American Oil and Gas Reporter - July 2016 - 79

SpecialReport: Big Data & Geostatistics

FWI Unravels Near-Surface Velocities
By Anthony A. Vassiliou,
Robert A. Meek,
Rodney Stromberg
and Lionel Woog
HOUSTON-The Spraberry/Wolfcamp
play in the Midland Basin holds an estimated resource potential of more than
75 billion barrels of oil equivalent, ranking
as the largest oil field in the United States
and the second-largest in the world. Pioneer Natural Resources is the largest operator in the Spraberry/Wolfcamp with
an inventory of more than 20,000 untapped
horizontal drilling locations.
Over the past five years, Pioneer has
transformed its Spraberry/Wolfcamp acreage
into a world-class horizontal resource play,
successfully developing highly prospective
stacked intervals within its 785,000-acre
lease position in the Midland sub-basin of
the greater Permian Basin, which spans
some 20 West Texas counties.
Pioneer's drilling program targets multiple, oil-rich intervals using advanced
horizontal drilling and hydraulic fracturing
techniques for optimal well performance
and capital efficiency. The Wolfcamp B
has been the company's primary target
in both its northern and southern areas,
but to date, Pioneer has successfully appraised six productive intervals within
its Spraberry/Wolfcamp leasehold: the
Wolfcamp A, B and D shale intervals,
the Lower and Middle Spraberry intervals,
and the Jo Mill. The Lower Spraberry is
estimated to have the most oil in place of
all the Midland Basin intervals.
While the subsurface depths of these
reservoir intervals range from 6,700 to
11,300 feet, the success of seismic imaging
at reservoir depth in the Spraberry/Wolfcamp play is highly dependent on detailed
imaging of the near-surface. Consequently,
Pioneer undertook a study in the southern
part of the Midland Basin to apply stateof-the-art 3-D acoustic full-waveform inversion (FWI) to create a detailed velocity
model of the near-surface using a wideazimuth 3-D dataset acquired over the
study area.

Midland Basin study area. Therefore, the
FWI-derived velocity model was used as
the input model in the prestack depth
migration to delineate the near-surface
geology and subtle image reflections, including the shallow Santa Rosa formation.
Located at depths between 800 and
1,200 feet, the Santa Rosa is a brackishwater-bearing sandstone and conglomerate
within the lower section of the Tertiaryaged Dockum group. Hydrologically separated from shallow freshwater aquifers
located at 300-600 feet, the brackish
water from Santa Rosa fluvial sandstone
has become a primary source of water
used to perform hydraulic fracturing stimulations in area wells after it is treated to
remove sulfates and reduce its salinity.
The Santa Rosa is difficult to map on
seismic. However, the operator recognized
that successfully delineating the sandstone
would not only allow more productive
water wells to be drilled, but the improved
shallow seismic response also would
allow the velocity model to be used as
input into the prestack depth migration
to more accurately map the underlying
Spraberry/Wolfcamp formations.
To accomplish this, near-surface complexity and velocity inversions required
advanced wave equation-based methods
to estimate near-surface velocities. Fullwaveform inversion is a relatively recent
development in oil and gas geophysics
technology, and has been applied mostly
to marine 3-D seismic data. In fact, there
have been very few studies published on
using full-waveform inversion on 3-D
land datasets, and to the best of our
knowledge, Pioneer's study was the first
application of acoustic 3-D waveform inversion to an unconventional producing
reservoir in the Midland Basin.
It is important to note that the project
achieved these objectives without using
any well information. The study also
demonstrated that the full-waveform
methodology used for 3-D marine seismic
datasets is not applicable to the land 3-D
seismic dataset used in this study.

Alternating Velocities
Shallow sonic logs indicate that the
near-surface consists of alternating layers
of significantly slow and fast velocities.
However, a refraction statics model could
not delineate the slower velocities in the

Acoustic FWI Technique
The Permian Basin's shallow geology
consists of Quaternary alluvial and aeolian
deposits overlying the high-velocity Cretaceous Edwards Limestone. Below the
limestone are low-velocity Cretaceous

Trinity sands; Triassic Copper Canyon,
Trujillo and Tecova sands and shales; the
Triassic Santa Rosa sandstone; Permian
Anhydrites; and Permian-age dolomites
in the Grayburg and San Andres formations. Because of the alternating fast and
slow velocities in this area, refraction
statics-derived velocity models are not
adequate for resolving the low-velocity
layers and lead to incorrect ray paths for
imaging.
While the brackish Santa Rosa aquifer
is used as a water source for hydraulic
stimulation, the underlying Guadalupian
San Andres Sand often is used as a water
disposal aquifer. The ability to accurately
delineate the Santa Rosa and San Andres
formations is important to accurately
placing water-producing and disposal
wells, as well as to provide the detailed
shallow velocity models necessary to
properly image the deeper Spraberry/Wolfcamp horizontal targets.
The 3-D seismic data used in the study
are a subset of a larger wide-azimuth
survey. The dataset includes 4,892 vibroseis shots and 4,245 channels, and
was acquired with maximum offsets of
18,000 feet. The seismic bandwidth is 395 hertz, and the common depth point
(CDP) bin spacing is 82.5 feet.
After the raw field shot gathers were
geometry-checked, a spherical divergence
correction was applied with a picked
brute velocity. The first-arrival travel
times were picked on the dataset and
subsequently inverted with nonlinear tomography from topography. The resulting
tomography travel time root mean square
error was 14.5 milliseconds.
After the travel time tomography, five
main steps were performed to apply:
* Tomostatics;
* Minimum phase conversion;
* Surface-consistent deconvolution
with a 180-millisecond operator and a
24-millisecond gap;
* Three passes of velocity analysis
at 0.5- x 0.5-mile spacing; and
* Surface-consistent scaling.
Since the full-waveform inversion
could not estimate velocities in the presence of residual reflection statics, the
computed residual reflection statics were
applied to the shot gathers prior to FWI.
The FWI technique applied in the
Midland Basin is acoustic, so surface
waves had to be removed. This consisted
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