Analysis has consistently shown that carbon capture and storage (CCS) has an important role in meeting emission-reduction targets (IPCC, 2018). CCS wells require special design considerations to ensure long-term zonal isolation when exposed to carbon dioxide (CO2) because a complex set of chemical reactions leads to carbonation and dissolution of conventional cement sheaths.

Several studies conducted into the long-term stability of different cement systems when exposed to wet supercritical CO2 and CO2-saturated water showed that the novel CO2- resistant cement system provides enduring zonal isolation. Properties investigated included permeability, porosity, mass evolution, CO2 degradation front, and compressive strength. Given its superior mechanical properties, the novel CO2-resistant cement system was selected for use in the first Australian offshore CCS Gular-1 appraisal well.

To ensure that the blend characteristics of the novel CO2-resistant cement system remained optimal, a stringent quality-control procedure was developed. The blend management process, supported by rigorous laboratory testing, covered the complete lifecycle of the blend. This lifecycle extended from sourcing chemical components, to blending the components in a bulk plant, to transporting the blend across land and sea, and ultimately, preparing the slurry mixing.

By adhering to the project management process, all primary cement jobs were successfully performed without incident using conventional cementing equipment and practices. The novel approach of blending the product locally at a fit-for-purpose facility reduced costs compared with previous methods of importing a preblended product prepared at a special centralized facility. Blend homogeneity was maintained during transfer from a sea vessel to the jackup rig, with minimal change in density between samples received from the bulk plant and samples received from the rig. This blending, which verified the initial blend flow capability and the robustness tests performed at a regional laboratory using specialized equipment, concluded the blend is suitable for offshore operations.

Selection of a suitable cement system to ensure long-term zonal isolation will prove essential to the continuing expansion of the CO2 injection market. Through this offshore CCS appraisal project, valuable best practices and lessons learned in design and execution have been captured. This paper presents the decision process used for selecting a suitable CO2-resistant cement system for Australia's first offshore CCS appraisal well, drilled by AGR as part of the CarbonNet Project in late 2019, as well as the project management processes implemented to ensure successful job execution. The experiences detailed in this paper will benefit other operators confronted by challenges associated with wells subjected to CO2 injection.

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