Quantitative Evaluation of Shale-Oil Recovery during CO 2 Huff-n-Puff at Different Pore Scales

Gas huff and puff (HnP) is one of the means to improve shale oil recovery, and the commonly used gases are CO 2 and N 2 . However, the current research on CO 2 and N 2 HnP enhancing shale oil recovery is not sufficient. This study compares the macroscopic exploitation and microscopic mobilization characteristics of CO 2 and N 2 HnP by the HnP experiments with a full-diameter core, gas chromatography−mass spectrometry (GC-MS) tests, and online nuclear magnetic resonance (NMR) tests. Moreover, the impact of cycle number on the oil recovery efficiency of HnP is analyzed. The results show that CO 2 HnP achieves a 7.23% higher final oil recovery factor (ORF) compared to N 2 HnP and CO 2 has a stronger injection capacity. Moreover, CO 2 has a better extraction ability, and the CO 2 HnP process can realize the storage of CO 2 . Furthermore, the lower limit of pore mobilization for CO 2 HnP is significantly lower, and the final ORF from micropores during CO 2 HnP is 20.98% higher than that during N 2 HnP. In summary, CO 2 HnP has a stronger recovery effect compared to N 2 HnP. As the cycle number grows, the incremental ORF decreases, while the growth rate of cumulative ORF slows down, and the produced gas−oil ratio (GOR) rises rapidly. Meanwhile, the gas injection volume increases and the gas−oil exchange ratio decreases. These indicate that the oil recovery effect and economic efficiency of HnP deteriorate with an increase in the cycle number.

Section: Methodsmentioning

confidence: 99%

“…S/V is the specific surface of individual pores, which is inversely proportional to the pore radius and can be expressed by the following equation: , S V = F normals r where r is the pore radius (μm) and F s is the pore dimensionless shape factor.…”

Section: Methodsmentioning

confidence: 99%

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

Liu,

Kang,

Zhang

et al. 2024

“…Under the condition of 120 h soaking time, the recovery of the crude oil in macropores increased with the increase of injection pressure. 34 Through the comparison of long core and short core huff and puff experiments, Wang et al observed that a greater pressure drop rate led to a lower recovery rate. Increasing the soaking time was found to be beneficial for enhancing the oil recovery rate.…”

Section: Introductionmentioning

confidence: 99%

“…Gao et al quantitatively evaluated the CO 2 huff and puff recovery of shale oil at different pore scales and different soaking times. Under the condition of 120 h soaking time, the recovery of the crude oil in macropores increased with the increase of injection pressure . Through the comparison of long core and short core huff and puff experiments, Wang et al observed that a greater pressure drop rate led to a lower recovery rate.…”

Section: Introductionmentioning

confidence: 99%

Insights into Enhancing Shale Oil Recovery Postfracturing with CO₂ Huff and Puff and Elastic Depletion Development Strategies

Li,

Huang,

Duan

et al. 2024

After hydraulic fracturing, shale reservoirs often undergo rapid production declines during depleted development, resulting in a low cumulative production. CO 2 huff and puff emerges as a promising method to enhance shale oil recovery postfracturing. This study conducted independent experiments to investigate multistage depressurization elastic depletion development and the CO 2 huff and puff method in shale oil. The effects of injection pressure, temperature, fractures, and other parameters on the CO 2 huff and puff recovery were analyzed using nuclear magnetic resonance (NMR). The impact of the CO 2 huff and puff on crude oil in different pores was elucidated. The results show that the fluid mobility of the shale reservoir is poor, with the recovery rate of elastic depletion development being less than 10%. Conversely, a CO 2 huff and puff can enhance the recovery rate by up to 20%. The recovery rate of shale oil through a CO 2 huff and puff correlates positively with injection pressure and temperature. The initial cycle of huff and puff contributes the most to crude oil recovery, reaching up to 50%. Under equivalent parameter conditions, the presence of fractures can increase the CO 2 huff and puff efficiency by 20% compared to the matrix core. Combined with NMR experiments, the lower limit of pores that oil can be produce in matrix and fracture cores in elastic depletion development is 200 and 150 nm, respectively. In the context of CO 2 huff and puff development, these values decrease to 30 nm for matrix cores and 15 nm for fractured cores.

“…Extensive work has been carried out on the application of gas HnP techniques in tight or shale reservoirs, − of which some concern the comparative EOR ability of N 2 and CO 2 HnP. Sheng et al examined the effect of gas composition on the HnP effectiveness in the Bakken reservoir using numerical simulation software, CMG-GEM.…”

Section: Introductionmentioning

confidence: 99%

N₂ and CO₂ Huff-n-Puff for Enhanced Tight Oil Recovery: An Experimental Study Using Nuclear Magnetic Resonance

Song

Zeng

et al. 2022

Gas huff-n-puff (HnP) is considered a promising method for enhanced oil recovery (EOR) in tight reservoirs. An accurate comparison of the EOR performance between N2 and CO2 HnP at the pore level is still lacking. In this work, N2 and CO2 HnP experiments were performed in tight cores with a nuclear magnetic resonance (NMR) instrument. The pore-level oil distribution at reservoir temperature and pressure was monitored online by NMR signals, and the ability of NMR technology to measure the oil recovery in different pores was explored. The result shows that both N2 and CO2 HnP can significantly improve tight oil recovery, but CO2 HnP presents better performance. There is less difference in oil recovery between N2 and CO2 HnP after the first cycle; however, the difference in oil recovery increases with increasing HnP cycles. The variation of the NMR transverse relaxation time (T 2) spectrum indicates that CO2 HnP is significantly more capable of enhancing oil recovery in large pores than N2 HnP, but N2 and CO2 HnP is comparable in their ability to enhance oil recovery in small pores. However, the above conclusion is not rigorous as a result of the error of NMR technology in measuring the oil recovery of specific pores. The composition of the remaining oil changes during gas HnP, resulting in no longer a fixed correspondence between the T 2 value and the pore size. This may cause the oil recovery calculated using the T 2 spectrum to be high for large pores and low for small pores, especially for CO2 HnP.