Shale oil and gas reservoirs, characterized by well-developed nano/micron-scale pores and fractures and high clay content, present significant wellbore stability challenges. Drilling fluid invasion can induce clay hydration and swelling, thereby weakening the rock mechanical properties. Concurrently, stress discontinuities alter the distribution of the stress field around the wellbore and cause stress concentration. Therefore, drilling into the formation instantaneously perturbs the in-situ stress, pore pressure, and fluid chemical potential, leading to a complex (fluid-solid-chemical) multi-physics coupling effect that exacerbates wellbore instability in shale formations ^[10]. Additionally, the inadequate lubricity of water-based drilling fluids introduces high friction and torque in long horizontal sections, increasing risks such as drag and stuck pipe. Consequently, optimizing the sealing, inhibition, and lubricating properties of water-based drilling fluids is critical for developing high-performance systems for shale oil and gas.

The well-developed network of nano/micro-scale pores and fractures in shale formations facilitates the rapid invasion of drilling fluid into the formation under the combined effects of differential pressure and capillary force. This phenomenon significantly exacerbates wellbore instability, leading to frequent occurrences of borehole collapse. Consequently, the development of high- performance plugging materials that can effectively mitigate drilling fluid invasion and prevent pressure transmission is of paramount importance for enhancing wellbore stability in shale oil and gas formations. Currently, plugging materials for water-based drilling fluids in shale oil and gas applications primarily encompass organic, inorganic, and organic-inorganic composite nano/micro-scale materials ^[11-13]. Organic nano/micro- scale plugging materials exhibit superior deformability, enabling them to undergo adaptive shape changes upon entering formation pores and microfractures. This mechanism ensures highly efficient plugging of the complex pore-fracture network in shale formations. Xie et al. ^[11] synthesized a spherical nano-plugging agent via emulsion polymerization. This agent effectively packed into the nano/micro-scale pores of the filter cake, yielding a smooth and compact surface and achieving a pore-plugging efficiency of 58.33%. Lei et al. ^[14] developed a self-crosslinking latex plugging agent. The agent utilizes a blend of particle sizes to physically fill the pore space in shale, while at elevated temperatures, the particles crosslink with each other, significantly reinforcing the compactness of the plugging layer. However, organic nano/micro-scale plugging agents suffer from poor thermal stability, often undergoing issues such as thermal softening, deformation, and decomposition at elevated temperatures. In contrast, inorganic plugging materials like SiO₂ and TiO₂ exhibit high rigidity and superior compressive strength, yet suffer from a strong tendency to agglomerate and poor dispersibility. Ahmed et al. ^[15] developed a SiO₂/g-C₃N₄ composite inorganic nanomaterial with excellent high-temperature dispersibility and achieved a plugging efficiency of 87.2% against a ceramic sand disk. Organic-inorganic nanocomposites have become the leading research focus in plugging agent development due to their unique capability to concurrently possess both rigidity and toughness. Lei et al. ^[16] designed a core-shell nano-plugging agent with a nano-SiO₂ core and a rosin-derived hydrophobic resin shell. This agent achieved a differential pressure of 24 MPa across the treated core, significantly enhancing its pressure-bearing and sealing capacity. Saleh et al. ^[17] synthesized a lamellar-structured plugging agent by grafting polyacrylic acid and melamine onto graphene nanosheets. This structured material effectively plugged shale pores, thereby preventing direct contact between the shale formation and the drilling fluid. Gao ^[18] developed a high-performance water-based drilling fluid system centered around a deformable micro-nano polymeric plugging agent. This system effectively mitigated the weakening of shale mechanical properties, thereby enabling the safe and efficient drilling of long horizontal sections. Researchers from Shaanxi Yanchang Petroleum (Group) Co., Ltd. developed a high-plugging- performance water-based drilling fluid system, designated HGW-1, with a density range of 1.10-1.30 g/cm³. The system was successfully deployed in a continental shale oil horizontal well in southern Yan'an, where it effectively resolved wellbore instability issues across the test section commencing at a measured depth of 1 505 m within the kick-off section corresponding to the inclination of 39.4°, to a measured total depth of 3 003 m ^[19].

Shale formations are typically rich in clay minerals, making them highly susceptible to hydration and swelling upon water contact. This frequently leads to downhole complexities such as borehole collapse, wellbore shrinking, and stuck pipe. Therefore, enhancing the inhibition of drilling fluids is critical to mitigating shale hydration and swelling, thereby maintaining wellbore stability. Montmorillonite, the predominant clay mineral in shale formations, is identified as the primary driver of shale hydration and swelling due to its large interlayer spacing, weak bonding forces, and pronounced swelling tendency ^[20]. Inorganic salts such as KCl and NaCl are common shale inhibitors that effectively suppress smectite hydration and swelling through cation exchange and osmotic pressure mechanisms. Shi et al. ^[21] demonstrated an inverse correlation between shale swelling extent and KCl concentration. While high concentrations of metal cations effectively inhibit shale expansion, they simultaneously induce bentonite flocculation and cause coiling of functional treatment agent molecular chains, consequently compromising drilling fluid performance. Organic salt inhibitors effectively inhibit shale hydration and swelling, and also significantly reduce drilling fluid activity. Consequently, water-based drilling fluid systems formulated with these salts often exhibit superior inhibition and thermal stability. Shaanxi Yanchang Petroleum (Group) Co., Ltd. developed a low-activity water-based drilling fluid system, designated PSW-2, using potassium formate as the primary inhibitor. The system demonstrated exceptional anti-collapse performance, evidenced by a shale rolling recovery rate exceeding 95% and a linear swelling rate of less than 1.38% ^[22]. Wang et al. ^[23] found that saikosaponins adsorb onto shale surfaces via hydrogen bonding to form a hydrophobic film, thereby inhibiting surface hydration. Separately, organic amine inhibitors suppress osmotic hydration by inserting into bentonite interlayers, where they reduce electrostatic repulsion and decrease interlayer spacing ^[24]. However, most organic amine inhibitors are prone to thermal degradation at elevated temperatures, resulting in a significant loss of inhibitory efficacy. This limitation restricts their application in deep and ultra-deep shale oil and gas development. Aramco developed MSil-N, a hybrid organic-inorganic layered nanomaterial inhibitor, which forms a robust and compact inorganic barrier on shale surfaces. This coating effectively suppresses both surface and osmotic hydration mechanisms ^[25].

Shale oil and gas horizontal sections are characterized by long lateral lengths, high inclination angles, and large drill string-to-formation contact area. These factors collectively contribute to elevated friction and torque during drilling, which frequently leads to downhole complexities such as drag and stuck pipe ^[26]. Consequently, enhancing the lubricity of water-based drilling fluids is essential for ensuring the safe and efficient drilling of long horizontal sections. The number and binding strength of adsorptive functional groups on lubricant molecules directly govern their lubricity performance. Inspired by the natural lubrication of earthworm mucus that reduces frictional resistance, Jiang et al. ^[27] employed a graft-through polymerization approach to incorporate adhesive functional groups into long-chain molecules. This molecular design enhances the lubricant's adsorption performance on both drill string and wellbore surfaces, achieving significant friction reduction. Sinopec Jianghan Petroleum Engineering Co., Ltd., in collaboration with Yangtze University, has developed an ultra-low friction water-based drilling fluid system. This system achieves a friction coefficient as low as 0.051 and increases the rate of penetration by 24.7% compared to oil-based systems. Deployed in Well Fuling X4, it set a new Chinese record for water-based drilling fluid operations in shale gas horizontal wells by drilling a 2 235-meter horizontal section, demonstrating the complete implementation of water-based fluid operations in the Fuling shale gas field. Lubricating films relying solely on physical adsorption face challenges of shear-induced desorption in the annulus, leading to significantly reduced long-term stability. In contrast, hydroxyl groups in alcohol and phenolic lubricants can form covalent coordination bonds with metal surfaces. Yang et al. ^[28] developed a bio-inspired lubricant that utilizes catechol groups to form bidentate coordination bonds with metal surfaces. This mechanism creates a robust, densely-packed lubricating film, achieving a friction coefficient as low as 0.06. Chen et al. ^[29] developed a high-efficiency lubricant, RNH-100, through the synergistic combination of organic polyols, high-temperature stabilizers, and polar group-containing compounds. This formulation enabled the creation of an ultra-low friction water-based drilling fluid system with a friction coefficient of 0.066. Field application in Well Y-11 in the Sichuan Basin demonstrated consistently low friction, which significantly improved the rate of penetration in the long horizontal section.

Oil-based drilling fluids, which use oil as the continuous phase, provide superior rheological properties, lubricity, and inhibition. Their exceptional stability under high-pressure/ high-temperature conditions makes them the preferred solution for drilling long horizontal sections in shale reservoirs. However, shale formations are prone to borehole collapse, wellbore shrinking, mud invasion and other downhole issues, which significantly compromise drilling efficiency and safety. Wellbore strengthening and selective flocculation technologies have proven effective in enhancing wellbore stability and optimizing drilling fluid performance. Meanwhile, the environmental impact of spent oil-based drilling fluids has become increasingly evident due to their high toxicity, poor biodegradability, and elevated disposal costs, underscoring the need to develop low-oil-water-ratio and synthetic-based drilling fluid alternatives.

During the drilling of long horizontal sections in shale reservoirs, the infiltration of oil-based drilling fluid into micro-fractures and pores alters the pore pressure distribution, and reduces both the rock strength and cementing strength, thereby inducing wellbore instability. Effective plugging of nano/micro-scale pores to reduce oil-based drilling fluid invasion and retard pressure transmission is critical for enhancing wellbore stability. Geng et al. ^[30] synthesized a low-surface-energy nano- plugging agent through copolymerization of 1-vinylimidazole and styrene. This agent increased the contact angle of oil on shale surfaces from 12° to 61°, reducing spontaneous imbibition by 52.7%. Core immersion tests confirmed its effectiveness in mitigating the weakening effect of oil-based drilling fluids on rock mechanical properties. To further suppress oil-based drilling fluid imbibition in shale, Du et al. ^[31] developed fluorosilane-modified hollow nano-silica particles. This material reduced the surface energy of oil-based filter cakes from 34.99 mJ/m² to 8.17 mJ/m² while increasing shale surface roughness from 0.26 μm to 2.39 μm. These modifications transformed the shale wettability from amphiphilic to dual-phobic, converting capillary attraction forces into capillary barriers and effectively mitigating the degradation of shale mechanical properties by oil-based drilling fluids. However, nano-plugging agents provide suboptimal performance in sealing micro-fractures due to their limited dimensions. To address this limitation, Bai et al. ^[32] developed lamellar two-dimensional graphene plugging agents with strong adsorption capacity. With an average lateral size of 2.16 μm, these materials achieved highly efficient sealing of shale micro-fractures. To address wellbore collapse and stuck pipe challenges in Longmaxi Formation shale with long horizontal sections, Sinopec Chongqing Fuling Shale Gas Exploration and Development Co., Ltd. implemented a synergistic plugging strategy using nano-graphene and multi-grade rigid micro-particles. This approach significantly reduced tripping obstruction time, shortened the average drilling cycle by 47.8%, and achieved a notably low average hole enlargement rate of just 4.07% in the horizontal section ^[33]. Conventional single-type plugging agents typically have a narrow particle size distribution, resulting in poor efficiency for multi-scale pores and fractures. To overcome this limitation, Zhao et al. ^[34] synthesized micro-nano polymer microsphere plugging agents with a wide particle size distribution via inverse emulsion polymerization. The resulting microspheres exhibited D₁₀ and D₉₀ values of 0.57 μm and 5.07 μm, respectively, achieving highly efficient sealing of multi-scale pores and fractures.

The waste centrifugate from oil-based drilling fluid for shale oil and gas operations contains high concentrations of low-quality solids characterized by fine particle size and strong oil-wet properties. Such solids cannot be effectively removed by conventional solid control equipment like shale shakers and centrifuges. The resulting paste-like centrifugate exhibits excessive viscosity, preventing its reuse in slurry preparation. This leads to substantially elevated transportation and disposal costs, along with significant environmental risks. Currently, flocculants are primarily employed to remove fine, low- quality solids, thereby enhancing the reuse rate of oil- based drilling fluids. Wang et al. ^[35] developed an oil- soluble flocculant through copolymerization of lipophilic and cationic monomers. This agent achieves selective flocculation by electrostatically adsorbing onto undesirable solid surfaces. Field testing at the Jimsar shale oil field demonstrated that this flocculant significantly improves the reuse rate of oil-based drilling fluids. Jing et al. ^[36] discovered that the highly oil-soluble compound dihexadecyl dimethyl ammonium chloride (DCDAC) adsorbs onto low-quality solids, competing with wetting agents in oil-based drilling fluids. Through interparticle entanglement, DCDAC effectively aggregates these low- quality solids, achieving a removal efficiency of 23.51%. Liu et al. ^[37] synthesized a hyperbranched polyamide flocculant through Michael addition and amidation reactions. The tertiary and secondary amine groups within the molecular structure serve as hydrogen-bonding donor units, enabling the flocculant to form three-dimensional network structures with solid particles. At a dosage of 0.1%, this flocculant achieved 80.6% removal efficiency for low-quality solids in oil-based drilling fluids. These flocculants function by treating contaminated oil-based drilling fluids to achieve regeneration. In contrast, Lyu et al. ^[38] developed an oil-based drilling fluid system capable of self-removing nano/micro-scale low-quality solids. This system enables real-time elimination of fine low-quality solids during drilling operations, significantly enhancing the drilling fluid reuse rate.

Due to operational safety and technical constraints, current oil-based drilling fluids for shale operations typically maintain high oil-water ratios, generally ranging between 80:20 and 90:10. Under high-density conditions, these ratios can even reach 95:5 or transition to all-oil systems. This results in elevated overall costs, compromised performance stability, and increased complexity in field maintenance and operational procedures. Development of innovative key additives for oil-based drilling fluids and low-oil-water-ratio drilling fluid systems can help to significantly reduce the overall costs relating to oil-based drilling fluid systems. Han ^[39] developed a novel multi-active-site emulsifier that enhances adsorption strength at the water/oil interface. This innovation enabled the formulation of low-ratio oil-based drilling fluid systems with oil-water ratios between 60:40 and 70:30. The formulated systems maintained satisfactory rheological properties and emulsion stability after undergoing 16-h thermal aging at 150 °C. Gao ^[40] developed a high-performance emulsifier from olefinic acids, organic amines, and alcohol ethers, exhibiting exceptional emulsification efficiency and stability. This emulsifier enabled the formulation of a high-density oil-based drilling fluid (2.0-2.2 g/cm³) with a reduced oil-water ratio of 65:35. The fluid demonstrated temperature resistance up to 200 °C and an emulsion-breaking voltage exceeding 800 V. It has been successfully deployed in 9 shale oil wells in the Liaohe Oilfield, significantly reducing overall drilling fluid costs. Ofei et al. ^[41] highlighted that low oil-water ratio oil-based drilling fluids in the North Sea region exhibit higher yield stress and superior suspension stability. CNPC Daqing Drilling & Exploration Engineering Co., Ltd. developed a drilling fluid system with a low oil-water ratio (75:35), which successfully resolved challenges such as oil-water separation and loss of rheological control under low oil content conditions. This system enabled Well GY2-Q9-H85 to achieve an average daily footage of 319.86 m, providing new momentum for the economic development of Gulong shale oil.

Synthetic-based drilling fluids (SBDFs), which utilize synthetic organic compounds (e.g., olefins, esters, ethers) as the continuous phase, show promise as ideal alternatives to conventional oil-based systems due to their superior biodegradability and lower waste disposal costs ^[42]. Gas-to-liquid (GTL) base fluids are synthesized from natural gas through the Fischer-Tropsch process, producing C₁₂-C₂₁ alkanes characterized by low aromatic content and low viscosity ^[43]. A synthetic-based drilling fluid system using GTL as the continuous phase was implemented in the third spud section of a pilot well in the Bonan shale oil block of the Shengli Oilfield. This system achieved an average hole enlargement rate below 2.5%, demonstrated effective wellbore stability, and significantly increased the rate of penetration ^[44]. Furthermore, biomass-derived synthetic base fluids, produced from vegetable oils employing processes such as esterification and transesterification, exhibit considerable potential for use in shale oil and gas drilling ^[42,45]. Fan et al. ^[46] developed a mixture consisting primarily of n-alkanes through esterification and hydrolysis of vegetable oils to eliminate unsaturated double bonds. Using this as the continuous phase, they constructed a biosynthetic-based drilling fluid with an 80:20 oil-water ratio. The system's toxic substance content and acute toxicity comply with industry standards. It has been successfully deployed in three wells at the HA Platform in the Changning shale gas field, meeting all operational requirements for horizontal shale gas drilling, logging, and casing running. Shengli Oilfield developed a synthetic-based drilling fluid system that provides both wellbore stabilization and reservoir protection to address the challenge of high clay content in shale oil reservoirs. This system enabled Well FY1-6-HF to set two records: deepest shale oil well (7 025 m) and longest horizontal section (3 055 m). This achievement supports the green and efficient development of shale oil in the Jiyang Depression.

The actual downhole temperatures encountered in deep to ultra-deep shale oil and gas horizontal wells pose significant risks of thermal degradation to oil-based drilling fluids and potential failure of downhole tool electronics. These challenges have prompted the development of specialized cooling technologies for oil-based drilling fluids. Current cooling technologies for oil-based drilling fluids are primarily categorized into two types: ground cooling devices and downhole phase change material systems. Ground cooling equipment represents the more widely adopted approach in field operations today. For instance, a ground cooling device employing combined air and water cooling was implemented in the Niuye-1 Block of the Shengli Shale Oil Field. After its deployment, the average circulating temperature of the drilling fluid was reduced by 10-15 °C, effectively mitigating the risk of tool damage under downhole high- temperature conditions ^[47]. In the southern Sichuan Basin, managing downhole temperature presented a major challenge for achieving one-trip drilling in deep shale gas operations. To address this, Yang et al. ^[48] independently developed a skid-mounted dual-plate heat exchange system for drilling fluid cooling. With a flow rate capacity of 30 L/s, this ground cooling system effectively reduced the temperature of pumped drilling fluid, lowered the bottomhole circulation temperature by more than 8 °C. Phase-change materials (PCMs), which store and release substantial latent heat through phase transitions within a specific temperature range, are critical components in drilling fluid cooling technologies ^[49]. Additionally, owing to the reversible nature of the phase-change process, microencapsulated PCMs can be cycled repeatedly, demonstrating significant application potential. Zhang et al. ^[50] fabricated ET@TiO₂ phase-change microcapsules using erythritol as the core material and tetrabutyl titanate as the shell precursor via a sol-gel method. Adding 10% of these microcapsules to a white oil-based drilling fluid achieved a temperature reduction of 11.7 °C, demonstrating their effective cooling capacity. Notably, the thermal conductivity of PCMs can be improved by doping with materials such as nano-graphite ^[51] and hexagonal boron nitride ^[52]. Su et al. ^[51] fabricated phase-change microcapsules (MEPN) using erythritol doped with nano-graphite as the core and polysulfone as the shell. The addition of 5% MEPN during drilling operations reduced downhole circulation temperature by 6-7 °C. Currently, this cooling technology based on phase-change microcapsules remains at the experimental stage and has not yet achieved large-scale industrial application ^[48].

Reservoir protection is a critical component in oil and gas exploration and development, as it directly determines well productivity and economic viability. Compared with conventional reservoirs, shale oil and gas formations present greater risks of severe formation damage due to complex geological conditions and intricate operational procedures. The primary damage mechanisms include: (1) solid-phase invasion and liquid-phase intrusion from drilling fluids, leading to particulate blockage damage; (2) aqueous phase trapping; (3) fluid sensitivity damage; and (4) stress sensitivity damage resulting from the inherently developed natural pore systems in shale reservoirs. Once formation damage occurs, it is difficult to completely eliminate through stimulation treatments, necessitating a strategy that treats prevention as the primary approach and remediation as a supplementary measure. Reservoir protection drilling fluids mitigate formation damage during drilling by synergistically inhibiting, plugging, controlling filtration, and balancing pressure, thereby preserving the reservoir's original permeability and productivity. Furthermore, reduced drilling-induced damage decreases the initiation pressure required for fracturing operations, enhances fracturing fluid coverage within the formation, and improves both stimulation effectiveness and ultimate recovery. Current reservoir protection drilling fluid technologies primarily include clay-free, under-balanced, and interface-modified drilling fluids.

Lost circulation control is critical for ensuring the safety and efficiency of drilling operations. Compared with conventional oil and gas reservoirs, shale reservoirs present greater challenges due to well-developed beddings and natural fractures, rapid lithological changes, and the presence of faults. When combined with active formation fluids and poor lost circulation material retention, these conditions readily induce natural fracture leakage and permeability leakage. Current lost circulation control technologies primarily include bridging, gel-based, responsive, and composite systems.

High-performance water-based drilling fluids are crucial for the safe and efficient drilling of shale formations, yet they face significant challenges in field applications. (1) Shale formations exhibit high clay mineral content, which readily undergoes hydration and swelling upon contact with water-based drilling fluids, leading to a reduction in the mechanical strength of the wellbore rock. The well-developed beddings and micro-fractures in shale reservoirs allow drilling fluids to cause pore pressure elevation and create a wedging effect. As conventional inhibitors often fail to effectively suppress clay hydration, and plugging materials struggle to form tight, durable sealing layers, wellbore collapse frequently occurs, resulting in extended drilling cycles. (2) During the drilling of long horizontal wells in shale formations, significant friction drag between the drill string and wellbore presents considerable challenges. Existing lubricants in water-based drilling fluids often perform inadequately under elevated temperatures, high pressures, and saline conditions, failing to effectively reduce friction. This limitation complicates directional drilling and casing running operations. (3) Shale gas reservoirs in the Sichuan-Chongqing region, such as the Permian Longtan and Silurian Shiniulan formations, contain substantial inherent carbon dioxide, creating risks of CO₂ gas channeling during development. When CO₂ invades water-based drilling fluids, it causes sharp increases in both viscosity and yield point. Current water-based systems demonstrate poor resistance to CO₂ contamination, typically requiring extensive fluid replacement with new mud that substantially increases drilling costs.

While oil-based drilling fluids effectively mitigate shale hydration and reduce wellbore instability risks, they face multiple challenges in shale oil and gas applications due to increasingly stringent environmental regulations, growing cost-control demands, and the continuous increase in drilling difficulty under complex geological conditions. (1) Oil-based drilling fluids use diesel or white oil as their base fluid. During drilling operations, fluid leaks, spills, and the discharge of waste materials can cause severe contamination of soil, water, and air, leading to significant ecological damage. In China, the oil-water ratios of conventional oil-based drilling fluids typically range from 65:35 to 90:10, which remains considerably higher than the low-ratio reversible emulsion drilling fluid system RIMO-FAZE developed by Schlumberger ^[7]. (2) The economic viability of oil-based drilling fluids is constrained by the substantial cost proportion of base oils, high additive costs, and persistently high maintenance and treatment expenses, which consequently affects their overall cost- effectiveness. (3) Under high-temperature, high-pressure, and high-salinity conditions, base oils and additives are prone to complex side reactions such as oxidation and degradation. These reactions lead to performance degradation of oil-based drilling fluids, manifested as significant fluctuations in viscosity, unstable gel strength, and ineffective cuttings transport, thereby increasing drilling risks. (4) PCMs absorb thermal energy through core material phase transitions, effectively reducing drilling fluid temperature and mitigating downhole tool damage in high-temperature environments. However, due to complex manufacturing processes and high costs, phase-change microcapsule-based downhole cooling while drilling technology remains at the experimental stage and has not yet achieved large- scale industrial implementation.

Although clay-free, under-balanced, and interface- modified drilling fluid technologies have achieved notable research advances and field applications, reservoir protection drilling fluid technology for shale oil and gas still faces considerable challenges and unresolved issues. (1) Current formation damage evaluation techniques primarily rely on laboratory core flow tests. However, parameters such as temperature, pressure, and flow rate in these experiments often deviate from actual downhole conditions, leading to significant discrepancies between laboratory-based damage assessments and real-world reservoir damage. (2) Clay-free drilling fluid systems often experience deterioration in rheological properties under high-temperature, high-pressure conditions encountered in long horizontal sections of deep shale formations. This degradation leads to ineffective cuttings transport, resulting in the accumulation of drill cuttings and the formation of cuttings beds within the wellbore. (3) The application of under-balanced drilling fluids necessitates precise pressure control equipment to maintain wellbore pressure below formation pressure, substantially increasing drilling costs. This under-balanced condition prevents filter cake formation on the wellbore wall. Consequently, if wellbore pressure unexpectedly exceeds formation pressure during drilling operations due to circumstances such as encountering lower than anticipated formation pressure or experiencing operational pressure fluctuations, the absence of a filter cake leaves the formation vulnerable to drilling fluid invasion and resultant reservoir damage. (4) Interface-modified drilling fluid technology reduces aqueous phase trapping damage by creating hydrophobic surfaces on rock formations. However, the modified films formed on rock surfaces may detach or degrade under downhole conditions of high temperature, high pressure, and formation fluid flushing. Their long-term stability and effectiveness require further validation.

Bridging, gel-based, responsive, and composite lost circulation control technologies have achieved notable progress and results in managing lost circulation in shale oil and gas reservoirs, yet they still face multiple technical challenges. (1) Complex reservoir geological conditions hinder real-time acquisition of loss zone parameters with existing downhole monitoring technologies, resulting in delayed identification of loss severity and location. This limitation prevents current lost circulation control technologies from rapid and precise treatment, potentially exacerbating formation damage. (2) Shale oil and gas reservoirs exhibit strong heterogeneity and contain well-developed micro-fractures, leading to diverse and complex loss scenarios. However, different lost circulation control technologies are only effective for specific loss types. These technologies require tailored adjustments to parameters such as particle size distribution and material shape according to the actual downhole conditions, thereby increasing the difficulty of effective treatment. (3) Deep shale oil and gas reservoirs present harsh high-temperature, high-pressure conditions where most LCMs experience performance degradation. This reduces sealing effectiveness and increases operational complexity, thereby resulting in low first-attempt success rates.

(1) Enhance the synergistic effects between system inhibition and plugging. To address wellbore spalling and collapse caused by clay mineral hydration and swelling, future water-based drilling fluid technology should focus on the synergy between nanoscale plugging and chemical inhibition. On one hand, high-performance two-dimensional or three-dimensional nanoscale plugging materials should be developed to achieve precise sealing of micro-fractures in shale reservoirs. On the other hand, hydrophobically modified materials, such as oxysilane- based modifiers, can be added to drilling fluids. These modifiers form dense hydrophobic films on clay surfaces via Si-O-Si bonds, effectively inhibiting the hydration and swelling of shale clay minerals.

(2) Enhance system lubricity to reduce friction. Conduct in-depth microscopic studies on lubrication mechanisms in long horizontal sections of shale reservoirs, thereby elucidating the formation and failure processes of lubricating films. Develop smart responsive lubricants that can autonomously enhance lubrication effectiveness in complex high- temperature, high-pressure environments. Create drilling fluid systems that integrate excellent lubricity with other superior performance characteristics.

(3) Enhance system contamination resistance. To address the deterioration of rheological and filtration properties in water-based drilling fluids caused by CO₂ gas channeling in shale oil and gas reservoirs, it is imperative to improve their resistance to CO₂ contamination. This will enable adaptive self-regulation under CO₂ invasion conditions.

(1) Reduce the system's oil-water ratio. To address the high cost of oil-based drilling fluids, future technological development should focus on decreasing the base oil proportion. This can be achieved by developing novel emulsifiers, such as nanoparticle-enhanced emulsifiers, which utilize the interfacial enhancement effect of nanoparticles to strengthen the oil-water interfacial film. This approach enables water-in-oil emulsions with low oil-water ratios to maintain high emulsion-breaking voltage.

(2) Develop environmentally friendly systems. To address the challenges of high oil content, significant environmental hazards, and complex treatment processes associated with spent oil-based drilling fluids, future technological development must enhance selective flocculation techniques for waste fluid treatment. Concurrently, greater efforts should be directed toward developing biosynthetic-based oil-based drilling fluids. These systems utilize chemically modified biomass oils, such as soybean or palm oil, to produce environmentally friendly base fluids, thereby reducing environmental risks at the source.

(3) Enhance high-temperature emulsion stability. To address the sharp decline in emulsion-breaking voltage and increased viscosity of oil-based drilling fluids under extreme high-temperature conditions, future technological development should prioritize the design of high-temperature resistant emulsifiers. These advanced emulsifiers must maintain interfacial activity at elevated temperatures, ensuring the fluid system retains higher emulsion-breaking voltage and thereby sustains reliable high-temperature stability.

(4) Streamline the manufacturing process of PCMs. To address the complex preparation methods and high costs of existing PCMs, future development of phase-change microcapsules should focus on process optimization by creating low-cost, high-coverage production techniques such as rotational granulation.

(1) Develop a real-time formation damage prediction and diagnostic expert system. Leveraging artificial intelligence algorithms and big data analytics, this system would construct models to identify the causes of formation damage, continuously monitor key parameters such as drilling fluid invasion volume and changes in reservoir permeability, and automatically generate tailored prevention and treatment strategies. This approach enables early detection and intervention of formation damage, thereby minimizing its impact on well productivity.

(2) Enhance research on system rheological control and reservoir compatibility. To address the challenges of difficult rheological control and poor suspension and cuttings transport capacity in clay-free drilling fluids, future efforts should prioritize developing novel, high- temperature resistant, and cost-effective rheology modifiers. This will ensure efficient cuttings transport performance of drilling fluid systems in long horizontal sections.

(3) Under-balanced drilling operations are more complex than conventional overbalanced drilling. Future under-balanced drilling fluid technology should evolve toward intelligent development through the integrated coordination of pressure control equipment, drilling fluid circulation systems, and pressure monitoring devices. This approach enables precise regulation of wellbore pressure, minimizes fluctuations, and prevents the occurrence of overbalanced conditions where wellbore pressure exceeds formation pressure.

(4) Future research on interface-modified drilling fluid technology should prioritize the long-term stability of modified surface films, with particular focus on evaluating their effectiveness under prolonged exposure to high-temperature, high-pressure, and high-shear conditions.

(1) Develop an intelligent lost circulation control expert system. This system would integrate downhole pressure sensors, flow monitoring devices, and logging-while- drilling data, utilizing Internet of Things technology to enable real-time acquisition and transmission of loss parameters (e.g. loss rate, loss volume and loss location), and wellbore pressure changes. It would rapidly identify loss signatures, accurately locate loss zones, promptly send early warnings to the surface control system with recommended treatment plans, and archive operational outcomes in a model training database to enhance prediction accuracy.

(2) Develop smart responsive LCMs. To address the limitation of single-technology approaches in meeting diverse formation requirements, future efforts should prioritize novel responsive LCMs capable of precise adaptation to changing downhole conditions (e.g., temperature, and pressure). These materials would achieve effective sealing through dynamic adjustments in their structure or properties.

(3) Create high-temperature high-pressure resistant LCMs. Solving the challenges of inadequate thermal resistance and unstable sealing barrier formation requires focused development of new materials combining thermal stability with pressure-bearing capacity. These advancements would effectively address seal failure in extreme downhole environments.