https://doi.org/10.3390/en16010454
“Tight oil and gas resources are widely distributed and play an important role in the petroleum industry. Due to its nanoscale pore-throat characteristics, the capillary effect is remarkable, and spontaneous imbibition is very beneficial to the development of low-permeability reservoirs. In this study, the imbibition experiments of 2D nano blackcard, nanoemulsion, and water were carried out, respectively. The pore-throat fluid distribution characteristics before and after core imbibition were analyzed with nuclear magnetic resonance technology, and the enhanced oil recovery effects of 2D nano blackcard nanoemulsion, and water were comprehensively evaluated. The results show that the final recovery factors of cores soaked in 2D nano blackcard (0.005 ωt%) and nanoemulsion (0.02 ωt%) or imbibed in water are 32.29%, 26.05%, and 7.19%, respectively. It can be found that 2D nano blackcard is the fluid with the best imbibition effect. In this work, a new type of 2D nano blackcard was proposed and identified as a functional imbibition fluid for enhanced oil recovery in tight reservoirs, providing a practical reference for the effective development of tight, low-permeability oil and gas reservoirs.”
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2.1.3. Instrument
2.2. Experimental Program
2.2.1. Nuclear Magnetic Resonance Theory
The hydrogen nucleus relaxation time of the nuclear magnetic resonance T2 spectrum can effectively characterize the pore radius of the reservoir rock. The longer the T2 relaxation time, the larger the corresponding pore radius, and vice versa [25]. The fluid in the large pores is less affected by the force of the core wall, so the relaxation rate is slow and the T2 relaxation time is long. The fluid in the small pores is relatively large under the force of the core wall, the relaxation rate is slightly faster, and the T2 relaxation time is short [26]. In order to facilitate the analysis and research, according to the method mentioned in the literature [27,28,29,30], the T2 relaxation time can be converted into the throat radius:
where R� is the throat radius, μm; T2 is the NMR T2 relaxation time, ms; and C is the conversion coefficient, ms/μm. The conversion coefficient is 14.1 ms/μm; under this conversion coefficient, the NMR T2 spectrum fits well with the conventional mercury intrusion curve, and the correlation is high.
T2 Relaxation Time/ms | Pore Radius/μm | Pore Type |
---|---|---|
T2 ≤ 1 | ≤2 | Micropore |
1 < T2 ≤ 10 | 2 < r ≤ 10 | Small pore |
10 < T2 ≤ 100 | 10 < r ≤ 20 | Middle pore |
100 < T2 ≤ 1000 | 20 < r ≤ 200 | Large pore |
2.2.2. Experimental Procedures
- (1)
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The cores were washed with oil, dried, weighed, and the gas-measured permeability and porosity were determined;
- (2)
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The cores were vacuum-saturated with water (24 h), weighed, and the NMR T2 profiles and 2D imaging in the saturated water state were determined;
- (3)
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The cores were dried (24 h), weighed, and saturated with fluorine oil (24 h), weighed, and the 2D imaging in the saturated fluorine oil state was determined;
- (4)
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The cores were put into water, nanoemulsion (0.02 ωt%), and 2D nano blackcard (0.005 ωt%) solution, respectively. The imbibition of the imbibition cores was observed at different times, and the oil volume was recorded at the same time;
- (5)
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The imbibition ends of the core sample were observed when the oil volume did not change. The surface of the core was dried. Then, the core was wrapped with plastic wrap and put it into the NMR instrument for the NMR T2 spectrum test. After the test, the two-dimensional pseudocolor imaging test was performed immediately. During the two-dimensional pseudocolor imaging test, the lateral center layer of the core was selected as the imaging operation layer, and the two-dimensional pseudocolor image was inverted by using the NMR signal changes in different areas of the core center plane;
- (6)
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The data was processed, images were drawn, and the experimental results were analyzed.
3. Analysis of the Results
3.1. Analysis of the Results of NMR Experiments
3.1.1. Pore Throat Characteristics and Fluid Distribution Based on T2 Spectrum
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