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Drilling Fluids: Reverse-circulation Drilling

Most everyone in the drilling industry is familiar with forward rotary drilling, but unless you are involved with larger-diameter wells (16-36 in.) used for irrigation, muni-cipal or industrial applications, you probably are not aware of the advantages and some of the disadvantages of reverse-circulation (RC) drilling. Advantages with reverse-circulation drilling are reduced energy requirements to remove the drill cuttings, a cleaner borehole, less damage to the water-bearing formation, and less time needed for well development.

First, let us look at the drilling requirements between forward rotary drilling compared to reverse-circulation drilling on a 24-inch borehole for removing the cuttings. With forward rotary drilling, we recommend an up-hole velocity between 80 feet and 120 feet per minute. This can vary with cutting size, materials and viscosity. Normally, if we do not have enough pump volume to give us the velocity needed to clean the borehole, we have to increase the viscosity/thickness of the fluid to assist carrying out the cuttings. The same can be said with reverse-circulation drilling, but most reverse-circulation drillers use air to lift the cuttings, allowing for a greater variance in the up-hole velocity. The cuttings are removed up the center of the drill pipe, usually 6-inch or 8-inch diameter rather than the annular area between the drill stem and the borehole.

Using an up-hole velocity of 90 feet per minute, for a 24-inch borehole with forward rotary drilling, we would need to pump 2,000 gallons per minute (gpm). Use the formula for calculating ascending velocity of drilling fluids:

= velocity of drilling fluid in ft/min

= pump discharge, gpm

= diameter of hole, inches

= outside diameter of drill pipe, inches

At a 1,000 gpm, our up-hole ascending velocity would be only about 45 feet per minute. To ensure we can remove the cuttings, our viscosity (thickness) of the drilling fluid needs to be increased, which leads to increased energy costs, added weights to the fluid, and increased equipment to remove the solids at the surface. The added drilling fluid (solids) leads to increased pressure on the formation, allowing greater migration of the fluids (solids) into the formation. This can lead to premature plugging, which can increase time and costs when developing the well. Using reverse circulation, we are removing the cuttings up the center of the drill pipe, so the borehole diameter is not the major factor compared to forward rotary drilling, as the drill pipe is a constant. Therefore, to have an up-hole velocity in a 6-inch pipe of 90 feet per minute would only require 100/150 cfm, or less than 200 gallons per minute. However, larger volumes normally are used, and I have seen rocks the size of baseballs coming out the discharge line. We have a flooded borehole, and the down-hole velocity is minimal. The drilling fluid normally is a bentonite or bentonite/polymer with very low viscosity (32 to 45 sec./qt.). The lower viscosity fluids and reduced weights minimize down-hole pressures, fluid loss and filter cake.

One of the main problems I come across with reverse-circulation drilling is using too little or no drilling fluids or no additives. In doing this, we are overlooking one of the main rules for completing a good well – if you cannot control the upper portion of the borehole, you will have problems completing the bottom portion of the borehole. What happens when drilling fluids and additives are not used is we have problems washing out the top part of the borehole in sand/gravel and un-consolidated formations, excess fluid loss, and potential for the borehole to cave in on the drill bit. When this happens, you lose all or most circulation, and you are stuck. I recommend keeping a good drilling fluids mixture with bentonite and additives in the upper portion of the borehole, above the static water level or water-bearing formation. When you get to the water-bearing formation, and there is a concern about viscous drill fluids, you can thin down, minimizing contact with the formation and still having a good safe filter cake in the upper portion of the borehole. I have seen reverse-circulation boreholes take tens of thousands of excess gallons of water because of a poor drilling fluids program, and, in many of those cases, have 50 percent or more overrun on gravel back, due to washing out portions of the borehole.
Courtesy ND

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