Recently, many breakthroughs have been made in tapping remaining potential of existing oil fields, efficient development of non-conventional resources, and upgrading oil production engineering equipment, which have alleviated the aggravation of resource degradation effectively. However, modern petroleum exploration technologies, particularly protection, remote sensing, detection, and control technologies under harsh well conditions, can hardly satisfy the need of emerging oil production technologies. In addition, as the exploration activities become gradually more complex, pollution of oil to environment is getting increasingly serious, restricting the sustainable development of the petroleum economy^{[1, 2, 3, 4]}. Therefore, there is a crying need to make use of emerging technologies in the petroleum industry to achieve sustainable oil development. In recent years, emerging technologies represented by functional micro-nano structures have become an important direction of technical development all over the world, because micro-nano structures have shown material characteristics and functional features different from those at macroscopic scale. Micro-nano structures offer a new solution and idea for solving problems that currently exist in the petroleum industry.

A functional micro-nano structure is defined as a structure aiming at the realization of a specific target having special functionality. Due to the micron-scale and even nano-scale size, functional micro-nano structures show different material characteristics from those observed at the macroscopic scale. In combination with optimized design and intelligent adjustment, such structures can achieve some special features or functions, such as large specific surface area, high thrust- weight ratio, and intelligent control of mechanical wave/electromagnetic wave transmission. Based on their functions and features, new functional micro-nano structures are categorized into mobile micro-nano structures, represented by micro-nano motors, and fixed micro-nano structures, represented by metamaterials.

Distinct typical application areas exist, based on different functions and features of functional micro-nano structures. At present, micro-nano motors that belong to mobile functional micro-nano structures are typically used in the fields of micro-nano scale medicine transmission and surgery, environmental rehabilitation, sensing detection, as well as micro-nano processing, etc. When the development needs of the petroleum industry are concerned, mobile micro-nano structures have a great potential to be used in oil detection, micropore oil displacement, petroleum pollution cleaning, oil exploration, and oil transmission pipeline cleaning, etc (Table 1). On the other hand, metamaterials as fixed functional micro-nano structures are typically applied in the adjustment of electromagnetic, acoustic, optical, and thermal features. At present, metamaterials are commonly used for vibration isolation, noise reduction, stealth (acoustic, optical, thermal), adjustment of transmission path, and super-resolution imaging, etc. Where the demand for petroleum equipment is concerned, such materials can be highly potentially applied in downhole equipment vibration isolation, special sensing elements, and pipeline protection, etc (Table 1).

Table 1

Table 1

Table 1 Typical micro-nano structures and their applications Type of micro-nano structure Typical example Typical application Potential application in petroleum industry Mobile Micro- nano motor Targeted medicine Oil detection Environmental rehabilitation Micropore oil displacement Sensing detection Oil pollution cleaning Micro-nano processing Oil exploration Nano surgery Oil transport pipeline cleaning Fixed Acoustic metamaterial Vibration isolation and noise reduction Downhole equipment vibration isolation Acoustic stealth Acoustic logging Super-resolution imaging Acoustic sensing element Acoustic path adjustment Acoustic flowmeter One-sided acoustic propagation Electromagnetic Antenna Downhole equipment signal communication Electromagnetic stealth High sensitivity and high precision element Absorbing materials Electromagnetic shield High-resolution focus Super-precision sensor High-resolution and holographic imaging Geological exploration Thermal metamaterial Thermal stealth Thermal protection for oil pipeline Thermal focuser Interference-free thermal sensor Thermal diode Downhole equipment thermal isolation

Table 1 Typical micro-nano structures and their applications

This paper introduces the development progress of functional micro-nano structures, using the examples of micro-nano motors and metamaterials, as well as explaining 3D printing as a critical manufacturing method for complex 3D micro-nano structures. The application prospects of these technologies in the petroleum industry are discussed.

2.1.1. Mechanism

A micro-nano motor as a sort of dynamic device at micro- or nano-scale can convert optical, electric, magnetic, and chemical energies from the environment into mechanical energy to achieve linear, circling, spiral, and other specific motions^[5]. According to the power sources, micro-nano motors can be classified into chemical-driving, external physical field-driving, and hybrid power-driving micro-nano motors^[6]. (1) A chemical-driving micro-nano motor mainly turns chemical energy of hydrogen peroxide, water solution, or acid solution into mechanical energy to move. The common chemical-driving modes include concentration gradient, automatic electrophoresis, and bubble driving^{[7, 8, 9, 10, 11, 12, 13, 14]}. Chemical-driving micro-nano motors have been widely applied in environmental rehabilitation, particle assembly, and inspection sensing, because of their simplicity, strong driving force, and easy manufacturability. (2) External physical field-driving micro-nano motors can convert external energies from light source, magnetic fields, or ultrasonic fields into mechanical energy to drive itself^{[15, 16, 17, 18, 19]}. This kind of motor doesn’ t need chemical fuel supplied, so they are highly biologically compatible and have broad application prospects in the biomedical industry. (3) A hybrid power-driving micro-nano motor combines the two driving mechanisms of chemical-driving and external physical field-driving. It can be driven by multiple driving forces and, accordingly, is applicable to a wider range than the former two^{[20, 21, 22, 23]}.

2.1.2. Current research

Current research on micro-nano motors focuses on the following aspects: (1) driving mechanism: to explore new driving modes, improve environmental adaptability, and enhance the application prospects of micro-nano motors; (2) collective behavior: to study mutual actions and group behaviors of micro-nano motors and explore the application prospects of micro-nano motors as a group; (3) potential applications: to explore potential applications of micro-nano motors in biomedicine, micro-nano sensing, and environmental protection as well as to study the potential applications in micro-nano processing and micro-operation, etc.

The application of metamaterials is one of the emerging orientations of development in the 21st century. Through orderly design of key physical dimensions of a material, it is expected to break the limit of some apparent natural laws and to obtain a micro-nano structure that has characteristics beyond original and ordinary physical characteristics of nature, so thermal visibility, acoustic, and optical wave directional-fixed frequency transmission can be achieved to satisfy some special requirements on material in special circumstances. Metamaterials are generally categorized into electromagnetic, thermal, and acoustic types, but some are categorized as dynamical or mechanical metamaterials based on the object of adjustment. The applications of metamaterials in the oil production and recovery industry offer a new research area to develop oil production technologies and high-end equipment.

2.2.1. Electromagnetic metamaterials

An electromagnetic metamaterial is a kind of composite material or structure having extraordinary electromagnetic characteristics that are not found in natural materials. Electromagnetic metamaterials are based on the law of electromagnetism, and the materials overall have extraordinary characteristics not found in natural materials by adopting a spatial structure designed with much smaller than the characteristic wavelength. To name a few, left-handed materials have negative dielectric constant and magnetic conductivity. Wave-absorbing metamaterials have negative magnetic conductivity and impedance matching with the air. Hyperlenses regulate electric field distribution of outgoing waves based on the phase change of electromagnetic waves.

Potential applications of electromagnetic metamaterials are introduced as follows^: (1) Metamaterial antennas feature high gain and high sensitivity and are capable of capturing weak electromagnetic signal changes; they can be widely applied to high-gain antenna, small antenna, radar and leaky-wave antenna wide-angle scanning^[24]. (2) Electromagnetic wave stealth, the resonance with the electromagnetic wave of a specific frequency band is achieved by structure design to consume the energy of electromagnetic wave and thus realizing the stealth of electromagnetic wave^[25]. (3) Wave-absorbing material realizes the match between metamaterial impedance and medium impedance of the environment (e.g. the air) through a proper structural design to minimize the reflection and transmission of the incident wave, to enhance the absorption rate and thus ultimately absorb the electromagnetic wave^{[26, 27]}. (4) Adjustable electromagnetic energy accumulation combines metamaterials with computer technology to obtain programmable electromagnetic metamaterial for the purpose of realizing demand-based adjustment of microwave energy. It is expected to be applied in terahertz radiation imaging, underground mineral detection, radar apparatus design, etc. in the future^[28]. (5) Anomalous reflecting surface is unequalizing the incident angle and the reflecting angle through a proper structural design. Relevant metamaterial optical apparatus are expected to be applied to solar energy receiving devices^[29]. (6) super-resolution imaging hyperlenses enable the metamaterial to break the diffraction limit for focusing and realize hyperlenses of the visible light band through proper structural design, which are anticipated to be widely applied in varied imaging fields^{[30, 31]}. (7) Holographic imaging metasurfaces enable the metamaterial to correct the defects of traditional holographic photos, including the impossibility of dynamic display, low resolution, etc. It is hopefully to be used to naked-eye 3D imaging and whole-system dynamic display^{[32, 33]}.

2.2.2. Thermal metamaterial

Thermal metamaterial is a kind of composite material or structure having extraordinary thermal characteristics that are not found in natural materials^{[34, 35]}. Thermal metamaterials are designed based on Pendry’ s theory on transform optics and the principle that thermal physical laws show form invariance in different coordinate spaces. The flux of thermal fields moves forward along a designed route by warping the space to design a thermal structure having non-uniform thermal conductivity coefficients and then to manufacture a functional part.

Potential applications of thermal metamaterials are introduced as below. (1) For thermal invisible cloak^{[36, 37, 38, 39, 40, 41]}, the researcher extends the concept of the optical invisible cloak to the passive steady-state thermal area as well as puts forward the design principle and a theoretical model for thermal invisible cloak. The research achievements will be widely applied to thermal isolation materials and structures. (2) As far as the thermal accumulator is concerned^{[40, 41, 42, 43]}, the researcher recommends a dual-function accumulator that accumulates electric and thermal flows simultaneously without disturbing the external field distribution. The research achievements will be widely applied in thermal flow adjustment, chip cooling, etc.

2.2.3. Acoustic metamaterial

Similarly, acoustic metamaterial is a kind of composite material or structure having extraordinary acoustic characteristics that are not found in natural materials. Acoustic metamaterials are designed based on the principle that acoustic waves transmit in media to achieve the extraordinary adjustment of acoustic waves by designing a special structure for a specific acoustic application.

Potential applications of acoustic metamaterials are: (1) Vibration damping and noise reduction: acoustic metamaterials with negative equivalent mass or equivalent elastic modulus can isolate acoustic waves of a specific frequency band to absorb the shock and reduce noise; and they are applicable to both military and civil fields. (2) Acoustic stealth, acoustic waves can be diffracted through coordinate alternation to realize acoustic stealth, which is expected to play a vital role in anti-detection of submarines and other underwater weapons and equipment. (3) Super-resolution imaging, evanescent waves can be converted into transmission waves through acoustic structural design to break the diffraction limit and achieve super-resolution imaging, this feature is applicable to medical acoustic imaging, and material defect detection, etc. (4) Acoustic wave transmission path adjustment, the acoustic wave transmission path, including extraordinary refraction, extraordinary reflection, acoustic wave focusing, conversion of transmission waves to evanescent waves, etc., can be adjusted as we wish through structural design of the acoustic metasurface, this characteristic is applicable to neo-type acoustic element development, etc. (5) One-sided acoustic transmission, one-sided flow of acoustic energy can be realized through acoustic structural design, which is applicable to the manufacture of acoustic diodes and one-sided acoustic glass, etc.^{[44, 45, 46, 47, 48]}

2.3.1. Traditional manufacture

The demands for high intelligence and informatization of biomedical and energy equipment technologies drive the development of core elements onto the paths of miniaturization, high precision, high reliability and functional integration. For instance, electromagnetic/acoustic/thermal stealth metamaterials, micro-nano robots, micro-nano sensors and executors, etc., are on the frontier of international academic circles and areas of intense research interest. They are also the strategic technologies highly valued by the world. However, the high-extent integration of functions, complexity of structure, 3D trends, and miniaturization also result in new challenges to micro-nano manufacture. Traditional micro-scale and nano- scale manufacturing technologies can only realize 2D or 2.5D characteristic fabrication instead of the manufacture of complex 3D structures at the micro-nano scale, although these technologies can, to some extent, satisfy the need for functional cross-scale micro-nano fabrications. The micro-scale manufacturing technologies include optical lithography, X-ray LIGA technology, bonding technology, while the nano-scale manufacturing technologies include nano-scale impressing, etching, atomic operation, etc. Traditional micro-scale and nano-scale manufacturing technologies are incapable of manufacturing and processing micro-spirals, micro-cavities, micro-trussed, and other characteristic structures. They can hardly satisfy the actual demands for material diversity, production efficiency, and controlled production cost, resulting in the existing problems, such as high manufacturing cost, long period of manufacture, strict conditions for manufacturing environment, complexities in assembly, etc.^{[49, 50, 51]} In summary, traditional manufacturing technologies are facing bottlenecks in high and new technology development regarding the manufacture of complex 3D structures in a cross-scale manner. There is an urgent need to explore new manufacturing methods to process and manufacture complex 3D micro-nano structures.

2.3.2. An emerging method for manufacturing micro-nano structures— 3D printing

3D printing, the mostly developed advanced manufacturing technology in recent years, is also called AM (Additive Manufacturing), RP (Rapid Prototyping), LM (Layered Manufacturing) and SFF (Solid Free-form Fabrication)^[49]. Different from traditional material removal technologies, 3D printing is characterized by layer-by-layer overlapping of materials based on a digital model to realize the “ bottom to top” material increase in manufacturing. 3D printing has shown incomparable technical advantages in fabricating complex 3D or composite (multi-materials) structures and the ones having high depth-to-width ratio. Besides, it is also widely used due to its low cost, high efficiency, diversified materials, and the direct forming mode, showing a huge potential of applications in aerospace, biomedicine, flexible manufacture metal circuits, embedded power supplies, sensing, micro-nano E& M systems, metamaterial, and other advanced areas^[45]. The Times lists 3D printing as the “ top 10 fast-growing industries in the U.S.” The Economist believes that 3D printing will “ drive the realization of the 3rd industrial revolution together with other digital production means.” In 2012, Barack Obama initiated 3D printing as the top 11 critical technologies for the manufacturing industry of the USA. In 2013, McKinsey & Company issued the disruptive technologies theory, i.e., advances that will transform life, business and global economy, in which 3D printing was ranked the 9th. In 2015, the State Council issued the Made in China 2025, which specifically stated that more support would be provided for 3D printing and other cutting-edge technologies and equipment^{[49, 50, 51]}.

Recently, 3D printing has experienced rapid development. A series of mature technologies have been derived, including stereolithography, TPP (two-photon polymerization) laser direct writing, FDM (fused deposition modeling), SLS (selective laser sintering), and powder bed and inkjet head 3D printing, etc. Meanwhile, the focus of research on 3D printing has shifted from manufacturing principles, processes, and equipment to 3D printing application technologies.

2.3.3. Typical applications of micro-nano 3D printing

The manufacture of complex micro-nano structures can be realized by photocuring, multi-material pool photocuring, TPP direct writing, and other 3D printing technologies.

(1) Metamaterials. The fabrication of extra-hard and extra- light metamaterials via 3D printing based on multiple overlapping of single curing layers and physical gas sedimentation technology can achieve a minimum load-bearing capacity that is 160 000 times larger than the dead weight of the material^[52]. Via 3D printing, the manufacture of metamaterials having special performance such as compressed reverse^[53] is highly possible in aerospace, rail transit, and the manufacture of high-end equipment.

(2) The manufacture of micro-scale mechanical parts. Making micro-scale mechanical parts with complex 3D structures or integrated micromechanical driving parts by 3D printing may play an important role in the field of micro-nano machinery manufacture^[54].

(3) Bio-truss. Cell culture frameworks, customized degradable intravascular stents and other biofunctional elements made by 3D printing would have huge application potential in the fields of bioscience and customized medical care industry^[55].

(4) Integrated electromechanical parts. Fabricating structural frameworks of E& M elements by photocuring 3D printing, manufacturing metal circuits, embedded power supplies, and processors etc. to realize integrated manufacture of complex 3D electromechanical parts^[56] would play an important role in the fields of robot and high-end E& M equipment, etc.

(5) Micro-nano robot. 3D printing can also be used to make micro-nano robots with complex 3D structures at micro-nano scale in an integrated manner^[57], which is of great importance to promote the development of micro-nano robot technology and to introduce relevant technologies into the areas of precision medicine and intelligent equipment, etc.

(6) New types of optical devices. Photonic crystals, optical lenses, metamaterial super-resolution lens, and other neo-type optical elements can be made by 3D structure integrated manufacture to overcome the deficiency of traditional manufacturing technologies^[58], such as lack of structural complexity, scale-crossing ability, low efficiency, and lack of difference in individual structures. 3D printing thus shows great promise in promoting the development of new technologies in the areas of optics and imaging, etc.

(7) Neo-type sensors and drivers made by 4D printing. The intention is to allow 3D printed structures to be adaptive to the ambient environment or time via integrated structure and material orderly design. Based on 2D printing of multi-material and composite materials or 3D printing of sensing elements with complex 3D structures^[59], 4D printing is to realize the construction of 4D structure^{[60, 61]} and to subsequently develop neo-type sensors and drivers. This technology is of critical value in the areas of aerospace, military and national defense, biomedicine, and high-end equipment, etc.

Engineering technologies and equipment in oil production are essential during the entire life cycle of oil/gas field exploration, covering several areas from well completion to injection, recovery, lifting, well workover, and surface engineering. Therefore, they are considered to be the key to efficient development of oil and gas. To eliminate the gap between supply and demand of resources in China, one of the critical paths is to comprehensively enhance and improve the engineering technologies and equipment in oil production. Here micro-nano structures, metamaterials, and biomimetic engineering technologies are expected to upgrade key technical equipment and downhole instruments, as well as to greatly reduce energy consumption and development and management costs, making a good reserve of new technologies for sustainable development of the petroleum industry.

Equipment surface corrosion, friction between fluids and inner walls of pipelines, and the capability of self-cleaning of equipment have become key topics in the fields of oil production, equipment, and pipe string anticorrosion, processing. They are also the key issues that need to be solved immediately. For instance, we can realize biomimetic structuring of the micro-nano composite structure on a metal surface using a direct and simple chemical method, forming lotus leaf-like micro-nano bulges of different sizes^[62]. With some substances having low surface energy, a special wettable metal surface that is hydrophobic, super-hydrophobic or super-oleophobic can be obtained. Such a surface has a static oil-water contact angle greater than 150° and rolling contact angle smaller than 2° . Therefore, we can guarantee oil resistance, wax resistance, acid and alkali corrosion resistance, reduction in flow resistance, and surface self-cleaning. This inspiration effectively improves anticorrosion performance of the surface of oil production equipment pipe strings, which realizes the self- cleaning of metal surfaces and reduces friction between fluids and the inner walls of pipelines^{[63, 64]}. In addition, the optimized organic-inorganic composite doping vario-property method can achieve an anticorrosion coating having a shell-modeled composite structure to make up for the shortage in resistance of organic matter against corrosive media. The mechanical strength of the composite material improves mechanical properties, including friction resistance; materials can be greatly enhanced by the synthesis of the shell-modeled layered buttress. Chemical decoration controls the formation of each layer of substance, ensuring that different anticorrosion capabilities are provided for different substances. Ideal bonding between layers is also ensured to establish an intelligent anticorrosion coating system adaptive to corrosive conditions in different regions. Such a technology featured in providing special wettability of equipment surface is expected to be widely adopted in many areas, including pipeline transmission systems, micro-flow control systems, oil-water separation equipment, hydraulic directional transmission motion, and deep-sea exploration. All of them represent the great potential and broad prospects for application.

Metamaterials can be obtained through the orderly design of a micro-nano structure to realize intelligent control of electromagnetic waves, acoustic waves, etc.; there are vast prospects for application in the field of oil production engineering. Such special characteristics are not found in traditional materials. To be specific, electromagnetic metamaterials are applicable to signal transmission and processing in complex environments as well as in environmental detection. Acoustic metamaterials can be used for vibration isolation and noise reduction for petroleum equipment. Thermal metamaterials are beneficial to thermal isolation as well as improvement of system stability and reliability for core E& M elements of downhole equipment. 3D printing is applied to in situ restoration of equipment and pipelines, rapid manufacture of components, and other traditional areas in the field of oil production equipment. Meanwhile, it is also a key technology for manufacturing micro-nano structures having complex 3D structures, including micro-nano motors, metamaterials, etc. In the future, micro-nano structures are anticipated to be a critical segment to be applied in the petroleum field, providing unprecedented solutions and ideas for solving existing technical bottlenecks.

At present, the water content of existing oil fields in China has generally exceeded 90%. Remaining oil is distributed in a complex manner. It becomes gradually difficult to implement oil displacement by water flooding. The data from inspection wells subject to polymer flooding displacement show that there is still a great amount of remaining movable oil that is highly scattered. In oil reservoirs that are not swept, the porosity of reservoir rocks has a small radius, great tortuosity, rough inner walls, and evident action of capillary force. Neither water flooding nor polymer flooding displacement can effectively sweep such an area, leading to complexities in exploration. In this case, the micro-nano material synthesis and functionalization technology provide good ideas and practicality to develop new technologies for improving the recovery rate in low permeation, ultra-low permeation as well as non-conventional oil and gas exploration.

The micro-nano material manufacturing technology not only stands for a size, but represents a design concept, an abrupt improvement in performance and the intelligent extent of new materials as well. The materials provide better integrated coordination performance if they are assembled into orderly structures of varied sizes. They also offer functional adjustment, including surface decoration available for graded structural materials. It is important to manufacture new micro-nano materials for improving the recovery rate of oil and gas based on nano materials.

A micro-nano motor that is the core part of an oil reservoir nano robot is characterized by low cost, high effectiveness and high biocompatibility. The motor enjoys vast prospects for applications in environmental monitoring, water quality restoration, oil removal, and environmental sensing. Micro-nano motors that are capable of converting chemical, optical, and electric energies into dynamic power or drive force might be adopted to detect oil reservoir and drive crude oil. A micro-nano motor enters the formation along with the flooding water. It senses and records information of oil reservoir and fluids (oil reservoir temperature, pressure, pore form, oil and water distribution, etc.) during the flow through the formation. It also collects the information according to characteristic variations in materials and recovers it when the produced fluid returns. Besides, based on fluid behaviors, a robot moves along with fluids in the remaining oil enrichment area under the action of different physical field drives. Via surface absorption and loaded catalyst, the robot performs in situ absorption and catalytic decomposition in oil reservoir, in addition to the separation of oil from water utilizing the special surface wettability as well as the combination with oil drip efficiently for directional transmission. Therefore, it can achieve the real-time monitoring and effective exploitation of the remaining oil and improvement in exploration of thickened oil and shale oil.

Non-conventional oil and gas reservoirs usually have small pores, complex pore structure, large specific surface area, and diversified types of clay minerals. Fluids, thus, usually show strong micro-flow effects (rarefaction, surface tensioning, low Reynolds number, multi-scale and multi-phase effects, etc.) when moving in narrow and small spaces of these reservoirs. Percolation theories based on the Darcy-Weisbach formula can no longer satisfy the demand for research and development. Therefore, there is an urgent need to develop a physical simulation model with adjustable aperture parameters. The model should be uniform, repeatable, and controllable for the research on percolation patterns in complex micro pore channels in reservoirs. In this sense, 3D printing is the most suitable tool to meet the demand.

By CT, FIB-SEM, and other experimental means, a real core micropore structure can be obtained and a nano-resolution 3D digital core model can be established. Then, we can print out the micro-nano scale core mock-up using a high-precision 3D printer. During the micro-flow simulation experiment on the printed core, we may also repeatedly print the micropore structure to provide standard repeatable samples for the experiment. Therefore, we can perform systematic experimental operations and theoretical analysis. Meanwhile, we can also realize the possibility for micro-flow mechanism research by establishing microscopic core experimental models at different scales via 3D printing. The permeation rate calculation formula under the absorption and diffusion effects can be explored. We can also describe the multi-scale flow behaviors of a specific non-conventional oil and gas reservoir more truthfully. A well testing analysis model and productive capability evaluation model can be set up by considering characteristics of the non-conventional oil and gas reservoir to provide experimental conditions for preparing and adjusting the exploration scheme, thus offering a solid theoretical foundation for interpreting reservoir testing data. In addition, we can use 3D printing and surface decoration technology to control pore wall wettability of the printed core and shield the disturbance of heterogeneous wettability, so that the impact of a single main factor on flow behavior of a fluid in a micro-nano pore structure can be clarified, even the effectiveness of different types of oil displacement agents can be quantitatively evaluated. The complexities in laboratory evaluation of performance of oil displacement agents can be solved. The cost for field experiment can be greatly reduced. The effectiveness and improvement plan for newly developed oil displacement agents can be forecast. Therefore, this would play a vital role in selecting oil displacement agents and to improve recovery efficiency.