Coating a self-healing underground tank for safe storage of CO₂ in chalk reservoirs

In the recent assessment report from the Intergovernmental Panel on Climate Change (IPCC), Carbon Capture and Storage (CCS) is mentioned as a promising way to mitigate climate change. There is currently intense interest in CCS related research. Beside the potential of underground CO2 storage, there are certain challenges such as CO2 capture, transport, storage safety, and so on, need to be addressed.

One of the critical challenges with CO2 storage in underground reservoirs is the potential for leakage back to the atmospheric environment. The North Sea chalk is mainly composed of calcite (CaCO3), for which the solubility in water is strongly enhanced by carbonation. Therefore, there is a dynamic reaction between CaCO3 and CO2 in aqueous solutions, which implies the importance of rock-fluid chemistry.

Given the chemical reaction and other risks of leakage, a proposed solution is to introduce a smart coating layer which is able to block the leaking pathways, stop the leakage and the associated reactions. This research aims to synthesize a smart self-coating polymer which would be activated by Ca2+ in the presence of water, chalk, and CO2.

In this purpose, two functional groups and two polymer chains were selected. Four species of polymers were prepared and were characterized by FTIR. Two of them showed promising results, in terms of surface coating, stability, solubility, strong binding to chalk surface and self-aggregation in presence of Ca2+.

The polymer, Poly(allylamine)-Nitrilotriacetic acid (PAA-NTA), showed specific properties that met the requirements as described earlier. Therefore, PAA-NTA was used for dynamic screening by employing CO2 bubbles experiment. This polymer was able to stop the reaction between CO2 and chalk successfully.

The thickness of coating layers was between 100 nm to 1 µm, according to SEM analyses. In addition to that, statistical analysis of the elemental information confirms the potential of PAA-NTA in binding with the chalk surface. It was crucial to test the blocking potential of the synthesized polymer in real reservoir conditions, so a CO2 core flooding experiment was designed.

In this experiment first, an outcrop Stevns Klint core sample was saturated with formation brine to equilibrate the chalk surface with a representative brine. Then polymer was injected into the core to flush out formation water and saturate the core. After that, pure supercritical CO2 at P=130 bar and T=70 °C was injected.

It was observed that supercritical CO2 sweeps almost all fluids (polymer) and no significant blockage was observed. However, in the next step, polymer and CO2 were simultaneously injected into the core to make sure that there is enough amount of both polymer and CO2 available in the rock pore space.

During this step a significant continuous blockage was observed. In order to evaluate the core condition compared to the first round of pure supercritical CO2 flooding, a second round was employed afterward. The second round of pure supercritical CO2 injection showed a massive blockage (permeability reduction/differential pressure increase) compared to the first round.

These results clearly support the potential of PAA-NTA on stopping the chalk dissolution in presence of CO2.