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

Abstract

AbstractStrainmeters 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 m2s−1 to 0.126 m2s−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.