Hydraulic fracturing (HF) behavior of Pocheon granite was investigated through a series of HF tests under 7 different pumping rates varying from 1 mm3/s to 100 mm3/s considering cleavage dependent anisotropy of granite. Sleeve fracturing tests were also performed to exclude water infiltration influence. Cylindrical samples with diameter of 50 mm and height of 100 mm were used in tests. Test results show that pumping rate has a significant influence on the HF behavior of granite. This is much related to the pre-existing microcracks in granite and water infiltration. There is a threshold pumping rate to fracture the granite specimen under certain stress conditions. When the pumping rate is lower than the threshold value, water infiltrates granite matrix with slow increment of injection pressure, and the specimen finally reaches a completely saturation state without fracturing. Injection pressure develops nonlinearly with time when water infiltrates specimen while it develops approximately linearly in sleeve fracturing tests where water infiltration was excluded. Moreover, X-ray computed tomography (CT) observations show that the higher breakdown pressure results in a larger aperture of fractures in HF tests. Induced fractures by HF are more tortuous and have more branching than those induced by sleeve fracturing.
Hydraulic fracturing (HF) behavior has been widely studied over a period of years on various rocks like sandstones, shales, granites and even artificial rocks. In laboratory study, the HF behavior especially the breakdown pressure was very much influenced by pumping rate or injection rate, and the influence was also related to rock type. However, there was no consistent conclusion on the breakdown pressure variation with pumping rate. Some reported an increase of breakdown pressure with increasing pumping rate (Haimson and Fairhurst, 1969) while some others reported an opposite finding in sandstone (Zeng and Roegiers, 2002). Solberg et al. (1977) pointed out there is no difference in breakdown pressures at very slow pumping rates of 0.33–3.3 mm3/s. Zoback et al. (1977) explained that anomalously high breakdown pressures at high pumping rates may be due to pressure losses at the propagating crack tip. This study mainly investigated influence of pumping rate on granite specimen and the reason behind based on laboratory experiments.
