Microbial electrolysis is a technology that uses living microorganisms as electro-catalysts in electrolysis cells. The technology can be used for wastewater treatment. Earlier, we proposed that microbial electrolysis be used to decentralize wastewater treatment and biogas production. Since this is a process that converts CO2 into organic compounds using electricity it can also be used for CO2 valorization. Besides methane, such electrolysis cells produce compounds such as acetic acid (vinegar), caproic acid, and others. It is then called microbial electrosynthesis.
However, the main problem with microbial electrolysis and electrosynthesis is the long start-up time. The start-up time is the time required for the microorganisms to form a biofilm on the electrode surface and to start producing the desired products. It can range from several weeks to several months, depending on the operating conditions and the type of microorganisms. This long start-up time limits the feasibility and the scalability of microbial electrosynthesis, as well as its economic and environmental benefits.
Now, scientists of the Wageningen University in the Netherlands presented new research, which aimed to reduce the start-up time of microbial electrosynthesis. By using a novel technique that involves alternating the direction of the catholyte flow through a three-dimensional electrode they were able to reduce the startup time to only ten days. They hypothesized that this technique enhances mass transfer and biofilm formation, and thus accelerates the CO2 reduction and the product formation. This was a start-up time reduction of 50%, compared to a conventional flow-through electrode.
The alternating electrolyte flow also reduced the power consumption to 136 kWh per kg of hydrogen. After 60 days, the local hydrogen concentration at the cathode was at a maximum of 600 μM, which indicates a better mass transport and thus a more active biofilm. The researchers speculated that the alternating catholyte flow improved mass transport, because the hydrogen could be better distributed over the cathode layers. In addition, the researchers think that alternating the flow refreshed potential “dead zones” in the cathode chamber.
However, there are limitations of the technique, such as the energy efficiency, the product selectivity, and the scalability of microbial electrosynthesis. Such limitations are typical for bench top experiments. We are therefore looking forward to see an industrial application of this new method.
In wastewater treatment, aeration is an energy-intensive but necessary process to remove contaminants. Pumps blow air into the wastewater to supply the microbes in the treatment tank with oxygen. In return, these bacteria oxidize organic substances to CO2 and hence remove them from the wastewater. This process is the industrial standard and has proven itself for over a century. If the researchers at Washington State University and the University of Idaho have their way, that is changing now.
In their project, the researchers used a unique microbial fuel cell system they developed to replace aeration. Their novel wastewater treatment system cleans wastewater with the help of microorganisms that produce electricity. These microbes are called electrophiles.
The work should one day lead to less dependence on the energy-intensive treatment processes. Most of the energy in such processes is consumed in the activated sludge and its disposal. The energy consumption in water treatment produces around 4-5% of anthropogenic CO2 worldwide. to put that in perspective, according to the Air Transport Action Group in Geneva, international air transport produced 2.1% CO2 in 2019. The researchers published their work in the journal Bioelectrochemistry. In addition to cutting green house gas emissions, lowering the energy consumption of wastewater treatment would save billions in annual operation and maintenance costs.
Microbial fuel cells allow microbes to convert chemical energy into electricity, much like in a battery. In wastewater treatment, a microbial fuel cell can replace aeration while capturing electrons from wastewater organics. These electrons themselves are in turn a waste product of the microbial metabolism. All living organisms strive to discharge their excess electrons. This process is known as respiration or fermentation. The electricity generated the microbes can be used for useful applications in the wastewater treatment plant itself. The technology kills two birds with one stone. On the one hand, the treatment of the wastewater saves energy. On the other hand, it also generates electricity.
Up until now, microbial fuel cells have been used experimentally in wastewater treatment systems under ideal conditions, but under real and changing conditions they often fail. Microbial fuel cells lack regulation that controls the potential of anodes and cathodes and thus the cell potential. This can easily lead lead to a system failure. The entire cell must then be replaced.
To tackle this problem, the researchers added an additional reference electrode to the system that enables them to control their fuel cell. The system becomes more flexible. It can either work as a microbial fuel cell on its own and consume no energy, or it can be converted so that less energy is used for aeration while it purifies the wastewater more intensively. Frontis Energy uses a similar control system for its electrolysis reactors.
The system was operated for one year without major issues in the laboratory as well as a pilot in a wastewater treatment plant in Idaho. It removed contaminants at rates comparable to those in a classic aeration tanks. In addition, the microbial fuel cell could possibly be used completely independent of grid power. The researchers hope that one day it could be used in small wastewater treatment plants, such as cleaning livestock farms or in remote areas.
Despite the progress, there are still challenges to be overcome. They are complex systems that are difficult to build. At Frontis Energy we specialize in such systems and can help with piloting and commercialization.
Forests are vital to our society. In the EU, forests make up around 38% of the total land area. They are important carbon sinks as they eliminate around 10% of EU greenhouse gases. Efforts to conserve them are a key part of EU climate targets. However, the increasing demand for forest products poses challenges for sustainable forest management.
According to a report recently published in the renowned science magazine Nature, the EU’s deforested area has increased by 49% and with it the loss of biomass (69%). This is due to large-scale deforestation, which reduces the continent’s carbon absorption capacity and accelerates climate change.
The analyzed a series of very detailed satellite data. The authors of the report show that deforestation occurred primarily on the Iberian Peninsula, the Baltic States, and Scandinavia. Deforestation of forest areas increased by 49% between 2016 and 2018. Satellite images also show that the average area of harvested land across Europe has increased by 34 percent, with potential implications for biodiversity, soil erosion and water regulation.
The accelerating deforestation could thwart the EU’s strategy to combat climate change, which aims in particular to protect forests in the coming years, the experts warn in their study. For this reason, the increasing use of forests is challenging to maintain the existing balance between the demand for wood and the need to preserve these key ecosystems for the environment. Typically, industries such as bioenergy or the paper industry are the driving forces behind deforestation.
The greatest acceleration in deforestation was recorded in Sweden and Finland. In these two countries, more than 50% of the increase in deforestation in Europe has been recorded. Next in line are Spain, Poland, France, Latvia, Portugal and Estonia, which together account for six to 30% of the increase, the study said.
Experts suggest linking deforestation and carbon emissions in model calculations before setting new climate targets. The increase in forest harvest is the result of the recent expansion of global wood markets, as evidenced by economic indicators for forestry, timber bioenergy and international trade. If such a high forest harvest continues, the EU’s vision of forest-based mitigation after 2020 could be compromised. The additional carbon losses from forests would require additional emission reductions in other sectors to achieve climate neutrality.
At Frontis Energy, we find the competition between bioenergy and this important carbon sink particularly disturbing, as both are strategies to mitigate global warming.
Much research has been done in order to reduce the use of fossil petroleum products as fuels. In that respect syngas (synthetic gas) seems as a great opportunity for sustainable energy developments. Syngas is the mixture composed of hydrogen (H2) and carbon monoxide (CO) as its main components. It represents an important chemical feedstock used widely for industrial processes for generating chemicals and fuels:
Global use of syngas in industrial processes.
Syngas can be produced from methane (CH4) in a reforming reaction with water (H2O), oxygen (O2) or carbon dioxide (CO2). The process called methane dry reforming (MDR) can be combined with carbon dioxide:
CH4 + CO2 → 2 H2 + 2 CO
It is an environmentally friendly path, turning two greenhouse gases into a valuable chemical feedstock.
However, the MDR is process requires chemical catalysts and high temperatures in the range between 700 − 1,000°C. Usually, it suffers from coke deposition and, in consequence, catalyst deactivation.
Some chemists have recently demonstrated that light, and not heat, might be a more effective solution for this energy-hungry reaction.
The photocatalytic solution
A team of researchers at the Rice University in Houston, Texas, together with colleagues from Princeton University and the University of California have developed superior light-stimulated catalysts that can efficiently power MDR reactions without any heat input. This work has been published in the prestigious journal Nature Energy.
They have reported a highly efficient and coke-resistant plasmonic photocatalyst containing precisely one ruthenium (Ru) atom for every 99 copper (Cu) atoms. The isolated single-atom of Ru obtained on Cu antenna nanoparticles provides high catalytic activity for the MDR reaction. On the other side, Cu antennas allow strong light adsorption and under illumination and deliver hot electrons to ruthenium atoms. The researchers suggested that both, hot-carrier generation and single-atom structure are essential for excellent catalytic performance in terms of efficiency and coking resistance.
The optimal Cu-Ru ratio have been investigated in synthesized series of CuxRuy catalysts with varying molar ratios of plasmonic metal (Cu) and catalytic metal (Ru), where x,y are atomic percentage of Cu and Ru. Overall, the Cu19.8Ru0.2 was the most promising composition in terms of selectivity, stability and activity. In comparison to pure Cu nanoparticles, the Cu19.8Ru0.2 mix exhibits increased photocatalytic reaction rates (approx. 5.5 times higher) and improved stability with its performance maintained over 20 h period. Calculations showed that isolated Ru-atoms on Cu lower the activation barrier for the methane dehydrogenation step in comparison to pure Cu without promoting undesired coke formation.
In addition, the research has been supported by different methods (CO-DRIFTS with DFT) in order to unravel and prove single-atom Ru structures on Cu nanoparticles occurring in Cu19.9Ru0.1 and Cu19.8Ru0.2 compositions.
The comparison between thermocatalytic and photocatalytic activity at the same surface for MDR has also been demonstrated. The thermocatalytic reaction rate at 726°C (approx. 60 µmol CH4 / g / s) was less than 25% of photocatalytic reaction rate under white-light illumination with no external heat (approx. 275 µmol CH4 / g / s). This enhancement in the activity is attributed to the hot-carrier generated mechanism which is predominant in the photocatalytic MDR. The role of the hot-carrier is an increase in C−H activation rates on Ru as well as improved H2 desorption.
The scientists also reported the catalyst achieving a turnover frequency of 34 mol H2 / mol Ru / s and photocatalytic stability of 50 h under focused white light illumination (19.2 W / cm2) with no external heat.
As the synthesized photocatalysts is primarily based on Cu which is an abundant element, this approach provides a promising, sustainable catalyst operating at low-temperatures for MDR. This allows cheaper syngas production at higher rates, bringing us closer to a clean burning carbon fuel.
An abandoned or unproductive oilfield can be reused for methane production from CO2 using renewable electrical power. Exhausted oilfields can be reactors for the conversion of renewable energy to natural gas using microbes. To achieve this, an oilfield can be made electrically conductive and catalytically active to produce natural gas from renewable power sources. The use of natural gas is superior to any battery because of the existing infrastructure, the use in combustion engines, the high energy density and because it can be recycled from CO2. Oilfields are superior to any on-ground production because of the enormous storage capacities. They are already well explored and these geological formations underwent environmental risk assessments. Lastly, the microbial power-to-gas technology is already available.
Process summary
Whole process (end-to-end via methane)
50% electrical efficiency
Energy density of methane
180 kWh kg−1
Storage capacity per oilfield
3 GWh day−1
Charge/Discharge cycles
Unlimited
Investment (electrodes, for high densities)
$51,000 MW−1
Cost per kWh (>5,000 hours anode lifetime)
<$0.01 kWh−1
Electrolyte
Seawater
The Problem
To address the problem of storing renewable energy, batteries have been proposed as a possible solution. Lithium ion batteries have a maximum energy storage capacity of 0.3 kWh kg−1. To date, this is considered the best trade-off between cost and efficiency but these batteries are still too inefficient to replace gasoline, which has a capacity of about 13 kWh kg−1. This makes battery driven cars heavier than conventional cars. Lithium air batteries are considered a possible alternative because they can reach theoretical capacities of 12 kWh kg−1 but technical difficulties have prevented them from being used for transportation.
In contrast, methane has an energy density of 52 MJ kg−1 corresponding to 180 kWh kg−1 which is second only to hydrogen with 500 kWh kg−1, not counting in nuclear energy. This high energy density of methane and other hydrocarbons along with their facile usage, is the reason why they are used in combustion and jet engines that drive nearly all transportation to date. While electrical cars seem to be a tempting green alternative, the fact that combustion engines and the fueling infrastructure are so wide-spread makes it difficult to switch.
In addition to the difficulty of changing habits, battery-driven electrical cars need other limited natural resources such as lithium. To equip all 94 million automobiles produced worldwide in 2017, 3 mega tons lithium carbonate would need to be mined annually. This is nearly 10% of the entire recoverable lithium resources of 35 mega tons worldwide. Although lithium and other metals can be recycled, it is clear that metal based batteries alone will not build the bridge between green energy and traditional ways of transportation due to the low energy densities of metals. And this does not even take into account other energy demands such as industrial nitrogen fixation, aviation or heating.
For Germany, with its high proportion of renewable energy, fuel for cars is not the only problem. As renewable energy is generated in the north, but many energy consumers are in the south, the grid capacity is frequently reached during peak production hours. A steadier energy output can only be accomplished by decentralizing the production or by energy storage. To decentralize production, homeowners were encouraged to equip their property with solar panels or windmills. As tax incentives phase out, homeowners face the problem of energy storage. The best product for this group of customers so far are again lithium ion batteries but investment costs of $0.10 kWh−1 are still unattractive especially because these products store the energy as electricity which can only be used for a short time and is less efficient than natural gas when used for heating.
Natural gas is widely used as energy source today and the global energy infrastructure is designed for natural gas and other fossil fuels. Increasing demand and limited resources for these fossil fuels were the main drivers of oil and gas prices during the last years, slowed by the recent economic crises and hydraulic fracturing (fracking). The high oil price attracted investors to recover oil using techniques that become increasingly expensive and are environmental risks such as deep-sea drilling or tar sand extraction. Ironically, the high oil price made costly renewable energies an economically feasible alternative and helped driving down their cost. Since habits are difficult to change and building an entirely new infrastructure only for renewable energies does not seem economically feasible today while CO2 drives global warming, a more realistic solution needs to be found.
Microbial Power-to-Gas could be a bridging technology that integrates renewable energy into the existing fossil fuel infrastructure. It reaches break even in less than 2 years if certain preconditions are met. This is accomplished by integrating methane produced from renewable energy into the current oil and gas producing infrastructure. The principal idea is to use carbon instead of metals as energy carrier because of its high energy density when bound to hydrogen. The benefits are:
High energy density of 180 kWh kg−1 methane
Low investments due to existing infrastructure (natural gas, oilfield equipment)
Carbon is not a limited resource
Low CO2 footprint due to CO2 recycling
Methane is a transportation fuel
Methane is the energy carrier for the Haber-Bosch process
Inexpensive catalysts further reduce initial investments
Low temperatures due to bio-catalysis
No toxic compounds used
No additional environmental burden because existing oilfields are reused
The solution
Methane can be synthesized by microbes or chemically. Naturally, methane is produced by anaerobic (oxygen-free) microbial biomass decomposition. The energy for biomass synthesis is provided by sunlight or chemical energy like hydrogen. In the case of methanogens (methane producing microbes), energy is harvested after CO2 and hydrogen were released from biomass decomposition following a 1-to-4 stoichiometry:
CO2 + 4 H2 → CH4 + 2 H2O
Without microbes, methane is produced by the Nobel-prize winning Sabatier reaction and several attempts are currently underway to use it on industrial scale. It is necessary to split water into hydrogen and use this to reduce CO2 in the gas phase. A major drawback of the Sabatier reaction is the need for high temperatures around 385ºC, and a nickel catalyst that becomes quickly spent. Methanogens use iron-nickel enzymes called hydrogenases to harvest energy from hydrogen, but do so at ambient temperatures.
Power-to-Gas concept for exhausted oilfields. Electrolysis catalyzes water splitting inside the oilfield producing methane gas and O2.
The future challenge will be to accelerate methane production rates as has been reported for a high temperature oilfield cultures. Besides increasing the temperature, the most obvious solution is to use a higher reactive surface and bringing both electrodes closer together. Using carbon brushes that are poor hydrogen catalysts but provide a higher surface for microbial attachment is one possibility. Methane production correlates with microbial cell numbers in the reactors.
The number of methanogens in microbial electrolysis reactors correlates with the electrode surface.
To overcome the problem of expensive carbon (and also steel) brushes for large scale applications,exhausted gas and oilfields could be used. They provide a high surface area and are usually economic liabilities and not assets. Methanogens inhabit oilfields where they carry out the final step in anaerobic petroleum degradation. Hence, oilfields can be seen as bioreactors at geological scale. Geological formations provide ideal conditions for producing, storing and extracting methane.
Open questions and potential solutions
Oilfield porespace volume
The Californian Summerland oilfield, for instance, has been abandoned and extensively studied in the past. It produced 27 billion barrels of oil and 2.8 billion m3 methane during its lifetime of 90 years. This maximum load of 3.5 billion m3 left the same volume of porespace filled with seawater behind. Only 2% of these pores are larger than 50 µm, which is necessary for microbial growth assuming dimensions of 1 x 2 µm of a methanogen cell. Experiments showed that the resulting 70 million m3 accessible porespace have a storage capacity of 35,000 TW. That is a lot of methane assuming a solubility of 0.74 kg methane m−3 seawater at 500 m water depth. All German off-shore windfarms together have a capacity of 7,000 MW. Obviously, the limiting factor is not the volumetric storage capacity of an oilfield.
Microbial methane production rates
But how fast can microbes produce methane in an hypothetical oilfield? Under optimal conditions, methanogens that grow on electrodes (typically the genus Methanobacterium or Methanobrevibacter) can produce methane at a rate of 100-200 nmol ml−1 day−1 (equals 2.24-4.48 ml l−1 day−1) depending on catalyst and potential. Using a production rate of 15 J ml−1 day−1 of methane (190 nmol ml−1 day−1), the entire microbially accessible oilfield (2%) has a capacity of 3.6 million MBtu per year. Microbes would theoretically consume 1 TWh per year for 3.6 million MBtu methane production if there were no losses and electrical power is translated into methane 1-to-1. A power generator of 121 MW would be sufficient to supply the entire oilfield at these rates. However, all German off-shore windfarms produce 7,000 MW meaning that only 3% off-peak power can be captured by our example oilfield. Therefore, the catalytic surface and activity must be increased to accelerate methane conversion rates.
Since methanogens produce methane from hydrogen, not only the 2% porespace big enough for cells can be used resulting in an increased catalytic surface to nearly 60%. A hydrogen catalyst needs to be found that does not out-pace methanogen growth to keep the reservoir pH within the limits of 6-8 required for methanogen growth. This hydrogen catalyst must be cheap and render an oilfield electrically conductive. A chemical formulation that mimics microbial hydrogen catalysis could be used. It has the potential to turn a non-conductive and non-catalytic oilfield into a conductive hydrogen catalyst sufficient to sustain methane production needed to store all of Germany’s electricity produced by off-shore windfarms. This catalyst is soluble in water when inactive. To become active, it coats mineral surfaces by precipitation that can be triggered by indigenous microbes or by electrical polarization. The investment would be $2.3 million per MW storage capacity ($16 billion for the entire 7,000 MW). Due to microbial growth, the catalytic activity of the system improves during operation and there is no need for the second component if an immediate production is not crucial. The investments made on the cathode side would then be as low as $600 per MW ($4.2 million for 7,000 MW).
Anodes
As the cathodic side of the reaction can be excluded as limiting factor, the anode needs to be designed. Several commercially available anodes such as mixed metal oxides (up to 750 A m−2) with platinum on carbon black or niobium anodes (Pt/C, 5-10 kA m−2) could be used. Anodes based on platinum are the most cost-efficient material available on the market. Investments made for Pt/C (10%, 6 mg cm−2) anodes will amount to $50,000 per MW ($350 million for 7,000 MW). However, the exact amount of Pt needed for the reaction still needs to be evaluated in an experiment because the corrosion rate at 2 V cell voltage is unknown. An often cited value for the lifetime of fuel cells is 5,000 hours and is used here to determine the costs per kWh. For 5,000 hours lifetime, the costs per kWh will be at the targeted limit of $0.01 but may be well below that because Pt/C anodes can be recycled and the Pt load may be reduced to 3 mg cm−2 (5%). Alternatively, steel anodes (SS316, 2.5 kA m−2, $54,000 per MW) can be used but it is unclear when steel anodes fail to electrolyze. In conclusion, the anodic side is the cost-driving factor. Hopefully, better water splitting anodes will lower these costs in future.
Cost estimation summary
Windfarms
Already in place
CO2 injection
Already occurred
Natural gas capturing equipment
Already in place
Microbial seed
Wastewater from oil rig
Cathode costs
$600 MW−1
Anode costs
$50,000 MW−1
Electrolyte (seawater)
Free
Total (>5,000 hours anode lifespan)
<$0.01 kWh−1
Energy and conversion efficiencies
The whole cell voltage for microbial power-to-gas reactions varies from 0.6 to 2.0 V, depending on cathodic rates, anodic corrosion and the presence of a membrane. Higher voltages will accelerate anode corrosion, again, making anodes the limiting factor. As the voltage decreases, methane production rates become slower but also more efficient. The voltage also depends on the pH of the oilfield. An oilfield that underwent CO2 injection as enhanced recovery method will have a low pH, providing better conditions for hydrogen production but not for microbial growth and must be neutralized using seawater. As stated above, the oilfield, being the cathode, is not limiting the the system. The use of Pt/C anodes eliminates the overpotential problem on the anode side. Hence, we can assume an ideal system that splits water at 1.23 V. However, the voltage is often 2 V due to anode and cathode overpotentials. Optimized cultures and cathodes produce about 190 nmol ml−1 day−1 methane which equals 0.15 J ml−1 day−1 using the energy of combustion of 0.8 MJ mol−1. The same electrolysis cell consumes 0.2 mW at a cell voltage of 2 V which equals 0.17 J ml−1 day−1 and the resulting energy efficiency is 91%. The anodes can be simple carbon brushes and the two chambers of the cell are separated by a Nafion™ membrane. The system can still be optimized by using Pt/C anodes and by avoiding membranes.
The overall electricity-methane-electricity efficiency also depends on the consumption side efficiency where methane is used in combustion engines and gas fired power plants. Such power plants frequently operate at efficiencies of 40- 60%. Assuming a reasonable power efficiency of 80% (see above), the overall electrical power recovery using gas fired power plants will be up to 50%. Besides the high efficiency of gas fired power plants, they are also easy to build and therefore contribute the a better power grid efficiency. Coal fired power plants can be upgraded to gas fired power plants.
Experimental approach
The conversion efficiencies of charge (Coulombs) transported across the circuit are usually between 70-100% in these systems depending on the electrode material. Another efficiency limitation could arise from mass transport inhibition. Mass transport can be improved by pumping electrolyte adding more costs for pumping which still have to be determined. However, since most oilfields undergo seawater injection for enhanced oil recovery the additional cost may be negligible. The total efficiency has yet to be determined in scale-up experiments and will depend on the factors mentioned above.
The reactor simulates oilfield conditions using sand as filling material under continuous flow of electrolyte.
Controlling the pH is crucial. Alkaline pHs significantly impede hydrogen production and therefore methanogenesis. This can be addressed by a software that monitors the pH and adjusts the potential accordingly. Addition of acids is not desired as this drives the costs. The software can also act as potentiostat that then fully controls the methane production process. To test the process under more realistic conditions, a drill core from an oilfield must be obtained.
Results show methane production in the simulation reactor. The appearance of methane in the anode compartment was a result of flow from the cathode to the anode, carrying produced methane with it.
Return of investment of the microbial power-to-gas process
The the microbial power-to-gas process in unproductive oilfields is economically superior to all other storage strategies because of the low start-up and operating costs. This is achieved because the major investments are the installation of oil- and gas production equipment and renewable power plants which are already in place as a precondition. These investments break even in a short amount of time.
But how can the microbial Power-to-Gas process accelerate the return of investment in renewable energy? Only 8 out of 28 active off-shore windfarms reported their investment costs. These 8 produce roughly half the overall power of 1,600 MW corresponding to $7 billion. While the maximum production of an oilfield with unlimited supply of electricity would yield hypothetical 3.6 million MBtu natural gas per year resulting a return of $13 million per year the real production is limited by off-peak power generated by renewable energy production. Assuming that the maximum annual methane production corresponds to 10% excess electrical power, $15 million per year can by generated by selling 4.3 million MBtu methane per year on the market. These are $15 million that are not lost during off-peak shutdowns. Clearly, this conservative estimate can help to compensate the investment in renewable energy earlier. It also decreases the investment risk because the investment calculations for new wind farms can be made on a more reliable basis.
In the example using all German windfarms (7,000 MW) this compensation roughly doubles. Using the $60 million generated by methane sales per year, the investment of $4 million for the cathodic catalyst and the $36 million for the Pt/C anodes are compensated for within less than a year. No other investments are required because the target oilfield already produced oil and gas and all necessary installation are in working condition. The target oilfield is swept using seawater as secondary extraction method. Electrical installations are in place for cathodic protection of production equipment in order to prevent microbial corrosion, which, however, may need to be upgraded to pass the now higher power densities. Moreover, CO2 is used from CO2 injection as tertiary enhanced oil recovery method. Only the pH may then need to be adjusted to sustain life by sweeping with seawater.
And this is not the end of oilfield storage capacity. In theory, an oilfield can store the entire amount of renewable energy produced in one year globally, allowing more than enough head room for future development and CO2 sequestration.
