The development of a new sonic correlation for UCS estimation from drilling data

Abstract

One of the most important characteristics of rocks in drilling operations is unconfined rock strength (UCS), which is critical in different aspects of drilling operations. Several laboratory-based correlations have been generated for specific rocks to estimate UCS from physical properties (such as transient time, porosity, and Young's modulus) of the rocks. In drilling analysis, when UCS information is required and direct methods for estimation of UCS are not available, it is common to use correlations that have been developed for other formations with the same or similar lithology. Obviously, the results of estimations based on UCS correlations for other formations will not be accurate and can affect subsequent analyses. Therefore, it is highly recommended to generate a correlation for the formation of interest, though it is not always possible to reach this goal from experimental works on core samples retrieved from the formation. In this study, a sonic correlation that shows that it can provide relatively better global estimation of UCS for limestone rocks is modified for one of the Iranian carbonate formations by determining new coefficients for the correlation based on drilling data. For this purpose, the drilling information recorded in mud logging data is analyzed to backward simulate the drilling process based on a modified penetration rate model and calculate the rock strengths of the formation. The apparent rock strength log generated from this calculation proceeds quality-controlled steps according to statistical and pattern recognition methods to eliminate the noises and fluctuations that normally exist while working with field data. Then, a new correlation is developed from the formation response to sonic logs and apparent rock strength log. Because this new correlation is originally generated for the formation of interest, UCS is estimated more accurately and analyses dependent on UCS show fewer errors. Copyright © Taylor & Francis Group, LLC.