he so-called reverse circulation refers to the process in which the rotating disc drives the drill bit at the end of the drill rod to cut and break the rock and soil in the hole during drilling. The flushing fluid flows into the bottom of the hole from the annular gap between the drill rod and the hole wall, cools the drill bit, and carries the cut rock and soil drilling slag back to the ground from the inner cavity of the drill rod. At the same time, the flushing fluid returns to the hole to form a circulation. Due to the much smaller diameter of the drill pipe cavity compared to the wellbore, the rising speed of mud and water in the drill pipe is much faster than in the normal circulation. It is not only clear water, but also can bring drilling slag to the top of the drill pipe, flow to the mud sedimentation tank, and recycle the purified mud for use.
Compared with normal circulation, reverse circulation has the advantages of much faster drilling speed, less mud required, less power consumption of the rotary table, faster hole cleaning time, and the use of special drill bits for rock excavation.
Reverse circulation drilling can be divided into gas lift reverse circulation, pump suction reverse circulation, and jet reverse circulation according to the flushing liquid circulation transportation method, power source, and working principle. Gas lift reverse circulation drilling, also known as compressed air reverse circulation drilling, works as shown in the following figure. Insert the drill rod into the borehole filled with flushing fluid, and use the rotation of the rotary disc ⑦ to drive the airtight square transmission rod ② and the drill bit ⑤ to rotate and cut the rock and soil. Compressed air is sprayed from the lower end of the drill rod nozzle ④, forming a mud sand water gas mixture lighter than water inside the drill rod with the soil, sand, etc. that have been cut. Due to the combined effect of pressure difference inside and outside the drill pipe and pressure air momentum, the mud sand water gas mixture rises together with the flushing liquid and is discharged into the ground mud pit or water storage tank through the pressure hose ⑥. Soil, sand, gravel, rock debris, etc. settle in the mud pit, and the flushing liquid flows into the manhole again.

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