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 CO2during 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.