Understanding the Hydromechanical Behavior of a Fault Zone From Transient Surface Tilt and Fluid Pressure Observations at Hourly Time Scales

Abstract

Flow through reservoirs such as fractured media is powered by head gradients which also generate measurable poroelastic deformation of the rock body. The combined analysis of surface deformation and subsurface pressure provides valuable insights of a reservoir's structure and hydromechanical properties, which are of interest for deep‐seated CO2 or nuclear waste storage for instance. Among all surveying tools, surface tiltmeters offer the possibility to grasp hydraulically induced deformations over a broad range of time scales with a remarkable precision. Here we investigate the information content of transient surface tilt generated by the pressurization a kilometer scale subvertical fault zone. Our approach involves the combination of field data and results of a fully coupled poromechanical model. The signature of pressure changes in the fault zone due to pumping cycles is clearly recognizable in field tilt data and we aim to explain the peculiar features that appear in (1) tilt time series alone from a set of four instruments and 2) the ratio of tilt over pressure. We evidence that the shape of tilt measurements on both sides of a fault zone is sensitive to its diffusivity and its elastic modulus. The ratio of tilt over pressure predominantly encompasses information about the system's dynamic behavior and extent of the fault zone and allows separating contributions of flow in the different compartments. Hence, tiltmeters are well suited to characterize hydromechanical processes associated with fault zone hydrogeology at short time scales, where spaceborne surveying methods fail to recognize any deformation signal.

Mechanical Response of Shallow Crust to Groundwater Storage Variations: Inferences From Deformation and Seismic Observations in the Eastern Southern Alps, Italy

“…The quasi simultaneity of modeled GWS and observed surface displacements ( Figure 4) suggests a quasielastic behavior, where water flow and deformation are decoupled. Following Schuite et al (2017), such behavior would require highly diffusive fractures (i.e., stiff fractures).…”

“…Following Schuite et al. (2017), such behavior would require highly diffusive fractures (i.e., stiff fractures).…”

“…In fact, while the effect of the direct stress can be considered instantaneous, pore‐pressure propagation exhibits a time lag between hydrological and seismological indicators, if pore‐fluid pressure variations are the main driver of earthquake rates modulation. The time lag τ is linked to both flow distance L and media/fracture diffusivity D by τ = L 2 / D (Schuite et al., 2017).…”

In situ hydromechanical responses during well drilling recorded by fiber-optic distributed strain sensing

“…In poroelastic theory, the deformation, such as soil consolidation, can induce "solid-to-fluid" coupled pressure change and fluid flow, whereas conversely, the fluid flow with pressure change can modify the effective stress of reservoir formation and cause "fluid-to-solid" coupled deformations (Cheng, 2016;Neuzil, 2003;Wang, 2017). The deformations could be the expression of fluid flow behavior in the reservoir and bear information regarding fluid flow and reservoir characteristics (such as permeability and compressibility) ( Barbour and Wyatt, 2014;Schuite et al, 2015Schuite et al, , 2017Schweisinger et al, 2009;. By monitoring strain changes of an aquifer, fluid-to-solid coupling can characterize the hydraulic parameters in the reservoir formation.…”

In situ hydromechanical responses during well drilling recorded by distributed fiber-optic strain sensing

“…In poroelastic theory, the deformation, such as soil consolidation, can induce "solid-to-fluid" coupled pressure change and fluid flow, whereas conversely, the fluid flow with pressure change can modify the effective stress of reservoir formation and cause "fluid-to-solid" coupled deformations (Cheng, 2016;Neuzil, 2003;Wang, 2017). The deformations could be the expression of fluid flow behaviour in the reservoir and bear information regarding fluid flow and reservoir characteristics (such as permeability and compressibility) (Barbour and Wyatt, 2014;Schuite et al, 2015Schuite et al, , 2017Schweisinger et al, 2009;. By monitoring strain changes of an aquifer, fluid-to-solid coupling can characterize the hydraulic parameters in the reservoir formation.…”