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| Titel |
Feasibility of estimation of vertical transverse isotropy from microseismic data recorded by surface monitoring arrays |
| VerfasserIn |
Davide Gei, Leo Eisner, Vladimir Grechka, Geza Seriani, Peter Suhadolc |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049582
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| Zusammenfassung |
| Hydraulic fracturing is a technique for increasing the reservoir permeability and consequently
the production rate of natural gas and oil. It involves injecting high-pressured fluids into the
reservoir formations to open existing or to create new fractures. This process induces
microseismic events. Before starting the injection operations, perforations are shot in the
stimulated well to create openings through the casing and establish communication between
the wellbore and the reservoir. These perforations can be used to invert seismic velocities as
their positions are known with a high precision.
Hydraulic fracturing treatments can be monitored with surface star-like arrays of receivers
centered at the wellhead (Duncan and Eisner, 2010). Arrival times recorded at the surface
from microseismic events or perforation shots can be inverted to estimate their origin times
and locations, as well as the velocity, the Thomsen anisotropy parameter δ (Thomsen, 1986)
and the anellipticity coefficient η (Alkhalifah and Tsvankin, 1995) of a vertically transversely
isotropic (VTI) medium. We consider the P-wave traveltime inversion for homogeneous VTI
media. This is a well established inversion technique in active seismic (e.g., Grechka and
Tsvankin, 1998; Grechka, 2009). The inversion algorithm minimizes the sum of time
residuals from the difference between the picked arrival times and traveltimes computed with
the nonhyperbolic moveout equation given by Alkhalifah and Tsvankin (1995) in the
form
2 2 –x2– ––––––2ηx4––––––
t(x) = t(0)+ VN2MO - VN2MO [t2(0)VN2MO + (1+ 2η)x2],
(1)
where, for microseismic applications, t(0) is the one-way vertical time, x is the offset (surface
projection of the source-receiver distance) and V NMO is the normal-moveout velocity.
The latter relates to the P-wave vertical velocity V P0 and to Thomsen parameter
δgs
V 2NMO = VP20(1 + 2δ).
For passive seismic monitoring, the measured arrival time ta(x) is given by
ta(x) = tt(x)+ t0,
(2)
where tt(x) is the traveltime in the subsurface and t0 is the origin time. The above
formulation assumes a homogeneous VTI medium, which is an acceptable approximation for
effective anisotropy of horizontally layered sedimentary rocks (Grechka and Tsvankin,
1998).
Measured arrival times can be affected by picking noise. Moreover, the locations of
perforation shots are known with a limited precision, which depends on the accuracy of a
well-deviation survey (e.g., Bulant et al. 2007). The vertical P-wave velocity can be obtained
from active seismic (e.g., check shots) and it can also contain errors. In this study, we
investigate the sensitivity of this inversion technique to inaccuracies in the input
parameters.
We compute synthetic arrival times by ray tracing and perturb them with Gaussian noise.
Inversions of these noisy arrival times show high sensitivity of the anellipticity coefficient η
and Thomsen parameter δ to the noise level, whereas the origin times are estimated
accurately. Large offsets of the receivers and their greater number along each line
improve the anisotropic parameters estimation. The root-mean-square of the time
residuals appears to provide a good indication of the quality of the inverted anisotropic
parameters.
Uncertainties in both the vertical velocity and the source depth strongly influence the
origin time, the anellipticity parameter, and Thomsen coefficient δ resulting from the
inversion procedure. The root-mean-square of the time residuals can be low even when the
three inverted quantities are grossly incorrect, which emphasizes the importance of using
accurate vertical velocity and source depth for the inversion.
References
Alkhalifah, T., and Tsvankin, I., 1995, Velocity analysis for transversely isotropic media,
Geophysics, 60, 1550-1566.
Bulant, P., Eisner, L., PšenÄík, I., and Le Calvez, J., 2007, Importance of borehole
deviation surveys for monitoring of hydraulic fracturing treatments, Geophysical Prospecting,
55 (6), 891-899.
Duncan, P., and Eisner, L., 2010, Reservoir characterization using surface microseismic
monitoring, Geophysics, 75 (5), 75A139-75A146.
Grechka, V., 2009, Applications of Seismic Anisotropy in the Oil and Gas Industry,
EAGE Publications, 171 pp., ISBN 978-90-73781-68-9.
Grechka, V., and Tsvankin, I., 1998, Feasibility of nonhyperbolic moveout inversion in
transversely isotropic media, Geophysics, 63, 957-969.
Thomsen, L., 1986, Weak elastic anisotropy, Geophysics, 51 (10), 1954-1966. |
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