Investigations on thermal spallation drilling performance using the specific energy method

Thermal spallation drilling is an alternative technology expected to be suitable for the development of oil and gas in hard rocks. In this paper, experiments on six types of rocks were performed to analyze characteristics of rock spallation and spall distribution. Additionally, a specific energy model for thermal spallation drilling was established. The analytical temperature results were compared with experimental values for validation. Finally, the specific energy of granite was compared with those of other rock breaking methods. Results show that it is important to control the lowering velocity of the coiled tubing (recommended in field application) to obtain the optimal penetration rate during spallation drilling. The degree of heterogeneity of rock may play an important role in the spall-ability. The shale has the largest average aspect ratio (diameter/thickness) of 30.97, with the following sequence being dolomite, limestone, granite and sandstone. The granite shows the highest penetration rate, while the limestone has the lowest. The specific energy values for sandstone, granite, shale and limestone are approximately 1000 MPa. Compared with other rock breaking methods, thermal spallation has advantages when drilling hard rocks (e.g., granite). All results in this paper can provide important implications for further study on the thermal spallation drilling.