Drill Bit Cooling and Cleaning
During the drilling process, drilling fluid (mud) circulates effectively to cool the drill bit. In the absence of mud circulation, the temperature of the drill bit rises by 1 to 2 degrees Celsius with every rotation at the bottom of the well. Assuming the bit rotates at 200 revolutions per minute, within a mere three minutes of drilling, its temperature would skyrocket to over 600°C-a scenario highly likely to trigger severe accidents such as bit burnout or sticking. Consequently, the circulating flow of the drilling fluid serves to efficiently dissipate the heat generated as the bit fractures the rock, thereby ensuring the bit remains adequately cooled and operates within its normal working parameters. Furthermore, the drilling fluid plays a crucial role in cleaning accumulated mud from the bit; by flushing the bit, the fluid effectively removes mud packs from the teeth and underside, ensuring the bit can continue to fracture rock efficiently and continuously, thereby enhancing overall drilling efficiency.
Cuttings Removal and Fluid Loss Control
As drilling progresses, the drill bit fractures the rock, generating a substantial volume of rock cuttings. To maintain drilling efficiency, these cuttings must be promptly flushed away from the wellbore bottom. This not only prevents the bit from re-grinding previously fractured cuttings-thereby increasing the rate of penetration-but also effectively prevents cuttings from adhering to the bit, which would otherwise impede its continued operation. The drilling fluid must possess specific properties-including appropriate specific gravity, viscosity, and shear strength-to ensure that the cuttings are successfully transported up to the wellhead. Additionally, by incorporating additives such as polyacrylamide and inorganic salts (e.g., potassium chloride and sodium chloride), the drilling fluid prevents the cuttings from hydrating and dispersing into the fluid phase; this ensures the cuttings remain intact and do not hydrate or disintegrate, thereby preventing an undesirable increase in the fluid's specific gravity. Fluid loss prevention and wellbore plugging are critical priorities in drilling fluid management; specifically, prior to penetrating salt-bearing formations (salt domes), the addition of salt-resistant inhibitors to the drilling fluid can effectively mitigate-or entirely prevent-contamination of the fluid by the salt formation.
Wellbore Stability and Water Influx Control
During the drilling process, particular attention must be paid to adjusting the properties of the drilling fluid before penetrating certain formations prone to collapse, in order to ensure the stability of the wellbore walls. When drilling through formations such as the Zhiluo and Fuxian Groups, it is essential to switch to a borehole-stabilizing mud system within 20 meters prior to entering any collapse-prone strata. Furthermore, mud fluid loss must be strictly controlled-kept below 5 mL-to ensure borehole wall stability. Additionally, when drilling in mature oilfields, wells penetrating the Yanchang Group oil zones often encounter high-pressure water influx from adjacent water-injection wells. To effectively mitigate this issue, a strategy may be employed involving the use of high-density mud to hydraulically fracture the water-bearing zone, followed by the injection of a controllable gel to permanently seal off the water source.
Hydrocarbon Influx and Lubrication
During drilling operations, should high-pressure hydrocarbon-bearing zones be encountered, the mud density must be increased in accordance with the client's specifications. In the event of a hydrocarbon influx or a blowout, the blowout preventer (BOP) must be closed immediately. Subsequently, the required mud density is calculated based on the downhole pressure to counteract the high-pressure zone and ensure well control safety. Furthermore, maintaining an adequate supply of lubricants during drilling helps reduce the resistance encountered during drill string tripping (lowering) and rotation, thereby increasing the rate of penetration (ROP) and ensuring the smooth execution of drilling operations.
Power Transmission and Rock Crushing
During drilling operations utilizing screw turbine assemblies, the drilling mud flows at high velocity through the turbine blades, driving both the turbine and the drill bit to rotate-a mechanism that significantly boosts the rate of penetration. When the drill bit is equipped with nozzles, the mud pumps generate high-pressure fluid jets that effectively fragment the rock at the bottom of the borehole, thereby facilitating highly efficient drilling. This unique combination of power transmission and rock-crushing mechanisms renders the drilling process both more efficient and more precise.
