Anisotropy and attenuation of crosshole channel waves from the Antrim Shale gas play, Michigan Basin

Abstract

We analyze the crosshole data from the Antrim Shale gas play at the MIT test site in the Michigan Basin. A crosshole seismic logging analysis has revealed the presence of strong transverse isotropy possibly due to the alignment of clay platelets along the bedding plane or sedimentary bedding, and strong continuity in the depth interval covered in this study.Ž . We find that the data are dominated by channel waves associated with low velocity zones waveguides identified in the sonic logs. Channel waves are characterized by their unique characteristics, such as dispersion and amplitude variation with depth. The channel-wave analysis has revealed the possible presence of azimuthal anisotropy at the top of the depth covered in this survey, which may be related to fracturing in the Antrim Shale formation. The data also show strong attenuation of S-waves characterized by very low frequency contents which may be due to viscoelastic properties of shaly sands or scattering by fractures. However, direct estimates of fracture parameters are not possible due to the limited data available. Finally, we have modeled the data with synthetic seismograms using a model with a 30% anisotropy. q

Anisotropy Effects on Microseismic Event Location

“…The combination of such elements of symmetry can result in eight anisotropy systems (e.g., GIBOWICZ and KIJKO, 1994). Transverse Isotropy (TI) is one type of anisotropy that has been extensively studied in soil and rock mechanics (e.g., TSVANKIN, 1996;LIU et al, 2000;RASOLOFOSAON, 2003). The existence of one axis of symmetry in this case, often along the vertical direction (VTI), reduces the number of independent elastic parameters to five.…”

Crosswell seismic waveguide phenomenology of reservoir sands & shales at offsets >600 m, Liaohe Oil Field, NE China

“…It is not clear why the Liu et al (2000) and Parra et al (2002) far‐offset crosswell seismic data do not appear to provide evidence for seismic‐waveguide action comparable to the 100 m offset data of Parra et al (1998) or the present 600 m offset Liaohe data. In the case of Liu et al (2000), perhaps the waveguide is simply not well enough defined; or perhaps the layer is, unknown to the investigators, offset or otherwise discontinuous. Further waveguide modelling may disclose whether the dispersion evidence of Liu et al (2000) is consistent with the broad LVL of moderate velocity contrast exhibiting relatively weak energy confinement.…”

“…The difference in far‐offset (>500–600 m) waveguide signals between those recorded at Liaohe and those recorded by Liu et al (2000) and Parra et al (2002) may be due to different degrees of physical property heterogeneity in the waveguide structures. Numerical simulations of 3‐D fault‐zone waveguide phenomena conducted by Leary et al (1991) and Igel et al (1997) suggest that heterogeneity within a waveguide LVL can destroy a waveguide signal.…”

“…Elsewhere far‐offset crosswell data have been recorded in shale and sand/shale formations, but the field evidence presented for waveguide action has been more oblique. Liu et al (2000) report crosswell seismic signals at 600 m offsets in shallow‐lying gas‐producing marine shales obtained with a prototype of the orbital vibrator source used at Liaohe. While the far‐offset signal was strong, the LVL was 50 m broad and not particularly well defined, and does not appear to have provided spatio‐temporal energy localization that can be clearly tied to sonic well‐log low‐velocity intervals.…”

“…In the case of Liu et al (2000), perhaps the waveguide is simply not well enough defined; or perhaps the layer is, unknown to the investigators, offset or otherwise discontinuous. Further waveguide modelling may disclose whether the dispersion evidence of Liu et al (2000) is consistent with the broad LVL of moderate velocity contrast exhibiting relatively weak energy confinement. A possible complication arises because the shale formations have strong layering anisotropy.…”