Quantifying flow reduction during injection of CO2 into legacy hydrocarbon reservoirs for CCUS

In the development of hydrocarbon fields, it is becoming known that CO₂ injection (which is sometimes done to improve hydrocarbon production) can cause pore blockage and wettability alteration by the promotion of asphaltene deposition. In hydrocarbon reservoirs, the result is poor oil recovery performance during carbon dioxide (CO₂) injection. If CO₂ is being injected into a legacy hydrocarbon reservoir (i.e., one that still contains residual oil) the same process will occur. Once again, the ability of fluid (this time supercritical CO₂) to flow will be impeded, but it is also possible that asphaltene deposition will also reduce the overall pore volumes in which CO₂ could otherwise be stored. In this work, the residual oil distribution and the permeability decline caused by organic and inorganic precipitation after miscible CO₂ flooding and water-alternating-CO₂ (CO₂-WAG) flooding have been studied by carrying out core-flooding experiments at high pressures and temperatures in an artificial three layer system. For simple CO₂ injection during CCUS operations, flooding experimental results indicate that the low-permeability layers retain a large oil production potential even in the late stages of production, which could impede CO₂ emplacement and provide significant heterogeneity, while the permeability decline due to asphaltene precipitation is more significant in high-permeability rocks. In contrast, we found that CO₂-WAG can reduce the influence of heterogeneity on the oil production, but it results in more serious reservoir damage, with permeability decline caused by CO₂–brine–rock interactions becoming significant. In addition, miscible CO₂ flooding has been carried out for rocks with similar permeabilities but different wettabilities and different pore-throat microstructures in order to study the effects of wettability and pore-throat microstructure on formation damage. Reservoir rocks with smaller pore-throat sizes and more heterogeneous pore-throat microstructures were found to be more sensitive to asphaltene precipitation, making these less attractive for CCUS reservoirs. However, rocks with larger, more connected pore-throat microstructures became less water wet due to asphaltene precipitation to pore surfaces, ultimately leading to a lower pore volume in which CO₂ can be stored. Taken together, there may be a case for not simply injecting CO₂ in CCUS operations, but alternating the CO₂ injection with injection of water in order to stabilise CO₂ flow and reduce formation damage by asphaltene precipitation.