FRICTIONAL MELTING OF SEDIMENTARY-ROCK DURING HIGH-SPEED DIAMOND DRILLING - AN ANALYTICAL SEM AND TEM INVESTIGATION

High-speed drilling at 900 rpm with a downhole motor-driven natural diamond drag bit has resulted in the extreme alteration of sedimentary rock in an offshore Canadian wildcat well. These effects, as revealed in the rock cuttings retrieved from the drilling fluid at surface, include discoloration (darkening), cataclasis, slickensiding and partial fusion. Calcareous siltstones are the most susceptible to alteration, especially melting, whereas sandstone and limestone cuttings mainly exhibit fracturing and surface glazing. Analytical scanning electron microscopy reveals that the calcareous siltstones have undergone decalcification via decarbonation and the loss of CaO to the drilling fluid. Decarbonation, combined with dehydration, resulted in the vesiculation and explosive fragmentation of certain cuttings. Analytical transmission electron microscopy shows that glass is present in the calcareous siltstone cuttings. Most of the modifications exhibited by the sedimentary rocks can be attributed to the effects of high strain-rate deformation and frictional heating at the bit-rock interface. That melting took place is supported by thermal calculations which indicate that, for a coefficient of friction of 0.5, temperatures of approximately 1200-degrees-C were realized at diamond-rock contacts. Such high temperatures can accelerate the wear of diamond and its substrate and hence shorten bit life, as well as obliterate potentially important geological information in the rock cuttings.