A comparative study on PDC drillbit performance with a 4D dynamic model on samples of late cretaceous chalk for North Sea applications

Abstract

The overall goal of this work has been to select the most suitable PDC bit for 8 ½” Chalk-sections located in the North Sea. The Ekofisk field is the largest field in the North Sea and has long sections with Chalk of late cretaceous age. This Chalk is considered to be soft, homogenous and relatively easy to drill. However, stick slip, poor hole cleaning and limited penetration rates have been observed. The main goal was therefore to optimize Chalk drilling by reducing stick slip and increasing penetration rates. Using sophisticated and advanced simulation software, several bits were simulated in order to determine the best recommendation for Chalk applications in the Ekofisk Field. In order to simulate a formation as close to the reservoir characteristics as possible, offshore Chalk was sampled from cores at NPD and studied in detail. At the same time several samples from offshore and onshore Chalk were collected and analyzed. Based on geological data as a whole and not only porosity, the Kansas Chalk (for medium porosity) and Ulster Chalk (for lower porosity) were the most similar ones to North Sea Chalk. These observations are valuable for future work. Based on the rock study, two different simulations were performed. #5 bits were first simulated in hard, compressive Lueders Limestone followed up by simulating #2 bits in medium-soft Austin Chalk. Through these extensive simulations, an MDi619LBPX PDC bit generated the lowest amount of vibrations with highest penetration rates, and was selected for chalk drilling applications.