In any carbon capture and sequestration (CCS) project, well cement plays a vital role as it provides the required zonal isolation and well integrity. Typical wellbore materials including well cement and formation rock will be exposed to a range of saturation mediums such as water and brine with different salinity levels. To date, ordinary Portland cement (OPC)-based well cement has been used. However, its survival has been questioned under CO2 sequestration conditions, as it experiences cement degradation and strength reduction in saline water. Therefore, this experimental work investigates the mechanical characteristics of geopolymer (G) as well cement and sandstone (S) as formation material. The mechanical behaviours of G, S and G-S composite materials in fresh water (W) and two concentrations of brine water (BW), 5% NaCl (5% BW) and 15% NaCl (15% BW), were studied. Based on the results, it was found that G, S and G S samples experience strength reduction in W and BW. However, the reduction rate of G is almost half of that of OPC-based oil well cement. In addition, the strength reduction rates of G and G-S were less in 15% BW compared to W and 5% BW, due to the lower alkali leaching rates from G in BW compared to W. Therefore, saline aquifers with high NaCl content are always favourable for G well cement. The S samples showed constant strength reduction regardless of the saturation medium, and hence NaCl does not show any significant effect on the mechanical behaviour of quartz-rich sandstone. The crack propagation stress thresholds were higher for G and G-S saturated in 15% BW compared to 5% BW. The S samples did not show major variation in crack propagation stress thresholds in W and BW. The ARAMIS strain measurement results showed that the maximum strain that wellbore materials experience at failure reduces with the introduction of brine. In addition, G and G-S undergo splitting failure, whereas S experiences shear failure in W and BW. (C) 2014 Elsevier Ltd. All rights reserved.