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

Abstract

For low-permeability sandstone reservoirs, CO 2 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 CO 2 and oil in the formation and further the CO 2 huff and puff performance. It is necessary to understand the recovery degree of various microscale pore sizes under different operational parameters during CO 2 huff and puff in tight sandstones. In this work, several experiments of cyclic CO 2 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 CO 2 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 CO 2 injection volume increase, the total recovery degree increases. When the CO 2 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 CO 2 huff and puff shall not be more than 3. This work is important to further understand the CO 2 huff and puff process for improving oil recovery in sandstone reservoirs with low permeability.

Huff-n-Puff Technology for Enhanced Oil Recovery in Shale/Tight Oil Reservoirs: Progress, Gaps, and Perspectives

“…54 When considering a fixed overall operating duration for recovery, however, shorter soak times and shorter puff times can result in better oil recovery, because more cycles can be done. In 2021, Zhao et al 279 discovered that, as the cycle number increases, the cyclic oil recovery diminishes, with the first cycle producing the majority of the oil. They indicated that the cycle number of CO 2 H-n-P should not exceed 3 under the experimental conditions of that kind of study.…”

Comparative Study on the Macroscopic Exploitation and Microscopic Mobilization Characteristics of CO2 and N2 HnP to Enhance Shale Oil Recovery

“…The distribution of the T 2 spectrum corresponds well with the pore distribution determined by the mercury intrusion experiment, and both show two wave peaks. In general, the pores can be classified into three categories based on the transverse relaxation time T 2 : ,− micropores, T 2 < 10 ms, corresponding to a pore radius r < 0.332 μm; mesopores, 10 ms < T 2 < 100 ms, corresponding to a pore radius of 0.332 μm < r < 3.320 μm; macropores, T 2 > 100 ms, corresponding to a pore radius r > 3.320 μm.…”

Pore- and Core-Scale Mechanisms Controlling Supercritical Cyclic Gas Utilization for Enhanced Recovery under Immiscible and Miscible Conditions in the Three Forks Formation

“…As a general rule, micropores have a relaxation time between 0 and 1 ms. Small pores, medium pores, and macropores are defined as those with relaxation times of 1–10 ms, 10–100 ms, and greater than 100 ms, respectively. The true pore classification method was used and is based on the pore classification types identified in earlier studies in shale reservoirs. − In compliance with NMR test and the current pore classification standard, the pores of experimental cores are divided into four categories: micropores (>0–007 μm), small pores (0.007–0.07 μm), medium pores (0.07–0.7 μm), and macropores (≫0.7 μm) on the scale of T 2 , as shown in Table . The remaining residual oil distribution during CO 2 HnP cycles can be studied using NMR spectroscopy to see how varying pore sizes affect enhanced oil recovery and the recovery factor in each pore size when different cycle numbers are used.…”

“…50−52 In compliance with NMR test and the current pore classification standard, the pores of experimental cores are divided into four categories: micropores (>0−007 μm), small pores (0.007−0.07 μm), medium pores (0.07−0.7 μm), and macropores (≫0.7 μm) on the scale of T 2 , as shown in Table 4. 52 The remaining residual oil distribution during CO 2 HnP cycles can be studied using NMR spectroscopy to see how varying pore sizes affect enhanced oil recovery and the recovery factor in each pore size when different cycle numbers are used.…”