Research On Converted-wave AVO Analysis And Fluid Identification Method In Weakly Anisotropic Media

Application of AVO and other pre-stack seismic data to predict oil and gas reservoirs has been widely used, which has made a lot of success stories in domestic and international oil and gas geophysical exploration and development. But due to the complex factors of AVO changes, conventional AVO analysis also exists multiple solutions, and multi-wave and multi-component seismic exploration information is an effective way to solve multiple solutions. The combination using of converted wave would greatly reduce multiple solutions, which is generally accepted by geophysicists. At the same time, comprehensive AVO attributes can get more accurate physical parameters of reservoir, therefore, the research and improvement of converted wave AVO technology is an urgent subject of reservoir seismic exploration.Building the geological-geophysical model scientifically and reasonably is the key issue for reservoir prediction and fluid identification. From the actual oil and gas reservior data analysis of the South China Sea deep water, the reservoir modeling based on two-phase media and anisotropic media is more scientific and reasonable than the conventional isotropic media. In order to make better use of pre-stack seismic information to accurately predict reservoir and identify fluid, we must first study PP wave and P-SV wave AVO in anisotropy media, according to which we can get lithology information and spatial distribution of reservoir. In this paper, the weak anisotropic media converted waves linear approximation formula is simplified, and AVO attribute analysis is done, while the weak anisotropic media P-SV wave elastic impedance is derived, the generalized anisotropic medium fluid identification factors are constructed, which are very meaningful.The paper first gives the advantage of converted wave AVO analysis, and summarizes converted wave reflection coefficient linear approximate formula in isotropic medium, recognizing that even with the same formula, accuracies of different models are not the same, which means that approximate formulas have some dependence on the model. For the four typical gas aquifer, P-SV wave AVA response characteristics are more complex than PP wave, and cannot distinguish the four types of gas-bearing aquifer, particularly did not identify first and second base, has better identification of the third category, indicating that the converted wave AVO is not very sensitive to the fluid composition of the reservoir. Reflection coefficient AVA response curve after normalized is monotonic increase or decrease, but also enlarge the exception, recognition effect for four types of AVO gas aquifer has increased, for former three categories, normalized reflection coefficient decreases with the increasing angle, and for the fourth category, the reflection coefficient is monotonically decreasing, but the reflection coefficient is positive.The most common converted wave reflection coefficient approximate formula in anisotropic media is given in 1999 by Vavrycuk, but the applicability of formula in the actual of AVO analysis is not strong. Therefore. Petr Jilek (2002) gives another PS wave reflection coefficient approximate formula expression for arbitrary anisotropic dielectric half-space, the derivation of the formula is based on the following assumptions:weak contrasts of elastic medium parameters across the interface and weak anisotropy in half-space, so this formula is called weak contrasts and weak anisotropy approximations, this formula is the function of the Thomsen anisotropic parameters, incident angle and azimuth. This resulting expressions can be applied for any combination of isotropic. VTI. HTI and orthorhombic halfspaces.. But it is complex, so we simplified orthotropic medium P-SV wave reflection coefficient formula, converted the anisotropy parameters, and degradated it to the VTI and HTI media, which are two typical anisotropic media.we commonly used it in combination with converted wave reflection coefficient in the isotropic approximation method, try to express the reflection coefficient as odd functions of sinθ.on the base of accuracy, we reserved it to one power.three power, five power and seven power, the paper called it a first order approximation, the third order, fifth-order, seven-order new approximate formula. Both are the combined-items of isotropic reflection coefficient and anisotropic disturbed item, through different models we compared the accuracy of several forms of expression, and made the corresponding error analysis, at the same time for different models the impact of factors of the reflection coefficient obtained corresponding laws.After obtaining the new approximate formula, we firstly attempt to normalize the P-SV wave new third-order approximation reflection coefficient in VTI and HTI media. Based on using of four classic case of isotropic AVO gas sandstone model parameters, adding to the anisotropy factors-Thomsen anisotropic parameters of the assignment, we draw gas/water sandstone response characteristics of AVA to get the effect of identification, obtained the similar results under the isotropic medium. Similar to the isotropic P-SV wave AVO gradient and intercept method, an example of P-SV AVO gradient attribute intercept is discussed in the VTI medium wave, and we obtain anisotropic converted wave positive and negative gradient case law with the law of isotropic media is similar, while the difference caused by the anisotropic parameter is the size of the intercept and gradient values. Forms of cross plots of different models are different, but their background trend can fit a straight line. The slope of the line can strike the average k value. After the research on AVO attributes analysis by new equation,we can get S wave velocity profile, density profile and shear modulus profile, which can provide a favorable way for qualitative analysis. When attribution parameters are constant, quantitative analysis and inversion of anisotropic parameters can be carried out.Many scholars have proposed different types of elastic impedance formula, the P-wave elastic impedance inversion of pre-stack data has become more sophisticated, while converted wave elastic impedance and elastic impedance in anisotropic media research is less than P-wave, so it needs to draw on the research based on the isotropic elastic impedance method and theory. Based on this, P-SV wave elastic impedance formulations in VTI and HTI were proposed by approximation formula.Then we present shale/gas sand of models to test the accuracy of P-SV wave elastic impedance.We normalized it with whitcombe's ideas, and formed converted wave elastic impedance formula in weak anisotropic media, and elastic impedance of the final formula of the main factors and rules are also summarized. The intersection of elastic impedance analysis also shows that recognition of reservoir types in terms of elastic impedance is more sensitive than the acoustic impedance.In order to predict reservoir by using AVO and multi-components seismic data, some fluid factors and fluid identification methods have been studied, seismic wave, shear wave and converted wave of comprehensive utilization of this also provides an effective way. These methods are used to extract linear Zoeppritz equation parameters, such as P wave and S wave impedance, velocity, Poisson ratio, but also can get some help to explain the parameters, such as elastic modulus. Lame constant, and some fluid factor. Generally, these fluids can better identify hydrocarbon region. Starting from the physical basis of Biot-Gassmann equations, we analyze the impact of filling the rock with different types of fluids on velocity and impedance, and in this basis list the common impedance fluid identification factors, which are used to identify four typical sandstones. Here recognition ability and sensitivity of the various factors on gas/water sandstone are analyzed. In order to introduce the anisotropic parameters and azimuthal angle into the equation of fluid factor, we insert the PP and P-SV wave elastic impedance into the fluid factor. The high sensitive fluid identification Fdam and low sensitive fluid identificationμpam are compared, which is easy to compare the sensitivity analysis of the fluid factor Fdam andμρam. Though three lithological model identification, we also analyze the Thomsen anisotropic parameters'effect and obtain the corresponding laws.Converted wave is the supplement for P-wave information, and at the same time AVO. elastic impedance and fluid identification are the key technology,joining the anisotropy factors for reservoir prediction, which can improve the accuracy of their reservoir interpretation, so our study is very meaningflul.