WO2015059026A2

WO2015059026A2 - Producing hydrocarbons under hydrothermal conditions

Info

Publication number: WO2015059026A2
Application number: PCT/EP2014/072221
Authority: WO
Inventors: Erling Rytter; Arne GRISLINGÅS
Original assignee: Statoil Petroleum As
Priority date: 2013-10-22
Filing date: 2014-10-16
Publication date: 2015-04-30
Also published as: GB2519521A; WO2015059026A3; GB201318657D0

Abstract

A method and apparatus for producing hydrocarbons from a subterranean formation. In a reservoir, liquid water is provided in the subterranean formation. The formation is heated to induce hydrothermal liquefaction of hydrocarbons. The liquefied hydrocarbons can then be produced from the subterranean formation. In-situ hydrothermal liquefaction allows production of hydrocarbons that could not otherwise be produced from subterranean reservoirs.

Description

Producing hydrocarbons under hydrothermal conditions

TECHNICAL FIELD The present invention relates to the field of producing hydrocarbons. BACKGROUND

In order to improve the efficiency of extracting hydrocarbons from subterranean formations, it is known to induce and/or extend existing fractures and cracks in the subterranean formation. Fractures may extend many meters and tens or even hundreds of meters from a main wellbore from which they originate.

As hydrocarbon-bearing formations are often disposed substantially horizontally, in many cases it is preferred to use horizontal drilling and tracking operations (inducing fractures in the formation) may be carried out on a single well. This may be accomplished by, for example, retracting open slots in a liner along the borehole. A common method to induce fractures is by hydraulic fracturing. In this case, a fluid is pumped into the formation via the wellbore at high pressures. The pressure can be up to around 600 bar, or in some cases even higher. The first fractures may be created by the use of explosive materials, and these are extended by the high pressure fluid. The most commonly used tracking fluid is water with added chemicals and solid particles. Typically the solids, termed proppants, make up 5-15 volume % of the tracking fluid, chemicals make up 1 -2 volume % and the remainder is water.

Other tracking fluids include freshwater, saltwater, nitrogen, C0₂ and various types of hydrocarbons, e.g. alkanes such as propane or liquid petroleum gas (LPG), natural gas and diesel. The tracking fluid may also include substances such as hydrogen peroxide, propellants (typically monopropellants), acids, bases, surfactants, alcohols and the like.

Considering the case where the tracking fluid is LPG, in order for LPG gas to be suitable for use in tracking of wells, it is necessary to form it into a gel so that, among other properties, it may transport proppants. A gel consistency is required to maintain suitable proppant dispersion. An advantage of this technology is the simplicity in disposal of the tracking fluid. After the tracking operation, the LPG reverts from a gel to a gas and escapes the borehole, leaving proppants in the fractures in order to hold the fractures open and prevent them from closing. Furthermore, during the change from (gel-like) liquid to gas form, the LPG volume increases greatly, thereby increasing the pressure in the formation and further extending fractures. It is thought that recovered LPG gas is suitable for reuse. Compared to many other methods of hydraulic tracking, the method based on LPG does not leave chemical substances in the soil, and also reduces the effect of reflux.

The chemicals added may comprise viscosifier agents and/or cross-linked polymers, often from natural vegetation like cellulose, that enhance the tracking fluid's ability to transport proppants into the reservoir and the fractures. Some chemicals also reduce the friction between the tracking fluid being pumped and the well conduits. Examples of suitable gelling agents are hydroxypropyl guars (of ionic or non-ionic type) and polyacryl imides. The tracking fluid may also be an emulsion created by mixing water with a liquid hydrocarbon. Another tracking fluid option is to form a foam, resulting from aeration of gels containing 70-80% of gas. After a tracking operation, the tracking fluid is returned, at least in part, back to the surface for reuse or disposal. This operation creates issues with handling the added chemicals and also with handling large amounts of water (where the tracking fluid is water-based). After fracturing the tracking fluid normally includes bacteria and hydrogen sulphide, which need to be safely handled.

Note that hydraulic fracturing is not the only means to stimulate hydrocarbon production in a subterranean reservoir. Other techniques include acid stimulation to dissolve part of the formation rock (typically carbonates like nahcolite), and steam injection in the steam assisted gravity drainage (SAGD) technique.

Hydrocarbons that can benefit from heat treatment are typically low viscosity or low mobility hydrocarbons such as bitumen, e.g. in oil sands, heavy oil, extra heavy oil, tight oil, kerogen and coal. Oils are often classified by their API gravity, and a gravity below 22.3 degrees is regarded as heavy, and below 10.0° API as extra heavy. Bitumen is typically around 8° API.

Shale reservoirs are hydrocarbon reservoirs formed in a shale formation, often denoted as shale oil, shale gas or oil shale. It can be difficult to extract the hydrocarbons from shale reservoirs because the shale formation is of low porosity and low permeability, and so fluid hydrocarbons may not be able to find a path through the formation towards a production well. This means that when a well is drilled into the formation, only those fluid hydrocarbons in proximity to the well are produced, as the other hydrocarbons further away from the well have no easy path to the well through the relatively impermeable rock formation. In order to improve hydrocarbon recovery from shale formations, the shale around the well is often hydraulically fractured. This involves propagating fractures through the shale formation using a pressurized fluid to create conduits in the impermeable shale formation. Hydrocarbon fluids can then migrate through the conduits toward the production well. In this way, recovery of hydrocarbons from the reservoir is improved because hydrocarbons that would not previously be able to reach the well now have a path to the well and can be produced.

The term "oil shale" refers to a sedimentary rock interspersed with an organic mixture of complex chemical compounds collectively referred to as "kerogen". The oil shale consists of laminated sedimentary rock containing mainly clay minerals, quartz, calcite, dolomite, and iron compounds. Oil shale can vary in its mineral and chemical composition. When the oil shale is heated to above 260-370 ^<€, destructive distillation of the kerogen (a process known as pyrolysis), occurs to produce products in the form of oil, gas, and residual carbon. The hydrocarbon products resulting from the destructive distillation of the kerogen have uses that are similar to other petroleum products. Oil shale is considered to have potential to become one of the primary sources for producing liquid fuels and natural gas, to supplement and augment those fuels currently produced from other petroleum sources.

Known in situ processes for recovering hydrocarbon products from oil shale resources describe treating the oil shale in the ground in order to recover the hydrocarbon products. These processes involve the circulation or injection of heat and/or solvents within a subsurface oil shale. Heating methods include hot gas injection, e.g. flue gas or methane or superheated steam, hot liquid injection, electric resistive heating, dielectric heating, microwave heating, or oxidant injection to support in situ combustion. Permeability enhancing methods are sometimes utilized including; rubblization, hydraulic fracturing, explosive fracturing, heat fracturing, steam fracturing, and/or the provision of multiple wellbores.

Heating fluids can be one of several types. Often a molten salt is used, such as a nitrate or carbonate salt, or a mixture of such salts. An example of a heating fluid is a mixture of 60% NaN0₃ and 40% KN0₃ with a melting point of 220 °C. This mixture can be heated to 450-650 preferably between 550-600 °C, before being piped into to the reservoir. The return temperature at the surface for reheating is typically around 250- 500 °C, preferably 300-450 °C. Other classes of suitable salts include carbonates, halides or other well-known anions. The counterion (cation) should be environmentally benign, essentially in the form of alkali, alkaline earth elements or sink. A further option is imidazolium based counterion if a low melting temperature is required. In general, a large size counterion gives a low melting point due to reduced coulomb interactions. The use of molten salts as a heat transfer fluid for heating a subsurface formation has been described in US 7,832,484, which also includes several examples of such salts. Note that it is also possible, with due consideration of cracking effects, to use a hydrocarbon as heating medium. The hydrocarbon can be in a gaseous or liquid form.

The heating fluid is returned to the surface. In the surface facilities, the heating fluid is reheated after having been cooled down in the reservoir formation. Furthermore, it may be necessary to remove unwanted impurities in the heating fluid that have been picked up in the reservoir. Certain aspects of U-shaped wellbores containing heating fluid in a closed loop heating system have been described in WO 2006/1 16096.

SUMMARY

It is an object to provide systems, methods and apparatus for improving the production of hydrocarbons from subterranean reservoirs, and in particular for improving the production of low mobility hydrocarbons. It has been realised that by allowing liquid water to remain in a subterranean formation, and heating the subterranean formation to a temperature and pressure suitable to induce hydrothermal reactions, hydrocarbons derived from low mobility hydrocarbons such as viscous oil, bitumen, kerogen and coal can be produced.

According to a first aspect, there is provided a method of producing hydrocarbons from a subterranean formation. In the reservoir, liquid water is provided in the subterranean formation. The formation is heated to induce hydrothermal liquefaction of hydrocarbons. The liquefied hydrocarbons can then be produced from the subterranean formation. In-situ hydrothermal liquefaction has the advantage of allowing production of hydrocarbons that could not otherwise be produced from subterranean reservoirs. As an option, the produced liquefied hydrocarbons are separated from a mixture of hydrocarbons, water, and any other fluid.

The method optionally includes draining a mixture of water and hydrocarbon from the reservoir.

The pressure in the reservoir is optionally reduced to release gaseous hydrocarbons.

As an option, fractures are introduced into the subterranean formation. Fractures are optionally introduced at a temperature of at least 100°C, preferably at least 150°C, more preferably above 200 °C. A proppant material may also be introduced into the fractures during a fracturing operation, the proppant material being selected to maintain the fractures after the fracturing operation. The proppant material optionally comprises at least one chemical arranged to provided catalysis of the hydrothermal liquefaction of the hydrocarbons in the subterranean formation. An advantage of introducing fractures is that it is easier to produce hydrocarbons, particularly in low permeability formations.

Examples of the types of hydrocarbons in the formation that is subject to hydrothermal liquefaction include any of kerogen, oil and coal.

As an option, the formation comprises at least 50% by volume shale.

The formation is optionally heated by injecting steam into the formation. Heating may be performed for a time period selected from the range of more than one week, more than one month, and more than one year.

The formation is optionally heated to a temperature selected from the range of between 150°C and 350 °C, 250 °C and 350 °C, and between 300 °C and 350 °C, or below 300 °C

The provided water optionally further comprises methanol or other alcohol, alternatively a neat alcohol.

The provided water may be at least partly water inherent in the reservoir, it may be at least partly a fluid used in a fracturing operation, and it may be at least partly condensed steam from heating of the reservoir. Furthermore, it may be any combination of the above.

The same wellbore is optionally used for heating, tracking and production. This reduces the costs of the operation, as fewer wells need to be drilled.

The method optionally further comprises inducing an elevated pressure in the subterranean formation, the pressure being selected from the range of more than 10 bar, more than 15 bar and more than 50 bar.

According to a second aspect, there is provided a system for producing hydrocarbons from a subsurface formation. The system comprises an injector for introducing liquid water into the subterranean formation, a heater for heating the formation to induce hydrothermal liquefaction of hydrocarbons, and a production well for producing the liquefied hydrocarbons from the subterranean formation.

The system optionally further comprises a separator for separating the produced liquefied hydrocarbons from a mixture of hydrocarbons, water, and any other fluid. As an option, the system comprises means for introducing fractures into the subterranean formation. As a further option, the system comprises a source of proppant material, the proppant material being arranged to be introduced into the fractures during the fracturing operation, the proppant material being selected to maintain the fractures after the fracturing operation.

Hydrocarbons that are subject to hydrothermal liquefaction are optionally selected from any of kerogen, oil and coal. The subterranean formation optionally comprises at least 50% by volume shale. As an option, the heater comprises a steam injector arranged to injecting steam into the subterranean formation.

According to a third aspect, there is provided a computer device comprising a processor for controlling the system described above in the second aspect, a memory and an interface connecting the computer with the system. According to a fourth aspect, there is provided a computer program comprising computer readable code which, when run on a computer device causes the computer device to control the system described above in the second aspect. According to a fifth aspect, there is provided a computer program product comprising a non-transitory computer readable medium and a computer program described above in the fourth aspect, wherein the computer program is stored on the non-transitory computer readable medium.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph of pressure against temperature showing the temperature and pressure region where hydrothermal conditions occur;

Figure 2 illustrates schematically a cross section view of a subterranean formation and a production facility comprising a production well and a heating well;

Figure 3 is a flow diagram showing exemplary steps; and

Figure 4 illustrates schematically in a block diagram an exemplary controller device. DETAILED DESCRIPTION Hydrothermal liquefaction is a process known for converting biomass into usable hydrocarbons. The hydrothermal reaction requires the presence of liquid water and sufficient temperature and pressure. In a regular production facility for producing hydrocarbons from a subterranean reservoir, it is typically a requirement that as much liquid water as possible is removed from the subterranean formation around the production well. However, using the present techniques, it is necessary to ensure that water is present in order to obtain a hydrothermal reaction.

Figure 1 is a graph showing the temperature and pressure region at which hydrothermal reactions occur. The water must remain in the liquid phase, and the temperature must be greater than around 150°C, with a pressure greater than around 5 bar. Above 374 °C and 22 MPa, water is in the supercritical state. This may enhance the extraction of soluble oil components. (1 bar is approximately 0.1 MPa).

In general, supercritical extraction is characterized by gas-like mass transfer coefficients and liquid-like solvating properties. Other particularly interesting supercritical fluids are carbon dioxide (31 °C; 7.5 MPa), methanol (240°C; 8.2 MPa), hexane (234 °C; 3.0 MPa) and benzene (562 °C; 4.9 MPa). Adding supercritical fluids for extraction of produced oil may enhance the oil yield and prevent decomposition of secondary products. Supercritical extraction can help dissolve soluble components in the hydrocarbon. However, supercritical water only to a limited extent contributes to hydrothermal breakdown of the chemical structure of subterranean hydrocarbons such as coal, kerogen or bitumen.

Liquid water is a natural constituent of most subterranean hydrocarbon reservoirs. Water is typically removed by pumping, or during thermal treatment water can flow in from surrounding rocks. Water in liquid form is often added to the subterranean formation during a fracturing operation, described above.

In a conventional subterranean formation, after a fracturing operation, it is considered desirable to remove liquid water to enhance efficient flow of gaseous or liquid hydrocarbon products.

It has now been found that allowing liquid water to remain in or being added to the reservoir can enhance production of hydrocarbons such as kerogen, oil or coal, if the temperature and pressure of the subterranean formation are at a level suitable for hydrothermal liquefaction of the hydrocarbons. Typical conditions are 250-350 °C and 10-20 MPa. Under these conditions, liquid water becomes a highly reactive medium promoting cleavage of chemical bonds of hydrocarbon molecules. Note that there is a resemblance in the types of large molecule hydrocarbons of biomass and kerogen or coal, in particular lignite and highly volatile bituminous types. Similar hydrothermal liquefaction therefore occurs for kerogen and coal as would occur for a hydrothermal liquefaction of biomass.

There is some overlap in the temperature and pressure required to induce hydrothermal liquefaction and the temperature and pressure required to optimise production of liquid hydrocarbons. Examples of such conditions are a temperature of 150-350 °C and 1 to 15 MPa. It is important to exclude temperatures and pressures at which water will be in neat gaseous or supercritical form.

An added advantage of the hydrothermal liquefaction process is that the produced oil is soluble in the water only to a limited extent, and can be transported to the surface by subsequent draining of the reservoir or following pressure release and evaporation. Note that hydrothermal liquefaction also may result in some gaseous components being formed at the reaction pressure. By draining, the energy intensive evaporation of water is eliminated. A particular important feature is that the produced oil will be less prone to polymerization, gum formation and coking. Liquid hydrocarbon production is therefore expected to increase significantly over standard thermal treatments of dry or water depleted reservoirs.

Depending on the conditions of the hydrothermal liquefaction process, heteroatoms such as oxygen, sulphur, nitrogen and phosphorous as well as metals are removed from the oil giving a product requiring less refining to fuel requirements.

Figure 2 shows an exemplary production facility 1 located at a surface above a subterranean reservoir 2. In this example, a production well 3 is provided and a heating well 4 is also provided. Any type of heating may be used, for example electrical heating, induction heating, injection of heating fluids such as steam or simply passing heating fluids through the heating well. In this example, a source of a heating fluid 5 is provided. Heating is carried out over a long period (typically months to years) in order to provide conditions suitable for hydrothermal liquefaction. The production well 3 or the heating well may be used to inject liquid water into the subterranean formation 2 in the event that there is insufficient liquid water in the subterranean formation 2.

Note that fractures 6 may also be provided. The fractures may extend in different directions and in variable lengths and geometries. For simplicity, only fractures between the heating well 4 and the production well 3 are shown. These are particularly useful in low permeability formations such as shale (or formation comprising at least 50% shale by volume), as they provide a path for liquid water to permeate the formation and thereby cause a larger volume of the subterranean formation 2 to be subjected to hydrothermal liquefaction. Furthermore, they allow paths for the produced liquid hydrocarbons to reach the production well 3. If hydraulic fracturing is used, then liquid water from the fracturing operation can be allowed to remain in the subterranean formation rather than being pumped out. The fracturing fluid, in addition to comprising water, may include proppants arranged to hold open the fractures. The proppants may include a chemical catalyst such as Zr0₂ or a zeolite to enhance the hydrothermal liquefaction of the hydrocarbons.

A separator 7 is provided at the production facility 1 for separating produced hydrocarbons from water and any other unwanted fluids. A controller 8 may also be required for controlling the temperature and pressure within the subterranean formation 2, for example by controlling the temperature of the heating fluid source 5.

A typical temperature range for the hydrothermal liquefaction operation is between 150°C and 350 °C, preferably 250 °C and 350 °C, and more preferably between 300 °C and 350 °C. A typical elevated pressure in the subterranean formation is typically more than 1 MPa, preferably more than 1 .5 MPa and more preferably more than 5 MPa.

Figure 3 is a flow diagram showing exemplary steps. The following numbering corresponds to that of Figure 3:

51 . A fracturing operation is performed in a subterranean reservoir using a water based fluid containing proppants. As described above, the proppants may include catalyst chemicals to enhance a subsequent hydrothermal liquefaction process.

52. The subterranean formation is heated in the presence of liquid water to induce hydrothermal liquefaction. As described above, this is typically in a range between 150 and 350°C and at a pressure greater than 1 MPa. This process may take weeks, months or years.

53. Hydrocarbons produced by hydrothermal liquefaction are produced.

54. The produced hydrocarbons are separated from any water and other unwanted fluids. Note that the hydrothermal liquefaction process described above may not be carried out in isolation. For example, a subterranean reservoir may contain a mixture of low viscosity hydrocarbons that can be produced without hydrothermal liquefaction and lower mobility hydrocarbons such as kerogen and coal that can he liquefied using hydrothermal liquefaction. A subterranean reservoir may therefore produce different types of hydrocarbon at different times.

The above described operation of in-situ hydrothermal treatment in a hydrocarbon containing reservoir can be subject to a number of variations that also in practice are improvements or simplifications. One such option is to use steam under pressure as heating medium and let the steam condense in the reservoir. The condensation enthalpy enhances heating of the reservoir. If there is no liner in the well, direct contact with the reservoir and hydrocarbons may provide suitable conditions for hydrothermal treatment. Subsequently, after completion of the desired reactions, the hydrocarbon can be produced from the same well by pumping out the water/oil mixture or pressure release. In other words, this is an option where the same wellbore is used for a multiple of:

i. Heating of the reservoir

ii. Providing water to the reservoir

iii. Fracking the reservoir.

iv. Conducting hydrothermal treatment

v. Producing hydrocarbons.

In this scheme a fracturing operation is conducted on the heated reservoir. This may result in more effective fracking than practiced today. Furthermore, the high pressure normally used during fracturing, up to 1000 bar, can enhance hydrothermal reactions when conducted at an elevated temperature. Alternatively, the fracturing operation may take place before heating.

The presence of water during hydrothermal treatment of kerogen, bitumen or coal leads to hydrolysis and cleavage of bonds between heteroatoms and carbon atoms. It is thus possible to break bonds under milder conditions, i.e. lower temperature, than for dry thermal cracking (pyrolysis). Reactions are typically expected to proceed via cleavage of ester and ether bonds or sulphur connecting structural units. In case of the hydrocarbon being highly aromatic in nature, hydrothermal degradation products can typically be catechol, phenol and cresol types of compounds, or in other words aromatic alcohols of different variations in number of alcohol groups, aliphatic side chains and their location on the aromatic ring. Polycyclic compounds may also occur of five and six membered rings with such substituents as mentioned. Some of these compounds and mixtures thereof are suitable as fuels or fuel components. From longer aliphatic chains bonded to other structural units through oxygen atoms, alpha-alcohols or fatty acids may be produced. Adding methanol to the water, or using pure methanol or other alcohol as a reagent, may favour certain reactions that are desired. For example, esterification of fatty acids will give esters well known as biodiesel.

Figure 4 illustrates schematically in a block diagram an exemplary controller 8 in the form of a computer device. The controller is provided with a processor 9 for executing instructions and sending them via an interface 10 to components of the system. For example, the processor might receive measurements from the system such as readings of the temperature of the heating fluid or formation, the pressure of heating fluid, the amount of water mixed with produced hydrocarbon, and on the basis of user input 1 1 or rules stored in a database 12, take corrective action. A non-transitory computer readable medium in the form of a memory 13 may also be provided that can be used to store the database 12. It may also be used to store a computer program 14 which, when executed by the processor 9, causes the controller 8 to control the system. Note that the computer program 14 may be provided from an external non- transitory computer readable medium in the form of a memory 15, such as a DVD disk, a flash drive and so on.

It should be understood that the term "hydrocarbon" present in the subterranean formation is used in a broad meaning of the term, i.e. not only covering material and compounds that are strictly composed of only hydrogen and carbon atoms, but also to a larger or smaller extent contains heteroatoms that typically are oxygen, sulphur or nitrogen, but also minor amounts of phosphorous, mercury, vanadium, nickel, iron or other elements can be present. The term "hydrocarbon" as produced by hydrothermal liquefaction is also used in a broad sense to cover products that contain heteroatoms, in particular oxygen. This hydrocarbon product will often be further treated in one or more processing steps to give a secondary or final product, e.g. to be shipped to a refinery or sold to a consumer.

It will be appreciated by the person of skill in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention.

Claims

CLAIMS:

1. A method of producing hydrocarbons from a subterranean formation, the method comprising, in the reservoir:

providing liquid water in the subterranean formation;

heating the formation to induce hydrothermal liquefaction of hydrocarbons; and producing the liquefied hydrocarbons from the subterranean formation.

2. The method according to claim 1 , further comprising separating the produced liquefied hydrocarbons from a mixture of hydrocarbons, water, and any other fluid.

3. The method according to claim 1 or 2, further comprising draining a mixture of water and hydrocarbon from the reservoir.

4. The method according to claim 1 , 2 or 3, further comprising reducing the pressure in the reservoir to release gaseous hydrocarbons.

5. The method according to any of claims 1 to 4, further comprising introducing fractures into the subterranean formation.

6. The method according to claim 5, wherein the fractures are introduced at a temperature of at least Ι ΟΟΌ, preferably at least 150°C, more preferably above 200 °C.

7. The method according to claim 5 or 6 further comprising introducing a proppant material into the fractures during a fracturing operation, the proppant material being selected to maintain the fractures after the fracturing operation.

8 The method according to claim 7, wherein the proppant material comprises at least one chemical arranged to provided catalysis of the hydrothermal liquefaction of the hydrocarbons.

9 The method according to any of claims 1 to 8, wherein the hydrocarbon subject to hydrothermal liquefaction is selected from any of kerogen, oil and coal.

10. The method according to any of claims 1 to 9, wherein the formation comprises at least 50% by volume shale.

11 . The method according to any one of claims 1 to 10, further comprising heating the formation by injecting steam into the formation.

12. The method according to any one of claims 1 to 1 1 , wherein the heating is performed for a time period selected from the range of more than one week, more than one month, and more than one year.

13. The method according to any one of claims 1 to 12, wherein the formation is heated to a temperature selected from the range of between 150°C and 350 °C, 250 °C and 350 °C, between 300 °C and 350 °C, and below 300 °C

14. The method according to any of claims 1 to 13, wherein the provided water further comprises methanol or other alcohol.

15. The method according to any of claims 1 to 14, wherein the provided water comprises at least partly water inherent in the reservoir.

16. The method according to any of claims 1 to 15, wherein the provided water comprises at least partly a fluid used in a fracturing operation.

17. The method according to any of claims 1 to 16, wherein the provided water comprises at least partly condensed steam from heating of the reservoir.

18. The method according to any of claims 1 to 17, wherein the same wellbore is used for heating, tracking and production.

19. The method according to any one of claims 1 to 18, the method further comprising inducing an elevated pressure in the subterranean formation, the pressure being selected from the range of more than 10 bar, more than 15 bar and more than 50 bar.

20. A system for producing hydrocarbons from a subsurface formation, the system comprising:

an injector for introducing liquid water into the subterranean formation; a heater for heating the formation to induce hydrothermal liquefaction of hydrocarbons; and

a production well for producing the liquefied hydrocarbons from the subterranean formation.

21 . The system according to claim 20, further comprising a separator for separating the produced liquefied hydrocarbons from a mixture of hydrocarbons, water, and any other fluid.

22. The system according to claim 20 or 21 , further comprising means for introducing fractures into the subterranean formation.

23. The system according to claim 22, further comprising a source of proppant material, said proppant material being arranged to be introduced into the fractures during the fracturing operation, the proppant material being selected to maintain the fractures after the fracturing operation.

24. The system according to any of claims 20 to 23, wherein the subterranean formation comprises hydrocarbons selected from any of kerogen, oil and coal.

25. The system according to any of claims 20 to 24, wherein the subterranean formation comprises at least 50% by volume shale.

26. The system according to any one of claims 20 to 25, wherein the heater comprises a steam injector arranged to injecting steam into the subterranean formation.

27. A computer device comprising:

a processor for controlling the system according to any of claims 20 to 26; a memory;

an interface connecting the computer with the system.

28. A computer program, comprising computer readable code which, when run on a computer device causes the computer device to control a system according to any of claims 20 to 26.

29. A computer program product comprising a non-transitory computer readable medium and a computer program according to claim 28, wherein the computer program is stored on the non-transitory computer readable medium.