A process for producing hydrogen gas from a subterranean light oil reservoir

Abstract

The present invention relates to process for producing hydrogen gas from a subterranean light oil reservoir.

Description

[0001] A PROCESS FOR PRODUCING HYDROGEN GAS FROM A SUBTERRANEAN LIGHT OIL RESERVOIR

[0002] Field of the invention

[0003] The present invention relates to process for producing hydrogen gas from a subterranean light oil reservoir.

[0004] Background of the invention

[0005] The hydrogen economy

[0006] To combat climate change and secure sustainable clean energy resources, one solution that is receiving ever-increasing attention, research and investment is the “hydrogen economy” where hydrogen replaces hydrocarbons as a fuel source. Hydrogen as an energy resource is a promising alternative to traditional fossil fuels because of its high energy density of 142 MJ / kg (Higher Calorific Value) in mass terms and zero carbon emissions. There is therefore huge and growing interest in the production of hydrogen in an environmentally beneficial manner. For the hydrogen economy to be viable and to have a meaningful environmental impact, huge amounts of hydrogen will be required, and multiple production methods are being developed. All will be required. However, there will be no overall environmental benefit and potentially a negative impact if the production of the hydrogen itself requires high levels of resources (such as energy or fresh water) or produces high amounts of carbon dioxide.

[0007] Hydrogen is currently produced on a large industrial scale and used in processes such as the reforming of heavy hydrocarbons, as in the production of oil from tar sands, or in industrial processes such as the production of ammonia to make fertilisers. Currently hydrogen is typically made by reforming methane or other hydrocarbons with steam or water. This process produces “synthesis gas” or syngas which is typically a mixture of hydrogen, carbon dioxide, methane and other light hydrocarbons in varying proportions depending on the process conditions and raw materials etc. This reaction has been extensively studied and optimised and is typically done at high temperatures between 400°C and 800°C, often using a variety of catalysts depending on the desired final products. Current production methods produce high levels (typically 8-12 kg) of carbon dioxide per kg of hydrogen as historically there has not been any focus on reducing CO2 emissions.

[0008] This is an energy intensive process and results in the production of high levels of carbon dioxide per kg of hydrogen. Carbon capture technologies can be used to deal with the CO2 emissions to produce clean hydrogen. However, unless the carbon dioxide is then sequestered or used elsewhere, carbon capture in isolation does not provide any environmental benefit.

[0009] A type of syngas can also be produced from the reaction of carbon (i.e., coal) with steam. This process was used historically to produce so-called “town gas”, a mixture of hydrogen and carbon monoxide, from coal to provide heating and light until replaced by electricity or natural gas. A very high temperature combustion bed of coal would be formed by the forced combustion of coal with air. Periodically the air supply would be shut off and replaced by steam injection. Water would react with carbon to form carbon monoxide and hydrogen by an endothermic reaction. Once the temperature of the hot coal bed had dropped to some minimum value, the steam would be replaced with air to heat the bed up again. This process produced hydrogen but with the associated production of very high levels of CO2, whether directly from combustion in the coal bed or when the carbon monoxide was subsequently burnt.

[0010] Utilisation of Oil Fields

[0011] Many oil fields in mature production areas, such as North-West Europe, have now reached the end of their economic oil production life. The extraction of oil from older fields is very typically assisted by a range of “Enhanced-Oil Recovery” or EOR techniques. Primary recovery of oil relies on the pressure within the reservoir / production system without major intervention. Secondary recovery typically uses injected water or gas to increase the recovery of the reserves by maintaining or increasing the reservoir pressure and / or introducing external fluid to force the oil to flow. Tertiary recovery uses a variety of additional techniques to improve the flow and recovery characteristic in the reservoir, including chemical injection or thermal technologies, such as in-situ combustion and fire flooding, which can lower the viscosity of the crude oil by heating and chemical modification. Gases can be injected or formed in-situ to increase pressure or react with the oil or otherwise change its properties to make it more suitable for extraction. Often, multiple different EOR techniques will be used in a field at different stages in the production lifetime of the field. However, there will always come a point when the amount of oil that is being extracted from the reservoir is insufficient in relation to the costs. Oil can be extracted technically long after it becomes uneconomic to produce conventionally. But eventually the value becomes less than the required production costs. A reservoir at this stage is typically referred to as “depleted”.

[0012] However, this does not mean that there is no oil left in the reservoir. In fact, frequently most of the oil in the reservoir is still present as residual oil. It just cannot be economically extracted as oil, or the conventional processes of oil production may be terminated for other reasons such as changes in the regulations governing oil production, or for instance social or environmental pressures, among other reasons. The term “residual oil” is taken to mean this oil which remains in the reservoir at the point at which conventional oil extraction ceases. Typically, the residual light oil is the light oil that remains in the depleted reservoir after primary recovery. The residual light oil may be light oil that remains in the depleted reservoir after primary and secondary recovery. The residual light oil may even be the residual light oil that remains in the depleted reservoir after primary, secondary and tertiary recovery.

[0013] However, if this residual oil can be used as a resource to produce hydrogen, preferably in a manner that can also limit and trap CO2 emissions, it can provide a valuable source of hydrogen. Limiting CO2 emissions is easier to achieve if the overall production of hydrogen is done in the most efficient, including the most energy efficient, manner possible. The term “overall” is used to describe all the steps or processes involved in the production of hydrogen. For example, this includes production of the energy used to separate or compress gases; or pump liquids for injection into the reservoir or to heat the reservoir. Producing hydrogen in an overall energy-efficient manner obviously reduces the amounts of carbon dioxide that are generated.

[0014] The gasification of hydrocarbons produces “synthesis gas” or “syngas”. The production of carbon monoxide from residual light oil in a reservoir can also be valuable to produce hydrogen as it can be reacted with water by the water-gas shift reaction (WGSR) to produce hydrogen. Processes that maximise the production of hydrogen in the syngas from in-situ gasification of the residual oil are especially beneficial Maximizing the hydrogen yield is especially beneficial. Therefore, processes that enhance the production of hydrogen from a hydrocarbon source in a low overall energy manner can give an enhanced and efficient overall production of hydrogen. The synthesis gas may be further treated during and / or after initial extraction. Such further treatments can include, for example, purification, passage through a WGSR and carbon dioxide capture, usage or storage. However, it is preferable for the syngas to contain high levels of hydrogen to simplify post-synthesis processing and purification.

[0015] Oil Types and Implications Thereof

[0016] Hydrocarbon reservoirs can include gas, light oil, heavy oil, and bitumen as well as the various coal types. Light oil describes lower molecular weight and lower viscosity oils. Light oil is typically defined by its API gravity (derived from density). In this document the term “light oil” means oil typically having an American Petroleum Institute (API) gravity of greater than 22 degrees. Light oil can have an API gravity of from greater than 34 degrees, or from 38 degrees or greater, or from greater than 38 degrees. Heavy oil typically has an API gravity of 22 degree or lower. The light oil may have an API gravity in the range of from greater than 22 to 50° API, or from 34 to 50° API, or from 38 to 50° API, or from greater than 38 to 50° API. Heavy oils contain higher molecular weight hydrocarbon species such as asphaltene, are much more viscous and have a lower API gravity, typically 22° API gravity or below. These different oils behave and react differently due to their chemical differences. Crude oil is a very complex mixture of hydrocarbons and the conditions that are suitable and relevant for one type of oil, such as heavy oil, will not necessarily be directly suitable or relevant to other oil types. For example, researchers have seen that the thermal activation energy of heavy oils very typically differs from that of light oil for partial oxidation reactions. At lower temperatures, light oil has a lower activation energy for partial oxidation. At higher temperatures this trend has been seen to reverse. Likewise, the conditions used in processes that utilise coal as the hydrocarbon source, such as in-situ gasification, cannot be simply re-applied to oil field conditions. Typically, the complexity of the various possible reaction pathways in processes such as the partial oxidation of oil means that modelling, often combined with empirical testing is often used. In addition, most of the processes that do include production of at least some hydrogen within oil fields, such as use of in-situ combustion to raise temperature and pressures or partial oxidation, are designed to increase oil recovery or to produce methane and other light hydrocarbons rather than the production of hydrogen. As such, process conditions to produce a high proportion of hydrogen in the syngas have not been of interest to most investigators. The production of any hydrogen is mostly an incidental result, rather than a primary objective, and hydrogen is often simply flared off along with other unwanted gases. Oil field gasification processes developed to date often focus on the production of gases from heavy oil, shale oil or even tar sand, fields. Heavy oil reservoirs are usually closer to the surface than light oil reservoirs and are usually at lower temperatures and pressures. Tar sands are typically surface mined. Hence, they are typically much easier to access and operate in. The temperature and pressure at which oil oxidation reactions and subsequent gasification reactions happen affect the reaction kinetics and pathways of the reactions and will change the composition of the resulting gases.

[0017] Attention and focus for production of hydrogen to date has also been on the more accessible and utilisable heavy oil reservoirs. The differences in the chemistries and conditions, especially the temperatures, pressures, and crude oil API gravity mean that processes and conditions suitable for heavy oil fields do not automatically apply to light oil reservoirs.

[0018] There are, however, many light-oil fields in locations such as North-West Europe, and elsewhere globally, which have infrastructure in place, but which have come to the end of their conventional oil producing life. Such fields are a valuable resource with the potential to produce large amounts of hydrogen. Oil fields, especially light oil fields which are very typically deeper and hence more sealed by rock, can also act as reservoirs for permanent carbon dioxide sequestration.

[0019] There is therefore a specific benefit in the development of processes and process conditions that can utilise light oil fields / reservoirs, especially those that are depleted, for the energy efficient production of hydrogen and that can work effectively at the higher pressures and conditions experienced in deeper oil fields.

[0020] Reduction of Carbon Dioxide Production

[0021] The classification of low-carbon hydrogen depends on the ratio of the quantity of carbon dioxide produced, and emitted to the atmosphere, to the quantity of hydrogen produced in the same process. For instance, in the EU since April 2023, to be considered low-carbon hydrogen the associated carbon dioxide emitted must be less than 3.38 kg CCh / kg H2. A particularly efficient and preferred approach to hydrogen production from subterranean oil reservoir gasification is to keep at least part of the carbon dioxide down in the reservoir, or to minimise the production of carbon dioxide in the first place. This reduces the amount of carbon dioxide that may need to be subsequently injected back into the reservoir or otherwise dealt with. One option to achieve this is to use selectively permeable membranes within the extraction pipes in the borehole which preferentially allow passage of gases apart from carbon dioxide through the membrane. Other gases can be dealt with at the surface.

[0022] Such an approach does not require the separation of gases to be highly efficient to provide worthwhile benefits so long as at least part of the carbon dioxide is prevented from reaching the surface. It is advantageous if any separation of gases in-situ can be done at lower temperatures. Many membranes, especially polymeric membranes, are not thermally resistant and / or physically robust at elevated temperatures and cannot be used in high temperatures, such as greater than 200°C, or greater than 300 °C.

[0023] It is also possible to subj ect the produced gases to additional treatment steps within the borehole prior to recovery to the surface. Such additional steps could include reacting carbon monoxide in the produced gas stream to produce hydrogen via the WGSR. This can be done by passing the produced gases through a catalyst material within the bore at relatively low temperature. Being able to carry out the WGSR at low temperatures, such as less than 200 °C, is very advantageous for the integrity of the borehole. Separating the reactions within the oil reservoir that produce the initial hydrogen and carbon monoxide from the further reaction of the carbon monoxide to produce hydrogen can allow each set of conditions to be individually optimised and thus increase the overall production of the valuable gases. It may be preferred to carry out the secondary treatment step before the separation of at least part of the carbon dioxide or simultaneously. A preferred feature is the use of a bifunctional membrane which can catalyse the reaction of carbon monoxide with water at low temperatures and preferentially allowing hydrogen to pass through.

[0024] Prior Art Relating to Hydrogen Production

[0025] Gasification of hydrocarbons and production of hydrogen by partial oxidation is known with many variants and production processes. Typically, such processes are carried out in surface reactors and the conditions and learnings are simply not applicable to the in-situ production of hydrogen. For example, the WGSR is typically carried out in a two-stage process with a high temperature and a low temperature step and using very specific catalysts. Such processes require specific reactors or control of reaction conditions and are not applicable to in-situ processes.

[0026] Advantageously, the process conditions identified by the inventors for the efficient production of hydrogen in light oil fields have not required the use of catalysts. Some processes require the injection of metal catalytic species into the well to help with the in-situ reactions of the oil, for example to catalyse the production of hydrogen. This adds cost and complexity. The process of the present invention can, and preferably does, avoid the need to use such metal catalytic species.

[0027] Some processes simply apply heat to a water saturated oil-bearing substrate to form hydrogen amongst other products. This does form a type of syngas but needs high temperatures to be most effective and is chemically different to partial oxidation. Such techniques typically work better for heavy oil. One reported approach uses RF (radio frequency) heating of the field. Other processes use resistance heating (often called ohmic heating). Another approach is to inject superheated steam into the reservoir, this is often known as steam reforming. Such processes require high amounts of energy, and the energy must come from somewhere. Very typically, the energy required for such processes will come from the burning of hydrocarbons. Such approaches can result in high emissions of carbon dioxide, given the lower conversion efficiency of the combustion energy into electricity. In addition, high process temperatures are not compatible with maintaining borehole integrity.

[0028] US11530603B2 (Proton Technologies) describes an in-situ process to produce hydrogen from hydrocarbon reservoirs. The gases produced in the reservoir are passed through a highly selective hydrogen permeable membrane such that only hydrogen is extracted to the surface. This does not allow for the presence of other gases, such as nitrogen from injected air, that may need to be removed from the well. Over time such gases would build up around the extraction bore.

[0029] In addition, US11530603B2 does not focus on the production of hydrogen and does not describe the unique problems associated with light oil fields. It does not describe the use of low temperature oxidation processes of less than 300 °C and teaches that oxidation should happen at between 400 and 700 °C. Lower temperatures are described but solely when ohmic (electrical resistance) heating, which does not involve combustion or partial combustion and hence results in different chemistries, is used. Ohmic heating uses high levels of electricity and is not energy efficient. Heating of water and hydrocarbons only, especially at lower temperatures, typically results in higher levels of hydrocarbon cracking and production of light hydrocarbon species and carbon oxides rather than optimised hydrogen production. High temperature oxidation processes in light oil form higher levels of carbon dioxide and can encourage loss of hydrogen in the reservoir by the reverse WGSR.

[0030] US7836957 describes a process to produce a synthesis gas comprising hydrogen from shale oil reservoirs based on the partial oxidation of oil by the injection of oxidizing gas. It teaches that it is important to use an oxidizing gas having greater than 50% oxygen. It gives no specific conditions or ranges or limits on conditions for such a process to maximise hydrogen production. It does not address issues associated with hydrogen production from light oil fields.

[0031] WO2019169492A1 (Proton Technologies) describes the in-situ production and extraction of synthesis gas comprising hydrogen from petroleum reservoirs. No details are given about process conditions and the problems associated with light oil fields are not recognised or addressed.

[0032] US7431084B1 (University of California) describes coal gasification to produce hydrogen, including the use of dual functionality membranes to catalyse the WGSR and selectively allow hydrogen to pass to the surface. Membranes include hydrophobic silica membranes and palladium containing membranes. No process conditions are given or described.

[0033] DE102022203221B3 relates to a method of extracting hydrogen from hydrocarbons. This method requires the presence of a particulate catalyst. This method requires the injection of a particulate catalyst and oxygen-containing gas into a reservoir. It teaches that temperatures in the reservoir need to be above 200 °C as temperatures below 200 °C “<7o not allow efficient activation of the hydrocarbons to form hydrogen”. Temperatures of 300 °C to 400 °C are especially preferred with an upper limit of 500 °C.

[0034] Problem solution statement

[0035] The combustion of fossil fuels has contributed to climate change and global warming through the emissions of carbon dioxide and other greenhouse gases. The world needs large volumes of hydrogen with low-carbon and ultimately net-zero carbon emissions (“net-zero hydrogen”), as discussed by the EU Hydrogen Strategy and many other such publications. Supply quantities from other low carbon hydrogen sources are, however, growing too slowly to meet global net-zero goals.

[0036] The invention as described in this application provides a viable solution. That is to produce hydrogen-rich syngas in an energy efficient manner in oil reservoirs, and to separate the produced syngas to produce high purity hydrogen. Preferably, the production of other gases such as carbon dioxide is minimised. The present invention can produce hydrogen-rich syngas with low to very low levels of carbon dioxide.

[0037] This may be applied in thousands of mature (such as depleted) oil reservoirs around the world. The importance of this is to rapidly increase global supplies of low carbon hydrogen in the immediate future to fill supply deficits to satisfy growing demand, while the production capacity increases more slowly of other low-carbon hydrogen production processes, such as hydrogen made from electrolysis of water using renewable power or other means.

[0038] There is a current and future need to produce hydrogen, preferably with low levels of carbon dioxide emissions, and with low levels of carbon monoxide, and preferably at very low temperatures, and preferably without the need for any particulate catalyst such as a metallic catalyst, and preferably without the injection of water into the reservoir. There remains an ongoing benefit to utilise oil fields, especially depleted light oil fields. The present invention provides a process and process conditions for the efficient production of hydrogen in a low energy / carbon dioxide manner from such fields and preferably including sequestration of produced carbon dioxide. The present invention seeks to maximize the yield of hydrogen, relative to other components of the syngas such as carbon dioxide and carbon monoxide.

[0039] Summary of the invention

[0040] The present invention provides a process for producing hydrogen gas from a subterranean light oil reservoir. The reservoir is connected to the surface by at least one injection borehole and at least one extraction borehole. The reservoir comprises residual light oil and connate brine. The process comprises the steps: (a) introducing an oxidizing gas into the injection borehole such that the oxidizing gas passes into the reservoir and contacts the residual light oil and connate brine present in the reservoir;

[0041] (b) initiating low temperature oxidation (LTO) reactions in the reservoir between the oxidizing gas, residual light oil and connate brine in a LTO reaction zone;

[0042] (c) maintaining the temperature of the LTO reaction zone in the range of from 60°C to 300°C and generating a low temperature oxidation (LTO) synthesis gas that comprises at least 10v / v% hydrogen;

[0043] (d) extracting the LTO synthesis gas from the porous reservoir via an extraction borehole; and

[0044] (e) separating and purifying the hydrogen from the LTO synthesis gas to form hydrogen gas.

[0045] Detailed description of the invention

[0046] Process for producing hydrogen gas

[0047] The process produces hydrogen gas from a subterranean light oil reservoir. The reservoir being connected to the surface by at least one injection borehole and at least one extraction borehole, and with the reservoir comprising residual light oil and connate brine. The process comprises the steps:

[0048] (a) introducing an oxidizing gas into the injection borehole such that the oxidizing gas passes into the reservoir and contacts the residual light oil and connate brine present in the reservoir;

[0049] (b) initiating low temperature oxidation (LTO) reactions in the reservoir between the oxidizing gas, residual light oil and connate brine in a LTO reaction zone; (c) maintaining the temperature of the LTO reaction zone in the range of from 60°C to 300°C and generating a low temperature oxidation (LTO) synthesis gas that comprises at least 10v / v% hydrogen;

[0050] (d) extracting the LTO synthesis gas from the porous reservoir via an extraction borehole; and

[0051] (e) separating and purifying the hydrogen from the LTO synthesis gas to form hydrogen gas.

[0052] Carbon dioxide may be introduced into the injection borehole. This introduction of carbon dioxide is described in more detail below.

[0053] The LTO synthesis gas may be retained and stored within the reservoir prior to being extracted via at least one extraction bore hole during step (d).

[0054] The storage of the LTO synthesis gas may be intended as part of a planned hydrogen storage facility for cycling gases into and out of the reservoir, or it may be a result of the operation of the process, wherein the extraction flow rate varies independently from the injection flowrate, and therefore syngas accumulates or depletes from the reservoir, while the reservoir acts as a storage vessel for hydrogen and other components of the syngas.

[0055] The process is designed to produce hydrogen and not to extract light oil from the reservoir. As such, typically no light oil is continuously extracted from the reservoir. Preferably, as little light oil as possible is extracted from the reservoir. The aim of this process is not to produce light oil as EOR, but to maximise production of hydrogen. In some instances, light oil may be produced for a period of time dependent on the reservoir behaviour and is likely to fluctuate.

[0056] The process is simple and avoids more complicated and expensive hydrogen extraction techniques. As such, typically no hydrogen-selective-membrane is inserted into the extraction borehole, and / or typically no particulate catalysts, and / or typically no catalytic metal species, are introduced into the reservoir via an injection borehole. Preferably, no particulate catalyst, such as a metallic catalyst, is introduced into the reservoir. Preferably, no water is introduced into the reservoir. By not introducing a particulate catalyst, such as metallic catalyst, into the reservoir the in-situ processes can operate more efficiently, maintaining reservoir flow integrity, and enhancing hydrogen recovery and conditions for chemical conversion of the hydrocarbons to hydrogen. Whilst particulate catalysts, such as metallic catalysts, can benefit specific reaction pathways to accelerate products, the process of the present invention, and especially the desired hydrogen yield obtained therefrom, does not require such a particulate catalyst, such as a metallic catalyst. There are benefits by not introducing a particulate catalyst, such as a metallic catalyst, into the reservoir. Such an injection of a particulate catalyst such as a metallic catalyst, into the reservoir risks plugging or blocking the permeability of the near-wellbore volume of the reservoir, which in turn will degrade the injectivity and hydrocarbon flow potential of the well and limit the physical contact of the oxidant gas with the crude oil. The process of the present invention can be used on a variety of reservoirs, including reservoirs with relatively low permeability as well as reservoirs with relatively high permeability.

[0057] Other advantages of not introducing particulate catalysts, such as metallic catalysts, into the reservoir, include the avoidance of issues associated with catalyst deactivation, avoids particulate agglomeration issues, avoids thermal instability of any catalyst, avoids catalyst migration issues, avoids any risk of chemical contamination, and reduces the burden of monitoring and maintaining the catalyst. Over time, particulate catalysts can deactivate due to sintering, poisoning, or coking, reducing their efficacy. By avoiding the introduction of a particulate catalyst into the reservoir, there is no need for continuous re-injection or costly regeneration methods for the catalyst: each regeneration risks further plugging, by injecting further particulates carried in water. Particulate catalysts can cluster together under high- temperature and high-pressure reservoir conditions, reducing their surface area and effectiveness while increasing the risk of pore blockage. Particulate catalysts can degrade or lose effectiveness under extreme in-situ combustion temperatures. Particulate catalysts can be carried away from the reaction zone (e.g., by fluid flow), reducing efficiency and making recovery difficult. Particulate catalysts may introduce unwanted metals or residues that may alter the reservoir chemistry in an undesirable manner. Different reservoirs may require different catalysts, since reservoir fluids vary geochemically; where possible a catalyst-free approach avoids this challenge and can be used in a much wider range of reservoirs. The absence of a particulate catalyst also avoids the need for continuous catalyst performance tracking or reinjection strategies. If a particulate catalyst, such as a metallic catalyst, is introduced into the reservoir, then preferably the concentration of the catalyst dispersed in the injected liquid is as low as possible, such as a concentration in the liquid of less than 0. Ig / L, or less than 0.05g / L, or less than O.Olg / L, or less than 0.005g / L, or less than O.OOlg / L., and preferably from above Og / L to less than 0. Ig / L, or from above Og / L to less than 0.05g / L, or from above Og / L to less than O.Olg / L, or from above Og / L to less than 0.005g / L, or from above Og / L to less than O.OOlg / L.

[0058] If a particulate catalyst, such as a metallic catalyst, is introduced into the reservoir, then preferably the particulate catalyst has a weight average particle of less than 100 nanometres, or even less than 10 nanometres. The particle size is typically measured by dynamic light scattering techniques, typically using equipment such as the Zetasizer Advance range from Malvern Panalytical.

[0059] Not introducing water into the reservoir also provides benefits by helping to control water saturation (Sw) and preventing the detrimental effects where hydrogen potential falls as water saturation increases. The absence of water introduction also minimises any unintended oil displacement or flushing of oil from the reaction zone due to water injection, thus preserving the hydrocarbon distribution and accessibility to the oxidant gas.

[0060] If water is introduced in the reservoir, then preferably, the amount of water present in the reservoir is less than 5% of the pore volume of the reservoir, preferably less than 4%, or less than 3%, or less than 2%, or even less than 1% of the pore volume of the reservoir, and preferably from above 0% to less than 5%, or from above 0% to less than 4%, for from above 0% to less than 3%, or from above 0% to less than 2%, or even from above 0% to less than 1% of the pore volume of the reservoir.

[0061] Hydrogen from external sources may be injected into and stored in the reservoir with the LTO synthesis gas prior to being extracted via at least one extraction bore hole during step (d).

[0062] Step (a) introduction of an oxidizing gas

[0063] Step (a) introduces an oxidizing gas into the injection borehole such that the oxidizing gas passes into the reservoir and contacts the residual light oil and connate brine present in the reservoir. The oxidizing gas is typically compressed prior to injection into the well. Step (b) initiation of low temperature oxidation (LTO) reactions

[0064] Step (b) initiates low temperature oxidation (LTO) reactions in the reservoir between the oxidizing gas, residual light oil and connate brine in a LTO reaction zone.

[0065] Preferably, during step (b) an initiator is introduced into the porous reservoir and initiates the LTO reactions. The initiator is described in more detail below.

[0066] Step (c) temperature maintenance of the LTO reaction zone

[0067] Step (c) maintains the temperature of the LTO reaction zone in the range of from 60°C to 300°C. The temperature is typically maintained for an extended period of time, such as weeks or even months. During step (c), a low temperature oxidation (LTO) synthesis gas is generated. The LTO synthesis gas comprises at least 16.5v / v% hydrogen, or at least 18v / v% hydrogen, or at least 20v / v% hydrogen.

[0068] During step (c), the LTO reactions may also produce carbon monoxide. The production of carbon monoxide is described in more detail below.

[0069] During step (c), the LTO reactions may also produce carbon dioxide. However, at the lower temperature range of the invention, much of this carbon dioxide will be retained within the reservoir due to density effects and dissolution into the reservoir fluids, especially the residual oil. This means that the residual oil present in the reservoir, not only acts as a fuel source for the LTO reactions to product hydrogen but will also act as an adsorbent for the carbon dioxide that is produced.

[0070] The LTO synthesis gas may additionally comprise from above 0v / v% to 65v / v%, or from above 0v / v% to 50v / v%, or from above 0v / v% to 40v / v%, or from above 0v / v% to 30v / v%, or from above 0v / v% to 20v / v%, or from above 0v / v% to 10v / v%, or from 30v / v% to 65v / v% carbon dioxide. Preferably, the carbon dioxide is subsequently separated and purified from the LTO synthesis gas to form carbon dioxide gas. Any suitable means can be used to separate the carbon dioxide, including amine absorbents, metal-organic framework adsorbents and hot potassium carbonate solution absorbents. The LTO synthesis gas may even be free of carbon dioxide. Preferably, carbon dioxide gas generated during step (c) is re-injected into the reservoir. As well as providing a convenient and robust location for the sequestering of carbon dioxide, injection of carbon dioxide as part of the oxidising gas can potentially increase the relative production of carbon monoxide.

[0071] Preferably, at least some of the carbon dioxide produced during step (c) dissolves in the reservoir fluids.

[0072] Preferably, during step (c) the pressure of the LTO reaction zone is maintained in the range of from 3xl06to 3xl07pascals.

[0073] Preferably, during step (c) the temperature of the LTO reaction zone is maintained in the range of from 60°C to 290°C, or in the range of from 60°C to 280°C, or in the range of from 60°C to 270°C, or in the range of from 60°C to 260°C, or in the range of from 60°C to 250°C, or in the range of from 60°C to 240°C, or in the range of from 60°C to 230°C, or in the range of from 60°C to 220°C, or in the range of from 60°C to 210°C, or in the range of from 60°C to 200°C, or in the range of from 60°C to less than 200°C, or in the range of from 60°C to 195°C, or in the range of from 60°C to 190°C, or in the range of from 60°C to 185°C, or in the range of from 60°C to 180°C, or in the range of from 60°C to 175°C, or in the range of from 60°C to 170°C, or in the range of from 60°C to 165°C, or in the range of from 60°C to 160°C, or in the range of from 60°C to 155°C.

[0074] Preferably, during step (c) the temperature of the LTO reaction zone is maintained in the range of from 70°C to 300°C, or in the range of from 70°C to 290°C, or in the range of from 70°C to 280°C, or in the range of from 70°C to 270°C, or in the range of from 70°C to 260°C, or in the range of from 70°C to 250°C, or in the range of from 70°C to 240°C, or in the range of from 70°C to 230°C, or in the range of from 70°C to 220°C, or in the range of from 70°C to 210°C, or in the range of from 70°C to 200°C, or in the range of from 70°C to less than 200°C, or in the range of from 70°C to 195°C, or in the range of from 70°C to 190°C, or in the range of from 70°C to 185°C, or in the range of from 70°C to 180°C, or in the range of from 70°C to 175°C, or in the range of from 70°C to 170°C, or in the range of from 70°C to 165°C, or in the range of from 70°C to 160°C, or in the range of from 70°C to 155°C. Preferably, during step (c) the temperature of the LTO reaction zone is maintained in the range of from 80°C to 300°C, or in the range of from 80°C to 290°C, or in the range of from 80°C to 280°C, or in the range of from 80°C to 270°C, or in the range of from 80°C to 260°C, or in the range of from 80°C to 250°C, or in the range of from 80°C to 240°C, or in the range of from 80°C to 230°C, or in the range of from 80°C to 220°C, or in the range of from 80°C to 210°C, or in the range of from 80°C to 200°C, or in the range of from 80°C to less than 200°C, or in the range of from 80°C to 195°C, or in the range of from 80°C to 190°C, or in the range of from 80°C to 185°C, or in the range of from 80°C to 180°C, or in the range of from 80°C to 175°C, or in the range of from 80°C to 170°C, or in the range of from 80°C to 165°C, or in the range of from 80°C to 160°C, or in the range of from 80°C to 155°C.

[0075] Preferably, during step (c) the temperature of the LTO reaction zone is maintained in the range of from 90°C to 290°C, or in the range of from 90°C to 280°C, or in the range of from 90°C to 270°C, or in the range of from 90°C to 260°C, or in the range of from 90°C to 250°C, or in the range of from 90°C to 240°C, or in the range of from 90°C to 230°C, or in the range of from 90°C to 220°C, or in the range of from 90°C to 210°C, or in the range of from 90°C to 200°C, or in the range of from 90°C to less than 200°C, or in the range of from 90°C to 195°C, or in the range of from 90°C to 190°C, or in the range of from 90°C to 185°C, or in the range of from 90°C to 180°C, or in the range of from 90°C to 175°C, or in the range of from 90°C to 170°C, or in the range of from 90°C to 165°C, or in the range of from 90°C to 160°C, or in the range of from 90°C to 155°C.

[0076] Preferably, during step (c) the temperature of the LTO reaction zone is maintained in the range of from 100°C to 300°C, or in the range of from 100°C to 290°C, or in the range of from 100°C to 280°C, or in the range of from 100°C to 270°C, or in the range of from 100°C to 260°C, or in the range of from 100°C to 250°C, or in the range of from 100°C to 240°C, or in the range of from 100°C to 230°C, or in the range of from 100°C to 220°C, or in the range of from 100°C to 210°C, or in the range of from 100°C to 200°C, or in the range of from 100°C to less than 200°C, or in the range of from 100°C to 195°C, or in the range of from 100°C to 190°C, or in the range of from 100°C to 185°C, or in the range of from 100°C to 180°C, or in the range of from 100°C to 175°C, or in the range of from 100°C to 170°C, or in the range of from 100°C to 165°C, or in the range of from 100°C to 160°C, or in the range of from 100°C to 155°C. Preferably, during step (c) the temperature of the LTO reaction zone is maintained in the range of from 120°C to 290°C, or in the range of from 120°C to 280°C, or in the range of from 120°C to 270°C, or in the range of from 120°C to 260°C, or in the range of from 120°C to 250°C, or in the range of from 120°C to 240°C, or in the range of from 120°C to 230°C, or in the range of from 120°C to 220°C, or in the range of from 120°C to 210°C, or in the range of from 120°C to 200°C, or in the range of from 120°C to less than 200°C, or in the range of from 120°C to 195°C, or in the range of from 120°C to 190°C, or in the range of from 120°C to 185°C, or in the range of from 120°C to 180°C, or in the range of from 120°C to 175°C, or in the range of from 120°C to 170°C, or in the range of from 120°C to 165°C, or in the range of from 120°C to 160°C, or in the range of from 120°C to 155°C.

[0077] Preferably, during step (c) the temperature of the LTO reaction zone is maintained in the range of from 130°C to 290°C, or in the range of from 130°C to 280°C, or in the range of from 130°C to 270°C, or in the range of from 130°C to 260°C, or in the range of from 130°C to 250°C, or in the range of from 130°C to 240°C, or in the range of from 130°C to 230°C, or in the range of from 130°C to 220°C, or in the range of from 130°C to 210°C, or in the range of from 130°C to 200°C, or in the range of from 130°C to less than 200°C, or in the range of from 130°C to 195°C, or in the range of from 130°C to 190°C, or in the range of from 130°C to 185°C, or in the range of from 130°C to 180°C, or in the range of from 130°C to 175°C, or in the range of from 130°C to 170°C, or in the range of from 130°C to 165°C, or in the range of from 130°C to 160°C, or in the range of from 130°C to 155°C.

[0078] Being able to produce hydrogen at lower temperatures has the important advantage of enabling the use in the reservoir of a much wider range of standard equipment rather than specialised, high-temperature resistant materials.

[0079] Typically, during step (c) the temperature of the LTO reaction zone is maintained at the required temperature range for an extended period of time, such as weeks or even months, preferably at least for 1 day, or at least 5 days, or at least 7 days, or at least 14 days, or at least 21 days, or at least 28 days, or at least 42 days, or at least 56 days.

[0080] The use of such low temperatures is required for the efficient production of hydrogen from light oil reservoirs. If the temperature is too low to initiate the partial oxidation reactions, the injected gas will simply act to simply increase pressure in the reservoir and will not usefully produce hydrogen. If the temperature is too high, the resulting synthesis gas contains lower levels of hydrogen and higher levels of carbon dioxide. In addition, the stresses placed on the equipment at these low temperatures is very significantly reduced.

[0081] During step (c), the temperature of the LTO reaction zone can be maintained by controlling the injection rate and oxygen content of the oxidizing gas being introduced into the injection borehole. This can further be affected by varying the extraction rate of the syngas at the extraction well, or by injecting other fluids at the injection well such as water or steam or gaseous or liquid fuels.

[0082] During step (c) the injection rate and / or oxygen content of the oxidizing gas being introduced into the injection borehole can be increased during this step. Preferably both the injection rate and oxygen content of the oxidizing gas are increased during the production of the LTO synthesis gas. This has been seen to best maintain the production of hydrogen throughout the working life of the reservoir.

[0083] Typically, the temperature of the LTO reaction zone can be measured directly in the bore, or can be measured indirectly by measurement of the extraction gas temperature at the surface and use of industry correlations to calculate the temperature down-hole.

[0084] Step (d) extraction of the LTO synthesis gas

[0085] Step (d) extracts the LTO synthesis gas from the porous reservoir via an extraction borehole. Any suitable extraction means can be used.

[0086] During step (d), the LTO synthesis gas can be extracted from the reservoir via an extraction borehole by means of an artificial lift mechanism.

[0087] Step (e) separation and purification of hydrogen

[0088] Step (e) separates and purifies the hydrogen from the LTO synthesis gas to form hydrogen gas.

[0089] Preferably, during step (e) the hydrogen from the LTO synthesis gas is separated and purified to form hydrogen gas. Typically, this is done by use of membranes. Preferably the LTO synthesis gas can be passed through further purification means either before, during or after removal of hydrogen such as a water-gas shift (WGS) reactor and a carbon dioxide separation unit.

[0090] Production of carbon monoxide

[0091] Preferably, during step (c) the LTO reactions also produce carbon monoxide. Preferably, the LTO synthesis gas comprises greater than 1.0v / v%, or greater than 2.0v / v%, or greater than or greater than 3.0v / v%, or greater than 4.0v / v%, or greater than 5.0v / v% carbon monoxide. Typically, the LTO synthesis gas comprises less than 15v / v% carbon monoxide or less than 12v / v% carbon monoxide.

[0092] Preferably, the carbon monoxide is separated from the remainder of the LTO synthesis gas, and subsequently reacted with water to undergo a water-gas shift (WGS) reaction to form hydrogen and carbon dioxide. This WGS reaction preferably occurs on the surface.

[0093] Introduction of carbon dioxide

[0094] Carbon dioxide can be introduced into the injection borehole such that the carbon dioxide passes into the reservoir and contacts the reservoir fluids. Preferably, at least some of the carbon dioxide dissolves in the reservoir fluid.

[0095] Preferably, the carbon dioxide is introduced into a separate injection borehole from that used to introduce the oxidizing gas into the reservoir. Preferably, the carbon dioxide is introduced into a separate region of the reservoir away from the LTO reaction zone. Preferably, the carbon dioxide is introduced into a region of the reservoir that has a temperature lower than the temperature of the LTO reaction zone. Injection of carbon dioxide into a lower temperature region of the reservoir contacts the carbon dioxide with lower temperature reservoir fluids comprising residual light oil and connate brine. The solubility of carbon dioxide in the reservoir fluids will be higher at lower temperatures. In addition, a greater quantity of carbon dioxide can be injected for a given volume at a specified pressure if the temperature is lower.

[0096] Preferably, at least some carbon dioxide can form part of the oxidizing gas and can be injected into the reservoir via the injection borehole. Typically, purified oxygen can be mixed with carbon dioxide to form the oxidizing gas. This can have advantages in terms of field simplicity and reduced equipment requirements and can potentially beneficially alter the LTO reactions and chemical equilibria.

[0097] Subterranean light oil reservoir

[0098] The reservoir is connected to the surface by at least one injection borehole and at least one extraction borehole. The reservoir comprises residual light oil and connate brine.

[0099] Preferably, the subterranean light oil reservoir is a subterranean depleted light oil reservoir.

[0100] Typically, a subterranean light oil reservoir is a subterranean depleted light oil reservoir when no more light oil can be extracted using primary recovery. Typically, the light oil that remains in the reservoir after primary recovery is known as residual light oil.

[0101] Typically, at least 90v / v%, or at least 95v / v%, or even at least 99v / v%, or even all, of the total light oil present in the reservoir during step (a) is residual light oil.

[0102] It may be preferred for the extraction borehole to be situated at a lesser depth than the injection borehole. This configuration allows the LTO synthesis gas to rise naturally towards the extraction point so as to improve concentration and subsequent collection. Since hydrogen is the least dense gas and carbon dioxide is much denser at typical reservoir conditions as well as at the required temperature ranges, a natural separation will take place resulting from buoyancy effects with hydrogen flowing up-dip away from carbon dioxide which will flow down-dip if not restricted. This choice of well placement may act as a natural separator and save energy in artificial separation.

[0103] Reservoir fluids

[0104] Reservoir fluids refers to the fluids present in the subterranean light oil reservoir. The reservoir fluids include the residual light oil and connate brine. Residual light oil

[0105] The residual light oil is light oil.

[0106] Light oil typically has an American Petroleum Institute (API) gravity of greater than 22 degrees. Light oil can have an API gravity of greater than 34 degrees, or even greater than 38 degrees. Whereas heavy oil typically has an API gravity of 22 degree or lower.

[0107] Typically, the residual light oil has an American Petroleum Institute (API) gravity of greater than 22 degrees.

[0108] Typically, the residual light oil is the light oil that remains in the depleted reservoir after primary recovery. The residual light oil may be light oil that remains in the depleted reservoir after primary and secondary recovery. The residual light oil may even be the residual light oil that remains in the depleted reservoir after primary, secondary and tertiary recovery.

[0109] Connate brine

[0110] Connate brine describes the solution of sodium chloride and other salts in water that is present in the pores of the rocks in the reservoir. Connate brine is the naturally occurring brine or water which is present in the reservoir, especially when the reservoir is depleted. Connate brine may also be a water mixture containing saline or fresh water which has been inj ected over the period of the extraction of oil or the extraction of hydrogen, including water which was produced from the reservoir and other water from external sources. Connate brine may also be the water which remains in the reservoir at the commencement of hydrogen extraction and may be present during the whole period of hydrogen extraction.

[0111] Injection gases

[0112] Injection gases are the gases that are injected into the reservoir. Typically, the injection gases are injected via one or more injection boreholes. Injection gases include the oxidising gas. Other injection gases can include carbon dioxide, this can include carbon dioxide that is generated by the LTO reactions and that is reinjected back into the reservoir. Oxidizing gas

[0113] Any suitable oxidizing gas can be used.

[0114] The oxidizing gas may comprise from 10v / v% to 100v / v% oxygen, or from 10v / v% to 45v / v%, or from 10v / v% to 40v / v%, or from 10v / v% to 35v / v%. Preferably the oxidizing gas comprises from 20v / v% to 35v / v% oxygen.

[0115] The oxidizing gas can be air. This offers high process simplicity but does typically result in high levels of nitrogen diluting the LTO synthesis gas. Increasing the proportion of oxygen in air to form the oxidizing gas will reduce the amounts of nitrogen being injected. Injection of some nitrogen can be beneficial to help with the flow of LTO synthesis gas out of the reservoir.

[0116] The oxidizing gas may comprise greater than 25v / v% oxygen, or greater than 35v / v% oxygen, or greater than 45v / v% oxygen, or greater than 55v / v% oxygen, or greater than 65v / v% oxygen, or greater than 70v / v%, or greater than 75v / v%, or greater than 80v / v%, or greater than 85v / v%, or greater than 90v / v%, or greater than 95v / v% oxygen. The oxidizing gas may even be pure oxygen.

[0117] Exhaust gases

[0118] Exhaust gases are the gases that are extracted from the reservoir. Typically, the exhaust gases are extracted via one or more extraction boreholes. An exhaust gas is the LTO synthesis gas. Another exhaust gas is nitrogen.

[0119] LTO synthesis gas

[0120] The LTO synthesis gas is generated during step (c). The LTO synthesis gas comprises at least 10v / v% hydrogen, preferably at least l lv / v%, or at least 12v / v%, at least 13v / v%, at least 14v / v%, or even at least 15v / v% hydrogen or even at least 16 v / v% or even at least 16.5v / v% hydrogen, or at least 17v / v%, or at least 18v / v%, at least 19v / v%, at least 20v / v%, or even at least 25v / v% hydrogen or even at least 30 v / v% hydrogen. The LTO synthesis gas typically comprises less than 50v / v% hydrogen. The LTO synthesis gas typically comprises less than 70v / v% methane, or less than 60v / v% methane, or less than 50v / v% methane, preferably less than 40v / v%, or less than less than 30v / v%, or even less than 25v / v% methane. The LTO synthesis gas will typically comprise greater than 10% methane. Preferably, the LTO synthesis gas comprises greater than 16.5v / v% hydrogen and less than 10v / v% carbon dioxide. Preferably, the LTO synthesis gas comprises less than 50v / v% carbon dioxide, or less than 40v / v%, or less than 30v / v%, or less than 20v / v%, or less than 10v / v%, or less than 5v / v%, or even less than 3v / v% carbon dioxide. The LTO synthesis gas may even be free of carbon dioxide. The LTO synthesis gas may comprise from 0v / v% to 3v / v% carbon dioxide, or from above 0v / v% to 3v / v% carbon dioxide. The composition of the LTO synthesis gas can be determined by measuring the composition of the exhaust gases and removing nitrogen, oxygen, and water from the calculation.

[0121] Hydrogen gas

[0122] The term “hydrogen gas” refers to higher purity hydrogen suitable for subsequent use in chemical and other processes. Hydrogen gas can be greater than 70v / v%, or greater than 80 v / v%, or greater than 90 v / v%, or greater than 95% v / v%, or greater than 99 v / v% hydrogen. Preferably, the hydrogen gas is a low-carbon footprint hydrogen gas, such as hydrogen having a CO2 footprint of less than 3.38 kg CCh / kg H2. Preferably, the hydrogen gas produced from the process is blue hydrogen.

[0123] Initiator for LTO reactions

[0124] The LTO reaction can be initiated by any suitable means. Suitable initiators include the injection of a self-heating material, such as linseed oil or other drying oil. A suitable initiator includes one or more of the following:

[0125] (i) injecting an air cushion, volume dependent on well configuration and reservoir characteristics.

[0126] (ii) injecting a crude oil cushion as “kindling” for initiating reactions in the reservoir’s residual crude oil.

[0127] (iii) injecting self-heating vegetable or mineral oil cushions or mixtures thereof.

[0128] (iv) injecting natural gas, methane, or heavier hydrocarbons with an ignition source to initiate reactions in kindling volume. Embodiments

[0129] 1. A process for producing hydrogen gas from a subterranean light oil reservoir, wherein the reservoir is connected to the surface by at least one injection borehole and at least one extraction borehole, wherein the reservoir comprises residual light oil and connate brine, the process comprises the steps:

[0130] (a) introducing an oxidizing gas into the injection borehole such that the oxidizing gas passes into the reservoir and contacts the residual light oil and connate brine present in the reservoir;

[0131] (b) initiating low temperature oxidation (LTO) reactions in the reservoir between the oxidizing gas, residual light oil and connate brine in a LTO reaction zone;

[0132] (c) maintaining the temperature of the LTO reaction zone in the range of from 60°C to 300°C and generating a low temperature oxidation (LTO) synthesis gas that comprises at least 10v / v% hydrogen;

[0133] (d) extracting the LTO synthesis gas from the porous reservoir via an extraction borehole; and

[0134] (e) separating and purifying the hydrogen from the LTO synthesis gas to form hydrogen gas.

[0135] 2. The process according to embodiment 1, wherein the subterranean light oil reservoir is a subterranean depleted light oil reservoir.

[0136] 3. The process according to any preceding embodiment, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 60°C to less than 200°C.

[0137] 4. The process according to any preceding embodiment, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 60°C to 180°C. 5. The process according to embodiments 1 or 2, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 90°C to 300°C.

[0138] 6. The process according to any preceding embodiment, wherein during step (c) the LTO reactions also produce carbon monoxide such that the LTO synthesis gas comprises greater than 5v / v% carbon monoxide.

[0139] 7. The process according to embodiment 6, wherein the carbon monoxide is separated from the remainder of the LTO synthesis gas and subsequently reacted with water to undergo a water-gas shift (WGS) reaction to form hydrogen and carbon dioxide.

[0140] 8. The process according to any preceding embodiment, wherein at least some of the carbon dioxide produced during step (c) dissolves in the reservoir fluids such that the LTO synthesis gas comprises less than 50v / v% carbon dioxide.

[0141] 9. The process according to any preceding embodiment, wherein carbon dioxide is introduced into the injection borehole such that the carbon dioxide passes into the reservoir and contacts the reservoir fluids, and wherein at least some of the carbon dioxide dissolves in the reservoir fluid.

[0142] 10. The process according to embodiment 9, wherein the carbon dioxide is introduced into a separate injection borehole from that used to introduce the oxidizing gas into the reservoir, and wherein the carbon dioxide is introduced into a separate region of the reservoir away from the LTO reaction zone.

[0143] 11. The process according to embodiment 10, wherein the carbon dioxide is introduced into a region of the reservoir that has a temperature lower than the temperature of the LTO reaction zone.

[0144] 12. The process according to any preceding embodiment, wherein during step (b) an initiator is introduced into the porous reservoir and initiates the LTO reactions.

[0145] 13. The process according to any preceding embodiment, wherein the LTO synthesis gas comprises greater than 20v / v% hydrogen and less than 10v / v% carbon dioxide. 14. The process according to any preceding embodiment, wherein the oxidizing gas comprises from 10v / v% to 30v / v% oxygen.

[0146] 15. The process according to any of embodiments 1-13, wherein the oxidizing gas comprises greater than 60v / v% oxygen.

[0147] 16. The process according to any of embodiments 1-13, wherein the oxidizing gas is air.

[0148] 17. The process according to any preceding embodiment, wherein during step (c) the pressure of the LTO reaction zone is maintained in the range of from 3x106to 3x107pascals.

[0149] 18. The process according to any preceding embodiment, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 130°C to 250°C.

[0150] 19. The process according to any preceding embodiment, wherein the LTO synthesis gas is retained and stored within the reservoir prior to being extracted via at least one extraction bore hole during step (d).

[0151] 20. The process according to any preceding embodiment, wherein hydrogen from external sources may be injected into and stored in the reservoir with the LTO synthesis gas prior to being extracted via at least one extraction bore hole during step (d).

[0152] 21. The process according to any preceding embodiment, wherein the extraction borehole is situated at a lesser depth than the injection borehole.

[0153] 22. The process according to any preceding embodiment, wherein during step (c) the temperature of the temperature of the LTO reaction zone is maintained by controlling the injection rate and oxygen content of the oxidizing gas being introduced into the injection borehole. 23. The process according to embodiment 18, wherein during step (c) the injection rate and / or oxygen content of the oxidizing gas being introduced into the injection borehole increases during step (c).

[0154] 24. The process according to any preceding embodiment, wherein during step (d) the LTO synthesis gas is extracted from the reservoir via an extraction borehole by means of an artificial lift mechanism.

[0155] 25. The process according to any preceding embodiment, wherein during step (c) the LTO reactions also produce carbon dioxide such that the LTO synthesis gas additionally comprises from 30v / v% to 65v / v% carbon dioxide, and wherein the carbon dioxide is subsequently separated and purified from the LTO synthesis gas to form carbon dioxide gas.

[0156] 26. The process according to embodiment 25, wherein the carbon dioxide gas obtained in claim 21 is re-injected into the reservoir.

[0157] 27. The process according to any preceding embodiment, wherein during step (e) the hydrogen from the low temperature synthesis gas is separated and purified to form hydrogen gas by means of a water-gas shift reactor and a carbon dioxide separation unit.

[0158] 28. The process according to any preceding embodiment, wherein no light oil is continuously extracted from the reservoir.

[0159] 29. The process according to any preceding embodiment, wherein no hydrogen-selective- membrane is inserted into the extraction borehole.

[0160] 30. The process according to any preceding embodiment, wherein no catalytic metal species is introduced into the reservoir via an injection borehole.

[0161] 31. The process according to any preceding embodiment, wherein the residual light oil has an API gravity of greater than 22 degrees. The process according to any preceding embodiment, wherein the at least 90v / v% of the total light oil present in the reservoir during step (a) is residual light oil. The process according to any preceding embodiment, wherein the hydrogen gas produced from the process is low-carbon hydrogen which is hydrogen produced in association with carbon dioxide emissions less than 3.38 kg carbon dioxide per kg of hydrogen. The process according to any preceding embodiment, wherein no particulate catalyst is introduced into the reservoir. The process according to any preceding embodiment, wherein no particulate metallic catalyst is introduced into the reservoir. The process according to any preceding embodiment, wherein no liquid water is introduced into the reservoir. The process according to any preceding claim, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 90°C to 300°C and the low temperature oxidation (LTO) synthesis gas that is generated comprises at least 16.5v / v% hydrogen.

[0162] Examples

[0163] The effect of controlling the temperature of the LTO reaction zone of a light oil reservoir on the composition of the resulting syngas was calculated using a finite difference discrete grid simulation technique and using CMG STARS software.

[0164] Reaction parameters and rate constants for the multiple different reactions involved in LTO were measured and calculated for a sample of the oil (having an API gravity of 34.5°) from the reservoir being modelled using air as the oxidising gas. This was undertaken using an Accelerating Rate Calorimeter (esARC from Thermal Hazard Technology7) to cany7out high pressure temperature scans at various temperatures. The calculated reaction parameters and rate constants were then used in modelling behaviour in the grids of the model.

[0165] Table 1 below shows that the simulated composition of syngas generated at different temperatures in the reaction zone for a light oil reservoir.

[0166] Table 1. NW Europe reservoir

Claims

Claims1. A process for producing hydrogen gas from a subterranean light oil reservoir, wherein the reservoir is connected to the surface by at least one injection borehole and at least one extraction borehole, wherein the reservoir comprises residual light oil and connate brine, the process comprises the steps:(a) introducing an oxidizing gas into the injection borehole such that the oxidizing gas passes into the reservoir and contacts the residual light oil and connate brine present in the reservoir;(b) initiating low temperature oxidation (LTO) reactions in the reservoir between the oxidizing gas, residual light oil and connate brine in a LTO reaction zone;(c) maintaining the temperature of the LTO reaction zone in the range of from 60°C to 300°C and generating a low temperature oxidation (LTO) synthesis gas that comprises at least 10v / v% hydrogen;(d) extracting the LTO synthesis gas from the porous reservoir via an extraction borehole; and(e) separating and purifying the hydrogen from the LTO synthesis gas to form hydrogen gas.

2. The process according to claim 1, wherein the subterranean light oil reservoir is a subterranean depleted light oil reservoir.

3. The process according to any preceding claim, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 60°C to less than 200°C.

4. The process according to any preceding claim, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 60°C to 180°C.

5. The process according to claims 1 or 2, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 90°C to 300°C.

6. The process according to any preceding claim, wherein during step (c) the LTO reactions also produce carbon monoxide such that the LTO synthesis gas comprises greater than 5v / v% carbon monoxide.

7. The process according to claim 6, wherein the carbon monoxide is separated from the remainder of the LTO synthesis gas and subsequently reacted with water to undergo a water-gas shift (WGS) reaction to form hydrogen and carbon dioxide.

8. The process according to any preceding claim, wherein carbon dioxide is introduced into the injection borehole such that the carbon dioxide passes into the reservoir and contacts the reservoir fluids, and wherein at least some of the carbon dioxide dissolves in the reservoir fluid.

9. The process according to claim 8, wherein the carbon dioxide is introduced into a separate injection borehole from that used to introduce the oxidizing gas into the reservoir, and wherein the carbon dioxide is introduced into a separate region of the reservoir away from the LTO reaction zone.

10. The process according to claim 9, wherein the carbon dioxide is introduced into a region of the reservoir that has a temperature lower than the temperature of the LTO reaction zone.

11. The process according to any preceding claim, wherein during step (b) an initiator is introduced into the porous reservoir and initiates the LTO reactions.

12. The process according to any preceding claim, wherein the LTO synthesis gas comprises greater than 20v / v% hydrogen and less than 10v / v% carbon dioxide.

13. The process according to any preceding claim, wherein the oxidizing gas is air.

14. The process according to any preceding claim, wherein during step (c) the pressure of the LTO reaction zone is maintained in the range of from 3x106to 3x107pascals.

15. The process according to any preceding claim, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 130°C to 250°C.

16. The process according to any preceding claim, wherein the LTO synthesis gas is retained and stored within the reservoir prior to being extracted via at least one extraction bore hole during step (d).

17. The process according to any preceding claim, wherein hydrogen from external sources may be injected into and stored in the reservoir with the LTO synthesis gas prior to being extracted via at least one extraction bore hole during step (d).

18. The process according to any preceding claim, wherein the extraction borehole is situated at a lesser depth than the injection borehole.

19. The process according to any preceding claim, wherein during step (e) the hydrogen from the low temperature synthesis gas is separated and purified to form hydrogen gas by means of a water-gas shift reactor and a carbon dioxide separation unit.

20. The process according to any preceding claim, wherein no light oil is continuously extracted from the reservoir.

21. The process according to any preceding claim, wherein no hydrogen-selective-membrane is inserted into the extraction borehole.

22. The process according to any preceding claim, wherein no catalytic metal species is introduced into the reservoir via an injection borehole.

23. The process according to any preceding claim, wherein the residual light oil has an API gravity of greater than 22 degrees.

24. The process according to any preceding claim, wherein the at least 90v / v% of the total light oil present in the reservoir during step (a) is residual light oil.

25. The process according to any preceding claim, wherein the hydrogen gas produced from the process is low-carbon hydrogen which is hydrogen produced in association with carbon dioxide emissions less than 3.38 kg carbon dioxide per kg of hydrogen.

26. The process according to any preceding claim, wherein no particulate catalyst is introduced into the reservoir.

27. The process according to any preceding claim, wherein no particulate metallic catalyst is introduced into the reservoir.

28. The process according to any preceding claim, wherein no liquid water is introduced into the reservoir.

29. The process according to any preceding claim, wherein during step (c) the temperature of the LTO reaction zone is maintained in the range of from 90°C to 300°C and the low temperature oxidation (LTO) synthesis gas that is generated comprises at least 16.5v / v% hydrogen.

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