Dear Hirak, although drilling wells it's not my area, I believe that the effectiveness of the reverse rotary method is due to the low velocity of the fluid (muddy water) on the walls of the well, sustended only by the hydrostatic pressure of the water column. In the mud rotary drilling method, even though one can use bentonite or similar fluids, the high velocity of the particles at the wall could disturb the inconsolidated soil in larger diameters. I hope i've helped.

There are multiple reasons that reverse circulation is preferable to direct circulation drilling. This is a bit complicated. In direct circulation drilling the drilling fluid travels down the drill stem and up the annulus of the borehole. The problems with that process are many. 1) The velocity of the fluid down the drill stem must be sufficient to deliver enough drill fluid to the bottom of the borehole to remove the cuttings, lubricate the bit and keep the borehole stable. That means that a large capacity pump is required. 2)To do that the amount of fluid required is related to the diameter of the annulus of the borehole and the diameter of the drill stem. The velocity is related to the pressure of the drill fluid within the drill stem. While drilling a large diameter borehole, the velocity within the annulus without drilling mud should be at least 100 ft per min. That means that a high pressure piston pump is the best tool. Piston pumps are expensive. 3) As the diameter of the borehole is increased the volume of fluid exiting the bit would need to be squared for a doubling of the diameter of the borehole. Therefore the pressure of the drill fluid exiting the drill bit will need to have high pressure so that the cuttings in the borehole can be brought to the surface. 4) That leads to the a problem of contamination of the aquifer. If one attempts to assist the removal of cuttings by adding drilling mud or allowing in-situ materials to act as a drilling mud, then one could suffer contamination of the aquifer with a thixotropic mud. The mud would then enter the aquifer, become gelled and cause the borehole to fail to produce water or limit the production of water. Over time the mud can travel to the well screen and block the screen entirely. 5) Because of the high volume of drill fluid required, erosion of the borehole walls is possible with direct circulation as the bit traverses the borehole. With reverse flow drilling the velocity of 100 ft per min is easier to attain and maintain. Because the drill fluid is always within the drill stem as it removes cuttings, the velocity of the drill fluid does not change with relation to the diameter of the borehole. It always is in relation to the diameter of the drill stem.

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