FIELD OF THE INVENTION

This invention relates to a method for the recovery of hydrocarbons from a natural gas subsurface reservoir with nitrogen injection, whereby the nitrogen is prepared in a chemical looping combustion reactor. It also relates to the method wherein a combination of nitrogen with CO ₂ is used for injection in the reservoir. The invention also relates to an integrated production system with nitrogen injection and a chemical combustion unit.

BACKGROUND OF THE INVENTION

Chemical looping combustion (CLC)

Chemical looping combustion of a fuel, e.g. a fossil fuel, with separation of CO ₂ for capture and storage is known. From a paper entitled "Effect of gas composition in Chemical-Looping Combustion with copper-based oxygen carriers: Fate of sulphur", by Forero et al in

International Journal of Greenhouse Gas Control 4 (2010) 762-770 it is known that CLC is an emerging technology for CO ₂ capture because separation of this gas from the other flue gas components (nitrogen in particular) is inherent to the process and thus no energy is wasted for the separation. It is indicated that natural or refinery gas can be used as gaseous fuels and they may contain different amounts of sulphur compounds, such as ¾S and COS. This paper by Forero et al presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of CH ₄ and ¾S as fuel. The influence of ¾S concentration on the gas product distribution and

combustion efficiency, sulphur splitting between the fuel reactor (FR) and the air reactor (AR) , oxygen carrier deactivation and material agglomeration was investigated in a continuous CLC plant (500Wth) . The oxygen carrier to fuel ratio was the main operating parameter affecting the CLC unit. Complete fuel combustion were reached at 1073 K working at fuel ratio values ≥ 1.5.

In another paper, by Tobias Proll et all, "Syngas and a separate nitrogen/argon stream via Chemical Looping Reforming - A 140 kW pilot plant study", in Fuel 89 (2010) 1249-1256, the production of synthesis gas, by reforming of methane or other light hydrocarbons of fossil origin is discussed. A dual circulating fluidized bed pilot plant was operated in chemical looping

reforming conditions at a scale of 140 kW fuel power with natural gas as fuel. A nickel-based oxygen carrier was used as bed material. In this paper it was noted that the oxygen in the air reactor can be completely absorbed by the solids as soon as the air reactor operating

temperature is higher than 900 °C (1173 K) .

Enhanced Gas Recovery (EGR)

Enhanced Gas Recovery (EGR) is a gas-gas displacement process for nearly depleted natural gas reservoirs by injecting e.g. waste gases. EGR by gas-gas displacement is seen as a promising way of prolonging the productive life and economic recovery of nearly depleted gas reservoirs. Nitrogen containing less than 10 ppm oxygen and/or CO ₂ containing less than 10 ppm oxygen, as well as flue gases (mixtures of CO ₂ , N ₂ , Ar and other trace compounds present in air) containing less than 10 ppm oxygen, can be used as injection gas, to purge the reservoirs .

When producing natural gas using EGR, at some point in time the gas that is injected will reach the

production well, thus diluting the produced natural gas stream. Typically up to 20% nitrogen can be tolerated in the produced stream when producing natural gas for pipeline gas. Only about 2% CO ₂ can be tolerated in the produced stream due to contractual limitations for pipeline gas. Hence, nitrogen is a preferred injection gas. EGR with CO2 has the advantage of CO2 storage (a greenhouse gas) . However, there may be operational problems when using CO2 associated with the corrosive nature of CO2 in the presence of water.

Nitrogen gas is commonly used in industry. It is typically produced by the fractional distillation of liquid air, or by mechanical means using gaseous air (i.e., pressurized reverse osmosis membrane or Pressure swing adsorption) . Commercial nitrogen is often a

byproduct of air-processing for industrial concentration of oxygen for steelmaking and other purposes. When supplied compressed in cylinders it is often called OFN (oxygen-free nitrogen) . For the use in EGR the oxygen content must be low, i.e., below 10 ppm, to avoid

corrosion problems in surface facilities and injection wells. This adds to the price of the commercial nitrogen. Enhanced Oil Recovery (EOR)

In Enhanced Oil Recovery (EOR) a more complete recovery of hydrocarbons may be achieved by artificial maintenance of formation pressure. This step for

increasing oil recovery involves the injection of fluid into (or near) an oil reservoir for the purpose of delaying the pressure decline during oil production.

Pressure maintenance can significantly increase the amount of economically recoverable oil over that to be expected with no pressure maintenance.

Nitrogen has long been successfully used as injection fluid for EOR and is widely used in oil field operations for gas cycling, reservoir pressure maintenance, and gas lift. The costs and limitations on the availability of natural gas and CO2 have in some cases made nitrogen an economic alternative for oil recovery by miscible gas displacement. From WO2011084581 (US2011146978) an enhanced oil recovery process is known that is integrated with a synthesis gas generation process, such as gasification or reforming, and an air separation process for generating (i) an oxygen stream for use, for example, in the syngas process or a combustion process, and (ii) a nitrogen stream for EOR use. It is indicated that air separation units suitable for use in this process are well-known to those of ordinary skill in the relevant art. Examples of well-known air separation technologies include, for example, cryogenic distillation, ambient temperature adsorption and membrane separations. In other words, the integration according to WO2011084581 is to utilize a conventional air separation unit and use both the oxygen stream and the nitrogen stream on-site. The economic benefit is therefore marginal. Moreover, there is no guarantee that the nitrogen stream is indeed oxygen-free.

From WO2008087154 a method is known for recovering hydrocarbons from a reservoir which includes separating air into a nitrogen-rich gas and an oxygen-rich gas, oxidizing a hydrocarbon fuel with at least part of the oxygen-rich gas to produce steam and a C02-rich gas, injecting at least part of the steam through an injection well into the reservoir to heat the hydrocarbons, purging the injection well with at least part of the nitrogen- rich gas, and injecting at least part of the C02-rich gas into the reservoir containing the heated hydrocarbons. In this reference, use is made both of the nitrogen

component of air and of the oxygen component of air while recovering hydrocarbons from a reservoir.

WO03016676 relates to an energy-integrated process scheme for the production of liquid hydrocarbons from light hydrocarbons. In the process an EOR process is combined with the production of liquid hydrocarbons. A mixture of oxygen and nitrogen (e.g. air) is separated, for example using an air separation unit. The nitrogen stream obtained is used for EOR, and the oxygen stream obtained is used for partial oxidation of a

hydrocarbonaceaous feed (e.g. associated gas) to produce syngas, which syngas is converted into liquid

hydrocarbons

WO03018958 relates to a process in which a nitrogen stream from an air separation unit is used for EOR, and the oxygen stream is used for the production of syngas.

Another EOR technique is referred to in US2012214106.

US2012214106 mentions a process in which steam is used in EOR, for example in steam assisted gravity drainage

(SAGD) . Paragraphs 74 to 77 of US2012214106 indicate that heat recovery is possible when performing chemical looping combustion (CLC) . Fuel reactor flue gases 90 may pass a heat recovery steam generator 190 to heat boiler feed water 200 to provide steam 210. Or air reactor flue gases 110 may pass through a heat recovery steam

generator 260 to heat boiler feed water 200 to provide steam 210. The steam 210 may be used in enhanced oil recovery, for example in SAGD.

WO2011084581, W02008087154 , WO03016676 and WO03018958 thus all relate to processes in which a nitrogen-rich gas from an air separation unit (ASU) is used for artificial maintenance of formation pressure during EOR. And

US2012214106 discloses a process in which steam is used in EOR, e.g. in SAGD, whereby the steam is generated in a heat recovery steam generator in a CLC line-up.

Aim for Enhanced Gas Recovery (EGR)

Hence, some methods have been developed for EOR, but the current invention relates to EGR (enhanced gas recovery) .

The current invention especially relates to the specific feature of EGR that it concerns a gas-gas displacement process for nearly depleted natural gas subsurface reservoirs, and that at some point in time the gas that is injected will reach the production well, thus diluting the produced natural gas stream.

The problem underlying the current invention is to find a means for maximum hydrocarbon recovery from a natural gas subsurface reservoir.

SUMMARY OF THE INVENTION

The present invention provides a process for the recovery of hydrocarbons from a natural gas subsurface reservoir,

wherein a nitrogen stream (i) containing less than 10 ppm oxygen is injected into the natural gas reservoir,

whereby the nitrogen stream (i) is prepared in and captured from a chemical looping combustion reactor wherein a fuel is combusted through use of an oxygen carrier with a gas stream (ii) containing both oxygen and nitrogen, and

wherein part of the hydrocarbons recovered from the natural gas reservoir are used as fuel in the chemical looping combustion reactor.

The present invention further provides an integrated production system with nitrogen injection for recovering hydrocarbons from a natural gas subsurface reservoir, comprising :

(i) one or more injection wells and one or more

production wells connected to a natural gas subsurface reservoir ;

(ii) a produced natural gas gathering system connected to a natural gas subsurface reservoir;

(iii) a chemical looping combustion unit comprising an air reactor and a fuel reactor; and

(iv) a compression unit,

wherein the air reactor is connected with the compression unit, and

wherein the compression unit is connected with at least one injection well, and

wherein the produced natural gas gathering system is connected with the fuel reactor.

With the method and with the production system of the present invention maximum hydrocarbon recovery from a natural gas subsurface reservoir can be obtained.

DETAILED DESCRIPTION OF THE INVENTION

Subsurface fossil-fuel wells come in many varieties. There can be wells that produce oil, wells that produce oil and natural gas, or wells that only produce natural gas. The current invention can be applied in respect of gas wells. The invention can be applied on land but also offshore. The invention can be applied, typically, when the production of the subsurface reservoir diminishes.

The current method relates to a method for recovering hydrocarbons from a natural gas subsurface reservoir.

The present invention provides a process for the recovery of hydrocarbons from a natural gas subsurface reservoir,

wherein a nitrogen stream (i) containing less than 10 ppm oxygen is injected into the natural gas reservoir,

whereby the nitrogen stream (i) is prepared in and captured from a chemical looping combustion reactor wherein a fuel is combusted through use of an oxygen carrier with a gas stream (ii) containing both oxygen and nitrogen, and

wherein part of the hydrocarbons recovered from the natural gas reservoir are used as fuel in the chemical looping combustion reactor.

The fuel that is combusted in the chemical looping combustion reactor may have a Wobbe index in the range of 3 to 40 MJ/Nm ³ , even of 5 to 27 MJ/Nm ³ .

The fuel that is combusted in the chemical looping combustion reactor preferably contains up to 95% by volume of nitrogen and/or carbon-dioxide. The oxygen to fuel molar ratio in the chemical looping combustion reactor preferably is in the range of 0.8 - 1.2.

The metal of the oxygen carrier preferably is

selected from Ni, Mn, Cu, Cd, Co, Fe, Mg or Ca, and wherein the metal is preferably deposited on a solid carrier .

The nitrogen stream (i) captured from the chemical looping combustion reactor preferably is routed to a polishing step to reduce the oxygen content to less than

10 ppm oxygen.

The gas stream (ii) containing both oxygen and nitrogen preferably contains more than 75 % by volume, preferably more than 85 % by volume, more preferably more than 90 % by volume of nitrogen.

The gas stream (ii) containing both oxygen and nitrogen preferably is enriched in nitrogen content by a membrane unit.

Preferably a flue gas stream is produced in the chemical looping combustion reactor and at least a part thereof is used in combination with the nitrogen stream (i) as injection gas for the natural gas reservoir. More preferably steam and/or power is produced during chemical looping combustion and at least a part thereof is used to compress the nitrogen stream (i) prior to injection, or to compress the gas stream (ii) containing both oxygen and nitrogen prior to the chemical looping combustion reactor .

The present invention further provides an integrated production system with nitrogen injection for recovering hydrocarbons from a natural gas subsurface reservoir, comprising :

(i) one or more injection wells and one or more

production wells connected to a natural gas subsurface reservoir; (ii) a produced natural gas gathering system connected to a natural gas subsurface reservoir;

(iii) a chemical looping combustion unit comprising an air reactor and a fuel reactor; and

(iv) a compression unit,

wherein the air reactor is connected with the compression unit, and

wherein the compression unit is connected with at least one injection well, and

wherein the produced natural gas gathering system is connected with the fuel reactor.

In the method of the present invention, a nitrogen stream (i) is prepared in and captured from a chemical looping combustion reactor (CLC reactor) wherein a fuel is combusted through use of an oxygen carrier with a gas stream (ii) containing both oxygen and nitrogen.

The method may, for example, include separating a nitrogen stream (i) from air, by oxidizing a fuel with the oxygen in the air in a chemical looping combustion reactor .

The combustion in the CLC reactor produces two separate streams: a nitrogen stream and a flue gas stream. The nitrogen gas mainly comprises nitrogen and preferably comprises, optionally after a polishing step, less than 10 ppm oxygen. The flue gas stream may comprise carbon-dioxide, steam, unburned hydrocarbons and/or reforming products such as carbon-monoxide and hydrogen. The fuel may be fully oxidized or partially oxidized.

A chemical looping combustion reactor, also referred to as chemical looping combustion unit, comprises an air reactor (AR) and a fuel reactor (FR) . The reaction in the fuel reactor can be slightly exothermic or endothermic, depending on the nature of the oxygen carrier. The main heat is produced in the exothermic reaction in the air reactor and can be used for the production of hot steam and for electric power.

The CLC unit, also referred to as CLC reactor, used in the present invention acts as a highly selective separator. It may be used to remove oxygen out of air, thereby producing a substantially oxygen-free nitrogen stream (i) , i.e. with less than 10 ppm oxygen. Moreover, this avoids contamination of the nitrogen stream (i) with flue gas components, such as CO ₂ and ¾0.

Chemical looping combustion (CLC) units are known. CLC units comprise an air reactor (AR) and a fuel reactor (FR) .

In the air reactor (AR) air or another stream

comprising oxygen and nitrogen are brought into contact with an oxygen carrier (OC) : a metal or metal oxide in a reduced state. In the AR, the oxygen in the stream containing both oxygen and nitrogen (e.g., air, but also streams containing less oxygen - e.g. a secondary flue gas from a combustion unit other than a CLC unit) reacts with the oxygen carrier. Near complete removal of the oxygen may be reached in one pass through one AR.

Alternatively, the air stream may be passed more than once through the AR until the oxygen therein is largely removed, or the air stream may be passed through more than one AR until the oxygen therein is largely removed. Additionally the air reactor exhaust stream, which is a nitrogen stream, can be routed to a polishing step that reduces the oxygen content to the required levels.

Preferably the oxygen content of the AR exhaust stream, i.e. the nitrogen stream, is reduced to less than 10 ppm by volume without the need of a polishing step.

Subsequently, in the fuel reactor (FR) the fuel is combusted with the use of the oxygen carrier (s),

generating steam and relatively pure carbon-dioxide. An advantage of the current method and production system is that maximum hydrocarbon recovery from a natural gas subsurface reservoir can be obtained. Before and during the time that the gas that is injected in the natural gas subsurface reservoir starts to dilute the produced natural gas stream, the current invention is advantageous. Especially when the produced natural gas stream becomes diluted the present invention results in a higher hydrocarbon recovery as compared to known methods and production systems.

Highly advantageous is the fact that it was now found that maximum hydrocarbon recovery can be obtained even from the moment that more than 20% nitrogen is present in the produced stream. This diluted natural gas cannot be used as pipeline gas. But with the present invention it does not need to be considered a waste stream. And it is also not necessary to take complicated measures to remove nitrogen from the diluted natural gas.

In the present method and production system natural gas with a very high nitrogen content, and thus a low

Wobbe index, can be fed to the fuel reactor of the CLC unit. When EGR starts to produce one or more natural gas production streams with more than 20% nitrogen,

this/these stream(s) is/are used as fuel for the CLC, and is/are further turned into valuable nitrogen stream(s) .

Another advantage is that the current method and production system are less complicated than known methods and production systems. They require a minimum amount of apparatuses and are economically attractive.

Preferably, the CLC is fully integrated, using the exhaust stream of the air reactor of the CLC for nitrogen injection in a natural gas subsurface reservoir, running the fuel reactor of the CLC on a hydrocarbon stream recovered from the natural gas subsurface reservoir, and using the power generated by the CLC unit for compression. EGR can even be continued when the

hydrocarbon stream recovered from the natural gas subsurface reservoir is a low value fuel having a low Wobbe index. This results in maximum hydrocarbon recovery from the natural gas subsurface reservoir.

One advantage achievable with a preferred embodiment of the current method and production system concerns energy efficiency. The CLC unit produces power (steam) . In a preferred embodiment this may be used to efficiently compress air to the CLC operational pressure and/or compress the produced nitrogen containing stream to a pressure suitable for injection.

Moreover, the CO2 rich flue gas stream produced as exhaust of the FR in the CLC unit may also be used as additional injection gas in a later phase of the EGR development, e.g. after 20% of the volume of the

subsurface reservoir has been purged with the nitrogen containing stream that was initially injected.

Another operating option that will result in a higher efficiency with respect to the nitrogen production is the usage of a gas stream comprising N ₂ , CO2 and O2 at a reduced O2 concentration, meaning less than 20 %v, but more than 10 ppm. For instance, this may be a flue gas stream of e.g. a separate combustion unit, instead of air. This will lead to a higher amount of 2 produced per amount of fuel combusted. In this embodiment, the

resulting air reactor exhaust will contain a mixture mainly of 2 and CO2. This nitrogen containing stream can be used directly or if required via an oxygen polishing step (discussed hereafter) for EGR.

The CLC reactor used in the present invention is schematically represented in Figure 1. In this figure: (A) represents the fuel. This may be fuel with a low

Wobbe index, for example gas recovered from a nearly depleted reservoir; (B) represents air or similar stream containing both oxygen and nitrogen. This stream may be compressed;

(C) represents air depleted in 0 ₂ ;

(D) represents steam (and power) ;

(E) represents flue gas.

And:

(1) represents a fuel reactor (FR)

(2) represents an air reactor (AR) wherein the

(depleted) oxygen carriers are oxidized and thereby loaded/regenerated

(3) represents a loop wherein oxygen loaded carriers and depleted oxygen carriers are re-circulated from (AR) to (FR) and from (FR) to (AR) respectively.

In Figures 2 to 6 various line-ups are shown. All embodiments concern Enhanced Gas Recovery (EGR) . Figures 4 and 5 are according to the present invention. Figures 2, 3 and 6 show line-up configurations that may be part of a line-up according to the present invention. In these figures :

1 Air feed

2 Compressed air (compression is optional)

3 Fuel gas feed

4 Hot nitrogen with less than 10 ppm oxygen

5 Hot flue gas (mainly CO2 and ¾0)

6 Boiler feed water

7 Hot pressurized steam

8 Nitrogen with less than 10 ppm oxygen

9 CO2 stream

10 CO2 stream

11 Compressed nitrogen for EGR

12 Injection gas into well head #1 ... well head #N (N being the number of well heads)

13 CO2 for EGR (in which case 11 also contains

compressed CO2 for EGR) 14 Produced natural gas from well #1 ... well #M (M being the number of wells)

15 Natural gas to treatment location

16 Sales quality gas

17 Natural gas with nitrogen content (fuel gas feed in addition to or instead of 3)

18 Oxygen low air

19 Oxygen rich air.

And:

A represents an optional compression unit

B represents a CLC unit

C represents a steam boiler

D represents a split, wherein CO ₂ stream is split for

EGR or disposal

E represents a compression unit operating at greater than reservoir pressure conditions

F represents an N number of injection well heads

G represents subsurface reservoir (s)

H represents a produced natural gas gathering system

I represents a produced natural gas conditioning

system

J represents a nitrogen enrichment unit (that

increases the nitrogen content of a feed air stream) .

In Figure 2 is shown Enhanced Gas Recovery with compressed nitrogen which comprises less than 10 ppm oxygen. In the line-up, air may be first compressed in unit A before it is introduced into the CLC unit B.

Compression is not mandatory. The CLC unit may be run at atmospheric conditions. The product streams of the CLC unit are hot and the energy may be recovered in for instance the steam boiler C.

Nitrogen with less than 10 ppm nitrogen may then be compressed in unit E and injected via injection well heads F into the subsurface reservoir. This may be a singular reservoir or a number of reservoirs.

Figure 3 is a modification, wherein (part) of the CO ₂ stream is compressed and also introduced in the

reservoir. Suitably, this is done after about 20% of the volume of the reservoir (s) has been purged with the nitrogen containing stream.

Figures 4 and 5 are according to the present

invention. Figures 4 and 5 are modifications of Figures 2 and 3, wherein is shown that part of the recovered gas stream is used as fuel gas feed. Part of the hydrocarbons recovered from a natural gas subsurface reservoir are used as fuel in the chemical looping combustion reactor. The fuel in the chemical looping combustion reactor may contain up to 95% by volume of ₂ and/or CO ₂ . Such commercially low value fuel has a Wobbe index in the range of 3 to 40 MJ/Nm ³ , preferably 5 to 27 MJ/Nm ³ .

In Figure 6 a line-up is shown wherein a unit is used that results in an oxygen enriched stream and an oxygen reduced stream.

Chemical-looping combustion technology with inherent separation of CO ₂ is known. It has been described in the aforementioned papers and the references cited therein. In principle, a metal oxide is used as an oxygen carrier to transfer oxygen from an air reactor to a fuel reactor.

Direct contact between fuel and combustion air is

avoided. In a typical CLC unit, the products of the combustion reaction, carbon dioxide and water, are kept separate from nitrogen and any remaining oxygen.

CLC conditions are known. Preferably a chemical- looping combustor is used, composed of two interconnected fluidized bed reactors, a fuel reactor (FR) and an air reactor (AR) , separated by a loop seal. The chemical- looping reactors (AR and FR) can be designed as risers or bubbling fluidized beds. The reactor designs are preferably optimized with respect to the nature of the oxygen carrier (e.g., density, and kinetics). The

reactors may further comprise a cyclone and a solid valve to control the solids fed to the fuel reactor (FR) , or similar equipment. In the FR, the fuel combustion is performed by an oxygen carrier, giving CO ₂ and ¾0. The solids in this respect are particles of the oxygen carrier. Depleted oxygen carrier particles overflow into the AR through another loop seal, preferably through a U- shaped fluidized loop seal, to avoid gas mixing between fuel and air. The loop seals are preferably fluidized by steam.

The loading of the oxygen carrier takes place at the AR, which can be designed as a riser or a bubbling fluidized bed depending on the nature of the oxygen carrier. In the present set-up, it is important to select conditions that also ensure complete absorption of the oxygen. ₂ leaves the AR, for instance, passing through a high-efficiency cyclone and a filter or similar

equipment. Optionally, the ₂ is routed to a polishing step, to remove any remaining traces of 0 ₂ . The recovered solid particles enter the upper loop seal that is

fluidized by steam setting the oxygen carrier ready to start a new cycle and avoiding the mixing of fuel and air out of the riser. The recovered solid particles may be partly sent to a reservoir of solids where heat is recovered in a bubbling fluidized bed heat exchanger. The oxygen carrier particles may be returned to the AR by gravity from the reservoir of solids located above a solids valve. Fine particles produced by

fragmentation/attrition in the plant are preferably recovered, for instance in filters that are located downstream of the FR and AR. Various oxygen carriers in CLC processes are known and suitable. The oxygen carrier particles are a cornerstone in the CLC technique. Important properties for oxygen carriers are high

reactivity in both reduction by fuel gas and oxidation by oxygen in the air, as well as high resistance to

attrition, fragmentation, and agglomeration.

Additionally, it is also an advantage if the metal oxide is cheap and environmentally friendly. Briefly, important criteria for a good oxygen carrier are the following:

(i) high reactivity with fuel and air;

(ii) low fragmentation and attrition, as well as low

tendency for agglomeration;

(iii) low production cost and environmentally benign;

(iv) be fluidizable and stable under repeated

reduction/oxidation cycles at high temperature.

Suitable metals include Ni, Mn, Cu, Cd, Co, Fe, Mg and Ca. The oxygen carrier (OC) in the current process is preferably a Cu-based oxygen carrier, an Fe-based oxygen carrier or an Ni-based oxygen carrier, more preferably a Cu-based oxygen carrier, with the metal deposited on a solid support. Generally, these metal oxides are combined with an inert which acts as a porous support providing a higher surface area for reaction, as a binder for

increasing the mechanical strength and attrition

resistance, and additionally, as an ion conductor

enhancing the ion permeability in the solid particles. AI2O3, S1O2, T1O2, and Zr02, are usually used as the inert support. They have the ability to increase the

reactivity, durability, and fluidizability of the oxygen carrier particles. The inert materials are believed to enhance positive properties among which the most

important are to maintain the pore structure inside the particle and inhibit migration of the metals, which could lead to sintering of oxygen carrier particles. Thus, in the current invention various supports may be used.

Preferably silica or alumina supports are used, more preferably Y-AI ₂ O ₃ . Suitable oxygen carriers may for instance be found in "Selection of Oxygen Carriers for Chemical-Looping Combustion", by J. Adanez et al in

Energy Fuels, 2004, 18 (2), pp 371-377. Also suitable as oxygen carriers are composite materials with NiO or Fe ₂ 03 as active phase and AI ₂ O ₃ -, iAl ₂ 0 ₄ - or MgAl ₂ 0 ₄ ~based support as well as materials based on the CaMn03-s perovskite structure. It is known that these materials typically have a high attrition resistance.

The reaction conditions in the AR are such as to convert the OC without adversely affecting the OC itself and without the generation of NO _x . The operational pressure of the CLC can be chosen freely, and follows from a cost optimization where the main parameters are reactor volume for a given capacity (which reduces with increasing reactor pressure) and air compression (which increases with increasing reactor pressure) . The pressure can be in the range from 1 to 10 bara, more preferably from 1 to 5 bara. As indicated above, the conditions should be selected such as to ensure complete adsorption of the oxygen. In other words, the residence time should be selected appropriately as well as the temperature and the oxygen carrier to fuel ratio. The FR might be

operated under slightly reducing conditions (sub- stoichiometric oxygen carrier to fuel ratio) to allow for full O ₂ removal in the AR. The temperature may vary from

700 to 1200°C, preferably from 850 to 950°C.

As indicated above, the gas stream (ii) containing both oxygen and nitrogen preferably contains less than 25 % by volume of oxygen, with the remainder being nitrogen or nitrogen and other inert gasses. More preferably it contains less than 15 % by volume of oxygen, more

preferably less than 10 % by volume. This may be achieved using a separation unit, as shown in Figure 6, or by using a stream which contains CO ₂ (e.g., exhaust stream of a separate combustion unit) . Preferably, the gas stream (ii) containing both oxygen and nitrogen contains more than 75 % by volume, preferably more than 85 % by volume, more preferably more than 90 % by volume of nitrogen .

The reaction conditions in the FR are such as to convert at least 90 vol%, preferably at least 95 vol% of the fuel with the oxidized OC without adversely affecting the OC itself. Partial combustion may be used for

chemical looping reforming as discussed in the paper by Tobial Proll et al, described herein before. If the flue gas from the FR is used to purge the subsurface

reservoir, then the fuel is preferably combusted fully, without the generation of partially combusted products. Suitably, the pressure and temperature in the FR and the AR are substantially the same (that is, they differ less than 1 bara, preferably less than 200 mbar in pressure) . Thus, the pressure can be in the range from 1 to 80 bara, more preferably from 1 to 10 bara. The temperature may vary from 700 to 1200°C, preferably from 850 to 950°C.

Preferably, the air to fuel molar ratio is in the range of 0.4 - 10, more preferably in the range of 0.6 - 3.0, most preferably in the range of 0.8 - 1.2.

Oxygen carrier to fuel ratios that are suitable for full combustions are known in the art and may be easily determined when carrying out a series of experiments. On the other hand, the ratio should be close to the ideal amount for full combustion of the fuel, however slightly reducing conditions in the FR might be required for full O ₂ removal in the AR. Suitable ratios to be used range from 0.2 - 10 (mol OC/mol fuel), more preferably from 0.4

- 5, most preferably from 0.8 - 1.2.

The waste stream from the AR is composed of ₂ that is essentially free of O ₂ , and trace amounts of other components present in air. It may be converted into pure 2 with an optional subsequent oxygen polishing step. For instance, the nitrogen may be treated in a catalyst bed to remove the detectable traces of oxygen.

The current invention concerns a method for Enhanced Gas Recovery. It is within the scope of the invention to have the CLC unit and other units at some distance of the well heads.