Evaluation of potential fracture-sealing materials for remediating CO2 leakage pathways during CO2 sequestration

This work presents experimental studies of four candidate fracture-sealing materials with the potential for sealing CO2 leakage pathways during CO2 sequestration, including paraffin wax, polymer-based gel, silica-based gel, and micro-cement. We investigated their ability to effectively seal CO2 injection-induced or natural fractures with widths from 1/4 mm up to 1 mm. All four materials significantly reduced fracture permeabilities. However, the micro-cement was the most effective sealant agent and was the only sealant material that was able to withstand large differential pressures, typical of what might be caused by CO2 injection. Visual inspection of the fracture surfaces revealed that both the gel and the wax-filled fractures had worm-holes which made them less effective as sealant agents. Based on the mechanical experiments conducted, the gels cannot be expected to withstand large pressure differentials in a fracture whereas the micro-cement can. Thus micro-cement is recommended for sealing of fractures if fracture width is above half a millimeter. The long-term chemical stability of polymer-based gel was assessed by exposing it to CO2 for 7 months at 20 °C without any apparent sign of degradation. The CO2 exposure does not seem to hinder the use of gel as a sealant material. Given that cement exposed to CO2 has been shown to be susceptible to alteration via carbonatation reactions, a potential injection scenario might be to inject cement first to create a barrier to differential pressures, and then follow with gel as a second injection fluid to create a chemically stable sealing agent to CO2 exposure or the usage of CO2 resistant micro-cements not investigated here.