Guangzhi Zhang

AVAZ inversion for fracture weakness parameters based on the rock physics model

Subsurface fractures within many carbonates and unconventional resources play an important role in the storage and movement of fluid. The more reliably the detection of fractures could be performed, the more finely the reservoir description could be made. In this paper, we aim to propose a method which uses two important tools, a fractured anisotropic rock physics effective model and AVAZ (amplitude versus incident and azimuthal angle) inversion, to predict fractures from azimuthal seismic data. We assume that the rock, which contains one or more sets of vertical or sub-vertical fractures, shows transverse isotropy with a horizontal axis of symmetry (HTI). Firstly, we develop one improved fractured anisotropic rock physics effective model. Using this model, we estimate P-wave velocity, S-wave velocity and fracture weaknesses from well-logging data. Then the method is proposed to predict fractures from azimuthal seismic data based on AVAZ inversion, and well A is used to verify the reliability of the improved rock physics effective model. Results show that the estimated results are consistent with the real log value, and the variation of fracture weaknesses may detect the locations of fractures. The damped least squares method, which uses the estimated results as initial constraints during the inversion, is more stable. Tests on synthetic data show that fracture weaknesses parameters are still estimated reasonably with moderate noise. A test on real data shows that the estimated results are in good agreement with the drilling.

A Novel Prestack AVO Inversion And Its Application

Prestack AVO inversion is multidimensional and ill posed, and is often strongly affected by noise and measurement uncertainty, so it is necessary to performing constraints in order to obtain steady and rational inversion results. A novel prestack AVO inversion method is developed based on linearized approximation of the Zoeppritz equation and Bayesian parameter estimation theory. Covariance matrix is used to describe the correlation degree between the parameters, Gaussian distribution is used for likelihood function and modified Cauchy distribution is used for prior distribution; P-wave impedance, S-wave impedance, density and rock physics relation are applied to constrain the inversion results, thus it is reliable to make the result more accurate and steady. Tests on synthetic and real data show that all inverted parameters are almost perfectly retrieved even the SNR is low.

Azimuthal Seismic Amplitude Difference Inversion for Fracture Weakness

Fracture weakness prediction is an important task in fractured reservoir analysis. We propose a new method to use seismic amplitude differences between azimuths to estimate the normal and tangential fracture weaknesses under the assumption that the anisotropic perturbation of the reflection coefficient is mainly induced by fractures. We first derive an expression of the reflection coefficient in terms of the normal and tangential fracture weaknesses for the case of an interface separating two fractured media. Then we use the linear fitting method to get the relationship between the two fracture weaknesses, and change the variables to precondition the inversion problem. The Bayesian framework, under the hypothesis of a Cauchy distribution prior information and a Gaussian distribution likelihood function, is employed to construct the objective function, and an initial low-frequency constraint is introduced to the objective function to make the inversion more stable. The conjugate gradient algorithm is adopted to solve the inverse problem. Tests on both synthetic and real data demonstrate that the normal and tangential fracture weaknesses can be estimated reasonably in the case of seismic data containing a moderate noise, and our inversion approach appears to be a stable method for predicting the fracture weaknesses.

Simultaneous inversion of petrophysical parameters based on geostatistical a priori information

The high-resolution nonlinear simultaneous inversion of petrophysical parameters is based on Bayesian statistics and combines petrophysics with geostatistical a priori information. We used the fast Fourier transform-moving average (FFT-MA) and gradual deformation method (GDM) to obtain a reasonable variogram by using structural analysis and geostatistical a priori information of petrophysical parameters. Subsequently, we constructed the likelihood function according to the statistical petrophysical model. Finally, we used the Metropolis algorithm to sample the posteriori probability density and complete the inversion of the petrophysical parameters. We used the proposed method to process data from an oil field in China and found good match between inversion and real data with high-resolution. In addition, the direct inversion of petrophysical parameters avoids the error accumulation and decreases the uncertainty, and increases the computational efficiency.

Seismic inversion for underground fractures detection based on effective anisotropy and fluid substitution

Underground fractures play an important role in the storage and movement of hydrocarbon fluid. Fracture rock physics has been the useful bridge between fracture parameters and seismic response. In this paper, we aim to use seismic data to predict subsurface fractures based on rock physics. We begin with the construction of fracture rock physics model. Using the model, we may estimate P-wave velocity, S-wave velocity and fracture rock physics parameters. Then we derive a new approximate formula for the analysis of the relationship between fracture rock physics parameters and seismic response, and we also propose the method which uses seismic data to invert the elastic and rock physics parameters of fractured rock. We end with the method verification, which includes using well-logging data to confirm the reliability of fracture rock physics effective model and utilizing real seismic data to validate the applicability of the inversion method. Tests show that the fracture rock physics effective model may be used to estimate velocities and fracture rock physics parameters reliably, and the inversion method is resultful even when the seismic data is added with random noise. Real data test also indicates the inversion method can be applied into the estimation of the elastic and fracture weaknesses parameters in the target area.

AVO simultaneous inversion using Markov Chain Monte Carlo

Summary A Bayesian model is developed to estimate P-wave reflection coefficient, S-wave reflection coefficient and density reflection coefficient using pre-stack seismic data. Using Markov chain Monte Carlo (MCMC) method, many samples of each unknown variable are obtained from the Bayesian model, which subsequently are used to infer the unknown variable. Generating S-wave velocity at random, and making use of Aki equation to get a simple model for the purpose of test the method. Moreover, a study based on seismic AVO data collected from an oil field located in South China, shows that the improved method is more precious for parameter estimation than traditional methods.

Seismic Data Analysis Based on Fuzzy Clustering

Seismic data contain a large amount of geological information, clustering analysis is an effective method to analyze seismic data; it has already become a valid analytical tool for lithology prediction and reservoir characterization; unsupervised fuzzy clustering based on fuzzy c-means algorithm is a very important technology in seismic data analysis, it is capable of creating useful character mappings of the data by reducing a large number of attributes down to one that can be visualized on a map; this kind of method can look for the center of each cluster and membership grades which belong to corresponding cluster; model test and actual application have shown the validity of this method

Azimuthal Seismic Amplitude Variation with Offset and Azimuth Inversion in Weakly Anisotropic Media with Orthorhombic Symmetry

Seismic amplitude variation with offset and azimuth (AVOaz) inversion is well known as a popular and pragmatic tool utilized to estimate fracture parameters. A single set of vertical fractures aligned along a preferred horizontal direction embedded in a horizontally layered medium can be considered as an effective long-wavelength orthorhombic medium. Estimation of Thomsen’s weak-anisotropy (WA) parameters and fracture weaknesses plays an important role in characterizing the orthorhombic anisotropy in a weakly anisotropic medium. Our goal is to demonstrate an orthorhombic anisotropic AVOaz inversion approach to describe the orthorhombic anisotropy utilizing the observable wide-azimuth seismic reflection data in a fractured reservoir with the assumption of orthorhombic symmetry. Combining Thomsen’s WA theory and linear-slip model, we first derive a perturbation in stiffness matrix of a weakly anisotropic medium with orthorhombic symmetry under the assumption of small WA parameters and fracture weaknesses. Using the perturbation matrix and scattering function, we then derive an expression for linearized PP-wave reflection coefficient in terms of P- and S-wave moduli, density, Thomsen’s WA parameters, and fracture weaknesses in such an orthorhombic medium, which avoids the complicated nonlinear relationship between the orthorhombic anisotropy and azimuthal seismic reflection data. Incorporating azimuthal seismic data and Bayesian inversion theory, the maximum a posteriori solutions of Thomsen’s WA parameters and fracture weaknesses in a weakly anisotropic medium with orthorhombic symmetry are reasonably estimated with the constraints of Cauchy a priori probability distribution and smooth initial models of model parameters to enhance the inversion resolution and the nonlinear iteratively reweighted least squares strategy. The synthetic examples containing a moderate noise demonstrate the feasibility of the derived orthorhombic anisotropic AVOaz inversion method, and the real data illustrate the inversion stabilities of orthorhombic anisotropy in a fractured reservoir.

AVAZ inversion for fluid factor based on fractured anisotropic rock physics theory

The fractured reservoirs show strong anisotropy.When the fracture is filled with different types of fluids,the seismic response characteristics of fractured reservoir are not the same.Starting from the anisotropic rock physics theory,this article introduces the identification factor that can effectively distinguish fracture fluid;studies the relationship of fluid factor to fluid types,saturation and crack aspect ratio;analyzes the seismic response characteristics when the fracture is full of different fluids;and ultimately puts forward the method to estimate the crack fluid factor based on AVAZ inversion.Firstly,the sensitivity of crack fluid factor is analyzed in this paper.Secondly,the features of fluid factor and fractured reservoirs reflection coefficient variation when the fracture is filled with different types of fluids are discussed.At last,well-logdata and complex fracture model are used to verify the inversion method.The results show that the method to estimate the crack fluid factor is reasonable and reliable.And the estimation results of the fluid factor can match the true value well when the synthetic seismic traces contain a random noise.

Coherence cube based on curvelet transform

The C3 coherence cube is a new seismic attribute, which has higher underground structural resolution than normal amplitude cube. Curvelet analysis is a better tool of timefrequency analysis than Wavelet analysis, and it has a very good time-frequency decomposition effect. In this paper, by combining Curvelet analysis with C3 coherence cube ,a good result is obtained in identifying different scale structural anomalies. With the method, the geological structure characteristic in the given frequency component can be identified, and the ability of identifying the geological body is improved. This result better represented complex and variable geologic body details. This paper introduces a new method for identified the fault and fracture system.