HR: 1330h
AN: S52E-0169    [PDF]
TI: Far Offset P-to-S ``Elastic Impedance" for Identifying Partial Gas Saturation (Fizz Water).
AU: Gonzalez, E
EM: ezequiel@pangea.stanford.edu
AF: Stanford Rock Physics Lab, Geophysics Dept., Stanford, CA 94305 United States
AU: * Mukerji, T
EM: mukerji@stanford.edu
AF: Stanford Rock Physics Lab, Geophysics Dept., Stanford, CA 94305 United States
AU: Mavko, G
EM: mavko@stanford.edu
AF: Stanford Rock Physics Lab, Geophysics Dept., Stanford, CA 94305 United States
AB: We present a formulation of P-to-S "elastic impedance" (PSEI), and demonstrate the possibility of using PSEI to discriminate between commercial gas saturations and fizz water. Attempting to seismically differentiate fizz water with low gas saturation from higher gas saturations is difficult. The abrupt reduction in Vp with the first few percent of gas controls the seismic response. Therefore, usually only the presence of gas but not the saturation can be detected with PP seismic. P-to-S converted waves has been suggested as a source of additional information. Wu (2000), and Zhu et al (2000) propose using PP and/or PS reflectivity. Landro et al (1999) derived "shear wave elastic impedance" (SEI), assuming weak contrast and small incidence angle. We use a formulation of PS "elastic impedance" (PSEI) not limited to small incidence angles, which at one specific angle (theoretically) gives a direct estimation of density. Although theoretically it is possible to get a direct value of density from PSEI at a single angle, clearly in practice it will be very difficult. Nevertheless, for mid-to-large offsets the density term dominates PSEI behavior. The "decoupling" between Vs and density contributions to PSEI can be exploited to discriminate different reservoir properties. As noted by Berryman et al. (2002) the linear behavior of density with saturation makes attributes that are closely related to density useful proxies for estimating saturation. To test the possibility of discriminating between fizz water and commercial gas saturations with PSEI, we used log data from an Eastern Venezuela well. PSEI values for incidence angles of $10^o$ and $50^o$ were calculated with original (Sw=1) and Gassmann simulated logs. Using correlated Monte Carlo simulations we calculated bivariate pdfs for all modeled Sw situations to estimate the Bayesian classification success rate. An advantage of using two PSEI attributes, instead of a combination of PP-PS is that the PP and PS time matching is avoided. The important result here is the real possibility of discriminating between different homogeneous gas saturations. Values of PSEI monotonically decrease with reduction of gas concentration. Consequently, it is possible to discriminate between fizz water and commercial gas concentration using PSEI.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5102 Acoustic properties
DE: 7200 SEISMOLOGY
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting