The leakage of CO2 through or along wellbores has been identified as one of the main challenges to secure underground CO2-storage. Currently used materials for sealing wellbores are commonly based on Ordinary Portland Cement, and the integrity of these materials can be a vulnerability during CO2-injection and -storage. Leakages may form through the cement, or along the cement-steel or cement-rock interfaces, as the result of chemical, thermal, or mechanical effects.

In order to successfully develop improved sealing materials, we need to identify critical properties that will ensure seal integrity. We also need to develop practical methods for measuring these properties under realistic conditions, and models that can be used for extrapolation. The CEMENTEGRITY project will perform experimental research that addresses the chemical, thermal and mechanical mechanisms that may damage wellbore integrity during CO2-injection and -storage on a range of different sealant material compositions. We will support this experimental work with numerical modelling. Through these activities, we will identify key properties that ensure long-term integrity of wellbore sealing materials, and we will also identify suitable methods for measuring these properties. Our findings can then be applied when developing new sealing materials for CO2-storage, to ensure the long-term integrity of underground CO2-storage reservoirs.

In WP 1 cement and a geopolymer sealant samples will be prepared and cured under conditions relevant for wellbore sealing in CCS applications, until plateau strength has been achieved. These specimens will then be confined in purpose-built flow-through apparatuses, and exposed to flow of CO2-saturated fresh water in parallel with super-critical CO2 over 6 months to study long term permeability response. In addition samples will be exposed in the same manner to flow of CO2-saturated fresh water to study post exposure impact on the sample matrix. Exposing samples in a forced flow-through apparatus will ensure that the full samples, rather than just the outer skins, are exposed to CO2 for a sufficiently long duration to allow chemical reactions to take place. Reference samples will be maintained at identical conditions without such exposure (fresh water only). 

After the exposure period, the Young’s modulus, Poisson’s ratio, unconfined compressive strength and tensile strength will be measured and compared to reference samples. In addition, a specially developed indentation test will be applied to map the penetration depth of impacted matrix so that damage progression speed can be estimated. SEM, XRD and other analysis techniques may be employed as required to study the effects of exposure on microstructure and mineral composition in areas of particular interest.

WP 2 studies the effects of selected impurities, and combinations thereof, in CO2 on the integrity of sealant materials, to further expand upon the exposure tests done in WP 1. We will expose samples of five different sealant compositions to both supercritical and dissolved CO2 bearing H2S and other impurities. Then, we will analyse the exposed samples using techniques such as SEM, EDX and XRD to determine the effects of exposure, and specifically the impact of impurities.

WP 3 investigates the effects of thermal variations on cement integrity under in-situ temperature and pressure encountered in CCS wells. We will use a triaxial deformation apparatus capable of mounting the sample (either intact or composite with analogous casing) in a vessel under in-situ conditions to perform injection-through experiments that induce thermal cycling or shocks. Then, we will conduct micro-CT scan and unconfined compression test to study how and where cracks initiate and grow in the cement, how de-bonding between casing and cement develops, and how the cracks and de-bonding affect the mechanical properties of cement.

WP 4 will firstly develop a reaction-transport numerical simulation model for the novel geopolymer sealant being developed as part of WP 6. The reaction processes, including ion transport, dissolution of precursors, and nucleation and growth of reaction products, will be simulated taking into account both kinetics and thermodynamics. Then, we will use this model to evaluate the effects of thermal cycling and carbonation on volume stability of the geopolymer by simulating the microstructural changes, mechanical changes, and potential cracking (by combining a mechanical model) induced by these two process. When cracking happens, leaching and self-healing may happen. We will thus apply our model to simulate the reaction processes of the leaching and self-healing, to evaluate the net effect of these two processes on the microstructure and long-term performance of the geopolymer.

WP 5 deals primarily with the assessment of the interfacial bond strength of five different sealants tested in this project with steel casing. Practical test methods will be developed during the course of the project, and their effectiveness will be assessed. In addition, WP5 aims to utilise the sealant materials themselves as sensors, using changes in the electrical conductivity of the sealant matrix to monitor changes in its sealing integrity. Similar methods will also be employed to monitor the conditions of the sealant-casing interface and how these are related to the bond strengths obtained from the destructive testing.

WP 6 aims to improve the chief attributes of a rock-based geopolymeric material recently synthesized at UiS, and to explore an optimum formulation for application of the developed GP within the acidic conditions typically observed in the injection wellbores of CO2 geosequestration. Key knowledge gaps with regards to the application of GP as wellbore sealants (e.g., stability of geopolymers and gels, permeability of geopolymer matrix, strength retrogression, and not fully condensed GPs) will be filled through conducting an in-depth literature review and experimental work. Accordingly, the optimum formulation of the GPs will be explored, and its performance will be examined through a set of laboratory-based experiments, and using analysis techniques such as XRD, EDS, SEM, CT-scanning, etc.

WP 7 will use the results obtained in the other six WP’s to identify critical properties that will ensure long-term integrity of sealant materials in CCS applications. We will then propose suitable methods and procedures with which these properties, and the impact thereon of exposure to CO2 and other components, can be measured consistently and thoroughly. Such procedures can then be applied when developing and testing novel compositions, or to determine the suitability of existing sealants under yet untested conditions. The identified properties and proposed methods will be compiled, and presented in an openly accessible report, as well as at relevant events.