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Experimental Investigation on the Imbibition Behavior of Nanofluids in the Tight Oil and Gas Reservoir through the Application of Nuclear Magnetic Resonance Method

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.”

2.1.3. Instrument

The static imbibition method was used in this study. MacroMR12-15H-I nuclear magnetic resonance equipment (sourced from Suzhou Niumag Analytical Instrument Co., Ltd. Suzhou, China) was used for the NMR measurements. NMR T2 spectra and two-dimensional imaging were scanned before and after being saturated with water and saturated with fluorine oil, respectively. Instruments such as beakers, vacuum saturation devices, and drying ovens were used in the saturated water and fluorine oil stages. LE204E/02 electronic balance (Mettler-Toledo Instruments Shanghai Co., Ltd. Shanghai, China) was used for weighing before and after the experiment. PDP200 gas permeability measuring instrument (Core Lab, Houston, TX, USA) was used for core permeability measurements, and PS-20A ultrasonic cleaner was used in the nanofluid preparation stage (Shenzhen Kejie Ultrasound Technology Co., Ltd. Shenzhen, China).

2.2. Experimental Program

2.2.1. Nuclear Magnetic Resonance Theory

Since the introduction of nuclear magnetic resonance technology to the petroleum industry in the 1990s [22], it has played an important role in describing complex reservoirs. From the NMR transverse relaxation time distribution, many important parameters can be derived, such as total porosity, effective porosity, irreducible water saturation, permeability, and pore-size composition and distribution, etc. [23,24]. It is an indispensable technical means in the evaluation of complex reservoirs.
In porous rocks, the amplitude of the NMR signal is proportional to the number of hydrogen atoms in the hydrogen-containing fluid. Therefore, this technique can be used to study the distribution of hydrogen-bearing fluids (oil or water) in porous rocks. During the time evolution of the dipole moment, there are two kinds of NMR relaxations: longitudinal relaxation (T1) and transverse relaxation (T2) (Liu et al., 2018). T2 spectroscopy is currently the most widely used NMR method because it can quickly and nondestructively obtain pore-throat scans.

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:

R=0.735T2/C�=0.735�2/�

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.

According to the size of the relaxation time, the pores are classified according to Hodot, and the pore types are divided into four categories: the radius of the micropore is less than or equal to 2 μm; the radius of the small hole is greater than 2 μm and less than or equal to 10 μm; the radius of the mesopore is greater than 10 μm and less than or equal to 20 μm; The pore radius is greater than 20 μm and less than or equal to 200 μm (Shi, 2018). According to previous research, there is a corresponding relationship between the relaxation time and the pore radius, as shown in Table 2 (Chen, 2020).
Table 2. Corresponding relationship between relaxation time and pore radius.
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
Magnetic resonance imaging can obtain cross-sectional, sagittal, coronal, and three-dimensional images of rock samples. The image signal represents the distribution of fluid in the core space. The brighter the image, the looser the core, and the higher the porosity and saturation. Conversely, the darker the image, the tighter the core, and the lower the porosity and saturation [31].
Magnetic resonance technology uses the principle of nuclear magnetic resonance plus a gradient magnetic field to detect the emitted electromagnetic waves. The principle is simply summarized as: core samples are divided into several thin layers, which are called slices. This process is called slice selection, and each slice can be divided into many small volumes, called voxels. A token is assigned to each voxel, a process called encoding or spatial localization. A radio frequency pulse is applied to a certain layer, and the nuclear magnetic resonance signal of the layer is received and decoded. Then, the size of each voxel nuclear magnetic resonance signal of the layer is obtained. The size of the voxel signal is displayed on the corresponding pixel of the fluorescent screen, and the signal size is represented by different gray levels. Finally, an image reflecting the size of each voxel NMR signal at the slice is obtained, that is, an MRI image [32].
The nuclear magnetic resonance image is a two-dimensional display of the three-dimensional space. The nuclear magnetic resonance scans the local and overall hydrogen-containing fluid conditions. The gray-scale pixel points correspond to the spatial position of the hydrogen-containing fluid (oil) in the core. Therefore, with the relative content of hydrogen fluid, the change in oil saturation can be obtained by comparing the gray value of the displacement image before and after [32]. The brighter the gray level, the higher the fluid saturation in the core; conversely, the darker the gray level, the lower the fluid saturation in the core. Therefore, the total amount of fluid in the core directly affects the strength of the NMR signal.
Both ordinary oil and formation water contain hydrogen, and it is hard to know the distribution of oil and water in the pores after the imbibition experiment. Therefore, fluorine oil without hydrogen elements was used for the imbibition experiments in this study. NMR scanning T2 map and two-dimensional imaging were used to observe the distribution of imbibition fluid entering the core. Since the density of fluorine oil is greater than that of water, we fixed the core at the bottom of the imbibition bottle, turned the imbibition bottle upside down, and carried out the imbibition experiments in both forward and reverse directions to compare and analyze the effect of gravity on imbibition. In the research process of this experiment, the nuclear magnetic resonance experiment combined with the mass method was used to analyze the final imbibition results. Then, the volume method was used to carry out the comparison and verification of the imbibition recovery degree, and to quantitatively characterize the spontaneous imbibition experiments of different imbibition liquids.

2.2.2. Experimental Procedures

The specific steps of the experiment were as follows:
(1)
The cores were washed with oil, dried, weighed, and the gas-measured permeability and porosity were determined;
(2)
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)
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)
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)
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)
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

Since the fluorine oil used in the experiment does not contain hydrogen, NMR signals are not generated by the fluorine oil, and NMR signals are all contributed by water. Therefore, the measured T2 relaxation time spectrum reflects the water pore-size distribution. The T2 relaxation spectrum collected during the imbibition process can reflect the change in the water content in the core pores. The larger the peak area, the greater the water content in the pores.
As shown in Figure 4, there is an obvious T2 spectrum after imbibition, indicating that the imbibition liquid replaced the fluorine oil. The water saturated in the core before imbibition is mainly distributed in the large and medium pore throats, accounting for about 60% and 30%, respectively. The small pore throats are less distributed, which conforms to the typical bimodal distribution characteristics of the ultra-low-permeability sandstone NMR T2 spectrum; After imbibition, most of the imbibition liquid is distributed in the large pore throat, accounting for more than 50%, and the fluorine oil in the large pore throat is replaced. The fluorine oil tends to flow out of the core from the large pore throats, and the residual fluorine oil is mainly distributed in the small and medium pore throats. From Figure 5a we can observe that the area enclosed by the T2 spectrum after imbibition represents the amount of oil–water exchange, which means the larger the area, the higher the imbibition efficiency and the higher the recovery degree. As shown in Figure 5b, the area of the T2 spectrum after imbibition is: Sblack card > Semulsion > Swater, corresponding to the recovery degree that: Rblack card > Remulsion > Rwater, indicating that the 2D nano blackcard can more significantly improve the imbibition efficiency than nanoemulsion and water.
Figure 4. (aT2 spectrum before and after black card imbibition; (b) fluid distribution frequency before and after black card imbibition; (cT2 spectrum before and after emulsion imbibition; (d) fluid distribution frequency before and after emulsion imbibition; (eT2 spectrum before and after water imbibition; (f) water fluid distribution frequency before and after imbibition.
Figure 5. (aT2 spectrum after saturated water; (bT2 spectrum after forward-directional imbibition.

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