Testing the self-healing capacity of sealant materials for subsurface storage applications.

The secure subsurface storage of fluids, whether energy carriers such as hydrogen or wastes such as CO₂ or nuclear waste, requires sealants that can ensure wellbore seal integrity over timescales of decades to millennia. Currently used sealants are typically based on Ordinary Portland Cement (OPC) technology, which results in brittle seals that may have limited ability to withstand aggressive chemical environments. These properties make it difficult to ascertain that such sealants will maintain seal integrity over the long lifetime of a subsurface storage reservoir, as temperature and/or pressure variations during operations; chemical attack; or geomechanical effects may induce leakage pathways through the seal, as well as along the interfaces between seal and wallrock, or between seal and steel, resulting in a loss of wellbore sealing integrity.

Self-healing sealant materials can be a key technology for ensuring long-term seal integrity in underground storage applications. Such sealants should interact with leaking fluids so that when leakage pathways do form, these pathways are sealed rather than widened. We present experiments in which we aim to test the self-healing capacity of different sealants, particularly for CCS applications, by exposing a reproducible simulated leakage pathway to a flow of CO₂(-bearing fluid) under in-situ conditions. Our simulated leakage pathway consists of a sawcut through a hardened sealant sample, propped with crushed, hardened sealant (though other materials can also be used). Until now, we have focused on OPC-based sealants with various mineral additives (such as olivine and brucite) that result in an increase in solid during carbonation; but other sealants not based on OPC, such as geopolymers, may also be tested. During flow exposure, the pressure drop across the sample is monitored to assess permeability changes. After the experiments, SEM is used to study microstructures and identify reaction products. The results of this experimental work are then used as input for numerical modeling studies that seek to simulate the observed interactions and can extrapolate obtained results beyond laboratory time and length scales. In our model, chemical alteration of the cement is coupled to mechanical deformation and fluid flow to capture the cement system's volume changes that will help mitigate leakages.