Time-Lapsed Visualization and Characterization of Shale Diffusion Properties Using 4D X-ray Microcomputed Tomography

Diffusion is an important mass transport mechanism in shale matrix, which usually has pore sizes ranging from molecular dimensions to micrometers. Better characterization of the diffusion properties is helpful in understanding the multiphysical mass transport process in shale. We present a method for measuring the local effective diffusivity of shale core plugs using four-dimensional (4D) X-ray microcomputed tomography (micro-CT). Liquidliquid diffusion of X-ray-opaque CH2I2 from a Permian Basin shale core plug into the surrounding X-ray-transparent toluene is monitored by 4D micro-CT imaging. The time-sequenced diffusion tomograms enable 4D visualization of the dynamic process. Local directional effective diffusivities are measured numerically from the micro-CT data using a mathematical method. The measured data are analyzed in relation to compositional variations of the sample. The DykstraParsons coefficient is used to quantify the degree of heterogeneity of the measured data at the subcore scale. We find that the diffusion in the Permian Basin subplug is uneven and influenced by matrix heterogeneities. Dense materials (e.g., pyrite) have low porosity and low horizontal effective diffusivity of around 1015 m2/s or below; light materials (e.g., fossil) have high porosity and high horizontal effective diffusivity of around 1014 m2/s or above. Compositional variation of the sample leads to porosity and mass transport property changes. 4D imaging and local diffusivity measurements identify the true heterogeneity of the shale sample, which is advantageous over static imaging. The measured local effective diffusivity enables us to infer smaller-scale characteristics and thus provides a means to relate microscale shale rock structure to macroscale transport properties.