Hydromechanical impact of carbon mineralisation in basalts

Permanent CO₂ storage in basalts by means of mineralisation is a promising cost-effective way to achieving reduction of carbon emissions in view of climate change mitigation. CO₂ is dissolved in water before injection in the subsurface, resulting in increased trapping safety, since solubility has already taken place. Storage of dissolved CO₂ in basalts at shallow depth has additional advantages such as rapid mineralisation (1-2 years), reduced drilling and monitoring cost and lower risk of leakage and induced seismicity events. However, large-scale application of this storage technology would require substantial amounts of water making it not ecologially viable. The use of seawater as a solute is an ideal alternative that is explored since recently in Iceland. Recent studies on basalt-seawater-CO₂ interaction showed that the efficiency of carbon mineralisation in seawater remains significant. Batch reactor testing revealed a total mineralisation of 20% of the initial injected CO₂ within five months, corresponding to carbonation rates similar to those observed in basalt-freshwater-CO₂ interaction experiments (lab and field).

Carbon mineralisation can substantially alter the pore space of the basaltic material, resulting in reduction of porosity, flow properties, and consequently overestimation of the injection and storage efficiency. While geophysical monitoring is not yet available, information on the reservoir properties of basalt remains limited. In this work, the impact of CO₂ mineralisation on the hydromechanical properties of a basaltic sample is studied. For the first time, injection of CO₂ dissolved in saline water is considered in view of a more ecological application of the technology at large scales. Fluid flow evolution before and after exposure to CO₂ dissolved in seawater is measured in terms of hydraulic conductivity and permeability under field-like conditions over a duration of 1 to 3.5 months. Permeability reduction of up to one order of magnitude suggests porosity decrease due to mineral precipitation after CO₂ exposure. X-ray tomographies of the tested cores reveal a maximum porosity decrease of 1.5% at the given resolution (50 μm/px). To better understand eventual modifications in the connected pore network after mineralisation, fluid flow simulations are performed on the 3D pore network of the material that is reconstructed from the acquired x-ray images. A double porosity is proposed: macro-porosity as visible from the tomographies (pores > 50 μm) and micro-porosity representing the solid matrix porosity (pores < 50 μm). To reproduce the post-CO₂ exposure flow, reduction of macro-porosity is not enough. Instead, a decrease of the solid matrix porosity is necessary by up to 30%. The experimental and numerical results suggest that mineralisation can substantially modify the pore space of the intact basaltic material and consequently impact storage efficiency if flow is not preserved.