A critical review of the experimental and theoretical research on cyclic hydraulic fracturing for geothermal reservoir stimulation

Hydraulic fracturing is necessary to stimulate deep geothermal reservoirs. Compared with traditional hydraulic fracturing (THF), cyclic hydraulic fracturing (CHF) decreases the breakdown pressure (BP), generates a more complex crack network and reduces the induced seismicity; thus, it has attracted increasing attention. In this work, state-of-the-art studies on CHF are comprehensively reviewed for the first time. Six CHF cyclic loading schemes are clarified, and their stimulation results are discussed. It is difficult to assess which loading schemes are the most advantageous because the reservoir boundary conditions and properties of rock specimens vary. The effects of critical influential factors on the stimulation results of CHF are reviewed in detail. A higher confining pressure generally produces longer cracks, and a lower-viscosity fracturing fluid more easily connects micropores and widens cracks. The existing theoretical and numerical models of CHF, which are based mainly on fracture mechanics and damage mechanics, are summarized and discussed. Based on this comprehensive review, some aspects through which the failure mechanism of CHF can be further understood are suggested. This work can benefit the application of CHF to exploit deep geothermal resources.