Determining Long-Term CO2-Containment Performance: Cement Evaluation
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 115707, "Assessing Long-Term CO2-Containment Performance: Cement Evaluation in Otway CRC-1," by Matteo Loizzo, SPE, and Sandeep Sharma, SPE, Schlumberger Carbon Services, originally prepared for the 2008 SPE Asia Pacific Oil and Gas Conference and Exhibition, Perth, Australia, 20-22 October. The paper has not been peer reviewed.
The full-length paper focuses on an in-depth evaluation of the annular material on the Otway CRC-1 well that is being used to inject CO2 in the CO2CRC pilot geological-storage project. The evaluation will draw on the design and job data, and on a detailed analysis of the high-resolution 3D cement-imaging log, to characterize the cement and ensure the long-term risk of containment breach is minimized.

Introduction
Cement slurries are exposed to a number of phenomena during mixing and placement that can lead to set-cement properties that are very different from their design value. Density-control problems (both for continuous and batch mixing), contamination, channeling, and fluid loss can and do cause slurry dilution/concentration and chemical incompatibility, which in turn can have a major negative effect on the capacity of cement to guarantee hydraulic isolation.
Currently it is debated if 10 m or more of competent cement, well bonded to casing and formation, would degrade during the expected isolation time frame for CO2-geological-sequestration wells (thousands to 10 thousand years). This is because competent cement, although reactive if exposed to CO2, has a very low permeability on the order of 0.5 to 5 µd. This low permeability means that most CO2 will travel by diffusion, a very slow process over the length scale of a meter. Cement with a high water/cement ratio, however, could have a much higher permeability, less resistance to CO2 aggression, and more-frequent defects related to slurry settling. Defects such as liquid channels in cement can provide direct pathways for CO2 leaks that could not be healed by calcite precipitation during the CO2 attack.
Some of these phenomena can be predicted, but cannot be controlled easily; others (such as fluid loss) can hardly be predicted at all. In any case, they belong to the class of fault-free risk, sometimes called residual risk: events causing substandard system performance that cannot be engineered away and that may happen even when the job is executed well. Mitigation measures must be adopted in this case to ensure a robust design. This is especially true for wells entering CO2-storage reservoirs, where storage containment is a key performance factor and CO2/cement reactions may cause leaks to grow over time. The full-length paper studies a well recently completed in Australia for CO2 injection, and identifies how fluid loss, contamination, and channeling affected the petrophysical properties (i.e., density, porosity, and permeability) of set cement.

Background
The Cooperative Research Centre for Greenhouse Gas Technologies is a joint venture engaged in research on carbon capture and storage, a proposed technology that captures CO2 from fossil-fuel power-plant effluents and injects it into geological formations for permanent storage.