Permeability characterization of the Soultz and Ogachi large‐scale reservoir using induced microseismicity

Abstract

Hydraulic fracturing is a common procedure to increase the permeability of a reservoir. It consists in injecting high‐pressure fluid into pilot boreholes. These hydraulic tests induce locally seismic emission (microseismicity) from which large‐scale permeability estimates can be derived assuming a diffusion‐like process of the pore pressure into the surrounding stimulated rocks. Such a procedure is applied on six data sets collected in the vicinity of two geothermal sites at Soultz (France) and Ogachi (Japan). The results show that the method is adequate to estimate large‐scale permeability tensors at different depths in the reservoir. Such an approach provides permeability of the medium before fracturing compatible with in situ measurements. Using a line source formulation of the diffusion equation rather than a classical point source approach, improvements are proposed for accounting in situation where the injection is performed on a well section. This technique applied to successive fluid‐injection tests indicates an increase in permeability by an order of magnitude. The underestimates observed in some cases are attributed to the difference of scale at which the permeability is estimated (some 1 km3 corresponding to the seismic active volume of rock compared to a few meters around the well for the pumping or pressure oscillation tests). One advantage of the proposed method is that it provides permeability tensor estimates at the reservoir scale.

Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)

“…Therefore, we highlight that fracturing or slip on pre-existing fractures-through natural tectonic deformation or anthropogenic stimulation (e.g. Audigane et al 2002;Evans et al 2005;Evans 2005;Lengliné et al 2017)-must outperform fracture sealing to maintain the permeability required for geothermal circulation in the Upper Rhine Graben.…”

Earthquake behavior and structure of oceanic transform faults

“…The relatively narrow range of spatial migration velocities between 0.1 and 1.0 kmhr-may be the most direct evidence of aseismic fault slip. Observations of seismicity triggered by borehole fluid injection (Audigane et al 2002;Shapiro et aL. 2005) and subsurface fluid flow from magma degassing (Hainzl & Ogata 2005) consistently show earthquake hypocentres that spread following much slower pore-pressure diffusion, with distances that increase proportional to t 1 1 2 at rates not exceeding metres per day.…”

“…The 0.1-1.0 kmhr-' migration rate associated with aseismic fault slip and slow events is significantly faster than the migration rate of earthquakes observed in regions of CO 2 degassing and borehole fluid injections. Seismicity initiated by fluid overpressure tends to reflect fluid diffusion timescales, with earthquakes spreading spatially proportional to t 112 and migration velocities not exceeding fractions of a kilometre per day (Audigane et al 2002;Hainzl & Ogata 2005;Shapiro et al 2005). Based on the disparity between migration rates associated with fluid diffusion and aseismic slip, hypocentral migration velocities observed during seismic swarms may be used to infer the specific stress transfer mechanism driving seismicity, even if direct observational evidence of the mechanism is not available.…”

Magnetotelluric monitoring of hydraulic fracture stimulation at the Habanero Enhanced Geothermal System, Cooper Basin, South Australia

“…Enhanced geothermal systems (EGS) are unconventional geothermal resources with low permeability and relatively high temperature (typically > 200°C), which require fluid stimulation to enhance hydraulic connectivity in existing fracture system (Audigane, et al 2002, Evans, et al 2005, Muñoz 2014. A significant number of EGS potential resources occur around the world in varying geological settings e.g.…”