The paper presents a new model of rock-bit wear, and refers to bits utilized in ultra-hot wells drilled in the geothermal area of Larderello, Tuscany, Italy. In particular, the model, conceived from field data, indicates that the correct evaluation of rock-bit wear, considered in the light of the knowledge of drilling parameters and downhole conditions, can improve the overall drilling performances through a better interpretation of the relationships among rock-bit, formation properties and bottomhole temperature. This paper investigates the detrimental effects, not yet very well understood, of high bottom hole temperature on sealing and reservoir systems of friction bearing rock-bits. A particular bearing wear model for friction bearing rock-bits utilized in ultra-hot holes has been considered and further verified via laboratory inspections on dull bits. A novel interpretation of flat worn cutting structure has been derived from the above wear model and viable recommendations for improving the cost of drilled interval are suggested. Also, rock-bit field data from several Italian geothermal wells are reported and analized as an example of field application.

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