Inversion of Anisotropy Parameters of the Overburden and Reservoir from Seismic Data
Inversion of anisotropy parameters of the overburden and reservoir from seismic data
Davide Gei, OGS, Italy
Most of the activity performed with Professor Santos was about seismic anisotropy and specifically about methods of inversion and analysis of seismic data to characterize the anisotropy properties of the different formations in sedimentary basins.
Traditionally the conventional processing of seismic data was based under the assumption of isotropic media. However, isotropic rocks are always an exception in nature. Sedimentary basins, the main targets in hydrocarbon exploration, are characterized by fine layering, lamination and often by the presence of shales. All of these characteristics cause seismic anisotropy.
At the moment, gas-shales are a main topic in hydrocarbon exploration. These rocks can be hydraulically stimulated through a technique called fracking. The process of fracking opens fractures and creates paths, which dramatically improve the permeability of the rock and consequently the hydrocarbon production rate. However, the fracturing of the rock is accompanied by microseismic events, which must be monitored during the reservoir stimulation. Particularly the events must be located in the subsurface to prevent out of pay fracturing.
Anisotropy plays a crucial rule in the process of correctly locating the microseismic events. We discussed about the most proper technique to invert seismic and microseismic data to obtain the anisotropy parameters of the overburden. Specifically, as vertical transverse isotropy (VTI) is, unarguably, the most common anisotropic model for sedimentary basins (Eisner et al., 2011) we focused on this model. In VTI media the formulation of non-hyperbolic reflection moveout proposed by Alkhalifah and Tsvankin (1995) is quite appropriate to invert seismic and microseismic data for the Thomsen anisotropy parameters (Thomsen, 1986; Gei et al., 2011). An inversion code was developed at OGS and is available.
A single set of vertical fractures embedded in a homogeneous medium give rise to a horizontal transverse isotropy (HTI) model. Fractured reservoirs are often modeled with HTI symmetries. Al-Dajani and Tsvankin (1998) and Grechka et al. (1999) proposed equations well approximating the non-hyperbolic reflection moveout of HTI media as a function of azimuth. In the near future we intend to compare the arrival times computed with these equations with picking times from synthetic seismograms computed with a Fourier forward modeling code (Carcione et al., 1992) and eventually invert field data (if available).
Bibliography
Al-Dajani A. and I. Tsvankin, 1998, Nonhyperbolic reflection moveout for horizontal transverse isotropy, Geophysics, 63(5), 1738–1753.
Alkhalifah, T. and I. Tsvankin, 1995, Velocity analysis for transversely isotropic
media, Geophysics, 60, 1550–1566, doi: 10.1190/1.1443888.
Carcione, J. M., D. Kosloff, A. Behle, and G. Seriani, 1992, A spectral scheme for wave propagation simulation in 3-D elastic-anisotropic media, Geophysics, 57, 1593- 1607.
Eisner, L., Y. Zhang, D. Gei, P. Duncan, M.C. Mueller and , M. P. Thornton, 2011, Effective VTI anisotropy for consistent monitoring of microseismic events, The Leading Edge, 30 (7), 772-776, ISSN: 1070-485X.
Gei, D., L. Eisner, and P. Suhadolc, 2011, Feasibility of estimating vertical transverse isotropy from microseismic data recorded by surface monitoring arrays, Geophysics, 76 (6), WC117, doi: 10.1190/geo2011-0028.1.
Grechka, V., I. Tsvankin and J. K. Cohen, 1999, Generalized Dix equation and analytic treatment of normal-moveout velocity for anisotropic media, Geophysical Prospecting, 47, 117-148.
Thomsen L., 1986, Weak elastic anisotropy, Geophysics, 51 (10), 1954-1966.