Methods for stimulating non-hydrothermal hydrogen generation from rocks

Abstract

Methods for stimulating hydrogen production from Fe(II)-bearing rocks and minerals under non-hydrothermal conditions (e.g., ≤200oC) by both increasing the rate and extent of hydrogen production and reducing the consumptive loss of hydrogen by abiotic or biotic reactions. The methods utilize a fluid (e.g., liquid) having low levels of oxidants such as oxygen, nitrates, sulfates and carbon dioxide (CO2), low levels of nutrients such as phosphate, a highly basic pH, and elevated Ca2+ or ammonium activity. In some embodiments of the methods, the fluid is free of any or all of oxidants, nitrates, sulfates, CO2, as well as the nutrient phosphate. By use of the terms "free of" and "essentially free of," what is intended is a level of no more than 1 ppm per oxidant.

Description

METHODS FOR STIMULATING NON-HYDROTHERMAL HYDROGEN GENERATION FROM ROCKSCROSS-REFERENCE

[0001] This application claims priority to U.S. provisional application no. 63 / 761,640 filed February 21, 2025 and titled “Design and Implementation of Reaction Conditions to Promote the Chemical and Biological Production of Hydrogen During Stimulated Water / Rock Reactions,” the entire disclosure of which is incorporated herein by reference for all purposes.BACKGROUND

[0002] Hydrogen is a versatile zero-carbon energy carrier that is increasingly sought in the chemical, transportation and energy sectors globally. Hydrogen is used as a feedstock for chemical synthesis (e.g., ammonium fertilizers) or to provide energy to processes that have been challenging to decarbonize, such as steel production. Hydrogen can also be used in fuel cells for transportation, heat and electricity generation. There is a need for hydrogen sources that are close to carbon neutral, rather than hydrogen produced from fossil carbon sources, to meet U.S. and international goals for net-zero CO2 emissions in the energy sector by 2050. One example is “green hydrogen” produced by electrolysis using renewable energy sources such as wind or solar. Green hydrogen projects are under development in the U.S., Europe, Australia, the Middle East and more. However, massive amounts of renewable electricity would be required to meet the growing global demand for hydrogen. Moreover, these sources of electricity have their own intensive resource demands, such as critical metals and materials, that may limit the amount of renewable energy that can be dedicated to hydrogen production. It is also challenging to generate “green hydrogen” at costs that are competitive with hydrogen produced from carbon-based sources such as steam-reforming of methane or petroleum.

[0003] Natural geological processes can produce zero-carbon hydrogen as a primary energy source. For example, hydrogen is slowly produced and stored through time in subsurface rock formations as radioactive elements in rocks that undergo spontaneous decay reactions that split water and produce hydrogen gas via radiolysis. In separate, more rapid processes, Fe(II)-bearing minerals in subsurface rock formations can reduce water to produce hydrogen gas. The1 Attorney docket CU2025-068-PCTlocations and amounts of previously formed hydrogen that are currently stored in the Earth’s subsurface is poorly constrained. Sometimes hydrogen can be detected as it exsolves from fluids and is released at seeps or in fault zones and other geological structures. Currently there is intensive exploration occurring globally, searching for geological “hydrogen systems” where source rocks have been reacted with deep fluids to generate hydrogen, where that hydrogen has migrated into a suitable reservoir with properties that allow for the storage and preservation of hydrogen. Some of these proposed systems are now being drilled and tested for hydrogen accumulations and gas flows.

[0004] There is also interest in whether or not the production of geological hydrogen can be successfully engineered. Fe(II)-bearing rocks can react with water in “real-time” to produce hydrogen. Thus there is increasing interest in designing “stimulated geological hydrogen” production by reacting fluids with abundant Fe(II)-bearing rock resources such as mafic and ultramafic rocks to produce hydrogen gas. However, the optimal fluid and rock compositions are not yet known. In addition, systems that can be most easily accessed for injection of fluids into target rocks are in shallower parts of the Earth’s crust where temperatures are low and microbial activity is expected to be pervasive, leading to the dual challenge of possible low rates of hydrogen production and high rates of biological hydrogen consumption.

[0005] Altogether, the extent of hydrogen production in natural and engineered systems depends on many factors, including the temperature of the rocks, the water / rock ratio, the fluid residence time, the chemical composition of the fluids, the chemical composition of the rocks, the permeability of the rocks, and the extent of biological hydrogen production or consumption. For engineered systems, the challenge is to promote the optimal reaction conditions that give rise to the fastest rates of hydrogen production and the highest yield of hydrogen produced per volume of rock at the prevailing temperatures. Unfortunately, some of the best-known reactions that couple the oxidation of mineral-derived Fe(II) to the reduction of water to produce hydrogen gas (e.g., serpentinization of olivine) are predicted to occur at maximum rates and extents at temperatures between 200-325°C. Reaching these high temperatures requires either conducting the water / rock reactions at great depth, in hydrothermally-active geological systems, or in rocks heated by large inputs of energy. Alternatively, to engineer hydrogen production from target Fe(ll)-bearing rocks at shallower temperatures, there is a need to find ways to increase the rates of reaction, especially for any efforts for economic production. Possible pathways to accelerate2 Attorney docket CU2025-068-PCTthe expected low rates of non-hydrothermal hydrogen production from target rocks will likely include ways to increase contact between rock and fluids (e.g., through fracturing technologies), increase the slow chemical rates of reaction anticipated at lower temperatures (e.g., through catalysis), or increase the thermal regime to achieve more maximal rates (e.g., through rockheating technologies or injection of heated fluids or steam). Effective methods designed to reduce loss of hydrogen through biological or abiotic reactions would also be a form of possible “stimulation.”

[0006] The present discussion provides improved methods for obtaining hydrogen from geological sources.SUMMARY

[0007] The present disclosure provides methods for stimulating hydrogen production from Fe(II)-bearing rocks and minerals under non-hydrothermal conditions (e.g., reactions <200°C), with some of the methods having the potential to increase the rate and yield of hydrogen production to temperatures as high as 325°C. The methods use synthetic or modified natural fluid compositions to hydrate and oxidize Fe(II)-bearing minerals and rock formations to produce H2 gas.

[0008] The fluid compositions can be designed to minimize consumptive loss of hydrogen to microorganisms that are naturally present under non-hydrothermal conditions when the rocks and fluids that are reacting together. Some fraction of those microorganisms will have the metabolic capability to consume or produce hydrogen, as well as to dissolve and / or precipitate minerals involved in hydrogen-generating reactions. The methods optimize fluid compositions that will give rise to higher net yields of hydrogen by reducing consumptive losses.

[0009] The fluids have low or very low levels of fluid-borne oxidants, since oxidants, when present, can control the conversion of Fe(II) to Fe(III) without yielding hydrogen, or can be used to abiotically or biotically consume the hydrogen that is generated, thereby reducing the total yield or “net” yield; these fluid-borne oxidants include oxygen, nitrates, nitrites, sulfates, poly sulfides and carbon dioxide. In some embodiments, the levels of these oxidants is below the single ppb range, and / or, in the nanomolar or lower range; it is understood that the acceptable or desired level of any of these and other oxidants will differ based on the composition of the rock formation. The fluids can also have low levels of nutrients, such as phosphate. The fluids may be3 Attorney docket CU2025-068-PCTneutral, alkaline, or hyperalkaline (e.g., pH 7-13), although in some embodiments the pH should be as high as possible. In some embodiments, the reaction between the fluid and the rock will push the pH higher; for example, when injected, the fluid may have a pH 7 but rapidly increases to >pH 9.5 downhole.

[0010] The fluids, having low dissolved oxidants and / or nutrients, and / or a high pH, create a “biological limitation” to dramatically reduce biological activity that consumes H2.

[0011] These approaches can be used in-situ (e.g., by injecting, circulating and extracting fluids and gas in fractures and connected porosity in the Earth’s subsurface Fe(II)-bearing rock formations) and ex-situ (e.g., by putting fluids in contact with crushed target rocks in a vessel, such as a reactor, and extracting hydrogen gas).

[0012] The rock types targeted as sources of hydrogen from these methods include, but are not limited to, peridotite, gabbro, basalt and banded iron formations. The minerals and metal alloys targeted as sources of hydrogen from these methods include but are not limited to, olivine, pyroxene, Fe(II)-brucite, siderite, magnetite, awaruite and other Fe(II)-bearing silicates, hydroxides and carbonates such as serpentine, greenalite, and green rust. The fluid composition is designed to increase the total yield and / or the rate of yield of hydrogen based on the rock.

[0013] The chemical and the biological methods use fluids designed to control mineral saturation states, such as control mineral solubility to induce / enhance dissolution and precipitation of target phases and their Fe(IVIII) ratio, control the redox state of the fluids, including dissolved oxidant availability that can lead to biotic or abiotic destruction of H2, and specific catalysis of the Fe(II)-oxidation reactions that yield hydrogen. The fluids also optimize conditions for chemical hydrogen generation from the rocks, reduce the amounts of preferred oxidants for H2-based metabolisms, as well as place the microorganisms under physiologically challenging conditions imposed by trace nutrient availability and the high pH. These methods, which together lead to chemical stimulation and “biological limitation,” alone or in combination, can increase the yield of “geological hydrogen” produced from reactions between rocks / minerals and water.

[0014] As one example, the fluid for stimulating hydrogen production has low levels of any or all of oxidants, such as oxygen, nitrate, nitrite, polysulfides, sulfates and CO2, nutrients such as phosphate, and has a pH above 9.5, and includes Ca2+. The level of any or all of these individual oxidants and nutrients is no more than 1 ppm, in other examples, no more than 1 ppb4 Attorney docket CU2025-068-PCTand preferably it is below detection (e.g., ppb to ppt levels depending upon analyte and methods of quantification), with Ca2+greater than 50 ppm. In some embodiments, the total level of total oxidants in the fluid is in the nanomolar range, whereas in other embodiments the level of each oxidant is in the nanomolar range. By use of the terms “free of’ and “essentially free of,” what is intended is a level of no more than 1 ppb or its equivalent nanomolar level. The preferred level of Ca2+is generally between 1 mM and 30 mM, where the maximum concentration is defined by the solubility of Ca2under the prevailing temperature and fluid composition conditions.

[0015] Ca(OH)2 is particularly useful, as both the addition of Ca2+and the OH’ base helps to draw the hydrogen-producing water / rock reactions forward by promoting secondary mineral formation. In addition to favoring the formation of Fe(III)-bearing oxides and Mg-silicates, such as magnetite and ferric serpentine, that can generate H2, the Ca(OH)2 addition can promote the formation of Ca-silicates that include Fe(III), such as (hydro)andradite; other Ca2+and Fe(III)-bearing phases such as epidotes and ferrites can also be favored depending upon the bulk rock geochemistry. At the same time, the combination of high Ca2+and high pH can also help to remove essential nutrients like phosphate, forming Ca-P minerals (e.g., apatite). The high pH provides physiological challenges to the metabolism of the microorganisms, which alone or together with the low phosphate availability, notably slows down microbial activity that might consume H2 and reduce net yields.

[0016] Additional Ca21effects can be promoted by including, in addition or as an alternate to Ca(OH)2, other Ca2+salts, such as CaCh, or by the addition of Ca-bearing minerals, or by targeting fluid circulation through a zone rich in Ca-bearing minerals in the mafic and ultramafic rocks, such as diopside or calcic plagioclase (primary Ca-bearing silicates) or xonotlite (which is a secondary Ca- silicate produced by prior water / rock reaction).

[0017] In one particular implementation, this disclosure provides a method that includes, at a temperature of no more than 200°C, reacting a rock having suitable Fe(II) minerals with a fluid to convert the Fe(II) to Fe(III) and thereby produce hydrogen, the fluid comprising no more than 1 ppm of any oxidant and having a pH 9.5 or greater (e.g., pH 10 and / or up to pH 13).

[0018] In another particular implementation, this disclosure provides a method that includes, in an enclosed space, exposing the rock to a fluid at a temperature of no more than5 Attorney docket CU2025-068-PCT200°C, the fluid comprising no more than 1 ppm of any oxidant and having a pH 9.5 or greater (e.g., up to pH 13); and collecting the hydrogen gas from the enclosed space.

[0019] In another particular implementation, this disclosure provides a fluid for injecting into a well for stimulation of hydrogen, the fluid comprising no more than 1 ppb of any oxidant, no more than 1 ppb phosphate, and having a pH 9.5 or greater (e.g., up to pH 13).

[0020] In yet another particular implementation, this disclosure provides a fluid for injecting into a well for stimulation of hydrogen, the fluid comprising no more than 1 ppb of any oxidant and no more than 1 ppb phosphate, with pH between 7 and 9.5, where the rock itself has sufficient pH-buffering capacity to quickly increase the pH of the fluid to 9.5-13. The pH-buffering capacity may be due to the presence of magnesium-iron brucite, calcium hydroxide, or other soluble base minerals.

[0021] These and other aspects of the methods to produce hydrogen described herein will be apparent after consideration of the Detailed Description and Figures herein. It is to be understood, however, that the scope of the claimed subject matter shall be determined by the claims as issued and not by whether given subject matter addresses any or all issues noted in the Background or includes any features or aspects recited in the Summary.BRIEF DESCRIPTION OF THE FIGURES

[0022] FIG. l is a schematic diagram of a method for producing hydrogen from a well in a rock formation.

[0023] FIG. 2 is a schematic diagram of a method for producing hydrogen from a plurality of rocks.

[0024] FIG. 3 is a graphical representation of an example of hydrogen production over time by reaction of an ultramafic rock with a synthetic groundwater fluid at 50°C contrasted against the same fluid modified to be highly basic (pH 12.8) by addition of Ca(OH)2.

[0025] FIG. 4 is a graphical representation of an example of hydrogen production over time by reaction of an ultramafic rock with fluids at 50°C, where dissolved nitrate is or is not present in the fluid.6 Attorney docket CU2025-068-PCT

[0026] FIG. 5 is a graphical representation of an example of hydrogen production by reaction of an ultramafic rock with fluids at 50°C over time with low (few micromolar) vs. high (almost millimolar) ammonium present in the fluid.

[0027] FIG. 6 is a graphical representation of an example of biological hydrogen consumption at 35°C by a mixed microbial community inhabiting I -rich fluids, where H2 loss (micromoles / liter / day) is calculated from corresponding methane and sulfide production in fluids that vary in terms of oxidant availability and pH.DETAILED DESCRIPTION

[0028] As indicated above, the present disclosure is directed to methods that increase the yield of “geological hydrogen” produced from reactions between rocks / minerals and water, both in-situ and ex-situ (e.g., in reactors). In the methods described herein, hydrogen production from Fe(II)-bearing rocks and minerals is stimulated by reaction with specific fluid compositions under non-hydrothermal conditions (e.g., reactions at <200°C); in some embodiments, the methods extend to temperatures potentially as high as 325°C. The methods provide optimized H2 production by having the aqueous geochemistry poised as close as possible to conditions where the H2-production reactions can continue to proceed. The system variables are manipulated to establish high dissolved H2 concentrations and the potential formation of a free gas phase. The fluid chemistry variables that are modified include fluid pH, redox state, oxidant availability, and activity of cations such as Ca2+and ammonium. Together, these affect mineral saturation states and the forward progress of H2-producing reactions.

[0029] The methods described herein are an engineering solution to producing hydrogen by “stimulating” reactions between optimized fluids and target Fe(ll)-bearing rocks. The methods include exposing target rock formations rich in reactive Fe(II)-bearing minerals to the fluid (e.g., fluid is injected), fluid is then stored or transferred through fracture permeability and connected porosity, hydrogen is produced via Fe-oxidation reactions, the fluid is extracted, and hydrogen is collected. In some embodiments, the temperature of the rocks is controlled by the local geothermal gradient (e.g., in the rock) and although the temperature may vary from near zero Celsius to temperatures greater than 325°C, the fluids of the present discussion are particularly directed to the optimization of reactions that are more common in the shallow7 Attorney docket CU2025-068-PCTsubsurface and where microbial life is expected to be abundant and active (e.g., <200°C). Other fluid compositions can be designed for other temperatures with the overall technology described herein.

[0030] The present methods increase H2 production rates at temperatures below 200°C, such as at 20-150°C, as well as at 200-325°C. In some embodiments, fluids can be injected into rocks that have been fractured (e.g., hydraulically or electrically fractured) to increase their permeability and reactive surface area. The methods can be in-situ (e.g., circulating the fluids in the Earth’s subsurface Fe(II)-bearing rock formations, e.g., in a well or between wells) or ex-situ (e.g., contacting the rocks in a reactor with the fluid, where temperature can be directly controlled). The methods are designed to address the fact that the rocks and fluids will naturally contain microbial organisms, some of which may have the potential to produce or consume hydrogen, at least for any conditions below 120°C. The goal is to reduce the biological H2-consuming activity through modification of the fluid chemistry, without requiring the addition of biocides or microbial metabolic inhibitors that only provide a short-lived effect and which can also negatively impact the rates of the Fb-producing water / rock reactions.

[0031] The aqueous geochemistry of the fluid will impact microbial activity, which in turn impacts the total hydrogen yield. In rocks and aquifer systems that are microbially-inhabited (e.g., subsurface rock formations below the known temperature limit of life, which is approximately 120°C), hydrogen oxidation with dissolved oxygen or nitrate or nitrite as “terminal electron acceptors” provides enormous amounts of energy for microbial growth, giving rise to rapid blooms of microorganisms. The high energetic yield also enables organisms to draw down the dissolved hydrogen concentrations to very low levels (<nanomolar), which is considered a “parasitic loss” of hydrogen. When only sulfate or poly sulfides or CO2 are present as the dominant oxidants in the groundwater, dissolved hydrogen can be maintained at higher levels (e.g., micromolar to millimolar), although other unwanted by-products such as hydrogensulfide or methane can be generated. Thus, the aqueous geochemistry of the fluid and how it evolves during water / rock reaction, will also strongly control which hydrogen consuming microbial metabolisms will be feasible and at what rates they operate.

[0032] As indicated above, the methods herein utilize a fluid (e.g., liquid) having particular chemical properties selected for increasing the mineral transformations that include the conversion of Fe(II) to Fe(III) associated with the production of H2, while also minimizing the8 Attorney docket CU2025-068-PCTprobability of microbial H2 consumption. For example, the fluid has low levels of oxidants such as oxygen, nitrates, sulfates and carbon dioxide (CO2). Nutrients such as phosphates are also minimized and can be eliminated or essentially eliminated. Prior to the discussion herein, injection fluid (sometimes referred to as virgin fluid) has been selected from available tap / drinking water, groundwater, seawater, municipal treated water, or desalinated water, all of which have some of these compounds present in the ppm range, sometimes in the 10 ppm range, sometimes even in the 100 ppm range. In a different way, unmodified fluids have these compounds present in the micromolar to millimolar range, whereas the modified fluids of these methods have these compounds in the nanomolar range, generally undetectable levels of these compounds.

[0033] In some embodiments of the methods described herein for production of H2, the fluid is free of or essentially free of any or all of the major oxidants noted, and with very low concentrations of nutrients such as phosphates; in other words, the original fluid has been treated to be free of or essentially free of any or all oxidants and have reduced concentrations of phosphates and other nutrients. By use of the terms “free of’ and “essentially free of,” what is intended is a level of no more than 10 ppb per oxidant, or in other embodiments, no more than 1 ppb total oxidants. In some embodiments, the total level of total oxidants in the fluid is below the nanomolar range. In other embodiments, the total level of oxidants is in the nanomolar range, whereas in other embodiments the level of each oxidant is in the nanomolar range. It is understood that the acceptable or desired level of any of these and other oxidants will differ based on the composition of the rock formation and / or the abundance of microbes.

[0034] The methods described herein utilize the fluid to hydrate and oxidize Fe(II)-bearing minerals and rock formations to produce H2 gas. The rock types targeted as sources of hydrogen from these methods include, but are not limited to, peridotite, gabbro, basalt and banded iron formations. The minerals and metal alloys targeted as sources of hydrogen from these methods include but are not limited to olivine; pyroxene; Fe(II)-bearing metal hydroxide minerals (e.g., brucite and green rust); Fe(II / III)-oxide minerals (e.g., magnetite); Fe(II / III) silicates such as serpentine, greenalite, and clay; Fe(II)-carbonate minerals (e.g., siderite and ankerite); and Fe(0)-minerals including awaruite and other metal alloys.

[0035] The chemical and the biological methods described herein include control of mineral saturation states (i.e., control mineral solubility to induce dissolution and precipitation of9 Attorney docket CU2025-068-PCTtarget phases and their Fe(TI / ni) ratio), control of the redox state of the fluids, including dissolved oxidant availability that can lead to (biotic or abiotic) destruction of H2, and specific catalysis of the Fe(II)-oxidation reactions that yield hydrogen. The use of these methods, alone or in combination, can increase the yield and the rate of yield of “geological hydrogen” produced from reactions between rocks / minerals and water.

[0036] Use of the fluids and the methods described herein can produce any or all of, in Fe(II)-rocks:chemical stimulation that enhances the in-situ production rate and total yield of geologic hydrogen in the subsurface;chemical stimulation that enhances hydrogen production rates and yields in ex-situ reactors;biological activity limitation that enhances the net in-situ production rate and total yield of geologic hydrogen;biological activity limitation that enhances the net hydrogen production rates and yields in ex-situ reactors;chemical stimulation that enhances the rate of production of hydrogen from geological rock formations containing Fe(0)-minerals and Fe(IT)-bearing minerals; chemical stimulation that enhances the rate of production of geological hydrogen from peridotite, gabbro, basalt, and banded iron rock formations;biological activity limitation that enhances the total yield of hydrogen from geological rock formations containing Fe(0)-minerals and Fe(II)-bearing minerals; biological activity limitation that enhances the total yield of geological hydrogen from peridotite, gabbro, basalt, and banded iron rock formations;chemical stimulation that increases the total yield of geologic hydrogen from targeted rock formations;biological activity limitation that increases the total yield of geologic hydrogen from targeted rock formations;chemical stimulation, which enhances the rate of transformation of Fe(II)-bearing minerals, that increases hydrogen production;chemical stimulation, which removes oxidants from the fluids, to enhance the amount of Fe(II)-oxidation by water that is coupled to hydrogen production;10 Attorney docket CU2025-068-PCTchemical modifications that remove oxidants from the fluids to suppress biological H2 consumption, thereby increasing the total yield from rock oxidation; chemical stimulation, which enhances the extent of Fe(III)-bearing secondary mineral formation from Fe(II)-bearing mineral reactants, that increases hydrogen production;biological controls, which removes oxidants from the fluids, that enhances the amount of Fe(II)-oxidation that is coupled to hydrogen production;

[0037] In the following description, reference is made to the accompanying drawing that forms a part hereof and in which is shown by way of illustration at least one specific implementation. The following description provides additional specific implementations. It is to be understood that other implementations are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense. While the present disclosure is not so limited, an appreciation of various aspects of the disclosure will be gained through a discussion of the examples, including the figures, provided below. In some instances, a reference numeral may have an associated sub-label consisting of a lower-case letter to denote one of multiple similar components. When reference is made to a reference numeral without specification of a sub-label, the reference is intended to refer to all such multiple similar components.

[0038] Turning to the figures, FIG. 1 shows an in-situ embodiment where fluid is injected downhole into a rock mass to stimulate hydrogen production. FIG. 2 shows an ex-situ embodiment where individual rocks (e.g., quarried and then crushed rocks) are exposed to fluid (e.g., liquid) purged with inert gas to stimulate hydrogen production from the rocks. Both of these environments are enclosed spaces in which the mineral-bearing rock is exposed to one of the fluids described herein.

[0039] FIG. 1 shows a gas-producing environment 100 where a well 102 is drilled into a rock formation R having multiple different layers or strata. The well 102 extends below the surface 110 of the rock and is sufficiently deep to reach the desired layer, often at least 100 meters, and often greater than 1000 meters, although the depth will depend greatly on the rock formation. The well 102 is sufficiently wide to receive therein a fluid (e.g., liquid) source 120 that provides the fluid to the depth of the well 102. The fluid source 120 may provide the fluid to an open section at the bottom of the well (e.g., at the end of casing or pipe) or may have multiple11 Attorney docket CU2025-068-PCTexits along the depth of the well 102 (e.g., perforations), e.g., to contact more rock within the “stimulation volume.” The well may be vertical in its entire length, vertical that transitions to horizontal at depth, a slant well, or any directional or deviated well. A well may operate alone or in connection with additional wells (e.g., paired wells or an array of wells). In any pairing or array of wells, fluid may be provided to one well and hydrogen extracted from that or a different well.

[0040] The fluid may be at pressure, e.g., 500 psi-5000 psi, in some embodiments as much as 10,000 psi, depending on, e.g., the depth of the well 102, the composition of the rock formation R, the composition of the fluid, the permeability of the rock formation and the natural temperature within the well. The temperature of the fluid prior to injection can be 200°C or less, such as at the surface temperature (e.g., 10-40°C); the fluid temperature may increase in-well. Hydrogen gas, produced by water / rock reactions and then released from the rock formation R by reaction with the liquid from the source 120, exits the well as a gas stream or by pumping as hydrogen dissolved in water 130 and is separated from the water to be collected in a vessel 135. The liquid from the source 120 facilitates the production of the hydrogen gas in one or more of the various ways described herein, as does the extraction of the hydrogen gas, helping to continue to pull the reactions forward.

[0041] FIG. 2 shows a gas-producing environment 200 having a vessel 202, such as a reactor, with an amount of rocks R present in its interior 204. A liquid 220 is also present in the interior 204, in contact with the rock R. The liquid has been purged with an inert gas (e.g., nitrogen, argon) or vacuum to remove oxidants such as O2 and CO2. The vessel 202 may rotate, tumble, or agitate to improve the rock exposure to the liquid. Any interior volume in the vessel 202 not filled by the rock R or the liquid 220 is filled by an inert gas, such as nitrogen or argon. Hydrogen gas, released from the rock R facilitated by the liquid 220, exits the vessel 202 as stream 230; in some embodiments, the stream 230 is composed of the liquid 220 with hydrogen gas entrained therein. The liquid 220 facilitates the production of the hydrogen gas in one or more of the various ways described herein, as does the extraction of the hydrogen gas, helping to pull the hydrogen-producing reactions forward. For an environment such as this, the temperature of the fluid can be higher than for the well environment of FIG. 1.

[0042] The fluid (e.g., liquid) composition, used in both environments 100, 200 describe above, is not virgin fluid, but fluid that has been modified to have a specialized composition to12 Attorney docket CU2025-068-PCTprovide more optimized reactions with Fe(0) and Fe(IT)-bearing rocks and minerals to Fe(III)-bearing phases to produce high purity H2 with minimal production of other gases, such as methane, nitrogen, carbon-dioxide (CO2), carbon-monoxide (CO) or hydrogen sulfide (H2S). The fluid composition can be adjusted to inhibit consumptive loss of H2, e.g., loss of Fhdue to abiotic reactions (e.g., thermochemical sulfate reduction), or due to the activity of microbes in or on the rock or in the fluid, which would oxidize the H2. The composition of the fluid will depend on the chemical composition and physical formation of the rock; for example, rocks and minerals such as peridotite, basalt, olivine, pyroxene, serpentine, brucite, magnetite, wustite and banded iron formations may each have their own optimal fluid composition. The fluid composition can be formed (e.g., the virgin fluid modified) on-site at the gas-producing environment 100, 200 or at a remote location. For example, the fluids can be subjected to gas-exchange, where the initial dissolved gases such as oxygen and carbon-dioxide are extracted exposing the fluid to a vacuum and / or inert “sparge gas” or “sweep gas”, such as nitrogen or argon using a gas-exchange membrane, or a gas-tight vessel, prior to injection or reaction.

[0043] The chemical mechanisms to stimulate H2 production focus on promoting secondary mineral formation. The rates of Fe-oxidation can be increased by enhancing the extent of Fe(III)-mi neral formation available from the Fe(IT). Fe(III)-b earing phases that can form from Fe(II) to accommodate hydrogen production include Fe(III)-b earing silicates (e.g., Fe(III)-serpentine, Fe(IIl)-clays, and Fe(III)-b earing garnet such as hydroandradite), ,Fe(IlI)-bearing oxides and hydroxides, such as magnetite, green-rust, maghemite, pyroaurite, iowaite, Fe(III)-brucite, ferrihydrite, lepidocrocite and goethite. Precipitation of these phases serves as a sink for Fe(III) and pull forward the Fe(II)-oxidation reactions that produce H2 (i.e., increase rates of Fe-oxidation).

[0044] The ion activity products can be increased to exceed the solubility of these Fe(III)-bearing minerals, through control of the aqueous chemical composition of the fluids, such as increasing Ca2+activity (e.g., by addition of Ca(OH)2 and / or CaCh), increasing pH by addition of other strong bases such as Na(OH) or KOH, and, by helping nucleate and grow / precipitate these minerals, using inorganic seed phases (e.g., diopside, andradite), or the incorporation of other trivalent metals (e.g., Al3+, Cr3+) that initiate the formation of Fe(IIl)-bearing secondary phases.13 Attorney docket CU2025-068-PCT

[0045] Additionally, as water / rock reaction proceeds, secondary minerals that occupy larger volumes than the precursor minerals can be formed, which thus lead to large differential stresses within the rock matrix, which can lead to fracturing and the generation of new reactive surface areas.

[0046] The biological controls employed to increase net hydrogen production focus on reducing the rate of consumptive activity by limiting nutrients and oxidants that can be coupled to H2 for metabolism. Hydrogen produced from water / rock reactions can be consumed by microbial organisms that couple the oxidation of H2 to the reduction of oxygen, nitrate, nitrite, sulfate, sulfur / poly sulfides, CO2, Fe(III)-phases and other oxidants or electron acceptors.Example H2-dependent metabolisms utilizing dissolved oxidants that can be minimized include O2 (e.g., “knallgas bacteria”), nitrate and nitrite (e.g., “denitrifiers”), sulfate (e.g., “sulfate reducing bacteria”), sulfur and polysulfides (e.g., “sulfur reducing bacteria”), and CO2 (e.g., methanogens and acetogens). To inhibit the loss of hydrogen, microbial ^-consumption can be suppressed by lowering or removing aqueous oxidants, for example, first through displacement by inert gas (e.g., nitrogen or argon) sparging and using gas exchange membranes that can achieve several orders of magnitude reduction in O2 and CO2 concentrations, then further removal of all aqueous oxidants by contacting with reactive reducing compounds such as high surface area zero-valent iron or Fe(II)-b earing hydroxides, and additionally by precipitation (e.g., removal of excess carbonate by reaction with Ca21at high pH).

[0047] Microbial ^-consumption can also be suppressed by removing nutrients from the fluids, for example, by precipitating phosphate at high pH and Ca2+concentrations. As a positive side effect, trace metals (e.g., Ni+2, Mo+2and other bio-essential metals) will also precipitate from the fluid at high pH.

[0048] For each of the approaches discussed above, the fluid composition that reacts with Fe(II)-bearing rocks is a factor in promoting net H2 generation, and thereby exerts important controls on mineral dissolution, Fe-oxidation and secondary mineralization processes. In addition to reducing oxidants (e.g., to less than 1 ppm total, preferably less than 1 ppb total), reducing nutrients, etc., other additives may be added to the fluid or the fluid composition can be otherwise altered.

[0049] As indicated above, the fluid is alkaline / basic and has low levels of oxidants, such as oxygen, nitrates, sulfates, and CO2. The pH is high, usually at least 8.5, and in many14 Attorney docket CU2025-068-PCTformulations varies between pH 9.5-13. The hyperalkaline fluids push the rocks to react faster to produce H2 and the microbial rate of H2 consumption drops above pH 9.5. The fluid pH can be adjusted by the addition of various bases, such as a hydroxide, to promote mineralization of secondary silicates, particularly Fe(III)-b earing phases. The fluid or the environment (e.g., the environment 100, 200 of FIGS. 1, 2) may be 10°C-200°C.

[0050] Calcium hydroxide (Ca(OH)2) is particularly beneficial and can be added as, e.g., granular lime or solutions made at the limit of Ca(OH)2 solubility (“milk of lime”). Ca(OH)2 can increase the pH to hyperalkaline (e.g., pH 11-13) and also help drive the formation of secondary phases that include Ca2+and enhance Fe(II)-oxidation. Additionally or alternately, highly crushed Ca2+bearing minerals can be added, or the fluid can be in pre-equilibrated by reaction with Ca2+-bearing minerals. One example of a suitable Ca-silicate mineral is diopside, which is common in mafic and ultramafic rocks. Increased Ca2+activity in the fluid may drive the formation of andradite and / or hydroandradite, which are Ca-bearing, Fe(III)-secondary phases that are coupled to the generation of H2. Ca(OH)2 also helps to remove nutrients like phosphate, e.g., by forming calcium-phosphate minerals, and also can remove trace elements that precipitate as hydroxides, thereby limiting biological activity. Ca(OH)2 also helps to remove any residual CO2 that may be present as an oxidant in the fluid by reacting with the carbonate ion and forming calcium carbonates at the high pH. FIG. 3 shows results from the exposure of two different peridotite rocks (named BA1B and BA3A) at 50°C to a synthetic groundwater composition where all oxidants had already been removed below detection, compared against the same fluid that was further modified by the addition of 30 mM Ca(OH)2. The fluid with the Ca(OH)2 addition showed significantly more H2 produced per day.

[0051] The fluids of this disclosure are formulated to promote water / rock reactions that produce H2 from Fe-bearing rock formations. The fluids can be based on synthetic fluids or virgin fluids derived from natural sources (e.g., tap water, groundwater, seawater, municipal treated water, desalinated water). For synthetic fluids, these formulations start with a base fluid that typically includes NaCl (0.4-30g / L) and <lg / L KC1, NH4CI, CaCh, MgCh, MgSCh and / or KHCO3, although greater concentrations of any of these base salts can be added depending upon the intended rock type and temperature of reaction. Typically MgSCf or KHCO3 are used sparingly because they do contain sulfate and carbonate. For virgin fluids derived from natural sources, the base fluids each possess a unique original salt composition that is measured and15 Attorney docket CU2025-068-PCTdefined before being chemically modified to obtain the desired formation. If needed, the synthetic or natural fluids are pH adjusted to be between 9.5 to 13, e.g., by the addition of Ca(0H)2 or NaOH. The fluids can be reacted with additional minerals, such as diopside, calcic plagioclase, (Mg / Fe)-brucite, or zero valent iron to buffer the fluid composition and oxidant availability.

[0052] Because oxidants such as oxygen, nitrates, sulfates and carbon dioxide divert reducing equivalents from Fe(II), diminishing the reaction with water to produce H2, the fluid is free of or essentially free of oxidants, whether specifically removed from source fluid by sparging and precipitation, or by limiting their addition. Oxidants can be removed from fluid by purging out atmospheric gases and replacing oxidants with an inert gas such as N2 or Ar.Alternatively, they can be removed by exposing the fluid to a vacuum. Either process can be through direct contact with vacuum / inert gas, or by using a gas-exchange membrane or membrane contactor. Additionally, chemical additives (e.g., strong reductants and / or bases) can be used to directly remove any oxidants including residual atmospheric oxygen, nitrate, nitrite, sulfate, or carbonate that otherwise would passivate the rock samples as well as scavenge Fe(II), and additives such as BaCl or ZnCl can be used to precipitate oxidants or other nuisance compounds.

[0053] Nitrates can also be removed from fluid by, e.g., ion exchange, reverse osmosis, or chemical reduction. Nitrates in the fluid will preferentially react with Fe(II) compared to H2O (which H2O is the source of H2), and produce N2 and NH3 instead of H2. When nitrates are reduced or removed, hydrogen production can proceed at more optimal rates and total yields. FIG. 4 shows that fluids bearing nitrate produce only limited quantities of hydrogen when reacting with a peridotite rock at 50°C, whereas in contrast, fluids that are free of nitrates produce substantial quantities of hydrogen from the same rocks at the same temperature. These results are likewise experienced with multiple, additional fluid compositions and varied minerals as the Fe(II)-source (e.g., Fe-bearing brucite reactions with nitrate bearing fluids).

[0054] In addition to the fluid being (hyper)alkaline and having reduced (low) or no levels of oxidants, the presence of ammonium cation (NHG) or other cations such as Al3+increases the production of H2. Ammonium is an example cation that can be incorporated into secondary silicates, that can pull the hydrogen-producing reactions forward and increase the rate of generation of H2. The ammonium can be added (e.g., as NH4CI) to achieve a concentration16 Attorney docket CU2025-068-PCTgreater than 0.3 mM NH4+(where the NH4 / NH3 will be strongly pH dependent in the optimal fluid conditions of pH 9.5-13). FIG. 5 shows the comparison of H2 production from fluids having elevated versus minimal concentrations of ammonium (several hundreds of micromolar versus a few micromolar) reacting with a peridotite rock at 40°C at pH 9.5.

[0055] In the fluids with hyperalkaline pH and low or no oxidants, strong physiological limitations are placed upon any active microorganisms in the rocks or fluids. Specifically, H2-consuming metabolisms will slow down; this “biological limitation” then leads to greater preservation of H2 produced from water / rock reactions. FIG. 6 shows the calculated hydrogen loss determined from three fluids containing mixed microbial communities provided with 2 atm of H2, where H2-I0SS is due to active microbial metabolisms such as methanogenesis (oxidizing H2 by reaction with CO2 to produce methane) and sulfate-reduction (oxidizing H2 by reaction with sulfate ions to produce H2S). Strong reductions in the biological H2 consumption rate occur as the fluid pH increases above pH 9.5 and oxidant availability decreases.

[0056] From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

[0057] Although the technology has been described in language that is specific to certain structures and materials, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific structures and materials described. Rather, the specific aspects are described as forms of implementing the claimed invention. Because many embodiments of the invention can be practiced without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.

[0058] Various features and details have been provided in the multiple designs described above. It is to be understood that any features or details of one design may be utilized for any other design, unless contrary to the construction or configuration. Any variations may be made.

[0059] The above specification and examples provide a complete description of the structure and use of exemplary implementations of the invention. The above description provides specific implementations. It is to be understood that other implementations are contemplated and may be made without departing from the scope or spirit of the present disclosure. The above17 Attorney docket CU2025-068-PCTdetailed description, therefore, is not to be taken in a limiting sense. While the present disclosure is not so limited, an appreciation of various aspects of the disclosure will be gained through a discussion of the examples provided.

[0060] Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties are to be understood as being modified by the term “about,” whether or not the term “about” is immediately present. Accordingly, unless indicated to the contrary, the numerical parameters set forth are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein.

[0061] As used herein, the singular forms “a”, “an”, and “the” encompass implementations having plural referents, unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and / or” unless the content clearly dictates otherwise.

[0062] Spatially related terms, including but not limited to, “bottom,” “lower”, “top”, “upper”, “beneath”, “below”, “above”, “on top”, “on,” etc., if used herein, are utilized for ease of description to describe spatial relationships of an element(s) to another. Such spatially related terms encompass different orientations of the device in addition to the particular orientations depicted in the figures and described herein. For example, if a structure depicted in the figures is turned over or flipped over, portions previously described as below or beneath other elements would then be above or over those other elements.18 Attorney docket CU2025-068-PCT

Claims

WHAT IS CLAIMED IS:

1. A method comprising:at a temperature of no more than 200°C, reacting a rock having Fe(II) minerals with a fluid to convert the Fe(II) to Fe(III) and thereby produce hydrogen, the fluid comprising no more than 1 ppm of any oxidant and having a pH 9.5 or greater during the reaction.

2. The method of claim 1, wherein the fluid comprises 1 mM to 30 mM Ca2+.

3. The method of claim 2, wherein the Ca2+is from Ca(OH)2, CaCk or other salt and mineral sources.

4. The method of claim 1, wherein the fluid further comprises greater than 0.3 mM ammonium cation.

5. The method of claim 1, wherein the fluid comprises no more than 1 ppb of any oxidant.

6. The method of claim 5, wherein the fluid comprises no more than 1 ppb of total oxidants.

7. The method of claim 1, wherein the fluid comprises no more than 1 ppm of phosphate.

8. The method of claim 7, wherein the fluid comprises no more than 1 ppb of phosphate.

9. The method of any of the previous claims, further comprising:injecting the fluid into at least one well in the rock;circulating the fluid through fractures; andcollecting hydrogen gas from a well.

10. The method of any of claims 1-8, further comprising:crushing the rock;in a vessel, exposing the crushed rock to the fluid, and19 Attorney docket CU2025-068-PCTcollecting hydrogen gas from the vessel.

11. A method of forming hydrogen gas from a rock, the method comprising:in an enclosed space, exposing the rock to a fluid at a temperature of no more than 200°C, the fluid comprising no more than 1 ppm of any oxidant and having a pH 9.5 or greater; and collecting the hydrogen gas from the enclosed space.

12. The method of claim 11, wherein the enclosed space is at least one well in the rock and exposing the rock to the fluid comprises injecting and circulating the fluid within the at least one well.

13. The method of claim 11, wherein the enclosed space is a vessel with the rock within the vessel.

14. The method of claim 11, wherein the fluid comprises at least 1 mM Ca2+.

15. The method of claim 11, wherein the fluid comprises 1 mM to 30 mM Ca2.

16. The method of claim 11, wherein the fluid further comprises at least 0.3 mM ammonium cation.

17. The method of claim 11, wherein the fluid comprises no more than 1 ppb of any oxidants.

18. The method of claim 17, wherein the fluid comprises no more than 1 ppb total of oxidants.

19. The method of claim 12, wherein the fluid comprises no more than 1 ppm of phosphate.

20. The method of claim 19, wherein the fluid comprises no more than 1 ppb of phosphate.20 Attorney docket CU2025-068-PCT21. A fluid for injecting into a well for stimulation of hydrogen, the fluid comprising no more than 1 ppb of any oxidant, no more than 1 ppb phosphate, and having a pH 9.5 or greater.

22. The fluid of claim 21, comprising no more than 1 ppb total oxidants.

23. The fluid of claim 21 or claim 22 comprising 1 mM to 30 mM Ca2+.21 Attorney docket CU2025-068-PCT

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