleted gas and at least one outlet for acidic gas-rich brine. The transport channel may have an angle of between 0 and 60° but is preferably horizontal. The brine is preferably sprayed with nozzles that direct the spray mainly in the flow direction of the gas being transported thus pushing it along the transport channel.

Optionally the transport channel further comprises a cooling zone upstream of the capture zone. Thus gas to be treated preferably passes through the cooling zone prior to entry to the capture zone. Optionally the transport tunnel further comprises a cleaning zone. Preferably this is downstream of the capture zone.

In the method and system of the invention utilising a transport channel, the gas to be treated enters the transport channel that would normally be void of processing equipment but which would lead to the acidic gas capture plant. Preferably shortly after entry into the transport channel, the gas is sprayed with cooling water in a cooling zone. Preferably the pressure of the cooling water is in the range 5-100 bars, e.g. 5-10 bars. This may be achieved using a pump before it is sprayed through nozzles. The cooling water preferably serves to push the gas through the transport channel. The cooling water is collected. Optionally brine may be used as the cooling water and in this case the collected brine is fed to a storage tank for subsequent pumping to the means for depressurising or is pumped directly to the means for depressurising.

The cooled gas preferably enters the absorption zone of the transport channel where it is sprayed with brine via nozzles. The brine is sprayed mainly in the flow direction of the gas and with a speed high enough to compensate for any pressure loss experienced in the first zone of the channel. The brine droplets move through the gas stream and absorb acidic gas therefrom. The acidic gas-rich brine is collected upstream, preferably by the use of a demister, and fed to a storage tank for subsequent pumping to the means for depressurising or is pumped directly to the means for depress u II SI fl g.

The gas continues downstream in the channel and may optionally enter a second absorption zone wherein it is sprayed with further brine. Again the brine is preferably sprayed from nozzles and with a speed that pushes the gas through the channel. The brine droplets are collected upstream and pumped to a subterranean formation for storage. The number of stages needed for absorption is a trade-off against the brine flow. Typically 2 or 3 zones will be present.

The acidic gas-depleted gas preferably enters a cleaning zone wherein the gas may be washed with water and/or other cleaning fluid. The cleaning fluid is sprayed via nozzles. The cleaned acidic gas-depleted gas may then be released, sent for further treatment or for storage prior to use.

The acidic gas-rich brine produced in the contacting step preferably comprises 1-8 mol % acidic gas, still more preferably 2-7 mol % acidic gas, e.g. 4-6 mol% acidic gas. The acidic gas-rich brine transported for depressurisation therefore preferably comprises 1-8 mol % 002, still more preferably 2-7 mol % 002, e.g. 4-6 rnol% 002.

Correspondingly the acidic gas-depleted gas preferably comprises 0-50%, still more preferably 0-20 % and yet more preferably 0-10 % of the acidic gas present in the original gas. The acidic gas-depleted gas preferably comprises 0-20 mol% acidic gas, still more preferably 0-15 mol% acidic gas, e.g. 0-10 mol % acidic gas. The acidic gas-depleted gas preferably comprises 0-20 mol % 002, still more preferably 0-15 mol % 002, e.g. 0-10 mol% 002. Generally this gas is sent for further processing, to storage for future use as fuel or released to the atmosphere. The amount of acidic gas present in the acidic gas-depleted gas partially depends on the gas treated in the method of the invention. The table below shows preferred levels of acidic gas present in acidic gas-depleted gas preferably achieved in the methods of the invention.

Source [002] [H2S] [002] mol% [H2S] mol% (mol%) mol% After After cleaning cleaning Flue gas from gas power 3-5 ci 0.3-1 <0.01 plant Flue gas from coal power 10-15 <1 1-2 <0.01 plant Gas from cement production 18-25 <1 1-5 <0.01 Gas from steam generator 6-10 <1 0.5-2 <0.01 Sour gas 5-40 1-40 0.05-3 C 4 ppm Acid gas 3-80 <1 0.05-3 C 4 ppm Syngas from coal gasification 20-50 <1 0.5-10 <4 ppm Syngas from natural gas 20-30 <1 0.5-10 <4 ppm reforming Some preferred methods of the invention further comprise pumping a portion of the acidic gas-rich brine into a subterranean formation for storage. Such methods are useful, for example, when more acidic gas than is required for hydrocarbon recovery is being produced. These methods preferably further comprise the steps of: (ia) dividing the acidic gas-rich brine into at least a first portion and a second portion; (ib) pumping the first portion of acidic gas-rich brine into a subterranean formation for storage; and (U) depressurising the second portion of acidic gas-rich brine to obtain pure acidic gas and brine.

Thus a further preferred method of the present invention comprises: (i) contacting a gas comprising an acidic gas with brine to produce an acidic gas-rich brine and an acidic gas-depleted gas; (ia) dividing the acidic gas-rich brine into at least a first portion and a second portion; (ib) pumping the first portion of acidic gas-rich brine into a subterranean formation for storage; (U) depressurising the second portion of acidic gas-rich brine to obtain pure acidic gas and brine and (iii) conducting hydrocarbon recovery using said pure acidic gas.

In preferred methods of the invention at least some of said brine used in step (i) derives from the subterranean formation into which said acidic gas-rich brine is pumped in step (ib). Still more preferably substantially all of the brine (e.g. all of the brine) used in step (i) derives from the subterranean formation into which said acidic-gas rich brine is pumped. Preferred methods of the invention therefore comprise the steps: (ai) obtaining brine from a subterranean formation; (i) contacting a gas comprising an acidic gas with brine to produce an acidic gas-rich brine and an acidic gas-depleted gas; (ia) dividing the acidic gas-rich brine into at least a first portion and a second portion; (ib) pumping the first portion of acidic gas-rich brine into the subterranean formation for storage; (ii) depressurising the second portion of acidic gas-rich brine to obtain pure acidic gas and brine; and (Di) conducting hydrocarbon recovery using said pure acidic gas.

This is highly beneficial for at least three reasons. First by extracting brine from a subterranean formation, the volume and space available for storage of acidic gas-rich brine is increased, thus increasing storage capacity. Second by pumping at least some of the acidic gas-rich brine back into the formation from which it derives the overall pressure in the formation is kept more constant and the acidic-gas therefore remains dissolved in the brine. Third using brine from the formation to be used for storage, ensures that the acidic gas-rich brine pumped into the formation has a higher density than the formation water and will therefore migrate to the bottom of the reservoir. This prevents the acidic gas, e.g. CO2 from migrating, i.e. it is effectively stored in place by solubility trapping. The CO2 is physically dissolved in the brine and can only escape therefrom by a decrease in the pressure in the formation or by an increase in its temperature. Such changes are, however, unlikely to occur and/or can be prevented from happening. This means that the acidic gas, e.g. CO2. will be significantly more safe compared to state-of-the-art methods wherein significant amounts of CO2 migrate to the top of the formation.

In preferred methods of the invention the acidic gas-rich brine pumped into the subterranean formation in step (ib) is a solution. In further preferred methods of the invention the acidic gas-rich brine pumped into the subterranean formation in step (ib) does not comprise solids. In particularly preferred methods of the invention, the acidic gas-rich brine is untreated prior to pumping into a subterranean formation for storage.

Thus preferably a portion of the acidic gas-rich brine produced in step (i) is directly pumped into a subterranean formation for storage.

In preferred methods and systems, the brine is pre-pumped out of the formation and stored in tanks prior to use. Alternatively, however, the brine may be pumped out of the formation at the same time as acidic-gas rich brine is being pumped into it. In this case the brine being pumped out of the formation is pumped from a relatively higher position (i.e. lesser depth) in the reservoir to the depth at which acidic gas-rich brine is being pumped into the reservoir. This avoids and minimises any mixing of the b ri n es.

Preferably the subterranean formation into which the acidic gas-rich brine is pumped is offshore. Still more preferably the subterranean formation into which the acidic gas-rich brine is pumped is a formation at a depth of at least 800 metres! more preferably at least 1000 metres. Deep subterranean formations are preferred as the solubility of acidic gases such as 002 in brine increases with increasing depth and pressure. As a result of the increased concentration of acidic gas, e.g. 002, in the brine it will generally be more dense than the brine present in the formation into which it is pumped. This is advantageous since it ensures that the acidic-gas rich brine is substantially stored at the bottom of said subterranean formation. The density of the acidic gas-rich brine is preferably at least 2%, still more preferably at least 4%, and yet more preferably at least 7% higher than the density of the original brine.

In further preferred methods of the invention the acidic gas-rich brine is untreated prior to depressurising. Thus preferably no chemicals are added to the brine prior to depressurising. Preferably no constituents are removed from the brine prior to depressurising.

In further preferred methods of the invention, depressurising is carried out in a flash unit. In the flash unit, the acidic gas, e.g. 002, is removed from the brine. The flash unit preferably comprises a valve, a flash drum or pipe and optionally a receiving vessel. In preferred flash units the flash drum or pipe is heated. The valve present in the flash unit is preferably a throttling valve. This enables the pressure of acidic gas-rich brine entering the flash unit to be reduced. The reduction in pressure causes the majority of the acidic gas dissolved in the brine to separate out in a vapour phase which is removed from the flash unit via a vapour outlet. The skilled man will readily be able to determine the necessary pressure reduction. The vapour outlet preferably transports the purified acidic gas to a receiving vessel.

In preferred flash units the flash drum or pipe is heated, e.g. by steam or water.

The temperature is selected according to the composition of the acidic gas-rich brine so that the acidic gas present in the brine is all essentially evaporated. Again the skilled man is readily able to determine a suitable temperature.

In preferred methods of the invention the pure acidic gas, e.g. 002 gas obtained in step (ii) has a purity of at least 99 %, more preferably at least 99.5 % and still more preferably at least 99.9 %.

In the methods of the invention the acidic gas is used in hydrocarbon recovery, preferably enhanced oil recovery or enhanced gas recovery. A preferred enhanced oil recovery technique is carbon dioxide flooding. In this technique 002 is injected into a subterranean formation to increase the output of hydrocarbon. Representative examples of CO2 based enhanced oil recovery techniques are gas injection, miscible flooding, supercritical CO2 flooding and in situ combustion. In gas injection the CO2. is injected into the reservoir to increase reservoir pressure and thereby increase the drive of hydrocarbon out of the reservoir. Miscible injection is a form of gas injection wherein the CO2 is miscible with the fluids, e.g. hydrocarbon, present in the reservoil. It improves oil recovery because it incleases reservoil pressuie and it impioves hydrocarbon displacement because the interfacial tension between hydrocarbon and water is reduced. The use of supercritical CO2 is a specific form of miscible injection.

In the supercritical state, more CO2 mixes with the hydrocarbon, reduces its viscosity and reduces its surface tension with the foimation. The pure C02, obtained in the method of the present invention may also be used in in situ combustion as a carrier gas, i.e. as a carrier gas for an oxidant such as 02 or air.

In a paiticulaily preferied method of the invention wherein the acidic gas is C02, the method further comprises mixing the pure CO2 with C02-rich brine to produce supersaturated brine. Such methods of the invention further comprise: (iib) mixing said pure 002 with C02-iich biine to produce brine that is super saturated with CO2. Paiticularly piefelably the hydrocarbon recovery, e.g. flooding, is conducted with the CO2 super saturated brine.

In preferred methods of the invention the brine produced in step (U) is recycled foi contacting with further gas.

The methods of the piesent invention may be operated continuously oi batchwise. Preferably the methods of the invention are operated continuously.

The methods of the present invention may be carried out using a system as heieinbefore described. In preferred systems the contact vessel is an absoiption column. In further preferred methods! the means for supplying said gas supplies said gas to the bottom of said contact vessel. In yet further preferred methods the means for supplying said brine supplies said brine to the top of said contact vessel.

In other preferred systems of the invention, the contact vessel is a horizontal tunnel comprising treatment sections oi a transpoit channel for transpoiting gas to be treated to an acidic gas capture plant.

Preferred systems comprise a means for cooling said brine prior to its introduction into the contact vessel. Further preferred systems comprise a means for compressing said brine prior to its introduction into the contact vessel.

Preferred systems of the invention further comprise a means to monitor the acidic gas content of said acidic-gas depleted gas. This is advantageous as it can be used to ensure that only gas having a sufficiently low level of acidic gas, e.g. C02, is released to the atmosphere or sent for storage prior to use as a fuel. If an acidic gas-depleted gas comprises a higher than desirable level of acidic gas, the gas can be routed back to the contact vessel or to a different contact vessel. Preferred systems of the invention therefore comprise a plurality of contact vessels connected in series.

In preferred systems of the invention the means for depressurising the acidic gas-rich brine is a flash unit or flash tank. The flash unit preferably comprises a valve, a flash drum or pipe and optionally a receiving vessel. In preferred flash units the flash drum or pipe is heated. The valve present in the flash unit is preferably a throttling valve.

The means for transporting the pure acidic gas into a subterranean formation for hydrocarbon recovery is fluidly connected to the outlet for pure acidic gas of the means for depressurising. The means is preferably conventional piping. The pure acidic gas may be pumped directly into a subterranean formation for hydrocarbon recovery but is more preferably pumped to a holding tank until it is required.

Preferred systems of the invention further comprise a mixing tank having an inlet for pure acidic gas fluidly connected to the outlet for pure acidic gas of the means for depressurising, an inlet for acidic gas rich brine and an outlet for acidic gas supersaturated brine. In such systems the inlet for acidic gas rich brine of the mixing tank is preferably fluidly connected to the outlet for acidic gas rich brine of the contact vessel. Thus further particularly preferred systems of the invention comprise: (f) a means for dividing the acidic gas-rich brine into at least a first portion and a second portion; (g) a means for transporting the first portion of the acidic gas-rich brine into a subterranean formation for storage; (h) a means for transporting the second portion of acidic gas-rich brine to the means for depressurising.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described with reference to the following non-limiting

examples.

Figure 1 shows a schematic diagram of a preferred method of the invention; Figure 2 shows a schematic diagram of a further preferred method of the invention; and Figure 3 shows a schematic diagram of a yet further preferred method of the invention.

DETAILED DESCRIPTION OF THE FIGURES

Referring to Figure 1 it shows a preferred method of the invention. Flue gas 100, generally cooled and compressed, is introduced at the bottom of the absorption column 101. From the inlet 102, the gas flows upwards in the absorption column and countercurrent to brine, in one or more contact sections, 103, 104. The brine is from the subterranean formation 115.

The illustrated absorption column is provided with two serially connected contact sections 103, 104 but any number of such sections may be included. After leaving the contact sections 103, 104 the gas is washed by a countercurrent flow of water in a washing section 105 to remove any impurities in the gas flow. Washing water is introduced via line 106 and sprayed at the top of the washing section 105. The water is collected at plate 107 below the washing section and removed via line 108.

CO2 depleted gas is withdiawn through line 109. The column 101 may comprise several water sections or other types of polishing/washing sections in the upper pad.

Typically 80-99 % of the original CO2 present in the gas is removed by absorption.

Brine is introduced into the column via line 110 and is sprayed on top of the upper contact section 104 by means of liquid distribution means. The brine flows through the upper contact section 104 and is collected at a plate 111. The brine is withdrawn via line 112 and sprayed at the top of the lower contact section 103. After flowing through the lower contact section 103 the brine and the CO2 absorbed thereto is collected from the bottom of the column and withdrawn via line 113.

The temperature of the brine in the absorption step is generally from about 5 to °C, for example from 10 to 15 °C. The overall pressure in the absorption step is generally from about 50 to 200 barg, preferably from about 80 to 150 barg.

The CO2 rich brine collected at the bottom of the column is then withdrawn to flash unit 114. The flash unit 114 comprises a throttling valve and a heated flash drum.

The throttling valve enables the pressure of C02-rich brine entering the flash unit to be reduced. The reduction in pressure and increase in temperature causes the majority of the CO2 gas dissolved in the brine to separate out in a vapour phase as pure CO2 which is removed from the flash unit via a vapour outlet connected to line 118. The 002 is optionally transported to a storage tank (not shown). Once the level of 002 in the storage tank reaches a certain level, it is pumped to a subterranean formation for hydrocarbon recovery. In Figure 1 hydrocarbon recovery is shown as occurring in the same formation from which brine is obtained. This is not necessary and indeed is unlikely to be the case.

The method and system of the present invention has a number of advantages as follows: -The CO2 capture process is simple and cost efficient -The brine may be extracted from the sea and/or a formation and is a plentiful and cheap solvent -The CO2 purification process is simple and cost efficient -The CO2 may be used on site or at a nearby site -The amount of 002 released to the atmosphere is reduced.

Referring to Figure 2, it shows a further preferred method of the present invention. Those features of the method shown in Figure 1 that are also present in Figure 2 are represented by the same reference numerals and are not further described. The main difference in Figure 2 is that the C02-rich brine that is produced in the contact vessel 101 is divided into a first and second portion in line 113. The first portion is transported to flash unit 14 wherein pure 002 and brine are produced. The brine is removed via line 116 and is optionally recycled to the absorption column, i.e. recycled into line 110 (not shown). The pure 002 is transported to a storage vessel via line 119. The second portion of 002 rich brine is transported into the subterranean formation 115 for storage. The CO2 rich brine is denser than the formation water present and therefore sinks to the bottom of the reservoir where the 002 is safely stored.

Referring to Figure 3, it also shows a further preferred method of the present invention. Again those features of the method shown in Figure 1 that are present in Figure 3 are represented by the same reference numerals and are not further described. The main difference in Figure 3 is that the pure 002 gas produced in flash unit 114 is transported to a mixing tank 120. A portion of 002 rich brine is also transported to the mixing tank 120 via line 121. Thus CO2 super saturated brine is produced. The CO2 super saturated brine is pumped into a subterranean formation for hydrocarbon recovery via line 122. In Figure 3 hydrocarbon recovery is shown to be occurring in the same formation from which brine is obtained. This is not necessary and indeed is unlikely to be the case.

Claims

CLAIMS: 1. A method of enhanced hydrocarbon recovery, wherein said recovery utilises captured acidic gas, comprising: (i) contacting a gas comprising acidic gas with brine to produce an acidic gas-rich brine and an acidic gas-depleted gas; (ii) depressurising said acidic gas-rich brine to obtain pure acidic gas and brine; and (Di) conducting hydrocarbon recovery using said pure acidic gas.
2. A method as claimed in claim 1, wherein said acidic gas is CO2.
3. A method as claimed in claim 1 or 2, wherein said gas is an exhaust gas or a fuel gas.
4. A method as claimed in any preceding claim, wherein said brine derives from a subterranean formation.
5. A method as claimed in any preceding claim, wherein said brine is untreated prior to contact with said gas.
6. A method as claimed in any preceding claim, wherein said contacting is in at least one absorption column.
7. A method as claimed in claim 6, wherein said contacting is in a plurality of absorption columns connected in series.
8. A method as claimed in any one of claims 1 to 5, wherein said contacting is in a horizontal tunnel comprising treatment sections.
9. A method as claimed in any one of claims 1 to 5, wherein said contacting is in a part of a transport channel for transporting gas to be treated to an acidic gas capture plant.
10. A method as claimed in any preceding claim, further comprising: (ia) dividing said acidic gas-rich brine into at least a first portion and a second portion; (ib) pumping said first portion of acidic gas-rich brine into a subterranean formation for storage; and (ii) depressurising said second portion of acidic gas-rich brine to obtain pure acidic gas and brine.
11. A method as claimed in claim 10, wherein at least some of said brine used in step (i) derives from the subterranean formation into which said acidic gas-rich brine is pumped in step (ib).
12. A method as claimed in claim 10 or 11, wherein said acidic gas-rich brine is untreated prior to pumping to said subterranean formation.
13. A method as claimed in any preceding claim, wherein said acidic gas-rich brine is untreated prior to depressurising.
14. A method as claimed in any preceding claim, wherein said depressurising is carried out in a flash unit.
15. A method as claimed in any preceding claim, wherein said pure acidic gas produced by depressurising has a purity of at least 99 %.
16. A method as claimed in any preceding claim, wherein said hydrocarbon recovery is enhanced oil recovery.
17. A method as claimed in any one of claims 1 to 15, wherein said hydrocarbon recovery is enhanced gas recovery.
18. A method as claimed in any one of claims 2 to 17, further comprising: (iib) mixing said pure CO2 with C02-rich brine to produce CO2 super saturated brine.
19. A method as claimed in claim 18, wherein said hydrocarbon recovery is conducted with said 002 super saturated brine.
20. A method as claimed in any preceding claim, wherein said brine produced in step (ii) is recycled for contacting with further gas.
21. A method as claimed in any preceding claim which operates continuously.
22. A system for enhanced hydrocarbon recovery, wherein said recovery utilises captured acidic gas, comprising: (a) a means for supplying a gas comprising acidic gas to a contact vessel; (b) a means for supplying brine to said contact vessel; (c) a contact vessel for contacting said gas with said brine to produce an acidic gas-rich brine and an acidic gas-depleted gas having an inlet for said gas, an inlet for said brine, an outlet for said acidic gas-depleted gas and an outlet for said acidic gas-rich brine; (d) a means for depressurising said acidic gas-rich brine having an inlet fluidly connected to said outlet for acidic gas-rich brine of said contact vessel, an outlet for brine and an outlet for pure acidic gas; and (e) a means for transporting said pure acidic gas into a subterranean formation for hydrocarbon recovery fluidly connected to said outlet for pure acidic gas of said means for depressurising.
23. A system as claimed in claim 22, wherein said contact vessel is an absorption column.
24. A system as claimed in claim 22, wherein said contact vessel is a horizontal tunnel comprising treatment sections.
25. A system as claimed in claim 22, wherein said contact vessel is a pad of a transport channel for transporting gas to be treated to an acidic gas capture plant.
26. A system as claimed in any one of claims 22 to 25, wherein said means for depressurising is a flash unit
27. A system as claimed in any one of claims 22 to 26, further comprising a means to monitor the acidic gas content of said acidic-gas depleted gas.
28. A system as claimed in any preceding claim, further comprising a mixing tank having an inlet for pure acidic gas fluidly connected to said outlet for pure acidic gas of said means for depressurising, an inlet for acidic gas-rich brine and an outlet for acidic gas supersaturated brine.
29. A method as claimed in claim 28, wherein said inlet for brine of said mixing tank is fluidly connected to said outlet for acidic gas rich brine of said contact vessel.
30. A system as claimed in any one of claims 22 to 29, further comprising: (f) a means for dividing said acidic gas-rich brine into at least a first portion and a second portion; (g) a means for transporting said first portion of said acidic gas-rich brine into a subterranean formation for storage; (h) a means for transporting said second portion of acidic gas-rich brine to said means for depressurising.