Experimental Investigation of the CO₂ Huff and Puff Effect in Low-Permeability Sandstones with NMR

For low-permeability sandstone reservoirs, CO₂ huff and puff is an effective method for increasing oil recovery. Commonly, sandstone formations with low permeability have diverse pore and throat sizes and a complex pore-throat structure, which essentially affects the flow characteristics of CO₂ and oil in the formation and further the CO₂ huff and puff performance. It is necessary to understand the recovery degree of various microscale pore sizes under different operational parameters during CO₂ huff and puff in tight sandstones. In this work, several experiments of cyclic CO₂ injection are conducted with sandstone core samples with low permeability. Before and after the injection, the T ₂ spectra of the sandstone cores are compared using the NMR technique. We then discuss the micro residual oil distribution and recovery degree in different pores, such as micropores (<1 ms), small pores (1-10 ms), medium pores (10-100 ms), and macropores (>100 ms). It is found that the recovery degree in the different pores increases as the pore size increases. Oil can be recovered more easily from macropores and medium pores during the cyclic CO₂ injection. The oil contained in micropores is relatively difficult to extract considering a high capillary force under immiscible conditions. It is found that the total recovery degree increases with the increase in soaking time. However, such a recovery degree increment in small pores is not as large as that achieved in medium and large pores. With the CO₂ injection volume increase, the total recovery degree increases. When the CO₂ injection volume is less than 1.5 PV, it is challenging to extract the oil from micropores and small pores. As the cycle number increases, the cyclic oil recovery decreases, and most of the oil is produced in the first cycle. This suggests that under the experimental conditions of this study, the cycle number of CO₂ huff and puff shall not be more than 3. This work is important to further understand the CO₂ huff and puff process for improving oil recovery in sandstone reservoirs with low permeability.