3D mapping of carbon dioxide-induced strain in coal using digital volumetric speckle photography technique and X-ray computer tomography

•Digital volumetric speckle photography, a 3D strain mapping technique, is presented.•DVSP is applied to the investigation of CO2 sorption-induced strain in coal.•The 3D mappings of gas induced strain in coal are presented.•The strain distribution in the coal is localized and highly heterogeneous.•Swelling and compression regions depend on microlithotype and microstructure.

Evaluation of sorption-induced strain in coal by gases uptake is an important part of the research on CO2 sequestration and enhanced coalbed methane operations. In this paper a 3D strain mapping technique called digital volumetric speckle photography (DVSP) is introduced and applied to the investigation of sorption-induced strain in coal with CO2 uptake with the help of X-ray computer tomography scanning in situ. For comparison strain gauges are also used to measure the strain during the overall process. By using DVSP, 3D strain evolution maps in coal exposed to CO2 at pressure 1.5 MPa as a function of sorption time are obtained. The results show that the strain distribution is localized and highly heterogeneous, and the swelling and compression regions are dependent on lithotype and microstructure. While the average volumetric strain from DVSP has a similar developmental trend as data from strain gauges, the magnitude is significantly less. This is attributed to the fact that the strain gauges' result is localized over the limited surface area covered by the gauges, whereas the DVSP result is the average of the entire volume.