One of the most common applications for CT is the cleanout and removal of fill materials that restrict flow through tubing or casing (at right). Fill material can impede production by blocking the flow of oil or gas. It also may prevent the opening or closing of downhole control devices such as sleeves and valves. Common sources of fill are sand or fine material produced from the reservoir, proppant materials used during hydraulic fracturing operations, debris from workovers and organic scale. Fill removal typically involves circulating a cleanout fluid, such as water or brine, through a jet nozzle run on the end of the CT. The circulating fluids carry the debris back to the surface through the annulus between the CT string and the completion tubing.