Elastic anisotropy of shales: the role of crack alignment and compliance ratio

Geophysics ◽  
2021 ◽  
pp. 1-18
Author(s):  
Jihui Ding ◽  
Anthony C. Clark ◽  
Tiziana Vanorio ◽  
Adam D. Jew ◽  
John R. Bargar
Keyword(s):  
Elastic Anisotropy ◽  
Rock Physics ◽  
Bedding Plane ◽  
Proposed Model ◽  
Anisotropy Parameters ◽  
Acoustic Velocities ◽  
Dip Angle ◽  
Physics Model ◽  
Rock Physics Model

Cracks, broadly defined as compliant discontinuities, are a major cause of elastic anisotropy. However, few models are available for quantifying crack properties relevant to anisotropy. We developed a rock physics model to quantify crack angular distribution and normal-to-tangential compliance ratio from pressure-dependent acoustic velocities measured in the laboratory. The proposed model utilizes a rectangular function of variable width and amplitude to extract the maximum dip angle of cracks, which is a direct quantification of crack alignment relative to the bedding plane. We tested the model on an organic-rich shale dataset and confirm that both crack alignment and compliance ratio strongly impact Thomsen anisotropy parameters, thus demonstrating the model as a useful tool for better understanding how cracks affect elastic anisotropy.

scholarly journals Rock physical characterisation of microstructural fabrics and elastic anisotropy for a shale oil reservoir

2020 ◽  
Vol 17 (2) ◽  
pp. 377-389
Author(s):  
Xiwu Liu ◽  
Zhiqi Guo ◽  
Qibin Zhang ◽  
Yuwei Liu ◽  
Haifeng Chen
Keyword(s):  
Elastic Anisotropy ◽  
Rock Physics ◽  
Physical Modelling ◽  
Velocity Model ◽  
Solid Matrix ◽  
Seismic Modelling ◽  
Anisotropy Parameters ◽  
Shale Reservoirs ◽  
Physics Model ◽  
Rock Physics Model

Abstract Rock physics models are constructed to describe elastic anisotropy of various fabrics in shales. A method is developed to invert elastic properties of the clay mixture in shales. Inversion results indicate that the clay mixture has abnormally low Vs and consequently leads to a very high Vp/Vs ratio, which is related to the presence of the soft interparticle medium in the clay mixture. Accordingly, a model is constructed to describe anisotropy clay mixture that consists of layering distributed illite-smectite particles, and a more compliant interparticle medium that has a bulk modulus similar to that of water and smaller nonzero shear modulus. Rock physical modelling indicates that an increase in the fraction of the soft interparticle medium (f-soft) leads to a dramatic increase in Vp/Vs ratios of the clay mixture. Meanwhile, the fraction of illite (f-illite) also shows a significant impact on elastic anisotropy of the clay mixture. Accordingly, a rock physical inversion scheme is further proposed to estimate the parameters f-soft and f-illite. Finally, based on these inverted parameters, elastic anisotropy values of various fabrics at different scale, including illite-smectite platelet, clay mixture, solid matrix of shale and shale rock, are computed by the constructed rock physics model. The estimated anisotropy parameters reveal lamination textures and can help in the evaluation of mechanical properties of shale reservoirs. Also, the obtained anisotropy parameters can provide an accurate velocity model for seismic modelling, seismic data processing and inversion.

scholarly journals Construction of a novel brittleness index equation and analysis of anisotropic brittleness characteristics for unconventional shale formations

2019 ◽  
Vol 17 (1) ◽  
pp. 70-85
Author(s):  
Ke-Ran Qian ◽  
Tao Liu ◽  
Jun-Zhou Liu ◽  
Xi-Wu Liu ◽  
Zhi-Liang He ◽  
...  
Keyword(s):  
Random Distribution ◽  
Rock Physics ◽  
Brittleness Index ◽  
Anisotropic Rock ◽  
Shale Formations ◽  
Shale Formation ◽  
Dip Angle ◽  
Physics Model ◽  
Rock Physics Model ◽  
Two Parameters

Abstract The brittleness prediction of shale formations is of interest to researchers nowadays. Conventional methods of brittleness prediction are usually based on isotropic models while shale is anisotropic. In order to obtain a better prediction of shale brittleness, our study firstly proposed a novel brittleness index equation based on the Voigt–Reuss–Hill average, which combines two classical isotropic methods. The proposed method introduces upper and lower brittleness bounds, which take the uncertainty of brittleness prediction into consideration. In addition, this method can give us acceptable predictions by using limited input values. Secondly, an anisotropic rock physics model was constructed. Two parameters were introduced into our model, which can be used to simulate the lamination of clay minerals and the dip angle of formation. In addition, rock physics templates have been built to analyze the sensitivity of brittleness parameters. Finally, the effects of kerogen, pore structure, clay lamination and shale formation dip have been investigated in terms of anisotropy. The prediction shows that the vertical/horizontal Young’s modulus is always below one while the vertical/horizontal Poisson’s ratio (PR) can be either greater or less than 1. Our study finds different degrees of shale lamination may be the explanation for the random distribution of Vani (the ratio of vertical PR to horizontal PR).

Frequency-dependent anisotropy due to two orthogonal sets of mesoscale fractures in porous media

2020 ◽  
Vol 221 (2) ◽  
pp. 1450-1467
Author(s):  
Da Shuai ◽  
Alexey Stovas ◽  
Jianxin Wei ◽  
Bangrang Di
Keyword(s):  
Seismic Anisotropy ◽  
Volume Fraction ◽  
Rock Physics ◽  
Wave Dispersion ◽  
Effective Porosity ◽  
Effective Frequency ◽  
Frequency Dependent ◽  
Anisotropy Parameters ◽  
Physics Model ◽  
Rock Physics Model

SUMMARY Seismic anisotropy can occur in rocks that have complicated internal structures and thin layering. Wave-induced fluid flow (WIFF) is one of the major causes of elastic wave dispersion and anisotropy. The principle goal of this paper is to combine the effects of WIFF and layer-induced anisotropy in orthorhombic (OTR) models that are often used in the seismic industry nowadays to describe azimuthal and polar anisotropy. We derive the effective frequency-dependent anisotropy parameters based on the Chapman model that accounts for the WIFF mechanism. First, we summarize two major problems to establish the link between the frequency-dependent seismic anisotropy and the multiple sets of fractures with different scales and orientations. Then we specify the multiple mesoscale fractures to be vertical and orthogonal so as to simplify the rock physics model to be an ORT medium. We also give the explicit expressions for the effective stiffness and the Thomsen-style parameters (vP0, vS0, ϵ1, ϵ2, γ1, γ2, δ1, δ2, δ3). Finally, we derive the effective frequency-dependent anisotropy parameters for ORT multiple layers using the Backus averaging under the approximation of weak contrast between layers. We also investigate the influence of frequency, fracture parameters (density and scale), effective porosity and volume fraction on the Thomsen-style parameters.

Multivariate probabilistic rock physics models using Kumaraswamy distributions

Geophysics ◽  
2021 ◽  
pp. 1-43
Author(s):  
Dario Grana
Keyword(s):  
Probability Density Function ◽  
Probability Density ◽  
Density Function ◽  
Random Variables ◽  
Rock Physics ◽  
Petrophysical Properties ◽  
Kumaraswamy Distribution ◽  
Model Predictions ◽  
Physics Model ◽  
Rock Physics Model

Rock physics models are physical equations that map petrophysical properties into geophysical variables, such as elastic properties and density. These equations are generally used in quantitative log and seismic interpretation to estimate the properties of interest from measured well logs and seismic data. Such models are generally calibrated using core samples and well log data and result in accurate predictions of the unknown properties. Because the input data are often affected by measurement errors, the model predictions are often uncertain. Instead of applying rock physics models to deterministic measurements, I propose to apply the models to the probability density function of the measurements. This approach has been previously adopted in literature using Gaussian distributions, but for petrophysical properties of porous rocks, such as volumetric fractions of solid and fluid components, the standard probabilistic formulation based on Gaussian assumptions is not applicable due to the bounded nature of the properties, the multimodality, and the non-symmetric behavior. The proposed approach is based on the Kumaraswamy probability density function for continuous random variables, which allows modeling double bounded non-symmetric distributions and is analytically tractable, unlike the Beta or Dirichtlet distributions. I present a probabilistic rock physics model applied to double bounded continuous random variables distributed according to a Kumaraswamy distribution and derive the analytical solution of the posterior distribution of the rock physics model predictions. The method is illustrated for three rock physics models: Raymer’s equation, Dvorkin’s stiff sand model, and Kuster-Toksoz inclusion model.

Porosity estimation based on rock skeleton general formula and comprehensive pore structure parameter – An application to a tight-sand reservoir

2021 ◽  
pp. 1-59
Author(s):  
Kai Lin ◽  
Xilei He ◽  
Bo Zhang ◽  
Xiaotao Wen ◽  
Zhenhua He ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Pore Structure ◽  
Seismic Data ◽  
Rock Physics ◽  
Structure Parameters ◽  
Pore Structure Parameter ◽  
Elastic Parameters ◽  
Physics Model ◽  
Rock Physics Model ◽  
Porosity Estimation

Most of current 3D reservoir’s porosity estimation methods are based on analyzing the elastic parameters inverted from seismic data. It is well-known that elastic parameters vary with pore structure parameters such as pore aspect ratio, consolidate coefficient, critical porosity, etc. Thus, we may obtain inaccurate 3D porosity estimation if the chosen rock physics model fails properly address the effects of pore structure parameters on the elastic parameters. However, most of current rock physics models only consider one pore structure parameter such as pore aspect ratio or consolidation coefficient. To consider the effect of multiple pore structure parameters on the elastic parameters, we propose a comprehensive pore structure (CPS) parameter set that is generalized from the current popular rock physics models. The new CPS set is based on the first order approximation of current rock physics models that consider the effect of pore aspect ratio on elastic parameters. The new CPS set can accurately simulate the behavior of current rock physics models that consider the effect of pore structure parameters on elastic parameters. To demonstrate the effectiveness of proposed parameters in porosity estimation, we use a theoretical model to demonstrate that the proposed CPS parameter set properly addresses the effect of pore aspect ratio on elastic parameters such as velocity and porosity. Then, we obtain a 3D porosity estimation for a tight sand reservoir by applying it seismic data. We also predict the porosity of the tight sand reservoir by using neural network algorithm and a rock physics model that is commonly used in porosity estimation. The comparison demonstrates that predicted porosity has higher correlation with the porosity logs at the blind well locations.

Sign in / Sign up

Export Citation Format

Share Document