Patent Description:
It has long been known that temperature within the earth sub-surface increases with depth and efforts have been made to "mine" this heat energy for heating and generating electricity. With the rising concerns of climate change and greenhouse gas emissions, governments and companies are looking for ways to reduce their energy intensity and carbon footprint. Renewable energy such as solar, wind and geothermal is often part of the technology portfolio especially with the rapid decrease of renewable energy prices due to significant research and mass deployment.

Oil and gas companies are among those most interested in reducing their energy intensity and carbon footprint so as to provide energy products with the lowest environmental impact. Many Oil and Gas (O&G) companies have adopted renewable energy in their operations seek to leverage O&G industry expertise to advance carbon capture utilization and sequestration. The O&G industry is particularly well suited to aid in carbon sequestration because of its significant underground reserves in which carbon can be stored, and its experience and knowledge of subsurface science and reservoirs.

The aims of carbon usage and sequestration with geothermal energy production can be simultaneously satisfied by using carbon dioxide (CO<NUM>) as a working fluid for capturing geothermal energy. This allows large volumes of carbon dioxide to be stored underground, used for energy capture, and recycled. However, the temperature and pressure of the CO<NUM> has to be maintained at elevated levels to capture geothermal energy efficiently. In this regard, supercritical CO<NUM> (CO<NUM> beyond the triple point in the temperature pressure phase graph, when the underground conditions allows it) has shown the potential to supersede many existing geothermal capture technologies.

CO<NUM> cycles efficiency is very dependent on the ambient conditions, especially if the ambient temperature goes above the CO<NUM> critical point, leading to energy intensive gas or supercritical CO<NUM> compression as compared to liquid CO<NUM> compression below the CO<NUM> triple point temperature. It has been reported by <NPL>" that an azeotropic ethane-CO<NUM> mixture can, under some conditions, be more efficient as working fluid in energy systems compared to pure CO<NUM>.

<CIT> discloses a geothermal energy generation system based on an underground reservoir. A working fluid is introduced to the reservoir, forcing methane out of solution, and heating a production fluid which can be extracted and used to create electricity.

In light of problems with using supercritical CO<NUM> under high ambient temperature conditions, the present disclosure provides systems and methods for adapting geothermal recovery to high temperature conditions by working fluid composition control.

In one embodiment, a system for geothermal energy recovery is provided. The system comprises a phase separator having an input port coupled to the extraction well and receiving an extracted working fluid including carbon dioxide and hydrocarbons therefrom, the phase separator operative to separate liquid and vapor portions the extracted working fluid, a controllable separator unit coupled downstream from the phase separator and receiving the vapor portion of the extracted working fluid therefrom, the separator unit being controllably operable to separate components of the received vapor portion based on chemical composition and to selectively mix the separated components into a modified working fluid based on detected process and ambient conditions of the extracted working fluid, wherein the modified working fluid (<NUM>) has a chemical composition that is optimized for energy recovery efficiency; and an expander coupled downstream from the controllable separator unit and operable to generate mechanical or electrical energy from expansion of the modified working fluid.

In some embodiments, the geothermal energy recovery system further comprises a condenser coupled downstream from the expander operable to cool the expanded working fluid, and a compression device coupled downstream from the condenser operable to increase the pressure of the working fluid received from the condenser to a pressure level suited for re-injection into the reservoir via the injection well. In certain implementations, additional carbon dioxide is introduced into the working fluid between the expander and the condenser or downstream the condenser depending on the additional carbon dioxide conditions.

The separator unit can be operated using a programmable electronic control unit that is configured to operate the separator unit to set a composition of the working fluid based upon detected ambient and process conditions received by the electronic control unit. In some implementations, when the electronic control unit determines that the extracted working fluid temperature in the cycle can be below the critical point temperature of carbon dioxide, the electronic control unit operates the separator unit to set a composition of the working fluid at between <NUM> carbon dioxide: <NUM> percent ethane and <NUM> percent carbon dioxide: <NUM> percent ethane depending on process or ambient condition. Conversely, if the electronic control unit determines that the extracted working fluid temperature is above the critical point temperature, the electronic control unit can operate the separator unit to set a composition of the working fluid to include carbon dioxide, ethane and at least one heavier hydrocarbon. In another embodiment, the electronic control unit can be operated to maximize the CO<NUM> sequestration and therefore operate the separator unit to maximize the recovery and rejection of the hydrocarbons and maximizing the recycle of CO<NUM>.

The present disclosure also provides a method of geothermal energy recovery. Embodiments of the method include steps of injecting carbon dioxide into a geothermal reservoir through an injection well, extracting a working fluid including previously injected carbon dioxide and hydrocarbons entrained in a flow of the carbon dioxide within the reservoir from an extraction well, separating components of the heated working fluid based on chemical composition, selectively mixing the separated components according to the current conditions of the extracted working fluid to produce an output modified working fluid that having a chemical composition that is optimized for energy recovery efficiency, and expanding the modified working fluid to generate mechanical or electrical energy.

In certain embodiments, the method further comprises separating the extracted working fluid according to phase into vapor and liquid streams prior to separating according to composition, wherein the vapor stream is thereafter separated based on chemical composition.

Embodiments of the method further includes condensing the expanded modified working fluid, compressing the condensed modified working fluid to increase pressure, and reinjecting carbon dioxide at elevated pressure into the reservoir through the injection well.

In certain implementations, the method can include introducing additional carbon dioxide into the modified working fluid prior to condensing or compressing.

The extracted working fluid stream can be separated into i) a carbon dioxide stream, ii) a light hydrocarbon stream, and iii) a heavy hydrocarbon stream. In some implementations, if the extracted working fluid has a temperature below the critical point temperature of carbon dioxide, the modified working fluid composition is set between <NUM> carbon dioxide: <NUM> percent ethane and <NUM> percent carbon dioxide: <NUM> percent ethane depending on process or ambient condition.

Conversely, if the extracted working fluid has a temperature above the critical point temperature, the modified working fluid composition is set to include carbon dioxide, ethane and at least one heavier hydrocarbon. In certain instances of the latter case, the modified working fluid composition is set to include <NUM> percent carbon dioxide, <NUM> percent ethane and <NUM> percent propane.

Embodiments of the method can further comprise heating the extracted working fluid to an elevated temperature by introducing the extracted working fluid with additional oxygen into an oxidation unit prior to separating the extracted working fluid based on composition. In additional embodiments, the method further comprising heating the extracted working fluid prior to separating components of the working fluid based on chemical composition.

These and other aspects, features, and advantages can be appreciated from the following description of certain embodiments of the invention and the accompanying drawing figures and claims.

Disclosed herein are methods and systems for recovering geothermal energy that use CO<NUM> and CO<NUM> hydrocarbon mixtures as a component of a working fluid. The efficiency of a supercritical CO<NUM> geothermal cycle is dependent upon operating and ambient conditions, especially when ambient temperature rises above the CO<NUM> critical point. Above the CO<NUM> critical point, energy compression can occur whereas only liquid CO<NUM> compression takes place below the triple point temperature. The disclosed method controls the composition and properties of working fluid by adding different types and amounts of hydrocarbons to the CO<NUM> based on the ambient conditions. The selective and controlled mixing allows for optimal cycle efficiency while reducing working fluid compression energy.

More specifically, the methods and systems herein control operation of a separator unit that separates components of the incoming working fluid extracted from the reservoir and then selectively mixes the separated components according to current process and ambient conditions before the working fluid is expanded and its energy is converted into mechanical or electrical energy.

<FIG> is a cross-sectional view of a geothermal recovery system in which the method and system of the present disclosure can be applied. The cross-sectional view shows a geological formation located below surface <NUM> which includes a hydrocarbon reservoir positioned beneath a caprock layer <NUM>. The reservoir includes a predominantly porous portion rich in liquid phase <NUM> and a porous portion rich in gaseous phase <NUM>. The liquid and gaseous rich reservoir portions <NUM>, <NUM> are located at a depth at which the temperature is elevated compared to the surface temperature and heat can be transferred to the reservoir materials. A geothermal recovery system <NUM> is positioned above the surface. The recovery system <NUM> receives carbon dioxide gas from a source <NUM> as well as an extraction working fluid stream in fluid communication with extraction well <NUM>. The system <NUM> includes elements such as pumps that can be used to inject carbon dioxide and other gaseous or liquid components down an injection well <NUM> into the reservoir. The carbon dioxide is injected into the reservoir and dissipates as a plume <NUM>. The injection serves two purposes. It is a way to sequester carbon dioxide underground. In addition, the carbon dioxide and other gases, as will be explained further, act as the working fluid <NUM> by absorbing the geothermal energy contained in the reservoir.

After the working fluid is injected through injection well <NUM> into the reservoir <NUM> (in <FIG>) the fluid migrates through various pathways (represented as dashed lines) toward an extraction well <NUM> through which the fluid is brought back to the surface. The working fluid is preferably supercritical CO<NUM> or a blend of hydrocarbons with CO<NUM>, the hydrocarbons ranging from C1 to C50 (in other words, hydrocarbon molecules having between one and <NUM> carbon atoms), preferably from C2 to C5. As seen in <FIG>, discussed, next, a working fluid stream <NUM> is output from extraction well <NUM>. The composition of the working fluid stream <NUM> can be different form the composition of the working fluid entering the reservoir via the injection because some of the components carried by the injected working fluid stream migrate to different areas of the reservoir and native fluids can be entrained in the flow into extraction well <NUM> as the working fluid migrates through the reservoir. The extraction well <NUM> and/or downstream components of the recovery system <NUM> include sensors to detect the current conditions of the extracted working fluid, including working fluid temperature and pressure.

Turning now to <FIG>, a schematic block diagram of an embodiment of a geothermal recovery system according to the present disclosure is shown. Shown at the bottom left, a heated working fluid stream <NUM> emerges from extraction well <NUM> and enters recovery system <NUM>. The working fluid <NUM> can be at pressures ranging between <NUM> and <NUM> bar and at temperatures ranging from <NUM> to <NUM> and typically contains both liquid and vapor components. Upon entering recovery system <NUM>, the working fluid <NUM> is fed to a phase separator <NUM> component. The phase separator <NUM> is designed to separate the liquid and vapor components of the working fluid <NUM>. The phase separator <NUM> can be implemented as a gravity-based or density-based separator that enables separation of a vapor stream from a gas-liquid mixture. The phase separator <NUM> diverts the liquid components and removes them from the system via stream <NUM>. In certain embodiments, the separated vapor components are output as stream <NUM> to a heat exchanger <NUM>. At the heat exchanger <NUM>, the working fluid vapor stream <NUM> is heated using an internal or external source. External heat sources can include a renewable energy source such as solar thermal heating device or an external combustion or oxidation system that can provide energy at temperature above the temperature of working fluid stream <NUM>. However, in alternative embodiments, if additional energy is unavailable, the system <NUM> can be operated without the heat exchanger <NUM>.

After being heated in embodiments that include heat exchanger <NUM>, a heated stream <NUM> exits the heat exchanger and is fed into a separation unit <NUM>. The separation unit <NUM> is operated to take as an input the heated stream <NUM> and to output, in a first outlet stream, a working fluid <NUM> with a selected composition. The selected composition is determined by an electronic control unit <NUM> (such as a programmable processor or specialized electronic control unit) which is configured to receive as input data current process, reservoir, and ambient conditions, including the temperature and pressure of the working fluid entering the system from the extraction well. The control unit <NUM> is also configured to determine, based on programmed instructions and the input data, a working fluid composition that reduces the compression energy of the working fluid while maximizing energy-recovery efficiency. The separation unit <NUM> first separates the input stream according to chemical composition into several output streams. Apart from the first output stream <NUM>, a second outlet stream <NUM> is composed of heavy hydrocarbons (mainly C2 and above) separated from working fluid and, in some embodiments, and a third outlet stream <NUM> is composed of the lightest hydrocarbons separated from the working fluid, mainly C1 and C2. In some implementations, the second and third outlet streams are combined as a single outlet stream. In some embodiments streams <NUM> and <NUM> are expanded separately or jointly to lower pressure to generate additional power. In some embodiments, the separated streams are then blended precisely with CO<NUM> in the first output stream to produce an output working fluid <NUM> that is optimized for energy recovery based on the current temperature and pressure conditions. In other embodiments, the separation unit <NUM> can provide directly the first output stream <NUM> with the desired composition by selectively removing the remaining components from the inlet stream <NUM> and drawing them out of the process through streams <NUM> and/or <NUM>.

The separation unit <NUM> can be implemented using an absorption, or adsorption unit or a membrane system that selectively separates the desired components. For example, separation unit <NUM> can comprise a membrane separation unit which enables the composition of output stream <NUM> to be adjusted by operating on different stage cuts with streams <NUM> and <NUM> as permeate streams. An example of a membrane material that can allow to do this type of separation is Poly(ether-b-amide) (Pebax). Alternatively, separation unit <NUM> can be implemented as a pressure swing adsorption (PSA) type unit that uses molecular gate type materials to separate C2, C3 from the rest of the mixture. In this case, CO<NUM> is removed along with the higher hydrocarbons (C3+) and any remaining moisture and other impurities or components. CO<NUM> is then separated from the rest of the components and mixed with the C2, C3 components previously separated. This allows adjustment of the output stream to be optimized for the current conditions.

The composition-controlled output stream <NUM> exiting separation unit <NUM> is fed to at least one expansion device <NUM>, such as a turbine, through which the heat energy of the stream is converted into mechanical or electrical energy that is extracted from the system. In some embodiments, the at least one expansion device <NUM> is implemented as a plurality of devices coupled in series or arranged in parallel. The working fluid emerges from the at least one expansion device <NUM> at lower pressure in stream <NUM>. The low pressure of stream <NUM> can range from <NUM> bar to <NUM> bar, and preferably from <NUM> bar to <NUM> bar. The low pressure stream <NUM> is fed to a condenser <NUM> that cools the working fluid to near ambient temperature or near cold sink temperature. Condensed stream <NUM> exiting the condenser <NUM> is then fed to a compression device <NUM>. The compression device <NUM> can be implemented as one or more pumps, compressors, turbo-pumps, and/or turbo-compressors. Stream <NUM> output from the compression device <NUM> emerges at a higher pressure relative to stream <NUM> and, at this elevated pressure, is suitable for injection (or re-injection) into the reservoir through injection well <NUM>.

An additional stream <NUM> comprising a fresh source of CO<NUM> or hydrocarbons can be introduced upstream or downstream of the condenser <NUM> (shown being introduced upstream in <FIG>) depending on the physical state of stream <NUM>. Separator outputs <NUM>, <NUM> can be processed further (for example, by additional hydrocracking) or the outputs can be recycled back to the reservoir through another injection well (not shown) or through the same injection well <NUM> via stream <NUM> downstream compression device <NUM>.

The efficiency of the recovery system is improved when the ratio of the amount of energy (work) that is input to compress the working fluid to the amount of energy output by the turbine is reduced. Table <NUM> provides ratios of compression work to turbine work for different working fluid compositions at different temperatures from a simplified energy recovery system, as shown schematically in <FIG>. As shown, a working fluid is first expanded in turbine <NUM>, then condensed in heat exchanger <NUM>, and next compressed in compression block <NUM>. Each row of Table <NUM> provides ratio data for a specific working fluid blend. The lower the ratio value, the higher the efficiency of the system. The inlet stream to the turbine <NUM> is at <NUM> and <NUM> bar for all runs.

The results shown in Table <NUM> demonstrate that the efficiency of CO<NUM> alone or in an <NUM>% CO<NUM>/<NUM>% ethane blend is higher than other more mixed compositions at condensing temperatures below the CO<NUM> critical point temperature (<<NUM>). At temperatures above the CO<NUM> critical point (><NUM>), at both subcritical and supercritical pressures, working fluids with additional ethane and heavier hydrocarbons such as propane are the most efficient.

The disclosed system and method enable the composition of the working fluid to be adjusted based on the fluid temperature in order to achieve the optimal efficiency. The adjustments can be made in response to ambient temperature changes or changes in reservoir conditions, for example. Simulations can be performed for the system <NUM> coupled to the reservoir for various ambient conditions and reservoir pressures to determine the optimal working fluids over temperature and pressure ranges. The control unit <NUM> of separation unit <NUM> can be configured with values and other parameters using data obtained from the simulations, in certain implementations. It is noted that in some embodiments the control unit can be incorporated as a component of the separation unit <NUM> itself.

When operating at condensing temperatures below the CO<NUM> critical point temperature, the separation unit <NUM> sets the composition of the working fluid to vary between pure CO<NUM> and an azeotropic mixture of CO<NUM> and ethane. When the condensing temperature exceeds the CO<NUM> critical point temperature, the separation unit <NUM> enables heavier hydrocarbons such as propane to enter the working fluid stream, facilitating the condensation of the working fluid to a liquid state at the elevated temperatures. By facilitating condensation to the liquid state, the required compression energy is minimized.

<FIG> is a schematic block diagram showing another embodiment of a geothermal energy recovery system according to the present disclosure. The recovery system <NUM> is adapted to provide higher power output than the arrangement of <FIG> by employing intrinsic heat generation. The geothermal recovery system <NUM> shown in <FIG> operates similarly to the system discussed above and shown in <FIG> except that in system <NUM>, an oxidation unit <NUM> is used instead of a heat exchanger. Except as discussed next, the system <NUM> can be arranged, connected and configured the same as the system <NUM>.

High pressure working fluid <NUM> output from phase separator <NUM> enters oxidation unit <NUM> which also receives an oxygen stream <NUM>. In some embodiments, oxidation unit <NUM> can receive a stream of water vapor <NUM> in addition to or as a substitute for oxygen stream <NUM>. The oxygen stream <NUM> oxidizes hydrocarbons in working fluid stream <NUM> in an exothermic reaction into CO<NUM> and water vapor, generating substantial heat. The output stream <NUM> that exits from the oxidation unit <NUM> is therefore at an elevated temperature with respect to stream <NUM>. Oxidation unit <NUM> can be implemented using a catalytic bed having a noble metal (Rh, Pt) based catalyst in geometries that provide sufficient surface area (such as monoliths, microliths, etc.) to ensure a low pressure drop through the system as well as to have selective oxidation of the heavier hydrocarbons and any other carbon-based species such as CO.

Depending on the characteristics and native fluid composition of the geothermal reservoir, oxidation unit <NUM> can provide additional heat to the system to increase the power output. Additionally, oxidation unit <NUM> can be utilized to complement the operation of separation unit <NUM> by selectively oxidizing specific species. For example, oxidation unit <NUM> can be implemented as an oxy-combustion system that non-selectively burns a specific amount of hydrocarbons present in the working fluid up to the depletion of the oxygen content. Additionally, the oxidation unit <NUM> can be provided with catalysts to promote complete combination of hydrocarbons into water, and also to reduce generation of carbon monoxide (CO). Alternatively, the amount of oxygen supplied through stream <NUM> can be restricted (e.g., through control valves in a known manner) to limit the consumption of the hydrocarbons to the desired level.

Oxidation unit <NUM> can be adapted to support the functions of the separation unit <NUM> by implementing the oxidation unit as a catalytic system that selectively oxidizes hydrocarbons. For example, if the amount of methane (CH<NUM>) in the reservoir is low while heavier hydrocarbons are more abundant, oxidation unit <NUM> can be to selectively oxidize CH<NUM>. Operation of the separation unit <NUM> (via controller <NUM>) can then be modified in tandem with operation of the oxidation unit, in the absence of light hydrocarbons, to separate only the heavier components of the input stream received from the reservoir. Alternatively, when the input from the reservoir has a significant amount of CH<NUM> and low amounts of heavy hydrocarbons (heavier than C2 or C3), the oxidation unit <NUM> can be adapted (with suitable catalysts and operating temperature) to selectively oxidize the heavier molecules or eventually reform the heavy molecules in the presence of steam or oxygen to produce a methane rich gas and CO<NUM>. In this case, the oxidation unit <NUM> provides a stream with light hydrocarbon and CO<NUM> to separation unit <NUM>. Separation unit <NUM> is then operated differently based on the different (lighter) input steam.

The remaining components of the embodiment of the geothermal recovery system are similar to those shown in <FIG>, and include an expansion device <NUM>, a condenser <NUM> and a compression device <NUM>.

<FIG> shows another embodiment of a geothermal recovery system according to the present disclosure. As in the other embodiments an extracted working fluid stream <NUM> is fed to phase separator <NUM> at which liquid and vapor portions of the working fluid are separated. A vapor stream <NUM> output from phase separator <NUM> is fed to heater exchanger <NUM> which outputs a heated working fluid stream <NUM>. However, at this point in the system, this embodiment differs from those shown in <FIG> and <FIG> in that the output from the heat exchanger <NUM> is fed to an expansion device <NUM> rather than to a separator unit <NUM>. In other words, the expansion device <NUM> is positioned upstream of the separator device <NUM>, which is the reverse in the other embodiments. The heated stream <NUM> enter into the expansion device <NUM> where it is expanded, producing work that is converted into mechanical or electrical energy. The resulting expanded and cooled working fluid exits the expansion device <NUM> as a low-pressure stream <NUM>. The low-pressure stream <NUM> is fed to the separation unit <NUM>. The separation unit <NUM> is operated to take as an input the low-pressure stream <NUM> and to output, in a first outlet stream, a working fluid <NUM> with a selected composition. Separation unit <NUM> operation is similar to the operation described in the embodiments of <FIG> and <FIG> for separation units <NUM> and <NUM>. However, separation unit <NUM> can be implemented using a different separation mechanism, such as, for example, distillation. The working fluid <NUM> output from the separation unit <NUM> is fed to condenser <NUM> and then to compressions device <NUM>, completing the cycle as described above.

One of the important advantages of the disclosed system and method is that it is adapted to operate in hot environments or when the cooling media temperature is above the CO<NUM> critical point (><NUM>). Higher temperatures are accommodated by controllably combining CO<NUM> with additional hydrocarbons to form azeotropic or other types of mixtures that allow for the partial condensation of the mixtures at temperatures above the CO<NUM> critical temperature. These measures optimize cycle efficiency and save compression energy, improving the overall performance of the geothermal recovery system and method.

It is to be understood that any structural and functional details disclosed herein are not to be interpreted as limiting the systems and methods, but rather are provided as a representative embodiment and/or arrangement for teaching one skilled in the art one or more ways to implement the methods.

It is to be further understood that like numerals in the drawings represent like elements through the several figures, and that not all components or steps described and illustrated with reference to the figures are required for all embodiments or arrangements.

It will be further understood that the terms "comprises" and "comprising", when used in this specification, specify the presence of stated features, integers, steps, operations, elements, or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or groups thereof.

Terms of orientation are used herein merely for purposes of convention and referencing and are not to be construed as limiting. However, it is recognized these terms could be used with reference to a viewer. Accordingly, no limitations are implied or to be inferred.

Claim 1:
A geothermal energy recovery system (<NUM>), coupled to a geothermal reservoir (<NUM>) via an injection well (<NUM>) and an extraction well (<NUM>), the system comprising:
a phase separator (<NUM>) having an input port coupled to the extraction well and receiving an extracted working fluid (<NUM>) including carbon dioxide and hydrocarbons therefrom, the phase separator operative to separate liquid (<NUM>) and vapor (<NUM>) portions of the extracted working fluid;
a controllable separator unit (<NUM>) coupled downstream from the phase separator and receiving the vapor portion of the extracted working fluid therefrom,
characterised in that:
the separator unit is controllably operable to separate components of the received vapor portion based on chemical composition and to selectively mix the separated components into a modified working fluid based on detected process and ambient conditions of the extracted working fluid,
wherein the modified working fluid (<NUM>) has a chemical composition that is optimized for energy recovery efficiency; and
an expander coupled downstream from the controllable separator unit and operable to generate mechanical or electrical energy from expansion of the modified working fluid.