Multiscale Characterization of Foamed Cement Using X-Ray Microcomputed Tomography

摘要

Foamed-cement systems are widely used in deepwater cementing operations because of their various favorable attributes compared to conventional cement systems. For instance, in the Gulf of Mexico, foamed cement is one of the most commonly used systems for shallow hazard mitigation. However, because current standard laboratory equipment cannot accurately simulate the foam-cementing process in the field, knowledge of the actual properties of foamed cement produced in field operations is limited. In this study, the microstructure of foamed cement produced using field equipment in yard tests is examined in detail. Set foamed cement samples were analyzed using X-ray micro-computed tomography (micro-CT) at different length scales with voxel resolution ranging from 2 to 20 μm. This study establishes the fundamental criteria and procedures necessary to obtain accurate gas bubble size distribution of foamed cement samples using micro-CT technology. The test results suggest that foamed cement should be analyzed at multiple length scales to obtain a better characterization of the gas bubbles in the sample. While a larger region of analysis is useful to obtain a statistically meaningful size distribution of the larger bubbles, small core samples (less than 0.5 in. diameter) and fine scan resolutions (5 μm or smaller) are typically required to obtain an accurate measure of the small gas bubbles in foamed cement. By comparing foamed cement produced using field equipment with that produced using the traditional multiblade laboratory blender (i.e., the standard API method), this study identifies the key characteristic differences of foamed cement based on different methods of generation. Analysis of the CT-scan images reveal that gas bubbles in foamed cement generated by field equipment have a wide size-distribution range, from less than 20 μm to more than 1000 μm, and the bubble size distribution appears to show little dependence on foam quality. Conversely, the gas bubble size distribution of foamed cement generated by the API method shows a completely different behavior, with both its distribution range and median varying significantly with foam quality. This research serves as a first step toward predicting the influence of gas bubble size distribution on the stability and various other properties of foamed cement to better understand the foam-cementing process in the field.