• Energy Research

Abstract In order to evaluate the long-term sealing performance of caprocks on geological CO2 storage, this study analyzed hydrological changes of various sedimentary rocks during geochemical processes. Mineral reactions for one month under the supercritical CO2-water-rock system affected rock's permeability and threshold pressure strongly depending on rock types. Such a divergence seems to be caused not by mineral compositions but by the internal structure of each rock. In other words, even if a caprock contains a large amount of carbonate minerals, their dissolution does not necessarily proceed over a wide range because the limited pore space within a caprock saturates the pore water with carbonates immediately. This suggests that volumetric changes of reactive minerals would be negligible on the evaluation of caprock's sealing ability. Rather, the wettability effect on mineral surface was proposed as a controlling factor of long-term sealing performance.

AbstractFor the long-term assessment of CO2 geological sequestration, the dissolution rate of feldspar was measured under various degrees of saturation based on observations of the time-course change of dissolved crystal surfaces on a nanoscale. We identified that the dependence of solution saturation state on anorthite dissolution rate follows to a L-shaped function, rather than to a linear function. Compared with a linear assumption, such a nonlinear form provides a much lower estimate of dissolution rate. This implies that the difference of rate formula can cause meaningful errors in the timescale evaluation of various geochemical processes. In this study, therefore, the effect of the function form of dissolution rate was also checked using the geochemical simulation code to analyze CO2 geological sequestration.

AbstractRelating to the assessment of the cap rock integrity on CO2 geological sequestration, we measured threshold pressure for supercritical CO2 under pressure and temperature conditions of 1,000 m depth (i.e., 10 MPa and 40 °C). The present study aims to quantify various factors which affect the variability of rock’s threshold pressure and to model the threshold pressure range of a cap rock. We prepared sintered compacts composed of uniformly-sized silica glass beads (0.2–10 μm), and examined a correlation between threshold pressure and permeability for each diameter. The result revealed that threshold pressure of sintered compacts increased drastically with a decrease of component particle size. Moreover, there existed a good linearity between the logarithm of threshold pressure and the logarithm of permeability. The fitted line is an important basis specifically from a viewpoint of safety assessment because it corresponds to the lower limit of threshold pressure of homogeneous rocks.

AbstractIn this study, we investigated effective pressure of dependence on hydraulic properties (i.e. threshold pressure and permeability) of Kazusa group mudstones, which is taken from Boso peninsula in Japan, at 40°C and effective pressures up to 20MPa. Additionally, to assess a correlation between these properties, a porosity change depending on effective stress is also examined. Our results demonstrates that for mudstones tested, the relationship between threshold pressure and permeability depends strongly on changes in pore structures as a function of effective pressure.

AbstractThreshold pressure Pcth, a parameter controlling rock's sealing performance in cases of CO2 geological sequestration, was studied in a supercritical CO2–water system under conditions of 1000 m depth. To provide a constraint to the variation range of rocks’ Pcth, the correlation between Pcth and permeability, k, was examined using sintered compacts composed of mono- and bi-dispersed spherical silica particles. The Knudsen number estimated for supercritical CO2 at 1000 m depth indicated that CO2 transmuted into a non-continuum at k < 0.1μD in the closest-packing structure. Future studies should consider whether the concept of Pcth is applicable in this situation.

AbstractDue to the strong interest in geochemical CO2-fluid-rock interaction in the context of geological storage of CO2 a growing number of research groups have used a variety of different experimental ways to identify important geochemical dissolution or precipitation reactions and – if possible – quantify the rates and extent of mineral or rock alteration. In this inter-laboratory comparison the gas-fluid-mineral reactions of three samples of rock-forming minerals have been investigated by 11 experimental labs. The reported results point to robust identification of the major processes in the experiments by most groups. The dissolution rates derived from the changes in composition of the aqueous phase are consistent overall, but the variation could be reduced by using similar corrections for changing parameters in the reaction cells over time. The comparison of experimental setups and procedures as well as of data corrections identified potential improvements for future gas-fluid-rock studies.

AbstractWe have been constructing a database named “Formation-water database” by accumulating chemical compositions of groundwater in deep aquifers in Japan. The study aims to evaluate potential for solubility trapping of deep groundwater for geological CO2 storage in Japan. The formation water of reservoir depth is found to be generally dilute in composition; regional average values of TDS are at about 1/5 to 3/5 of the seawater. The dilute composition of deep groundwater indicates its high potential in CO2 solubility trapping (in average, more than 90% solubility as compared to pure water).

AbstractTo improve prediction accuracy of the long-term geochemical simulation, we measured carbonate growth rates at carbonated or bicarbonated springs as a natural analogue field of CO2 geological sequestration. Seeding experiments, specially focusing on the effect of the Mg-ion as an inhibitor of calcite growth, indicated that the formation phase changed from calcite to aragonite with an increase of the Mg/Ca ratio, whereas that neither dolomite nor magnesite grew in the present range of the Mg/Ca ratio corresponding to typical aquifer conditions. On the other hand, there was no correlation between the calcite growth rate and the Mg/Ca ratio: the impact of the Mg ion alone seems to be diminished in nature. Regarding this, we also found that the calcite growth rate at field conditions was much lower than the literature values mainly based on laboratory experiments. These results suggest that various other factors besides the Mg-ion affect the natural carbonate kinetics intricately; which in turn can cause a significant error on the predicted timescale of the mineral trapping.