New mechanisms of rock-bit wear in geothermal wells

This paper presents the recent results of an investigation on failure mode and wear of rock-bits used to drill ultra-hot and abrasive sections of wells located in the geothermal area of Larderello (Italy). A new wear mechanism, conceived from field data analysis and dull bit evaluation, has been identified and a new configuration of rock-bits has been developed and tested. Test runs indicated a significant reduction of the drilling costs. Rock-bit wear is very often a critical aspect in drilling of both geothermal wells and ultra-hot holes for oil and gas production. Short life on bottom, strong cutting structure wear and unacceptable undergage are typical of rock-bits run through hole sections characterized by high temperature and/or formation abrasiveness. The role of high bottom hole temperature on rock-bit performances seems not yet very well understood: sofar, only drillability and formation abrasiveness are generally considered to account for poor drilling performances. In this paper, the detrimental effects of high temperature on sealing and reservoir system of friction bearing rock-bits have been investigated. In particular, a new bearing wear model for friction bearing rock-bits utilized in hot boreholes has been identified 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 the design of a new configuration of rock-bits are suggested, in order to reduce the cost of the drilled feet through an appropriate well planning and rock-bit design. Moreover, thanks to a major rock-bit manufacturer, some test bits, designed in the light of the above criteria, have been prepared and field tested successfully. Conclusions of the paper suggest that the correct evaluation of rock-bit wear can help to improve the overall drilling performances and to minimize drilling problems through a better interpretation of the relationships amongst rock-bit, formation properties and downhole temperature. The paper reports as well the results of the above successful field application, nowadays considered as a standard practice in Italian geothermal fields. An accurate analysis of drilling cost reduction is also presented and supported by a thorough comparison of several bit records from Italian geothermal wells. Key issues of the paper: 1) Mechanisms of rock-bit wear in ultra-hot and abrasive sections of geothermal wells. 2) Innovative indications for rock-bit design and bit selection in drilling engineering. 3) Cost reduction and drilling optimization from field experience.