**Laura Wasch \* and Mariëlle Koenen**

TNO, Princetonlaan 6, 3584 CB Utrecht, The Netherlands; marielle.koenen@tno.nl **\*** Correspondence: laura.wasch@tno.nl; Tel.: +31-64966157

Received: 31 July 2019; Accepted: 16 October 2019; Published: 22 October 2019

**Abstract:** Driven by concerns for safe storage of CO2, substantial effort has been directed on wellbore integrity simulations over the last decade. Since large scale demonstrations of CO2 storage are planned for the near-future, numerical tools predicting wellbore integrity at field scale are essential to capture the processes of potential leakage and assist in designing leakage mitigation measures. Following this need, we developed a field-scale wellbore model incorporating (1) a de-bonded interface between cement and rock, (2) buoyancy/pressure driven (microannulus) flow of brine and CO2, (3) CO2 diffusion and reactivity with cement and (4) chemical cement-rock interaction. The model is aimed at predicting leakage through the microannulus and specifically at assessing methods for CO2 leakage remediation. The simulations show that for a low enough initial leakage rate, CO2 leakage is self-limiting due to natural sealing of the microannulus by mineral precipitation. With a high leakage rate, CO2 leakage results in progressive cement leaching. In case of sustained leakage, a CO2 reactive solution can be injected in the microannulus to induce calcite precipitation and block the leak path. The simulations showed full clogging of the leak path and increased sealing with time after remediation, indicating the robustness of the leakage remediation by mineral precipitation.

**Keywords:** CO2 leakage; cement; well integrity; leakage remediation; TOUGHREACT; reactive transport modelling
