An estimation of rock strength using a drilling performance model: A case study in Blacktip field, Australia

Abstract

This paper presents the results of a case study performed in Well BT-1 in the Blacktip field; an offshore field located in northern Australia. An analytical model based on mud logging data was used to estimate a formation’s strength in Well BT-1. The drilled well is vertical and the input data to the model included the rate of penetration, rotary speed, weight on bit and torque values of the bit obtained from mud logging data. The final model was completed in several steps: estimation of the bit constants from either lab or field data, backward calculation of rock strength using a drilling performance model, smoothing of the estimated log to obtain the apparent rock strength log and classifying the formations into zones with similar strength values. The results of a log-based approach carried out in Well BT-1 were used to do the modelling. The results presented here indicate that the drilling performance model is able to predict the formation strength reasonably well. Based on the results, care should be taken when the model is used for the intervals where the bit is worn and dull, since within these intervals the model tends to over-estimate the rock strength.