This page introduces main researches on CO₂ geological storage which we have conducted.

• Seismic Wave and Specific Resistance Tests Using Sandstone Samples

• Analysis of Two-phase Flow in Porous Rock by the Lattice Boltzmann Method

• Evaluation of Mineral Fixation by Geochemical Reaction

Seismic Wave and Specific Resistance Tests Using Sandstone Samples

Using samples of sandstone, which is the typical bedrock used as the reservoir in aquifer storage, the change of seismic waveforms (P-wave) before and after CO₂ injection was measured, and a decline of its velocity was confirmed. The obtained results were utilized in the analysis of seismic tomography.

Using the same sandstone samples, the specific resistance (electric resistance per unit volume) between electrodes was measured before and during CO₂ injection. Because CO₂ does not conduct electricity, the specific resistance will increase if CO₂ flows in. In this test, it was observed that the specific resistance gradually increased when CO₂ was injected from the lower side of the sandstone sample.

From these tests, it was confirmed that CO₂ injected into the deep saline aquifer can be monitored through the changes of seismic wave velocity and specific resistance.

Analysis of Two-phase Flow in Porous Rock by the Lattice Boltzmann Method

This study aims to understand microscopic flow phenomena of water and CO₂ in a porous rock by numerical analysis using the lattice Boltzmann method.

Basic data, such as irreducible water saturation and residual gas saturation is required to estimate long term CO₂ behavior and evaluate storage capacity and safety. To obtain these data, it is thought to be effective to carry out the laboratory experiment using field cores and microscopic numerical flow analysis to explain the result.

The lattice Boltzmann method has the following features; first, it is easy to set complex boundary conditions in the interior of porous solid because grid points are computed independently and secondly, it is suitable for the parallel computing. These features enable to understand microscopic flow.

In this study, the two-phase flow in the interior of porous solid is analyzed with different cavity structures and injection pressures under the temperature and pressure conditions of subsurface environment or the laboratory experiments.

Evaluation of Mineral Fixation by Geochemical Reaction

The calcium concentration in the solution was measured by conducting a reaction test using core samples and the formation water obtained from the Nagaoka test site. It was found that, with the lapse of time, calcium dissolved out from the core samples and the calcium concentration in the solution increased. With the decrease of hydrogen ions as calcium dissolves out, the formation water is neutralized and it tends to dissolve CO₂. If the pH of formation water returns to near neutral, calcium carbonate will be deposited (such as calcite). Hence, there is the possibility to eventually produce mineral fixation which is a stable storage form of CO₂.

At the Nagaoka site, formation water was sampled at a depth of 1,118m where CO₂ is stored and at a depth of 1,108.6m where no change was found via both specific resistance and sonic logging. From such analysis, it was found that bicarbonate ions (HCO₃) increased at 1,118m, indicating that CO₂ was dissolving at that depth. Calcium and iron were also found to increase, which means that a reaction between CO₂ and minerals was proceeding. Based on the aforementioned, it is considered that mineral fixation will advance more rapidly than predicted at the Nagaoka site.