Anisotropy
Analysis in Shale Using Advanced
Sonic Data — Bakken Case Study
Incorporating anisotropy
in
seismic
data processing will
improve accuracy in prestack depth migration, amplitude variation with offset
(AVO) and hydraulic fracture monitoring, among which hydraulic fracing plays a
critical role in production enhancement of tight reservoirs such as Bakken
Formation. Considering shales as being vertically transverse isotropic (VTI),
three Thomsen
anisotropy
parameters with five independent stiffness
coefficients could be used to fully characterize them. So far, different
methods are applied to measure these parameters directly such as walkaway
VSP(vertical
seismic
profiling), multi-offset and multi-azimuthal VSP. Although
these surveys can provide us reliable anisotropic models, various constrains
including the cost could limit recording the VSPs. Recent innovations in the acquisition
of broad band sonic waveforms and dispersion analysis have changed the
industry’s perspective on cost of
anisotropy
analysis.
In this paper, advanced sonic data in two producing
wells in Bakken Fm, each drilled through different geologic features has been
acquired and analyzed. One is located on the crest of Nesson anticline, the
major geologic structure in the area. The other is away from the anticline and
none of the Nesson geologic feature is visible. First, a total of five
stiffness moduli were estimated from the Stoneley, two flexural waves and
formation density. Second, the Thomsen parameters in three members (upper,
middle and lower) of Bakken Fm. were calculated and compared one another. It
has been found that the Upper and Lower Bakken are highly VTI because of the
clay platy particles, while the middle member is isotropic or slightly
anisotropic. The well on the top of the anticline shows a higher degree of
anisotropy
in the middle member; and we interpreted this as the presence of
fractures due to the folding. Finally, data inversion of Stoneley and two
flexural waves was performed to create slowness radial profile around the
borehole and in the far field to elaborate the idea of existence of intrinsic
fractures and to discriminate them from the drilling induced ones. The well on
the crest of the anticline shows higher amount of variations in the Stoneley
wave slowness. Stoneley wave is proved to be sensitive to the mobility and
could be an indicator of higher permeability.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California