A casing scraper comprises a generally cylindrical body (1) adapted for connection at one end to a drill string, having a plurality of scraping or scouring elements (2) disposed around the outer surface thereof. A pipe (6) extends through the cylindrical body (1). One end of the pipe (6) is adapted for connection to a source of cleaning fluid and the other end is connected to a tail pipe (7). The tail pipe (7) has at least one opening (8) in it through which cleaning fluid may be circulated into the well casing. The casing scraper further comprises filtration structure (4) located above the openings (8) in the tail pipe (7) and remotely operable structure (9, 10) for selectively directing the flow of cleaning fluid from passing outside the filtration structure (4) to passing through the filtration structure (4).