Surfaces heterogeneity is a well-investigated fact and it represents a fundamental characteristic of the rock surfaces. Many factors are responsible for this surfaces heterogeneity such as the existence of impurities, surface roughness, the arrangement of the local crystalline, and the variation in crystal faces. Although the presence of clay minerals heterogeneity has been proven previously, its effect on the wettability measurements and the CO2 storage capacity has not been studied yet. Thus, in this paper, we therefore systematically measured the contact angle (wettability) of pure clay minerals (i.e. montmorillonite, illite and kaolinite) and mixed clay mineral (i.e. 14 wt% kaolinite, 48 wt% illite and 38 wt% montmorillonite) for CO2/brine systems. For both pure and mixed clay minerals, advancing and receding contact angles were measured at various pressures (5 MPa, 10 MPa, 15 MPa and 20 MPa) and temperatures (305 K and 333 K). The results show that clay minerals heterogeneity has a significant effect on the wettability measurements. The contact angles increase with increasing pressure for both pure clays and mixed clay. However, there is a slight increase in contact angles of pure clays as the pressure increases, compared to the mixed clay. For instance, the advancing contact angles have been increased from 77⁰ to 110⁰, 53⁰ to 67⁰, and 41⁰ to 57⁰ for pure montmorillonite, illite and kaolinite, respectively, by increasing the pressure from 5 MPa to 20 MPa. Meanwhile, the contact angle of mixed clay has been increased from 47⁰ to 93⁰ (i.e. the wettability of the mixed clay minerals has been altered from weakly water-wet to intermediate-wet) at the same pressures increase (from 5 MPa to 20 MPa). Furthermore, the results illustrate that the contact angle of both pure and mixed clay slightly reduce with increasing temperature from 305 K and 333 K. Thus, we conclude that clay minerals heterogeneity affects the clay wettability and leads to increase the contact angle at high pressures. Consequently, clay minerals heterogeneity reduce the CO2 storage capacity and containment security at high pressures. This study has important implications for deep geological carbon sequestration, CO2 dynamics and spreading in the geological reservoir.

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