Improving Saline Aquifer Caprock Integrity in CO2 Geosequestration Using Waste Plastics
Authors: Chen Hsieh , Zhu S. Sherry 
Published: September 2024

Abstract
CO2 mineralization is very safe for long-term CO2 geosequestration but requires huge amounts of resources. CO2 storage in saline aquifers requires relatively fewer resources but is highly dependent on the formation/seal integrity. Using waste plastics-CO2 gels to promote CO2 mineralization selectively on saline aquifer caprocks is a cost-effective approach that uses minimal resources to maximize safe and long-term CO2 geosequestration capacity. This paper demonstrates the proposed methodology with lab experiments and reservoir simulations.
In lab experiments, we placed amine-based waste plastics in simulated high-temperature and high-pressure downhole environments with brine and supercritical CO2. Thermal gravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR) analysis were used to characterize the gels/scales formed after polymer degradation. Using field-scale reservoir simulations, we demonstrated the workflow and explored the relevant field operational conditions of using waste plastics-based CO2 gels/scales to improve saline aquifer caprock integrity in CO2 geosequestration. The reservoir simulations were realized with three key developments: (1) relative permeability and capillary pressure of CO2-brine two phase system, (2) porosity-permeability relation for evolving pore space, and (3) gel/scale formation kinetics.
Lab experiments revealed that hydrolysis of the amine-based waste plastics pellets in downhole conditions transforms a heterogeneous mixture to a solution which react with CO2 to form solids of poly(calcium adipate) and amine-promoted formation of calcium carbonate. These strong solid residues may be used in improving saline aquifer caprock integrity in CO2 geosequestration operations. The reservoir simulations were performed in two steps: (1) injection of waste plastics fragments into saline aquifers that can form gels with the presence of excessive divalent cations near caprocks, and (2) injection of CO2 plumes into saline aquifers with caprocks with or without improved integrity and monitored the CO2 geosequestration amounts. In the present idealized cases with a caprock leakage path 10 meters away from the CO2 injection well, improving the caprock integrity by injecting 75 kg of waste plastics gels resulted in an extra 70 MMtCO2 storage in 100 years with 1 MMtCO2/year injection. In addition, we studied the waste plastics injection strategy for different gel formation kinetics, with injecting (1) low concentrations for long durations for fast gelation kinetics, and (2) high concentrations for short durations for slow gelation kinetics. Finally, we explored the applications in fresh aquifers by the co-injection of both waste plastics fragments and divalent cations. In all scenarios, we were able to improve the caprock integrity for the safe and long-lasting CO2 geosequestration.
Using both lab experiments and reservoir simulations, we demonstrated the applicability of using waste plastics to improve the saline aquifer caprock integrity in CO2 geosequestration. We believe this technology has great potential to provide a low-cost approach to reduce both CO2 emission and waste plastic pollution synergistically.