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Studying the quantity and origin of CO 2 emitted by back‐arc mud volcanoes is critical to correctly model fluid‐dynamical, thermodynamical, and geochemical processes that drive their activity and to constrain their role in the global geochemical carbon cycle. We measured CO 2 fluxes of the Bledug Kuwu mud volcano on the Kendeng Fold and thrust belt in the back arc of Central Java, Indonesia, using scanning remote sensing absorption spectroscopy. The data show that the expelled gas is rich in CO 2 with a volume fraction of at least 16 vol %. A lower limit CO 2 flux of 1.4 kg s −1 (117 t d −1 ) was determined, in line with the CO 2 flux from the Javanese mud volcano LUSI. Extrapolating these results to mud volcanism from the whole of Java suggests an order of magnitude total CO 2 flux of 3 kt d −1 , comparable with the expected back‐arc efflux of magmatic CO 2 . After discussing geochemical, geological, and geophysical evidence we conclude that the source of CO 2 observed at Bledug Kuwu is likely a mixture of thermogenic, biogenic, and magmatic CO 2 , with faulting controlling potential pathways for magmatic fluids. This study further demonstrates the merit of man‐portable active remote sensing instruments for probing natural gas releases, enabling bottom‐up quantification of CO 2 fluxes.

The CO 2 is regarded to be an excellent solvent for miscible flooding. However, it is still facing a main problem which is the high mobility. Microbubbles with their unique characters offer some advantages for CO 2 EOR application. Different pore throat size filters were used to generate different dominant sizes of microbubbles that were injected into sandpacks under tertiary condition. Microscopic analysis was carried out to visualize the presence, stability and behavior of microbubbles inside the solution and porous media. The microbubbles with a dominant size of 10-50 µm showed additional 26.38% of oil recovery, showing their advantages over a larger dominant size of microbubbles up to 5.28% of oil recovery. The injection with larger microbubbles with a dominant size of 70-150 µm showed 27.5% of higher injection pressure than with a smaller dominant size of microbubbles, showing their advantage in gas blocking ability. In the heterogeneous porous media experiment, the recovery volume ratio between low-and high-permeability sandpacks was increased from 1:57 during water flooding to 1:4 during the CO 2 microbubble injection with 74.65% of additional recovery from a low-permeability zone, showing the microbubble gas blocking capability to change the flow pattern inside heterogeneous porous media.

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The production of fly ash as a solid waste of coal combustion increases with electricity demand growth in Indonesia. Fly ash is usually discarded in landfills due to its lack of utilization. Poor handling of the material can cause pollution and harm to human health. One potential of fly ash that can be further explored is as an alternative source of rare earth elements. The use of citric acid in the recovery process will be more environmentally friendly. The magnetic phase of fly ash is used as it is more favorable for the leaching process due to the smaller amount of acid-resistant components. This research aims to study the leaching mechanisms, evaluate the effect of temperature and acid concentration, and determine the appropriate kinetics model. Magnetic fly ash of less than 38 µm was leached using 300 mL of citric acid with an S/L ratio of 1:10 at a 400 rpm stirring rate. The leaching experiments were carried out for 4 h and samples were taken at the designated time. Acid concentration of 0.5 M, 1 M, 1.5 M, and 2 M was used, while the temperature was varied from 25 ºC, 45 ºC, 65 ºC, and 75 ºC to 90 ºC. The results show that acid concentration does not affect La, Ce, and Y recovery. Meanwhile, the temperature has a significant impact where the recovery increases as temperature elevates. Leaching at lower temperatures (25 ºC and 45 ºC) fits the Z-L-T kinetics model, while at higher temperatures (65 ºC, 75 ºC, and 90 ºC) it follows the Kröger-Ziegler model.