Dynamic micro-CT imaging of diffusion in unconventionals

Diffusion is a key mechanism of gas recovery from unconventional reservoirs. Spatial and temporal monitoring of the progress of diffusion would also aid in characterizing the local variations in pore size and tortuosity, of broad relevance to all unconventionals and transport processes. In this study the technique recently developed for contrast- enhanced, micro-CT static imaging of the pore volume distribution in shale plugs, after saturation of their accessible pore space with the X-ray opaque liquid diiodomethane (CH2I2), was extended to dynamic imaging of diffusion over these micron-centimeter scales. The CH2I2-saturated plug was immersed in the miscible, X-ray transparent liquid toluene, for diffusion to be monitored by continuous micro-CT scanning via the progressive advance in attenuation loss. This sequence of dynamic tomograms was then spatially registered to the static tomograms of the dry state and the initial CH2I2-saturated state to visualize and quantify the CH2I2 attenuation decrease at each voxel over time. Results are presented for vertically cored, cleaned sub-plugs of 3-12 mm diameter of two shale gas and one shale oil samples.