APPLYING VACUUM SATURATION TO STUDY THE PORE STRUCTURE OF TIGHT SHALES

The steep decline in productivity of tight shale gas plays are, in part, macroscopic consequences of the nano-scale pore structure of the shale matrix. Vacuum saturation is an integral method in a full suite of techniques used to investigate the pore structure and properties of geologic and man-made media. In addition to fluid saturation as a means of sample preparation for a number of experimental approaches (e.g., porosity measurement, saturated diffusion), this study illustrates the utility of using the vacuum saturation approach to studying the edge-only accessible porosity distribution of tight shales, which is linked to the steep production decline. A custom apparatus has been designed for vacuum saturating samples of porous media. Consideration was given to chamber material properties (e.g., strength, outgassing) in order to strike a balance between cost, usability, and vacuum/pressure reliability. Beyond improving upon the vacuum efficiency of our previous apparatus, the new design adds the ability to apply positive (beyond atmospheric) pressure subsequent to fluid immersion, resulting in a more complete saturation of the nano-sized pores. With the new apparatus, samples were saturated with both oil and water wetting solutions containing both non-sorbing and sorbing tracers. After freeze drying, the tracers occupying the edge-connected, evacuated pore spaces were mapped using micro-scale laser ablation – inductively coupled plasma – mass spectrometry.