Hydraulic fracturing tests on Pocheon granite cylinders at seven different injection rates varying from 1 to 100 mm 3 /s were conducted. They were compared with sleeve fracturing tests in which borehole was sleeved, and therefore, water infiltration influence was excluded. Hydraulic fracturing behavior of granite is significantly influenced by water infiltration, which is closely related to the preexisting microcracks in granite as well as the cleavage anisotropy. There was a threshold injection rate to fracture the granite specimen under given stress conditions. When the injection rate is below the threshold, water infiltrated granite matrix with slow increment of injection pressure, and the specimen finally reached a full saturation without fracturing. Injection pressure developed nonlinearly with time during water infiltration, while approximately linearly when infiltration was excluded. For both hydraulic and sleeve fracturing tests, breakdown pressure increases with increasing injection rate. The breakdown pressures by sleeve fracturing were more than two times higher than those in hydraulic fracturing. X-ray computed tomography (CT) observations show that induced fractures are along the weaker cleavage parallel to the direction of the vertical stress. The higher breakdown pressure results in a larger aperture of fractures in hydraulic fracturing tests.
Laboratory hydraulic fracturing tests on cubic granite specimens with a side length of 100 mm were performed under true triaxial stress conditions combined with acoustic emission monitoring. Six different injection schemes were applied to investigate the influence of the injection scheme on hydraulic performance and induced seismicity during hydraulic fracturing. Three of these schemes are injection rate controlled: constant rate continuous injection (CCI), stepwise rate continuous injection (SCI), and cyclic progressive injection (CPI); the other three are pressurization rate controlled: stepwise pressurization (SP), stepwise pulse pressurization (SPP) and cyclic pulse pressurization (CPP). The test results show that the SPP scheme achieves the highest increase in injectivity among the six schemes. The CPI scheme generates the lowest induced seismicity while the improvement in injectivity is the least pronounced. The CPP scheme allows increasing injectivity and decreasing induced seismicity, and is suggested as a promising alternative injection scheme for field applications. Thin section microscopic observations of fractured specimens show that intragranular fractures splitting microcline, orthoclase and quartz grains dominate the hydraulic fractures independent of the injection scheme. The SPP scheme creates the largest fracture length, which explains the highest injectivity value among all schemes. Tests with relatively low magnitude of maximum AE amplitude correspond to short fracture length and small portions of intragranular fractures in microcline grains. Quartz grains are more fractured than microcline and orthoclase grains, and quartz chips (natural proppants) are frequently observed adjacent to hydraulic fractures. The laboratory test results show the potential for hydraulic fracture growth control in field applications by advanced fluid injection schemes, i.e. cyclic pulse pressurization of granitic rock mass.
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