[anakaine]

Sorry mate, but thats quite the underinformed reply. You have pote tially hundreds of metres of drill string sitting above the face of the drill. Material is being vaporised or cut, and depending on whether this setup is traditional circulation or reverse circulation, you're going to have vaporised material, cut material, and sidewall material appearing at the cutting face. You do need a lot of gas to move it out of thw way, even if its dust. You have only the hole down the centre of the rotating string, or the hole between the outer and inner steel jacket of the string through which to send gas. That in its own is not a teivially small space. You need to swap rods, which necessarily introduces a depressurisation pathway and event every 6m, and of course you need enough pressure to lift cuttings. Gas recapture will be very difficult and dirty, so recirculation is going to be extremely troublesome, not to mention escape to formation.