Current techniques for deciding when to pull out worn bits are based on speculation rather than reliable engineered methods. Two concepts have been previously reported in the literature to incorporate the effects of drilling parameters on forecasting the life of drill bits. Bit tooth flatness and specific energy approaches were used for assessing the bit tooth wear and predicting the rock formation and its properties being drilled. However, drilling shale formations as well as encountering abnormal geologic formations especially unconformities, reduces the reliability of these methods.
In this paper, a modified technique based on combined bit dullness and specific energy have been used as trending tools for determining the status of the drill bit even in cases where drill torque data is unavailable.
As case studies, currently producing oil wells from southern Iraq are analysed for bit wear. Estimated results of bit wear for each bit run were correlated with the qualitative field bit tooth flatness revealing a successful key index for the suitable time to pull out worn bits. Furthermore, literature values of unconfined compressive strength (UCS) of the drilled formations were compared with the computed mechanical specific energy (MSE) to validate the obtained results. Good agreement was observed making the study encouraging.
The analysis is promising for evaluating drill bit selection and predicting the type of formation being drilled.