Stress-Dependent Pore Deformation Effects on Multiphase Flow Properties of Porous Media

Amir H Haghi*,Stephen Talman,Richard Chalaturnyk

doi:10.1038/s41598-019-51263-0

Abstract

Relative permeability and capillary pressure are the governing parameters that characterize multiphase fluid flow in porous media for diverse natural and industrial applications, including surface water infiltration into the ground, CO2 sequestration, and hydrocarbon enhanced recovery. Although the drastic effects of deformation of porous media on single-phase fluid flow have been well established, the stress dependency of flow in multiphase systems is not yet fully explored. Here, stress-dependent relative permeability and capillary pressure are studied in a water-wet carbonate specimen both analytically using fractal and poroelasticity theory and experimentally on the micro-scale and macro-scales by means of X-ray computed micro-tomography and isothermal isotropic triaxial core flooding cell, respectively. Our core flooding program using water/N2 phases shows a systematic decrease in the irreducible water saturation and gas relative permeability in response to an increase in effective stress. Intuitively, a leftward shift of the intersection point of water/gas relative permeability curves is interpreted as an increased affinity of the rock to the gas phase. Using a micro-scale proxy model, we identify a leftward shift in pore size distribution and closure of micro-channels to be responsible for the abovementioned observations. These findings prove the crucial impact of effective stress-induced pore deformation on multiphase flow properties of rock, which are missing from the current characterizations of multiphase flow mechanisms in porous media.

Highlights

VPMi and VPM defined as proxy model’s initial and stress-dependent pore volume, respectively. This process was repeated iteratively until the pore strain of the proxy model became equal (±0.1%) to its corresponding experimental value and the representative model used for further 3D analysis
We implemented the 3D calculation of pore size distribution, fractal dimension, and degree of anisotropy using the CT Analyser program within Bruker microCT 3D Suite Software
Pore size is defined as the diameter of the largest sphere which meets two conditions: (1) the sphere encloses the point, and (2) the sphere is entirely located within the pore

Summary

Introduction

The original X-ray images were segmented μCT images were filtered using median into black and white binary areas representing pores and grains, respectively. A grayscale index smaller than its original value for the rock at zero effective stress was selected manually. Otsu’s thresholding method[60] in 3D space was used initially to define the original grayscale index and match the model’s 3D porosity with experimentally measured porosity at zero effective stress.

Results

Conclusion