Hydraulic fracture propagation in a heterogeneous stress field in a crystalline rock mass

Abstract

Abstract. As part of the In-situ Stimulation and Circulation (ISC) experiment, hydraulic fracturing (HF) tests were conducted in a moderately fractured crystalline rock mass at the Grimsel Test Site (GTS), Switzerland. The aim of these injection tests was to improve our understanding of processes associated with high-pressure fluid injection. A total of six HF experiments were performed in two inclined boreholes, where the surrounding rock mass was accessed with twelve observation boreholes, which allow high-resolution monitoring of fracture fluid pressure, strain and micro-seismicity in an exceptionally well-characterized rock mass. A similar injection protocol was used for all six experiments to investigate the complexity of the fracture propagation processes. At the borehole scale, these processes involved newly created tensile fractures intersecting the injection interval while at the cross-hole scale, the natural network of fractures dominated the propagation process. The six HF experiments can be divided into two groups based on their injection location (i.e., south or north to a brittle ductile shear zone), their similarity of injection pressures and their response to deformation and pressure propagation. The injection tests performed in the south connect upon propagation to the brittle ductile shear zone. Thus, the shear zone acts as a dominant drain and a constant pressure boundary. The experiments executed north of the shear zone, show smaller injection pressures and larger backflow during bleed-off phases. From a seismic perspective, the injection tests show high variability in seismic response independent of the location of injection. For two injection experiments, we observe re-orientation of the seismic cloud as the fracture propagated away from the wellbore. In both cases, the main propagation direction is normal to the minimum principal stress direction. The re-orientation during propagation is interpreted to be related to a strong stress heterogeneity and the intersection of natural fractures striking different than the propagating hydraulic fracture. The seismic activity was limited to about 10 m radial distance from the injection point. In contrast, strain and pressure signals reach further into the rock mass indicating that the process zone around the injection point is larger than the zone illuminated by seismic signals. Furthermore, strain signals indicate not just single fracture openings but also the propagation of multiple fractures. Transmissivities of injection intervals increase about 2–4 orders of magnitudes.

Hydro-mechanical processes and their influence on the stimulation effected volume: Observations from a decameter-scale hydraulic stimulation project

“…Similar observations for all six HS experiments are documented in the appendix. The seismological responses of the rock mass and shear zones are covered in detail by Villiger et al (2019), and thus only a brief summary will be presented in this paper.…”

“…The shear zones contain few discrete fractures (i.e., brittle discontinuities) inside which formed during low temperature retrograde deformations of the shear zones (Wehrens et al, 2016). Note that the minor S1 shear zone HS8 in Figure 2 was not included in the geological model presented by Krietsch et al (2018b) as it was only recently localized from borehole intersection data and seismicity detected during HS8 (Villiger et al, 2019). The younger set, 10 referred to as S3, includes subparallel brittle-ductile shear zones with an average orientation of 183/65 ( Figure 2b).…”

“…Induced seismic events were located using an anisotropic velocity model based on manually picked P-wave onsets. For more details on the seismic monitoring and event localization, see Doetsch et al (2017), Gischig et al (2018) and Villiger et al (2019). 15…”

“…In this paper, we focus on the hydro-mechanical-coupled (HM) observations made during the six hydraulic shearing (HS) experiments conducted in February 2016. An overview of the six hydraulic fracturing (HF) experiments that were conducted 35 within the same rock mass in May 2016 is given in Dutler et al (2019). HF experiments conducted on a smaller scale as part of the stress characterization program are presented by (Gischig et al, 2018;Jalali et al, 2018a;Krietsch et al, 2018c).…”

“…HF experiments conducted on a smaller scale as part of the stress characterization program are presented by (Gischig et al, 2018;Jalali et al, 2018a;Krietsch et al, 2018c). The analysis of observed induced seismicity is presented by Villiger et al (2019). An extensive description of the overall project, stimulation experiments, characterization steps, and monitoring setup are presented by Doetsch et al (2017).…”