Time‐Dependent Deformations of Sandstone During Pore Fluid Pressure Oscillations: Implications for Natural and Induced Seismicity

Abstract

In tectonically active regions, natural seismicity is often correlated with the seasonal hydrology, suggesting that cyclic loading variations might trigger seismicity. Moreover, recent field observations suggest that cyclic fluid injection strategies into geological reservoirs could produce less seismicity than monotonic injections. Here we present 10 brittle creep laboratory triaxial experiments that bring new constraints on fluid-rock interactions during cyclic pore fluid variations. The experiments were performed on Fontainebleau sandstone with various pore fluid pressure conditions: (i) with constant pore fluid pressure levels from 1 to 10 MPa, at constant Terzaghi effective pressure (P c − P f = 30 MPa); and (ii) with cyclic (sinusoidal) pore fluid pressure oscillations of varying amplitudes (from 0 to 8 MPa) and periods (from 30 to 3,000 s) around a mean value of 5 MPa. During deformation, the rock's mechanical properties and the high-frequency acoustic emission signals were monitored to investigate the physics underlying the rupture processes. Under macroscopically drained conditions, rather than their amplitude, the period of the oscillations appeared to strongly affect the rock sample strength, time-to-failure, and dilatancy behavior. Moreover, even for small variations of pore fluid amplitude, and at all pore fluid pressure period, pore fluid pressure and acoustic emissions were strongly correlated. Our experiments demonstrate that pore fluid pressure oscillations may strongly affect rocks mechanical behavior and associated seismic activity.

Radon degassing triggered by tidal loading before an earthquake

“…A similar phenomenon was observed in the water injection experiment conducted after the earthquake 43 , 44 . In addition, the deformation of quartz-rich sandstone samples is dependent on the frequency of pore-pressure oscillation rather than its amplitude 49 ; samples exposed to lower-frequency pore-pressure oscillation exhibited substantial deformation. Accordingly, it can be considered that modulation of the crustal stress regime during 1990–1994 amplified only the radon signal related to the K1 tide.…”

Fault Reactivation During Fluid Pressure Oscillations: Transition From Stable to Unstable Slip

“…For the experiments where fluid pressure oscillated, we investigated the correlation between fast audible stress drops (hereafter referred to as stick-slip or unstable slip) and AE events with the fluid pressure oscillations. First, for each experiment, stick-slip and AE events were split with time intervals corresponding to the period of the fluid pressure oscillation as described in Noël et al (2019). This allowed for the estimation of the distribution of events over the period of a fluid pressure oscillation.…”

“…Indeed, high-pressure fluid injection can reduce the effective normal stress (σ n − P f ) acting on a fault, bringing the system closer to failure. If the effect of a linear increase in fluid pressure on fault reactivation has been widely studied (e.g., French et al, 2016;Passelègue et al, 2018;Rutqvist et al, 2016;Scuderi & Collettini, 2018), only little is known about cyclic fluid pressure perturbations (Chanard et al, 2019;Farquharson et al, 2016;Noël et al, 2019). In addition, while the quasistatic reactivation of faults is well understood, the influence of such fluid pressure perturbations on the slip behavior of faults remains poorly constrained.…”

“…With the growing interest in the use of georeservoirs, such cyclic stimulation might be of great interest for future injections. However, only a small number of experimental studies have been performed with the aim of increasing the comprehension of such phenomena, and those were mainly conducted on intact rock samples (Chanard et al, ; Farquharson et al, ; Noël et al, ; Zhuang et al, ).…”

Seismic and Microseismic Signatures of Fluids in Rocks: Bridging the Scale Gap

“…Monitoring MS (field scale) or AE (laboratory scale) activity has been successfully used to remotely characterize rock fracturing and slip events (e.g., hydraulic fracturing, shear fracturing/faulting, and reactivation), as well as fluid migration through porous and/or fractured rocks when local pore pressure perturbations impact the in situ stress field. Most recently, numerous studies focused on the interplay between fluids, fractures, and seismicity have been published (e.g., Barbosa et al, ; Brown & Ge, ; Cueto‐Felgueroso et al, ; De Barros et al, ; Dempsey et al, ; Dempsey & Suckale, ; Diehl et al, ; Fazio et al, ; Goodfellow et al, ; Improta et al, ; Jeanne et al, ; Levandowski et al, ; Noël et al, ; Rivet et al, ; Segall & Lu, ; Zhang et al, ). This highlights the growing interest for such issues in the geoscience community, which is clearly driven by an increased need to better address actual problems encountered during fluid injection/withdrawal operations.…”