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Hydraulic fracturing experiment · Injection rate · Acoustic emission · Hydraulic fracture morphology ·
Fluorescent tracer · Shale reservoirs
Abstract:
Hydraulic fracturing has been widely used to enhance reservoir permeability during the extraction of shale gas. As one of
the external input parameters, injection rate has a significant impact on formation breakdown pressure and the complexity of
hydraulic fractures. To gain deeper insights into the effect of injection rate on breakdown pressure and fracture morphology,
we conducted five hydraulic fracturing experiments on Changning shale in the laboratory. We used five different injection
rates between 3 and 30 mL/min to fracture cylindrical core samples with 50 mm in diameter and 100 mm in length. We
monitored acoustic emissions and surface displacements during the tests, and analyzed the fracture pattern post mortem by
using a fluorescent tracer. We find a semi-logarithmic relationship between the breakdown pressure and the injection rates.
Second, we find that it is the injection rate that dictates sample deformation and crack formation during breakdown rather than
the fluid volume injected during the whole process. The analysis of amplitudes and frequency of acoustic signals indicates
that hydraulic fracturing of Changning shale is overall dominated by tensile fractures (> 60%). However, at low injection
rates, shear events are facilitated before rock breakdown. On the other hand, high injection rates result in reducing fracture
tortuosity and surface roughness due to limited fluid infiltration in the relatively short injection window. We close this study
with a conceptual model to explain the difference between fluid infiltration (low injection rates) and the loading rate effect
(high injection rate) in low-permeability shale rocks. The findings obtained in this study can help to adjust injection rates in
the field to economically and safely produce gas from shale.
