4. Mine Applications

4.1 Mine Applications of Spherical Nanofluids

The application cases of spherical nanofluids in mines include but are not limited to the following situations.

① Shengli Oilfield conducted multiple field experiments on nanofluid injection technology in 2002 and 2010, respectively. In 2002, Gudong Oil Production Plant conducted two nanoscale polymer blocking and water blocking field tests. After the construction was completed, the water content of the two corresponding oil wells decreased by an average of 15%, resulting in a total increase of 1.0×10³ tons of oil. Since 2010, Chunliang Oil Production Plant has successfully applied the polysilicon nanomaterial injection technology 16 times. By changing the rock wettability to increase water phase permeability and reduce water injection pressure, the effective injection rate of this technology application is 86%.

② In 2005, the Shuanghe Oil Production Plant in Henan Oilfield conducted 5 field tests of nanoscale water blocking agents, with a 100% effective success rate in water blocking. The ineffective water injection volume of the measure wells was reduced by 25653.5 m³, and the corresponding production wells increased oil by 3293.5 tons. The effective period of nanoscale water blocking agents for water blocking was as long as 23 months; In 2010, Henan Oilfield selected four well groups in Wangji and Guchengbi 124 blocks to carry out nano deep profile control and flooding field experiments. Among them, the profile control and flooding effects of Wang32 and Chai 9 well groups were significant, and the water injection profile of the water injection wells was significantly improved. The injection pressure increased, and during the effective period of the measures, a total of 551.4 tons of oil increased and 12535.6 cubic meters of precipitation occurred.

③ In 2019, the KL21-B1 well in Bohai Oilfield conducted a field test of biological nano polycrystalline silicon for pressure reduction and injection enhancement. After the measures were taken, the water injection index of KL21-B1 well increased from 5.5 m³/(MPa·d) to 28 m³/(MPa·d), indicating that the biological nano polycrystalline silicon system has a good pressure reduction and injection enhancement effect.

④ In 2014, Castilla and Chichimene oil fields in Colombia conducted a new technology field experiment for oil-based nanofluids. The CNA and CNB wells in Castilla oilfield were injected with 31.8 m³ and 23.9 m³ of nanofluids, respectively. After 12 hours of shut-down, the average oil production increased by 43.725 m³/d; The CHA and CHB wells in Chichimene oilfield were injected with 13.7 m³ and 17 m³ of nanofluids, respectively. After 12 hours of shut-down, the oil production of the CHA and CHB wells increased by 49.3 m³/d and 13.8 m³/d, respectively.

4.2 Mining Applications of Sheet-like Nanofluids

There are many studies on sheet-like nanofluids indoors, but there are few application cases in mines. The following mainly lists the application effects of 2D nanoblack cards in domestic oil fields.

4.2.1 Field Application Cases of Fractured Carbonate Reservoirs

①. In 2019, the TK7-459 well group in Tahe Oilfield carried out a 2D nano black card flooding test. The well group is a composite karst area of underground rivers and faults, with good connectivity, reservoir temperature of 130℃, and formation water mineralization degree of 2.4×10⁵ mg/L, crude oil viscosity 9×10⁴ mPa·s at 50℃. The underground river of the well group is connected, partially filled, and shows two directions of effectiveness during the unit water injection period. TK7-459 well injected a total of 3150 m³ of 0.005 wt% 2D nano black card dispersion system. The injection of 2D nano black card dispersion system began to take effect less than a month ago, transitioning from "1 injection and 2 production" to "1 injection and 4 production". The average water content of the well group decreased from 82% to 25.7%, and the average daily production increased from 6.2 t to 17.7 t. The cumulative increase in oil production was 1248.6 tons, with an increase of 2.758 million yuan and an input-output ratio of 1:6.3.

②. TK779 well in Tahe Oilfield has already injected 50000 m³ of water in the early stage, with zero oil pressure and casing pressure, resulting in unidirectional water channeling. When 2.0×10³ m³ of 2D nano black card dispersion system was injected, the oil pressure increased to 4 MPa (maximum 7.4 MPa) and the casing pressure reached 7 MPa (maximum 9.8 MPa); After injecting 4.9×10³ m³ of 2D nano black card dispersion system, all three adjacent wells were affected, and "1 injection and 1 production" became "1 injection and 3 production". As of the statistical date, the cumulative oil increase of the well group was 223.9 tons, and the average water content decreased from 82% to 25.7%.

③. The TP251X well in Tahe Oilfield has multi-level and multi-scale channels between the wells, and the reservoir is distributed along the fault. During the injection of 2D nano black card dispersion system, the oil pressure and sleeve pressure continued to climb. Before injection, the water content of TP7-1 remained at 100%, and the oil production remained at 0. After injecting the 2D nano black card dispersion system, the water content decreased from 100% to 0. From September 5, 2019, self spraying production began, with a self spraying oil production of 20 t/d. Before injecting the 2D nano black card dispersion system, the oil production of the well group was 4 t/d. After injecting the 2D nano black card dispersion system, the daily oil production of the well group reached 31 t/d.

4.2.2 Examples of Field Applications in Sandstone Reservoirs

①. In December 2019, the Dugu 20 well group in Liaohe Oilfield introduced 2D intelligent nano black card technology. The Dugu buried hill belongs to a fractured buried hill reservoir. The Dugu 20 well was reinjected in 1986, with a water injection well section of 2242-2564 meters and an effective thickness of 267 meters. There are a total of 7 oil wells in the surrounding area. The dynamic data of oil and water wells shows that the injected water mainly flows in the southeast direction of the strip, with obvious water channeling passages. After two months of injecting the 2D nano black card oil displacement system, the well group's production increased from 94.3 m³/d to 100.3 m³/d, the oil production increased from 15.6 m³/d to 20.3 m³/d, and the water content decreased from 88.5% to 79.3%.

②. After the 2D nano black card oil displacement system injection was carried out in the Zhao 52-44 well area of the low-permeability reservoir in Daqing Oilfield in 2020, the average single well water content in the block decreased from the lowest of 87.4% in the water drive stage to 78.4%, and the average single well oil production increased from the highest calibrated 0.8t/d to 1.5t/d. As of the statistical date, the net increase in crude oil production due to the effect of 2D nano black cards is 830.5 tons.

③. After the 2D nano black card flooding system was injected into the Xin154-1 well area of the low-permeability reservoir in Shengli Oilfield in 2020, the comprehensive water content in the block decreased from 92.6% in the water flooding stage to 84.9%, and the comprehensive oil production increased from the calibrated 5.6 t/d to 10.5 t/d, with an effective period of more than 1 year. As of the statistical date, due to the injection of 2D nano black card oil displacement system, the crude oil production has increased by 1260 tons.

4.3 Bottlenecks Restricting the Large-scale Application of Nanofluids in Mines

Research has found that there are many publicly published research results on the application of nano oil displacement systems in improving oil recovery, but most of them are only indoor research results, and there are few reports on the application of nano oil displacement systems in mines. Although indoor research has proven that nano oil displacement technology is a promising new technology for improving oil recovery, there are still multiple issues that limit the large-scale application of nano oil displacement technology in oilfield sites. There are currently two main bottleneck issues in the promotion of nanofluids from indoor experiments to reservoir applications:

①．Lack of efficient nano oil recovery systems for unconventional oil reservoirs. At present, low concentration (<1000 mg/L) spherical nanomaterials that have been successfully applied in mines are mainly used as enhancers and cannot be used alone as oil displacement systems for field experiments. When spherical nanofluids exert wedge-shaped permeability, their concentration is high(˃10 wt%), and at low concentrations, they cannot exert the important oil displacement mechanism of wedge-shaped permeability, resulting in unsatisfactory oil displacement efficiency. In addition, the contact form between spherical nanomaterials and oil-water interfaces is "point-to-point" contact, while the contact form between two-dimensional sheet-like nanomaterials and oil-water interfaces is "face-to-face" contact, which limits the micro motion of two-dimensional sheet-like nanomaterials such as rotation at the oil-water interface, enhances their influence on the properties of oil-water interfaces, and is expected to break through the technical limitations of spherical nanomaterials.

②．Two dimensional sheet-like nanomaterials have their unique characteristics compared to spherical nanomaterials, but the theoretical and technical research on the development of two-dimensional sheet-like nanofluids, the mechanism of improving oil recovery, and the integration of field pilot experiments is not yet systematic, and requires deeper exploration and research.

5. Conclusion

This article summarizes the research and application progress of nanofluids in improving crude oil recovery, including the synthesis methods of nanomaterials, stability evaluation methods of nanofluids, mechanism of nanofluid in improving crude oil recovery, and progress in nanofluid field applications.

1). The nanomaterials applied in the field of improving crude oil recovery mainly include surface single wettability nanomaterials and surface hydrophilic oil amphiphilic nanomaterials, and amphiphilic nanomaterials have higher interfacial activity, which has greater potential for improving crude oil recovery;

2). The stability evaluation methods for nanofluids mainly include Zeta potential method, pH control method, and precipitation method. High stability nanofluids are the prerequisite and most basic condition for applying nanofluids to improve oil recovery;

3). The six main mechanisms of nanofluids improving oil recovery are reducing interfacial tension, changing wettability, reducing crude oil viscosity, improving foam stability, structural separation pressure, and reducing pressure and increasing injection;

4). Spherical nanofluids only form structural separation pressure when their concentration exceeds 10 wt% and 8 wt% SDS is added, while sheet-like nanofluids can form structural separation pressure at ultra-low concentrations (0.005 wt%), which is also one of the main oil displacement mechanisms of sheet-like nanofluids;

5). Compared to spherical nanofluids, sheet-like nanofluids have greater potential for improving crude oil recovery. However, the theoretical and technical research on the development of two-dimensional sheet-like nanoflooding systems, the mechanism of enhancing oil recovery, and the integration of field pilot experiments is not yet systematic, and further exploration and research are needed.