Between conventional mud drilling and straight air drilling, there is another method that is well adapted to many parts of the country: reverse circulation drilling. In this method, the fluid and cuttings flow up the drill pipe instead of down, as with conventional drilling. The two most common uses are for large holes — 20-inches and larger — and in severe lost circulation zones where normal circulation can’t be maintained.
For large holes, where lost circulation isn’t a problem and circulation can be maintained, a jet eductor (JED) system is used. This uses a pump to move high-pressure fluid through a jet system to pull a vacuum on the drill pipe. Mud and cuttings return through the drill pipe into the pit. Water is added to keep fluid flowing back into the hole. If the hole is drinking too much water, it is common to use a bentonite slurry to help build wall cake downhole. This system works best in holes with a high fluid level, usually less than 20 feet.