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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]

Seismic inversion of amplitude versus offset (AVO) data in viscoelastic media can potentially provide high-resolution subsurface models of seismic velocities and attenuation from offset/angle seismic gathers. P - and S -wave quality factors (Q), whose inverse represent a measure of attenuation, depend on reservoir rock and pore fluid properties, in particular, saturation, permeability, porosity, fluid viscosity and lithology; however, these quality factors are rarely taken into account in seismic AVO inversion. For this reason, in this work, we aim to integrate quality factors derived from physically based models in AVO inversion by proposing a gradient descent optimization-based inversion technique to predict the unknown model properties (P - and S -wave velocities, the related quality factors and density). The proposed inversion minimizes the non-linear least-squares misfit with the observed data. The optimal solution is iteratively obtained by optimizing the data misfit using a second-order limited-memory quasi-Newton technique. The forward model is performed in the frequency–frequency-angle domain based on a convolution of broad-band signals and a linearized viscoelastic frequency-dependent AVO (FAVO) equation. The optimization includes the adjoint-state-based gradients with the Lagrangian formulation to improve the efficiency of the non-linear seismic FAVO inversion process. The inversion is tested on synthetic seismic data, in 1-D and 2-D, with and without noise. The sensitivity for seismic quality factors is evaluated using various rock physics models for seismic attenuation and quality factors. The results demonstrate that the proposed inversion method reliably retrieves the unknown elastic and an-elastic properties with good convergence and accuracy. The stability of the inverse solution especially seismic quality factors estimation relies on the noise level of the seismic data. We further investigate the uncertainty of the solution as a function of the variability of the initial models. [ABSTRACT FROM AUTHOR]

While hydro-mechanical coupling in rocks is generally well understood, exotic rock poroelastic responses—such as unexpected dependence to fluid diffusion time and low skeleton moduli—have been reported. Hydro-mechanical coupling, or poroelasticity, explains how fluid-saturated rocks respond to either confining or fluid pressure variations. This coupling is usually inferred from the apparent mechanical and hydraulic properties: mechanical properties determine the strain level experienced by the rock when submitted to pressures at a timescale when fluid pressure is equilibrated, which is in turn ruled by hydraulic properties. However, the coupling between properties might not always be straightforward, particularly for rocks in which two distinct families of pore types coexist: spherical pores and cracks. Comparing it with reported laboratory data sets on pressure-dependent hydraulic and elastic properties in sandstones of different porosity confirms that the well-known simple concepts of networks in parallel apply and yield opposite dependencies for the two properties on the two pore families. Because hydro-mechanical coupling implies that the two properties—that depend differently on the pore families—will be interdependent, we further apply the same concept of parallel network. It yields that, although under apparent drained conditions, typical poroelasticity experiments could underestimate the rock compressibility |${C_{\mathrm{ bp}}}$|⁠ , measured as a response to fluid pressure variation, and underestimate the related skeleton (or unjacketed) bulk modulus |${K_\mathrm{ s}} = \ 1/{C_\mathrm{ s}}$|⁠. [ABSTRACT FROM AUTHOR]

Seismic wave attenuation is frequency dependent in rocks saturated by two fluid phases and the corresponding scaling behaviour is controlled primarily by the spatial fluid distribution. We experimentally investigate the frequency scaling of seismic attenuation in Berea sandstone saturated with two fluid phases: a liquid phase, water, and a gas phase, air, carbon dioxide or nitrogen. By changing from a heterogeneous distribution of mesoscopic gas patches to a homogeneous distribution of pore scale gas bubbles, we observe a significant steepening of the high-frequency asymptote of the attenuation. A transition from one dominant attenuation mechanism to another, from mesoscopic wave-induced fluid flow to wave-induced gas exsolution dissolution (WIGED), may explain this change in scaling. We observe that the high-frequency asymptote, for a homogenous pore scale gas bubble distribution, scales in accord with WIGED. [ABSTRACT FROM AUTHOR]

Biot's theory describes the coupled solid-fluid interaction in a porous medium. In the earth sciences, poroelasticity is essential in many applications, for example, in hydrogeology, in seismic monitoring of CO2 reservoirs, etc. In the present work, we solve Biot's equations for particle velocity and stress in the time domain with a finite difference approach on a staggered grid. Depending on the medium's properties, besides the elastic compressional and shear waves, one may observe the propagating slow wave or the diffusive static slow mode. In such a case, when the diffusive slow mode is present, the time-stepping should be very small in order to reach a stable simulation of Biot's equations. We present a new treatment of this problem based on the so-called pseudo-transient method. The idea is that at each time step, another pseudo iteration causes the slow mode to attenuate quickly. As a result, very fine time-stepping is unnecessary and the time-stepping is controlled by a standard Courant stability condition for the fast P-wave. Furthermore, we accelerate a finite-difference code using an NVIDIA graphics card with the CUDA programming language. [ABSTRACT FROM AUTHOR]