Measurements of CO2 trapping in carbonate and sandstone rocks

CO2 storage in saline aquifers (sandstone/carbonate types) has been proposed as a promising solution to help reduce CO2 emissions to the atmosphere. CO2 will likely be stored as a dense, supercritical (sc.) phase. There are different mechanisms by which CO2 could be stored safely underground; structural and stratigraphic trapping, dissolution trapping, capillary trapping, and mineral trapping. I study capillary trapping. We assume that in the middle of a CO2 plume, many kilometres in extent, the CO2, brine and rock have been in mutual contact for several years. In these circumstances, the degree of capillary trapping is determined by a displacement of CO2 by brine under these equilibrated conditions. Reproducing such conditions in the laboratory poses a challenge. I have measured the first trapping curve, the relation between initial and residual CO2 saturation, for carbonates in the literature, as well as contributing to the first data on sandstones. For capillary trapping experiment, the porous plate method was used during primary drainage. Two sandstones (Berea and Doddington) and two types of carbonates (Ketton and Indiana) were studied. These experiments were conducted at temperatures of 33, 50, and 70 ˚C and 9 MPa pressure, which matches the conditions observed for several current and planned storage sites. Two displacement steps, primary drainage and water flooding were followed to reach residually trapped CO2 saturation. The isothermal de-pressurization method was used to measure the amount of scCO2 residually trapped. The drainage capillary pressure curve, the Leverett J-function and the trapping curve were measured. During capillary trapping experiments, the brine was equilibrated with CO2 to achieve immiscible displacement. We used a stirred reactor, to equilibrate CO2 with brine. The solubility of CO2 in brine was also measured using the isothermal depressurization method and compared with data in the literature.In Berea sandstone the trapping curves at 33, 50 and 70˚C were compared. We showed that temperature (density) variation has no effect on the saturation of scCO2 that is residually trapped. In Doddington sandstone our result was consistent with that from a micro-flow cell in which the trapped scCO2 was imaged using an X-ray source at the pore scale. We find that significant quantities of the CO2 can be trapped, with residual saturations up to 35%, but less than in analogue experiments where oil is displaced by brine. Hence, it is hypothesized that scCO2-brine systems in sandstones are weakly water-wet with less trapping than the more strongly wetting analogues. Capillary trapping in carbonates is very challenging. In carbonates, another step was required, where brine/CO2/carbonate will be equilibrated together before running the capillary trapping experiment. The apparatus used for sandstone rocks was modified so that the geochemical reaction between CO2/rock was accounted for. Samples are taken and analysed to ensure that the brine/CO2 mixture is saturated with carbonate minerals. In Indiana, the CO2 trapping curve for scCO2 at 50 ˚C and 9 MPa was compared with that of gaseous CO2 at 50 ˚C and 4.2 MPa. A scCO2 residual trapping endpoint of 23.7% was observed in Indiana for scCO2, with a smaller trapping end point in Ketton limestone. This indicates a slightly less trapping of scCO2 in carbonates than in sandstone. There is also less trapping for gaseous CO2 (endpoint of 18.8%). The system appears to be more water-wet under scCO2 conditions, which is different from the trend observed in Berea; the greater concentration of Ca2+ in brine at higher pressure was hypothesised to lead to more water-wet conditions. Our work indicates that capillary trapping could effectively store CO2 in carbonate aquifers

Modeling and numerical investigation of the performance of gas diffusion electrodes for the electrochemical reduction of carbon dioxide to methanol

In this study, a model was built to investigate the role of Cu2O-/ZnO-based gas diffusion electrodes in enhancing the reduction of carbon dioxide into methanol inside an electrochemical cell. The model was simulated using COMSOL Multiphysics software and validated using experimental results. It showed reasonable agreement with an average error of 6%. The model demonstrated the dependence of the methanol production rate and faradic efficiency on process key variables: current density (j = 5-10 mA cm-2), gas flow rate (Qg/A = 10-20 mL min-1 cm-2), electrolyte flow rate, and CO2 gas feed concentration. The results showed a maximum methanol production rate of 50 -mol m-2 s-1 and faradic efficiency of 56% at -1.38 V vs Ag/AgCl. From the economic point of view, it is recommended to use a gas stream of 90% or slightly lower CO2 concentration and an electrolyte flow rate as low as 2 mL min-1 cm-2.The authors would like to convey special thanks to Prof. Mai Kamal El-Din for her willingness to share her knowledge and expertise that are of significant relevance to this work. J.A. gratefully acknowledges the financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) under Ramon y Cajal program (RYC-2015-17080). The authors from ́ the Chemical Engineering Department, Cairo University, gratefully acknowledge the financial support provided by the Science and Technology Development Fund (STDF) of Egypt under project ID #11872. R.M.E.-M. acknowledges the support from the Oil and Green Chemistry research center and the Enhanced Oil Recovery Lab, Suez University, Egypt, and STDF (Science and Technology Development Fund) [Project ID 12395]