Impacts of salinity on CO₂ spatial distribution and storage amount in the formation with different dip angles

Formation dip angle and the distortion of salinity affect the spatial distribution and storage capacity of carbon dioxide (CO₂). In this numerical study, based on an actual CO₂ injection demonstration project (Shiqianfeng group in the Ordos Basin) in China, CO₂ was injected for a period of 20 years at four different formation dip angles (0°, 5°, 10°, 15°). In conjunction, some salinity values were chosen, ranging from saturation salinity to no salinity. A three-dimensional (3D) model was established to systematically explore the influence of different formation dip angles and salinities on the CO₂ spatial distribution and storage amount. The simulation results showed that larger salinity and higher pressure near the injection well will lead the CO₂ gas-phase saturation and mass fraction to be smaller for a given formation dip angle. When salinity is held constant at the saturation value, a larger dip angle will cause a smaller CO₂ gas saturation in the upper right units of the injection well, and a larger gas saturation in the lower left units at the 20th year of CO₂ injection. For large salinity values (full, half, and quarter saturation salinity), the larger the formation dip angle is, the greater the CO₂ total storage amount. For smaller salinity values (0.00 and 0.03), a transition point existed (at 8 and 18.2 years) during the 20-year injection period. Before the transition point, the CO₂ total storage amount also increases with the dip angle. After the transition point, however, the larger the formation dip angle is, the smaller the CO₂ total storage amount becomes. In addition, a lower salinity may lead to the earlier appearance of the transition point.