Non-invasive quantification of throat-size distribution and corresponding capillary pressure

Abstract

Pore-throat-size distribution (PSD) controls fundamental petrophysical properties regulating the fluid flow in porous rocks and impacts characterization of water and hydrocarbon reserves. However, the methods available for estimating PSD (e.g. mercury injection porosimetry, gas adsorption/condensation, and electron microscopy) require rock samples and, thus, are not adequate for field applications using borehole geophysical measurements. Therefore, the aims of this paper include (1) appraising pore geometry features (e.g., tortuosity, PSD) using multi-physics formation data, (2) introducing a new non-invasive process to estimate PSD through the combined interpretation of permittivity, resistivity, as well as nuclear magnetic resonance (NMR) measurements, and (3) demonstrating that the new process can be applied to reliably estimate capillary pressure curves and displacement pressure.We introduce a process based on a new equation for non-invasive estimation of PSD in porous media. This novel throat-size equation requires information obtained through the analysis of NMR and electric measurements. We calculate the distribution of pore sizes from the T2 distribution by assuming the pores can be approximated by spheres. The pore-to-throat size ratio and the tortuosity are estimated from the combined analysis of permittivity, resistivity, and NMR measurements. Then, we can use the pore-size distribution and the pore-to-throat ratio to calculate the PSD. The new non-intrusive process relies on fundamental petrophysics and entirely removes the necessity of calibration efforts. Results demonstrate that the relative differences between the calculated mean pore-throat sizes against the corresponding values obtained from mercury injection capillary pressure (in core samples) and the generalized network modeling results (in pore-scale images) are approximately 10% and 40%, respectively. These differences are not expressive since the effective pore-throat sizes can vary by orders of magnitude in different rock types.