Water Imbibition and Oil Recovery in Shale: Dynamics and Mechanisms Using Integrated Centimeter-to-Nanometer- Scale Imaging

Water imbibition, and the associated oil displacement, is an important process in shale oil reservoirs after hydraulic fracturing and in water -based enhanced oil recovery (EOR). Current techniques for water imbibition measurement are mostly black-box-type methods. A more explicit understanding of the water imbibition/oil recovery dynamics and geological controls is in demand. In this paper, a mul-tiscale imaging technique that covers centimeter to nanometer scale (i.e., core to pore scale), integrating neutron radiography, microcom-puted tomography (micro -CT), and scanning electron microscope (SEM) is applied to investigate the water imbibition depth and rate and the cause of heterogeneity of imbibition in shale samples. The dynamic processes of water imbibition in the 1 -in. (25.4 -mm) core sample were explicitly demonstrated, and the imbibition along the matrix and imbibition through microfractures are distinguished through neu-tron radiography image analysis. The causes of observed imbibition heterogeneity were further investigated through micro -CT and SEM image analysis for 1.5 -mm diameter miniplug samples from different laminas of the 1 -in. core samples. Imbibition depth and rate were calculated on the basis of image analysis as well. Estimation of oil recovery through water imbibition in shale matrix was performed for an example shale field. This innovative and integrated multiscale imaging technique provides a white/gray- box method to understand water imbibition and water -oil displacement in shale. The wide span of the length scale (from centimeter to nanometer) of this technique enables a more comprehensive, accurate, and specific understanding of both the core -scale dynamics and pore -scale mechanisms of water imbibition, oil recovery, and matrix-fracture interaction.

Automated summary

This paper applies a multiscale imaging technique to investigate the depth and rate of water imbibition in shale samples and the causes of imbibition heterogeneity. By using neutron radiography, micro-CT, and SEM, the dynamic process of water imbibition and the distinction of imbibition in matrix and microfractures are demonstrated. The causes of imbibition heterogeneity are further investigated through image analysis, and the depth and rate of imbibition are calculated. This innovative technique provides a comprehensive understanding of water imbibition and water-oil displacement in shale.