Insights Into Complex Subdecimeter Fracturing Processes Occurring During a Water Injection Experiment at Depth in Äspö Hard Rock Laboratory, Sweden

Abstract

We investigate the source characteristics of picoseismicity (Mw < −2) recorded during a hydraulic fracturing in situ experiment performed in the underground Äspö Hard Rock Laboratory, Sweden. The experiment consisted of six stimulations driven by three different water injection schemes and was performed inside a 28‐m‐long, horizontal borehole located at 410‐m depth. The fracturing processes were monitored with a variety of seismic networks including broadband seismometers, geophones, high‐frequency accelerometers, and acoustic emission sensors thereby covering a wide frequency band between 0.01 and 100,000 Hz. Here we study the high‐frequency signals with dominant frequencies exceeding 1000 Hz. The combined seismic network allowed for detection and detailed analysis of 196 small‐scale seismic events with moment magnitudes MW < −3.5 (source sizes of decimeter scale) that occurred solely during the stimulations and shortly after. The double‐difference relocated hypocenter catalog as well as source parameters were used to study the physical characteristics of the induced seismicity and then compared to the stimulation parameters. We observe a spatiotemporal migration of the picoseismic events away and toward the injection intervals in direct correlation with changes in the hydraulic energy (product of fluid injection pressure and injection rate). We find that the total radiated seismic energy is extremely low with respect to the product of injected fluid volume and pressure (hydraulic energy). The radiated seismic energy correlates well with the hydraulic energy rate. The obtained fault plane solutions for particularly well‐characterized events signify the reactivation of preexisting rock defects under influence of increased pore fluid pressure on fault plane orientations in good correspondence with the local stress field orientation.

Normalized Radiated Seismic Energy From Laboratory Fracture Experiments on Opalinus Clayshale and Barre Granite

“…Comparison of normalized radiated seismic energy with other laboratory and field studies. Aspo data collected from study by Kwiatek et al (); field and other lab data collected from study by Goodfellow et al (). HF = hydraulic fracturing; FPB = four‐point bending; UNI = uniaxial compression.…”

“…In contrast, the proportion of volume reduction (−ISO) focal mechanisms in shale is higher than granite, suggesting that closure of pore space or bedding planes are significant factors in shale behavior even in tensile stress regimes as generated by HF or beam bending. Kwiatek et al (2018); field and other lab data collected from study by Goodfellow et al (2015). HF = hydraulic fracturing; FPB = four-point bending; UNI = uniaxial compression.…”

How to Reduce Fluid-Injection-Induced Seismicity

“…In Figure 8, hydraulic test parameters like FBP, RFP, flow rate and injected volume are compared with the total number of acoustic emission events localized (color bars) for different rock types and injection schemes. In Figure 9, the localized acoustic emission events are plotted in space together with tunnel geometry and fracturing borehole, F1 and three monitoring boreholes (Kwiatek et al 2017b). In the deeper section of the horizontal borehole, F1 three hydraulic tests are carried out in Ävrö granodiorite, two conventional continuous injection tests and one progressive cyclic injection test.…”

“…The first test, HF1 starts in the deeper part of the 28 m long, horizontal borehole (25 m), and the last test HF6, is operated about 5 m from the onset of F1 at the tunnel wall. Figure 9 shows the relocated AE hypocenters from the in-situ triggered recordings (Kwiatek et al 2017b). Color coding of fractures is the same as in Figure 8.…”

“…Fig. 9 Locations of 197 AE hypocentres (colour spheres) from six hydraulic testing intervals (colour disks) viewed in the direction of maximum horizontal stress at 410 m depth in Äspö HRL (Kwiatek et al 2017b, snapshot of movie). In the conventional tests with green (HF1), blue (HF2) and yellow injection intervals (HF6) the AE hypocentres form a planar structure (colour dots).…”