American Oil and Gas Reporter - December 2015 - 68

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SpecialReport: Well Stimulation & Completion Technology
FIGURE 4
Log Display Summary of Datasets Used to Define Upper
And Lower Horizontal Target Zones

Lower Eagle Ford
Upper Target_T

Upper Target_B

Lower Target_T
Lower Target_B
Buda

bed boundaries and high vertical stress
anisotropy.
Bed boundary and vertical stress
changes dictate vertical fracture growth
and fracture failure type, with weaker or
more ductile zones forming more layerparallel or inclined shear-dominated fracture planes versus competent mechanically
brittle zones. This may be a major factor
in limiting vertical proppant transport
and subsequent fracture connectivity,
thereby resulting in overall restrictions
to effective reservoir pay thickness.

XRF And SEM Analyses
XRF data were used to correlate key
redox sensitive trace elements to coreand log-derived reservoir parameters, including TOC, porosity, mineralogy, gas
chemistry (wetness), and SRA outputs.
Overall elemental enrichment of vanadium
(V) was observed in the upper section of
the Lower Eagle Ford, with enrichment
of copper (Cu) in the lower section. Cu
enrichment corresponded to higher TOC
and clay-rich lithofacies, with V enrichment associated with calcite-rich lithofacies
and geochemical indications of free (mobile) hydrocarbons.
An observed relationship between geochemical parameters and lithofacies is
consistent with results from other Eagle
Ford studies with similar thermal maturities. The observed elemental associations
are interpreted to correspond to similar
associations described in modern upwelling pelagic shelf settings with cyclic
68 THE AMERICAN OIL & GAS REPORTER

disoxic to euxinic conditions.
An overall progression from disoxic
to euxinic conditions is proposed within
the Lower Eagle Ford section, likely allowing for differing kerogen genesis and
subsequently giving rise to observed gas
wetness variations. This also implies that
pore types and geometries should vary,
which involves the interplay of organic
matter types (kerogen and bitumen) and
lithofacies, with certain developed pores
being more interconnected and conductive
to flow versus others, thereby favoring
higher-matrix deliverability into the induced hydraulic fracture network.
Cuttings and rotary sidewall core plugs
were examined with SEM, producing
more than 150 2-D images and four 3-D
SEM volumes. The dominant observed
pore network was organic-associated
spongy pores with secondary pendular
(bubble) pores. Pore distributions were
observed to be consistent with log-corecomputed porosity as well as many geochemical trends.
The high frequency of geochemical
data (acquired and sampled every five
feet) and their strong correlation to logcore parameters and SEM pore metrics
enabled the identification of stratigraphic
zones with interpreted high "matrix deliverability units."
From this interpretation, the high
degree of potential flow segregation or
compartmentalization within the Lower
Eagle Ford was evident, with relatively
high deliverability units being no more

than 10-50 feet thick.
Analyzing 3-D digital volumes revealed
mean pore diameters of 0.11-0.14 micron,
maximum diameters of 0.53-0.58 micron,
and computed permeabilities of 30-115
nanodarcies. The largest observed pores
are associated with pendular organic and
calcite inter- and intragranular pores, both
of which tend to dominate in the upper
portion of the Lower Eagle Ford. A large
quantity of pores is expected to exist
below 2-D/3-D SEM resolution within
sections of high TOC. This is interpreted
to account for the higher computed logderived/log-core porosity in the lower
portion of the Lower Eagle Ford, which
is inferred to have the highest hydrocarbon
storage capacity within the section.
Figure 4 shows a log display summary
of datasets used in defining the upper
and lower horizontal target zones. From
left to right, the log curves show gamma
ray, log-derived and XRF facies, log/cuttings TOC, log porosities with red-shaded
effective porosity, 2-D SEM total porosity
and percentage of porosity associated
with organic matter, key XRF trace elements, SRA dataset utilized to describe
matrix deliverability units, and gas chromatography mud gas and calculated gas
wetness.

Microseismic Results
A permanent buried array was utilized
within the development area to monitor
the six-well multizone staggered test.
The buried array consisted of 90 stations,
each with three channels at varying depths
(270 channels). The array covered a total
area of approximately 15 square miles.
The main deliverables from microseismic acquisition were estimating overall
fracture geometry (specifically determining
propped versus unpropped stimulated
reservoir volume (SRV)), and well-towell interaction behavior.
Propped versus unpropped SRV was
determined by the microseismic contractor's proprietary workflow, in which fractures were modeled per microseismic
event, focal mechanism, seismic moment,
rock rigidity, and injected fluid volume
to create a discrete fracture network
(DFN). The DFN-modeled fractures were
filled utilizing a mass balance method to
create a propped DFN that was translated
to a volume grid to create a propped
SRV (PSRV).
The two groups of wells were drilled
in mirrored staggered configurations, with
wells within the same target zone spaced
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