Experimental Investigation of Drillability Indices of Thermal Granite After Water-Cooling Treatment

Abstract

Understanding the drillability indices of thermal granite under various water-cooling conditions is of great significance for deep drilling and wellbore stability during the extraction of deep geothermal energy. In this paper, we report the results of micro-drilling tests, indentation hardness tests, friction-and-wear tests as well as conventional physico–mechanical tests on thermal granite after water-cooling treatment, and the relationships between mechanical strength and drilling parameters of granite are discussed based on statistical analysis. In addition, the micro-characteristics of thermal and water-cooling defects in granite were observed via scanning electron microscopy. With increase in thermal temperature, the conventional physico–mechanical parameters and indentation hardness of thermal granite after water-cooling decreased linearly, while the average values of drilling rate, plasticity coefficient, and the mass losses of granite specimens and the grinder increased exponentially. The average P-wave velocity, uniaxial compressive strength, tensile strength and indentation hardness decreased by 84.9, 66.2, 73.3 and 66.1%, respectively, when the granite was heated to 600 °C. At 600 °C, the wellbore wall of granite collapsed during the micro-drilling tests and the friction-and-wear tests, and the average width and density of micro-cracks of thermal granite increased to 20.54 μm and 4.82 mm/mm2. The average width and density of micro-cracks in thermal granite under various water-cooling conditions developed gradually with thermal temperature, which was the main reason for the degradation in the drillability indices of granite. Strong links exist between the mechanical strength and drilling parameters of granite after water-cooling, and the drilling parameters of thermal granite can be estimated by the use of mechanical strength with given empirical equations. This study provides a theoretical basis for the geothermally accurate simulation and engineering of wellbore stability for deep hot dry rock drilling.