In the exploration and development of energy sources such as oil and natural gas, drilling operations are a crucial step in obtaining underground resources. Drilling fluid, as the indispensable ‘blood’ of drilling engineering, plays a vital role. It not only directly affects the efficiency and quality of drilling operations but also relates to the safety and cost of the entire drilling project.
Carrying and Suspending Rock Cuttings
During drilling, rock cuttings generated by the drill bit breaking rocks continuously enter the well. The primary function of drilling fluid is to act like a diligent ‘porter,’ using its circulating flow to carry these rock cuttings from the bottom of the well to the surface. Through the action of the drilling pump, the drilling fluid rises at high speed within the drill pipe, reaching the wellhead and entering a settling tank where the rock cuttings settle and separate. The purified drilling fluid is then injected back into the well for reuse.
When drilling operations need to be suspended for various reasons, drilling fluid plays a crucial role in suspending rock cuttings. At this time, the drilling fluid, with its viscosity and shear force, suspends the rock cuttings within it, preventing them from settling and accumulating at the bottom of the well, thus avoiding the formation of a cuttings bed. Once a cuttings bed forms, it increases the frictional resistance of the drill string, leading to increased drill bit torque and potentially causing serious accidents such as stuck pipe, thus affecting the normal progress of drilling operations.
Cooling and Lubricating the Drill Bit and Drill String
During the rock-breaking process, the drill bit generates a large amount of heat due to friction. If this heat cannot be dissipated in time, the drill bit temperature will rise sharply, leading to accelerated wear and decreased hardness, thereby reducing drilling efficiency and service life. Drilling fluid, during its circulation, continuously flows over the drill bit, carrying away heat and cooling it, ensuring the drill bit operates at a suitable temperature and maintains good drilling performance.
Simultaneously, drilling fluid also forms a lubricating film between the drill string and the wellbore, and between the drill bit and the rock, reducing frictional resistance. This is like putting a ‘lubricating coat’ on the drilling equipment, reducing the rotational torque of the drill string and the resistance during tripping in and out of the hole, making drilling operations smoother, reducing equipment wear and energy consumption, and improving drilling efficiency.
Balancing Formation Pressure
Different formation depths underground contain different pressure systems. To prevent wellbore kicks and blowouts caused by pressure imbalances, drilling fluids need to balance formation pressure. This is achieved by adjusting the drilling fluid density to balance the resulting fluid column pressure with the formation pore pressure. When the drilling fluid density is appropriate, the fluid column pressure effectively inhibits formation fluids (such as oil, natural gas, and water) from entering the well, ensuring safe drilling operations.
If the drilling fluid density is too low, the fluid column pressure will be lower than the formation pressure, allowing formation fluids to enter the well, leading to kicks or even blowouts. This not only wastes resources and pollutes the environment but also poses a serious threat to drilling equipment and personnel safety. Conversely, if the drilling fluid density is too high, the fluid column pressure will exceed the formation fracturing pressure, causing wellbore rupture and leakage accidents, similarly affecting the normal operation of drilling.
Stabilizing the Wellbore
Wellbore stability is crucial for drilling operations. During drilling, the rocks surrounding the wellbore are soaked and eroded by the drilling fluid. If the drilling fluid is of poor quality, it can lead to rock instability, causing collapse and rockfall. High-quality drilling fluid forms a thin, tough filter cake on the wellbore surface. This filter cake acts like a protective film, preventing water from the drilling fluid from further penetrating the formation, reducing erosion and soaking of the wellbore rocks, and thus maintaining wellbore stability.
Furthermore, some chemical components in the drilling fluid can react with the wellbore rocks, enhancing their strength and stability, further improving the wellbore’s resistance to collapse. A stable wellbore provides a safe wellbore environment, facilitating normal drill bit progress and subsequent well completion operations.
Transferring Hydraulic Energy
When drilling fluid flows at high speed within the drill pipe, it possesses a certain amount of hydraulic energy. This energy can be ejected through nozzles on the drill bit, forming a high-speed water jet. This high-speed water jet can directly impact the rock, assisting the drill bit in breaking the rock and increasing drilling speed. The effect of hydraulic energy is particularly pronounced when drilling into formations with high hardness and abrasiveness.
Simultaneously, hydraulic energy can also be used to clean the drill bit and wellbore, promptly flushing away fine rock cuttings generated by the drill bit breaking rocks, keeping the wellbore clean, creating favorable working conditions for the drill bit, and further improving drilling efficiency.
Drilling fluid plays multiple crucial roles in drilling engineering. It is not only a versatile tool for carrying rock cuttings and cooling and lubricating drilling equipment, but also a guardian for balancing formation pressure, stabilizing the wellbore, and transmitting hydraulic energy. With the continuous development of drilling technology, the performance requirements for drilling fluids are becoming increasingly stringent.
