Salinity effect on micro-scale contact angles using a 2D micromodel for geological carbon dioxide sequestration

One of the most effective methods of mitigating carbon dioxide emission is geological sequestration, in which an enormous amount of CO2 captured from the main sources of CO2 is injected into deep underground layers for long-term storage. The most capacious of these storage sites are saline aquifers. The capacity and safety of these storage sites are governed by capillary force. Among the parameters influencing capillary pressure, the wett-ability of rock, which is quantified by the contact angle, has the highest uncertainty. Measuring the contact angle in real conditions inside the pores of a rock is ideal, but technical issues force researchers to measure the contact angle of a bubble of CO2 or a water/brine droplet on a flat surface, which cannot model the real fluid configuration inside the pores. In our experimental study, we used a transparent glass 2D micromodel with randomly patterned channels in order to inject CO2 and brine with different salinities, then measured the contact angle of the water-CO2 interface on glass channel wall surfaces. Our pore-scale dynamic contact angle results showed that by increasing the pressure from 1 to 8 MPa (equivalent to increasing the depth from 100 to 800 m, respectfully), the receding and advancing contact angles increased for all brines with different salinities (0.1, 1, and 3 molarity of NaCl). For example, the receding contact angle increased from similar to 60 degrees to similar to 90 degrees with the increased pressure for brine with 1M salinity. A higher contact angle, which indicates lower capillary pressure, increases the risk of the leakage through ground layers and the migration of CO2 back to the surface. Moreover, increased salinity results in an increased contact angle for a specific pressure, which must be considered for the safety of any storage site. Also, contact angle hysteresis, which is an important parameter for immobilizing CO2 bubbles inside of the rock pores, increases with pressure and decreases with salinity.