Method and system for CO2 enhanced oil recovery

Methods of Enhanced Oil Recovery (EOR) from an oil reservoir by CO2 flooding are disclosed. One method comprises producing a well stream from the reservoir; separating the well stream into a liquid phase and a gas phase with a first gas/liquid separator, wherein the gas phase comprises both CO2 gas and hydrocarbon gas; cooling the gas phase with a first cooler; compressing the gas phase using a first compressor into a compressed stream; mixing the compressed stream with an external source of CO2 to form an injection stream; and injecting the injection stream into the reservoir. Systems for EOR from an oil reservoir by CO2 flooding are also disclosed.

The present invention relates to improved methods and systems for Enhanced Oil Recovery (EOR) from an oil reservoir by CO2flooding (CO2EOR). In particular, it relates to utilising back-produced CO2from a production well stream.

CO2EOR is a secondary or tertiary recovery method for oil production, in which CO2is injected into an oil reservoir to increase the oil recovery rate. The injection can be performed either by CO2injection alone or in combination with traditional gas and/or water injection. The use of CO2for EOR purposes has a potential to increase the oil recovery rate of a reservoir by up to 5 to 15 percentage points. It is also an effective method to store considerable amounts of CO2underground, making this process a climate change mitigating measure. Many existing applications of CO2EOR are for onshore oil fields, however its use in offshore fields is also being developed.

Some known methods utilise CO2back-produced from the oil reservoir itself, instead of or in addition to CO2from an external source. For example WO 2016/108697 teaches a method whereby a gas/liquid separator separates the gas phase from a well stream, CO2is then separated from this gas phase by a membrane separator, the CO2is then compressed and cooled and injected into the reservoir for EOR. Another method is taught in WO 2014/170466. In this method, the gas phase of a well stream is separated from the fluid, and the gas is further separated into a CO2stream and a hydrocarbon stream. These are then recombined in desired quantities in order to provide a composition comprising specific CO2and hydrocarbon components. The method comprises cyclically alternating between injecting a first composition comprising substantially pure CO2and the CO2/hydrocarbon composition to provide enhanced EOR.

However, there are various issues with such methods. The separation of CO2from the gas phase can be complex and expensive, particularly since the CO2content in the gas phase is highly dynamic, from a few mole % just after breakthrough increasing up to 80-90 mole % or more. Not many technologies are applicable, particularly in an offshore, topside or subsea, application. Indeed, many difficulties exist in the practical implementation of the methods described in the prior art, for example in terms of avoiding hydrate formation in the separated gas and the mixture of separated gas and external CO2, the highly dynamic operational conditions for the main process equipment (such as separators, heat exchangers and compressors), material issues in the liquid pipeline and process equipment in the existing oil processing facility due to high CO2content in the liquid phase and compressor requirements.

The present invention seeks to address these problems.

According to a first aspect, the present invention provides a method of Enhanced Oil Recovery (EOR) from an oil reservoir by CO2flooding, comprising: producing a well stream from the reservoir; separating the well stream into a liquid phase and a gas phase with a first gas/liquid separator, wherein the gas phase comprises both CO2gas and hydrocarbon gas; cooling the gas phase with a first cooler; compressing the gas phase using a first compressor into a compressed stream; mixing the compressed stream with an external source of CO2to form an injection stream; and injecting the injection stream into the reservoir.

This method provides significant advantages over the prior art. Firstly, the gas phase processed into an injection stream and injected into the reservoir comprises both CO2and hydrocarbons. In other words, after separating the gas phase from the liquid phase, the CO2and hydrocarbon gas present in the gas phase are not further separated into individual streams and only the CO2stream utilised as in prior art methods, Instead, the complete gas phase is processed and formed into an injection stream.

Thus, at each stage in the method, and at the point of injection into the reservoir, the gas phase/injection stream comprises both CO2and hydrocarbons. Moreover, it may be considered that the entire (or substantially the entire) gas component separated by the first gas/liquid separator is injected (either as gas or after being condensed) into the reservoir. In other words, no gas fraction, in particular no hydrocarbon fraction, is removed. The present inventors have discovered that it is not necessary to separate these gases, thus avoiding the complex and costly CO2separation step frequently found in the prior art. This is particularly the case in relation to oil reservoirs with a low Gas to Oil Ratio (GOR), since the higher pressure required to inject a CO2+hydrocarbon stream is less problematic. Thus, preferably the method is utilised with a low GOR oilfield. Moreover, CO2separation in offshore environments is particularly challenging, thus the method of the invention provides particular advantages in relation to such environments. The injection, in the invention, of a stream formed from the complete gas phase simplifies the process and provides a commercially viable solution.

It will be appreciated that this first aspect of the invention defines a method carried out after CO2breakthrough into the well stream has occurred, in other words CO2is being back-produced from the reservoir into the well stream. Thus, CO2is present in the gas phase separated by the first gas/liquid separator, and is then, after the various processing steps, re-injected into the reservoir to enhance the oil recovery by CO2flooding. Prior to this method of the invention being carried out, in a first phase of operation prior to CO2breakthrough, in order to inject CO2into the reservoir, CO2must be supplied only from an external source. An aspect of the invention including also this first phase of operation is described later.

It will be appreciated that by “an external source of CO2” means CO2not produced from the reservoir, but rather provided from a source external to the EOR process. The CO2may be pure CO2, and may come from natural sources of CO2or CO2captured from industrial processes. For example, CO2may be CO2captured from industrial processes such as cement production and ammonia production, or captured from exhaust gases from power and heat production, onshore or offshore. The external CO2may be gaseous CO2, but is most preferably liquid CO2.

Mixing with external CO2is particularly beneficial in a first period after CO2breakthrough. In this early phase the flow rate of the gas from the well stream may be quite low and comprise a large proportion of methane and the external CO2is needed to maintain injection flow-rate. After CO2breakthrough, the content of CO2in the well stream will increase, and the need for external CO2will decrease. At some point, the back-produced CO2may be sufficient that in a further phase of operation, after the method of the invention, the supply of external CO2is stopped.

Mixing the gas phase with an external source of CO2provides further significant advantages. In a first period after CO2breakthrough, the gas phase flow from the first compressor (the output of the compressor is generally gas, possibly with small amount of liquid) will be low and contain high concentrations of methane. This gas should preferably be condensed prior to injection into the reservoir. However, a very high pressure from the first compressor would be required for condensation by sea-water alone, and there would be a high risk of hydrate formation. However, by mixing the gas phase with external CO2(preferably liquid CO2), the gas phase may condense/dissolve during the mixing process. Consequently, after mixing, the injection stream may preferably be a liquid phase or a gas+liquid phase.

In situations where the injection stream is a gas phase (or has a significant proportion of gas phase) after mixing, a subsequent cooling step may be carried out by a second cooler to condense the gas fraction so that the injection stream is in the liquid phase (or is substantially in the liquid phase with only a low percentage of gas).

Alternatively, the compressed gas phase leaving the first compressor may be cooled and condensed into a liquid phase or liquid+gas phase by a second cooler prior to mixing.

The second cooler may be an active cooler, preferably with seawater circulation. The operational parameters, mainly temperatures, need to be controlled to avoid hydrate formation.

By cooling/condensing the compressed stream, its density increases and thereby the required injection pressure is reduced (high density gives a pressure increase down the injection piping).

The first compressor may preferably be a liquid tolerant compressor since liquid may form after the first cooler. If the compressor is not liquid tolerant, an additional gas/liquid separator may be required upstream the compressor. Most likely, an additional liquid pump would be required to bring the liquid phase back into the main gas liquid separator or directly into the liquid being transported to the oil processing facility. Such complexity can be avoided by using a liquid tolerant compressor.

Preferably the first cooler is an active cooler so that the cooling temperature may be controlled in order to both prevent hydrate formation and control the compressor inlet temperature.

Hydrates are to be avoided both in the first and second coolers since they can create flow blockages. The hydrate formation temperature will decide the minimum temperature in the cooling process. The hydrate formation temperature will depend on the gas composition. In a first period after CO2breakthrough, the gas phase will have a high methane content and the hydrate formation temperature will be highest. Later, when the gas phase comprises more CO2, the hydrate formation temperature will decrease. For optimum injection, it is desired to have as low as temperature as possible (low temperature means high density) but it is necessary to keep above the hydrate formation temperature. For the actual compositions and operational pressures, the hydrate temperature may typically be in the range of 5-25° C.

Generally, the gas phase separated by the first gas/liquid separator will comprise a small amount of water vapour (i.e. evaporated water) in addition to CO2and hydrocarbon gas, or the gas phase may be saturated with water. Whilst the gas phase may be dehydrated to remove the water, thereby reducing the corrosive effect of the gas phase, preferably the gas phase is not dehydrated so as to avoid the associated process complexity.

The liquid phase separated by the first gas/liquid separator will generally comprise water (formation water), oil and dissolved CO2.

Typically, the well stream is choked, generally to a pre-defined pressure, prior to separating the well stream into a liquid phase and a gas phase. This will release a gas from the well stream, which is then separated by the first gas/liquid separator. The pressure to which the well stream is choked determines the partial pressure/content of CO2in the gas-phase, and the content of CO2in the liquid phase. A lower pressure means a lower CO2content in the liquid. One skilled in the art would readily appreciate how to select a suitable pre-defined pressure dependent on the particular scenario.

The separation pressure will influence the compressor requirements and the power required for the stream to be injected. It will determine whether, if the liquid phase is sent to an oil processing facility, the liquid phase needs to be pressure boosted or not. If pressure boosting is required, a pump will be provided for the liquid phase.

Moreover, the separation pressure will determine whether carbon steel can be used in piping used to convey the liquid phase downstream the separator, e.g. piping connecting with an oil processing facility, or whether corrosion resistant materials are required. The higher the pressure, the more CO2there will be in the liquid phase. Due to the corrosive effect of CO2, if the CO2in the liquid phase is too high, some pipeline materials such as carbon steel will suffer from corrosion to an unacceptable extent. Thus, at higher pressures, the larger amounts of CO2in the liquid phase requires downstream piping to be manufactured from corrosion resistant material, such as stainless steel. Thus, in one embodiment, the liquid phase is transported to an oil processing facility through corrosion-resistant, e.g. stainless steel, piping. Other corrosion-resistant materials may be used, such as nickel based alloys, but these are generally more expensive. Copper based alloys may also be viable. Material selection may also depend on temperature. Whilst the need for corrosion-resistant materials may have some disadvantages, the higher pressure means that additional pumping for the liquid phase may not be required. Moreover, if lower pressures were used in order to avoid the need for corrosion-resistant materials, a more technically complex compressor solution would be required.

In one embodiment, prior to separating the well stream into a liquid phase and a gas phase, the well stream is heated. This is preferably by means of a heat exchanger, more preferably by a heat exchanger utilising heat supplied by the first compressor so as to minimise the external energy requirement.

Whilst the method of the invention can be used with onshore reservoirs, it has particular application for offshore oil reservoirs, for example offering the particular advantages discussed above. In the case of offshore (i.e. subsurface) reservoirs, the entire method of the invention may be carried out subsea.

Alternatively, at least the steps of separating the well stream, cooling the gas phase and compressing the gas phase may in fact be carried out above the sea (topside), preferably on a platform or floater. The steps of mixing and cooling the injection stream may also be carried out above the sea.

In another embodiment, the step of separating the well stream is carried out subsea, whilst the steps of cooling the gas phase and compressing the gas phase are carried out above the sea, preferably on a platform or floater. This reduces the equipment topside and the amount of hydrocarbon inventory topside. This makes it easier to use an unmanned topside without a flare system. Furthermore, by carrying out the gas/liquid separation at the seabed, the liquid phase can be sent directly to the oil processing facility, and an extra liquid riser from the separation process topside to the seafloor (and then up to the oil processing facility) is avoided.

The well stream gas flow rate after CO2breakthrough will be highly dynamic (mainly increasing), especially in a first period, before a more stable situation is reached. To handle this dynamic situation, after compressing the gas phase, part of the compressed gas phase may be recycled into the well stream upstream the gas/liquid separator. Alternatively, the compressor recycle flow can be mixed into the gas phase downstream the gas/liquid separator. This compressor recycle provides more stable conditions for the separator operation, as it allows the separator to operate within narrower gas and liquid load ranges during the lifetime of the oil reservoir, which simplifies the operation and control of the separator.

Furthermore, after compressing the gas phase, part of the compressed stream may be used to form an anti-surge flow which is directed into the gas phase downstream the first gas/liquid separator and upstream the first cooler. Alternatively, gas from downstream the compressor may be mixed with the well stream upstream the first gas/liquid separator. In one embodiment, a combined compressor recycle and anti-surge line may be provided.

The injection stream may be pumped by a booster or injection pump to increase the pressure thereof prior to injection in the reservoir. Such a pumping step will generally be carried out after cooling e.g. by the second cooler. If the injection stream is liquid, this can be pumped by one common pump. Pumping is advantageous since it requires less energy/power than compression.

In one embodiment, the first compressor may in fact comprise two compressors arranged in series (or, it may be considered that after compressing the gas phase with a first compressor, the compressed stream is then further compressed with an additional compressor). Thus, after cooling the gas phase with the first cooler, it is compressed in two stages. Compression in more than one stage may be desirable if the required pressure ratio is higher than can be achieved by one compressor. However, it is preferable to use only one compressor if possible, in order to minimise cost and complexity.

It will be appreciated that the injection stream is CO2-rich, since it comprises both CO2from the well stream and CO2from an external source. In one example, at the point of injection into the reservoir, the injection stream comprises 85 to 95 mole % CO2.

As mentioned previously, following gas/liquid separation, the liquid phase may be transported to an oil processing facility. Generally, this will be an existing oil processing facility. Since the liquid phase will still comprise some dissolved CO2, it will be corrosive, thus preferably the liquid phase is transported through corrosion-resistant piping.

As mentioned above, if it were desired to use non-corrosion resistant piping to transport the liquid phase, a low pressure could be used in the first gas/liquid separator to reduce the CO2content in the liquid phase to a level allowing the use of e.g. carbon steel piping. However, this would require a more technically complex compressor solution. The present inventors have found a solution to this problem, by using a multiple-stage separation process. In such a preferred process, after separation in the first gas/liquid separator, the liquid phase is choked to a lower pressure (i.e. a pressure lower than with which it exits the first gas/liquid separator) such that a second gas phase comprising CO2and hydrocarbon gas is released from the liquid phase. This second gas phase and liquid phase are separated in a second gas/liquid separator. By doing this, the partial pressure of CO2in the liquid phase leaving the second gas/liquid separator can be made low enough to allow for a carbon steel pipeline transporting the liquid phase e.g. to an oil processing facility, and also within the oil processing facility itself. In other words, since there is less CO2in the liquid phase, the liquid phase is less corrosive, so corrosion-resistant piping is not necessarily required and carbon steel can instead be used.

The particular partial pressure of CO2which is low enough to allow for a carbon steel pipeline will depend e.g. on temperature, oil and formation water component. However, in one example, carbon steel may be used with a CO2pressure below 5 barn (bar absolute) (500 kPa).

The second gas phase separated by the second gas/liquid separator is preferably combined with the gas phase separated by the first gas/liquid separator. Preferably this combining is carried out prior to cooling by the first cooler and compressing by the first compressor. Thus, this combined gas phase is then cooled by the first cooler, compressed by the first compressor; mixed with an external source of CO2and injected into the reservoir.

Generally, prior to being combined with the first gas phase, various processing steps are carried out on the second gas phase. In one embodiment, after being separated by the second gas/liquid separator, the second gas phase is cooled with a third cooler and then compressed, before it is combined with the gas phase separated by the first gas/liquid separator. The second gas phase may be compressed by one compressor. However, the gas flowrate from the second gas/liquid separator is substantially lower than that from the first gas/liquid separator, and to bring this gas up to the same pressure as the first gas phase multiple compressors are most likely needed. Thus, two or more compressors arranged in series may be used. These may be smaller than the first compressor compressing the first gas phase. If the total pressure ratio is low enough, intermediate cooling between the compressors is not needed, but may be required for higher pressure ratios. The compressors are preferably liquid tolerant, especially the first of two compressors arranged in series. Alternatively, dry gas compressors may be used, but upstream separators/scrubbers would then be required. The power requirement for two small compressors may be less than 10% of the main (first) compressor.

The operational conditions of the compressor(s) compressing the second gas phase are most likely constant enough that compressor recycle is not necessary. However, situations may arise where compressor recycle is needed, and in this case, after compressing the second gas phase, part of the second gas phase is recycled into the liquid phase upstream the second gas/liquid separator. Alternatively, it may be recycled into the second gas phase downstream the second gas/liquid separator.

After compressing the second gas phase, an anti-surge flow may be formed from part of the second gas phase. This may be directed into the second gas phase downstream the second gas/liquid separator and upstream the third cooler; or upstream the second gas/liquid separator.

As an alternative to compressing the second gas phase with a compressor, the pressure of the second gas phase may be increased by an ejector prior to being combined with the first gas phase separated by the first gas/liquid separator. The ejector can advantageously be powered by motive gas flow from downstream the first compressor. This simplifies the process and removes the need for the third cooler.

Whilst the second separation step using a second gas/liquid separator is described above as an optional feature in relation to the first aspect of the invention, this two-stage separation process is seen as an invention in its own right. Thus, accordingly, in a second aspect, the invention provides a method of Enhanced Oil Recovery (EOR) from an oil reservoir by CO2flooding, comprising: producing a well stream from the reservoir; separating the well stream into a liquid phase and a first gas phase with a first gas/liquid separator; reducing the pressure of the liquid phase to release a second gas phase and separating this second gas phase from the liquid phase with a second gas/liquid separator; combining the first and second gas phases into a combined gas phase; cooling the combined gas phase with a first cooler; compressing the combined gas phase into an injection stream with a first compressor; and injecting the injection stream into the reservoir.

It will be appreciated that many of the various preferred and optional features described above in relation to the first aspect of the invention are also applicable to this second aspect. Some of these will now be described, however the particular advantages of the preferred features may not be repeated here for brevity; instead, reference may be made to the advantages described above in relation to the first aspect.

This aspect of the invention is not limited to the gas phase comprising both CO2gas and hydrocarbon gas. Thus, this method may be utilised in situations where CO2gas is separated from the hydrocarbon gas. Whilst, as described above in relation to the first aspect, it is advantageous to use the entire gas phase and not only separated CO2, the method of the second aspect will provide advantages even where CO2is separated from hydrocarbons.

Moreover, this aspect of the invention is not limited to an external source of CO2being mixed with the compressed stream from the first compressor. Whilst, as described above in relation to the first aspect, this mixing with external CO2offers various advantages, the method of the second aspect will provide advantages independently of the use of external CO2.

By carrying out the two-stage gas/liquid separation of the second aspect of the invention, the partial pressure of CO2in the liquid phase leaving the second gas/liquid separator can be low enough to allow for a carbon steel pipeline transporting the liquid phase e.g. to an oil processing facility, and also within the oil processing facility itself. In other words, since there is less CO2in the liquid phase, the liquid phase is less corrosive, so corrosion-resistant piping is not necessarily required and carbon steel can instead be used. This is highly advantageous. The particular partial pressure of CO2which is low enough to allow for a carbon steel pipeline will depend e.g. on temperature, oil and formation water component. However, in one example, carbon steel may be used with a CO2pressure below 5 bara (500 kPa).

Moreover, this is advantageous in terms of the oil processing facility which would likely experience corrosion problems with high levels of CO2. Thus, the two-stage separation process is also advantageous in avoiding the need for significant modification to the oil processing facility.

Whilst the second aspect of the invention describes a two-stage gas/liquid separation, it will be appreciated that in some situations more than two stages may be provided. In such a multi-stage gas/liquid separation process, for example, after separating the second gas phase from the liquid phase with a second gas/liquid separator, the pressure of the liquid phase may be further reduced to release a third gas phase. This third gas phase may be separated from the liquid phase with a third gas/liquid separator. All of the first, second and third gas phases may be combined into a combined gas phase which is then processed by the first cooler, first compressor and then injected into the reservoir.

It will be appreciated that this second aspect of the invention defines a method carried out after CO2breakthrough into the well stream has occurred, in other words CO2is being back-produced from the reservoir into the well stream. Thus, CO2is present in the gas phase separated by the first gas/liquid separator, and is then, after the various processing steps, re-injected into the reservoir to enhance the oil recovery by CO2flooding. Prior to this method of the invention being carried out, in a first phase of operation prior to CO2breakthrough, in order to inject CO2into the reservoir, CO2must be supplied only from an external source. An aspect of the invention including also this first phase of operation is described later.

In one embodiment, the first gas phase and the second gas phase each comprise both CO2and hydrocarbon gas. They may each also further comprise water vapour. The injection stream injected into the reservoir preferably comprises both CO2and hydrocarbons, in other words no separation process has been carried out to separate and use only the CO2, however as described above the method can be used even in cases where the CO2is separated.

The first cooler is preferably an active cooler so that the cooling temperature may be controlled in order to both prevent hydrate formation and control the compressor inlet temperature.

Furthermore, the first compressor may preferably be a liquid tolerant compressor since liquid may form after the first cooler. If the compressor is not liquid tolerant, an additional gas/liquid separator may be required upstream the compressor. Most likely, an additional liquid pump would be required to bring the liquid phase back into the main gas liquid separator or directly into the liquid being transported to the oil processing facility. Such complexity can be avoided by using a liquid tolerant compressor.

Whilst, as mentioned above, this second aspect of the invention does not require an external source of CO2to be mixed into the injection stream. In one preferred embodiment such an external source of CO2is mixed into the injection stream prior to injecting the injection stream into the reservoir. This external source of CO2may be gaseous CO2, but more preferably is liquid CO2.

The injection stream into which the external source of CO2is mixed may comprise a gas phase, and the step of mixing the external source of CO2into the injection stream may cause the gas phase of the injection stream to at least partially condense or dissolve in the external source of liquid CO2.

The method may further comprise cooling the injection stream with a second cooler, preferably by active cooling, either before or after the external source of CO2is mixed into the injection stream. This cooling step may condense at least part of a gas phase in the injection stream into liquid.

In one embodiment, the first compressor may in fact comprise two compressors arranged in series (or, it may be considered that after compressing the gas phase with a first compressor, the compressed stream is then further compressed with an additional compressor). Thus, after cooling the gas phase with the first cooler, it is compressed in two stages. Compression in more than one stage may be desirable if the required pressure ratio is higher than can be achieved by one compressor. However, it is preferable to use only one compressor if possible, in order to minimise cost and complexity.

Preferably, the well stream is choked to a pre-defined pressure prior to separating the well stream into a liquid phase and a first gas phase. This will release a gas from the well stream, which is then separated by the first gas/liquid separator. The pressure to which the well stream is choked determines the partial pressure/content of CO2in the gas-phase, and the content of CO2in the liquid phase. A lower pressure means a lower CO2content in the liquid.

Prior to separating the well stream into a liquid phase and a first gas phase, the well stream may be heated. This is preferably done via a heat exchanger, more preferably by a heat exchanger utilising heat supplied by the first compressor so as to minimise the external energy requirement.

Whilst the method may be used with onshore reservoirs, the method is particularly useful for offshore reservoirs. In the case of an offshore reservoir, the entire method of this aspect of the invention may be carried out subsea. Or, the step of separating the well stream into a liquid phase and a first gas phase may be carried out subsea, and the subsequent steps are carried out above the sea, preferably on a platform or floater. Or, at least the following steps may be carried out above the sea, preferably on a platform or floater: separating the well stream into a liquid phase and a first gas phase with a first gas/liquid separator; reducing the pressure of the liquid phase to release a second gas phase and separating this second gas phase from the liquid phase with a second gas/liquid separator; combining the first and second gas phases into a combined gas phase; cooling the combined gas phase with a first cooler; and compressing the combined gas phase into an injection stream with a first compressor.

The liquid phase from the second gas/liquid separator, comprising oil, water and dissolved CO2will generally be transported to an oil processing facility. The second gas/liquid separation process will have sufficiently reduced the CO2content in the liquid so that corrosion-resistant piping is not required to transport the liquid. Instead, carbon steel piping may be used. However, the liquid will still cause some low level of corrosion, and to protect against this a corrosion control method may be used such as injecting a film forming corrosion inhibitor into the liquid phase to limit the corrosion rate of the pipeline and process equipment and piping.

Preferably, the second gas phase is cooled by a third cooler and then compressed prior to being combined with the first gas phase. This compression may be carried out by one compressor, or in two stages by two (or more) compressors arranged in series. After compressing the second gas phase, part of the compressed second gas phase may be used to form an anti-surge flow which is directed into the second gas phase downstream the second gas/liquid separator and upstream the third cooler.

As an alternative to compressing the second gas phase with a compressor, the pressure of the second gas phase may be increased by an ejector prior to being combined with the first gas phase separated by the first gas/liquid separator. The ejector can advantageously be powered by motive gas flow from downstream the first compressor.

The well stream gas flow rate after CO2breakthrough will be highly dynamic (mainly increasing), especially in a first period, before a more stable situation is reached. To handle this dynamic situation, after compressing the gas phase, part of the compressed gas phase may be recycled into the well stream upstream the gas/liquid separator. Alternatively, the compressor recycle flow can be mixed into the gas phase downstream the gas/liquid separator. This compressor recycle provides more stable conditions for the separator operation, as it allows the separator to operate within narrower gas and liquid load ranges during the lifetime of the oil reservoir, which simplifies the operation and control of the separator.

Furthermore, after compressing the gas phase, part of the compressed stream may be used to form an anti-surge flow which is directed into the gas phase downstream the first gas/liquid separator and upstream the first cooler. Alternatively, gas from downstream the compressor may be mixed with the well stream upstream the first gas/liquid separator. In one embodiment, a combined compressor recycle and anti-surge line may be provided.

The injection stream may be pumped by a booster or injection pump to increase the pressure thereof prior to injection in the reservoir. Such a pumping step will generally be carried out after cooling e.g. by the second cooler. If the injection stream is liquid, this can be pumped by one common pump. Pumping is advantageous since it requires less energy/power than compression.

In the first and second aspects of the invention described above, the process of recycling part of the compressed gas in order to provide more stable operation in the presence of a highly dynamic gas flow rate is described as an optional feature. However, this is more generally applicable and is considered as an invention in its own right. Thus, according to a third aspect, the invention provides a method used in Enhanced Oil Recovery (EOR) from an oil reservoir by CO2flooding, comprising: producing a well stream from the reservoir; separating the well stream into a liquid phase and a gas phase with a gas/liquid separator; compressing the gas phase using a compressor; and recycling part of the compressed gas phase into the well stream upstream the gas/liquid separator or into the gas phase downstream the gas/liquid separator.

It will be appreciated that many of the various preferred and optional features described above in relation to the first and second aspects of the invention are also applicable to this third aspect. Some of these will now be described, however the particular advantages of the preferred features may not be discussed here to avoid repetition; instead, reference may be made to the advantages described above in relation to the first and second aspects.

This aspect of the invention is not limited to the gas phase comprising both CO2gas and hydrocarbon gas. Thus, this method may be utilised in situations where CO2gas is separated from the hydrocarbon gas. Whilst, as described above in relation to the first aspect, it is advantageous to use the entire gas phase and not only separated CO2, the method of the third aspect will provide advantages even where CO2is separated from hydrocarbons.

Moreover, this aspect of the invention is not limited to an external source of CO2being mixed with the compressed stream from the first compressor. Whilst, as described above in relation to the first aspect, this mixing with external CO2offers various advantages, the method of the third aspect will provide advantages independently of the use of external CO2.

By providing a compressor recycle flow, more stable conditions for the gas/liquid separator operation and the compressor operation are provided, by allowing the separator to operate within narrower gas and liquid load ranges during the lifetime of the oil reservoir, which simplifies the operation and control of the separator. Such a method may be used as part of any method of EOR by CO2flooding, where a compressor is used, in order to provide such advantages.

After compressing the gas phase, part of the compressed gas phase may be used to form an anti-surge flow which is directed into the gas phase downstream the gas/liquid separator and upstream the cooler. In one embodiment, a combined recycle and anti-surge line may be provided to recycle the part of the compressed gas phase and also provide the anti-surge flow. This reduces the pipework required.

The gas phase may be cooled with a first cooler, prior to compressing the gas phase. Part of the compressed gas phase may be recycled into the gas phase downstream the gas/liquid separator and upstream the first cooler.

The first compressor may preferably be a liquid tolerant compressor since liquid may form after the first cooler. If the compressor is not liquid tolerant, an additional gas/liquid separator may be required upstream the compressor. Most likely, an additional liquid pump would be required to bring the liquid phase back into the main gas liquid separator or directly into the liquid being transported to the oil processing facility. Such complexity can be avoided by using a liquid tolerant compressor.

The well stream may be choked to a pre-defined pressure prior to separating the well stream into the liquid phase and the gas phase. This will release a gas from the well stream, which is then separated by the first gas/liquid separator. The pressure to which the well stream is choked determines the partial pressure/content of CO2in the gas-phase, and the content of CO2in the liquid phase. A lower pressure means a lower CO2content in the liquid. Part of the compressed gas phase may be recycled into the well stream upstream of the gas/liquid separator and downstream the choke.

In one embodiment, the gas phase comprises CO2and hydrocarbon gas. It may also comprise a small amount of water vapour, or the gas phase may be saturated with water.

The first cooler is most preferably an active cooler so that the cooling temperature may be controlled in order to both prevent hydrate formation and control the compressor inlet temperature.

In one embodiment, prior to separating the well stream into a liquid phase and a gas phase, the well stream is heated. This is preferably by means of a heat exchanger, more preferably by a heat exchanger utilising heat supplied by the first compressor so as to minimise the external energy requirement.

Whilst the method of the third aspect can be used with onshore reservoirs, it has particular application for offshore oil reservoirs.

Whilst, as mentioned above, this third aspect of the invention does not require the mixing of an external source of CO2, in one preferred embodiment an external source of CO2is mixed into the gas phase to form an injection stream. This injection stream is then injected into the reservoir. This external source of CO2may be gaseous CO2, but more preferably is liquid CO2. The injection stream may be cooled with a second cooler. Its pressure may be increased by pumping prior to injecting into the reservoir.

The liquid phase may be transported to an oil processing facility. Generally, this will be an existing oil processing facility.

As described previously, prior to the method of the first aspect of the invention being carried out (and indeed prior to any of the methods of the invention being carried out), there will generally be an earlier phase of operation prior to CO2breakthrough in which CO2is supplied only from an external source. Thus, in another aspect, the invention provides a method of Enhanced Oil Recovery (EOR) from an oil reservoir by CO2flooding, comprising: a first phase comprising injecting CO2from a source external to the oil reservoir into the reservoir, and a second phase comprising any of the above described methods.

Preferably, prior to commencement of the second phase, it is determined that the back-produced CO2in a well-stream produced from the reservoir exceeds a threshold value or it is determined that the CO2/methane ratio in a well-stream produced from the reservoir exceeds a threshold value.

If it is determined that the CO2or CO2/methane ratio does not exceed a threshold value, and thus the second phase has not yet commenced, the method may further comprise directing the well stream from the reservoir to an oil processing facility. Thus, during the first phase of operation, the well stream is directed to the oil processing facility.

In the first phase of operation when CO2is supplied only from an external source, the pressure of the external CO2may not be sufficient for direct injection into the reservoir, so preferably the method comprises pumping the external CO2to a sufficient pressure prior to injection. This pressure will depend on the pressure in the reservoir at the injection point, the necessary excess pressure to drive the CO2into the reservoir, the static pressure increase from the injection template to the injection point and the pressure drop in the injection pipe.

In the first phase of operation in which only CO2from an external source is injected into the reservoir, the injection stream therefore comprises 100 mole % CO2. During the second phase of operation, the injection stream may comprise 85-95 mole % CO2.

There may be a third phase of operation after the second phase wherein the mixing of external CO2is stopped. Preferably, the method includes the step of determining whether the CO2in the injection stream exceeds a certain value in the second phase, and if this value is exceeded, the mixing of external CO2is stopped. This value represents a sufficient quantity of backproduced CO2that external CO2is not required.

The invention also extends to systems arranged to perform any of the methods as described above.

In one further aspect, the invention provides an enhanced oil recovery system, comprising: a producer arranged to produce a well stream from a reservoir a first gas/liquid separator arranged to separate the well stream into a liquid phase and a gas phase comprising both CO2gas and hydrocarbon gas; a first cooler arranged to cool the gas phase; a first compressor arranged to compress the cooled gas phase into a compressed stream; a mixer arranged to mix the compressed stream with an external source of CO2to form an injection stream; and injection piping arranged to inject the injection stream into the reservoir.

This is a system corresponding to the method of the first aspect of the invention described above. Many advantages described in relation to the method and its preferred features are also clearly applicable to this system and its preferred features as described below. However, not all of these advantages will be described here in order to avoid repetition.

The first cooler may be an active cooler. The first compressor may be a liquid tolerant compressor.

The system may further comprise a choke arranged upstream the first gas/liquid separator to choke the well stream to a pre-defined pressure.

The system may further comprise a heat exchanger arranged upstream the first gas/liquid separator to heat the well stream. In this heat exchanger the well-stream may be heated in heat exchange with warm gas from the compressor discharge, provided e.g. by suitable piping.

Whilst the system may be used with an oil reservoir in any location, it is preferably configured for operation with an offshore reservoir. In such a case, the system may therefore be located entirely subsea. In another embodiment, the first gas/liquid separator, first cooler and first compressor is in fact located above the sea, preferably on a platform or a floater. In a further embodiment, the first gas/liquid separator is located subsea whilst the first cooler and first compressor are located above the sea, preferably on a platform or floater.

The system may further comprise a recycle line connecting between downstream the first compressor and either upstream the first gas/liquid separator or downstream the first gas/liquid separator. The recycle line is arranged to recycle part of the compressed gas phase into either the well stream upstream the first gas/liquid separator, or the gas phase downstream the first gas/liquid separator.

The system may also further comprise an anti-surge line connecting between downstream the first compressor and a point downstream the first gas/liquid separator and upstream the first cooler. The anti-surge line is arranged to provide an anti-surge flow of compressed gas phase into the gas phase downstream the first gas/liquid separator and upstream the first cooler. A combined recycle and anti-surge line may be provided.

In one embodiment, the first compressor may in fact comprise two compressors arranged in series (or, it may be considered that downstream the first compressor is an additional compressor) to provide compression in two stages. Compression in more than one stage may be desirable if the required pressure ratio is higher than can be achieved by one compressor. However, it is preferable to use only one compressor if possible, in order to minimise cost and complexity.

A second cooler may be provided downstream the first compressor to cool the compressed gas phase prior to being input to the mixer, or downstream the mixer to cool the injection stream. A booster pump or injection pump may be provided downstream the second cooler to pump the injection stream, prior to the injection stream being supplied to the injection piping for injection into the reservoir.

Corrosion-resistant piping is preferably used within the implementation of the system of this aspect of the invention. The system may further comprise piping arranged to transport the liquid phase to an oil processing facility. Preferably, this piping is corrosion-resistant, for example made form stainless steel.

In one particularly preferred embodiment, the system further comprises a second choke arranged to choke the liquid phase to a lower pressure so as to release a second gas phase. A second gas/liquid separator is then arranged to separate the second gas phase from the liquid phase. Piping may be provided to connect the second gas phase with the first gas phase such that the gas phases mix together.

A third cooler may be arranged downstream the second gas/liquid separator to cool the second gas phase. Further, a compressor may be arranged downstream the third cooler to compress the second gas phase. Such components would generally be provided upstream any piping connecting the second gas phase with the first gas phase.

In yet another aspect, the invention provides an enhanced oil recovery system, comprising: a producer arranged to produce a well stream from a reservoir; a first gas/liquid separator arranged to separate the well stream into a liquid phase and a gas phase; a choke arranged to reduce the pressure of the liquid phase so as to release a second gas phase; a second gas/liquid separator arranged to separate the second gas phase from the liquid phase; piping or a mixer arranged to combine or mix the first and second gas phases into a combined gas phase; a first cooler arranged to cool the combined gas phase; a first compressor arranged to compress the combined gas phase into an injection stream; and injection piping arranged to inject the injection stream into the reservoir.

This is a system corresponding to the method of the second aspect of the invention described above. Many advantages described in relation to the method and its preferred features are also clearly applicable to this system and its preferred features as described below. However, the advantages will not all be described here in order to avoid repetition.

In this system, the first cooler is preferably an active cooler. The first compressor is preferably a liquid tolerant compressor.

The system may further comprise an external source of CO2, preferably liquid CO2. A mixer may be arranged to mix CO2from the external source of CO2with the injection stream.

A second cooler may be provided. In one embodiment this is arranged upstream the mixer to cool the injection stream prior to mixing with the CO2, and in another embodiment it is arranged downstream the mixer to cool the injection stream after mixing with the CO2. Preferably, the second cooler is an active cooler. A booster pump or injection pump may be provided downstream the second cooler to pump the injection stream, prior to the injection stream being supplied to the injection piping for injection into the reservoir.

A second choke may be arranged to choke the well stream upstream the first gas/liquid separator to a predefined pressure.

Piping may be provided to transport the liquid phase to an oil processing facility. Due to the two stage separation, the liquid phase will comprise a low enough amount of CO2that corrosion resistant piping may not be required. Consequently, the piping may be made of carbon steel. However, the liquid will still cause some low level of corrosion, so to protect against this a source of film forming corrosion inhibitor may be arranged for injection into the liquid phase.

In one embodiment, the system further comprises a third cooler arranged downstream the second gas/liquid separator to cool the second gas phase. Furthermore, a compressor may be arranged downstream the third cooler to compress the second gas phase. Alternatively, instead of a compressor for compressing the second gas phase, the system may comprise an ejector arranged to increase the pressure of the second gas phase. The ejector is preferably powered by motive gas flow from downstream the first compressor.

In yet another aspect, the invention provides a system for use in enhanced oil recovery, comprising: a producer arranged to produce a well stream from a reservoir; a gas/liquid separator arranged to separate the well stream into a liquid phase and a gas phase; a compressor arranged to compress the gas phase; a recycle line arranged to direct compressed gas from downstream the compressor to either upstream the gas/liquid separator or downstream the gas/liquid separator, such that the recycle line recycles part of the compressed gas phase into either the well stream upstream the gas/liquid separator, or the gas phase downstream the gas/liquid separator.

This is a system corresponding to the method of the third aspect of the invention described above. Many advantages described in relation to the method and its preferred features are also clearly applicable to this system and its preferred features as described below. However, not all of these will be described here in order to avoid repetition.

In one embodiment, a cooler is arranged upstream the compressor to cool the gas phase. Preferably, this is an active cooler.

The system may further comprise an anti-surge line arranged to direct compressed gas as an anti-surge flow from downstream the compressor to downstream the gas/liquid separator and upstream the cooler. In one embodiment, the anti-surge line and the recycle line may be provided as a combined line.

A choke may be arranged downstream the gas/liquid separator to reduce the pressure of the well stream. In this case, the recycle line may be arranged to direct compressed gas into the well stream upstream the gas/liquid separator and downstream the choke.

Preferably, the system further comprises an external source of CO2, preferably liquid CO2. In this case, a mixer may be arranged to mix CO2from the external source of CO2into the gas phase to form an injection stream.

In one embodiment, a second cooler is arranged to cool the injection stream. Optionally, a pump may be arranged to pump the injection stream. Generally, the system further comprises injection piping for injecting the injection stream into the reservoir.

The invention further provides an enhanced oil recovery system, comprising: an external source of CO2; injection piping arranged to inject the external source of CO2into an oil reservoir; a device for monitoring a CO2content or CO2/methane ratio back-produced in a well stream from the oil reservoir; a device for determining when a CO2content or CO2/methane content threshold is exceeded; and a system according to any of the embodiments described above; wherein the system further comprises piping to bypass the elements of the system according to any of the embodiments described above downstream the producer and direct the well stream to an oil processing facility, in the event that the threshold is not met. Preferably a pump is provided for pumping the external source of CO2prior to injection in the reservoir.

It will be appreciated that features described above in relation to certain aspect(s) of the invention may be equally useful when applied to other aspect(s) of the invention, and vice versa.

It will be noted that the described embodiments relate to offshore CO2EOR processes, however the skilled person will appreciate that the embodiments may equally be employed in onshore fields.

FIG. 1aillustrates an EOR system1of an embodiment of the invention wherein the entire process is carried out subsea. A well stream4is produced from an oil reservoir2by the production tubing3and passed out via well head5. At point6it is determined whether CO2breakthrough has occurred yet. If it has not, then the well stream, numbered4a, is directed to an existing top-side oil processing facility7. Reference numeral10indicates sea level and numeral22indicates the sea floor. In this case, since there is no back-produced CO2, imported CO212from an external source (preferably liquid CO2) forms stream13which is injected into the reservoir2via injection piping15, to provide EOR. This may be considered as a first phase of operation.

If CO2breakthrough has occurred, i.e. CO2gas is now being back-produced, then the well stream, numbered4b, is directed to subsea process unit8. This may be considered as a second phase of operation. Embodiments of this process unit will be described later with reference toFIGS. 3 to 5. In this process unit8a gas phase11comprising CO2, and hydrocarbon gas and small amounts of dissolved water is separated from a liquid phase9comprising oil and water. The oil/water stream9is provided to the oil processing facility7. The gas phase11exits the process unit8and is mixed with imported CO212(preferably liquid CO2) from an external source, to form an injection stream13. Further process steps are carried out on this stream (not shown) and then it is provided via injection well-head14to injection piping/injector15, which injects the injection stream13into the reservoir2. Whilst the mixing of the gas phase11with the imported CO212into injection stream13is shown outside subsea process unit8, this may in fact typically be part of subsea process unit8.FIG. 1billustrates the system ofFIG. 1a, but wherein these steps are incorporated into a complete subsea process unit8′, incorporating also those processes of unit8.

FIG. 2aillustrates an EOR system20of an embodiment in which part of the main process is carried out topside on a separate installation (platform or floater). Essentially, in this embodiment, the entire subsea process unit8ofFIG. 1is instead located above sea level10, i.e. topside, and forms topside process unit19. The other parts and processes of the system20are the same as those ofFIG. 1, and so will not be described again here. As withFIG. 1a, whilst the mixing of the gas phase11with the imported CO212into injection stream13is shown subsea, outside topside process unit19, it may in fact be part of process unit19. Thus, this part of the process may also be carried out topside.

FIG. 2bis a modified version of the embodiment ofFIG. 2a, wherein some processing steps are carried out subsea, and some carried out topside. As withFIGS. 1 and 2a, in this EOR system30, a well stream4is produced from an oil reservoir2by the production piping3and passed out via well head5. At point6it is determined whether CO2breakthrough has occurred yet. If it has not, then the well stream, numbered4a, is directed to a top-side oil processing facility. In this case, since there is no back-produced CO2, imported CO212only from an external source forms stream13which is injected into the reservoir2via injection piping15, to provide EOR.

If CO2breakthrough has occurred, i.e. CO2gas is now being back-produced, then the well stream, numbered4b, is directed to gas/liquid separator16, which separates the well stream4binto a liquid phase9comprising oil and water and a gas phase18comprising CO2and hydrocarbon gas and dissolved water.

The liquid phase9is directed to the oil processing facility17. The gas phase18is supplied to a topside process unit19above the surface, for example on a platform or a floater. This topside process unit19carries out various further process steps, resulting in a gas phase11comprising CO2and hydrocarbon gas which is mixed with imported CO212from an external source, to form injection stream13. Further process steps are carried out on this stream13(not shown) and it is then provided via injection well-head14to injection piping/injector15, which injects the stream13into the reservoir2.

As withFIG. 1a, whilst the mixing of the gas phase11with the imported CO212into injection stream13is shown subsea, outside topside process unit19, it may in fact be part of process unit19. Thus, this part of the process may also be carried out topside.

Optionally, in the embodiment ofFIG. 2b, the topside process unit19may carry out a further gas/liquid separation step on the gas phase18that it receives. In this case, the separated liquid phase21exits the topside process unit19, mixes with the liquid phase9from the gas/liquid separator16, and is input to the oil processing facility7.

FIG. 3illustrates an EOR system40of an embodiment in which corrosion-resistant piping and equipment is required for the liquid phase downstream the separator16. This embodiment is based on the general system configuration ofFIG. 1b, wherein the entire process is carried out subsea. Parts common to both Figures are given the same reference numbers. The components within the dotted line box numbered8′ inFIG. 3form the subsea process unit8′ ofFIG. 1b. The entire system may be known as an “EOR process facility”.

In the system ofFIG. 3, a well stream4is produced from an oil reservoir2by production piping3. It will be determined from an analysis of the well stream4whether CO2breakthrough has occurred yet, as described in relation toFIG. 1, though this is not illustrated inFIG. 3for simplicity. If it has not, then the well stream, is directed to a top-side oil processing facility (again, not shown in this Figure), which may be considered as a first phase of operation. In this case, since there is no back-produced CO2, imported CO212from an external source forms stream13′ which is injected into the reservoir2via injector15, to provide EOR. This may be considered as a second phase of operation.

If, as is likely, the pressure of the external CO2source is not sufficient for direct injection into the reservoir, a booster pump36is provided to increase the pressure prior to injection. The booster pump36delivers sufficient pressure to inject the CO2into the reservoir. The pressure required will depend on the pressure in the reservoir at the injection point, the necessary excess pressure to drive the CO2into the reservoir, the static pressure increase from the injection template to the injection point, and the frictional pressure drop in the injection pipe.

FIG. 3also illustrates a subsea cooler34through which the stream13passes; however it is not necessary to cool the stream13if it comprises only external CO212, so in this situation the cooler34will be inactive. Since the cooler34is not required for external CO212only, in an alternative configuration the external CO212could be supplied downstream the cooler34.

Once CO2breakthrough has occurred, i.e. CO2gas is now being back-produced, then the well stream4is directed to various process equipment which together form a “subsea process unit”8. The point at which the well stream4should be directed to the subsea process unit8may be determined based on the composition of the well stream. For example, a certain gas composition, particularly a certain CO2/methane ratio may be expected once CO2breakthrough has occurred. In this initial phase after CO2breakthrough, the methane content in the gas will be high and the CO2content low. Also, the total gas flow will be low, compared with later life.

First, the well stream4is choked by choke25to a pre-defined pressure, and is then directed to gas/liquid separator16. The selection of the pressure level provided by the choke25will decide the partial pressure/content of CO2in the gas-phase and the CO2content in the liquid phase produced by the gas/liquid separator16. A lower pressure will reduce the CO2content in the liquid. The separation pressure will also influence the compressor requirements (compressor30, discussed later) and the power required for the gas to be injected, and will decide if the liquid phase9sent to the oil processing facility needs to be pressure boosted or not. If pressure boosting is required, a pump will be provided for liquid phase9(not shown inFIG. 3).

Moreover, the separation pressure will determine whether carbon steel can be used in the piping downstream the separator16(i.e. the piping connecting with the oil processing facility) or whether corrosion resistant materials are required. The higher the pressure, the more CO2there will be in the liquid phase9. Due to the corrosive effect of CO2, if the CO2in the liquid phase9is too high, some pipeline materials such as carbon steel will suffer from corrosion to an unacceptable extent. Thus, at higher pressures, the larger amounts of CO2in the liquid phase9requires the downstream piping to be manufactured from corrosion resistant material, such as stainless steel. In the embodiment ofFIG. 3, the CO2content in the liquid phase9is high enough that the downstream piping must be made of corrosion resistant materials. Whilst this may be disadvantageous, the higher pressure means that no additional pumping is required for the liquid phase9(though in other embodiments a pressure boost may be required as discussed above).

However, in another embodiment, the separation pressure could be lowered to a level where corrosion resistant materials are not necessary, and thus the downstream piping could be made of carbon steel. A pump would then be required to increase the pressure of the liquid phase9after leaving the separator. Such embodiments are described later with reference toFIGS. 4 and 5.

Corrosion-resistant materials will always be required in the EOR process facility (i.e. the whole system ofFIGS. 3, 4 and 5except for the piping for liquid phase9inFIGS. 4 and 5) due to the separated gas phase comprising dissolved water, unless the gas phase is dehydrated.

Continuing the discussion ofFIG. 3, the gas phase26separated by the separator16comprises both CO2and hydrocarbon gas and dissolved water. This is, if necessary, cooled in a subsea cooler27. Preferably, this is an active cooler, here shown with pump circulation by a sea-water pump18, so that the temperature can be adequately controlled to avoid hydrate formation and optimise the gas temperature prior to later mixing with external CO2. The cooled gas29is input to compressor30which increases the pressure thereof, forming cooled, compressed gas11. Preferably, the compressor30is a liquid tolerant compressor since liquid may form after the cooler27. If the compressor is not liquid tolerant, an additional gas/liquid separator may be required upstream the compressor to separate any liquid that has formed during cooling (not shown in the Figure). An additional liquid pump may also then be needed to bring the liquid phase back to the main gas/liquid separator16or directly to the liquid phase9being directed to the oil processing facility.

InFIG. 3, one-stage compression is shown utilising a single compressor30. However compression in more than one stage (i.e. by more than one compressor in series) is also possible and may be used if the required pressure is higher than can be achieved by one compressor. However, for simplicity, a design such as that illustrated requiring only one compressor30is preferable (this is also less expensive).

The well stream gas flow rate after CO2breakthrough will be highly dynamic (mainly increasing) especially in the first period of operation, before a more stable situation is reached. To give an example, if the operational time for the CO2EOR facility is 10 years after CO2 breakthrough, the largest dynamics would happen in the first 1 to 2 years. To handle this dynamic situation, a compressor recycle is provided. As can be seen, a recycle flow32from downstream the compressor30is directed into the well stream4upstream the gas liquid separator16. This provides more stable conditions for the separator operation, as it allows the separator to operate within narrower gas and liquid load ranges during the lifetime of the oil reservoir2, which simplifies the operation and control of the separator. Alternatively, the compressor recycle flow32can be mixed into the gas26downstream the gas/liquid separator16.

To protect the compressor against surge, an anti-surge line31is also provided. Gas from downstream the compressor30is directed into the separated gas26upstream from the cooler27. Alternatively, gas from downstream the compressor30may be mixed with the well stream4upstream the gas/liquid separator16. It will be appreciated that in one embodiment, a combined compressor recycle and anti-surge line may be provided.

Downstream the compressor30, the gas phase11is mixed at mixer33with CO212from an external supply. The pressure of the external CO2and the compressed gas phase11needs to be balanced. In a first phase after CO2breakthrough, the gas flow11from the compressor will be low and contain high concentrations of methane. This gas needs to be condensed prior to injection into the reservoir2. However, a very high pressure from the compressor would be required for condensation by sea-water alone, and there would be a high risk of hydrate formation. However, by mixing the gas11with the external CO212, the gas11will condense/dissolve in the external CO2during the mixing process or in subsequent cooling by cooler34. Thus, the compression requirement is lower.

The process temperatures are controlled by both sea-water coolers34,27to avoid hydrate formation. It is desirable to reach a lower temperature after the mixing and cooling, to increase the density of the fluid, preferably liquid, leaving the cooler34, but at the same time stay above the hydrate formation temperature. Therefore, the cooler34is preferably an active cooler, with sea water circulation by a sea water pump35. In an alternative embodiment, the gas11is cooled prior to (rather than after) being mixed with the external CO212.

The pressure of the fluid leaving the cooler34is increased by booster pump36, then the resulting fluid13′ comprising a high proportion of CO2is injected into the reservoir2via injection well head14and injection piping15, yielding enhanced oil recovery. Typically, the proportion of CO2 in the injection fluid13′ will be between 85-95 mole % of the total fluid (though this will be case specific). The CO2is ultimately back-produced via production tubing3and recycled through the process again.

After some time, the gas flow rate from the reservoir2will stabilise and contain more and more CO2, up to 80-90 mole % or more. When the gas flow rate increases, the required amount of external CO212reduces.

Whilst in the embodiment ofFIG. 3the entire process is carried out subsea (as in the general configuration ofFIG. 1b), the mixing of external CO2and the processes downstream of this could be carried out subsea whilst the remainder of the method is carried out topside on a platform or floater, e.g. as inFIG. 2a. Or, the external CO2could also be taken topside for mixing, cooling and pumping. Or, the gas/liquid separation could be carried out subsea, the mixing with external CO2and the downstream processes carried out subsea, and the remainder of the method carried out topside (as inFIG. 2b).

It will be appreciated that these values are approximate, and are for one particular example only.

FIG. 4illustrates an embodiment of an EOR system50in which a secondary separation process is carried out. Much of this system50is the same as that ofFIG. 3, and will not be described again here. Parts common to both Figures are given the same reference numbers. The difference between the embodiments ofFIGS. 3 and 4is that inFIG. 4, there is a secondary gas/liquid separation process. In other words, the gas/liquid separation is carried out in multiple stages, in this embodiment two stages, but in other embodiments the system may be extended to more than two stages.

Gas/liquid separator16, as inFIG. 3, separates the well stream4into gas phase26comprising both CO2and hydrocarbon gas, and a liquid phase. However, the liquid phase is not then directed directly to an oil processing facility as stream9. Instead, the liquid phase41from the gas/liquid separator16is choked down to a lower pressure by choke42which results in the formation of more gas. The reduced pressure gas/liquid flow43is input to a further gas/liquid separator44, which separates the gas phase47. The partial pressure of CO2in the gas phase results in CO2content in the liquid phase45being low enough to allow for a carbon steel pipeline to the oil processing facility and in the oil processing facility itself. In other words, since there is less CO2in the liquid45, the liquid is less corrosive, so corrosion-resistant piping is not required and carbon steel can instead be used. Due to the reduced pressure of the liquid45, an export pump46is provided to pump the liquid, as liquid9, to the oil processing facility.

The liquid phase45will still be corrosive to some extent though, as some CO2will still be dissolved in it, so a corrosion control method such as the injection of a film forming corrosion inhibitor may be used to limit the corrosion rate of the pipeline and process equipment.

The gas stream47is, if required, cooled by cooler48, here shown as an active cooler with sea-water circulation by sea water pump49. However in other embodiments a passive cooler may be used. The pressure is likely to be low enough that hydrates are not an issue, so active cooling may be less essential.

The flow rate of the gas stream47from the gas/liquid separator44is substantially lower than that of the gas stream26from the gas/liquid separator16. To bring the latter up to the same or similar flow rate/pressure as the former, more than one compressor is required if the required pressure ratio for the compression is too high for one compressor. Thus, the cooled gas stream51from the cooler48is compressed by compressor52to form compressed stream53, followed by compressor54to form further compressed stream55. If the total pressure ratio is low enough, intermediate cooling between the compressors is not needed, but may be required for higher pressure ratios. The compressors are preferably both liquid tolerant compressors, or at least the compressor52should be a liquid tolerant compressor. Optionally, dry gas compressors may be used, and if so then upstream separators/scrubbers will be needed.

The compressors52and53are smaller than compressor30, and the power requirement is typically less than 10% of that of the compressor30. The operational conditions of compressors52and53will likely be constant enough to avoid the need for compressor recycle, but if not a compressor recycle system similar to shown inFIG. 3could be introduced

To protect the compressors52and53against surge, and anti-surge line56is provided. Gas from downstream compressor53is directed into the separated gas47upstream from the cooler48. Alternatively, gas from downstream the compressor53may be mixed with the liquid phase43upstream the gas/liquid separator44.

In other embodiments, more than two compressors may be necessary.

The compressed gas55is mixed into separated gas stream26, to form combined gas stream56. This is then processed in the same way as in gas stream26inFIG. 3, and ultimately injected into the reservoir2.

FIG. 5illustrates an alternative embodiment of an EOR system in which a secondary separating process is carried out. Much of this system is the same as that ofFIG. 4, and will not be described again here. Parts common to both Figures are given the same reference numbers. The difference between the embodiments ofFIGS. 4 and 5is that inFIG. 5, the compressors52and53are replaced with an ejector48.

In this embodiment, the gas stream47is directed to ejector48. Ejector48is powered by motive gas flow49from downstream the compressor30. The ejector utilises this high pressure gas flow49to increase the pressure of stream47. This significantly simplifies the system, and may also remove the need for any intermediate cooler. Since the ejector motive gas flow49is taken from downstream the compressor30, this will be ultimately be recycled through the compressor30, in addition to the compressor recycle flow32. Thus, more gas might be recycled through the compressor30in the embodiment ofFIG. 5than in the embodiment ofFIG. 4. This could potentially increase the compressor power requirement.

In some embodiments, more than one ejector may be used.

Whilst in the embodiments ofFIGS. 4 and 5the entire process is carried out subsea (as in the general configuration ofFIG. 1), the subsea process unit8could instead be located topside on a platform or a floater (as inFIG. 2a) or onshore, instead of subsea. Or, either or both of the gas/liquid separations could be carried out subsea but the other process steps carried out topside (as inFIG. 2b). Moreover, whilst in the embodiments shown the reservoir2is an offshore reservoir, the process is equally applicable to onshore reservoirs.