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Abstract

Hydraulic fracturing (HF) in crystalline rock has become increasingly important in geothermal development, especially for enhanced geothermal system (EGS). However, induced or triggered earthquakes reported from EGS sites is one of the main technical hurdles encountered. New hydraulic treatments with minimal environmental impact (i.e., controlled and mitigated induced seismicity) are of great interest. The replacement of conventional HF, which employs continuous injection, by cyclic HF (CHF) that produces cycles of alternating high and low injection rates or injection pressures is suggested to assist reduction of induced seismicity. Multiscale demonstration of the cyclic hydraulic treatment was conducted within the Work Package 5 of the EU Horizon 2020 international collaboration project “Demonstration of soft stimulation treatments of geothermal reservoirs” (Acronym: DESTRESS). Proof of concept of cyclic treatment by laboratory hydraulic fracturing under X-ray CT observations was led by Korea Institute of Civil Engineering and Building Technology (KICT). We developed experimental techniques and performed a series of hydraulic fracturing equipment allowing for different sizes of rock samples and various injection schemes to be tested. Laboratory HF and CHF tests on intact granite cores containing preexisting microcracks were performed under both biaxial and true triaxial stress conditions, combined with acoustic emission (AE) monitoring. Injectivity of fractured samples were measured by injection test for evaluation of hydraulic performance. Computed tomography and thin section microscopy were applied for grainscale observations on hydraulic fractures to help further understand hydraulic fracturing mechanism. Experimental findings show that CHF systemically reduced the breakdown pressure (BP) by ~20% and the maximum amplitude of AE by ~14 dB on average, compared with conventional HF. At the grain scale, intragranular fracturing dominated regardless of the injection pattern, whereas intergranular fractures between quartz and feldspar grains were more frequently observed in CHF, which explains the reduction in BP. Cyclic injection tends to form fracturing paths of least resistance thus to mitigate maximum amplitude of AE during fracturing. In addition, CHF creates complex fractures with more branches. However, CHF increases injectivity less than conventional HF and this is likely due to the lack of single predominant fracture in CHF fractured samples. Fractures generated in conventional HF contributed greatly to the increase of fluid flow. A modified CHF consisting of combination of cyclic injection and pulse pressurization at the peak of each cycle was tested and gave an improvement in both injectivity and decreasing induced seismicity, and is suggested as a promising alternative injection scheme for cyclic hydraulic treatment.