Experiment study on cyclic injection schemes for the stimulation of an Enhanced Geothermal System (EGS) site

The concept of cyclic injection for the mitigation of large-magnitude fluid-injection induced events during hydraulic fracturing was first applied to the Enhanced Geothermal System (EGS) project in Pohang South Korea on 2017. The application of cyclic injection helped to keep the maximum seismicity under the threshold value of Mw 2.0 defined in the traffic light system. Although these results are encouraging, there is still a need of more field tests to validate this concept, especially with those with larger injection volumes. Within the framework of DESTRESS project, which is funded by the HORIZON 2020 European research program, this concept of cyclic injection will be tested at a second EGS site in Geldinganes, Iceland. The EGS project was drilled on 2001, and it is expected to be stimulated this year. A couple of injection schemes have been considered, and their application will be monitored closely through an extensive array of stations, and manage through an advanced traffic-light system. In this work, some main features of these injection schemes planned for the Geldinganes site were tested at the laboratory scale. The cyclic hydraulic fracturing tests evaluate the effect of cyclic injection, not only at a small, and middle term, but also at the large term (repeated sets of cycles). The tests were carried out using cubic granite samples with 100 mm length per side and a 5 mm diameter borehole drilled on top of the specimen up to a length of 70 mm. For all experiments we used tap water as injection fluid, and the acoustic emission activity was monitored with an array of 8 Nano sensors distributed on the lateral faces of the sample. Results of the acoustic emission activity, injectivity, and fracture geometry are discussed in detail.