Literature Interpretation | Dong Xu, Unconventional Oil and Gas Research Institute, Northeast Petroleum University《SPE》The Impact of Fractures on the Recovery of Low-Permeability Rocks—1H Nuclear Magnetic Resonance Study

Abstract

 

 

Hydraulic fracturing technology can enhance the oil production from tight formations, and the impact of fractures on rock porosity recovery is a key area of research. The rock pore structure undergoes changes during fracturing, especially during laboratory-created fracture formation, affecting mass transfer between matrix pores and fractures. These changes need to be considered to accurately assess the impact of fractures on fluid migration through the pore network. Directly comparing the results before and after fracturing can lead to misleading conclusions. This study uses deuterium and guar gum to prepare fracture-filling materials that do not invade matrix pores and do not produce detectable ¹H NMR signals. Samples with fractures filled using this new material were tested and compared with unfilled samples to obtain the NMR characteristics of fractures. These were then isolated and eliminated in subsequent N2 and CO2 injection experiments to analyze the mechanism of fluid migration in the pore-fracture dual system. The experimental results show that: 1) Fractures reduce gas sweep efficiency, which can be partially alleviated by injecting N2 instead of CO2, as N2 can elastically support small pores. However, the total recovery rate with pure N2 injection is significantly lower than that with CO2; 2) Filling fractures increases pore recovery rates.

Experimental Equipment and Methodology

 

 

The low-field nuclear magnetic resonance equipment used in this study is the Low-Field NMR Core Analysis System (Medium-sized NMR Imaging Analyzer) manufactured by Suzhou Newmai Analysis Instruments Co., Ltd., as shown in Figure 1. The low-field NMR is used to monitor gas injection processes during enhanced oil recovery. 1) Oil-saturated matrix sample gas injection experiment (Dong, 2020a, 2020b); 2) Fractured sample gas injection experiment, Brazilian Fracture Method (BDM) for fracture creation, oil-saturated to determine the total porosity distribution after fracturing; 3) Filled fracture sample gas injection experiment, deuterium and guar gum configured as fracture filling agents to determine fracture distribution and content. The initial NMR T2 curves for four samples are shown in Figure 2.

Figure 1. Medium-sized NMR Imaging Analyzer

Figure 2. Pre-fracture sample saturated light oil T₂ spectrum (J-1 and J-2 from the Jimusaer depression, J-3 and J-4 from the Xihu depression)

Experimental Results

 

 

Fracture Distribution

The complete T₂ distribution of the fractured open fractures obtained through guar gum filling experiment (T₂ spectrum orange filled area, Figure 3), with newly added large fractures on the right side of the T₂ spectrum. Small fractures extend to T₂ = 1 ms. Fracturing changes the matrix pore structure (M0 vs. G0), with varying degrees of change in matrix pore amplitude and boundaries. Therefore, clearly defining fracture and matrix pore distribution helps accurately evaluate the impact of fractures on fluid migration.

Figure 3. Fracture T₂ distribution (Q1 and Q2 represent the boundaries of large, medium, and small pores)

Pore Increase Before and After Fracturing

The pore variation rate before and after fracture filling was calculated (Figure 4). PVF (blue) reflects the improvement in total porosity due to fracturing, and PVG (red) reflects the amount of matrix pores converted into fractures. Fracturing improved the pore volume of micropore-developed rock samples (J-1 and J-2) more significantly, but the proportion of matrix pores converted to fractures was low. For macropore-developed rock samples (J-3 and J-4), the opposite conclusion was found: the improvement in total porosity was moderate, but the proportion of matrix pores converted into fractures was high. PVF was obtained by comparing the accumulated NMR signal amounts of M0 and F0, while PVG was obtained by comparing M0 and G0 accumulated NMR signal amounts.

Figure 4. Pore variation rate before and after fracture filling

Impact of Fracture Filling on Flow

Fracturing changes the matrix pore structure, and the pore size classification method obtained from the original samples is no longer applicable. This study divides pores into large, medium, and small categories based on fracture size percentiles to calculate pore occurrence (e.g., medium pores Q₁ < T₂ < Q₂). It should be noted that fracturing does not introduce new oil volume; therefore, when calculating fracture recovery rates, the extra oil volume filled must be adjusted (F0 – M0). The post-fracture pore recovery rate data are shown in Figure 5, and the pre-fracture matrix pore recovery data can be found in Dong, 2020a, 2020b.

Figure 5. Gas injection T₂ spectra of fractured rock samples (‘G₆ N₂-CO₂‘ is the sixth round N₂-CO₂ injection spectrum for the fracture-filled sample G₂)

Using the pre-fracture M₀ recovery rate as a baseline, the increased recovery Ru under fracture and gas combination was compared (Figure 6). The matrix rock samples M₀ injection of N₂-CO₂ performed better than pure CO₂ (gray, Dong, 2020a). Compared to the pure CO₂ injection model, the N₂-CO₂ injection for fractured samples performed well in micropore-developed rocks (J-1 and J-2), but poorly in macropore-developed samples (J-3 and J-4). This is likely due to the elastic support effect of N₂ molecules in small pores. Fractures store a large amount of gas, especially CO₂, which weakens the gas diffusion energy in matrix pores, resulting in a lower overall recovery (red). Fracture filling treatment increases the gas sweep efficiency in matrix pores, increasing recovery rates (blue). In the short term, fracturing significantly boosts production; however, fracture gas storage negatively impacts long-term development.

Figure 6. Incremental recovery under fracture filling and gas combination modes

Related References

 

 

1) Dong Xu, Shen Luyi*, Golsanami Naser, Liu Xuefeng, Sun Yuli, Wang Fei, Shi Ying, Sun Jianmeng. How N2 injection improves the hydrocarbon recovery of CO2 HnP: An NMR study on the fluid displacement mechanisms. Fuel. 2020a. 278:118286.

2) Dong Xu, Shen Luyi*, Liu Xuefeng, Zhang Pengyun, Sun Yuli, Yan Weichao, Sun Jianmeng. NMR characterization of a tight sand’s pore structures and fluid mobility: An experimental investigation for CO2 EOR potential. Marine and Petroleum Geology. 2020b. 118:104460.

3) Liu Xuefeng, Dong Xu*, Golsanami Naser, Liu Bo, Shen Luyi W., Shi Ying, Guo Zongguang. NMR characterization of fluid mobility in tight sand: Analysis on the pore capillaries with the nine-grid model. Journal of Natural Gas Science and Engineering. 2021. 94.