Predicting the reactivity of cement-rock CO2 during annulus leakage and remediation by clogging (PRECALC)

Summary

This project explored the long-term impact of CO2 effects at a well interface and the remediation techniques that need to be developed to demonstrate possibilities for preventing CO2 migration to overlying aquifers or to the surface in the unlikely event of leakage by using predictive models. The study focused on a method for CO2 mobility control by injecting a CO2-reactive and CO2-consuming solution into a wellbore annulus to form solid reactants that clog the pore space, reduce permeability and minimise leakage. It was concluded that several model modifications were necessary to improve the predictive behaviour. 

Background

The CCS directive of the European Commission requires investigation of the CO2 fate in storage reservoirs and of potential leakage pathways over decades and millennia after abandonment. Mechanical damage such as cement interface de-bonding result in the formation of such leakage pathways in wells. This de-bonding could be the result of Portland cement shrinking when hardening or due to stresses that occur during operation. Other experimental research has revealed preferential cement alteration by CO2 along the cement–rock interfaces. This could enhance the permeability of the de-bonded interfaces.

Project objective

Developments in TNO’s ‘Structural Integrity’ Early Research Programme (ERP) had suggested that natural clogging of cement-rock micro-annuli by mineral precipitation could seal leakage pathways, but only if CO2 migration was slow. Furthermore, experimental and modelling work conducted in the EU MIRECOL project showed that fluid injection could cause intentional clogging above a caprock leak, thereby reducing leakage up to 95%. These results offered potential scope for stimulating precipitation at the cement-formation interface, in case of leakage detection, by injecting a fluid with a fit-for-purpose composition. Inducing permanent closure of leak paths by ‘intentional clogging’ requires a careful choice of fluids as the clogging material should remain stable under natural subsurface conditions in the long-term. The results raised several crucial questions for leakage mitigation at the cement-formation interface: 

1. Which geochemical processes occur before and during CO2 leakage at the cement-formation interface?

2. What are the crucial factors that impact such geochemical mechanisms (e.g. mineralogy, temperature, pressure, salinity)? 

3. What are the requirements for injecting fluids to mitigate leaks at the cement-formation interface by intentional clogging? 

The PRECALC project sought to answer these questions by building, calibrating, and running reactive transport models (RTM) of core plug samples exposed to CO2-saturated brine in an autoclave reactor. These core plug samples were composed of 50% caprock and 50% well cement.

Project results

The project modelled an experiment of rock and wellbore cement being exposed to CO2-saturated brine in an autoclave reactor to evaluate the reaction potential of cement in contact with CO2, the surrounding rock minerals, and the pore water. In the experiments, patterns of different reactive zones were observed. MoReS-PHREEQC and TOUGHREACT simulations showed some differences in chemical reactions, especially for the pH evolution, and it was recommended to evaluate the kinetic rates of the mineral reactions further, in the future, using a larger surrounding bath. The report also suggested that enabling a gas phase in the MoReS-PHREEQC model should be considered as a potential future improvement to the model. No results for clogging were achieved due to numerical instabilities and the report concluded that further model development steps were required to study the intentional clogging process. The project team also investigated several state-of-the-art C-S-H solubility models and concluded that the non-ideal binary solid solution model of Walker et al shows the best agreement with a compilation of experimental solubility data. The report recommended using this C-S-H model in future reactive transport modelling work, to include a more detailed representation of the cement phase compositional range and solubility.