Zhiqi Guo

A shale rock physics model for analysis of brittleness index, mineralogy and porosity in the Barnett Shale

We construct a rock physics workflow to link the elastic properties of shales to complex constituents and specific microstructure attributes. The key feature in our rock physics model is the degrees of preferred orientation of clay and kerogen particles defined by the proportions of such particles in their total content. The self-consistent approximation method and Backus averaging method are used to consider the isotropic distribution and preferred orientation of compositions and pores in shales. Using the core and well log data from the Barnett Shale, we demonstrate the application of the constructed templates for the evaluation of porosity, lithology and brittleness index. Then, we investigate the brittleness index defined in terms of mineralogy and geomechanical properties. The results show that as clay content increases, Poissons ratio tends to increase and Youngs modulus tends to decrease. Moreover, we find that Poissons ratio is more sensitive to the variation in the texture of shales resulting from the preferred orientation of clay particles. Finally, based on the constructed rock physics model, we calculate AVO responses from the top and bottom of the Barnett Shale, and the results indicate predictable trends for the variations in porosity, lithology and brittleness index in shales.

Anisotropy parameters estimate and rock physics analysis for the Barnett Shale

The rock physics model is an important tool for the characterization of shale reservoirs. We propose an improved anisotropic rock physics model of shale by introducing clay lamination (CL) index as a modeling parameter in effective medium theories. The parameter CL describes the degree of preferred orientation in distributions of clay particles, which depends on deposition and diagenesis history and determines intrinsic anisotropy of shales. Those complicated parameters of sophisticated methods that are difficult to quantify are substituted by CL. The applications of the proposed rock physics method include the inversion for anisotropy parameters using log data and the construction of a rock physics template for the evaluation of the Barnett Shale reservoir. Results show reasonable agreement between the P-wave anisotropy parameter e inverted by the proposed method and those measured from core samples. The constructed rock physics templates are calibrated on well log data, and can be used for the evaluation of porosity, lithology, and brittleness index defined in terms of mineralogy and geomechanical properties of the Barnett Shale. The templates predict that the increase in clay content leads to the increase in Poissons ratio and the decrease in Youngs modulus on each line of constant porosity, which confirms the consistent and reveals quantitative relations of the two ways of defining the brittleness index. Different scenarios of mineralogy substitutions present the varied layout of constant lines on the templates.

Research on anisotropy of shale oil reservoir based on rock physics model

Rock physics modeling is implemented for shales in the Luojia area of the Zhanhua topographic depression. In the rock physics model, the clay lamination parameter is introduced into the Backus averaging theory for the description of anisotropy related to the preferred alignment of clay particles, and the Chapman multi-scale fracture theory is used to calculate anisotropy relating to the fracture system. In accordance with geological features of shales in the study area, horizontal fractures are regarded as the dominant factor in the prediction of fracture density and anisotropy parameters for the inversion scheme. Results indicate that the horizontal fracture density obtained has good agreement with horizontal permeability measured from cores, and thus confirms the applicability of the proposed rock physics model and inversion method. Fracture density can thus be regarded as an indicator of reservoir permeability. In addition, the anisotropy parameter of the P-wave is higher than that of the S-wave due to the presence of horizontal fractures. Fracture density has an obvious positive correlation with P-wave anisotropy, and the clay content shows a positive correlation with S-wave anisotropy, which fully shows that fracture density has a negative correlation with clay and quartz contents and a positive relation with carbonate contents.

Rock physics model-based prediction of shear wave velocity in the Barnett Shale formation

Predicting S-wave velocity is important for reservoir characterization and fluid identification in unconventional resources. A rock physics model-based method is developed for estimating pore aspect ratio and predicting shear wave velocity Vs from the information of P-wave velocity, porosity and mineralogy in a borehole. Statistical distribution of pore geometry is considered in the rock physics models. In the application to the Barnett formation, we compare the high frequency self-consistent approximation (SCA) method that corresponds to isolated pore spaces, and the low frequency SCA-Gassmann method that describes well-connected pore spaces. Inversion results indicate that compared to the surroundings, the Barnett Shale shows less fluctuation in the pore aspect ratio in spite of complex constituents in the shale. The high frequency method provides a more robust and accurate prediction of Vs for all the three intervals in the Barnett formation, while the low frequency method collapses for the Barnett Shale interval. Possible causes for this discrepancy can be explained by the fact that poor in situ pore connectivity and low permeability make well-log sonic frequencies act as high frequencies and thus invalidate the low frequency assumption of the Gassmann theory. In comparison, for the overlying Marble Falls and underlying Ellenburger carbonates, both the high and low frequency methods predict Vs with reasonable accuracy, which may reveal that sonic frequencies are within the transition frequencies zone due to higher pore connectivity in the surroundings.

An improved method for the modeling of frequency-dependent amplitude-versus-offset variations

A proper description of the frequency-dependent seismic amplitude variation versus offset (AVO) responses should consider the effect of both the layered structure of a reservoir and the dispersive and attenuated property of the media in the reservoir. We propose an improved method to seamlessly link the rock physics modeling and the calculation for frequency-dependent reflection coefficients based on propagator matrix method. The improved AVO modeling method is implemented in frequency-wavenumber domain, and can accurately considers dispersion and attenuation that described by complex and frequency-dependent elastic properties predicted by rock physic models. Therefore, the improved method avoids errors resulting from truncating imaginary parts of the elastic properties as adopted by the conventional Zoeppritz-equation-based method. Moreover, the proposed method considers the intrinsic contribution of the layered structure to frequency-dependence of AVO responses, which has been ignored by current conventional methods. In addition, the method provides an efficient way to calculate seismograms for a dispersive and attenuated layered model. Finally, modeling results show the applicability of the improved method for the interpretation of complex frequency-dependent abnormalities, and indicate the potential for fluid detection in a layered reservoir.

A rock physics model for the characterization of organic-rich shale from elastic properties

Kerogen content and kerogen porosity play a significant role in elastic properties of organic-rich shales. We construct a rock physics model for organic-rich shales to quantify the effect of kerogen content and kerogen porosity using the Kuster and Toksöz theory and the self-consistent approximation method. Rock physics modeling results show that with the increase of kerogen content and kerogen-related porosity, the velocity and density of shales decrease, and the effect of kerogen porosity becomes more obvious only for higher kerogen content. We also find that the Poisson’s ratio of the shale is not sensitive to kerogen porosity for the case of gas saturation. Finally, for the seismic reflection responses of an organic-rich shale layer, forward modeling results indicate the fifth type AVO responses which correspond to a negative intercept and a positive gradient. The absolute values of intercept and gradient increase with kerogen content and kerogen porosity, and present predictable variations associated with velocities and density.

Seismic signatures of reservoir permeability based on the patchy-saturation model

Modeling of seismic responses of variable permeability on the basis of the patchy-saturation model provides insights into the seismic characterization of fluid mobility. We linked rock-physics models in the frequency domain and seismic modeling on the basis of the propagator matrix method. For a layered patchy-saturated reservoir, the seismic responses represent a combination of factors, including impedance contrast, the effect of dispersion and attenuation within the reservoir, and the tuning and interference of reflections at the top and bottom of the reservoir. Numerical results suggest that increasing permeability significantly reduces the P-wave velocity and induces dispersion between the high- and low-frequency elastic limit. Velocity dispersion and the layered structure of a reservoir lead to complex reflection waveforms. Seismic reflections are sensitive to permeability if the impedance of the reservoir is close to that of the surroundings. For variable layer thickness, the stacked amplitudes increase with permeability for high-velocity surrounding shale, whereas the stacked amplitudes decrease with permeability for low-velocity surrounding shale.

An AVO inversion method in the frequency domain based on a layered model—a Bakken Shale case study

Conventional AVO techniques are based on the linear approximation of Zoeppritz equations for the inversion of elastic properties across a subsurface interface. However, for a layered model consisting of internal multiple layers beyond seismic resolution, it can be difficult to use the traditional AVO inversion methods due to the practicalities of picking amplitudes, resolution problems, and thin-layer effects. We propose an improved AVO inversion method applied to a layered model for inversion of layer thickness and properties by incorporating the frequency content of the wavelet. The proposed method uses the propagator matrix method as a theoretical description of the frequency-dependent reflection coefficients for the layered model in an inversion scheme. We test the proposed method on a single layered model with synthetic data to produce an inversion for layer thickness and porosity, and then investigate the feasibility of the method for the characterization of the fracture zone in the Bakken formation. For the inversion of the reference crack density and layer thickness of the fracture zone in the Bakken formation, the minima of the objective functions generate inversion results that indicate a reasonable fit with the true model parameters. The inversion error may result from the intrinsic complexity of reflections from a layered model, in which several different combinations of layer thicknesses and associated properties may produce similar frequency-dependent coefficients. By contrast, in the multicomponent model data example, the inversion of converted PS-wave data seems to be less stable compared to PP-wave data. The potential of the proposed AVO inversion method may include applications to complex models, such as a sandstone/shale interbedded system, or a formation that presents internal heterogeneity.

Anisotropy rock physics model for the Longmaxi shale gas reservoir, Sichuan Basin, China

The preferred orientation of clay minerals dominates the intrinsic anisotropy of shale. We introduce the clay lamination (CL) parameter to the Backus averaging method to describe the intrinsic shale anisotropy induced by the alignment of clay minerals. Then, we perform the inversion of CL and the Thomsen anisotropy parameters. The direct measurement of anisotropy is difficult because of the inability to measure the acoustic velocity in the vertical direction in boreholes and instrument limitations. By introducing the parameter CL, the inversion method provides reasonable estimates of the elastic anisotropy in the Longmaxi shale. The clay content is weakly correlated with the CL parameter. Moreover, the parameter CL is abnormally high at the bottom of the Longmaxi and Wufeng Formations, which are the target reservoirs. Finally, we construct rock physics templates to interpret well logging and reservoir properties.

Azimuthal seismic responses from shale formation based on anisotropic rock physics and reflectivity method: a case study from south-west China

Due to intrinsic anisotropy related to preferred alignment of clay particles and the existence of vertical or high angle fractures, shales usually present orthorhombic anisotropy. The objective of this study was to build anisotropic rock physics models for shales at the seismic scale. Based on the well-log and Formation Micro Imager (FMI) log data from a shale formation in the Sichuan Basin in south-west China, we derive an orthorhombic model at the seismic scale by using Schoenberg and Helbig’s method and generalised Backus averaging method in the rock physics workflow. In order to understand the relationship between physical properties of the rock physics model and seismic wave propagation, we apply the simplified reflectivity method to calculate seismic responses for amplitude variation with azimuth (AVAz) analysis. The method is based on the scheme of anisotropic reflectivity method which is commonly used to simulate the full-wave field in stratified anisotropic media, in analogy with the formula of horizontal slowness components in Schoenberg and Protázio’s method. The AVAz analysis is conducted on the seismograms of PP-wave, radial and transverse components of PS-wave. The results show that overburden effects caused by wave propagation in anisotropic media can’t be ignored. Azimuthal variations in amplitudes of both PP-wave and radial component of PS-wave can be used to indicate strikes of fractures, while PS-wave appears to be more sensitive. We have constructed an anisotropic rock physics model at the seismic scale for shales, and applied it to the Longmaxi Shale in the Sichuan Basin, south-west China. The simplified reflectivity method is proposed to calculate seismic responses for amplitude variation with offset and azimuth (AVAz) analysis.