by Millan Arcia Einstein (Copyright in process)

Disclaimer: This paper is a summary of a full project aimed at monetizing carbon emissions. The full project was submitted to the Elon Musk Carbon removal competition under the XPRIZE organization in 2021.

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A novel, efficient, and cost-effective process for carbon sequestration-mineralization, and; to a less extent methanation, has been conceptually developed. Our process promises to sizably reduce final costs, as both, energy and key components needed to trigger key reactions will be provided at literally no cost. Although our main objective is to mineralize and sequestrate CO2, in situ methanation is believed to be partially achievable because of our technique.

During 2007 a novel methodology for improved condensate recovery (ICR) was proposed by Millán (1) et al., using cyclic supercritical CO2 injection. To our knowledge, it was the first time this kind of technique based on supercritical CO2 interactions with hydrocarbon (HC) systems, was proposed for ICR purposes. The novel method allowed for formation damage removal, high molecular weight liquid property enhancement, and reservoir re-pressurization, taking advantage of a cyclic and positive pressure and temperature gradient between foreign and in situ species, while creating a safe and reliable highway for downhole CO2 sequestration.

For carbon transformation, sequestration; and in a less extent, methanation, our attention is centered on injecting continuous volumes of supercritical CO2 into depleted gas (HCG) formations (P=>0), with high illite-clay content. We take advantage of the reservoir itself to efficiently promote needed reactions at low cost, and high efficiency, at subsurface conditions, using the same reservoir and its components as a reactor.

Mineral carbonation or mineralization is the process of absorption of CO2 via multiple contacts with certain alkaline oxides. In our case, we focus our attention on dolomitic-limestone-derived calcium and magnesium oxides reactions. Normally at surface conditions mineral carbonation may occur within an extended period because of a low CO2 concentration in the atmosphere, at the expense of high energy consumption. However, the carbonation reaction process can be accelerated in an environment with high CO2 concentration, and abundantly available surface to volume ratio of alkaline calcium and magnesium oxides, to accelerate carbonation at low energy costs.

In our specific case, we will be dealing with gas-to-solid mineral carbonation. For the case of calcium oxide (CaO), there is a significant reduction in the CO2 carrying capacity as temperature decreases below certain range, therefore, the amount of heat supplied to the system is fundamental in the reactivity, economics, and final success of the CO2 transformation potential. Until now, CaO sorbent absorption although effective, has shown inefficient, costly, and slow; with a high level of infrastructure, demanding temperature in the range of 390°C and 510°C, translating into large energy related CAPEX exposure.

Methanation is an exothermic reaction by which carbon oxides can be converted into methane through hydrogenation. Methanation reaction tends to be optimum at high temperature and low pressure. At low temperature, it often requires the presence of a metal acting like catalyst. Like mineral carbonation, it’s an energy intensive process, where typical operating temperatures vary from more than 150°C to ~550°C (302°F and 1,022°F), with optimum pressure around 400 psi to 500 psi.

During our process, temperature and pressure requirements for mineral carbonation, and methanation can be satisfied via downhole combustion, sizably reducing costs associated with energy and needed mineral components. Temperature gradient can be achieved by promoting downhole ignition via the concurrent injection of air in the presence of low pressure, remaining natgas fractions, residual water, and key lithologies.

Our method benefits from the significant subsurface potential to not only speed up and accelerate needed interactions, but also to reduce capital exposure. The proposed technology is a safe carbon negative solution, as only a small portion of the CO2 species will be released back into the environment. To the best of our knowledge, no work or process of this nature has been previously done or proposed. We believe this is a promising technique to achieve in situ mineral carbonation, CO2 sequestration, and to a less extent; methanation.

Detailed Process Description

The process targets depleted HCG reservoirs, normally containing remaining water saturation, and fractions of different other gases such as C1-C2+. This combination, together with the chosen formation lithology focused on abundantly occurring dolomitic-limestone, naturally fracture formations rich in calcium and magnesium, emphasizing on those with high nickel/aluminum-rich-illite content, constitutes the backbone for the success of our technique. Illite can contain nickel–aluminum oxide (NiO-Al2O3), which is a natural catalyst needed for methanation.

Addressing the mineralization-sequestration process, our method takes advantage of the same reservoir and its components as a reactor to reproduce at subsurface conditions, a similar chain of process of that of a surface DAC sorbent-based system, but in a more robust, energy efficient, and less costly approach, allowing for a reliable, safe, and cost competitive alternative for subsurface CO2 transformation, and sequestration. 

Together with prevailing subsurface pressure (P) and temperature (T) conditions, our process will ultimately reduce the time, costs, and effort necessary, to generate the amount of heat, overall energy, and substrate requirements to precipitate, maintain, and regenerate the life extent for the sorbent, and to escalate the CO2 sequestration, and final transformation process with minimum CAPEX exposure, and maximum efficiency.

We expect to target dolomitic-limestone fractured formations, and depleted deep hydrocarbon gas (HCG) reservoirs, with predictable temperatures beyond 104°C (>220°F). The sub-hydrostatic nature of the depleted gas reservoirs allows for the accommodation of substantial and continuous CO2 volumes, whose upper limit will be calibrated against hydrostatic pressure conditions to ensure final integrity.

Initial reservoir pressure and temperature conditions could be about ~400 psi and over ~104°C (>~220°F). At the end of the process, upper reservoir pressure level will be dictated by the specific hydrostatic conditions. Away from the wellbore, CO2 rock system dissolution is anticipated to be accelerated by smaller velocity gradients.

A continuous volume of supercritical CO2 will be injected into the formation at constant volume. Injection pressure (P) will be optimized below the formation parting pressure (FPP), eliminating the likelihood for inconvenient near wellbore artificial formation fracturing, and/or upward migration along the wellbore. Once the pressure gradient reaches a pre-established critical level close to the FPP, injection will be discontinued.

During the initial stage or soaking period, the presence of increasing CO2 fractions in the presence of an interstitial or reduced water saturation environment, will result primarily into a strong carbonic acid solution, which will dissolve mainly the calcite components in the formation. The dissolution of calcite components will progressively result in the precipitation of calcium hydrogen carbonate, or calcium bicarbonate.

During the first-soaking period we expect that the combination of interstitial water salinity (>3,000 ppm NaCl), and increasing pressure conditions, will facilitate the incoming fractions of supercritical CO2 to be more easily absorbed12 onto the rock surface. Potentially this dissolution may cause formation damage, dislodging, or even irreversible flow-blocking, mainly in the mixing and combustion regions. For this reason, our process focusses on naturally fractured formations, where multi-directional flow normally occurs. Additionally, while ongoing dissolution it could also promote the presence of new flow paths. At a later stage, derived reactions leading to in-situ mineral calcination will be undertaken to maximize CO2 absorption and mineralization. For this purpose, well-to-well downhole combustion via a controlled air injection will be triggered, aided by the presence of remaining in-situ C1-C2+ fractions.

CO2 absorption via multiple contacts with specific alkaline oxides will be enabled, as calcium oxide (CaO) can be extracted from calcined dolomite. Dolomite has been proven2 to be a potentially advantageous alternative for CO2 capture because of its fast calcination kinetics, and high carbonation reactivity of the dolomitic CaO. Around one gram of CaO can absorb ~0.8 grams of CO2. So far, this has been achieved at surface conditions at the expense of a significant amount of energy consumption, as it requires temperatures varying between 390°C and 510°C to react at a reasonable rate. This temperature gradient can easily be achieved by controlled downhole combustion at literally no cost. Downhole combustion is believed to be the key to overcome the slow kinetics involved into the mineral-to-CO2 reactions, while favoring the precipitation and regeneration cycle for the sorbent, by providing the energy and the mineral substrate needed, to accelerate the carbon dioxide mineralization process, at relatively low cost, and low effort. The gas-solid carbonation potential of both calcium hydroxide and calcium oxide has been previously3 studied for both; isothermal and non-isothermal reactions. Findings confirm that under non-isothermal conditions; such as in our case, carbonation potential substantially increases.

Calcium oxide hydration and dehydration is a reversible reaction essential for CO2 storage/mineralization purposes, where energy is normally released (exothermic) during hydration and stored (endothermic) during dehydration; it’s an energy intensive process. The idea behind downhole combustion is to trigger in situ reactions, favoring the precipitation of key minerals, such as Ca and Mg, taking advantage of reservoir mineral components. Calcination temperatures can be developed by a controlled injection of air, to promote together with the in situ (fuel) gas species, the needed ignition and combustion process, capable of developing instantaneous temperatures in the range of ~350°C to ~1,200°C. The amount of heat, and temperature requirement during each cycle will be controlled by adjusting the ratio of injected air to in-situ available fuel species.

The potential for additional CO2 fractions to be generated during the calcination period is minimized by the relatively small calcination time, compared against the soaking and mineralization periods. At full scale, calcination to soaking/mineralization ratio could be about 1:30, or one calcination period per every 30 days (1 day per month). The final extent will be firmed up during a laboratory pilot test already defined for this purpose.

After each combustion cycle naturally occurring or externally injected water re-saturation, will stimulate the precipitation of abundant calcium and magnesium hydroxide, which in turn will maximize the CO2 consumption rate aided by the right P, T, and sorbent (CaO) precipitation conditions in the previous cycle. This energy intensive process can be satisfied within the reservoir by downhole combustion. The reaction between calcium oxide (CaO) and carbon dioxide (CO2) occurs in two stages. The specific reaction between calcium hydroxide and CO2 shows the following balanced equation;

Ca (OH)2 + CO2→ CaCO3 + H2O

By managing air injection, we intend to provide downhole steady temperatures about ~510°C as required by the equation preceding previous reaction, where calcium hydroxide decomposes into calcium oxide (CaO+H2O) plus water, to form calcium carbonate in the presence of carbon dioxide. For the reaction between magnesium hydroxide and CO2, the balanced equation is as follows;

  Mg (OH)2 + CO2MgCO3 + H2O

The reaction above will thermally decompose to form magnesium carbonate (MgCO3) in the presence of carbon dioxide (CO2) at temperatures ~390°C. Like the previous case, by managing air injection we aim to provide the required downhole steady temperatures, by the equation preceding previous reaction, where magnesium hydroxide decomposes into magnesium oxide (MgO+H2O) plus water. While CaO is an active sorbent, MgO remains normally inert, although favoring the carbonation potential2 of dolomitic CaO, turning into a synergistic approach. This whole process constitutes the basis for not only sequestering CO2 but also for facilitating its dissolution, mineralization, and permanent sequestration. Pressure and temperature conditions of about ~400 psi in sub hydrostatic reservoirs is initially expected, while final pressure conditions will be dictated by the near hydrostatic gradient. Under normal pre-downhole-combustion operational conditions, temperatures are anticipated to vary from 100°C to up to 150°C. This excludes the calcination/regeneration period.

Apart from the soaking and normal operation period, because of the combination of initially low reservoir pressure (~400 psi), and high temperature (700°C >T>350°C) developed during in situ combustion, in situ water is expected to be in the near-vapor phase or in the near-supercritical region (depending on salinity). The presence of water vapor has been found4 to benefit the speed of sorbent conversion, hence the mineral absorption rate of CO2.

The likelihood for decarbonation to occur after in-situ mineralization will be minimized, by keeping the maximum temperature under certain threshold, experimentally previously established around ~700°C, in a reduced CO2 partial pressure environment. If needed, decarbonation can be triggered by raising the reservoir temperature beyond this limit.

As for methanation, although it is not our core interest it is possible that the Sabatier reaction naturally triggers during the extent of our process, as all key mineral components are available, while pressure, and temperature conditions can be met. Through this reaction, carbon oxides can be converted into methane through hydrogenation in the presence of certain catalysts. During our process, all these elements will be abundantly available within the reservoir; heat, hydrogen, nickel or aluminum oxide components acting as catalysts, and CO2. Methanation is an energy intensive process where typical operating temperature requirements vary between more than 150°C and ~550°C (>302°F and 1,022°F), with optimum pressure between 400 psi to 500 psi, according to the following exothermic reaction:

CO2 + 4H2 → CH4 + 2H2O

As the net amount of heat increases, available fractions of naturally occurring NiO/Al2O3 contained in certain clay minerals such as Illite, in the presence of hydrogen (C1, C2+), carbon dioxide (CO2), and/or carbon monoxide (CO), reduces to metallic nickel/Aluminum in a highly exothermic reaction, to produce methane plus water. Methane is expected to migrate by gravity segregation towards the topmost portion of the formation. This reaction is believed to take place particularly downstream in the combustion region, between the injection and the monitoring wells, where higher hydrogen concentrations are expected.

Operational Feasibility & Scalability

According to the Worldbank5 around 150 billion cubic meters (BCM) of toxic gases were flared during 2019. This volume decreased during 2020 towards 142 BCM due to the pandemic. By considering only the CO2 fractions on existing oil & gas industry flared gases, without accounting for the massive emissions of other industrial (steel, thermoelectric, construction) emissions, it is possible to exceed several megatons of CO2 removed from the atmosphere every year. For the megaton volume scenario, accounting for unit volume conversion from standard conditions of 25°C, and 14.7 psi (1 atm), to expected injection conditions at the sand-face of ≥100°C (≥212°F) and >400 psi, four (4) wells will be required, each injecting some 14 million cubic feet of CO2 per day (MMCfD).

Injection volume is considered a conservative figure, as depending on the specific well architecture, intrinsic rock properties, and reservoir thermodynamic conditions, injection volumes can be further optimized towards 20 to 25 MMCfD per well, proportionally reducing the total number of wells required, hence final costs. Wells can be distributed randomly worldwide or focused on any specific location to further enhance economics. To maximize injection rates, downhole CO2 volumetric efficiency, and minimize the number of wells, highly inclined and/or horizontal multi-stage wells are considered.

A localized loop of CO2 compression, pipelines, and air injection facilities serving different wells is considered. Pipelines with specific non-reactive material can be used to ensure corrosion and operational risk-failure is minimized. Our project has several components of CAPEX reduction as facilities are planned on site serving multiple wells, or clusters of wells, minimizing transport and processing costs. Preliminary cost estimates suggest that unit cost per MT of CO2 sequestered and mineralized runs around $20.


  • A new carbon negative solution including CO2 mineralization-sequestration, and in a less extent carbon methanation, is proposed. Although carbon methanation is not our core interest, we expect this process to potentially develop ahead of the combustion zone.
  • Previous methods based on rock/sorbent absorption although effective, has shown inefficient, low volume, slow, with high level of energy requirements, and high sorbent replacement rate, translating into large infrastructure needs, time consumption and high capital costs, to escalate the process to the mega and gigaton level.
  • Our method takes advantage of the significant subsurface potential to not only speed up and accelerate needed reactions and optimize energy requirements, but also to reduce capital exposure and time, by using the same reservoir and its components as a reactor.
  • During the first stage, favorable P & T gradients are expected to accelerate CO2-in-water dissolution rate and minimize rock dissolution rate towards the wellbore, while maximizing the CO2-in-rock dissolution away from the wellbore.
  • After the first stage is completed, the method calls for downhole combustion🡺calcination🡺regeneration, to trigger and enhance in situ reactions favoring CO2 mineralization, and potentially; methanation.
  • Calcination temperatures can be reached by a controlled injection of air, to promote downhole combustion. After downhole ignition, naturally occurring (or externally supplied) water re-saturation will stimulate, and accelerate the precipitation of calcium and magnesium hydroxide, which in turn will favor the final CO2 consumption-mineralization rate.
  • Several lab tests are required to fine tune applicable dynamic variables cycle-to-cycle, during full field implementation. A pilot test is proposed based on a lab reactor designed for this purpose.
  • Our approach is technically doable, timely achievable, as well as financially robust, while ensuring compliance with safety standards and long-term sustainability for the project.
  • At field conditions, facilities and operations are envisaged as eco-friendly, cero discharge, close loop, and energy efficient, with minimum impact on the ecosystem.


  1.; A novel improved condensate recovery method by cyclic supercritical CO2 injection, by Millan Arcia Einstein, et al.; Latin American & Caribbean Petroleum Engineering Conference, Buenos Aires, Argentina, April 2007.
  2.!divAbstract; Thermal decomposition of dolomite under CO2: insights from TGA and in situ XRD analysis; PCCP journal, 2015.
  3.; Gas-solid carbonation of Ca(OH)2 and CaO particles under non-isothermal and isothermal conditions by using a thermogravimetric analyzer: Implications for CO2 capture; Montes-Hernandeza, R. Chiriacb , F. Tocheb , F. Renarda; University of Grenoble.
  4.; Study of Calcination-Carbonation of Calcium Carbonate in Different Fluidizing Mediums for Chemical Looping Gasification in Circulating Fluidized Beds; ECI Digital Archives, Bau et al.; 2011.
  5.; Global Gas Flaring Tracker Report; The WorldBank, 2020.


M: thousands

MM: millions

CfD: cubic feet per day

HC: total hydrocarbon (oil + gas)

HCG: gas

Conversions & Base Conditions:

Standard Conditions: 14.69 psi and 25°C

Injection Conditions: 4,000 psi and ~100°C

One cubic meter: 1,000 liters

One mole CO2: 44 grams 

1 tonne CO2: 19,634 Cubic feet (Cf) @ 25°C and 14.69 psi

1 m³: 35.3 Cubic feet (Cf)

About the author;
Millan Arcia Einstein: Is a Senior Upstream Oil & Gas Global Adviser, and Reservoir Engineering Subject Matter Expert with over 40 years of successful exposure in the oil industry. Currently he is the Managing Director for Energy and Sequestration at Fractal-Software [ Fractal Software- Multiple Industries. Flexible Solutions ( ]; a Big Data, Artificial Intelligence start up, also specialized in energy consulting. Mr. Millan has been C-level manager with PDVSA and Advisor/Consultant to YBPF-Bolivia, KOC-Kuwait, PEME-Mexico, Schlumberger, and Core laboratories among others. He holds a Petroleum engineering degree from the University of Oriente-Venezuela, Master of Science degree in Petroleum Engineering, and PhD level specialization courses in Fluid Flow through Porous Media, both from the University of Oklahoma. He has been exposed to several Oil & Gas assets worldwide; including Middle East, México, EEUU, Bolivia, Colombia, Ecuador, and Venezuela. He has published over 11 highly specialized technical papers internationally, and more than 300 energy related articles in a number of journals, blogs and newspapers. He has also been quoted in SPglobal,, Plattsblog, Platts, The Slush Pit (Oklahoma Oil & Gas News), Energy Economist UK, La Vanguardia Spain, Stabroeknews & Kaieteurnews in Guyana, Sputnik in Russia, and Los Angeles Times in the US; among others.
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