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8753854 
Meetings & Symposia 
Drill-bit metamorphism: Recognition and impact on show evaluation 
Wenger, LM; Pottorf, RJ; Macleod, G; Otten, G; Dreyfus, S; Justwan, H; Wood, ES 
2009 
Society of Petroleum Engineers (SPE) 
7 
4565-4573 
English 
With petroleum exploration and development progressing into deeper and more challenging environments, drilling technology has provided the means to effectively and safely reach and complete desired targets. Approaches such as turbo-drilling with polycrystalline-diamond compact (PDC) bits or diamond-impregnated bits increase penetration rates, lessen drill-bit changes, and are more easily steered in directional drilling, but also generate artifacts that may hinder recognition of indigenous hydrocarbons and interfere with geochemical interpretations. The higher bit speeds with turbo-drilling compared to rotary drilling (e.g., ∼ 1000 vs. 300 rpm, respectively) often generate additional heat that may essentially fuse rock cuttings with drilling mud, thermally crack oil-based drilling fluids, and produce both hydrocarbon and non-hydrocarbon artifacts. Bit-generated rock textures and pulverized drill cuttings, mixed with drilling mud cause difficulty in evaluation of lithology, age, and shows. The recognition of indigenous vs. artifact products is crucial to determining the presence and characteristics of hydrocarbons encountered in the well bore. These can be distinguished through analyses of mud gas and fluid-inclusion volatiles (FIV) analyses of drill cuttings. Analyses of mud gases show that gases typically not found in natural gases are generated during drill-bit metamorphism (DBM) including CO, ethene, and propene. Gases are rich in methane and C2-C4+ hydrocarbon gas components and may include H2S, CO2 and carbonyl sulfide (COS). Laboratory experiments suggest that thermal cracking of oil-based drilling mud can be the major contributor to these gases. Cracking of base oil alone generated wet gas with little CO2 and no H2S, while cracking of base oil in drilling mud generated wet gas with abundant CO2 and H2S. Drilling mud additive components such as lignosulfonate are likely sources of the generated CO2 and H2S. Recognition of DBM is crucial to effective drilling operations and evaluation of indigenous hydrocarbons. Copyright 2009, Society of Petroleum Engineers.