The most important and basic function of the drilling mud is to carry the cuttings broken by the bit to the surface through its own circulation, so as to keep the borehole clean, make the drill-down unimpeded and ensure that the bit can always contact and break the new formation at the bottom of the hole, which does not cause repeated cuttings and maintains safety and fast drilling. When the circulation is stopped for some reasons, the drilling mud will suspend the drilling cuttings in the drilling mud, so that the drilling cuttings will not sink quickly and prevent the occurrence of sand sticking.
The drilling mud with good performance can form a thin and tough mud cake on the well wall with the help of the filtration effect of the liquid phase, so as to stabilize the drilled formation, prevent the liquid phase from invading into the formation, and weaken the hydration expansion and dispersion degree of the shale. At the same time, in the process of drilling, it is necessary to adjust the drilling mud density to balance the formation pressure, so as to prevent the occurrence of complex conditions such as well collapse and blowout.
During drilling, the drill bit constantly rotates and breaks the rock stratum under the high temperature, which produces a lot of heat. At the same time, the drilling tool constantly rubs against the well wall to produce heat. It is through the continuous circulation of drilling mud that the heat is absorbed in time, and then brought to the ground and released into the atmosphere. Thus it plays the role of cooling the drill bit and the drilling tool, prolonging its service life. Due to the drilling mud, the drill bit and the drilling tool rotate in the liquid, so the friction resistance is reduced to a great extent, and it has a good lubrication effect.
The drilling mud impacts the bottom hole at a very high velocity at the bit nozzle, which improves the drilling speed and the rock breaking efficiency. High pressure jet drilling makes use of this principle, that is, high pump pressure drilling. It makes the high-speed jet formed by the drilling mud produce strong impact force on the bottom of the well, thus significantly increasing the drilling rate. When drilling with turbodrill, the drilling mud flows through the turbine blade with high velocity from the drill pipe, which makes the turbine rotate and drive the bit to break the rock.