Long-term well bore integrity in Otway: Integrating ultrasonic logs, cement petrophysics, and mechanical analysis

Abstract

The CO2CRC pilot project in the Otway basin of Australia has recently started injecting supercritical CO2 into a depleted gas reservoir, in what is the first geological storage project in the southern hemisphere. For CO2 geological storage to be successful, every effort should be made to ensure that CO2 doesn’t find its way back to the atmosphere for a very long time - possibly centuries. Potable aquifers and other permeable formations (e.g. hydrocarbon deposits) must also be protected against CO2 contamination. Wells are generally recognized as a weak spot in CO2 storage, where containment can break down. This is because cement, steel and elastomers can be corroded by CO2, and the ageing process will be accelerated by any defects in the cement sheath. In particular, containment must be assured across impermeable geologic barriers - the caprock. In Otway’s case, an objective of the project was to contain CO2 below a relatively short shale interval (around 150 ft in length). This represents the primary barrier, backed up by a much longer section higher up the well. This paper discusses how the integration of 3D ultrasonic imaging together with an advanced understanding of cement petrophysical properties brings about a detailed picture of the cement sheath. Caprock properties and borehole detailed shape and features can also be characterized through formation evaluation logging. The actual in-situ cement properties can then be used in a mechanical analysis of the cement-rock-casing system in order to assess the occurrence of cement defects (such as cracks and microannuli). This methodology is applied step-by-step over the short section of cement that must assure isolation across the primary barrier, providing an accurate input to long-term leak models that will estimate the risk to containment over hundreds of years.