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Abstract:
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.
