1.4 Drilling Fluid Leakage Prevention and Plugging Technology

During the process of deep and ultra deep drilling, the leakage prevention and plugging performance of drilling fluid must adapt to the high-temperature and high-pressure underground environment, ensuring effective sealing and plugging under extreme conditions, and maintaining wellbore stability.

1.4.1 Bridge material leak prevention and sealing technology

Bridge material leak prevention and plugging technology is one of the powerful means to solve the problem of wellbore leakage in fractured formations. It mainly involves compounding bridge plugging materials of different shapes and sizes with drilling fluid at different concentrations to form a plugging slurry. The plugging slurry bridges, accumulates, and fills the cracks, forming a crack plugging layer that blocks fluid pressure transmission and fluid medium passage.Common bridging materials include walnut shells, calcium carbonate, fibers, mica flakes, etc.However, the commonly used bridging and plugging materials have poor matching between particle size and formation leakage channel size. Due to factors such as gravity settlement and internal erosion, bridging and plugging materials are not easily retained in large cracks with large crack width and high longitudinal extension, especially in caves, resulting in low pressure bearing capacity of the plugging layer.Bridge plugging is a commonly used and effective method of plugging, but currently commonly used bridge plugging materials have weak high-temperature resistance, poor particle size and crack scale gradation, and poor stability of the bridging plugging layer due to particle accumulation in cracks, resulting in ineffective crack plugging or high risk of re leakage, making it difficult to meet the increasing demand for plugging in ultra deep oil and gas drilling.Baodan et al. optimized and prepared high-temperature bridging and plugging formulas suitable for different crack openings by controlling the particle size distribution and concentration of different types of bridging and plugging materials. The high-temperature pressure sealing ability can reach up to 15 MPa.

The crack sealing layer formed by bridging and plugging materials becomes more unstable and damaged in complex environments such as high temperature, high pressure, and high ground stress, leading to difficulty in achieving the expected success rate of plugging and crack sealing effect.Kang Yili et al. established a high-temperature aging performance evaluation method and index system for bridging and plugging materials in deep and ultra deep well drilling by analyzing the morphology, particle size distribution, and mechanical properties of walnut shells, calcium carbonate, and other bridging and plugging materials.Zhu Jinzhi et al. proposed a new method for predicting the particle size distribution of bridging plugging materials and formulations using segmented cubic Hermite interpolation based on measured data of particle size distribution of a single bridging plugging material.The bridging and plugging technology requires high compatibility between material particle size and crack size. During construction, it is prone to "seam sealing" or "loss inside the seal", and subsequent drilling is prone to backflow and re leakage. Zhang Peiyuan proposed the "gradual method" of bridging and plugging with bridging material particle size increasing from small to large and concentration increasing from low to high. Six wells were constructed on site, and the success rate of plugging was 100%.

1.4.2 Leakage prevention and plugging technology of gel materials

Polymer gel leakage prevention and plugging materials are mainly used to form high-strength gel with three-dimensional network structure by chemical crosslinking reaction or intermolecular interaction to plug the leakage channel of drilling fluid in complex formations.For large fractures and cavernous loss formations, it is difficult for plugging materials such as bridging to stay in them to form a plugging layer, while polymer gel has strong deformation and retention characteristics, which can overcome the defects of granular materials and achieve ideal plugging effect.Gel plugging materials are mainly divided into chemical gel and cross-linked polymer gel.Chemical gel is mainly formed by chemical reaction, while cross-linked polymer gel is formed by cross-linking reaction of polymer and cross-linking agent. At present, many gel materials have been studied and applied in practical engineering, such as polyacrylamide gel, polyvinyl alcohol gel, etc. These materials have their own characteristics in terms of cross-linking reaction, high temperature resistance, and environmental friendliness.Bai et al. developed a dual network self-healing polymer gel plugging material based on hydrophobic association and ionic bond, which effectively improved the pressure bearing plugging ability of the lost formation.Jia et al. have prepared environment-friendly and strength enhanced nano silica based composite gel for temporary plugging of high-temperature reservoirs, which has good mechanical and elastic properties and is conducive to enhancing the plugging effect.Lecolier and others have formed solid particle reinforced gel slug plugging technology based on the crosslinked polymer plugging agent developed by Lecolier, combined with fibers, rigid particles, etc., which has achieved good field application results in malignant loss wells in Louisiana and northern Iran.Baker Hughes has developed a magnesium oxide based thixotropic inorganic gel plugging material MAX-LOCK, and optimized the setting time of gel based on factors such as formation leakage rate, temperature, leakage channel size, etc., forming a gel plugging technology specially used to deal with carbonate lost circulation formation, which has a good plugging effect in carbonate malignant lost circulation wells in the Middle East.

1.4.3 Curing leak prevention and sealing technology

The solidifiable leak prevention and plugging technology refers to pumping the prepared solidifiable plugging slurry into the wellbore, and under the action of pressure difference, the plugging slurry enters the leakage channel and solidifies under the action of ground temperature, forming high-strength sealing for cracks.The pressure bearing capacity of solidified plugging slurry is high, and it is currently widely used in malignant well leakage sites.Curing leak sealing materials have a wide range of sources, low cost, high strength, simple preparation process, and high bonding strength after solidification. However, their construction safety risks are high, their ability to resist high salinity formation water pollution is poor, and they are easily diluted by fluids, resulting in poor solidification effect.Compared with conventional bridging plugging materials, curable plugging materials have high pressure bearing capacity and good solidification performance. During cyclic drilling, the plugging layer will not be damaged by the pressure of the drilling fluid column, which can greatly reduce the number of plugging construction times.Solidifiable leak prevention and sealing technology is one of the important technologies in the field of leak prevention and sealing, which mainly relies on materials with solidification ability to achieve the sealing of cracks or fissures.At present, various curable leak proof and plugging materials such as cement, epoxy resin, polyurethane, etc. have been developed.These materials have excellent physical sealing and solidification bonding abilities, which can effectively seal the leakage channels of the formation and improve the pressure bearing capacity of the formation.The successful application of curable leak proof and plugging materials on site has significantly reduced the possibility of well leakage accidents.Halliburton has developed a "one bag" plugging technology to seal and prevent channel loss, control malignant well leakage, and have a pressure sealing capacity of up to 20 MPa. It can block cracks with a width of 3-25 mm and has good temperature resistance.Natural fractures are widely developed in the X structure of the Yinggehai Basin in the western South China Sea, with a maximum formation temperature of 204℃ and a maximum formation pressure coefficient of 2.19. The formation has low pressure bearing capacity and a narrow safety density window. During the drilling process, complex situations such as malignant well leakage are prone to occur. The intermittent cement squeezing process of "trial squeezing cleaning solution+injection squeezing cement slurry" was used for high-pressure plugging in more than 10 wells, with a success rate of 100%, ensuring the safe and smooth drilling of the ultra-high temperature and high-pressure well section of the X structure in the Yinggehai Basin.

1.4.4 Leak prevention and sealing technology for composite materials

Composite sealing materials are usually composed of multiple components combined in a certain proportion, such as fibers, particles, and flakes. These components play their own roles in composite sealing materials and together form a leak prevention and sealing system with excellent performance.Fibers and other materials can provide good reinforcement and toughness; Particles and other substances can fill and seal small pores and cracks; Flakes and other materials can play a good bridging and sealing role in the process of plugging.Composite material leak prevention and plugging technology has gradually developed in recent years with the advancement of drilling technology and the challenges of complex geological conditions.Composite material plugging can fully leverage the advantages of different types of plugging materials to achieve good plugging effects.At present, the main composite slug modes are bridge plug+curable, bridge plug+high water loss, gel+cement, etc.Han Cheng et al. developed a high-density "bridge plug+high water loss" composite plugging technology with a density of 2.30 g/cm³ for the high-temperature, high-pressure, and narrow safety density window (less than 0.10 g/cm³) formations in the Yingqiong Basin, which increased the safety density window of the formation to 0.16 g/cm³ and significantly improved drilling efficiency.Liu Wentang et al. synthesized micro nano scale deformable spherical gel and compounded it with mica sheet, fiber and other materials to obtain a composite plugging material with good matching with oil-based drilling fluid, which has been successfully applied in shale gas wells in Fuling area and achieved good plugging effect.Wang Guangcai et al. achieved tight packing and compaction of the sealing layer by introducing rigid sealing materials of different particle sizes (walnut shells, cottonseed shells, mica sheets) and deformable particles (sawdust), improving the pressure bearing capacity of the formation and forming a sealing system with dual functions of leakage prevention and sealing. They conducted on-site tests in 5 wells in the hinterland of Flame Mountain, reducing the average leakage by 74.3% compared to the same block, saving 93.5% of the sealing loss time, and shortening the drilling cycle by 57.8%.

2.Problems with Deep and Ultra Deep Drilling Fluid Technology

2.1 Problems with high-temperature, high salt, and high-density water-based drilling fluid technology

High temperature resistant water-based drilling fluid technology is an important technical means to address the challenges of high temperature environments in drilling engineering, but it still faces some problems in practical applications.① Insufficient stability in high temperature (above 200℃, the same below) environments can lead to thermal degradation of polymers and other additives in drilling fluids, resulting in a decrease in drilling fluid performance. At the same time, it may also cause phase separation of different components in drilling fluids, affecting the uniformity and stability of drilling fluids.② In high-temperature environments, the viscosity of drilling fluid is often difficult to control and may become too high or too low, affecting drilling efficiency and wellbore stability. Meanwhile, high-temperature resistant water-based drilling fluids may exhibit poor shear dilution at high temperatures and shear rates, which is not conducive to improving drilling speed and carrying rock debris.③ As the temperature increases, it becomes difficult to control the filtration loss of drilling fluid, leading to instability of the wellbore and damage to the reservoir. Additionally, the quality of the formed filter cake may deteriorate under high temperature conditions, affecting its protective and leak proof functions.④ Some high-temperature resistant water-based drilling fluids may react with minerals or fluids in the formation, leading to a decrease in formation permeability or blockage. In terms of protecting reservoirs, some high-temperature resistant water-based drilling fluids may not meet specific requirements, resulting in reservoir damage and reduced production capacity.

2.2 Problems with High-temperature Resistant Oil-based/Synthetic based Drilling Fluid Technology

High temperature resistant oil-based/synthetic based drilling fluid technology plays an important role in drilling operations in deep wells, ultra deep wells, and complex formations. Although foreign technology in this area is relatively mature and China is constantly catching up and making some progress, there are still some problems that need to be solved.① Under high temperature environment, the components of oil-based/synthetic based drilling fluid may undergo chemical or physical changes, leading to unstable performance, such as increased viscosity, gel, etc. Meanwhile, as the temperature increases, the rheological properties of drilling fluid become difficult to predict and control, which may affect drilling efficiency and wellbore cleanliness.② Choosing the appropriate emulsifier to maintain the emulsion stability of drilling fluid at high temperatures is a challenge. The performance of emulsifiers directly affects the stability and service life of drilling fluids. After drilling is completed, demulsification becomes more difficult in high-temperature environments, which may lead to environmental pollution and increased waste disposal costs.③ Oil based/synthetic based drilling fluids react with certain minerals or fluids in the formation, resulting in a decrease in formation permeability or the formation of solid precipitates. The compatibility between oil-based/synthetic drilling fluid and cement slurry is a key issue in cementing operations. Incompatibility may lead to a decrease in cementing quality or damage to wellbore stability.④ Oil based/synthetic based drilling fluids may cause environmental pollution during use, especially in marine drilling, and the potential impact on marine ecosystems requires special attention. At the same time, certain components may decompose and produce toxic or flammable gases in high-temperature environments, increasing safety risks.