Oil drill bits aren't like hand drill bits. They're 3 gears at angles to each other connected to a LONG hollow pipe (up to FIVE MILES long) with about a HALF-MILLION POUNDS of weight on the top. The diameter of the gear set is wider than the pipe. As the gears spin from the pipe being turned, they GRIND rock.
Obviously, there has to be a way to get rid of that. A thick chemical called "drilling mud" is pumped down the pipe and comes back up the space between the outside of the pipe and the drilled hole and it carries up the ground rock, etc., with it.
If you're pumping 20 gallons a minute down and 20 gallons is coming back up, it's all rock, etc. If only 10 is coming back up it's because you hit an underground cave. They switch to cheap "filler" and just keep pumping until the cave is filled, then go back to the expensive drilling "mud".
If 30 gallons is coming back up, either you've hit water or GAS or OIL. If you don't control the oil/gas--you get a little thing called a "blowout" --
THINK OF THE EIFFEL TOWER WITH 50,000 GALLONS OF BURNING OIL SHOOTING UP FROM UNDER IT UNDER HIGH PRESSURE ...
THAT'S WHY you want to know how much is being pumped per minute and how much has been pumped total ...
