GB2471862A - Extracting and upgrading heavy hydrocarbons using supercritical carbon dioxide - Google Patents

Abstract

A process for upgrading a heavy hydrocarbon mixture involves: i) contacting the heavy hydrocarbon mixture with supercritical or near-supercritical CO2under conditions suitable to produce a pumpable lighter fraction and a heavier residue from said hydrocarbon mixture; and ii) utilising at least a part of the heavier residue obtained in step (i) to generate steam for use in further extraction of the heavy hydrocarbon mixture and/or to produce electricity, wherein the CO2used in step (i) and/or generated in step (ii) is either captured and recycled or is stored subsurface (e.g. permanently stored subsurface).

Description

Process This invention relates to a process for the partial refinement and upgrading of heavy hydrocarbon mixtures, e.g. extra heavy oils and bitumen, using supercritical or near-supercritical carbon dioxide. In the process described herein, the original heavy hydrocarbon mixture is fractionated into a heavier residue which is used for the on-site production of steam and/or electricity and a pumpable lighter fraction which may be further refined or upgraded. The process also enables the capture and recycling of carbon dioxide for use as a supercritical fluid extraction solvent and for permanent subsurface storage.

Heavy hydrocarbons, e.g. extra heavy oils and bitumen, represent a huge natural resource of the world's total potential reserves of oil. Present estimates place the quantity of heavy hydrocarbons reserves at several trillion barrels, more than 5 times the known amount of the conventional, i.e. non-heavy, hydrocarbon reserves.

However, extra heavy hydrocarbon reserves are generally difficult or impossible to recover and process by conventional means, the hydrocarbons being characterised by very high viscosities and low API (American Petroleum Institute) gravity as well as high levels of unwanted components such as asphaltenes, trace metals and sulphur. As an added disadvantage, higher volumes of hydrogen are required during upgrading of heavy hydrocarbons to produce products for the market, especially for the automotive fuel market. Furthermore, the ecological impact of recovering and refining heavy hydrocarbon mixtures can be very heavy, especially where shallow-well or surface reserves are utilised, such as for example the oil sand deposits of Northern America, e.g. in Alberta, Canada.

Methods have been developed to extract and process heavy hydrocarbon mixtures. Enhanced Oil Recovery (EOR) techniques such as steam injection, Cyclic Steam Stimulation (CS S) and Steam Assisted Gravity Drainage (SAGD) are known for the purpose of recovery. Such methods rely on high temperatures to reduce the viscosity of the heavy hydrocarbon mixtures. Oil displacement using gas injection is also known. Here, gas such as carbon dioxide (CO2), natural gas or nitrogen is injected into the reservoir, whereupon it expands and pushes additional oil to a production well. The injected gas can also dissolve in the oil to lower its viscosity and improve its flow rate, but such EOR methods are most applicable in a conventional light oil context.

A particular problem with the extra heavy oils and bitumens recovered from heavy hydrocarbon reservoirs is the need to improve flowability of the oil through partial upgrading andlor refining of the oil before transportation by pipeline.

Upgrading of the oil is conventionally achieved by dilution with a lighter hydrocarbon such as naphtha (dilbit) or a synthetic oil (synbit). However, the need to transport a diluent to the heavy oil production site or to transport the heavy oil to an upgrading facility represents a significant economic and ecological disadvantage.

Alternatively, the extracted heavy hydrocarbon mixture may be partially or fully refined or upgraded on-site, i.e. using a surface processing plant located close to the production well. Examples of known processes for the upgrading and refining of heavy hydrocarbon mixtures are described in US patent No. 4,483,761 (Paspek, Jr.) and in international patent application WO 2008/055 155 (Chevron U.S.A. Inc.).

Such processes use water (or other hydrogen-containing materials) in the supercritical state for the upgrading, e.g. cracking andior reforming, of a heavy hydrocarbon mixture. However, these processes usually require specialist equipment such as high pressure and high temperature reactors which are expensive to acquire and maintain.

The supercritical fluid state describes the state of a substance in a non-solid state when at a temperature and pressure at or above its critical point. The critical point describes the endpoint of the liquid-vapour coexistence line on the phase diagram for that substance; for carbon dioxide the critical point occurs at a temperature of 31.1 °C (304.1 K) and a pressure of 72.8 atm (7.39 MPa).

Supercritical fluids are known to have interesting properties, for example they have high diffusion rates and low viscosities and they can act as powerful solvents.

A number of supercritical fluids have been suggested or investigated for potential use as solvents for extracting heavy hydrocarbons over the years. Propane, ethane, ethene and CO2 have all been suggested for use as supercritical extraction solvents for heavy hydrocarbons. It is currently believed that supercritical ethane markedly outperforms CO2 as a solvent for the processing of complex hydrocarbon mixtures such as bitumens and heavy oils (Rose et a!., md. Eng. Chem. Res., 2000, , 3875-3883).

Despite recent advances in the field of heavy hydrocarbon recovery and processing, there still exists an acute need for a commercially-viable process for refining and upgrading heavy hydrocarbon mixtures in a cheap, quick and an as environmentally-clean as possible fashion. The present inventors have devised such a process which, it is believed, constitutes the first self-contained and commercially viable process for upgrading heavy hydrocarbon mixtures to meet pipeline specification. In particular, the inventors have found that supercritical CO2 is a surprisingly versatile extracting agent and that the composition of the upgraded oil can be controlled to an unexpected degree by varying the extraction conditions.

Accordingly, in a first aspect the invention provides a process for upgrading (or partially upgrading) a heavy hydrocarbon mixture, said process comprising: i) contacting the heavy hydrocarbon mixture with supercritical or near-supercritical CO2 under conditions suitable to produce a pumpable lighter fraction and a heavier residue from said hydrocarbon mixture; and ii) utilising at least a part of the heavier residue obtained in step (i) to generate steam for use in further extraction of the heavy hydrocarbon mixture and/or to produce electricity; and iii) optionally further upgrading or refining the pumpable lighter fraction, e.g. by known catalytic refining processes such as catalytic hydrocracking, wherein the CO2 used in step (i) and/or generated in steps (ii) and/or (iii) is either captured and recycled (e.g. after cleaning and compression) or is stored subsurface, preferably stored permanently subsurface.

The value of the present invention resides particularly in a combination of technologies. In particular, the process of the invention provides a complete solution, utilising hydrocarbon sources that were hitherto not viable to process, whilst minimising the environmental impact of the processing of the materials. It is especially efficient since the process is self-contained as to the fuel for powering the processing plant and/or providing heat or steam for the extraction of feedstock from the hydrocarbon reservoir and is also self-contained in that diluents are not required to provide an upgraded oil that is pumpable from the site of production to a processing or finishing plant. Furthermore, the process does not require large quantities of water which will be subsequently contaminated and ultimately have to be disposed of. In addition, the CO2 produced by the combustion of the heavy residue can by captured and either used in the supercritical fluid extraction (SCFE) process step or it can be injected into the production reservoir for EOR purposes or for carbon dioxide sequestration.

Further advantages of the process of the invention include the reduction in catalyst poisoning components in the pumpable lighter fraction, thereby making it more suitable for catalytic upgrading; the production of a pumpable lighter fraction which is not contaminated with toxic solvent residues; and the ability to extract thermally labile compounds from the hydrocarbon mixture without damage owing to the relatively low temperatures that need to be employed.

The terms "upgrading" and "refining" are well known to the person of skill in the art. "Upgrading" generally refers to the process of altering a hydrocarbon mixture to have more desirable properties, referring for example to providing lighter, synthetic crude oils from heavier oils by hydrogen addition. "Refining" encompasses the processes of separation and purification by distillation and fractionation, as well as reforming and cracking.

Reference to "permanent" subsurface storage of CO2 would be understood by the skilled person to relate to sequestration of CO2 for an extended period of time in a suitable subsurface storage site, e.g. in a geological formation such as a depleted hydrocarbon reservoir. A small amount of leakage of CO2 from a subsurface storage site could be expected over time, but a well chosen and managed geological site could provide permanent storage of over 95% of injected CO2 over a period of hundreds or even thousands of years.

By "hydrocarbon mixture" is meant a combination of various types of molecules that contain carbon atoms, some with hydrogen atoms attached. It is understood that a hydrocarbon mixture may comprise a large number of molecular species having a wide range of molecular weights. Heavy oils according to the invention will also typically include atoms besides hydrogen and carbon, such as sulphur, nitrogen and oxygen, as well as metals such as iron, nickel and vanadium in the heavier parts of the mixture. Generally, at least 90% by weight of the hydrocarbon mixture consists of carbon and hydrogen.

The term "heavy hydrocarbon mixture" according to the present invention would be understood by the skilled person. Examples of heavy hydrocarbon sources include bitumens, oil shales and oil sand deposits. Terms such as "light", "lighter", "heavier" etc. are to be interpreted relative to "heavy". Heavy hydrocarbon mixtures of the invention are especially those which have an API gravity of less than about 25 (degrees), preferably of less than about 20, e.g. less than about 18, 15, 13 or 10. It is particularly preferred that the API gravity of the heavy hydrocarbon mixture is from between about 5 and about 20, e.g. between about 6 and about 17, between about 7 and 14, or between about 8 and 12 especially preferably between about 9 and about 12, i.e. about 10. The process of the present invention is particularly applicable to heavy hydrocarbon mixtures having an API gravity of less than 10 degrees, e.g. to extra heavy crude oils or bitumen.

The term "supercritical" has particular meaning in the art, i.e. within the area of the phase diagram beyond the critical point of the substance in question. In the present context the term "near supercritical" means having a state sufficiently close to the supercritical state that the substance possesses useful properties which are particularly characteristic of the supercritical state. Examples of such properties are density, viscosity, solubility and solvent extraction capability.

The present inventors postulate that a heavier parent crude quality oil or hydrocarbon mixture will, at a defined temperature around or above the supercritical temperature, require a higher pressure for a given extraction process to occur.

Accordingly, pressure and temperature may be optimised according to the parent crude oil quality in question to obtain a defined extraction product. Therefore, in a preferred embodiment, the extraction process is performed at a suitably high temperature to bring the viscosity of the parent hydrocarbon mixture down to a liquid level state for the extraction procedure to work properly. This temperature may be determined by the skilled person based on trial extractions at different temperatures and is preferably set as low as possible to maximise efficiency.

At a constant temperature, increasing the pressure will increase the density of the C02, whilst at constant pressure decreasing the temperature will result in the same. Hence, once the lowest possible temperature has been chosen, the pressure will preferably be defined to yield the desired hydrocarbon product through extraction of the best parts of the parent hydrocarbon mixture. Thus, the temperature and the pressure can be optimised for each sample for extraction in order to obtain the optimum commercial product from the extraction process. At a constant temperature, a higher pressure will result in a denser CO2 that will gradually cut deeper into a given crude oil quality and result in a product with a heavier refinery yield structure (more of the heavier fractions). In a preferred embodiment of the invention, the temperature should be low enough to provide a pumpable lighter fraction with the desired properties, but high enough to minimise the time taken for the extraction process to work.

In one embodiment of the invention the temperature of the CO2 in the near-supercntical state is only slightly below the critical temperature. Preferably, the CO2 is in the supercritical state at a temperature above the critical temperature.

Preferably the pressure is at or above the critical pressure.

In one embodiment of the invention, the temperature of the near-supercritical CO2 is between about 25°C and 31°C, e.g. about 28°C. In a preferred embodiment, the temperature of the supercritical CO2 is between about 31°C and 70°C, e.g. between about 31°C and 4 1°C, between about 32°C and 55°C or between about 33°C and 45°C, e.g. about 50°C or about 35°C.

In a further embodiment, the pressure of the supercritical or near-supercritical CO2 is between about 7.0 and 30 MPa, e.g. between about 7.4 and 15 MPa, especially at a pressure of about 8.0 MPa, about 10 MPa or about 12 MPa.

Preferably, the pressure of the supercritical or near-supercritical CO2 is less than about 25 MPa, especially less than about 20, 18 or 15 MPa. Especially preferred are conditions where the near-supercritical CO2 is at a temperature of about 28 °C and a pressure of about 7.4 MPa or about 12 MPa. Further preferred are conditions where the supercntical CO2 is at a temperature of about 35 °C and a pressure of about 8.0 MPa or about 12 MPa.

It has surprisingly been found that the pressure of the supercritical or near-supercritical CO2 in step (i) of the process according to the invention is a particularly important parameter for tunability of the characteristics of the fraction produced.

Use of the pressures described above, e.g. at a constant temperature slightly above the critical temperature, facilitates the control of the process and especially the control of the final product quality of the two resulting fractions. The composition of the pumpable lighter fraction can be optimised for marketability and market value, feedstock value etc. and the heavier residue can be tailored for availability as fuel. The split ratio between the pumpable lighter fraction and heavier residue can therefore be carefully controlled by the density of the supercritical CO2 and adjusted according to the desired result; the conditions described above yielding fractions with particularly advantageous properties. It is particularly preferred that the temperature of the process remains essentially constant and as low as possible, while the pressure is adjusted to tune the ratio between the between the pumpable lighter fraction and heavier residue.

By "pumpable lighter fraction" is meant a hydrocarbon mixture with a lower density and viscosity that has an improved quality over that of the source heavy hydrocarbon mixture. This improved quality product can be pumped through a pipeline using conventional equipment with little or no additional heating, i.e. it is pumpable at ambient temperature and pressure. A pumpable lighter fraction is especially a hydrocarbon mixture which requires little, e.g. essentially no, additional solvent to be added in order to render the mixture pumpable.

The "heavier residue" produced in step (i) of the first aspect of the invention will have a greater viscosity and a lower API gravity than the heavy hydrocarbon source material. Such heavier residues may be partly or completely solid at ambient temperature and pressure. Preferably, heavier residues are separated from the lighter pumpable fraction in a form suitable for further use, e.g. precipitated from the SCFE solvent stream following a reduction in the pressure applied to the solvent.

The heavier residue separated in step (i) of the first aspect of the invention is used to provide power for the process itself as well as for other purposes. Preferably the power supplying apparatus, e.g. a boiler, is located close to the SCFE apparatus andlor close to the production wells. The residue is transported to the power supplying apparatus either via pipeline, e.g. under elevated temperature, or piecemeal, e.g. by conveyor. The residue may be treated before burning, e.g. to reduce the sulphur content, and may be formed into pellets before being used to provide power. The heavier residue is used to produce steam and/or electricity. The steam can be used for oil recovery purposes, e.g. in a SADG process, and the electricity may be used to power the plant. In this way, the process is self-contained in that it reduces or eliminates the need for other fuel sources.

The gasses produced by burning the heavier fraction are subject to separation to remove the CO2 and the remaining components of the gas stream may be cleaned and/or further separated by methods known in the art. Carbon dioxide capture, cleaning and recycling technology for use on an industrial scale is known. In particular, CO2 has conventionally been used in large volumes in the oil industry, e.g. for enhanced oil recovery. Subsurface storage of CO2 is also known and the skilled person would be able to assess the suitability of any given geological fonnation for oil recovery or for permanent CO2 storage.

The process of the invention may be carried out in a continuous fashion or as a batch process. When a continuous process is employed, the heavy hydrocarbon mixture and supercritical CO2 may be combined in a reaction vessel and the heavier residue continuously removed from said vessel, e.g. by precipitation from the supercritical CO2 stream or by mechanical removal from the vessel, e.g. from the bottom of the vessel. Alternatively, when a batch process is used, a series of extraction chambers may be employed in sequence, each chamber potentially being at a different stage of the process, such that the overall process is quasi-continuous.

In one embodiment of the invention, the hydrocarbon mixture is obtained directly from a shallow reservoir, e.g. by hot production techniques known in the art.

Coupling the hot production techniques for hydrocarbon mixture extraction directly with the upgrading process of the present invention results in particular benefits of economy and energy savings.

The term "shallow reservoir" means a hydrocarbon reservoir which is unsuitable for employing supercritical CO2 extraction technology directly, i.e. a reservoir which is at an insufficient depth to provide the temperature and pressure conditions for CO2 to exist in the supercritical state and/or which possesses insufficient geological stability to pressurise and/or heat the CO2 to supercritical conditions in situ. In particular, shallow reservoirs in the context of the invention are those wherein hydrocarbons are removable by steam assisted production 9..

technologies (e.g. SAGD) from a depth below the surface of more than about 50 m but less that a depth equivalent to a reservoir pressure and temperature equal to the supercritical point of CO2.

In an alternative embodiment, the process of the invention is used as a stand-alone process.

In a related aspect of the invention, a process for producing a pumpable crude oil is contemplated, said process comprising the process for upgrading a heavy hydrocarbon mixture as described above and further comprising the step of separating the pumpable lighter fraction from the heavier residue. In a preferred embodiment, the pumpable lighter fraction is further separated from the majority of, e.g. all of the supercritical or near-supercritical CO2. The pumpable crude oil may be further refined or upgraded before transport (e.g. by pipeline). Alternatively or additionally the pumpable crude oil may have been partially or fully refined or further upgraded after separation from the heavier residue andlor the CO2.

In a further aspect, the invention provides a pumpable crude oil producible (e.g. produced) by a process as defined herein. In particular, the pumpable crude oil of the invention is characterised by a lighter refinery yield structure compared to the residual part, with a dominance of fractions such as middle distillate (light and heavy gas oils and kerosene) and lighter fractions (gas and light and heavy virgin naphtha). The pumpable crude oil may contain more paraffinic components relative to aromatic components and particularly polar (NSO) components. Hence, the pumpable crude oil has improved properties and lower density and viscosity compared to the parent heavy hydrocarbon mixture. The residual, heavier fraction is complementary to the pumpable, lighter fraction and is dominated by an atmospheric residue (vacuum gas oil and vacuum residue) containing asphaltenes and trace metals.

The pumpable crude oil of the invention preferably comprises a large proportion of middle distillate, e.g. at least 45% by weight of the product is kerosene, light gas oil and heavy gas oil. Preferably, the pumpable crude oil comprises at least 50% by weight, especially at least 60%, 70%, 80% or 90% by weight, of middle distillate. The pumpable crude oil preferably comprises at least 10% by weight of kerosene, e.g. at least 15% or at least 20% by weight. The pumpable crude oil also preferably comprises at least 35% by weight of light gas oil, e.g. at least 40%, 45% or 50% by weight.

Furthermore, the pumpable crude oil of the invention comprises a small proportion of atmospheric residue, i.e. little of the extra heavy fraction, e.g. less than 45% by weight of the product is vacuum gas oil and vacuum residue. Preferably, the pumpable crude oil comprises less than 40% by weight, especially less than 35% by weight, of atmospheric residue. The pumpable crude oil preferably comprises less than 15% by weight of vacuum residue, e.g. less than 10% or less than 5% by weight.

By "Gas" is meant a hydrocarbon fraction having a boiling point of less than 5°C. By "light virgin naphtha" is meant a hydrocarbon fraction having a boiling point of between 5°C and 90°C and by "heavy virgin naphtha" is meant a hydrocarbon fraction having a boiling point of between 90°C and 180°C. By "kerosene" is meant a hydrocarbon fraction having a boiling point between about 180°C and 240°C; by "light gas oil" is meant a hydrocarbon fraction having a boiling point between about 240°C and 320°C and by "heavy gas oil" is meant a hydrocarbon fraction having a boiling point between 320°C and 375°C. By "vacuum gas oil" is meant a hydrocarbon fraction having a boiling point between about 3 75°C and 525°C and by "vacuum residue" is meant a hydrocarbon fraction having a boiling point of greater than about 525°C.

One advantage of the process of the present invention is that the pumpable crude oil has a lower level of contaminants than the parent heavy hydrocarbon mixture. Accordingly, in one embodiment, the pumpable crude oil of the invention has a sulphur content of less than 50% of that of the parent heavy hydrocarbon mixture. For example, the upgraded hydrocarbon product of the invention may comprise less than 4%, preferably less than 3% and especially preferably less than 2.5% by weight of sulphur.

In a yet further aspect, the invention provides an apparatus adapted for use in a process as described herein. The apparatus of the invention comprises one or more reactors; means for introducing supercritical CO2 into said reactors; means for separating a heavy hydrocarbon mixture into at least two fractions; and optionally means for recycling the C02, means for pressurising and storing the CO2 subsurface and means for producing energy from the heavier residue.

Apparatus according to the invention may be used to produce a lighter and pumpable oil from extra heavy oil or bitumen. A contact/high pressure cell may be employed for use in a batch process according to the invention. In figure 1, heavy hydrocarbon 11 is transferred to a holding/warming tank 12. The heavy hydrocarbon may be provided directly by a SAGD process 13. CO2 and H2S 14 may be released by aquathermolysis of the heavy hydrocarbon 11. The warm heavy hydrocarbon is then transferred to optional furnace 15. Flue gas/combustion exhaust 16 from the furnace 15 may be collected. The heavy hydrocarbon is then transferred to a high pressure batch extractor 17 in which supercritical CO2 18 is used to extract a pumpable crude oil from the heavy hydrocarbon. Two phases are created in the batch extractor, a supercritical CO2 phase 19 and a residual heavier phase 110. The supercritical CO2 phase 19, which carries lighter, extracted oil, is transferred to a low (e.g. atmospheric) pressure separator 111 wherein it separates into a low pressure gaseous CO2 phase 112 and an extracted oil phase 113. The extracted oil phase 113 is transferred to a holding tank 114 for pipeline transport or for further upgrading.

The residual heavier phase 110 is transferred from the high pressure batch reactor 17 to a heated heavy residue holding tank 115. This residue may be used as fuel for the optional furnace 15 or for the production of steam and electricity 116.

Flue gasses 117 from the steam and electricity generation, along with the gasses released from aquathermolysis 14 and the flue gas/combustion exhaust 16 are fed to a CO2 cleaning and pressurisation apparatus 118. Waste gasses and sulphur 119 from the CO2 cleaning and pressurisation apparatus can be handled appropriately.

The cleaned CO2 may be used for EOR or may be permanently stored subsurface andlor it can be recycled 121 and transferred to a supercritical CO2 holding tank 122 for use in further extraction of heavy residue.

A further aspect of the invention contemplates the use of supercritical or near-supercntical CO2 in a process for upgrading a heavy hydrocarbon mixture as described herein, wherein said CO2 is ultimately captured and either recycled or stored subsurface, e.g. permanently stored subsurface. -12-

The invention is illustrated by the appended figures and following non-limiting examples in which: Figure 1 shows a schematic drawing of a supercritical CO2 extraction plant for heavy oil and bitumen; Figure 2 shows the results of slimtube experiments with an extra heavy oil (<100 API) under controlled temperature and pressure conditions, where (A) shows tentative refinery yield structures; (B) shows pressure changes; (C) shows weight percent of sulphur; (D) shows the C/H mass ratio; and (E) shows the sample composition by latroscan across the sample tubes; Figure 3 exemplifies refinery yield structures obtained from extraction experiments with extra heavy crude oil having <10° API (see Example 1). The original extra heavy oil (second bar) is diluted to pipeline specifications (left bar) and is upgraded to variable degrees with supercntical CO2 (right bars); and Figure 4 shows the results of contact experiments with 25° and 190 API crude oils under different conditions (see Example 2).

In the figure legends: "VRES" denotes vacuum residue; "VGO" denotes vacuum gas oil; "ARES" and "Atm. Resid" denote atmospheric residue, i.e. VRES and VGO; "HGO" denotes heavy gas oil; "LGO" denotes light gas oil; "KERO" denotes kerosene; "HVN" denotes heavy virgin naphtha; and "LVN" denotes light virgin naphtha. The terms "SATS", "ARO" and "Polars" respectively denote saturates, aromatics and polar components.

Example 1 -Slimtube experiment An experiment was carried out to determine the effects of continuously contacting extra heavy oils with supercritical CO2. The experiment was carried out in a typical slimtube made of Hastelloy C. The slimtube, packed with glass beads, had the following characteristics: -Column length: 1830 cm -Bulk volume: 380 ml -Pore volume: 137 ml -Porosity: 36% -13 - -Permeability: 4478 mD During the experiments the product could be constantly sampled (collected and weighed) before analysis by HTSimDist (simulated distillation for refinery yield structure), latroscan (for saturates, aromatics and polar components) and an element analyzer (for C, H, S and N according to ASTM D-5291 and D-5762). Sulphur (ASTM D-4294) and density (ASTM D-4052/5002) could also be analysed.

An extra heavy crude/bitumen quality hydrocarbon (<100 API) was extracted at 50°C employing a pressure at 15 MPa in a long-time slimtube experiment with supercritical CO2 as the mobile phase. At the end of the experiment the pressure was increased to 25 MPa. The following results were observed: In the first part of the experiment (50°C and 15 MPa pressure) the produced hydrocarbon quality became gradually richer in the middle distillate fractions (kerosine, light and heavy gas oils), from 15-20 wt% at the start, up to 75-80 wt% at the end of this period, whilst the atmospheric residue (vacuum gas oil and vacuum residue) at the same time was gradually reduced from around 80 wt% at the start to about 20 wt% at the end. The change in quality of the produced hydrocarbon is postulated to be due to moving from a push to a real extraction of the extra heavy crude oil/bitumen in the slimtube.

The ratio between carbon and hydrogen was during this period gradually reduced from approximately 8.7 in the parent hydrocarbon mixture to about 6.9-7.0 at the end of the 150 bar pressure period, i.e. the degree of hydrogen saturation was gradually increased in the recovered product compared to the parent hydrocarbon mixture. The product also showed a decreasing trend as to sulphur-containing components in the product, going from approx 5 wt% sulphur in the parent hydrocarbon mixture to an average of about 2.2 wt% sulphur in the extracted product at the end of the same pressure period. SARA analyses (saturates, aromatics, resins and asphaltenes) were also performed on the sampled product batches, showing that the saturate/aromatic/polar fraction composition changed from 27/46/27 to 5 1/37/12 over this phase of the experiment. The results support a selective recovery of good quality components in the products obtained by supercritical extraction of a extra heavy crude oil/bitumen sample, and the very best results were obtained when there was a good contact between the parent hydrocarbon mixture and the supereritical CO2 phase.

When the pressure was increased to 25 MPa, the CO2 became denser and started to cut deeper into the parent crude oil, resulting in a heavier refinery yield structure. Here, the atmospheric (long) residue increased again to an average of about 70 wt%, whilst the level of sulphur reached approximately 4 wt%. The relative composition as to saturated, aromatic and polar components reverted to a ratio closer to that of the parent hydrocarbon mixture whilst the carbon/hydrogen ratio reached a level of 8.3. Figure 2 shows the results from the slimtube experiment, demonstrating a gradual improvement in oil quality over time.

Upgrading using supercntical CO2 achieved a better refinery yield than conventional dilution of the extra heavy fraction.

It can be concluded that an extra heavy crude oil/bitumen can be extracted with supercritical CO2 at 50°C with pressures between 15 and 25 MPa, yielding a high value, pumpable product.

Example 2 -Supercritical CO2 extractions Experiments were carried out with different crude oils in a closed stainless steel cell under temperature and pressure control. The cell was equipped with valves to recover the lower oil phase and an upper C02-phase, and a window in the cell made it possible to see the two phases and the boundary between them.

Crude oils having API gravities of 190 and 25° were contacted with CO2 in the pressure cell and the upper (C02) and the lower (crude oil) phases could be sampled for compositional analyses, densities and viscosities. Temperature and pressure conditions employed were a) Temperature of 28°C, pressure of 7.4 MPa; b) Temperature of 35°C, pressure of 8.0 MPa; c) Temperature of 35°C, pressure of 12 MPa; and d) Temperature of 28°C, pressure of 12 MPa.

The experiments were carried out within a few hours and the results, shown in figure 4, demonstrate that extraction with supercritical CO2 at relatively low temperatures and pressures allows for selective upgrading of the yield structure. -15-

Claims (9)

1. Claims I. A process for upgrading a heavy hydrocarbon mixture, said process comprising: i) contacting the heavy hydrocarbon mixture with supercritical or near-supercritical CO2 under conditions suitable to produce a pumpable lighter fraction and a heavier residue from said hydrocarbon mixture; and ii) utilising at least a part of the heavier residue obtained in step (i) to generate steam for use in further extraction of the heavy hydrocarbon mixture and/or to produce electricity, wherein the CO2 used in step (i) and/or generated in step (ii) is either captured and recycled or is stored subsurface (e.g. permanently stored subsurface).
2. 2. A process according to claim 1, said process further comprising: iii) upgrading or refining the pumpable lighter fraction, e.g. by catalytic hydrocracking.
3. 3. A process according to claim 1 or claim 2 wherein the API gravity of the heavy hydrocarbon mixture is less than about 15 degrees.
4. 4. A process according to any one of claims 1 to 3 wherein the temperature of the CO2 in step (i) is between about 25 and 70 °C.
5. 5. A process acëording to any one of claims Ito 4 wherein the pressure of the CO2 in step (i) is between about 7.4 and 30 MPa.
6. 6. A process for producing a pumpable crude oil comprising a process as claimed in any one of claims 1 to S and further comprising the step of separating the pumpable lighter fraction from the heavier residue and optionally separating the pumpable lighter fraction from the majority of the supercritical or near-supercritical CO2.

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7. 7. A pumpable crude oil producible by a process as claimed in claim 6.
8. 8. An apparatus adapted for use in a process according to any one of claims I to 6.
9. 9. Use of supercntical or near-supercritical CO2 in a process for upgrading a heavy hydrocarbon mixture, wherein said CO2 is ultimately captured and either recycled or stored subsurface (e.g. permanently stored subsurface).