The focus of this study was to evaluate the effect of heat treatment and increasing confining pressure on the CERCHAR abrasiveness index (CAI) and Sievers' J-value (SJ-value) on selected granitic rocks from Garam Chashma Pluton (GCP) and Kafiristan Pluton (KP) formations. The results of SJ-value test were further utilized for the computation of specific energy (S.E) for rotary drilling. Moreover, petrographic analyses were also performed to observe the behaviour of the creation of cracks introduced in rock fabric due to progressive heat treatment. Results show that for GCP and KP granites, CAI values decreased with an increase in temperature up to 500 °C while at 600 °C an increase in CAI values is observed which is attributed to a α–β quartz crystal transition, whereas SJ values (or penetration) kept on increasing despite an increase in the CAI value. Specific Energy (S.E.) values decreased with increasing temperature due to the thermal degradation of rocks. Further CAI and S.E. values have an increasing relationship with confining pressure, while SJ values have a decreasing relationship with confining pressure. It can be established from the findings of this study that overheating of the granitic rocks increases their abrasivity after a certain temperature. Therefore, using a combined thermo-mechanical drilling technique for the exploitation of deep geothermal reservoirs, thermal treatment may be provided in such a way as to avoid the occurrence of α–β quartz crystals transition limit. Highlights: Selected granite samples from Garam Chashma Region, Chitral Pakistan were heat treated. The heat-treated granite samples were subjected to CERCHAR abrasivity and Siever's J-value tests at ambient pressure. The heat-treated granite samples were subjected to CERCHAR abrasivity and Siever's J-value tests at elevated confining pressures to simulate in-situ pressure conditions. Effects of increasing temperatures and confining pressures on both the CERCHAR abrasivity test and Siever's J-value tests were analysed to develop meaningful statistical relationships. [ABSTRACT FROM AUTHOR]