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Sequestering Carbon Dioxide in Rock

  1. Apr 28, 2018 #1

    BillTre

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    About possibly using peridotite to capture large amounts of CO2 out of the air.
    Nice pictures, very little chemistry.
    NY Times story here.
     
  2. jcsd
  3. Apr 30, 2018 #2
    Artificial CO2 sequestering is all well and good but generally seems slow and difficult and usually requires energy to perform the processes. The article is not very detailed and is questionable e.g. it says the 40 GTonnes of CO2 are generated (supposedly;by by humans ?) each year while other sources put it at about 10 to 20 Gtonnes. From a physicist point of view the proposed method of pumping and circulating CO2 saturated water at high pressure into the substrate to interact with the peridotite does't seem very efficient. Even if the elevated temperature accelerate the reaction the surface area available to accesses the mineral is limited and once the surface of the bore hole is saturated then you have to depend on the diffusion of the CO2 to deeper levels which decreases the absorption rate.

    (disclaimer: The following calculation is subject to spurious arithmetic errors and I welcome others to verify it.)
    Moving the water through the bore is an issue to. CO2 is absorbed in water at a concentration of 2g/kg or 1mole/44L @ 15 deg C. to sequester 1 G tonne of CO2 one would need to 1012/ m3 of water. On a yearly basis that is moving 31,700 m3/sec. to handle that flow volume one would need about 1000 pipes 2m in diameter. Since this is only the first pass in the recirculation process it does not seem like a promising concept.

    IMH these techniques only sidetracks efforts to find the true solution which is the reduction of the combustion of fuels as a source of energy.
     
  4. May 12, 2018 #3
    For the last dozen years or so I have been designing, permitting, drilling, operating and monitoring acid gas injection wells for oil&gas facilities.

    These wells safely and efficiently sequester waste CO2 and H2S from natural gas processing plants. The wells inject from 1 to over 15 million cubic feet per day of these gasses, at depths from 5,000 to over 15,000 feet, in saline deep aquifers

    Although these projects are small relative to fossil-fuel power plans, they do make these sites much safer, cleaner and easier to operate.
     
  5. May 15, 2018 #4

    Tom.G

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  6. May 15, 2018 #5
    Tom.G:

    Thanks for the link.

    As production gas wells age, pressure declines over time as good old P=nRT/V. V is the gas reservoir (finite), R and T don't change, hence P~n where n is the available moles of gasses. One expects the inverse to occur in injection wells as a finite volume accepts more and more gas (n). In many carbonate (limestone, dolomite) reservoirs, the surface injection pressure of acid gas injection wells has been observed to decrease over time against reasonably stable injection rates.

    How can this happen? The V in any reservoir is limited by the porosity (% pore space) in the reservoir rock. Acid gases CO2 and H2S react with the existing reservoir fluids (saline waters) to form various C and S acids that then can attack the limestone and dolomites, increasing the reservoir porosity and hence the available reservoir volume. Acid treatment in wells (about 10,000 gallons of 5-15% HCL) is a very common completion/stimulation procedure for improving carbonate reservoir porosity.

    Not every well is lucky. Original formation fluid chemistry, especially the ionic concentrations of Ca and Mg as well as overall TDS, can decrease the formation of C/S acids in the fluids. Reservoir pressure and temperature,easily up to 6000 psi and 180-200 F in the Permian Basin where I work, also works in mysterious ways in affecting porosity. Did I mention that the injected acid gases are in the supercritical phase when they encounter the formation fluids?

    No more AGI wells for me this month. Off to a long week of rafting the Grand Canyon.
     
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