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Geological Storage of CO2 with membrane-based Direct Air Capture to form Carbon Sinks

June 17, 2021. By Kolemann Lutz

Japanese researchers investigated the cost and feasibility of geological storage of m-DAC membranes to sequester significant amoutns of low-purity CO2 from the atmosphere to deremine thatgeological carbon storage is economically, low power, and environmentally method at greater depths.

Earth’s soil contains about 2,500 gigatons of carbon, which is more than three times the amount of carbon in the atmosphere comprising 0.04% CO2 and more than four times the carbon stored in all living plants and animals. Terrestrial Soil Organic Matter (SOM) is assumed to be between 50 to 58% carbon with the remaining soil composition of H, O, N, P and S.

Carbon capture and utilisation have potential to yield a 15% cumulative reduction in GHG emissions by 2060.Geological carbon sequestration involves pumping and storing CO2 into subsurface geological formations. Underground pore space can store, reduce, and manage vast quantities of CO2, especially in rocks where hydrocarbons were extracted. Since the 1980’s, CO2 has been injected into declining oil fields to increase petroleum recovery. Gaseous and liquid hydrocarbons have long been stored underground by many countries to flexibly respond to supply-demand imbalances and manage the gas supply chain. Potential storage sites suggested include declining oil fields, saline aquifers, and unmineable coal seams. Geological carbon storage brings many advantages including large volume, low storage cost, reductions in mass and infrastructure, and minimal transmission distance. Although, leakage back into the atmosphere and high injection pressures may become a concern in saline-aquifer storage, which have the largest identified carbon storage potential. Trapping mechanisms include hydrodynamic trapping, solubility trapping, and mineral trapping that can result when CO2 interacts with water, gases, and local minerals. As the CO2 dissolves into the local matter, the CO2 becomes immobilized underground, reducing the risk of leakage back into the atmosphere.

Direct air capture (DAC) of CO2 can be performed anywhere, including at the geological storage site. The geological sequestration approach does not require high energy and cost purification processes because the atmospheric impurities, O2 and N2, are not hazardous. Typical carbon sequestration projects inject supercritical CO2 into reservoirs deeper than 800m because sCO2 is in supercritical state and approximately 600X denser than the gaseous phase at pressures typical of these depths (>7.38MPa) on Earth. While low-purity CO2 can be captured and injected directly into geological formations, the cost of capturing high-purity (98%) CO2 typically used for geological storage is high. To determine the efficiency of geological storage, we must evaluate the density of low-purity CO2 at pressure and temperature conditions typical of storage reservoirs.

In a new study published in June 2021, researchers from Kyushu University and the National Institute of Advanced Industrial Science and Technology, Japan, investigated the cost and feasibility of geological storage of DAC membranes to sequester low-purity CO2 mixed with nitrogen (N2) and oxygen (O2), which were assumed to retain their atmospheric proportions (4 Nitrogen molecules:1 Oxygen molecule). CO2 separation by membranes requires less energy consumption, has smaller space requirements, and uses simpler setups.

Researchers conducted molecular dynamic simulations using GROMACS version 5.1.4. to model density diffusivity and adsorption behavior of the storage efficiency of CO2-N2-O2 mixtures at three different temperature and pressure conditions, corresponding to depths of 1,000 m, 1,500 m, and 2,500 m at the Tomakomai CO2 storage site in Japan. The corresponding three temperature and pressure conditions were 38deg C and 10MPa (1,000m), 52.5deg C and 15MPa, (1,500m), and 80deg C and 25MPa(2,500m).

Researchers determined the density of CO2-N2-O2 mixtures and evaluated the costs to determine that low-purity CO2 geological storage is economically and environmentally method. Low-purity CO2 into shallow formation (~1000m depth) is not efficient considering density of CO2 increases at deeper depths and deeper reservoirs. Although, the available emptied underground geological formations are usually smaller at greater depths due to compaction and diagenesis of reservoir rocks.

“Because of the ubiquity of ambient air, direct air capture has the potential to become a ubiquitous means of carbon capture and storage that can be implemented in many remote areas, such as deserts and offshore platforms.”, mentioned by the study’s lead author, Professor Takeshi Tsuji at the Department of Earth Resources Engineering, Kyushu University.

Low temperatures and high pressure in remote, offshore environments could improve efficiency of CO2 storage. 100 gigatons of CO2 could be stored in Japanese islands alone, which equates to 100 years of total CO2 emissions from Japan. 95% of injected CO2 into basaltic rock at CarbFix site in Iceland has transformed to carbonate minerals or stone within less than 2 years, suggesting a large permanent mineral sequestration storage method.

It is critical to monitor the status of low-purity CO2 in reservoirs to manage and prevent leakage. While mixtures can be monitored using time-lapse seismic surveys, new systems such as existing fiber optic cables and distributed acoustic sensor technology can create a virtual sensor array tens of km long at low cost for continuous CO2 monitoring.

On Mars, it may be possible to pump carbon into subsurface (eventually depleted) water reservoirs that are natively low in temperatures. If underground water reservoirs can store up to half the water volume of the Atlantic ocean from crustal hydration, that would imply up to 155.2 million cubic km of water or 4.1 x 10^19 gallons of H2O that presently exist in subsurface Martian liquid water reservoirs, which upon depletion would hold the potential to form large geological pore spaces for deep carbon dioxide sequestration similar to Earth.

In order to store the sCO2 underground, the pressure can be increased in the underground geological formations on Mars with high pressure air/gas source (HPGS) such as a Compressor or internal pressure generator to spawn a hyperbaric underground reservoir.

Unpressurized lava tubes are too shallow near the surface for carbon sequestration to form carbon-based soil ecosystems and organic matter. Although, pumping carbon dioxide into sealed, pressurized lava tubes may hold the potential to facilitate mineral sequestration, by trapping carbon in the form of solid carbonate salts, which can affect soil productivity by influencing soil pH, structure, WHC and water flow. Pressure transducer sensors can be utilised to measure and convert pressure into electrical signals , which could also be used for the pressurization of nearby habitats.

If depleted water reservoirs detected from orbital radar instruments are investigated for carbon storage, further research will be required to estimate the location, volume, of water and hydrocarbon reservoirs on Mars. The analysis of permeability of pore space and carbon in local regolith and reservoir minerals, as well as water drilling locations can be evaluated introduce subsurface habitable environments for chemolithoautotrophic microbes, which naturally sequester up to 22% of total carbon delivered to Earth’s mantle.

Geological storage of CO2 may hold the potential to become more long term projects to complement the terraforming movement by sequestering large amounts of carbon and supporting a nitrogen and oxygen-rich atmosphere on planet Mars.


“Geological storage of CO2–N2–O2 mixtures produced by membrane-based direct air capture (DAC)” by Takeshi Tsuji, Masao Sorai, Masashige Shiga, Shigenori Fujikawa, Toyoki Kunitake, 1 June 2021, Greenhouse Gases: Science and Technology.


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