Alkali feldspar dissolution in response to injection of carbon dioxide

The dissolution of alkali feldspar in CO2-saturated aqueous solutions has been investigated in a series of laboratory experiments in the temperature range 22-200 degrees C and at CO2-pressures from 0.3 to 20 MPa. To mimic the situation when CO2 is injected into feldspar-containing reservoirs, the unsaturated aqueous solutions were allowed to react with the solid phases before CO2 was injected. In the higher end of the temperature range, CO2-injection lead to precipitation of aluminium hydroxide and kaolinite. Thus, while alkalis were released at rates comparable to other studies at similar temperature and pH ranges, Al was lost from the fluid rather than gained. Based on fluid chemistry, precipitation of Al-Si phases occurred down to at least 100 degrees C, but was delayed after the injection. This may be because the fluids were not fully saturated with feldspar at injection or may reflect sluggish nucleation. In either case, the experiments show that aluminous precipitates are likely to form in natural feldspathic reservoirs as a consequence of CO2 injection. Kaolinite precipitation proceeds slower than feldspar dissolution, leading to kaolinite-supersaturated solutions. This implies that precipitation kinetics may control the extent of feldspar dissolution in a sandstone reservoir. Plotting the measured dissolution rates against inverse temperature has revealed changes in slope at around 100 degrees C, indicating that different mechanisms dominate the dissolution above and below this temperature.