Tracking Fluid Flow in Shallow Crustal Fault Zones: 2. Insights From Cross‐Hole Forced Flow Experiments in Damage Zones

Abstract

Cross-hole fluid injection tests were performed on shallow (0.5 km depth) crustal fault zones to characterize their internal flow structures on scales of 3-30 m. The data were acquired at the Grimsel Rock Laboratory, Switzerland, after the zonal isolation of damage zones in boreholes equipped with multipacker systems. We show that 80% of the pressure responses detected evolved as a power function of time due to fracture-controlled flow and were best described by a fractional model with a flow dimension (n) in the range of [1.3,1.5]. The scaling of characteristic times (t c ) and intermediate transitions in the flow dimension is shown to be inconsistent with the trend expected for normal diffusion, indicating a diffusion slowdown process caused by the spatial integration of structurally different damage zones. An analysis of the entire data set points to a subdiffusive regime where the mean squared displacement of pressure fronts, ⟨r 2 (t)⟩, scales as ⟨r 2 (t)⟩ ∝ t 0.59 , similar to an anomalous diffusion process on synthetic fractal systems. We characterize this diffusion slowdown by calculating a network connectivity exponent ( ) of 1.4, from which we estimate a fractal dimension of d f = 2.23 for the tested fault zones. Such a value close to 2.0 supports the view of fault damage zones as hierarchical systems promoting flow channeling. Our results provide further support to theoretical models of fractional flow and their application to fractured media in nature.

Tracking Fluid Flow in Shallow Crustal Fault Zones: 1. Insights From Single‐Hole Permeability Estimates

“…Similar fractures have been reported for parallel, low‐displacement (<10 m) faults separated by a few meters (Martel et al, , Figure 3a), and extensive field studies have identified and characterized linking damage zones from exhumed faults (Kim et al, ). If permeable, such damage zones may allow fluids to migrate across subparallel faults and play a key role in promoting fault‐controlled flow (Brixel et al, ).…”

“…For initial characterization, we deployed temporary straddle packer systems to screen the main structures identified from borehole image logs. This survey informed the final design of the pressure monitoring boreholes by defining the position and length of permanent observation intervals (for cross‐hole testing, see Brixel et al, ). Once completed, the downhole installation of 0.2‐m‐long packer seats and epoxy resin sections enabled recording fluid pressure in eight intervals in dedicated pressure monitoring boreholes.…”

Computing Localized Breakthrough Curves and Velocities of Saline Tracer from Ground Penetrating Radar Monitoring Experiments in Fractured Rock

“…The ISC experiment volume and its fracture network are fluid-saturated. Fluid flow was shown to be primarily fault-controlled in well testing experiments by Brixel et al [13], who found permeability to be strongly decreasing (from 10 −13 m 2 to 10 −21 m 2 ) within 1 m to 5 m from the fault cores. Brixel et al [12] determined the equivalent hydraulic apertures of single fractures in the rock volume to range from 2 to 130 µm, with a mean of 30 µm.…”

“…Therefore, additionally to the monitoring performed during the stimulations, pre-and post-stimulation characterizations were carried out with various geological, hydrogeological, and geophysical methods. Aside from GPR, as described here, by Giertzuch et al [30] and by Doetsch et al [39], extensive hydrologic testing was performed, tracer tests were run, and borehole logs were acquired, e.g., [7,12,13,38,[40][41][42][43].…”

“…Equally limited by reservoir access locations are hydraulic tests that can be used for reservoir characterizations. Besides determining hydraulic parameters such as transmissivities, permeabilities and hydraulic conductivities (e.g., [12,13]), such tests also allow for investigations on flow paths and their connectivity (e.g., [14]), they are dependent on discrete observation locations. Therefore, for the characterization of flow and transport processes in fractured rock, a method that can remotely retrieve information on tracer transport with high spatial resolution at various locations in the subsurface without the need of direct access points would be desirable.…”