Top-Of-Line Corrosion in Legacy and Abandoned Wells for CCS Applications

Abstract

One of the risks associated with carbon capture and storage (CCS) strategy is the integrity issue of the legacy/abandoned wells, where the condensation of acidic gases may lead to the top-of-line corrosion within the wellbore. The main objective of this work is to assess this problem with the help of physics-based models for phase behavior, pH, heat transfer and corrosion rates that could help in screening and risk-assessment of the candidate fields for CCS. First, the thermodynamics of formation brine upon ingression of CO2 in abandoned well is modelled using the PR78 equation of state (EOS) with asymmetric Huron-Vidal mixing rule for CO2-brine system. An empirical concentration/pressure dependent expression for the interaction coefficient for CO2-salt system are developed and validated for the three salts: NaCl, KCl and CaCl2. Second, a pH model is developed based on the dissociation constants of carbonic acids. Third, a heat transfer model is employed to obtain the condensation rates at given reservoir conditions. Finally, the condensation rate models are used in an empirical corrosion rate model for a representative reservoir scenario. The main results are as follows: The prediction of CO2 solubility in brine from the phase equilibria model (developed here) matches very well with experimental data, within the accuracy of 5.34% for single salt and 4.14% for mixed salts. The water condensation rate (WCR) across the wellbore height changes with the brine composition. With a given ionic strength (of 3 molality), the highest WCR value (0.45 g/m2/s) corresponds to the case of pure CaCl2-based brine while the lowest WCR value (0.35 g/m2/s) corresponds to the case of pure NaCl-based brine. Most importantly, the models presented here are shown to be valid for a wide range of pressure, temperature and salinity conditions, so as to be directly applicable to assess the candidacy of the field for CCS applications. The novelty of this work lies in the development a fast, convenient, and easy-to-implement physics-based modeling framework to evaluate the corrosion rates within legacy/abandoned wells. Especially, the thermodynamic modelling of brine solution containing a mixture of NaCl, KCl and CaCl2 salts, where a simple concentration/temperature dependent binary interaction between CO2 and salt component are determined. While the framework is developed for screening the candidacy of the field for CCS applications, it could easily be extended to screening for geothermal and other subsurface applications.