Introduction
The Antarctica occupies nearly one-tenth of the land of the Earth, but of where 99.68% is covered by ice sheets with an average thickness of 1829 m [1]. As the last undeveloped continent on the Earth, the Antarctica contains numerous scientific mysteries and resources [2]. Beyond 220 types of mineral resources have been discovered in the Antarctica, including crude oil, natural gas, iron, coal, copper, diamond, thorium, plutonium, uranium, etc. [3⇓-5]. Currently, the estimated crude oil and natural gas reserves in the Antarctica are approximately (7.95-15.90)× 109 m3 and (3-5)×1012 m3, respectively [6-7]. In addition, the potential resource of hydrates in the continental margin is about (4.7-4.8)×1013 m3 [8]. Subglacial bedrock samples were obtained by rapid drilling through Antarctic ice sheets to determine rock exposure age and structural features. These bedrock samples play a significant role in investigating the geological structure, exploring the mineral resources, revealing the evolution of ice sheets, and evaluating future climate changes in the Antarctica.
During the drilling process, drilling fluid needs to have a specific density to maintain wellbore stability. Therefore, the quality of drilling fluid is the dominant factor in determining drilling rate, core state and drilling success rate. The average surface temperature in the Antarctic region is extremely low (-60 °C to -50 °C) [9-10], the underground temperature varies in an extensive range (-55°C to -2 °C) due to complex structures (snow, ice, and rock layers from top to bottom), challenging the performance of drilling fluids. Existing drilling fluids are insufficient at the tolerance of low temperature and wellbore stability, resulting in severe ice debris accumulation while drilling at ultra-low temperature, and consequently wellbore collapse and stuck pipe issues. In addition, drilling fluid is corrosive, resulting in critical damage to drilling motor and control system and threating operators' health [11].
Due to the limitation of drilling fluids performance, only five times of subglacial bedrock drilling operation (the deepest one is to 681 m) have been accomplished around the Antarctic coast by three countries, namely China, Russia, and the USA. A small number of subglacial core samples (the longest core is only 8 m) have been taken, which cannot fulfill the scientific research needs in the Antarctica. In this paper, we reviewed the publications regarding the characteristics of drilling in the Antarctica, the requirements of drilling fluids at low-temperature in the Antarctic, and the research development of low-temperature drilling fluids to summarize challenges and future research trends of drilling fluids at low temperature in the Antarctica.
1. Drilling characteristics and challenges in the Antarctic region
1.1. Drilling characteristics in the Antarctic region
The Antarctic ice sheet comprises snow layer, bubble ice layer, brittle ice layer, glacier ice layer, warm ice layer, ice-rock interlayer, and subglacial bedrock from top to bottom. Meltwater and unfrozen glacial till may exist in the ice-rock interlayer and the subglacial bedrock, and other structures (such as subglacial lakes) may present in part of the Antarctic region. Therefore, drilling jobs in the Antarctica primarily involve drilling in snow, ice, and subglacial rock formations [12].
(1) Drilling in the snow layer. The thickness of the Antarctic snow layer is approximately 100 m. Snow thicknesses at South Pole, Vostok, Dome C, and Byrd are 115, 95, 100, and 64 m, respectively. The snow-covered layer has large porosity and high permeability due to its relatively short existence. The thickness of snow is correlated to the humidity and the accumulation rate of the ice sheet [13]. Due to the high permeability of the snow layer, drilling fluid is difficult to form closed circulation in the wellbore, and compressed air or low-temperature drilling fluid is easy to leak through the gaps between the wellbore and the bottom hole. Snow debris is accumulated at the bottom hole and compacted into ice slag, wrapping the drill bit and sticking the drill pipe. Therefore, the casing is commonly required for isolation, and casing shoes can prevent drilling fluid leakage into the snow layer. In addition, air drilling operation is an option for the thin snow layer cases.
(2) Drilling in the ice layer. Ice is a nonlinear rheological material that may trigger yield and creep deformation at small stress, leading to wellbore diameter shrinkage and wellbore collapse or stuck pipe accidents. Therefore, low-temperature drilling fluid with a specific density is utilized to balance the pressure in the ice layer and maintain the wellbore stability [14]. In the ice layer, the temperature increases as depth increases, resulting in the occurrence of warm ice layers. Ice cuttings prefer to accumulate as drilling in warm ice layers, leading to stuck pipe, wellbore closure, etc.
(3) Drilling in the subglacial rock formation. Subglacial rock formations are compound porous media composed of various mineral particles, ice blocks, unfrozen water and air with a high percentage of water vapor. These structural and component features make subglacial rock formations susceptible to temperature. When drilling fluid contacts with the subglacial formation, heat exchange occurs with the frozen rock formation around the wellbore, resulting in ice melting in the pores. Moreover, the ice melting range extends as time goes on, leading to the weakening of mechanical properties of the rock formation (Table 1 ), enhanced deformability, and wellbore instability and other events [15-16]. Therefore, it is essential to maintain the original state of the rock formation along the wellbore for fast and safe drilling in subglacial formations.
Table 1. Mechanical properties of rock under frozen and ablation states [16] |
| Lithology | Frozen state | Ablation state | ||||
|---|---|---|---|---|---|---|
| Internal friction coefficient | Cohesion force/MPa | Elastic modulus/MPa | Internal friction coefficient | Cohesion force/MPa | Elastic modulus/MPa | |
| Granite, diorite, andesite | 0.78 | 0.60 | 200 | 0.75 | 0.30 | 110 |
| Granite*, diorite*, andesite * | 0.73 | 0.35 | 140 | 0.70 | 0.15 | 90 |
| Gneiss | 0.84 | 0.65 | 330 | 0.80 | 0.35 | 180 |
| Limestone | 0.67 | 0.30 | 120 | 0.65 | 0.20 | 80 |
| Sandstone | 0.23 | 0.40 | 160 | 0.70 | 0.25 | 100 |
| Sandstone * | 0.68 | 0.24 | 120 | 0.65 | 0.15 | 75 |
| Clay*, shale | 0.62 | 0.20 | 100 | 0.60 | 0.10 | 50 |
| Clay, shale * | 0.57 | 0.15 | 40 | 0.55 | 0.07 | 20 |
Note: *—The water content of the sample was increased during sample preparation and then frozen to increase the ice content of the sample. |
1.2. Drilling challenges in the Antarctic region
By summarizing the characteristics of drilling at low temperature in the Antarctic region, there are four drilling challenges in the Antarctic region: (1) Extreme climate conditions. The annual average temperature is lower than -55 °C, and the temperature in the ice layer varies in an extensive range (from -55 °C to -2 °C). Thus, the extreme climate conditions challenge the capabilities of drilling fluids on tolerance to low temperature and the adaptability to an extensive span of temperature. The Antarctic region is characterized by long winter, short summer, and polar nights. Because the construction period is only about 40 d in the Antarctic, high-performance drilling fluids are essential to adapt to low-temperature environments. (2) The geological conditions are complex in the Antarctic region, and the drilling strata include snow, ice, and subglacial rock formations. The high permeability of the snow layer causes lost circulation, stuck pipe, and other accidents. When drilling the ice layer, creep is tended to occur, and the safety density window is narrow. The warm ice layer is prone to stuck pipe in the wellbore due to ice cuttings accumulation. During the drilling in the subglacial formations, frozen soil tends to melt, resulting in wellbore collapse. (3) The Antarctic region is located at the south pole of the Earth, off the beaten track, lacks infrastructure facilities, and is challenging to provide logistical support. Therefore, the volume and reuse of drilling fluid is requested to a higher standard. (4) Protecting the ecological environment of the Antarctic region is a common responsibility for all humankind. However, the Antarctic region is fragile and sensitive, and the ecosystem has a low carrying capacity. Therefore, stricter requirements are essential for improving environmentally friendly drilling fluids [17⇓-19].
2. Performance requirements of low-temperature drilling fluids in the Antarctic region
The performance parameters of drilling fluids used in polar drilling were collected in the University of Copenhagen report [20] (Table 2 ). The density of the ice layer in the Antarctic region is 910-925 kg/m3. Thus, the density of the low-temperature drilling fluid used in the Antarctic region should be 920-950 kg/m3, and its capable of adjusting ±25 kg/m3 in density is preferred [21]. The lower viscosity of low-temperature drilling fluid is required for better performance. Due to the high time cost in the Antarctic region, high viscosity reduces the speed of tripping operation in wellbore. In order to reduce the tripping time, the space between the drill tool and wellbore should be enlarged to reduce the viscous resistance. However, a large-diameter wellbore slows down the drilling rate, increases cuttings, and increases energy consumption. Moreover, it is unnecessary to consider the impact of drilling fluid rheology on cuttings-carrying capability because the density of ice is less than that of the drilling fluid. Therefore, the viscosity of the low-temperature drilling fluid needs to be lower than 23.75 mPa·s [21]. In addition, the freezing point of the low-temperature drilling fluid used in the Antarctic region should be lower than the lowest temperature in the wellbore and the air temperature outside the drilling shed (a place for storing drilling fluid). Therefore, the freezing point of the drilling fluid should be less than -55°C. It’s also required that the low-temperature drilling fluid has high chemical stability and is environmentally friendly. Water-based drilling fluids cannot be applied under low-temperature conditions. The applicable low-temperature drilling fluids for the Antarctic region are generally oil-based or synthetic-based[22].
Table 2. Primary performance of low-temperature drilling fluids in the Antarctic region [20] |
| Performance | Impacts on drilling | Requirements for drilling fluids |
|---|---|---|
| Density | Improper density can lead to wellbore instability and even stuck drill pipe | Density equal to or slightly higher than the density of ice layer at -55 °C |
| Viscosity | High viscosity reduces the tripping speed of drilling tools, increases total drilling time while increasing drilling costs | Viscosity below 23.75 mPa·s at -55 °C |
| Frost resistance | Freezing point above -55 °C may result in drilling fluid to freeze during storing or circulation | Freezing point lower than -55 °C |
| Stability | Unstable chemical properties can lead to changes in drilling fluid performance during storage, transportation and operation | Chemically stable |
| Compatibility with polymers and metals | Corrosion can cause damage to drill pipe, cables or other drilling tools | Non-corrosive |
| Volatility and flammability | High volatility pollutes work environment, clothes, ice cores, while low ignition point brings fire risk | Low volatility, high ignition point |
| Water (ice) solubility | Water solubility can lead to dissolution of well wall and formation of ice mud, even stuck drill pipe | Water-insoluble |
| Toxicity | Toxicity may pose a threat to the health of wellsite personnel | Non-toxic or low toxicity |
| Environmental performance | Poor environmental friendliness leads to pollution to environment | Environmentally friendly |
| Economy | High prices lead to higher drilling costs | Low price, easily available |
3. Research progress of low-temperature drilling fluids in the Antarctic region
3.1. Petroleum-based drilling fluids
Low-temperature petroleum-based drilling fluids are primarily based on petroleum products, such as diesel, kerosene, dearomatized hydrocarbon solvents, isoparaffin solvents, etc., supplemented by weighting agents.
Diesel and kerosene are the earliest used base fluids for the petroleum-based drilling fluids. From 1967 to 1968, the United States drilled a well to 2164 m for the first time (only drilled into the ice layer) using diesel as drilling fluid at the Byrd Station in the Antarctica. Later, drilling operations using different types of kerosene supplemented by various weighting agents continued in the Antarctica [23⇓⇓-26]. The operating temperature of these base fluids was -13 °C to -7 °C, and they were highly irritating, toxic and corrosive, so they were rarely used in drilling in the Antarctic recently.
Subsequently, dearomatized hydrocarbon solvents (ExxsolTM D30, ExxsolTM D40 and ExxsolTM D60) and isoparaffinic solvents (IsoparTM K) which are less toxic, readily biodegradable and better tolerant to low temperature gradually replaced traditional kerosene-based drilling fluids. The performance parameters of these drilling fluids are listed in Table 3 . Since 1996, these two types of drilling fluids have been applied to various locations in the Antarctic, such as Dome C, Berkner Island, Queen Maud Land in the Antarctica, and the West Ice Sheet [23,27⇓⇓ -30]. The drilling depth reached 3405 m (only into the ice layer). Although the performance of the drilling fluids has been dramatically enhanced, ice cuttings accumulated and plugged the wellbore during the drilling.
Table 3. Primary performance parameters of solvents ExxsolTM D series and IsoparTM K [31] |
| Solvent | Density at 15 °C/ (kg•m-3) | Viscosity at 20 °C/ (mPa•s) | Pour point/°C | Flash point/°C | Mass fraction of aromatic hydrocarbon/% | Aniline point/°C | Evaporation rate/% |
|---|---|---|---|---|---|---|---|
| ExxolTM D30 | 762 | 0.572 | <-55 | 29 | 0.001 | 64 | 44.0 |
| ExxolTM D40 | 775 | 0.744 | <-55 | 42 | 0.003 | 67 | 14.0 |
| ExxolTM D60 | 792 | 1.022 | <-55 | 63 | 0.060 | 70 | 3.4 |
| IsoparTM K | 763 | 1.404 | <-18 | 54 | 0.003 | 83 | 6.0 |
The density of petroleum-based drilling fluid is from 800 to 850 kg/m3 below -30 °C, while the density of ice is 910 to 925 kg/m3. Therefore, petroleum-based drilling fluids need to mix with fluorocarbons or other compounds with significantly larger densities than ice to achieve the required density of low-temperature drilling fluids [32].
From 1967 to 1968, Americans used trichloroethylene (C2HCl3) at the Byrd Station in the Antarctic for the first time as a weighting agent. Next, perchloroethylene (C2Cl4) was utilized as a weighting agent at Law Dome in the Antarctic from 1987 to 1993 [25-26]. However, perchloroethylene was substituted by chlorofluorocarbons (such as trichlorofluoromethane CFC-11 [23] and trichlorotrifluoroethane CFC-113 [33]) due to its high toxicity. Dichlorofluoroethane (HCFC-141b) has a more negligible destruction effect on the ozone shield and has a density of 1332.5 kg/m3 below -30 °C, which is miscible with petroleum products at any ratio. Thus, it is widely applied and replaces chlorofluorocarbons CFC-11 and CFC-113 [34⇓-36]. The physicochemical properties of weighting agents with halogenated hydrocarbons used for drilling in the Antarctic are shown in Table 4 .
Table 4. Physicochemical properties of weighting agents with halogenated hydrocarbons used for drilling in the Antarctic [20, 31] |
| Halogenated hydrocarbons weighting agents | Density at 20 °C/ (kg•m-3) | Viscosity at 25 °C/ (mPa•s) | Ozone consumption coefficient | Greenhouse coefficient |
|---|---|---|---|---|
| C2HCl3 | 1464 | 0.791 | 1.00 | 4850 |
| C2Cl4 | 1625 | 1.363 | 1.00 | 4700 |
| CFC-11 | 1487 | 0.625 | 1.00 | 4600 |
| CFC-113 | 1575 | 1.024 | 0.90 | 5000 |
| HCFC-141b | 1240 | 0.516 | 0.11 | 630 |
The Montreal Protocol on Substances that Deplete the Ozone Layer, finalized in 1987 at the United Nations, entirely phased out chlorofluorocarbons. Later, researchers have developed several environmentally friendly weighting agents, such as hydrofluoroether HFE-7100 [18] (density of 1530.5 kg/m3) as a weighting agent with low toxicity, low viscosity, and no flash point. However, laboratory tests indicated that the mixture of IsoparTM K and HFE-7100 would separate into two phases as temperature was below -45 °C, limiting the application of this mixture in low-temperature ice layer [31]. The mixture comprising succinic acid, glutaric acid, and adipic acid diisobutyl ester with a fraction of 2:4:3 has a density of 960 kg/m3 [28]. The mixture is a colorless, odorless, less toxic and biodegradable weighting agent. Previous investigations indicated that it was readily miscible with ExxsolTM D40 solvent. The mixture has high viscosity at low temperature (18 mPa·s at -30 °C), which is not a suitable candidate as a component of drilling fluids in the deep ice layer in the Antarctic [31,37].
In general, weighting agents used in the existing low- temperature petroleum-based drilling fluids still have some issues: high viscosity at low temperature, high toxicity, and severe environmental pollution. A breakthrough should be made to the weighting agent for petroleum- based drilling fluid since it restricts the application of low-temperature drilling fluids in the Antarctic region.
3.2. Alcohol-based drilling fluid
Ethylene glycol aqueous solution and ethanol aqueous solution are two types of alcohol-based drilling fluids that have been utilized for drilling operation in the Antarctic ice layers [38-39]. The hydrophilic nature and ice melting feature of these drilling fluids can eliminate stuck pipe accidents caused by ice cuttings [40-41]. For instance, during the drilling in the Dome C area, the drill bit encountered the warm ice layer (temperature at -10 °C). A small amount of ice melted due to heat released from the drilling process, then re-froze on the drill bit and drilling tools, resulting in stuck drill pipe. Finally, the ethylene glycol aqueous solution was used to solve the problem of drilling in the warm ice layer and can prevent meltwater from re-freezing on the drill bit. However, this type of drilling fluid cannot overcome all the problems related to stuck pipe accidents in the ice layer. In addition, the largest depth of drilling operation using this drilling fluid was only 412 m (the Soviet Union in 1972), and the quality of the ice cores was less desired. There may be four factors causing the above result: (1) The viscosity of ethylene glycol and ethanol aqueous solution was very large at low temperature, which had a severe hindering effect on drilling operation. (2) Ethylene glycol and ethanol aqueous solution could dissolve ice, leading to corrosion on the wellbore. (3) With the temperature changing in the wellbore (for instance, due to convection in the well), the aqueous solution froze to muddy-shaped ice debris in the wellbore, and strongly impacted the drilling efficiency. Ice debris would block the wellbore after stopping drilling operation. (4) The hydrophilicity of ethylene glycol and ethanol aqueous solutions corroded ice cores and degraded the quality [27,42 -43].
3.3. Ester-based drilling fluid
N-butyl acetate is characterized by low-temperature tolerance and possesses appropriate density and viscosity at low temperature. The density at -50 °C is close to 970 kg/m³, which can be used to drill in the ice layer without adding weighting agents. The viscosity at -50 °C maintains below 3 mPa·s. N-butyl acetate has been utilized as a drilling fluid in the Antarctic drilling operations by multiple counties (such as the United States, Japan, China, etc.) [11,44⇓⇓ -47]. However, n-butyl acetate as a drilling fluid has two obvious drawbacks, which is harmful to physical and mental health and highly corrosive [17,48]. Ethyl butyrate, propyl butyrate, butyl butyrate, n-pentyl butyrate, and n-hexyl acetate also have good low-temperature properties, and their densities and viscosities at low temperature (density of 932.5-959.0 kg/m3 at -55 °C and 3.0-11.7 mPa·s in viscosity) can meet the requirements for drilling in the Antarctic region, and they are environmentally friendly, miscible at low temperature, and can be formulated according to specific application cases [49⇓⇓-52]. The physical parameters of these esters with low molecular weight are presented in Table 5 . However, esters with low molecular weight are flammable, explosive, and irritating to eyes and skin. Currently, they are still in laboratory research, and have not been verified by field application.
Table 5. Physical parameters of esters with low molecular weight [49] |
| Description | CAS No. | Molecular formula | Relative molecular mass | Freezing point/°C | Boiling point/ °C | Density at 20 °C/ (kg•m-3) | Viscosity at 25 °C/ (mPa•s) | Vapor pressure/ kPa | Flash point/ °C | Mass concentration in water at 20 °C/% |
|---|---|---|---|---|---|---|---|---|---|---|
| Butyl acetate | 123-86-4 | C6H12O2 | 116 | -76.8 | 126.1 | 882.4 | 0.73 | 1.20 | 22.0 | Slightly soluble |
| Ethyl butyrate | 105-54-4 | C6H12O2 | 116 | -93.3 | 121.3 | 879.0 | 0.71 | 1.51 | 25.0 | 0.7 |
| Propyl propionate | 106-36-5 | C6H12O2 | 116 | -76.0 | 122.4 | 881.0 | 0.70 | 1.43 | 24.4 | 0.5 |
| Butyl butyrate | 109-21-7 | C8H16O2 | 144 | -91.5 | 166.6 | 869.0 | 1.01 | 0.24 | 53.0 | Insoluble |
| N-amyl butyrate | 540-18-1 | C9H18O2 | 158 | -73.2 | 186.4 | 870.0 | 1.20 | 0.08 | 57.0 | Soluble |
| N-hexyl acetate | 142-92-7 | C8H16O2 | 144 | -80.9 | 171.5 | 871.8 | 1.10 | 0.19 | 43.0 | 0.4 |
From 2006 to 2016, several drilling operations were conducted using the drilling fluid formulated from coconut oil heptyl ester derivatives (ESTISOLTM) with environmentally friendly feature and low-temperature tolerance. The retrieved ice cores have good quality, and there was no stuck pipe accident due to ice debris accumulation during drilling [42,53]. Meanwhile, coconut oil heptyl esters have a great potential to act as weighting agents for low-temperature drilling fluids, such as coconut oil heptyl esters 165 and F2887 [31,37]. The primary physical parameters of ESTISOLTM esters are listed in Table 6 . However, ESTISOLTM-based drilling fluid has the disadvantages of high viscosity, strong corrosion, and great irritation at low temperature. For instance, coconut oil heptyl ester 240 has high viscosity at low temperature, which reaches 30.0 mPa·s at -35 °C [54].
Table 6. Physical properties of ESTISOLTM esters [31] |
| ESTISOLTM classification | Raw material source | Viscosity at 25 °C/ (mPa•s) | Density at 20 °C/ (kg•m-3) | Pour point/°C | Flash point/°C | Boiling point/°C |
|---|---|---|---|---|---|---|
| 140 | Synthesis | 1.3 | 870 | -93 | 75 | 199 |
| 165 | Synthesis | 3.0 | 1100 | <-30 | 81 | 180-190 |
| 240 | Vegetable | 4.0 | 855 | <-50 | 730 | 250-290 |
| F2887 | Synthesis | 7.0 | 1083 | <-10 | 167 | >280 |
3.4. Silicone oil-based drilling fluid
Methyl silicone oil is a colorless, odorless, non-toxic, water-insoluble, and non-volatile inert solvent, with the advantages of small viscosity-temperature coefficient and low-temperature tolerance. The density of methyl silicone oil with low relative molecular mass at low temperature is comparable to the density of ice. The viscosity of methyl silicone oil is around 8.8 mPa·s at -50 °C, which fulfills the requirements of ice drilling in the Antarctica[55]. However, the price of methyl silicone oil is 5 to 10 times higher than that of normal low-temperature drilling fluids, severely restricting its use in the Antarctic region. Methyl silicone oil has never been applied in any drilling projects in the Antarctic region.
4. Technology development trend of low-temperature drilling fluids in the Antarctic
The existing low-temperature drilling fluids used in the Antarctic region commonly possess the disadvantages in insufficient low-temperature tolerance, weak environment protection, inadequate performance of anti-collapse and anti-ice debris accumulation. They cannot fully adapt to drilling activities under the complex subglacial conditions in the Antarctic region. Therefore, it is urgent to investigate the mechanisms of low-temperature drilling fluids for the Antarctic region, develop new materials for low-temperature drilling fluids, and establish an environment-friendly low-temperature drilling fluid system. Results of the research can provide theoretical and technical supports for scientific research and resource exploration and development in the Antarctic continent. Future research trends on low-temperature drilling fluids in the Antarctic region will focus on the following five aspects.
(1) Study the experimental approach of low-temperature drilling fluid. Due to the particularity of drilling operation in the Antarctic region, conventional evaluation methodology and experimental instruments are no longer applicable for assessing the performance of drilling fluids. It is essential to execute investigations to experimental approaches and the development of experimental setups, including developing performance testing equipment for drilling fluid at low temperature, such as the equipment for testing the rheology, filtration and plugging of drilling fluid, and to establish a methodology for evaluating the performance of drilling fluid at low temperature; developing instruments to detect the physical and chemical interactions (heat and mass transfer, etc.) between low-temperature drilling fluid and ice/rock layers, to establish experimental approaches for evaluating the stability between low-temperature drilling fluids, ice layers and rock layers; developing visualizing equipment for the accumulation and blockage of ice debris in low-temperature drilling fluid and establishing an evaluation method for the accumulation and blockage of ice debris in low-temperature drilling fluid; establishing an analysis method for the microscopic mechanism of treatment agents in low-temperature drilling fluid.
(2) Investigate the acting mechanism of low-temperature drilling fluids. The environment of the Antarctic surface is extreme, and the Antarctic subglacial geological conditions are complex. Therefore, the drilling safety is severely endangered by plenty of issues occurred during the drilling process, such as rapid wellbore closure caused by the moving ice layer, wellbore blockage triggered by accumulation of ice debris, leakage of drilling fluid at the ice-rock interlayer, and wellbore instability caused by interaction between drilling fluid and rock formation. It is urgent to conduct research on the mechanisms of interaction between the low-temperature drilling fluid and ice/rock layers, the mechanisms of hydraulic fracturing on the wellbore at the ice layer, the mechanisms of blockage caused by ice debris accumulation, and the mechanisms of leakage and plugging of low-temperature drilling fluid.
(3) Research and develop base fluids for low-temperature drilling fluids. The primary research objective of the Antarctic drilling fluids is to develop non-toxic, economic, and environmentally friendly base fluid for low-temperature drilling fluids. The new low-molecular-weight fatty acid esters are characterized by appropriate density and viscosity properties at low temperature. Further research should focus on the performance of dissolving ice and low volatility. With excellent physical and chemical properties, low-molecular-weight silicone oil is a potential low-temperature drilling fluid suitable for the Antarctic region. It is essential to improve technologies further to reduce the cost of silicone oil-based drilling fluids and improve their adaptability. For instance, technologies such as optimizing the synthesis method of low-molecular-weight silicone oil and developing the formula of silicone oil with other solvents.
(4) Develop new materials for low-temperature drilling fluids. New materials for low-temperature drilling fluids need to be developed to solve issues like wellbore closure, stuck pipe, wellbore leakage, and wellbore instability during drillings. Such as weighting agents for low-temperature drilling fluid, low-temperature phase change materials, plugging materials with low-temperature tolerance, wellbore stabilizers, anti-agglomerant for ice debris, and lost circulation materials.
(5) Develop low-temperature drilling fluid systems and operating procedures in the field. No research has been conducted on the compatibility and synergistic interaction between the base fluid and the treatment agent in the drilling fluid system at low temperature. The establishment and performance modulation of low-temperature drilling fluid system is still in the initial stage. Standard specifications for the preparation and workflow have not been established for Antarctic drilling fluid. It is urgent to conduct in-depth research on low-temperature drilling fluid systems and construct environment-friendly low-temperature drilling fluid systems. Meanwhile, a multi-functional integrated control method is required to enhance the wellbore stability and improve the cutting carrying capacity when drilling into ice layers and ice- rock interlayers. Furthermore, specifications of on-site operation are established to provide technical support for Antarctic drilling.
5. Conclusions
Rapidly drilling through the Antarctic ice sheet and retrieving subglacial bedrock samples play a significant role in studying the geological structures of the Antarctica and exploring mineral resources in the Antarctica. A suitable low-temperature drilling fluid is the key to fast drilling through the Antarctic ice sheet. After years of development, progresses have been made on low-temperature drilling fluids for the Antarctic region. Multiple types of drilling fluids have been developed, such as low-temperature petroleum-based drilling fluid, ethanol/ethylene glycol-based drilling fluid, ester-based drilling fluid, and silicone oil-based drilling fluid. However, these drilling fluids have problems such as insufficient low-temperature tolerance, low environmental performance and weak wellbore stability, etc. It is essential to investigate the mechanisms of low-temperature drilling fluids and develop environment-friendly base fluids and related additives for low-temperature drilling fluids. An environment-friendly low-temperature drilling fluid system is required to be established. Furthermore, a multi-functional integrated control method should be established to ensure wellbore stability and improve cuttings-carrying capacity when drilling into ice layers and ice-rock interlayers, and specifications of on-site operation will be established. These studies will provide core technologies, promote technological innovation, expand strategic space, and improve China’s dominance in the Antarctic region.