Confinement Correction to Mercury Intrusion Capillary Pressure of Shale Nanopores.

Sen Wang,Farzam Javadpour,Qihong Feng

doi:10.1038/srep20160

Abstract

We optimized potential parameters in a molecular dynamics model to reproduce the experimental contact angle of a macroscopic mercury droplet on graphite. With the tuned potential, we studied the effects of pore size, geometry, and temperature on the wetting of mercury droplets confined in organic-rich shale nanopores. The contact angle of mercury in a circular pore increases exponentially as pore size decreases. In conjunction with the curvature-dependent surface tension of liquid droplets predicted from a theoretical model, we proposed a technique to correct the common interpretation procedure of mercury intrusion capillary pressure (MICP) measurement for nanoporous material such as shale. Considering the variation of contact angle and surface tension with pore size improves the agreement between MICP and adsorption-derived pore size distribution, especially for pores having a radius smaller than 5 nm. The relative error produced in ignoring these effects could be as high as 44%—samples that contain smaller pores deviate more. We also explored the impacts of pore size and temperature on the surface tension and contact angle of water/vapor and oil/gas systems, by which the capillary pressure of water/oil/gas in shale can be obtained from MICP. This information is fundamental to understanding multiphase flow behavior in shale systems.

Highlights

Shale—a typically fine-grained sedimentary rock having ultra-low permeability and previously regarded as inaccessible—has received extensive attention, owing to its enormous hydrocarbon reserves and economical production rate after fracking
We have demonstrated that the contact angle, θHg, and liquid-vapor surface tension, γHg, of mercury droplets strongly depend on the pore size
The variations of contact angle with pore size and temperature are studied for water in shale nanopores through molecular dynamics (MD)

Summary

Introduction

Shale—a typically fine-grained sedimentary rock having ultra-low permeability and previously regarded as inaccessible—has received extensive attention, owing to its enormous hydrocarbon reserves and economical production rate after fracking. Approaches commonly employed to determine the pore size distribution (PSD) of shales include low-pressure adsorption (LPA) using N2 and CO2, mercury intrusion capillary pressure (MICP) measurement, nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM) image analysis[3,6,7]. Taking into account that the surface tension of a liquid droplet is strongly dependent on the curvature[11] and the contact angle varies with the pore size[12], we hypothesized that the common procedure for MICP analysis will lead to a significant error for shale pore characterization. Study of the variations of surface tension and contact angle of mercury with shale pore size is vital for its accurate characterization This technique may serve as an efficient tool to estimate the capillary pressure of water/oil/gas of shales under sedimentary conditions. Our results, which highlight the need for the calibration of contact angle and surface tension at the nanoscale, have implications for the characterization of shale pore structure but more generally for multiphase flow modeling in nanoporous materials

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Results

Conclusion