September 17, 2021
The so-called reverse circulation means that when the drilling rig works, the rotary table drives the drill bit at the end of the drill pipe to cut and break the rock and soil in the hole. The flushing fluid flows into the hole bottom from the annular gap between the drill pipe and the hole wall, cools the drill bit, carries the cut geotechnical drilling slag, and returns to the ground from the inner cavity of the drill pipe. At the same time, the flushing fluid returns to the borehole to form a circulation. Because the inner cavity of drill pipe is much smaller than the diameter of wellbore, the rising speed of mud water in drill pipe is much faster than that of positive circulation. It is not only clean water, but also can carry the drilling slag to the top of the drill pipe and flow to the mud sedimentation tank. The mud can be recycled after purification.
Compared with the positive circulation, the reverse circulation has the advantages of fast drilling speed, less mud required, less power consumption of the rotary table, fast hole cleaning time, and using a special drill bit to drill and dig rock.
Reverse circulation drilling can be divided into gas lift reverse circulation, pump reverse circulation and jet reverse circulation according to the circulating transmission mode, power source and working principle of flushing fluid.
Gas lift reverse circulation drilling is also called pneumatic reverse circulation drilling, and its working principle is as follows:
Put the drill pipe into the drilling hole filled with flushing fluid, drive the airtight square transmission rod and drill bit to rotate and cut the rock and soil through the rotation of the rotary table, spray compressed air from the nozzle at the lower end of the drill pipe, and form a mud sand water air mixture lighter than water with the cutting soil and sand in the drill pipe. Due to the combined action of internal and external pressure difference of drill pipe and air pressure momentum, the mud sand water gas mixture and flushing fluid rise together and are discharged into the ground mud pool or reservoir through the pressure hose. Mud, sand, gravel and rock debris settle in the mud pit, and the flushing fluid flows into the hole.