ABSTRACT:

Hydraulic fracturing tests on cubic granite samples with a side length of 50 mm were conducted under a true tri-axial test equipment combined with acoustic emission (AE) monitoring. The applied three principal stresses are 20 MPa (

s

v), 20 MPa (

s

H) and 10 MPa (

s

h), respectively. Experimental results show that cyclic hydraulic fracturing (CHF) can help reduce breakdown pressure as well as induced seismicity, compared to the conventional hydraulic fracturing. However, injectivity enhancement effect is less pronounced. Re-injection tests were conducted on the fractured samples by applying multiple incremental injection rates step-by-step. Propagation of existing fractures and creation of new fractures were confirmed, without inducing larger AE compared to continuous injection. Moreover, based on computed tomography (CT) imaging, fractures along grain boundaries are more frequently observed in CHF, particularly at large number of cycles and re-injection tests with multiple injection rates. Fractures cutting across mineral grains dominate as a result of conventional HF.

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