Rock-physics relationships between inverted elastic reflectivities

Thomas, Michelle; Ball, Vaughn; Blangy, J. P.

doi:10.1190/tle35050438.1

Cited by 23 publications

(3 citation statements)

References 15 publications

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“…To do so, it is common practice to employ a linearized approximation of Zoeppritz' equations (e.g., Mavko et al, 1998) to retrieve impedances and velocities from reflection coefficients. We employ two classic approximations due to their widespread presence in the literature and considering that the information they can infer from seismic data may be different due to the different assumptions employed by the respective authors (e.g., Thomas et al, 2016). One of the approximations we employ is that of Fatti et al (1994), which in its two-term version approximates the P-wave reflectivity R P P as function of incidence angle θ as…”

Section: Ava Modellingmentioning

confidence: 99%

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

et al. 2023

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Detecting the presence of gaseous formation fluids, estimating the respective volumes, and characterizing their spatial distribution is important for a wide range of applications, notably for geothermal energy production. The ability to obtain such information from remote geophysical measurements constitutes a fundamental challenge, which needs to be overcome to address a wide range of problems, such as the estimation of the reservoir temperature and pressure conditions. With these motivations, we compute the body wave velocities of a fractured granitic geothermal reservoir formation with varying quantities of steam to analyze the seismic signatures in a partial saturation context. We employ a poroelastic upscaling approach that accounts for mesoscale fluid pressure diffusion (FPD) effects induced by the seismic strain field, and, thus, describes the governing physical processes more accurately than standard representations. Changes in seismic velocities due to steam saturation are compared with changes associated with fracture density variations, as both are plausible results of pressure changes in geothermal reservoirs. We find that steam saturation has a significant impact on P-wave velocities while affecting S-wave velocities to a significantly lesser extent. This contrasting behavior allows to discriminate between fracture density and steam saturation changes by means of P- and S-wave velocity ratio analyses. To evaluate the potential of seismic methods to provide this information, a canonical geothermal reservoir model is employed to compute Rayleigh wave velocity dispersion and seismic reflection amplitude vs angle (AVA) curves. These studies reveal that AVA analyses allow to differentiate changes in fracture density from changes in steam saturation. We also note that Rayleigh-wave-based techniques are much less sensitive to steam content changes than to fracture density changes. Comparisons with elastic approaches show that including FPD effects through the use of a poroelastic model is crucial for the reliable detection and characterization of steam in fractured geothermal reservoirs.

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Section: Ava Modellingmentioning

confidence: 99%

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

et al. 2023

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“…The impact of bias caused by 2T fitting (e.g. Thomas et al., 2016) is, however, often ignored in 2T AVO projections.…”

Section: Introductionmentioning

confidence: 99%

Sandstone detection using three‐term amplitude variation with offset projection: A case study

Asaka

2023

Geophysical Prospecting

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Seismic amplitude variation with offset projection is widely used in the oil and gas industry to extract information contained in amplitude variation with offset responses.Traditional two-term amplitude variation with offset projection, based only on intercept (A) and gradient (B), has been used in various ways, including hydrocarbon detection and the estimation of target properties such as water saturation. Three-term amplitude variation with offset projection, in which A, B and curvature (C) are blended using two projection angles (χ and ψ), can also be used for the same purposes and has some advantages over the two-term methods. The effectiveness of the three-term amplitude variation with offset projection in the detection of sandstone has been investigated using field data. A sandstone detection workflow is proposed, which includes (1) defining the target attribute, (2) optimizing projection angle in the χ-ψ analysis plane painted with the target attribute and (3) confirming/refining the selected projection angle using the two-term amplitude variation with offset projection-C cross-plot. The impact of anisotropy is taken into consideration, and the sandstone index is introduced as the target attribute. The sandstone detection workflow was applied for both synthetic and field data, which illustrates that three-term methods offer an important perspective on noise in the amplitude variation with offset attributes: Noise in the third term is correlated with noise in the other terms so that including C can actually suppress noise to some extent, even if C looks unusable. Moreover, two-term amplitude variation with offset projections are a subset of three-term amplitude variation with offset projections; therefore, if C adds no information, this fact is revealed as the worst-case outcome in the three-term analysis; inclusion of C will never degrade results and there is no reason to exclude C a priori.

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“…For this reason, linear approximations of the full Zoeppritz equations (see Thomas et al 2016) are routinely used to solve the AVA inversion problem, but a linear forward model might not be sufficiently accurate to describe the relation between seismic data and elastic parameters at far sourcereceiver offsets (i.e., incidence angles higher than 30°-35°) and significant elastic contrasts at the reflecting interface. In these cases, oversimplified forward operators could result in biased model parameter estimations.…”

Section: Introductionmentioning

confidence: 99%

Hamiltonian Monte Carlo algorithms for target- and interval-oriented amplitude versus angle inversions

Aleardi

Salusti

2020

GEOPHYSICS

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A reliable assessment of the posterior uncertainties is a crucial aspect of any amplitude versus angle (AVA) inversion due to the severe ill-conditioning of this inverse problem. To accomplish this task, numerical Markov chain Monte Carlo algorithms are usually used when the forward operator is nonlinear. The downside of these algorithms is the considerable number of samples needed to attain stable posterior estimations especially in high-dimensional spaces. To overcome this issue, we assessed the suitability of Hamiltonian Monte Carlo (HMC) algorithm for nonlinear target- and interval-oriented AVA inversions for the estimation of elastic properties and associated uncertainties from prestack seismic data. The target-oriented approach inverts the AVA responses of the target reflection by adopting the nonlinear Zoeppritz equations, whereas the interval-oriented method inverts the seismic amplitudes along a time interval using a 1D convolutional forward model still based on the Zoeppritz equations. HMC uses an artificial Hamiltonian system in which a model is viewed as a particle moving along a trajectory in an extended space. In this context, the inclusion of the derivative information of the misfit function makes possible long-distance moves with a high probability of acceptance from the current position toward a new independent model. In our application, we adopt a simple Gaussian a priori distribution that allows for an analytical inclusion of geostatistical constraints into the inversion framework, and we also develop a strategy that replaces the numerical computation of the Jacobian with a matrix operator analytically derived from a linearization of the Zoeppritz equations. Synthetic and field data inversions demonstrate that the HMC is a very promising approach for Bayesian AVA inversion that guarantees an efficient sampling of the model space and retrieves reliable estimations and accurate uncertainty quantifications with an affordable computational cost.

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Rock-physics relationships between inverted elastic reflectivities

Cited by 23 publications

References 15 publications

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

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

Sandstone detection using three‐term amplitude variation with offset projection: A case study

Hamiltonian Monte Carlo algorithms for target- and interval-oriented amplitude versus angle inversions

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