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

Mechanical Compliance of Individual Fractures in a Heterogeneous Rock Mass From Production‐Type Full‐Waveform Sonic Data

The mechanical fracture compliance is of interest in a number of geoscientific applications. Seismic borehole methods, especially full-waveform sonic (FWS) data, have indicated their potential to infer the compliance of macroscopic fractures under in situ conditions. These approaches rely on the assumption of a homogeneous background embedding the fractures and, as of yet, compliance estimates for individual fractures are limited to static FWS measurements. In this work, we assess the potential of inferring the compliance of individual fractures from standard, production-type FWS data in the presence of background heterogeneity. We first perform a comparative test on synthetic data to evaluate three approaches known as the transmission, phase, and group time delay methods. The results indicate that the former two produce adequate compliance estimates for scenarios with a strongly heterogeneous background or a damage zone around the fracture. These two methods are then applied to two FWS data sets acquired before and after a hydraulic stimulation campaign in a crystalline rock, which allows to test them on natural and man-made fractures. The transmission method turned out to be unsuitable for the considered data due to its reliance on amplitudes. Conversely, the travel time behavior remained stable and the phase time delay method produced robust and consistent estimates. The results for a newly created hydro-fracture imply the capability of resolving remarkably small compliance values of the order of 10−14 m/Pa. This estimate is one order-of-magnitude smaller than that for the natural fracture, which may help to distinguish between these two fracture types

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

Fracture Unclogging: A Numerical Study of Seismically Induced Viscous Shear Stresses in Fluid‐Saturated Fractured Rocks

Dynamic shaking imposed by passing seismic waves is able to promote various hydrological processes in fractured reservoirs. This is often associated with seismically‐induced fracture unclogging due to mobilization of deposited colloids in the fracture network which, in turn, affects permeability at the reservoir scale. Numerous laboratory and field studies pointed out that fracture unclogging can be initiated when viscous shear stresses in the fracture fluid are in the range of 0.1‐1 Pascals. In this numerical study, we compute viscous shear stress in a fluid‐saturated fractured medium due to the action of passing P‐ and S‐waves. We perform a sensitivity analysis in terms of fluid, fracture, and host rock physical properties as well as seismic wave characteristics. Our results show that seismically‐induced viscous shearing increases with frequency and seismic strain and can be in the order of those initiating fracture unclogging for typical seismic strains and frequencies. S‐waves tend to produce viscous shearing approximately two times larger than P‐waves and, for anisotropic distribution of fractures, it is extremely dependent on the direction of wave propagation. Moreover, larger viscous shearing is expected for more viscous fluids and stiffer host rocks. Regarding the fracture network distribution, for the same fracture density, the presence of longer fractures drastically increases the potential of fracture unclogging at seismic frequencies. The fracture aperture distribution, on the other hand, can also affect the development of viscous shearing. Fractures with correlated distributions of contact areas exhibit an order of magnitude larger viscous shearing than uncorrelated ones

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

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

Estimation of Fracture Compliance From Attenuation and Velocity Analysis of Full‐Waveform Sonic Log Data

In fractured rocks, the amplitudes of propagating seismic waves decay due to various mechanisms, such as geometrical spreading, solid friction, displacement of pore fluid relative to the solid frame, and transmission losses due to energy conversion to reflected and transmitted waves at the fracture interfaces. In this work, we characterize the mechanical properties of individual fractures from P wave velocity changes and transmission losses inferred from static full‐waveform sonic log data. The methodology is validated using synthetic full‐waveform sonic logs and applied to data acquired in a borehole penetrating multiple fractures embedded in a granodioritic rock. To extract the transmission losses from attenuation estimates, we remove the contributions associated with other loss mechanisms. The geometrical spreading correction is inferred from a joint analysis of numerical simulations that emulate the borehole environment and the redundancy of attenuation contributions other than geometrical spreading in multiple acquisitions with different source‐receiver spacing configurations. The intrinsic background attenuation is estimated from measurements acquired in the intact zones. In the fractured zones, the variations with respect to the background attenuation are attributed to transmission losses. Once we have estimated the transmission losses associated with a given fracture, we compute the transmission coefficient, which, on the basis of the linear slip theory, can then be related to the mechanical normal compliance of the fracture. Our results indicate that the estimated mechanical normal compliance ranges from 1 × 10−13 to 1 × 10−12 m/Pa, which, for the size of the considered fractures, is consistent with the experimental evidence available.Fil: Barbosa, Nicolás D.. Universite de Lausanne; Suiza. Universidad de Ginebra; SuizaFil: Caspari, Eva. Universite de Lausanne; SuizaFil: Rubino, Jorge German. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Greenwood, Andrew. Universite de Lausanne; SuizaFil: Baron, Ludovic. Universite de Lausanne; SuizaFil: Holliger, Klaus. Universite de Lausanne; Suiza. Zhejiang University; Chin

Seismic signatures of partial steam saturation in fractured geothermal reservoirs: Insights from poroelasticity

Detecting the presence of gaseous formation fluids, estimatingthe respective volumes, and characterizing their spatial distributionare important for a wide range of applications, notably forgeothermal energy production. The ability to obtain such informationfrom remote geophysical measurements constitutes afundamental challenge, which needs to be overcome to addressa wide range of problems, such as the estimation of the reservoirtemperature and pressure conditions. With these motivations, wecompute the body wave velocities of a fractured granitic geothermalreservoir formation with varying quantities of steam toanalyze the seismic signatures in a partial saturation context.We use a poroelastic upscaling approach that accounts for mesoscalefluid pressure diffusion (FPD) effects induced by the seismicstrain field, and, thus, describes the governing physicalprocessesmore accurately than standard representations. Changesin seismic velocities due to steam saturation are compared with changes associated with fracture density variations, as both areplausible results of pressure changes in geothermal reservoirs.We find that steam saturation has a significant impact on P-wavevelocities while affecting S-wave velocities to a significantlylesser extent. This contrasting behavior allows us to discriminatebetween fracture density and steam saturation changes by meansof P- and S-wave velocity ratio analyses. To evaluate the potentialof seismic methods to provide this information, a canonical geothermalreservoir model is used to compute the Rayleigh wavevelocity dispersion and seismic reflection amplitude variationwith angle (AVA) curves. These studies reveal that AVA analysesallow differentiating changes in fracture density from changesin steam saturation. We also note that the Rayleigh-wave-basedtechniques are much less sensitive to steam content changes thanto fracture density changes. Comparisons with elastic approachesindicate that including FPD effects through the use of a poroelasticmodel is crucial for the reliable detection and characterizationof steam in fractured geothermal reservoirs.Fil: Quiroga, Gabriel E.. Universite de Lausanne; SuizaFil: Rubino, Jorge German. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Solazzi, Santiago Gabriel. Universite de Lausanne; Suiza. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Barbosa, Nicolás D.. Universite de Lausanne; SuizaFil: Favino, Marco. Universite de Lausanne; SuizaFil: Holliger, Klaus. Universite de Lausanne; Suiz