Sometimes the flow of gas up through the production casing would not be fast enough to lift particles out of the well that could clog production. Or it may be that the particles would erode the vital well casing. In either case,or for other reasons, an additional metal "tubing" is lowered down the well inside the production casing. It is shown as orange in the image at the right. The gas escapes to the surface and then to market only through the tubing. The narrower tubing increases the velocity of the gas flowing out of the well to lift the troublesome particles, and it protects the casing. There is still pressure between the inside of the casing and the outside of the tubing even though the gas there is not moving.