RC, or reverse circulation, drilling is a tried and true drilling method in certain circumstances. Drillers usually use it on large-diameter holes because it is faster and easier to clean the hole. The fluid and cuttings flow up the drill pipe and down the annulus. In softer formations, this causes less washout and makes sampling faster and more accurate because cuttings are not contaminated by sloughing formation off the hole walls. They also come up the drill pipe very fast, giving an accurate formation log.
There are two general methods of RC. First is flooded reverse, where fluid is introduced through a flow ditch to the hole to keep it full. This works well in fairly unconsolidated formations that need some hydrostatic head. The trick here is to have enough water supply to keep up with any losses to the formation. In extreme loss situations, high-viscosity mud can be introduced to help stabilize the hole, but it might take A LOT of mud. Usually in these situations, we drill with direct circulation until we can get enough casing set to get into more stable formations. This RC method can be used with either a large centrifugal pump to maintain flow, or air lift. A centrifugal pump system will need a “rock catcher” between the Kelly hose and the pump to catch cobbles big enough to damage the pump.