The poor rock drillability of the conglomerate has restricted the development of the abundant conglomerate reservoirs in Xinjiang. Lack of downhole rock drillability makes drilling operators usually tend to use conservative drilling parameters when drilling the conglomerate formation. The conventional logging parameter methods have significant errors because of the serious heterogeneity of conglomerate formation. An experimental study is conducted to investigate the correlation between the rock drillability of the conglomerate and its components. The bottom hole cores have been collected which have varied mineralogy, the volume fraction of gravel (Vg), the median grain size (D), and the compressive strength. Rock drillability is measured under the combined pressure conditions by Rollow's micro-bit method. Through the experiment, the influence of different components on the rock drillability has been obtained. The results indicate that the volume fraction of gravel (Vg) and the median grain size (D) have a better correlation with rock drillability. Vg has a linear correlation with rock drillability of conglomerate, and D has a logarithmic correlation with rock drillability of conglomerate. A model for rock drillability of the conglomerate based on different components has been derived. The novelty of the new model considers the influence of components on the rock drillability of the conglomerate for the first time. The findings contribute significantly to optimize the bit selection and drilling parameters, which provides an efficient and economic drilling operation.

1. Introduction

Rock drillability is a comprehensive characterization value to describe the difficulty of rock being invaded and destroyed, it was first proposed by Tillson (White, 1969). It is an intuitive, accurate, and easily accessible parameter to evaluate the rock mechanics of formation currently drilled in drilling engineering, and an important reference for bit selection (Yarali and Kahraman, 2011). Therefore, the technology to accurately evaluate the rock drillability plays an important role in improving the rate of penetration and reducing drilling costs in the oil and gas drilling operation. Head classified the rock drillability for the first time, he used the time of 1/16 inch drilling depth obtained by micro bit as an index to classify the formation successfully. The drilling parameters were 417lbs WOB and 110RPM. This method is the rock drillability classification DCN (Head, 1951). This method and its similar way have a huge advantage in conglomerate mechanic properties research. Because it can clearly obtain the components of the cuttings shearing down from the conglomerate. Therefore, it can help researchers to select rock-breaking regions of similar components to avoid errors caused by strong heterogeneity.

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