Utilization of fly ash-based geopolymer for well cement during CO2 sequestration: A comprehensive review and a meta-analysis

Global warming is a pressing issue caused by the increase in greenhouse gas emissions, with CO2 contributing to 64% of total emissions. To reduce anthropogenic CO2 emissions, several options have been proposed, including CO2 sequestration. A key requirement for a successful and sustainable geo-sequestration process is the use of appropriate zonal isolation provided by the cementing material used between the annular surface and injection well. Although, Ordinary Portland Cement (OPC) is typically used as the well cement, it has shown failures during the process, including degradation issues, carbonation, shrinkage and microcracking, increased permeability in CO2-rich environments, and loss of sealing properties in a short period of time. To address these problems, fly ash (FA)-based geopolymers have been introduced as a better well cement replacement. This study provides a comparative review between OPC and FA-based geopolymers in the context of CO2 sequestration. The review comprehensively analyses the behaviour of FA-based geopolymer cement with its chemical composition, the impact of preparation conditions on the mechanical behaviour of geopolymers, and CO2 permeability through FA-based geopolymer. Furthermore, a meta-analysis was conducted to develop statistical models for predicting the pertinent hydro-mechanical properties of FA-based geopolymer, including dry density, compressive strength, autogenous shrinkage strain, and CO2 permeability during the geo-sequestration process. The outcomes of the meta-analysis can aid decision-making regarding the appropriateness of applying FA-based geopolymer as a replacement for OPC to conduct a sustainable and safe CO2 geo-sequestration process under proper isolation conditions.