Poroelastic Response of a Fractured Rock to Hydrostatic Pressure Oscillations.

Poroelastic coupling between fractures and the surrounding rock is important to numerous applications in geosciences. We measure the in‐situ fluid pressure and local strain response of a fractured carbonate sample to hydrostatic pressure oscillations. A linear poroelastic model that represents the rock sample is parameterized using X‐ray imaging and ultrasonic wave transmission measurements. The numerical solution, based on Biot's quasistatic equations, is consistent with the measured frequency dependent dispersion of the apparent bulk modulus of the background matrix and the in‐situ pore pressure response, which is caused by fluid pressure diffusion from the compliant fractures into the stiffer matrix. The observed fluid pressure diffusion is causally related to the numerically quantified intrinsic attenuation at seismic frequencies, which is a major contributor to the dissipation of seismic waves. Our analysis supports the use of a simple approximation of fractures as compliant and planar inclusions in numerical simulations based on linear poroelasticity. Plain Language Summary: Fractures control the flow of fluids through rocks as well as their mechanical properties. Finding ways to accurately simulate coupled hydro‐mechanical processes in fractured rock is important to a variety of applications in geosciences (e.g., subsurface storage of carbon dioxide or enhanced geothermal energy extraction). Physics‐based simulations require accurate parameterization and validation against experiments. In our experiment on a fluid saturated fractured rock sample, we applied an oscillating confining pressure to the sample and measured the corresponding deformation and the change in the pore fluid pressure in a fracture and the porous matrix. By adjusting the frequency of the oscillations, we observed a divergence in the pore pressure amplitude in the fracture and the matrix, which is a consequence of flow from the fracture into the porous matrix becoming restricted at elevated frequencies. The laboratory measurements were in close agreement with the results of our simulations, which were based on a simplified model of the rock sample. The outcome of our work supports the use of a widely applied approximation of fractures as simple planar inclusions in numerical simulations based on linear poroelasticity. Key Points: We observe the poroelastic coupling of fractures to the rock matrix in in‐situ fluid pressure measurements during stress oscillationsThe geometrically complex fractures can be modeled as compliant and planar poroelastic inclusionsThe numerically quantified intrinsic seismic (<100 Hz) attenuation is due to fluid pressure diffusion, which is experimentally observed [ABSTRACT FROM AUTHOR]