Wettability of common rock-forming minerals in a CO2-brine system at reservoir conditions

Wettability plays a major role in subsurface fluid migration, however little empirical evidence besides qualitative relative permeability data in this context is known about the characteristics of carbon dioxide (CO2). Core analysts use several methods combined for use in the oil industry, including the USBM (U.S. Bureau of Mines) method and Amott test, supported visually by images from an environmental scanning electron microscope (ESEM) at near-vacuum conditions. Unlike contact angle measurements between CO2 and brine, the combined methods cannot readily utilize CO2 at supercritical state which is crucial for understanding sequestration phenomena. Air-brine in those tests is considered
as the analogue, which may not be appropriate.

As a part of an ongoing investigation into CO2 sequestration, this work focuses on
evaluating the wetting state of CO2 at reservoir conditions with contact angle
measurements. There is limited consensus among experimentalists in this field, and conclusions are drawn from a limited data set. This paper provides further observations to leverage the assessment of CO2 wettability.

We have extended prior investigations in a cross-disciplinary manner by preparing pure mineral samples of common rock forming minerals consisting of quartz, orthoclase, labradorite, calcite and biotite. These were benchmarked against processed quartz and mica plates which are commonly used for this type of test. This method may also be of benefit to the Petroleum industry for investigating how wettability varies with mineralogy; aiding the development of improved chemical design for enhanced oil recovery.

The contact angle of CO2 through the brine phase was measured using the captive bubble technique for plain and carbonated brine. Brine carbonation did not show a significant difference in CO2 contact angles. The findings of this investigation between 800-2000 psi at 40°C show strongly water-wet to water-wet conditions for all the gas and supercritical measurements of CO2. Our measured data shows that the mica and calcite substrates become more water-wet as pressure is reduced, whereas the quartz and biotite substrates
become more water-wet by pressurizing the system. A comparison of the measured contact angles for N2 and CO2 shows that CO2 is less water wet than N2 at low pressure conditions. There is not a clear trend at high pressure conditions.

We have identified a challenge, which is that supercritical CO2 cannot be irrefutably characterized by the Amott-USBM method, and propose that contact angle measurement is the most appropriate direct observation of wettability. A novel extension to this method has been developed to provide insight into the workings of realistic reservoir minerals.