Wellbores used in underground production and storage activities, including carbon capture and storage (CCS), are typically sealed using sealants based on Ordinary Portland Cement (OPC). However, leakage along these seals or through them during CCS operations can pose a significant threat to long-term storage integrity. In this review paper, we explore the potential of geopolymer (GP) systems as alternative sealants in wells exposed to CO₂ during CCS. First, we discuss how key parameters control the mechanical properties, permeability, and chemical durability of GPs based on different starting materials as well as their optimum values. These parameters include the chemical and mineralogical composition, particle size, and particle shape of the precursor materials; the composition of the hardener; the chemistry of the full system (particularly the Si/Al, Si/(Na+K), Si/Ca, Si/Mg, and Si/Fe ratios); the water content of the mix; and the conditions under which curing occurs. Next, we review existing knowledge on the use of GPs as wellbore sealants to identify key knowledge gaps and challenges and the research needed to address these challenges. Our review shows the great potential of GPs as alternative wellbore sealant materials in CCS (as well as other applications) due to their high corrosion durability, low matrix permeability, and good mechanical properties. However, important challenges are identified that require further research, such as mix optimization, taking into account curing and exposure conditions and available starting materials; the development of optimalization workflows, along with building larger data sets on how the identified parameters affect GP properties, can streamline this optimization for future applications.

Subsurface mineralization of CO₂ by injection into (hydro-)fractured peridotites has been proposed as a carbon sequestration method. It is envisaged that the expansion in solid volume associated with the mineralization reaction leads to a build-up of stress, resulting in the opening of further fractures. We performed CO₂-mineralization experiments on simulated fractures in peridotite materials under confined, hydrothermal conditions, to directly measure the induced stresses. Only one of these experiments resulted in the development of a stress, which was less than 5% of the theoretical maximum. We also performed one method control test in which we measured stress development during the hydration of MgO. Based on microstructural observations, as well as XRD and TGA measurements, we infer that, due to pore clogging and grain boundary healing at growing mineral interfaces, the transport of CO₂, water and solutes into these sites inhibited reaction-related stress development. When grain boundary healing was impeded by the precipitation of silica, a small stress did develop. This implies that when applied to in-situ CO₂-storage, the mineralization reaction will be limited by transport through clogged fractures, and proceed at a rate that is likely too slow for the process to accommodate the volumes of CO₂ expected for sequestration.