Experimental investigation on effect of water film thickness in unsaturated sandstone cores on CO2 transport during geologic storage

Abstract

Characteristics of the water distribution and film thickness within pores control CO2 transport and determine the effectiveness of geologic storage. Although studies of adsorbed water films on solid surfaces are abundant, reports about the dependence of CO2 transport behavior on film thickness under geologic CO2 storage conditions are still lacking. In this study, the water film thickness was estimated through water adsorption equilibrium experiments on four tight sandstone samples at varying relative humidities. The calculated results reveal that the adsorbed water exerts a greater influence on smaller pores, as micro- and mesopores are first filled with condensed water, while macropores only contain absorbed water films, and that the measured critical brine film thicknesses are strongly controlled by the clay mineral content. The influence of water film thickness on CO2 transport was evaluated based on the DLVO theory for cylindrical pores. Considering the mobility difference of water films in different pores, the effective disjoining pressure measured by the most probable pore radius can be regarded as the preliminary basis for the selection of reservoir or caprock and evaluation of the sealing performance of rock. Increasing the water film thickness results in a smaller effective pore radius and flowable pore space, and the movement of water in pore channel causes an electroviscous effect. Both conditions have a negative effect on CO2 movement and eventually decrease the CO2 effective permeability.