Wettability Evaluation of a CO<sub>2</sub>/Water/Bentheimer
Sandstone System: Contact Angle, Dissolution, and Bubble Size
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Abstract
The success of CO<sub>2</sub> storage
in deep saline aquifers and
depleted oil and gas reservoirs is largely controlled by interfacial
phenomena among fluid phases and rock pore spaces. Particularly, the
wettability of the rock matrix has a strong effect on capillary pressure,
relative permeability, and the distribution of phases within the pore
space and thus on the entire displacement mechanism and storage capacity.
Precise understanding of wettability behavior is therefore fundamental
when injecting CO<sub>2</sub> into geological formations to sequestrate
CO<sub>2</sub> and/or to enhance gas/oil production. In this study,
the contact angles of Bentheimer sandstone/water/CO<sub>2</sub> or
flue gas have been evaluated experimentally using the captive-bubble
technique in the pressure range from 0.2 to 15 MPa. The experiments
were conducted using different compositions of aqueous phase with
respect to CO<sub>2</sub>, i.e., unsaturated and fully saturated.
It has been shown that a reliable contact-angle determination needs
to be conducted using a pre-equilibrated aqueous phase to eliminate
dissolution effects. In the fully saturated aqueous phase, the Bentheimer
sandstone/water system is (and remains) water-wet even at high pressures
against CO<sub>2</sub> and/or flue gas. In these systems, the data
of the stable contact angle demonstrate a strong dependence on the
bubble size, which can be mainly explained by the gravity (buoyancy)
effect on bubble shape. However, the surface nonideality and roughness
have significant influence on the reliability of the contact-angle
determination. The results of this study prove that in order to avoid
the dependency of the contact angle on the bubble size in these systems,
the effect of gravity (buoyancy) on bubble shape has to be considered
by calculation of the Bond number; for systems characterized by Bond
numbers less than 0.9, the influence of the bubble radius on the contact
angle becomes insignificant. The experimental results show that, in
contrast to quartz, the phase transition of CO<sub>2</sub> from subcritical
to supercritical has no effect on the wettability of the Bentheimer
sandstone/water system, which originates from differences in the surface
charges of quartz and Bentheimer sandstone. In an unsaturated system,
two dissolution regimes are observed, which may be explained by density-driven
natural convection and molecular diffusion