Experimental Study for the Validation of Drilling Optimization Model for Improved Performance in Hard Rock Formations

Abstract

ABSTRACT: Lower Rate of Penetration (ROP) is one of the key challenges while drilling through deeper hard rock formations to access geothermal reservoirs in an advanced geothermal system (AGS). Rotary percussive drilling (RPD) becomes crucial for such systems in increasing ROP. The transferability of hammer drilling operations in deeper and harder rock formations, the mechanisms behind the improved drilling performance, and a nuanced understanding of the drilling dynamics of the method remains to be a critical challenge for a full-scale application of RPD in advanced geothermal systems. This study addresses these challenges through experimental validation of a developed bit performance index (BPI) model. The BPI model was developed using a reward function, the outcome is either rewarded or penalized. The concept of reward function usually exists in optimization and machine learning models. Using the combination of bit geometry, RPM, and hammer frequency, the function predicts optimum drilling dynamic parameters to maximize the ROP under given conditions. A series of experiments, limited to atmospheric pressure and ambient confining stress, were conducted using state-of-the-art hammer drop equipment developed in the laboratory to validate BPI, using dome-shaped diamond percussion inserts on granite rock samples. Efficiency estimates and volume removed by impacting the rock surface at different locations while moving away from the center of the crater were determined through these experiments. The results followed the BPI model – maximum efficiency and volume removal at the edge of the crater as compared with impacts at the center and center-edge. This study provides a novel approach to the characterization of drilling performance using RPD and gives a pathway for improved drilling performance through optimization of drillstring dynamics in deeper, harder rock formations for geothermal well drilling. 1. Introduction Rotary Percussion Drilling (RPD) had been widely used in the industry, before the introduction of rotary rigs in the mid-1930s, for its potential to drill faster in hard formations like granites. In RPD, a down-the-hole hammer is driven by either air or fluid (water) through drill pipes, which also acts as a cleaning mechanism for downhole cuttings.