Seismic attenuation and velocity dispersion in heterogeneous partially saturated porous rocks

Using a numerical approach, we explore wave-induced fluid flow effects in partially saturated porous rocks in which the gas-water saturation patterns are governed by mesoscopic heterogeneities associated with the dry frame properties. The link between the dry frame properties and the gas saturation is defined by the assumption of capillary pressure equilibrium, which in the presence of heterogeneity implies that neighbouring regions can exhibit different levels of saturation. To determine the equivalent attenuation and phase velocity of the synthetic rock samples considered in this study, we apply a numerical upscaling procedure, which permits to take into account mesoscopic heterogeneities associated with the dry frame properties as well as spatially continuous variations of the pore fluid properties. The multiscale nature of the fluid saturation is taken into account by locally computing the physical properties of an effective fluid, which are then used for the larger-scale simulations. We consider two sets of numerical experiments to analyse such effects in heterogeneous partially saturated porous media, where the saturation field is determined by variations in porosity and clay content, respectively. In both cases we also evaluate the seismic responses of corresponding binary, patchy-type saturation patterns. Our results indicate that significant attenuation and modest velocity dispersion effects take place in this kind of media for both binary patchy-type and spatially continuous gas saturation patterns and in particular in the presence of relatively small amounts of gas. The numerical experiments also show that the nature of the gas distribution patterns is a critical parameter controlling the seismic responses of these environments, since attenuation and velocity dispersion effects are much more significant and occur over a broader saturation range for binary patchy-type gas-water distributions. This analysis therefore suggests that the physical mechanisms governing partial saturation should be accounted for when analysing seismic data in a poroelastic framework. In this context, heterogeneities associated with the dry frame properties, which do not play important roles in wave-induced fluid flow processes per se, should be taken into account since they may determine the kind of gas distribution pattern taking place in the porous roc

Seismic attenuation: effects of interfacial impedance on wave-induced pressure diffusion

Seismic attenuation and dispersion in layered sedimentary structures are often interpreted in terms of the classical White model for wave-induced pressure diffusion across the layers. However, this interlayer flow is severely dependent on the properties of the interface separating two layers. This interface behaviour can be described by a pressure jump boundary condition involving a non-vanishing interfacial impedance. In this paper, we incorporate the interfacial impedance into the White model by solving a boundary value problem in the framework of quasi-static poroelasticity. We show that the White model predictions for attenuation and dispersion substantially change. These changes can be attributed to petrophysically plausible scenarios such as imperfect hydraulic contacts or the presence of capillarit

Permeability effects on the seismic response of gas reservoirs

In this work, we analyse the role of permeability on the seismic response of sandstone reservoirs characterized by patchy gas-water saturation. We do this in the framework of Johnson′s model, which is a generalization of White′s seminal model allowing for patches of arbitrary geometry. We first assess the seismic attenuation and velocity dispersion characteristics in response to wave-induced fluid flow. To this end, we perform an exhaustive analysis of the sensitivity of attenuation and velocity dispersion of compressional body waves to permeability and explore the roles played by the Johnson parameters T and S/V, which characterize the shape and size of the gas-water patches. Our results indicate that, within the typical frequency range of exploration seismic data, this sensitivity may indeed be particularly strong for a variety of realistic and relevant scenarios. Next, we extend our analysis to the corresponding effects on surface-based reflection seismic data for two pertinent models of typical sandstone reservoirs. In the case of softer and more porous formations and in the presence of relatively low levels of gas saturation we observe that the effects of permeability on seismic reflection data are indeed significant. These prominent permeability effects prevail for normal-incidence and non-normal-incidence seismic data and for a very wide range of sizes and shapes of the gas-water patches. For harder and less porous reservoirs, the normal-incidence seismic responses exhibit little or no sensitivity to permeability, but the corresponding non-normal-incidence responses show a clear dependence on this parameter, again especially so for low gas saturations. The results of this study therefore suggest that, for a range of fairly common and realistic conditions, surface-based seismic reflection data are indeed remarkably sensitive to the permeability of gas reservoirs and thus have the potential of providing corresponding first-order constraint

Squirt flow in partially saturated cracks: a simple analytical model

This is a pre-copyedited, author-produced PDF of an article published in Geophysical Journal International following peer review. The version of record ``Santiago G Solazzi, Simón Lissa, J Germán Rubino, Klaus Holliger, Squirt flow in partially saturated cracks: a simple analytical model, Geophysical Journal International, Volume 227, Issue 1, October 2021, Pages 680–692’ is available online at: https://doi.org/10.1093/gji/ggab249

Impact of fracture clustering on the seismic signatures of porous rocks containing aligned fractures

The presence of fractures in a reservoir can have a significant impact on its effective mechanical and hydraulic properties. Many researchers have explored the seismic response of fluid-saturated porous rocks containing aligned planar fractures through the use of analytical models. However, these approaches are limited to the extreme cases of regular and uniform random distributions of fractures. The purpose of this work is to consider more realistic distributions of fractures and to analyze whether and how the frequency-dependent anisotropic seismic properties of the medium can provide information on the characteristics of the fracture network. Particular focus is given to fracture clustering effects resulting from commonly observed fracture distributions. To do so, we have developed a novel hybrid methodology combining the advantages of 1D numerical oscillatory tests, which allows us to consider arbitrary distributions of fractures, and an analytical solution that permits extending these results to account for the effective anisotropy of the medium. A corresponding numerical analysis indicates that the presence of clusters of fractures produces an additional attenuation and velocity dispersion regime compared with that predicted by analytical models. The reason for this is that a fracture cluster behaves as an effective layer and the contrast with respect to the unfractured background produces an additional fluid pressure diffusion length scale. The characteristic frequency of these effects depends on the size and spacing between clusters, the latter being much larger than the typical spacing between individual fractures. Moreover, we find that the effects of fracture clustering are more pronounced in attenuation anisotropy than velocity anisotropy data. Our results indicate that fracture clustering effects on fluid pressure diffusion can be described by two-layer models. This, in turn, provides the basis for extending current analytical models to account for these effects in inversion schemes designed to characterize fractured reservoirs from seismic data.Fil: Barbosa, Nicolás D.. Universite de Lausanne; SuizaFil: Rubino, Jorge German. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Caspari, Eva. Universite de Lausanne; SuizaFil: Holliger, Klaus. Universite de Lausanne; Suiz

Fluid pressure diffusion effects on the seismic reflectivity of a single fracture

When seismic waves travel through a fluid-saturated porous medium containing a fracture, fluid pressure gradients are induced between the compliant fracture and the stiffer embedding background. The resulting equilibration through fluid pressure diffusion (FPD) produces a frequency dependence of the stiffening effect of the fluid saturating the fracture. As the reflectivity of a fracture is mainly controlled by the stiffness contrast with respect to the background, these frequency-dependent effects are expected to affect the fracture reflectivity. The present work explores the P- and S-wave reflectivity of a fracture modeled as a thin porous layer separating two half-spaces. Assuming planar wave propagation and P-wave incidence, this article analyzes the FPD effects on the reflection coefficients through comparisons with a low-frequency approximation of the underlying poroelastic model and an elastic model based on Gassmann's equations. The results indicate that, while the impact of global flow on fracture reflectivity is rather small, FPD effects can be significant, especially for P-waves and low incidence angles. These effects get particularly strong for very thin and compliant, liquid-saturated fractures and embedded in a high-permeability background. In particular, this study suggests that in common environments and typical seismic experiments FPD effects can significantly increase the seismic visibility of fractures.Fil: Barbosa, Nicolás D.. Universite de Lausanne; SuizaFil: Rubino, Jorge German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. University of Western Ontario; CanadáFil: Caspari, Eva. Universite de Lausanne; SuizaFil: Milani, Marco. Universite de Lausanne; SuizaFil: Holliger, Klaus. Universite de Lausanne; Suiz

Fractures in Low‐Permeability Rocks: Can Poroelastic Effects Associated With Damage Zones Enhance Their Seismic Visibility?

Fluid pressure diffusion (FPD) between a fracture and a porous permeable background can increase the normal compliance of the fracture and, thus, its reflectivity. However, many fractured environments of interest are associated with background rocks that can be regarded as largely impermeable for the the typical frequencies employed in seismic surveys. Nonetheless, there is evidence to suggest that the seemingly ubiquitous presence of damaged zones (DZs) associated with fractures may provide the necessary hydraulic communication between fractures and their immediate surroundings for FPD to occur. Here, we assess the pertinence of this phenomenon. To this end, we consider a 1D elastic-poroelastic model, which comprises a poroelastic system consisting of a fracture embedded in adjacent DZ layers. This system is enclosed in an impermeable background represented by two elastic half-spaces. We calculate the frequency-dependent P-wave reflectivity at normal incidence at the background-DZ interface for different permeabilities, thicknesses, and porosities of the DZ. We also evaluate the corresponding normal fracture compliance. Our results show that, when accounting for the presence of a DZ surrounding an individual fracture, FPD effects between these regions induce a higher seismic reflectivity and a higher normal compliance compared to that of a hydraulically isolated fracture. This, in turn, implies that, even in largely impermeable environments, the seismic visibility of fractures can be enhanced through FPD enabled by the presence of DZs.Fil: Sotelo, Edith. Universite de Lausanne; SuizaFil: Rubino, Jorge German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Solazzi, Santiago Gabriel. Universite de Lausanne; Suiza. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Barbosa, Nicolás D.. Universite de Lausanne; Suiza. Universidad de Ginebra; SuizaFil: Holliger, Klaus. Universite de Lausanne; Suiz

Seismic Signatures of Fractured Porous Rocks: The Partially Saturated Case

Seismic attenuation and phase velocity dispersion due to mesoscopic fluid pressure diffusion (FPD) have received increasing attention due to their inherent sensitivity to the hydromechanical properties of monosaturated fractured porous media. While FPD processes are directly affected by key macroscopic properties of fractured rocks, such as fracture density and fracture connectivity, there is, as of yet, a lack of comprehension of the associated characteristics when multiple immiscible phases saturate the probed fractured medium. In this work, we analyze the variations experienced by P and S wave attenuation and phase velocity dispersion when CO2 percolates into an initially brine-saturated fractured porous rock. We study such variations considering a simple model of a porous rock containing intersecting orthogonal fractures as well as a more complex model comprising a fracture network. In the latter, we simulate the flow of a CO2 plume into the medium using an invasion percolation procedure. Representative samples are subjected to numerical upscaling experiments, consisting of compression and shear tests, prior to and after the CO2 invasion process. Results show that fracture-to-background FPD is only sensitive to the presence of CO2, which decreases its effects. However, fracture-to-fracture FPD depends on both the overall CO2 saturation and the fluid distribution within the fracture network. While the former modulates the magnitude of the dissipation, the latter can give rise to a novel FPD process occurring between CO2-saturated and brine-saturated regions of the fracture network.Fil: Solazzi, Santiago Gabriel. Universite de Lausanne; Suiza. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Hunziker, Jürg. Universite de Lausanne; SuizaFil: Caspari, Eva. Universite de Lausanne; Suiza. Montanuniversität Leoben; AustriaFil: Rubino, Jorge German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Favino, Marco. Universite de Lausanne; SuizaFil: Holliger, Klaus. Universite de Lausanne; Suiza. Zhejiang University; República de Chin

Permeabilidad hidráulica y permeabilidad sísmica de medios porosos altamente heterogéneos

En el presente trabajo se realiza un análisis comparativo entre las curvas de atenuación y dispersión de muestras de roca con heterogeneidades mesoscópicas y las curvas que se obtienen al reemplazar el campo de permeabilidad heterogéneo por un campo homogéneo equivalente. Las curvas de atenuación y dispersión son estimadas utilizando un test de compresión, considerando una roca altamente heterogénea con heterogeneidades de tipo fractal. Para los campos homogéneos equivalentes se utilizan la media aritmética y la media armónica calculadas a partir del campo de permeabilidad, así como también la permeabilidad hidráulica efectiva estimada mediante un experimento numérico de flujo (test hidráulico). El análisis propuesto permite concluir que la permeabilidad hidráulica efectiva coincide con la permeabilidad sísmica en el rango de bajas frecuencias, lo que constituye un aporte importante ya que permite relacionar parámetros sísmicos con propiedades hidráulicas de medios porosos altamente heterogéneos.In the present work we perform a comparative analysis between the attenuation and dispersion cu rves of rock samples having mesoscopic heterogeneities and the curves obtained when the heterogeneous permeability field is replaced by an equivalent homogeneous permeability field. The attenuation and dispersion curves are estimated for a rock sample having fractal heterogeneities. As equivalent homogeneous permeability values the arithmetic and harmonic means computed from the heterogeneous permeability field are used, and also the effective hydraulic permeability obtained using a flow numerical experiment (hydraulic test). The proposed analysis allows to conclude that the effective hydraulic permeability coincides with the seismic permeability in the low frequency range, which is an important contribution because it allows to relate seismic parameters and hydraulic properties of highly heterogeneous porous media.Universidad Nacional de La Plat

Impact of poroelastic effects on the inversion of fracture properties from amplitude variation with offset and azimuth data in horizontal transversely isotropic media

<jats:p> The identification and characterization of fractures is an important objective in many areas of earth and environmental sciences. Amplitude variation with offset and azimuth (AVOAz) analysis of seismic reflection data is a key method for achieving these tasks. Theoretical and experimental studies have shown that the presence of pore fluids together with the strong mechanical contrast between the fractures and their embedding background give rise to wave-induced fluid flow (WIFF) effects. This implies that the effective stiffness tensor of a fluid-saturated fractured rock defining its seismic response becomes viscoelastic and frequency-dependent. In spite of this, AVOAz analysis typically relies on end-member-type elastic stiffness models that either assume a relaxed (i.e., equilibrated) or unrelaxed (i.e., unequilibrated) state of the wave-induced fluid pressure in the rock. In general, however, neither the appropriateness of the chosen model nor the associated errors in the inversion process are known. To investigate this topic, we have considered a poroelastic medium containing parallel vertical fractures and generate synthetic seismic AVOAz data using the classic Rüger approximations for PP-wave reflection coefficients in horizontally transversely isotropic media. A Markov chain Monte Carlo method is used to perform a Bayesian inversion of the synthetic seismic AVOAz data. We quantify the influence of WIFF effects on the AVOAz inversion results when elastic relaxed and unrelaxed models are used as forward solvers of inversion schemes to estimate the fracture volume fraction, the elastic moduli, and the porosity of the background rock, as well as the overall weakness of the medium due to the presence of fractures. Our results indicate that, when dealing with single-frequency data, relaxed elastic models provide biased but overall better inversion results than unrelaxed ones, for which some fracture parameters cannot be resolved. Improved inversion performance is achieved when using frequency-dependent data, which illustrates the importance of accounting for poroelastic effects. </jats:p&gt