1176 FUNDAMENTAL STUDY TO EVALUATE THE POTENTIAL FOR CO2 MINERALIZATION IN DEEP UNDERGROUND MINES *A. Sainoki1, X. Yang1, H. Ito1 1Faculty of Advanced Science and Technology, Kumamoto University, Japan, (*Presenting author: atsushi_sainoki@kumamoto-u.ac.jp) ABSTRACT It is not uncommon that the host rock of ultra deep underground mines is composed of ultramafic rocks, such as basalt and peridotite. Previous studies have shown the potential of such ultramafic rocks reacting with CO2, resulting in the precipitation of carbonate minerals. It is to be noted, however, that such experiments were performed under the range of temperatures exceeding 100℃, meaning that the results obtained cannot be simply applied to conditions of ordinary deep underground mines. In this regard, the present study aims to develop a novel technology to enhance the chemical reaction between ultramafic rocks and CO2 so that carbonate minerals are precipitated under the temperature and stress conditions of deep underground mines. To address this, Bacillus subtilis, which is a special type of bacteria with carbonic anhydrase (CA), is employed. CA is an enzyme that catalyzes the dissolution of CO2 into water and the generation of carbonate ions. A series of CO2 sealing test with ultramafic rocks was performed under a pressure of 9 MPa whilst varying parameters, such as the presence of CA and the timing of adding CA. The result indicated that the presence of CA can enhance the precipitation of carbonate minerals, but its efficiency is not as significant as expected due to the fact that the microbial solution works as a buffer, thereby hindering the dissolution of divalent ions from the rock when CO2 is injected into the solution. To solve this problem, a two-stage injection method was proposed, whereby the timing of adding CA, i.e., microbial solution, was intentionally delayed by one week so that sufficient divalent ions are released from the rock in the carbonated water. The experimental result demonstrated that the two-stage injection method clearly increase the efficiency of the carbonate mineral precipitation. KEYWORDS CO2 geological storage, carbonate precipitation, microbial enzyme, ultramafic rocks 1. INTRODUCTION Carbon capture and sequestration (CCS) has been attracting attention in the world as a countermeasure to reduce the amount of CO2 emission to the atmosphere and achieve The Paris Agreement. Currently, more than 70 ongoing projects are in operation with more than 40 projects under construction. It is generally recognized that one of the difficulties involved in the initiation of CCS projects is to find an appropriate geological layer with high permeability overlain by an impermeable geological layer that does not intersect with major faults and/or fractured shear zones, which could lead to the leakage of injected CO2. Hence, it is of paramount importance in ascertaining such geological layers and monitoring injected CO2, but this can lead to a delay and/or an increase in the cost. In this regard, abandoned deep underground mines have a great potential to store CO2, as an alternative to such ordinary geological layers used for CCS, when the host rock is composed of ultramafic rocks. Recent studies have revealed that carbonate minerals are precipitated when such mafic rocks have a contact with carbonic water, due to the chemical reaction between carbonate ions and those released from the rock (Cao et al., 2024). However, such experiments are predominantly performed under high temperature ranges more than 100℃. Under low temperature ranges, the chemical interaction proceeds at a slow pace.
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