Reactive transport modeling of mineral carbonation in unaltered and altered basalts during CO2 sequestration

The consequences of excessive carbon dioxide generation are beginning to manifest due to intense fossil fuel usage. Global warming and extreme weather among other negative effects are evident around the globe. To large extent, the scholarly community has taken this matter seriously and various counteractive measures have been proposed. For instance, development of green energy systems has garnered pace and electric vehicles have been developed and commercialized among other measures. In addition, carbon sequestration technologies have also been proposed. Of the various carbon sequestration approaches available, carbon capture and storage in deep geological formations has been found to be an effective means to combat excessive carbon emissions; the combination of physical and chemical trapping mechanisms has yielded positive results in terms of CO₂ sequestration.

Research has shown that the nature and chemical composition of the host rock dictate its long-term carbon capture capability. Basalt- a rock rich in iron and other metals that react to form stable carbonates via the reaction involving basalt mineralization has also been proposed for CO₂ sequestration. Unfortunately, commercial scale implementation is yet to be examined. Also, existing publications have not factored several transport processes in their models.

There is need for excellent predictive models that work over longer time frame so as to comprehend the serpentinization of unaltered basalt that is a common occurrence around the globe. To address this, Professor Ramesh Agarwal at the Washington University together with scientists at China University of Geosciences (Dr. Danqing Liu, Dr. Yilian Li and Dr. Sen Yang) have developed a predictive model in DOE code TOUGHREACT using the data from core-scale static batch experiments and the field-scale Carbfix project. Their goal was to develop a model that could evaluate the mineral carbonation efficiency of unaltered and altered basalt over a longer time frame. Their work has been currently published in International Journal of Greenhouse Gas Control.

The research team established models for the unaltered basalt (flood) and the altered basalt using the DOE reactive transport code TOUGHREACT, which has been widely used in the simulation of fluid flow through pores and fractures in the sedimentary host rocks and wellbore cements. The researchers employed available mineral carbonation volume fraction and distribution with micro-CT scans from the static batch experiment for their model validation. The impact of basalt conductivity and the CO₂ injection rate on the CO₂ carbonation, porosity/permeability variation and CO₂ plume evolution were analyzed.

The authors observed that unaltered basalt has higher carbonation efficiency than altered basalt both in the carbonation rate and in extent due to serpentine kinetic limitations for aqueous phase CO₂ injection. In addition, they noted that the carbonates mineralization extent and rate increased with the content of olivine minerals in basalt. Compared to the injection rate, the impact of reservoir conductivity was found to be significant on mineral carbonation in both the unaltered basalt and altered basalt; higher conductivity was more advantageous for CO₂ mineralization under certain circumstances.

In summary, a predictive model for evaluating the CO₂ carbonation efficiency and distribution in the pore and fractured structure both at core-scale and field-scale under conditions relevant to CO₂ injection in unaltered and altered basalt was developed and validated. In an interview with Advances in Engineering, Professor Ramesh Agarwal, the corresponding author, emphasized that the CO₂ injectivity rate in basalt should be carefully assessed at sites based on the reservoir conductivity and whether the basalt is altered or unaltered and the extent of alteration. Overall, in the study, a reliable reactive transport model for CO₂ sequestration in basalt was developed and validated.

There are many basalt sites around the world. It is very costly to conduct field scale experiments. The laboratory scale experiments provide limited information and are not reliable. The computational model provided by Professor Agarwal and his team can be employed to conduct field scale computations to determine the feasibility and economic viability of CO₂ sequestration in basalt at a given site. This work should also provide impetus for further development and refinement of the model.