Characterization of heterogeneities in porous media using constant rate air injection porosimetry

This paper elucidates the pore-scale physics of the gas invasion into a vugular heterogeneous porous media, using a constant-rate air injection (CRAI) method. For very low injection rates, the magnitude of capillary pressure at the pore scale determines the pathways selected by the invading gas. Because the gas invades relatively larger pores, or vugs, the gas pressure registered near the face of injection suddenly drops and then builds up again to invade the largest pore throat connecting the invaded vug to adjacent pores. The pressure trace enables identifying the volume and size distribution of macroscopic heterogeneities, such as vugs or clusters of high permeability, surrounded by a matrix of low permeability. One of the improvements obtained by this work was to carefully detect the characteristic times on the porosimetry signal, during the invasion of non-wetting phase into a vug and away from the vug from which a vug volume was formulated. Flow visualization results obtained by conducting CRAI experiments in sintered glass bead models validated the rational of this method that offers an inexpensive way for characterizing the pore structure of heterogeneous porous media. Furthermore, we have advanced the understanding of a pressure trace, obtained during the porosimetry process by differentiating different signatures of invasion of the non-wetting phase into water-filled vugs, and have modified the formulation of vug sizes based on these invasion mechanisms. Results of vug size characterization are in good agreement with the actual vug sizes in the micromodels. The CRAI porosimetry has applications for routine core analysis in determining vugs with a size greater than 1 mm3 in porous media when accessed by a gas phase during constant rate gas injection. Core samples with a pore volume up to 10 cm3 can be analyzed by CRAI porosimetry.

► Identified the volumes of vugs and heterogeneous regions of higher permeability.
► Analyzed the constant rate air injection porosimetry signal.
► Modified the formulation of pore spaces based on the porosimetry signal.
► Differentiated possible mechanisms of invasion into water-saturated
► pores by air.