Carbon dioxide sequestration in underground formations is being considered on a massive scale to reduce the amount of CO2 emitted into the atmosphere. However, a better understanding of the chemical and physical interactions with formation fluids and rock is necessary before implementing sequestration in a depleted reservoir, aquifer, or during enhanced oil recovery operations. These interactions are affected by many parameters, including pressure, temperature, brine salinity, and CO2 injection rate. They can be evaluated by the change in water composition before and after injection, or from the change in CO2 injectivity over time. CO2 may affect the permeability positively due to carbonate rock dissolution, or negatively due to precipitation of reaction products, mainly CaCO3.
CO2 dissolves in the formation brine, generating carbonic acid, which dissolves carbonate rock. Dissolution impacts brine composition, which affects solubility. Calcium carbonate may tend to precipitate with changing concentration of bicarbonates. Precipitation may occur in either EOR operations or during primary CO2 sequestration. Injectivity changes are a concern during EOR operations, while storage capacity and seal integrity are primary concerns during CO2 sequestration. This paper addresses the effect of the pressure, injected CO2 volume, and the CO2 to water volumetric ratio during water alternating gas (WAG) operations on precipitation and the relative impact of dissolution or precipitation on the formation injectivity.
A core flood study was conducted using limestone cores. CO2 was injected under supercritical conditions at a pressure of 1,300 psi, and a temperature of 200°F. Core effluent samples were collected and the concentration of calcium ions was measured.
The results showed that the outlet pressure had the greatest effect on the bicarbonate presentation. Keeping the outlet pressure below the supercritical pressure of CO2 minimized precipitation of calcium carbonate. Additionally, maintaining the CO2/water volumetric ratio above 1.0, reduced bicarbonate formation in the formation water and helped in minimizing precipitation of calcium carbonate.