CPV code 71331000 designates Drilling-mud engineering services in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
71331000 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 71330000 (Miscellaneous engineering services).