Impression blocks, a.k.a. “confusion” blocks, have been around since people first started running any tool in a well. This simple device is designed to reveal the identity, size and shape of the top of a “fish,” any piece of equipment left in a hole that should be removed. These fish can range from pieces of broken pipe, to tools that have been dropped, to sand or scale fill in the bottom of the hole. The impression block has long been used to identify what the fish is and what needs to be done to remove it from the hole. Impression blocks are simple devices. A piece of a soft metal, usually lead, about ⅜-in. (10-mm) thick, is mounted on a steel carrier block. The lead or babbitt is generally melted, poured into a mold that includes the block and filed or polished until it is circular with the same OD as the block with a flat face.
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