Modeling strain and pore pressure associated with fluid extraction: The Pathfinder Ranch experiment

Abstract

Strainmeters can be subject to hydrologic effects from pumping of nearby water wells, depending on the state of the local rock. Strain signals associated with hydrology are generally not used and regarded as troublesome because they are much larger than most tectonic signals (e.g., tides or slow slip episodes in Cascadia), but here we show that fluid extraction leads to detectable strain and pore pressure signals, which we use to constrain valuable material properties of the rock, namely the hydraulic diffusivity and elastic shear modulus. We collected multiple years of pump activity at two active water wells near a pair of Plate Boundary Observatory borehole strainmeters in southern California. These data demonstrate clearly the connection between fluid extraction and deformation: the onset of transient strains and pore pressures is strongly correlated with both the onset of fluid extraction, and the sizes of the transient signals are strongly correlated with cumulative extraction volumes. These data also suggest that the instruments are a possible tool for remote monitoring of fluid injection and withdrawal. Based on poroelastic modeling, we find estimates of hydraulic diffusivity (0.061 m 2 s −1 to 0.126 m 2 s −1 ) which are consistent with data for fractured igneous rock, and estimates of shear modulus (39.7 MPa to 101 MPa) which are comparable to data for shallow granodiorite-expected to be weak from weathering, and other sources of damage (e.g., faulting). We infer that crustal rock in this region is drained at shallow depths by pervasive, hydraulically conductive fractures: as a result of changes in applied stress, fluid flow will occur rather than a sustained change in pore fluid pressure.

Pore pressure sensitivities to dynamic strains: Observations in active tectonic regions

“…As was mentioned previously, the regression-based results represent an apparent modulus; hence, the true reductions in modulus are likely multiple orders of magnitude smaller than those seen in Figure 10, for example. But in consideration of recent tomographic imaging [Allam and Ben-Zion, 2012;Allam et al, 2014], observations from dense seismic array deployments [Zigone et al, 2014], space geodetic studies , inferences from poroelastic modeling [Barbour and Wyatt, 2014], and the data presented here, it is highly likely that significant reductions in modulus are ubiquitous around the San Jacinto Fault. More work is needed, though, to firmly establish the magnitudes and spatial extent and of these reductions.…”

“…For B082, 15 of 54 estimates during this time period are deemed spurious, whereas at B084 only 3 of 54 are deemed spurious. The greater variability in pore pressure response and number of spurious points at B082 is due to daily and subdaily effects associated with anthropogenic fluid extraction [Barbour and Wyatt, 2014].…”

The Effects of Varying Injection Rates in Osage County, Oklahoma, on the 2016Mw 5.8 Pawnee Earthquake

“…Elevated levels of differential stress at centroid depths are apparent in both the homogeneous elasticity and layered elasticity cases, but the maximum difference between the two solutions is less than 1 kPa; hence, even with homogeneous elastic structure, stress localization is controlled by the hydraulic diffusivities in each layer. The pronounced dependence of both pore-pressure and stress changes on hydraulic diffusivity is a direct consequence of interrelated hydromechanical parameters (Kümpel, 1991;Barbour and Wyatt, 2014).…”

Inferring field‐scale properties of a fractured aquifer from ground surface deformation during a well test

“…In contrast, relative deformation instruments like tiltmeters and strainmeters were introduced in an innovative way to apprehend both short‐ and long‐term subsurface flow phenomena, first in volcanology where the signal is strong due to large inflating cavities [ Dzurisin , ] and later in hydrogeology thanks to technical improvement of these instruments [ Boudin et al , ; Hisz et al , ]. In particular, much effort has been dedicated into assessing how they can be used to characterize properties of groundwater flow, whether fluxes are man driven [ Chen et al , ; Barbour and Wyatt , ; Fabian and Kümpel , ] or natural [ Braitenberg , ; Jahr et al , ]. Furthermore, tiltmeters are very efficient when it comes to capture transient responses to subsurface progressive pressure variation, which is a convenient way to spot productive zones without having the need for supplementary boreholes [ Vasco et al , ].…”

“…Well F30, located at a few hundreds of meters west from the pumping site, was used as the nonmoving reference point for the least squares adjustment of the optical leveling data. are man driven [Chen et al, 2010;Barbour and Wyatt, 2014;Fabian and Kümpel, 2003] or natural [Braitenberg, 1999;Jahr et al, 2009]. Furthermore, tiltmeters are very efficient when it comes to capture transient responses to subsurface progressive pressure variation, which is a convenient way to spot productive zones without having the need for supplementary boreholes [Vasco et al, 2001].…”