Comprehensive data set of in-situ hydraulic stimulation experiments for geothermal purposes at the Äspö Hard Rock Laboratory (Sweden)

Abstract. In this article, a high-resolution acoustic emission sensor, accelerometer and broadband seismometer array data set is made available and described in detail from in-situ experiments performed at Äspö Hard Rock Laboratory in May and June 2015. The main goal of the hydraulic stimulation tests in a horizontal borehole at 410 m depth in naturally fractured granitic rock mass is to demonstrate the technical feasibility to generate multi-stage heat exchangers in a controlled way superior to former massive stimulations applied in enhanced geothermal projects. A set of six, sub-parallel hydraulic fractures is propagated from an injection borehole drilled parallel to minimum horizontal in-situ stress, and monitored by an extensive complementary sensor array implemented in three inclined monitoring boreholes and the near-by tunnel system. Three different fluid-injection protocols are tested: constant water injection, progressive cyclic injection, and cyclic injection with a hydraulic hammer operating at 5 Hz frequency to stimulate a crystalline rock volume of size 30 x 30 x 30 m at depth. We collected geological data from core and borehole logs, fracture inspection data from impression packer, acoustic emission hypocenter tracking and tilt data, as well as quantified the permeability enhancement process. The data and interpretation provided through this publication is an important step both, in upscaling laboratory tests, and downscaling field tests in granitic rock in the framework of enhanced geothermal system research.