AbstractWall loss due to adhesive wear between a rotating drillstring tooljoint and the softer inner surface of a steel casing is often encountered while drilling deviated oil and gas wellbores. Despite the significance of the problem in the drilling industry, there is little data on casing wear efficiencies in the public domain. A large body of data collected during a joint industry project was deemed proprietary to participants who sponsored the private project (DEA-42). In this paper, we outline an approach to determine the wear efficiency of unlubricated surfaces by using the roughness parameters and material properties of the wearing surface. Our method combines the classic Greenwood & Williamson (1966) approach with the Archard-Rabinowicz (1953) interpretation of wear efficiency. The Greenwood & Williamson study models surface morphology as an ensemble of randomly distributed asperities. Archard (1953) interprets wear efficiency as the probability of a wear particle being created during an encounter of asperities between sliding surfaces. By using these notions we derive a formula for wear efficiency of an unlubricated surface as a function of the standard deviation of its summit height distribution, the summit radius, the hardness and contact modulus. Our predicted wear efficiencies are in the range of 1.5 - 5 ×10-3. These values agree well with published results for similar metals sliding on each other without lubrication. Future work along these lines will consider the effect of lubricants on adhesive wear efficiency.