Interplay between Rock Permeability and the Performance of Huff-n-Puff CO2 Injection

The CO2 huff-n-puff experiments are often conducted on a rock sample with a given permeability. However, there is a need for understanding the production performance of CO2 huff-n-puff over a range of rock permeability values. In this study, CO2 huff-n-puff corefloods were conducted by using 30 cm long artificial cores over a permeability range between 0.7 and 240 mD. After that, the cores and produced oil were analyzed by NMT tests and gas chromatography tests. The effects of rock permeability on primary parameters, such as ultimate oil recovery, gas and oil ratio (GOR), pressures, residual oil distribution, and produced oil composition, were studied in detail. The experimental results indicate that the overall CO2 huff-n-puff efficiency increases with permeability, while the production dynamics also changes with permeability. The oil production is greater and realized faster in high permeability core samples than low-permeability rocks; hence, maintaining the same production efficiency for low-permeability samples needs more production cycles and a longer production time. Fortunately, the GOR of CO2 huff-n-puff in low-permeability samples is lower, which is favorable in long-term production. In contrast, a larger produced GOR is realized in high-permeability core samples, especially beyond the optimal cycle. Moreover, although the CO2 front occurs at a shorter distance from the inlet as rock permeability decreases, CO2 huff-n-puff can simultaneously produce oil from different pore sizes of different permeability core samples. The permeability of core samples also has a significant influence on the composition of the produced oil. The CO2 extraction capability is stronger in samples with a lower permeability.