High-temperature resistant water-based drilling fluid in China has evolved in three stages: calcium-treated drilling fluid, sulfonated drilling fluid, and polysulfonated drilling fluid. The calcium-treated drilling fluid inhibits clay dispersion by calcium ions and has improved filtration and rheological properties at high temperature. The sulfonated drilling fluid, prepared with sulfonated lignite and sulfonated phenolic resin as additives, can resist high temperature up to 180 °C, but has a poor salt and calcium resistance, and it is mainly applied in deep freshwater drilling ^[7]. The polysulfonated drilling fluid incorporates temperature- and salt-resistant polymers on the basis of sulfonated drilling fluid, with improved rheological and filtration properties, and enhanced temperature resistance to 200 °C or above ^[8]. Polysulfonated drilling fluid has been studied mostly to optimize its performance and improve its applicability. For example, the proportions of polymers with different molecular weights (MWs) and the bentonite content are adjusted to improve the rheological properties of the drilling fluid. The volume of high-MW polymer encapsulating agent is increased or appropriate amount of inorganic salt is added to enhance the inhibitive properties of the drilling fluid. In addition, depending on the characteristics of deep well sections and intervals, the types of additives in the polysulfonated drilling fluid can be adjusted to achieve the optimal drilling results.

Polysulfonated drilling fluid is the popular high-temperature resistant water-based drilling fluid system in China now. It can resist a temperature up to 220 °C in case of salt water and up to 240 °C in case of fresh water ^[9]. Well Songke 2 in the Songliao Basin, NE China, with a well depth (WD) of 7 018 m and a bottomhole temperature of 241 °C, was successfully drilled using the high-temperature resistant potassium polysulfonated and high-temperature resistant polysulfonated drilling fluid systems, setting a new record of drilling at the highest temperature (241 °C) in China then. Well Luntan 1 in the Tarim Oilfield, NE China, with a WD of 8 882 m and a bottomhole temperature of 175 °C, was successfully drilled using the potassium polysulfonated drilling fluid system, creating the deepest well onshore in Asia ^[9]. Table 1 lists the typical applications of deep and ultra-deep high-temperature resistant polysulfonated water-based drilling fluids in China.

Lignosulfonate water-based drilling fluid is the commonly-used high-temperature resistant sulfonated water-based drilling fluid in other countries. It demonstrated a temperature resistance up to 260 °C when it was applied in drilling high-temperature deep wells in the Pontchartrain Lake, Louisiana, the United States ^[9]. Baker Hughes synthesized a series of high-temperature resistant sulfonated high-MW polymers and high-temperature stabilizers by using 2-acrylamide-2-methylpropanesulfonic acid and monomers such as acrylamides and alkyl acrylamides with different long chains, and produced a polysulfonated drilling fluid system by adding inorganic salt or other agents, which has been satisfactorily applied in Well Yacheng 21-1-3 in the South China Sea ^[10].

The research on high-temperature resistant polymer drilling fluid technology started early and has progressed rapidly oversea. Research, development and application of polymer drilling fluids can be traced back to the 1960s ^[1]. Currently, a significant progress has been made in research on high-temperature resistant polymer water-based drilling fluids, and innovative additives are consistently developed to improve the stability of drilling fluid performance in extreme conditions such as high temperature and high salt, providing an important technical support for drilling deep and ultra-deep wells. M-I SWACO developed a polymer drilling fluid system with a temperature resistance of 232 °C and a density of 2.20 g/cm³, which is resistant to high temperature, high salt and pollution, and has little damage to reservoirs ^[11]. Halliburton and Shell jointly developed a high-temperature resistant polymer water-based drilling fluid technology, which is well performed in hole cleaning and can increase the average rate of penetration (ROP) by more than 8 times in drilling horizontal wells in hard sandstone, and this technology has been successfully applied in drilling at a bottomhole temperature of 193.3 °C in Hail Oilfield, the United Arab Emirates ^[12]. M-I SWACO developed a polymer water-based drilling fluid system with a temperature resistance up to 204 °C and a density up to 2.30 g/cm³, which improves greatly in anti-pollution and reservoir protection and maintains a good settling stability under long-term high temperature aging conditions ^[9]. Wandji et al. ^[13] designed a new type of high- density polymer nano-composite material - bentonite hybrid system, which is thermally stable and prepared by incorporating the thermo-thickening/amphoteric vinyl- functionalized modified nano-silica (VMS) as a viscosifier. This system remains a viscosity retention rate over 60% at 220 °C and 20% mass fraction of NaCl, with a low viscosity attenuation. It has been satisfactorily used in drilling under high-salt and high-temperature conditions.

In China, the research on high-temperature resistant polymer water-based drilling fluids has been progressing. The stability of drilling fluids in extreme environments such as high temperature is improved by developing new additives, upgrading fillers, and incorporating modified techniques. Typical high-temperature resistant polymer drilling fluid systems that have been successfully applied in China include non-dispersible low-solid-phase polymer system, biomimetic polymer system, amphiphobic high- efficiency polymer system, and high-temperature resistant high-density polymer saturated brine system ^{[6,14 -15]}. Xuan et al. ^[14-15] developed a series of biomimetic drilling fluid materials, and established the theory and technology of biomimetic polymer drilling fluids, which were applied in horizontal drilling in the Su53 block of Sulige Gasfield, showing remarkable results with an average ROP enhanced by 27% and the drilling fluid cost reduced by 26.4%. In addition, Huang et al. ^[6] designed and developed three types of high-temperature resistant additives for polymer water-based drilling fluids, and established an ultra-high-temperature resistant high-density polymeric saturated brine-based drilling fluid system, which exhibits an excellent plugging and lubricating performance.

The high-temperature resistant oil-based drilling fluids are mainly prepared with various grades of diesel oil and mineral oil as the base oils, together with high-temperature resistant emulsifier, flow pattern regulator, filtrate reducer and weighting materials. Other agents can be added depending on specific geological conditions, in order to ensure the wellbore integrity and deal with lost circulation in reservoirs.

The high-temperature resistant oil-based drilling fluid technology is relatively sophisticated overseas. Owning to variety of base fluids and upgrading of high-temperature resistant additives, this technology has been used widely. M-I SWACO developed an ultra-high- temperature resistant white oil-based drilling fluid by designing the polyfunctional group structure of polyether materials. This drilling fluid was stably performed before and after hot rolling at 300 °C during laboratory experiment, and it has been successfully applied in the Kuqa piedmont block of the Tarim Basin, NW China, Well F7H in western South China Sea, and Well Gulong 1 in Daqing Oilfield ^[16]. Halliburton developed a diesel-based drilling fluid system with fatty acid derivative emulsifier and tall oil filtrate reducer, which has a temperature resistance up to 260 °C and a strong resistance to salt water invasion, and has been applied in drilling in the Kuqa piedmont in the Tarim Basin ^[17].

In China, research on high-temperature resistant oil-based drilling fluids started later, but has evolved quickly in recent years. These efforts have helped breaking the monopoly of foreign oilfield service companies in the high-temperature resistant oil-based drilling fluid technology, and addressing the challenges from the extremely thick salt-gypsum layers, high-pressure saline layers and mud shales in the Kuqa pediment block of Tarim Basin, shale gas blocks in southwestern China, southern margin of Junggar Basin, and Shunbei area of Tarim Basin. Table 2 shows the typical applications of oil-based drilling fluids in ultra-deep wells in China ^[9,18]. Among the applications of high-temperature resistant oil-based drilling fluids in China, Well Guole 3C in the Tarim Oilfield is the deepest horizontal well (WD of 9 396 m), and Well Pengshen 6 in the Sichuan Basin, SW China, is the deepest vertical well (WD of 9 026 m); Well Tatan 1 in southwestern Sichuan Basin records the highest temperature, with a bottomhole temperature of 216 °C upon completion; and Well Letan 1 in the Junggar Basin establishes the highest drilling fluid density (2.68 g/cm³) ^[9]. Wang et al. ^[19] developed a high-density and high-salt resistant oil-based drilling fluid system with a temperature resistance up to 220 °C, a density of 2.60 g/cm³, and a resistance to salt water invasion capacity of 45%, which was applied in the fourth spud-in of Well Keshen 1101 in the Kuqa pediment to effectively avoid the occurrence of spill and loss in the same zone. Qin et al. ^[20] synthesized a primary emulsifier for high-temperature oil-based drilling fluid using tall oil fatty acids and maleic anhydride as main raw materials, and accordingly formulated a high-temperature resistant oil-based drilling fluid system, which was tested successfully in the drilling of shale gas wells on the Wei 204H5 platform in the Weiyuan block of the Sichuan Basin and has been widely promoted and applied. Zhao et al. ^[21] developed a high-temperature resistant and high-density oil-based drilling fluid system, which was successfully applied in the fourth spud-in of Well Dabei 12X in the Kelasu structural belt, Kuqa Depression, Tarim Basin, by providing a solution to drilling in high-temperature and (ultra-) high-pressure salt-gypsum strata.

The synthetic-based drilling fluids adopt a biodegradable synthetic base oil as continuous phase, instead of traditional mineral oil or diesel oil, thus reducing the environmental pollution. The synthetic-based drilling fluids have demonstrated good lubrication, carrying capacity and anti-collapse ^[22], and they can meet the needs of drilling in complex formations through continuous optimization of rheology. These fluids have been successfully applied in several fields, especially offshore fields and complex fields.

Li et al. ^[23] applied the synthetic-based drilling fluid system in the inclined and horizontal sections of Well H6-6 during the third spud-in in the Changning block, enabling a standardized operation mode of environmental protection and safe and fast drilling in pressure-sensitive strata, and thus providing a solid technical foundation for shale gas development in the Changning block. Wang et al. ^[24] developed the SBM-II synthetic-based drilling fluid system by using special fluid-loss agent PF-FLB and gas-made oil PF-SGO. This system has the characteristics of low fluid loss, good rheology, strong inhibitive property, and high stability, and has been successfully applied in Well A4 of Bohai BZ25-1 oilfield.

The temporary-plugging reservoir protection drilling fluids prevent the reservoirs from damage by temporarily plugging the oil and gas layers during drilling to avoid invasion of the solid phase and liquid phase of drilling fluids into the reservoir ^[28]. As to the mechanism of reservoir protection, solid particles in drilling fluids are relied to form a near-wellbore temporary shielding zone with a near-zero permeability under a positive pressure difference. This shielding zone prevents further invasion of the solid phase and filtrate of drilling fluids and cement slurry into the reservoirs and thus avoids pollution to reservoirs.

With efforts in more than half a century, scholars around the world have launched three generations of temporary-plugging reservoir protection drilling fluid technology, i.e. shielding temporary plugging, fine temporary plugging, and physico-chemical membrane temporary plugging, indicating improved reservoir protection effects and significant economic benefits ^{[5,14 -15,29]}. Sun et al. ^[29] illustrated the basic principles and research progress of reservoir protection technologies such as physical particle temporary plugging and chemical film-forming temporary plugging, and proposed the future direction of tight/shale reservoir protection technologies. Jiang et al. ^{[5,14 -15]} introduced bionics into the theory of drilling fluids to protect reservoirs, and developed the bionic temporary-plugging reservoir protection technology of "super-amphiphobic, biofilm and synergistic effect" applicable to various reservoirs, which reduces the damage to reservoirs with different permeabilities and improves the single well production. It has been popularized and applied in China’s major oilfields such as Shengli, Tarim, and Dagang, with remarkable results.

The liquid casing reservoir protection drilling fluid technology is applied by adding special additives into drilling fluids to reduce their density and viscosity, thereby reducing their pressure and friction in reservoirs, so that the reservoirs are protected ^[30]. "Liquid casing" is a special fluid used in drilling, and it forms a protective film on the borehole wall to function as traditional casing does, but work more flexibly and efficiently. Liquid casing generally consists of drilling fluid or completion fluid with specific properties, which can adhere tightly to the borehole wall and form an isolation layer to avoid reservoir damage during drilling and downhole operations.

Existing studies on liquid casing reservoir protection drilling fluid technology primarily focus on the solutions to improve the stability and durability of liquid casing and optimize its protection on reservoirs. Jiang et al. ^[31] developed a novel water-based drilling fluid using super-amphiphobic inhibitor, borehole strengthening agent and bonding lubricant synthesized by bionic technologies, together with other additives. This novel drilling fluid can form liquid casing while drilling to protect the reservoir, and it has been successfully applied in more than 1 000 complex wells in China (incl. shale gas wells in the Zhaotong block of the Sichuan Basin, shale oil wells in the Bohai Bay Basin, tight gas wells in the Sulige block of the Ordos Basin, tight gas wells in the Songliao Basin, tight oil wells in the Junggar Basin, and coalbed methane wells in Shanxi) and Chad. Compared with oil-based drilling fluids and other high-performance water-based drilling fluids used in the same block, this technology reduces the average rate of wellbore collapse by 82.6%, lost circulation by 80.6%, and blockage/sticking by 80.7%, and increases the ROP by 32.8% and the production by more than 1.5 times, suggesting remarkable application results.

The clay-free water-based drilling fluids are prepared with water as a continuous phase and no solid components, and by adding chemical additives to realize good rheological, inhibitive and environmental properties. They protect the reservoirs mainly by reducing solid content, inhibiting hydration, mitigating filtration and invasion, and providing effective lubrication and drag reduction. In foreign studies, clay-free water-based drilling fluid is mainly prepared by adding clay inhibitors and high-molecular polymers into fresh water, such as Halliburton's clay-free water-based drilling fluid system with a temperature resistance up to 180 °C, and M-I SWACO's VeraTherm system with a temperature resistance up to 204 °C ^[32]. In China, the scholars have developed clay-free water-based organic salt drilling fluid, and clay-free water-based weak gel drilling fluid, etc., which are all designed for mitigating reservoir damage and improving drilling efficiency. Specifically, the organic salt drilling fluid combines high-molecular polymer and soluble salt to effectively protect the reservoir; the weak gel drilling fluid can form a weak gel structure to enhance the carrying capacity and borehole stability. Ma et al. ^[33] developed a high-temperature resistant clay-free organic salt reservoir protection drilling fluid system with the temperature resistance up to 200 °C by using the additives such as high-temperature resistant flow pattern regulator, high-temperature resistant colloid protecting agent, polyamine inhibitor and potassium formate weighting material, and the system has been successfully applied in 3 horizontal wells in tight sandstone gas reservoirs in the Mi 38 block of Shenmu gas field, the Ordos Basin. Shi et al. ^[34] developed the cesium formate clay-free drilling fluid system with a temperature resistance up to 220 °C, and a density up to 2.37 g/cm³, which exhibits good rheology and lubrication, and can effectively avoid barite settling in high-density condition, except for its disadvantage of high cost.

The clay-free synthetic-based drilling fluid technology is an important object in the drilling sector. This technology realizes a significant improvement in the stability and environmental property of drilling fluids by optimizing the formula to remove the clay component. In recent years, a significant progress has been made in research on the clay-free synthetic-based drilling fluid technology ^[35-36]. A variety of new clay-free synthetic- based drilling fluid systems with excellent borehole stability, low drag, and performance stability at high temperature have been developed for drilling in complex strata. Wang et al. ^[35] developed a flow pattern regulator for synthetic-based drilling fluids, and prepared a high-temperature resistant and high-density clay-free synthetic-based drilling fluid system with a temperature resistance up to 180 °C and the maximum density of 2.0 g/cm³, which exhibits high demulsification voltage before and after aging, small filtration, and excellent rheological properties, and can meet the requirements of field operation. Zhang et al. ^[36] used the biomass synthetic base fluid LAE-12 to prepare a clay-free synthetic-based drilling fluid with a temperature resistance up to 150 °C and a density of 1.20-2.50 g/cm³, which demonstrated good results in its first field test in Well ZJ204H in the Sichuan Basin.

The bridging material LCC technology provides a powerful solution to the lost circulation in fractured strata. The plugging slurry is formed by mixing bridging materials of different shapes and sizes with drilling fluids at different concentrations, and it bridges, accumulates and fills in fractures to form a plugging layer near the fracture entrance, blocking the transfer of drilling fluid pressure and fluid medium ^[37]. The common bridging materials include walnut shell, calcium carbonate, fiber, and mica flake. However, the particle sizes of these bridging materials poorly match with the sizes of the leaking channels in strata. Due to the influence of gravity settlement and erosion inside fractures, etc., bridging materials are difficult to retain in large fractures with large widths and high vertical extensions, especially in karst caves, so the plugging layers in these fractures are low in pressure-bearing capacity ^[38].

Bridge plugging is a common and effective LCC technique. Nevertheless, the common bridging lost circulation materials (LCMs) have weak high-temperature resistance, poor compatibility of particle sizes with fracture scales, and poor stability of plugging layers formed by particle stacking in fractures, leading to ineffective fracture plugging or high risk of repeated leakage. Thus, these LCMs cannot meet the increasing demand for LCC in ultra-deep drilling. Bao et al. ^[39] optimized the high-temperature resistant bridging formulas for fractures with different apertures by controlling the particle size distribution and concentration of various LCMs, showing a reliable plugging capacity at high temperature and high pressure up to 15 MPa. The fracture plugging layer formed by bridging LCMs becomes more instable in complex environments such as high temperature, high pressure and high in-situ stress, leading to the plugging success rate and performance far behind expectations ^[40-41]. Kang et al. ^[40] established the high-temperature aging performance evaluation method and index system for bridging LCMs used in deep and ultra-deep drilling after analyzing the morphology, particle size distribution and mechanical properties of bridging LCMs such as walnut shell and calcium carbonate. Based on the measured data of particle size distribution of a single bridging material, Zhu et al. ^[41] proposed a new method to predict the particle size distribution of bridging LCMs and formulas by using the segmental cubic Hermite interpolation method. The bridge plugging technology requires a good matching between material particle sizes and fracture scales, and is likely to witness the phenomenon of plugging at the fracture tip or loss within the plugged fracture during plugging operations, possibly accompanied by backflow and repeated fluid loss during subsequent drilling. Zhang ^[42] proposed a progressive bridge plugging technology, with fluid injecting while being prepared with bridging materials in ascending orders of particle size and concentration, and this technology has been applied in 6 wells, with a success rate of up to 100%.

Polymer gel LCMs are mainly used to form the high-strength gel with a 3D network structure by chemical crosslinking reaction or molecular interaction to plug the leaking channels in complex strata ^[43]. For large fractures and cavernous or vuggy lost-circulation formations, bridging materials and other LCMs cannot retain to form plugging layers, while polymer gels highly capable of deformation and retention have no defects of granular materials and can achieve an ideal plugging performance ^[44].

Gel LCMs include chemical gels, which are formed mainly by chemical reaction, and crosslinked polymer gels, which are formed by crosslinking reaction between polymers and crosslinkers. Currently, a variety of gel materials, such as polyacrylamide gel and polyvinyl alcohol gel, have been developed and applied in the industry. These materials have their own characteristics in crosslinking reaction, high-temperature resistance, and environmental protection. Bai et al. ^[45] presented a double network self-healing hyrogel based on hydrophobic association and ionic bond, which effectively improves the plugging under pressure in lost-circulation strata. Jia et al. ^[46] synthesized an environmental and strength-enhanced nanosilica-based composite gel for well temporary plugging in high-temperature reservoirs, which has good mechanical and elastic properties to enhance the plugging effect. Lecolier et al. ^[47] developed the solid particle enhanced gel slug plugging technology based on the crosslinked polymer as the lost circulation material, in combination with fiber and rigid particles, which has been successfully applied for lost circulation treatment in Louisiana and northern Iran. Baker Hughes launched MAX-LOCK, an MgO-base thixotropic inorganic gel LCM, and optimized the gel curing time by the factors such as fluid-loss rate, temperature and size of leaking channel in strata, thus forming a specific gel LCC technology for carbonate reservoirs, which has well performed in wells with serious lost circulation in carbonate reservoirs in the Middle East ^[37].

The curable material LCC technology realizes a high- strength plugging of fractures by using the prepared curable plugging slurry, which is injected into the wellbore and then enters the leaking channels under the pressure difference before it is cured under the geothermal action. The curable plugging slurry has a high pressure-bearing capacity after curing and is widely used in wells with serious lost circulation ^[48]. Curable LCMs are characterized by diverse sources, low cost, high strength, simple preparation process, and high cementation strength, but they are liable to high safety risks in operations, poor in resistance to pollution by high-salinity formation water, and susceptible to fluid dilution which will impede their curing effects. Compared with conventional bridging LCMs, curable LCMs have higher pressure-bearing capacity and better curing performance. Moreover, the plugging layer is not destroyed under the pressure of drilling fluid column in the process of circulation drilling, so that the plugging operations can be reduced greatly.

The curable material LCC technology is critical in the industry, and it realizes plugging of cavities or fractures by virtue of curable materials. There are a variety of curable LCMs under the categories of cement, epoxy resin and poly-urethane ^[49]. These materials are well capable of physical plugging and curing cementation, and they can effectively plug the leaking channels in strata and improve the pressure-bearing capacity of strata. Field applications have demonstrated a significant reduction of possible lost circulation accidents. Halliburton developed a one-bag plugging technology to plug the leaking channels and control the lost circulation, and the technology can effectively plug the fractures with widths of 3-25 mm under a pressure up to 20 MPa, and has a good temperature resistance ^[49]. In the X structure of the Yinggehai Basin, western South China Sea, natural fractures exist universally, the formation temperature reaches up to 204 °C, and the formation pressure coefficient peaks at 2.19, suggesting low formation pressure-bearing capacity and narrow safety density window. These conditions may probably lead to complexities such as lost circulation. To address these challenges, the “cleaning fluid + cement slurry" intermittent squeeze cementing process was adopted in over 10 wells for plugging under pressure, showing a 100% success. This practice ensured safe and smooth drilling of the ultra-high temperature and high-pressure formations in the X structure.

The composite LCMs are usually prepared by mixing various components (e.g. fibers, particulates and flakes) in a certain proportion. These components play their respective roles in composite LCMs, and jointly constitute a LCC system with an excellent performance. Fibers function for enhancement and toughness. Particulates can fill and block micro pores and fractures. Flakes are good bridges and seals in plugging process.

The composite material LCC technology has emerged gradually along with the advancement of drilling technology and to deal with challenges from complex geological conditions. It integrates the advantages of various LCMs for purpose of ideal plugging. Currently, the popular composite slug processes include bridge plug + curing, bridge plug + high water loss, and gel + cement. For the formations with high temperature, high pressure and narrow safety density window (<0.10 g/cm³) in the Yingqiong Basin, Han et al. ^[50] proposed a high-density (2.30 g/cm³) bridge plug + high water loss composite plugging technology, which increases the safety density window to 0.16 g/cm³, thereby contributing a significant improvement of drilling efficiency. Liu et al. ^[51] synthesized a nano-micro deformable spherical gel, and combined it with mica flake and fiber to obtain a composite LCM with a good compatibility with oil-based drilling fluids, which was successfully applied in shale gas wells in Fuling area. Wang et al. ^[52] introduced some rigid LCMs of different particle sizes (e.g. walnut shell, cottonseed shell, and mica flake) and deformable particles (e.g. sawdust) to obtain close packing and consolidation of the plugging layers, thereby enhancing the pressure-bearing capacity of the formations. The resulting LCC system has been tested in 5 wells in the hinterland of Huoyan Mountain, with the average loss volume, lost time and drilling period reduced by 74.3%, 93.5% and 57.8%, respectively, compared with other wells in the same block.