Variation of Drilling Rate Index (DRI) with temperature and its relationship with thermal damage on 'Prada' limestone

Abstract

This research evaluates the variation in the Drilling Rate Index (DRI) and its relationship with thermal damage on thermally treated rocks. Samples from 'Prada' limestone, a lower cretaceous formation in the southern Pyrenees (Lleida, Spain), were subjected to temperatures of 105, 300, 400 and 500 °C and then cooled at a slow rate. Ultrasound P-wave velocity tests were performed before and after heating the samples to evaluate the thermal damage experienced by the rock. Sievers' J miniature drill and brittleness tests were conducted on intact and thermally treated samples, and then resulting SJ and S20 values were combined to determine DRI. The obtained results show that thermal treatment allowed an increase of 34% in the DRI of 'Prada' limestone at 500 °C. DRI exhibited the same variation trend than S20, so we can conclude that thermal variation in DRI is more influenced by S20 than by SJ in 'Prada' limestone. We also report a strong relationship between DRI and P-wave velocity, confirming a tight dependence between the drilling performance and the thermally induced damage on the limestone. The observed substantial improvement in the drillability of the rock when heated, measured in terms of DRI value increase, could help in the advance on the development of thermally assisted mechanical excavation methods.