We Challenges in shale-reservoir characterization by means of AVA and AVAZ

Transcription

1 We Challenges in shale-reservoir characterization by means of AVA and AVAZ N.C. Banik* (WesternGeco), M. Egan (WesternGeco), A. Koesoemadinata (WesternGeco) & A. Padhi (WesternGeco) SUMMARY In most basins, shale reservoirs are bounded by limestone and carbonates at the top and the base. Consequently the isotropic, boundary reflection coefficients are high. Also, shale is anisotropic with or without existing fractures and the anisotropy is high. As a result, the approximate reflection coefficient equations that are used in the standard amplitude vs. angle of incidence (AVA) and azimuth (AVAZ) provide qualitatively and quantitatively erroneous results. These are illustrated in the paper through comparing the results of approximate and exact boundary reflection coefficients as a function of angle of incidence and azimuth for isotropic, vertical transverse isotropy (VTI), horizontal transverse isotropy (HTI), and orthorhombic media. Based on these observations, we recommend using the anisotropic fullreflectivity method for AVA and AVAZ analysis and as the forward modeling engine of the AVA and AVAZ inversion methods.

2 Introduction Amplitude versus angle of incidence (AVA) and shot-receiver azimuth (AVAZ) are becoming increasingly important in shale and other unconventional reservoir characterization. The seismic and petrophysical attributes generated through AVA and AVAZ analysis and inversion work are being used in identifying reservoir quality and completion quality of these heterogeneous reservoirs. The study presented in this work examines the accuracy of the conventional AVA and AVAZ analysis methods in unconventional reservoirs, especially in shale reservoirs, which are known to exhibit high anisotropy and sharp impedance contrasts at the boundaries. We consider here only plane-wave reflection coefficients for compressional wave arrival and reflection (R pp ). The shale in shale reservoirs is highly anisotropic, with or without fractures. Without fractures, shale anisotropy is transversely isotropic, with the symmetry axis generally aligned with the vertical. With in-situ fractures, shale anisotropy will be of orthorhombic type. In Figure 1, we show the Marcellus and Utica shale outcrops in New York. The layering and almost vertical nature of fractures are evident. A combination of transverse isotropy with vertical and horizontal axes of symmetry makes the reservoir system orthorhombic. Figure 1 Marcellus and Utica shale outcrops in New York. Source: http/// In conventional sand reservoirs, the kinematic effects of anisotropy are accounted for through anisotropic imaging with shale treated as the main overburden; the reservoir itself is treated as isotropic; all reservoir characterization work through AVA analysis and inversion is conducted assuming that the reservoir itself is isotropic. In shale resource plays, shale itself is the reservoir; therefore, the consideration of vertical transverse isotropy (VTI) in AVA analysis is very relevant. In this paper one of the questions we address is how significant VTI effect is in shale-reservoir AVA analysis and inversion. A second, equally relevant issue is high reflectivity at the boundaries of the shale layer. Most shale reservoir basins have the shale layer enclosed with high-impedance carbonates. This is probably due to deposition in a shallow-water, anoxic environment, which is characteristic of shale reservoir formation. The large (>0.1) boundary reflectivity makes the conventional Aki-Richard type AVA equations (Aki and Richard, 1980) untenable for proper AVA analysis and inversion. The question here is how acceptable the approximate AVA equations are in the cases of isotropy and VTI anisotropy. These are also examined in details in this paper. Because the anisotropy in shale is quite high, in this paper we also examine the validity of approximate, linearized VTI, horizontal transverse isotropy (HTI), and orthorhombic equations for extracting anisotropic attributes in shale. The AVAZ characteristics arising in media with HTI and orthorhombic symmetries are especially important in identifying and characterizing in-situ or induced fractures in shale as well as in other unconventional and in fractured carbonate reservoirs.

3 We have studied several well-data-based synthetic seismograms. We find that (1) shale anisotropy is very important for P-wave AVO characterization through the gradient and the curvature terms, and (2) fitting the AVA and AVAZ events with Aki-Richard (1980) or Rüger-type approximate equations (Rüger, 1997) provide qualitatively and quantitatively incorrect attributes. We, therefore, recommend using exact reflectivity equations for AVA and AVAZ analysis and inversion work involving shale reservoirs. We present and discuss the results in detail in the paper. Theory and Method In conventional AVA and AVAZ, equations are based on approximations assuming that the change in the elastic parameters at the solid-contact boundaries is small, so one can neglect second and higherorder terms of changes in elastic parameters in the Taylor series expansion of the exact reflectivity expressions. Thus, in the isotropic case, one has Aki-Richard (Aki and Richard, 1980) approximation of the exact Zoeppritz equation for AVA. Similar equations have been derived for VTI media by Banik (1987) and Rüger (1997), using Thomsen parameters (Thomsen, 1986) δ and ε after linearizing the exact equation of Daley and Hron (1977). The anisotropy parameter γ comes into play in the compressional wave reflection coefficient only when the anisotropy is of HTI or of lower-order symmetry. Rüger (1997) extended the work to interfaces between two HTI media. The standard forms of the VTI and HTI AVA and AVAZ equations are also available in Rüger (2001). Alternative forms of approximate equations are available in the literature, but they can be algebraically reduced to the form given by Rüger (2001). For media with lower-order symmetry, approximate equations are also available (see, for example, Schonberg and Protazio 2001). Because no approximation is done with respect to the angle of incidence, these equations are assumed to be good at all angles below the critical angle. However, this is true only when the assumptions with regard to the changes in elastic parameters, including the anisotropy parameters, are satisfied. Chen et al. (2001) verified the inaccuracies of the linearized, three-parameter Rüger (1997) approximate equation for VTI media and suggested the use of higher order terms through analysis of the exact results with a regression method. Innanen (2012) discussed inadequacies of linearized equation for P-S conversion. In our study we compare the approximate compressional reflection coefficient as a function of angle of incidence and azimuth for VTI and HTI media, R pp V (θ), R pp H (θ,φ), and that for orthorhombic media, R pp O (θ,φ), with their exact expressions, appropriate for shale reservoirs. The computation of exact reflection coefficients for VTI media is based on the method developed by Graebner (1992). For HTI and orthorhombic media cases we use the method by Schoenberg and Protazio (1992). In both the methods the theoretical development begins with solving the Christoffel equations (see, for example, Rüger, 2001) on both sides of the reflector to obtain the slowness and polarization vectors for the incident, reflected and transmitted waves at different angles. Two 3 x 3 impedance matrices are constructed for each medium. Finally, all four matrices are combined to yield reflection and transmission coefficients for a given plane wave. We follow Rüger s conventions (2001) for anisotropic parameters and model all wave types at any angle and azimuth. For model parameters, we use the upper medium as an isotropic medium, but the elastic parameters are obtained from well data in the Marcellus basin. The lower medium is anisotropic VTI or HTI. The parameters for VTI anisotropy have been chosen based on the surface seismic and VSP data in Marcellus shale. HTI anisotropy parameters were chosen appropriately. Model parameters are shown in Table 1. In the table, V p and V s are vertical compressional wave and shear wave velocities and δ v, ε v refer to Thomsen parameters in equivalent VTI models.

4 Results In Figure 2, we show comparisons among exact VTI, approximate VTI, exact isotropic, and approximate isotropic cases for the same upper medium. The lower medium has zero or nonzero VTI anisotropy. It is clear that the results are qualitatively different. First, if we treat the shale as isotropic, then the P-wave reflection coefficient decreases with angle of incidence, while treating the shale as VTI will result in an increase in amplitude with increasing angle of incidence. If we treat the medium as isotropic and use the approximate equation, then the inaccuracies in the AVO character are larger. The use of the approximate VTI equation overshoots the results of exact VTI AVA character. Figure 2 Comparison of VTI and Isotropic results for exact and approximate equations. In Figures 3 and 4, we present the results for the same isotropic upper layer, but this time the lower layer has HTI anisotropy. In Figure 3, we plot the exact and approximate reflection coefficients as a function of the angle of incidence for different angles of the source-receiver azimuth. We note, at incidence angles greater than 50 0 and azimuths in the range 0 to 60 0, the exact AVA characters are opposite to those given by the approximate equation (Equation 6.7 of Rüger, 2001). This is revealed dramatically when we plot AVAZ in Figure 4, at a constant angle of incidence of At smaller angles of incidence, the AVAZ characters remain the same but quantitative variation of the reflection coefficient with angle of incidence and azimuth change appreciably. Figure 3 Comparison of HTI exact and approximate reflection coefficients.

5 Figure 4 Comparison of HTI exact and approximate reflection coefficient as a function of the azimuth angle for 56 0 angle of incidence. Conclusion We studied similar other cases, including the cases of the lower medium being orthorhombic. In all of these cases, we come to the conclusion that, as long as the interface of shale has elastic parameters including the anisotropy parameters, as we see in the Marcellus and in other basins, the exact equations must be used to represent the AVA and AVAZ characters in shale reservoir characterization. The use of approximate equations for shale reservoir characterization is very likely to provide wrong characterization through AVA and AVAZ studies. The magnitude of contrasts in any of the relevant elastic parameters at the boundaries of shale facies is the determining factor for both qualitative and quantitative accuracy of the approximate equations. References Aki, K. and Richards, P. G. [1980] Quantitative seismology: Theory and practice. W. H. Freeman and Company, Banik, N. [1987] An effective anisotropy parameter in transversely isotropic media. Geophysics 52, Chen H, Castagna, J. P., Brown, R. L., and Ramos, A. C. B.[2001] Three parameter AVO crossplotting in anisotropic media. Geophysics 66, Daley, P. F. and Hron, F. [1977] Reflection and transmission coefficients for transversely isotropic media, Bulletin of the Seismological Society of America, 67, Graebner, M. [1992] Plane-wave reflection and transmission coefficients for a transversely isotropic solid. Geophysics 57, Innen, K. A. [2012] AVO theory for large contrast elastic and inelastic in pre-critical regimes, SEG Annual International Conference Extended abstract, Las Vegas, USA. Ruger, A. [1997] P-wave reflection coefficients for transversely isotropic models with vertical and horizontal axis of symmetry. Geophysics 62, Rüger, A. [2001] Reflection coefficients and azimuthal avo analysis in anisotropic media, SEG Geophysical monograph series no. 10. Schoenberg, M., and Protazio, J. [1992] Zoepprittz rationalized and generalized to anisotropy, Journal of Seismic Exploration, 1,