1.2 High Temperature Resistant Oil-based/Synthetic based Drilling Fluid Technology

Compared to water-based drilling fluids, high-temperature resistant oil-based/synthetic based drilling fluids have significant advantages in wellbore stability, lubricity, anti jamming, inhibition of shale hydration dispersion, inhibition of formation clay slurry formation, and optimal and fast drilling, making them the primary choice for various difficult high-temperature deep well drilling.

1.2.1 High temperature resistant oil-based drilling fluid

Anti high temperature oil-based drilling fluid mainly uses various types of diesel and mineral oil as base oils, combined with anti high temperature emulsifiers, flow regulators, fluid loss agents, and weighting materials to form a drilling fluid system. It can be matched with other treatment agents according to different geological conditions to ensure the integrity of the wellbore and effectively deal with reservoir leakage.At present, foreign high-temperature resistant oil-based drilling fluid technology has become relatively mature, with a variety of base fluids, in-depth research and development of high-temperature treatment agents, and widespread application of drilling fluid systems.M-I SWACO company has developed a high-temperature resistant white oil-based drilling fluid by designing the multifunctional structure of polyether materials. The performance of the drilling fluid remains stable before and after indoor hot rolling at 300℃, and has been successfully applied in the Kuche Mountain Front Block in Tarim, F7H well in the western South China Sea, and Gulong 1 well.Halliburton Company has developed a diesel based drilling fluid system with fatty acid derivative emulsifier and Tarot oil filtration reducer as the core, which can resist temperature of 260℃ and salt water invasion, and has been successfully applied in Kuqa piedmont drilling in the Tarim Basin.

Although the research on high-temperature resistant oil-based drilling fluids in China started late, it has developed rapidly in recent years, gradually breaking the monopoly of foreign oil service companies on high-temperature oil-based drilling fluids and overcoming the challenges posed by thick salt gypsum layers, high-pressure saltwater layers, and shale in the Kuqa Mountains, southwestern shale gas, southern Junggar margin, and northern Tarim region. Table 2 shows typical cases of using oil-based drilling fluids in ultra deep wells in China.At present, the deepest horizontal well for the application of high-temperature resistant oil-based drilling fluid in China is the Guole 3C well in Tarim Oilfield, with a depth of 9396 meters; The deepest vertical well used is the Pengshen 6 well in the Sichuan Basin, with a depth of 9026 meters; The highest temperature well used is Well 1 in southwestern Sichuan, Sichuan Basin, with a bottom temperature of 216℃ after drilling; The highest density well used is Letan 1 Well in the Junggar Basin, with a density of 2.68g/cm³.In addition, Wang Jianhua et al. developed a high-density and high salt water resistant oil-based drilling fluid system with a temperature resistance of 220℃, a density of 2.60g/cm³, and a salt water invasion capacity limit of 45%. During its application in the fourth section of the Keshen 1101 well in Kuqa Mountain, it effectively overcame the problem of leakage in the same layer.Qin Yong et al. synthesized a high-temperature resistant emulsifier for oil-based drilling fluid using tall oil fatty acids and maleic anhydride as the main raw materials, and constructed a set of high-temperature resistant oil-based drilling fluid based on this as the core. The experiment was successfully carried out in the shale gas well drilling process of Wei204H5 platform in Weiyuan block, Sichuan Basin, and has been widely promoted and applied.Zhao Wen and others developed a high temperature resistant and high-density oil-based drilling fluid system, which was successfully applied in the fourth well section of Dabei 12X well in Kelasu structural belt of Kuqa depression, Tarim Basin, and solved the drilling problem of high temperature, high pressure and ultra-high pressure salt gypsum beds in this structural belt.

1.2.2 High temperature resistant synthetic drilling fluid

Synthetic drilling fluid uses biodegradable synthetic base oil as the continuous phase, replacing traditional mineral oil or diesel, reducing environmental pollution.In terms of performance, synthetic based drilling fluids have shown good lubricity, rock carrying capacity, and anti collapse performance. By continuously optimizing their rheological properties, they can meet the drilling needs of various complex formations.Synthetic based drilling fluids have been successfully applied in multiple oil and gas fields, especially in offshore oil and gas fields and complex formation oil and gas fields where they perform well.Li Na and others successfully applied a synthetic based drilling fluid system in the three deviated and horizontal sections of the H6-6 well in Changning. This system successfully achieved a dual standardized production mode that balances clean and environmentally friendly drilling with safe and rapid drilling in pressure sensitive formations, laying a solid technical foundation for the development of shale gas in Changning.Wang Wei et al. prepared SBM-II synthetic based drilling fluid system using special water loss agent PF-FLB and gas to oil PF-SGO. The drilling fluid system has the characteristics of low filtration loss, good rheological properties, strong inhibition and stability, and has been successfully applied in Well A4 of Bohai BZ25-1 oilfield.

1.3 Reservoir Protection Drilling Fluid Technology

As early as 1959, Monaghan et al. first proposed the concept of oil and gas reservoir damage and methods to prevent the interaction between freshwater and clay. Since then, "damage and protection of oil and gas reservoirs" has attracted high attention from industry experts and scholars.Oil and gas reservoir damage refers to the phenomenon of decreased permeability of oil and gas reservoirs during the entire process of drilling, completion, downhole operations, and oil and gas field exploitation. Due to the initial contact between drilling fluid and oil and gas reservoirs, and the complex composition of drilling fluid, the physical and chemical balance of oil and gas reservoirs is greatly disrupted, resulting in severe damage to the oil and gas reservoirs.Therefore, how to reduce or even avoid the damage of drilling fluid to oil and gas reservoirs has become a research hotspot. The reservoir protection drilling fluid technology aims to reduce the damage of drilling fluid to oil and gas reservoirs during the drilling process.

1.3.1 Temporary plugging type oil and gas reservoir protection drilling fluid

Temporary blocking type oil and gas reservoir protection drilling fluid is a drilling fluid that temporarily blocks the oil and gas reservoir during the drilling process to prevent solid and liquid phases in the drilling fluid from invading the reservoir, thereby protecting the reservoir from damage.The main mechanism of reservoir protection is to use solid particles in drilling fluid to quickly form a shielding temporary plugging zone with near zero permeability near the wellbore wall under a certain positive pressure difference.This temporary blocking strip can effectively prevent the solid phase and filtrate in drilling fluid and cement slurry from continuing to invade the oil and gas reservoir, thereby avoiding pollution to the oil and gas reservoir.At present, scholars at home and abroad have conducted research work for more than half a century, and have successively established three generations of temporary plugging drilling fluid technologies for protecting oil and gas reservoirs, including shielding temporary plugging, fine temporary plugging, and physical and chemical film temporary plugging, gradually improving the effectiveness of protecting oil and gas reservoirs and achieving significant economic benefits.Sun Jinsheng and others elaborated on the basic principles and research progress of reservoir protection technologies such as physical particle temporary plugging and chemical film-forming temporary plugging, and provided future development directions for tight/shale oil and gas reservoir protection technologies.Jiang Guancheng and others were the first to introduce biomimetics into the theory of drilling fluids for protecting oil and gas reservoirs. They developed a biomimetic temporary plugging protection technology that is suitable for different oil and gas reservoirs, including "super double thinning, biofilm, and synergistic enhancement", which reduces damage to oil and gas reservoirs with different permeabilities, greatly improves single well production, and has been widely applied in major oil fields such as Shengli, Tarim, and Dagang, with significant results.

1.3.2 Liquid casing oil and gas reservoir protection drilling fluid

Liquid casing oil and gas reservoir protection drilling fluid technology is a special drilling fluid technology that reduces the density and viscosity of the drilling fluid by adding special additives to the drilling fluid, reducing its pressure and friction on the oil and gas reservoir, thereby avoiding damage to the oil and gas reservoir.Liquid casing "is a special fluid used in the drilling process, which can form a protective film around the wellbore, similar to traditional casing, but more flexible and efficient.Liquid casing is typically composed of drilling or completion fluids with specific properties that can tightly adhere to the wellbore, forming an isolation layer to protect the reservoir from damage during drilling and other downhole operations.

The current research on drilling fluid technology for protecting oil and gas reservoirs with liquid casing mainly focuses on how to improve the stability and durability of liquid casing, as well as optimize its protective effect on oil and gas reservoirs.Jiang et al. have developed a new drilling fluid technology that can form liquid casing while drilling to protect reservoir by using bionic super double diluent, bionic wall consolidation agent, bionic bonding lubricant, and other processing agents. It has been successfully applied in more than 1000 complex oil and gas wells, including Zhaotong shale gas in Sichuan Basin, shale oil in Bohai Bay Basin, Sulige tight gas in Ordos Basin, Songliao tight gas in Songliao Basin, the Junggar Basin tight oil, Shanxi coalbed gas, and Chad oilfield.Compared with oil-based drilling fluids or other high-performance water-based drilling fluids used in the same block, this technology can reduce the average well collapse accident rate by 82.6%, wellbore leakage by 80.6%, complex situations of blockage and drilling by 80.7%, increase mechanical drilling speed by 32.8%, and increase production by more than 1.5 times, with significant effects.

1.3.3 Non soil water-based/synthetic based oil and gas reservoir protection drilling fluid

The technology of soilless water-based drilling fluid mainly utilizes water as a continuous phase and does not contain solid components. By adding various chemical treatment agents, it has good rheological, inhibitory, and environmental performance.The reservoir protection mechanism mainly includes reducing solid content, inhibiting hydration, reducing filtration and damage, providing good lubrication and drag reduction, etc.The research on soilless water-based drilling fluid technology abroad mainly involves adding clay inhibitors and high molecular weight polymers to clean water, such as the soilless water-based drilling fluid system developed by Halliburton, which can withstand temperatures up to 180℃;The VeraTherm system developed by M-I SWACO has a temperature resistance of 204℃.In terms of soilless water-based drilling fluid technology, Chinese scholars have mainly developed soilless water-based organic salt drilling fluid, soilless water-based weak gel drilling fluid, etc. These drilling fluids are aimed at reducing reservoir damage and improving drilling efficiency.Among them, organic salt drilling fluid effectively protects the reservoir through the combination of high molecular weight polymers and soluble salts; The weak gel drilling fluid enhances the rock carrying capacity and wellbore stability by forming a weak gel structure.Ma Yingwen et al. constructed a high-temperature resistant organic salt reservoir protection drilling fluid system using materials such as high-temperature flow regulators, high-temperature protective adhesives, polyamine inhibitors, and potassium formate weighting agents. The system has a temperature resistance of 200℃ and has been successfully applied in three horizontal wells in the tight sandstone gas reservoir of Mi38 block in Shenmu gas field, Ordos Basin.The cesium formate soilless drilling fluid system developed by Shi Kaijiao and others has a temperature resistance of up to 220℃ and a density of up to 2.37g/cm³. It has good rheological and lubricating properties and can effectively avoid the settling problem of barite under high-density conditions, but it is expensive.

The technology of soilless synthetic based drilling fluid is an important research direction in the current drilling field.This technology optimizes the formula to remove clay components from traditional drilling fluids, significantly improving the stability and environmental friendliness of drilling fluids.In recent years, researchers at home and abroad have made significant progress in the field of soilless synthetic based drilling fluid technology, successfully developing various new soilless synthetic based drilling fluid systems. These systems have excellent wellbore stability, low friction, and high temperature stability, and can be applied to drilling operations in complex formations.Wang Xudong and others have developed a flow regulator suitable for synthetic based drilling fluids and established a high-temperature and high-density soil free synthetic based drilling fluid system. It can withstand temperatures of 180 ℃ and has a maximum density of 2.0g/cm³. Before and after aging, it has high demulsification voltage, low filtration loss, and excellent rheological properties, which can meet the requirements of on-site construction.Zhang Yi et al. developed a soilless synthetic drilling fluid based on biomass synthetic base liquid LAE-12, which has a temperature resistance of 150℃ and an adjustable density of 1.20-2.50 g/cm³. It showed good results in the first field test of Zhongjiang 204H well in Sichuan Basin.