2.2 Oil Based Drilling Fluid for Deep and Ultra Deep Wells

2.2.1 Deep and Ultra Deep Well Oil-based Drilling Fluids at Abroad

Compared to water-based drilling fluids, oil-based drilling fluids have obvious advantages in wellbore stability, lubricity, anti sticking and anti sticking, inhibition of shale hydration dispersion, inhibition of formation clay slurry making, and optimal and fast drilling, making them the primary choice for drilling various difficult high-temperature deep wells and various complex structural wells. With the continuous increase of well depth, the requirements for the temperature resistance of oil-based drilling fluids are becoming increasingly high. Since the 1970s, a series of high-temperature resistant oil-based drilling fluid treatment agents and systems have been gradually developed. The high-temperature resistant oil-based drilling fluid mainly consists of various types of diesel and mineral oil as the base oil, combined with high-temperature resistant emulsifiers, flow pattern regulators, fluid loss agents, and weighting materials. It can be matched with other treatment agents (reservoir protection agents, high-temperature resistant plugging materials, etc.) for different geological conditions to ensure the integrity of the wellbore and effectively respond to reservoir leakage. Typical cases of foreign oil-based drilling fluid applications are shown in Table 3, which lists the characteristics and on-site application examples of oil-based drilling fluid systems in deep and ultra deep wells of major foreign companies.

At present, the high-temperature oil-based drilling fluid technology with resistance to 260℃ in foreign countries is relatively mature, with a variety of basic fluids, in-depth research and development of high-temperature resistant treatment agents, and widespread application of drilling fluid systems. In order to improve the settling stability of oil-based drilling fluid, micro powder barite with a particle size range of 0.1-10μm and micro manganese material with a particle size range of 0.5-2μm were developed, greatly improving the settling stability of the system while also possessing the advantages of low friction. This technology has been successfully applied in the OP-11 well of the Oduptu oilfield in Sakhalin, Russia, with a horizontal displacement of 11475 meters and no downhole complications caused by drilling fluid settlement.

In recent years, inorganic nanomaterials have become a hot topic in the research and development of treatment agents. Swark Company has developed polyether carboxylic acid emulsifiers and nano flow type regulators with high temperature resistance based on the multifunctional structure of polyether materials. They are compounded with a small amount of high-performance organic soil and oil wetting agent to form ultra-high temperature resistant white oil-based drilling fluid. The various properties of the drilling fluid remain stable after indoor hot rolling at 300℃.

In the latest achievements, Halliburton has developed a series of high-density fluorinated treatment agents based on the special wetting properties and high-temperature resistance of fluorinated materials, including perfluoropolyether based fluids (PFPE), fluorinated emulsifiers with high-temperature resistance, and polytetrafluoroethylene based fluid loss agents (PTFE). And based on this as the core, a perfluoropolyether based drilling fluid is constructed, with a theoretical temperature resistance of 315.6℃. At the same time, it has advantages such as low solid content, low corrosion to drilling tools, good wellbore stability, and strong anti pollution ability. This technology is still in the laboratory research stage.

Considering the difference in temperature difference between deep wells in mainland and deep water,oil service companies such as Swark, Halliburton, Baker Hughes, and Xinyuan Energy have respectively developed constant rheological drilling fluids. Using linear olefins, linear paraffin, or gas to oil as the base fluid, combined with constant rheological flow type regulators such as polyfatty acid derivatives, a constant rheological drilling fluid is formed with parameters such as viscosity, dynamic shear force, and ECD that are less affected by temperature. The maximum temperature difference range of the constant rheological temperature is 4-163℃. This drilling fluid has the characteristics of low settlement risk and low leakage risk, providing technical support for safe and efficient drilling in deep water environments.

2.2.2 Deep and Ultra Deep Well Oil-based Drilling Fluid at China

Although the research on oil-based drilling fluids in China started late, it has developed rapidly in recent years, gradually breaking the monopoly of foreign oil service companies against high-temperature oil-based drilling fluids. We have overcome the challenges posed by the presence of thick salt gypsum layers, high-pressure saline layers, and shale in the Kuqa front of Tarim, southwestern shale gas, southern margin of Junggar, and Shunbei area of Tarim. Typical cases of domestic oil-based drilling fluid applications are shown in Table 4.

At present, the deepest horizontal well used for high-temperature resistant oil-based drilling fluid in China is the Guole 3C well, with a depth of 9396 meters (vertical depth of 8057 meters); The deepest applied vertical well is Pengshen 6, with a depth of 9026 meters; The highest temperature well applied is Ta Tan 1 well, with a bottom temperature of 216℃ after drilling; The highest density well used is Le Tan 1 well, with a density of 2.68 g/cm³. 

Wang Jianhua et al. developed a single chain multi group efficient emulsifier based on fatty acid amides and a fluid loss additive based on humic acid amides, forming a high-density and high salt water invasion resistant oil-based drilling fluid system with a temperature resistance of 220℃, a density of 2.6 g/cm³, and a salt water invasion resistance capacity limit of 45%. In the on-site application process of Keshen 1101 well in front of Kuqa Mountain, the fourth spud well effectively overcame the problem of leakage in the same layer. During this period, a drainage and depressurization process was adopted, and a total of 1129.98 m³ of saline water was discharged. The rheological properties, filtration effect, emulsification stability, and settlement stability of the drilling fluid were maintained in good condition, and the drilling was ultimately completed smoothly.

In response to the problem of drilling fluid settlement caused by high temperature and large fluctuations in formation pressure, a certain proportion of fine powder barite with a particle size of 0.1-10 μm was used for weighting, effectively solving the settlement risk during drilling with a density of 1.25 g/cm³ drilling fluid at 216℃ in Well Tatan 1, and ultimately successfully completing the drilling. Chuanqing Drilling has developed a set of high-temperature resistant oil-based drilling fluid technology, successfully overcoming challenges such as ultra-deep, ultra-high temperature, ultra-high pressure, and high sulfur content, and ultimately completed the drilling of the deepest vertical well in Asia, Pengshen 6 well, with a completion depth of 9026 meters and a bottom hole temperature of 197℃.

CNOOC has developed a deep-water constant rheological drilling fluid system FLATPRO with a constant rheological temperature of 3-180℃, with a temperature resistance of over 200℃ and a density of 2.2 g/cm³, based on the developed flow pattern regulator PF-MOVIS, viscosity increasing agent PF-FSGEL, and fluid  loss agent  PF-MOHFR. Successfully carried out drilling operations in 7 deep/ultra deep water wells in China, including the LS-W well. The deepest operating depth on site was 2619 meters, and the drilling fluid performance remained good during the drilling period.

3. Development Trends of Deep and Ultra Deep Drilling Fluids

3.1 Water-base Drilling Fluids

There is still a gap between domestic and foreign high-temperature resistant water-based drilling fluids in terms of temperature and salt resistance, and research is needed in terms of high-temperature resistance mechanisms, high-temperature treatment agents, and high-performance environmental protection treatment agents.

(1) . Conduct research on the failure mechanism of water-based drilling fluid treatment agents under ultra-high temperature conditions in deep and ultra deep layers, including high temperature pyrolysis leading to difficult control of viscosity reduction and filtration loss, and high temperature excessive crosslinking leading to viscosity increase; Research on the interaction mechanism between drilling fluid and wellbore rocks, including the inhibition mechanism under high temperature conditions, and the interaction between rock minerals and water.

(2) . Study the effects of functional groups or inert atoms, polymer molecular weight, and multi branched structure on the performance of treatment agents, and guide the design of environmentally friendly high-temperature treatment agents. Consider combining inorganic nanomaterials (such as nano silica, carbon nanotubes, graphene, etc.) with organic materials to achieve a dual improvement in temperature resistance and functionality.

(3) . Reserve the type of ultra-high temperature resistant water-based drilling fluid to form a water-based drilling fluid system with a temperature resistance of over 240℃, a density greater than 2.2 g/cm³, and resistance to saturated salt water. Strengthen the rheological properties, filtration reduction, and sealing properties of water-based drilling fluid under high temperature conditions.

3.2 Oil-base Drilling Fluids

At present, the technology of oil-based drilling fluids for deep and ultra deep wells in China is still in a catching up position compared to foreign countries, and further research is needed in terms of mechanism of action, base oil, high-temperature treatment agents, and related supporting technologies. 

(1) . Conduct research on the high-temperature stability mechanism of oil-based drilling fluid treatment agents and systems, the interaction mechanism between oil-based drilling fluid and wellbore rocks, and the relationship between rheological properties and settlement stability.

(2) . Carry out the research and development of environmentally friendly base oils, and further support the research of core treatment agents such as emulsifiers,fluid loss agents, sealing agents, and flow pattern regulators with high-temperature resistance, to solve the contradiction between temperature resistance and environmental protection. At the same time, we are developing specialized treatment agents for oil-based drilling fluids to cope with different complex downhole situations, including high-temperature plugging agents, viscosity reducing and diluting agents, sulfur removal agents, lubricants, and release agents.

(3) . Form a high-temperature resistant oil based drilling fluid system with a temperature resistance of 300℃, strengthen the emulsification stability, rheological stability, filtration loss reduction, and sedimentation stability of the system under ultra-high temperature conditions.

(4) . Develop national environmental protection drilling fluid standards, guide waste reduction and treatment, and promote the green development of drilling fluid.

4. Conclusion

The development of drilling fluid technology for deep and ultra deep wells in foreign countries has been relatively mature. The on-site application temperature of oil-based and water-based drilling fluids has reached over 260℃, and they have formed drilling fluid treatment agents and systems that can cope with different complex situations.

The technology of deep and ultra deep drilling fluids in China has made significant progress in recent years, but it is still in a catching up position. Some technologies of water-based drilling fluids have reached international leading levels, but they are still in the indoor research and development stage; However, there has been no breakthrough in the series of technologies for oil-based drilling fluids, especially those that resist temperatures above 260℃.

In the future, it is necessary to increase research efforts in the mechanism research of deep and ultra deep well drilling fluid technology, the development and system construction of anti ultra-high temperature drilling fluid treatment agents, and the response to different complex types of environmental protection treatment agents and plugging materials, in order to provide strong technical support for safe and efficient drilling of deep and ultra deep reservoirs.