The Effect of Pore Structure on CO2 Relative Permeability
Authors: Carpenter Chris 
Published: June 2016

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 18125, “The Impact of Rock Pore Structure on CO2 Relative Permeability,” by Adel Traki and Mehran Sohrabi, SPE, Heriot-Watt University, prepared for the 2014 International Petroleum Technology Conference, Kuala Lumpur, 10–12 December. The paper has not been peer reviewed.

In this study, the authors use measured CO2/brine relative permeability data available in the literature to study the behavior of the data obtained for various rocks. These measured CO2 relative permeabilities show large variations in the values of relative permeability and also in the trend of the relative permeability curves. The results of this study will allow identification of rocks that would be more suitable for CO2 injection on the basis of the pore structure and the distribution of the pores inside the rock.

Introduction

In much of the literature, the relative permeability of CO2 has been studied for each formation separately and the main factors considered to affect CO2 relative permeability include fluid saturation, hysteresis, and interfacial tension (IFT). As for a group of formations with different rock types, the difference in CO2 relative permeability curves is mainly attributed to rock-type parameters. However, it has been found that even in a set of samples extracted from different formations in the same rock type or from a single formation, there is diversity in CO2 relative permeability curves. Pore structure or quality has been assumed to be responsible for the observed disparity, but no detailed explanation has been offered as to how it could result in different CO2 relative permeability curves for a set of formations in the same rock type. In this work, the authors introduce an improved concept of pore and throat distribution that will be used to interpret the observed differences in CO2 relative permeabilities.

Improved Pore-Size-Distribution Concept

In the authors’ concept, the rock consists of pores and throats (channels). The summation of the cross-sectional area of channels (throats) represents the absolute permeability, while the total volume of pores represents the porosity of the rock. For example, invading the pores (initially filled with brine) by CO2 constitutes the main part of CO2 saturation, whereas invading the channels or throats is directly linked to CO2 relative permeability. The throats and pores in a rock exist in different sizes. According to this concept of pore and throat definition, the authors suggest two types of distribution: throat-size distribution and pore-size distribution.

Throat-Size Distribution. Throat-size distribution refers to the percentage represented by each throat size in absolute permeability, not in the total number of throats. If the percentages of throat-size distribution are close to each other for each size, it can be said that there is a normal throat-size distribution, but if the percentages of some throat sizes are very high while those for others are very low, an abnormal throat-size distribution is indicated. The throat-size distribution affects the rock quality because, if the percentage of large throats is high, fluid flow through the rock will be much easier. Inside the rock, if the percentages of throat size in different directions are not close to each other, then heterogeneity exists. Consequently, the throat-size distribution should be verified in the direction of flow and in other directions.

Pore-Size Distribution. Pore-size distribution refers to the percentage represented by each pore size in total porosity. If the percentages of pore-size distribution are close to each other, it can be said that there is a normal pore-size distribution, but if some percentages are very high while others are very low, an abnormal pore-size distribution is indicated.

Throat/Pore Connections. If the throats and pores are connected and both have the same size, the system will appear as consisting of throats only and can be termed a similar pore/throat connection, but if the connected throats and pores have different sizes, the system can be described as a dissimilar pore/throat connection. The pores and throats are connected and feed each other such that the throats may be first and the pores subsequent and vice versa. On the basis of these distribution and connection concepts, the authors study the relative permeability of CO2 and its associated saturation.