The current focus of reducing CO₂ in the atmosphere to lessen its effects on global warming has led many researchers to explore the use of subsurface reservoirs for its retention either through adsorption, absorption, or mineralization. Organic shales used for recovering hydrocarbon production are currently considered a potential target for gas sequestration due to the kerogen comprising the matrix. The large surface area and the swelling properties of kerogen creates a significant gas storage capacity, and because it is an elastomer, it can significantly influence the mechanics of the rock. However, these properties can change as the kerogen is thermally transformed into hydrocarbons. As a result, measuring the gas retention properties of kerogen can be very difficult due to the complex, nanoscale mineral matrices of the rock. Also, chemically separating and recovering unaltered kerogen from the rock can be challenging as well. Therefore, uncertainty remains over the chemical and physical properties that influence gas storage capacity in kerogen, which is important to understand for predicting CO₂ storage potential in organic shales. To address this challenge, a series of hierarchical carbons have been created from waste tea as a proxy for kerogen. To prepare the samples, bulk green tea leaves were first boiled in deionized water, and subsequently dried, crushed, and sieved to 150 µm. The ground waste tea was then chemically treated in 1.0 M HCl and then dried again in an oven at 100°C in preparation for roasting. Equal aliquots of the treated waste tea were then roasted in an inert atmosphere under argon for two hours at temperatures between 225°C to 1000°C. Results demonstrate that hierarchical carbon created using this method exhibits similar chemistry, molecular properties, and surface area as that of kerogen as it is thermally transformed as measured by infrared spectroscopy. Evidence of this similarity is also shown by CO₂ gas adsorption isotherms which also exhibit an increase in gas storage capacity due to increasing porosity as verified by scanning electron microscope images. These trends and properties suggest waste tea prepared as hierarchical carbon could be used as a synthetic substrate to study and understand the mechanisms of gas retention in organic shales. It also may serve as testing material for developing chemical oxidizers of kerogen for increasing the gas storage capacity of future organic shale repositories of anthropogenic CO₂.

Poorna Srinivasan

Geologist, Aramco Americas - Houston Research Center

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