CO2-Resistant and Self-Healing Cement: Prior Art and New Concept Validation
Authors: Daou Fatma , Engelke Bernardo Soares , de Miranda Cristiane Richard , L. de Oliveira Jeferson , Guillot Dominique Jean 
Published: May 2016

Abstract
Establishing and maintaining the integrity of oil & gas wells present significant challenges, particularly in a CO2 environment. The degradation of conventional Portland cement by the CO2 acidic environment, combined with the possible damage caused to the cement sheath by physical changes and mechanical stresses, may compromise the integrity of a well.
In response, the industry has directed efforts toward a "smart" system that would adapt its structure to the surrounding environment. The appropriate system should present chemical resistance to CO2 and would also possess autorepair capabilities under certain conditions as a response to CO2-leaking fluids. Although cements that are self-healing to hydrocarbons have been used for about 10 years, nothing exists in the industry with respect to CO2.
In a collaborative initiative, an operator and a service company have undertaken research to help overcome well integrity and cementing challenges in presalt wells in Brazil. One key objective is to provide innovative well cementing solutions that can be mixed and placed in a conventional manner. Another focus is utilization of special materials that allow the cement sheath to heal itself, upon contact with CO2 fluid, if failures occur after the cement is set or in presence of a microannulus.
Initially, several options were investigated to allow selecting a material that would expand in the presence of CO2 fluids, close the fracture, and reestablish the integrity of the well.
Key candidate materials were identified, targeting those applicable to oil well cementing. Results from the first screening phase fulfill the initial criteria and validate the concept of the CO2 self-healing cement technology. A possible application is for cementing wells into which CO2 is injected, stored, or extracted. Another key application is in wells where reservoirs naturally contain a fair amount of CO2, as is the case of the presalt wells in Brazil.

Introduction
For oil & gas well integrity applications, the cement sheath provides mechanical support and hydraulic isolation between the zones or formation layers to prevent contamination. For the specific case of the CO2 oil well cement, the matrix allows zonal isolation thanks to its very low permeability, chemical resistance to the acidic environment, and good bonding to both borehole and casing. However, over time, the cement sheath can deteriorate and become permeable due to apparition of fissures or microcracks. Alternatively, debonding from the casing or the wellbore may occur, creating a microannulus. These damages create preferable pathways for migration of underground fluids and, therefore, affect the integrity of the well. The presence of a self-healing material would allow re-establishing the integrity of the cement sheath upon contact with the leaking fluid.
Today, the industry proposes different solutions to keep cement sheath integrity in hydrocarbon oil/gas environment (Le Roy-Delage and Martin-Al-Khatib 2012) or to resist to CO2 attack. Although these existing technologies have proven their efficiency, none of them allows restoration of well integrity after damage triggered by CO2 fluid. It is the objective of this collaborative study to provide well cementing systems that can be placed by pumping in the normal manner and which contain materials that would swell upon contact with CO2 fluid to allow the cement sheath to adapt its structure in response to the surrounding environment.
Initial key requirements for the target technology are the following:The cement composition needs to be chemically resistant to CO2 and/or acid brines.The cement should have very low permeability to avoid corrosive fluid invading the matrix.