NIUMAG
Low Field NMR Study the Effect of Fractures on the Recovery of Low-Permeability Rock
Hydraulic fracturing technology can improve crude oil production in tight formations, and the impact of fractures on rock pore recovery is the focus of research. Rock pore structure changes during fracturing, especially lab-made fractures, affecting the mass transfer between matrix pores and fractures. These changes need to be taken into account to accurately assess the impact of fractures on pore fluid migration; directly compare samples The results before and after fracturing can lead to some misleading conclusions. In this study, heavy water and guar gum were used to configure the fracture filling material, which did not invade matrix pores and did not produce detectable NMR signals. By testing the samples filled with this new material and comparing the measurement data of the unfilled samples, the NMR characteristics of the fractures can be obtained, which can be isolated and eliminated for the analysis of the fracture-to-hole-fracture two in the subsequent N2 and CO2 huff and puff experimental studies. Influence mechanism of fluid transport in metasystems.
Low-field NMR analysis experimental results shows:
- Fractures will reduce the sweep efficiency of gas, which can be partially alleviated by injecting N2 instead of CO2, N2 can elastically support small pores, but the total recovery rate of pure N2 huff and puff is significantly lower than that of CO2.
- Filling fractures will increase pore recovery.
Experimental equipment and methods
The NIUMAG low-field NMR core analysis system (medium size MRI analyzer) used in this study is shown in Figure 1.
Medium size NMR analyzer (low-field NMR core analysis system)
Low-field NMR monitoring the process of gas injection huff and puff oil displacement.
- Gas injection huff and puff experiments on saturated oil matrix samples (Dong, 2020a, 2020b).
- The gas injection huff and puff experiment of fracturing samples, Brazilian splitting method (BDM) to create fractures, and saturated oil to determine the total pore distribution after fracturing.
- The gas injection huff and puff experiment of the filled fracture samples was carried out, and the crack filler was configured with heavy water and guar gum to determine the distribution and content of cracks.
The initial NMR T2 curves of the four samples are shown in Figure 2.
Fig. 2. T2 spectrum of saturated light oil samples before fracturing (J-1 and J-2 are taken from Jimsar Sag, J-3 and J-4 are taken from Xihu Sag)
Low-field NMR experiment results
1) Fracture distribution (low-field NMR analysis)
The complete T2 distribution of the fractured fractures was obtained through the guar gum filling experiment (the orange filled area of the T2 spectrum, Figure 3). Some large-sized fractures were newly added on the right side of the T2 spectrum, and the tiny fractures could extend to T2=1ms. Fracturing changes, the matrix pore structure (M0 vs. G0), with varying matrix pore amplitudes and boundaries. Therefore, clarifying the distribution of fractures and matrix pores is helpful to accurately evaluate the impact of fractures on fluid migration.
Figure 3. Fracture T2 distribution (Q1 and Q2 are the boundaries between large, medium and small pores)
2) Pore increase before and after fracturing
The porosity change rate before and after fracture filling is calculated (Fig. 4). PVF (blue) reflects the improvement effect of fracturing on total porosity, and PVG (red) reflects the amount of matrix pores converted into fractures. The effect of fracturing on the pore volume of rock samples with developed micropores (J-1 and J-2) is more obvious, but the proportion of matrix pores converted into fractures is low. The macropore-developed rock samples (J-3 and J-4) have the opposite conclusion, the improvement effect of total pore volume is average, but the proportion of matrix pores converted into fractures is high. Among them, PVF is obtained by comparing the accumulated nuclear magnetic signal amounts of M0 and F0, and PVG is obtained by comparing the accumulated nuclear magnetic signal amounts of M0 and G0.
Figure 4. Porosity change rate before and after fracture filling
3) The effect of fracture filling on flow (low-field NMR analysis)
Fracturing changes, the matrix pore structure, and the pore size classification method based on the original sample is no longer applicable here. In this paper, the textiles of fracture size are used to divide the pores into three categories: large, medium and small to calculate the pore occurrence (such as medium pores Q1)
Figure 5. Gas injection huff and puff T2 spectrum of fractured rock sample (‘G6 N2-CO2’ is the sixth round of N2-CO2 huff and puff spectrum of fracture filling sample G0)
Taking the M0 recovery rate before fracturing as the base value, the incremental recovery rate Ru under the combination of fractures and gas is compared (Fig. 6). The effect of N2-CO2 injection in the matrix rock sample M0 is better than that of pure CO2 (gray, Dong, 2020a). Compared with the pure CO2 huff and puff mode, the effect of N2-CO2 injection in the fracturing samples is better in the micropore-developed rock samples (J-1 and J-2), but the effect is poor in the macropore-developed samples (J-3 and J-2). 4), which is presumed to be related to the elastic supporting effect of N2 molecules on the pores. Fractures store a large amount of gas, especially CO2, which weakens the kinetic energy of gas diffusion in the matrix pores and reduces total production (red). Fracture filling treatment can increase the sweep efficiency of the gas in the matrix pores and increase the recovery factor (blue). In the short term, fracturing will greatly increase production; however, the gas storage capacity of fractures will adversely affect long-term development.
Figure 6. Incremental recovery in combined fracture filling and gas injection mode
Relevant literature (low-field NMR analysis):
1) Dong Xu, Shen Luyi*, Golsanami Naser, Liu Xuefeng, Sun Yuli, Wang Fei, ShiYing, Sun Jianmeng. How N2 injection improves the hydrocarbon recovery of CO2HnP: 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, SunJianmeng. NMR characterization of a tight sand’s pore structures and fluidmobility: 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, GuoZongguang. 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.