On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity

Abstract

Abstract. To characterize the stress field at the Grimsel Test Site (GTS) underground rock laboratory, a series of hydrofracturing and overcoring tests were performed. Hydrofracturing was accompanied by seismic monitoring using a network of highly sensitive piezosensors and accelerometers that were able to record small seismic events associated with metre-sized fractures. Due to potential discrepancies between the hydrofracture orientation and stress field estimates from overcoring, it was essential to obtain highprecision hypocentre locations that reliably illuminate fracture growth. Absolute locations were improved using a transverse isotropic P-wave velocity model and by applying joint hypocentre determination that allowed for the computation of station corrections. We further exploited the high degree of waveform similarity of events by applying cluster analysis and relative relocation. Resulting clouds of absolute and relative located seismicity showed a consistent east-west strike and 70 • dip for all hydrofractures. The fracture growth direction from microseismicity is consistent with the principal stress orientations from the overcoring stress tests, provided that an anisotropic elastic model for the rock mass is used in the data inversions. The σ 1 stress is significantly larger than the other two principal stresses and has a reasonably welldefined orientation that is subparallel to the fracture plane; σ 2 and σ 3 are almost equal in magnitude and thus lie on a circle defined by the standard errors of the solutions. The poles of the microseismicity planes also lie on this circle towards the north. Analysis of P-wave polarizations suggested double-couple focal mechanisms with both thrust and normal faulting mechanisms present, whereas strike-slip and thrust mechanisms would be expected from the overcoring-derived stress solution. The reasons for these discrepancies can be explained by pressure leak-off, but possibly may also involve stress field rotation around the propagating hydrofracture. Our study demonstrates that microseismicity monitoring along with high-resolution event locations provides valuable information for interpreting stress characterization measurements.

The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in situ stimulation experiment

“…Schorlemmer et al (2005) examined the b values of earthquakes in different stress regimes and found that lower values correlated with areas of higher differential stress. Similar trends have been reported for induced seismicity (Bachmann et al, 2012) but also in tectonic earthquakes (Tormann et al, 2014(Tormann et al, , 2015Spada et al, 2013) and laboratory experiments (Amitrano, 2012;Goebel et al, 2012). Thus, it was hypothesized that b values are related to local stress conditions (Scholz, 2015), or -in the context of induced earthquakes -to a combination of pressure and stress conditions.…”

“…Bachmann et al (2011) and Mena et al (2013) developed several statistical models and tested them in a pseudo-prospective manner using the Basel seismicity dataset. More complex models including physical considerations and stochastic processes (so-called hybrid models) were developed to include information on the reservoir behavior and from the spatiotemporal evolution of seismicity (Goertz-Allmann and Wiemer, 2013;Kiràly et al, 2018). Mignan et al (2015) evaluated reported insurance claims arising from the Basel induced seismicity in order to infer procedures for evaluating risk based on induced seismic hazard estimates.…”

“…2). The first phase (2015-2016) was a pre-stimulation phase that aims to characterizing the rock volume in terms of geological and structural conditions, the local stress state (Gischig et al, 2018), hydraulic and thermal properties, and fracture connectivity, all of which are essential for the design of the experiment and the interpretation of experimental results. In addition, during the pre-stimulation phase, a monitoring system was established that allows capturing the seismohydromechanical response at high spatial and temporal resolution.…”

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

“…The feasibility to monitor fracture processes of hydraulic fracturing in situ were demonstrated by Eisenblätter (1988), Niitsuma et al (1993), and Manthei et al (2003). The results from recent in situ experiments showed the efficiency of combined seismic monitoring (Gischig et al, 2018;Kwiatek et al, 2011;López Comino et al, 2017;Nakatani et al, 2008;Plenkers et al, 2010).…”

Transmissivity Changes and Microseismicity Induced by Small‐Scale Hydraulic Fracturing Tests in Crystalline Rock

“…Also, at the Basel geothermal project, a 400-fold irreversible permeability enhancement was observed after stimulation (Häring et al, 2008). Since during HF, permeability enhancement tends to be reversible, permanent fracture opening is achieved in shale gas fields by flushing sand or other particles (proppants) into the fractures along with the fluid (Gidley et al, 1989 geothermal energy exploitation, HS is the favored method because no proppant, or chemical additives to increase fluid viscosity for better proppant spreading, have to be used.…”