Thermodynamic Modeling of Carbonation of Blended Cements for Wellbore Integrity
Authors: Rahman Farzana , Ferron Raissa Douglas 
Published: August 2024

ABSTRACT:
Carbon capture and storage (CCS) holds promise for mitigating global greenhouse gas emissions by capturing carbon dioxide (CO2) from industrial sources and storing it in deep geological formations. However, the potential impacts of CCS on oil well cements, critical components for wellbore integrity, demand careful evaluation. These cements are employed in primary, remedial cementing and plugging and abandonment operations of oil and gas wells. Alteration of cement hydration products via carbonation poses threat to wellbore integrity since it leads to leaching of cement matrix. Although carbonation of Portland cement is an inevitable process due to its favorable thermodynamic conditions, its negative impacts can be mitigated by optimizing the chemical composition of blended cements, tailored to specific thermodynamic environments. These blended cements incorporate supplementary cementitious materials (SCMS), such as silica, calcined clay, limestone, fly ash etc, to promote pozzolanic reactions. These pozzolanic reactions in blended systems produce chemically resistant hydration products. These hydration products differ from those of neat Portland cement due to the blend's initial lower calcium-to-silica (C/S) ratio. Blending cements with a low C/S ratio have been shown to produce hydration products that are less susceptible to CO2 attack. This study investigates the carbonation behavior of blended cements using thermodynamic modeling. Predicted carbonated phase assemblages indicate blended cements with SCMs are expected to provide wellbore integrity in the long run under static conditions. Thermodynamic modeling of neat and blended cements will provide us with insight to understand the progress of carbonation and the effectiveness of SCMs in blended cements for improving carbonation resistance.

1. INTRODUCTION
Recent years have seen a spike in global interest to reduce carbon footprint in hopes of mitigating climate change. More actions are being taken by global society towards a low carbon future. Keeping that in mind, the energy industry is adapting to the emerging needs of the 21st century by means of effective sustainable practices and development. Oil and gas wells require high temperature and pressure (HTHP) resistant cement sheath that provides structural integrity and zonal isolation to these wells. At the end of their service life, these geothermal wells require plugging and abandonment (P&A) in the form of cement sealant to prevent hydrocarbon leakage. It is estimated that over 2 million of unplugged abandoned wells in U.S. are leaking pollutants such as methane into groundwater reservoir, sea, and ocean, causing severe environmental pollution (Marcacci, 2020). Thus, advanced cement performance is required for geothermal cementing to mitigate issues related to leaky wellbore (both active and abandoned). Additionally, cement production itself is a carbon intensive process, which accounts for at least 8% of global CO2 emission. The cement industry needs to cut down its emission by at least 16% by 2030 to comply with the Paris Agreement (Tigue, 2022). Although concrete construction is considered as an avenue for such a reduction, one area that is overlooked is the cement used in the wellbores.