The present invention relates to a system for production of renewable energy. More specifically, the invention relates to a system that uses heat from a reservoir to produce electricity.

There is an ongoing, massive effort to electrify the continental shelf for reducing CO2 emissions from the production of petroleum resources. Offshore wind turbines have been shown to be one possible source of offshore energy, but there is a need for supplements and alternatives.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

The present invention relates to a system that uses the heat from produced fluids, such as petroleum resources, produced water, which is a by-product from the production of petroleum resources, and treated produced water, which has been injected into the well to increase production of remaining petroleum resources in the well, to indirectly generate electricity, as will be explained with reference to the figure below. Produced water may also include additives/chemicals such as glycol, de-oxygenation chemicals, surfactants, anti-lime scale chemical, soap means, etc.

More specifically, the invention relates to a system for production of electricity from fluids flowing out from a well, the system comprising:

P30391PC00 description and claims for PCT - a well from which one or more fluids flow;

- a heat exchanger for receiving one or more of the fluids flowing from the well;

- a closed loop of CO2, the CO2 being heated and pressurized in the heat exchanger;

- a turbine driven by pressurized CO2; and

- an electric generator connected to the turbine.

In certain embodiments the closed loop of CO2 may be fully self-circulating based on the thermosiphon principle, meaning that circulation may be based on natural convection from heat transfer and not be dependent on a pump.

The fluids flowing out from the well may be fluids produced by the well. Alternatively, the fluids may be circulated into the well to be heated and back. It is also envisioned that the system according to invention may be used with other types of heat sources, such as waste heat from industrial processes near a sea or lake, other thermal sources, etc.

In one embodiment, the heat exchanger may be placed at or near a seabed. This may be advantageous to use the thermal energy in the produced fluids at a location near the wellhead and for reducing equipment footprint on a topside installation. In other embodiments, the heat exchanger may be placed in the water column above the seabed, such as within 10, 20 or 30 meters from the seabed, or in other embodiments higher up in the water column and even near the surface.

In one embodiment the turbine and electric generator may be provided on an offshore rig, platform, vessel or barge. This may be advantageous for producing the electricity near its position of use, potentially reducing the need for complicated infrastructure for transfer over distances and power losses along the way. In an alternative embodiment, also the closed CO2 loop, the turbine and the electric generator may be provided on the seabed, such as on a subsea template. The electric power produced subsea may be fed to a topside installation, such as a rig, through a power cord or the power cord may extend to land. In an alternative embodiment, the turbine and electric generator may be provided subsea, such as at a subsea template.

P30391PC00 description and claims prepared for PCT In one embodiment the closed loop may include a pipe extending from the downstream side of the turbine, through a water column and to the heat exchanger, whereby seawater may be used as a cooling medium for the depressurized CO2.

One of the advantages of using a closed loop of CO2, is that it is easy to control the CO2 and to keep the piping system, turbine etc. clean, as opposed to using CO2 directly from the well.

In one embodiment, the system may further comprise a separator for separating different produced fluids from each other. The separator is optional in the sense that the produced fluids, at the start of a well's lifetime may typically be void or substantially void of produced water. With time, the amount of produced water in the produced fluids will typically increase, whereby a separator may be useful. The separator may be positioned downstream of the heat exchanger, whereby the heat in the combined produced fluid may be used to heat and pressurize the CO2. In an alternative embodiment, the separator may be located upstream of the heat exchanger. In some embodiments, more than one separator may be provided in the system. E.g. the system may be provided with a separator both upstream and downstream of the heat exchanger.

In one embodiment, the system may comprise a double riser through which pressurized CO2 and produced fluids, such as petroleum resources, may flow in separate conduits from the heat exchanger and, optionally the separator, to the rig, vessel or platform. The double riser may extend all the way from the heat exchanger, and optionally separator, to the rig, vessel, or platform, or it may extend a part of the distance therebetween. By letting the closed loop CO2 flow in separate, parallel conduits, energy/temperature in the pressurized CO2 may be substantially maintained from the heat exchanger and to the turbine and generator, which may be useful for increasing circulation efficiency in the closed loop of CO2 and to enable self-circulation. The conduits in the double riser may be arranged substantially concentrically, such as with the pressurized CO2 flowing in the centre and the produced fluids flowing in an annulus between the central pipe and an outer, surrounding pipe. The double riser may preferably be isolated so as to reduce, or more preferably minimize, heat loss from the double riser to sea. In an alternative embodi-

P30391PC00 description and claims prepared for PCT merit, pressurized COz and produced fluids may flow in separate pipes/risers from the heat exchanger (and optionally separator) towards the turbine and generator. At least the pipe with the pressurized COz may be isolated so as to reduce, or preferably minimize, heat loss to the surrounding sea water.

In one embodiment the system may further comprise a reinjection line extending from the separator and down into the reservoir for re-injection of un-used fluids. The non-used fluids may typically include gas and/or produced water.

In one embodiment, the system may comprise a plurality of wells and a well collector for receiving produced fluids from the plurality of wells, the well collector being provided upstream of the heat exchanger.

In the following is described an example of a preferred embodiment illustrated in the accompanying drawing, wherein:

Fig. 1 shows an embodiment of a system according to the invention.

In the following, reference numeral 100 will be used to denote a system according to the invention. The figure is shown schematic and simplified, and the various features therein are not necessarily drawn to scale.

In the shown embodiment, fluids, here in the form of petroleum resources and produced water, are flowing from a reservoir, generally indicated at 2, and out from a well 1, through a wellhead 3 on a seabed 5. The produced fluids may, depending i.a. on the depth of the reservoir, typically have a temperature in the range from 50°C to 200°C, or even higher. From the wellhead 3 the produced fluids flow through pipe 7 and optionally through a well collector/manifold 9 collecting fluids from a plurality of wells of which only one is shown in the figure. The other wells 1, if present, may be similar to or different from the well 1. The heated fluids flow from the well collector 9, through pipe 11 and into a heat exchanger 13. In the heat exchanger 13, the thermal energy of the produced fluids is used to pressurize CO2 provided in a closed CO2 loop 15 as will now explained. Notably, in the shown embodiment, the closed loop 15 does not include a pump, as the CO2 is fully self-circulating by the thermosiphon principle. The pressurized CO2 flows up through an

P30391PC00 description and claims prepared for PCT inner tube 17 of a double riser 19 and upwardly to a surface installation, here in the form of a rig 21 floating on pontoons 22. The heat exchanger 13 may typically be provided at depths in the range of 30-300 meters, though the invention is not limited to such depths. Downstream of the heat exchanger 13 the produced fluids flow through pipe 23 and enter a separator 25 in which they are separated into their different constituents. The subsea separator 25 is optional in the sense that the produced fluids may include only hydrocarbons, such as oil and/or gas, which may typically be the case in an early phase of production from a new well. Unused fluids, typically produced water, may optionally be re-injected back into to the reservoir 2 through a re-injection line 27. Petroleum products flow out from the separator 25 into pipe 29 and further into an outer pipe 31 of the double riser 19, where the petroleum products flow up in an annulus 34 between the inner pipe 17 and the outer pipe 31, thus surrounding the pressurized COz and contributing to maintaining the energy in the pressurized COz on its way from the heat exchanger 12 to the turbine 35. Preferably the double riser 19 is isolated to minimize, or at least significantly reduce, heat loss to the surrounding seawater. The petroleum products are to be collected and processed at a production facility highly schematically indicated at 33 on the rig 21. The pressurized CO2 reaches a turbine 35 at which it expands and depressurises to rotate the turbine 35 and generate electricity by means of an electric generator 37 via a shaft 39, as will be understood by the skilled person. After depressurisation, the CO2, i.e. downstream of the turbine, is circulated back to the heat exchanger 13 through a single riser 41. The single riser 41 will functions as a gas cooler (heat exchanger) where the CO2 is cooled by the surrounding sea water 43. The single riser 41 may optionally be provided with fins or other protruding members, here highly schematically indicated at 45, on the outside of the single riser 41 to increase the total area of contact with the sea water 43 for more efficient heat transfer / cooling. In the shown embodiment, the closed CO2 loop exchanges heat without being in direct contact with the produced fluids and without any need for resupply. In alternative embodiments, the gas cooler (single riser) may take other forms than straight/vertical riser. E.g. the gas cooler may be a coiled pipe, a bent pipe or another pipe loop, which increases the length of the pipe 41 and thereby increases heat exchange / cooling. Simply summarized, one may say that it may be beneficial to minimize heat transfer for the pressurized CO2 upstream of

P30391PC00 description and claims prepared for PCT the turbine 35 and to maximized heat transfer / cooling of the de-pressurized CO2 downstream of the turbine 35 to optimize the efficiency of the system 100.

Simulations have been run to verify the potential efficiency of the proposed solution. In the simulations the temperature at the seabed was set to 8-10°C, while the temperature of the produced fluids flowing from the well 1 was set in the range to 50°C - 100°C in different simulations. Depth at seabed 5 was varied in the range 50-400 metres. The diameter of the inner tube 17 upstream of the turbine 35 was varied in the range 200-300 millimetres, while the diameter of the return pipe 41 downstream of the turbine was set in the range 350-540 millimetres. The volumetric flow of COzwas varied from 3.2 m ³ /s at a mass flow of 582 kg/s to a volumetric flow of 6.8 m ³ /s at a mass flow of 1618 kg/s. The flow velocity of CO2 was set to 30 m/s in all simulations. The pressure at the turbine 35 inlet was set in the range 120 - 252 bar while the pressure at the turbine outlet was set to 44 bar in all simulations. The electric generator power was calculated to be 25MW, while the corresponding calculated efficiency was around 89% in all simulations.

Any positional indications refer to the position shown in the figures.

In the figures, same or corresponding elements are indicated by same reference numerals. For clarity reasons, some elements may in some of the figures be with-out reference numerals.

A person skilled in the art will understand that the figure is just a principal drawing. The relative proportions of individual elements may also be distorted.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The mere fact that certain measures are recited in mutually different dependent claims

P30391PC00 description and claims prepared for PCT does not indicate that a combination of these measures cannot be used to advantage.

P30391PC00 description and claims prepared for PCT