American Oil and Gas Reporter - September 2018 - 86

SpecialReport: Reservoir Modeling & EUR
As actual geologic properties vary
from these best cases, the analytic model
estimates the cumulative impact on production.
From a geologic perspective, true vertical depth tends to be the best statistical
predictor of relative well performance in
both simple cross-plots and full multivariate
analysis. One-two miles of rock lying on
top of the Wolfcamp formation with pressure gradients of 0.5-0.8 psi/foot results
in reservoir pressures of several thousand
psi, which keep gas in solution and drive
high fluid flow rates.

The advantage of modeling each major
production driver is that the engineering
effects can be normalized to produce a
customized Wolfcamp B sweet spot map
driven by depth, regional oil/gas/water
cuts and average gamma ray readings
(Figure 5A). While providing a valuable
acreage valuation tool, a static sweet spot

map will lessen in value over time as
wells are drilled and produced. Since dynamic wellbore spacing and production
timing is known by the model, the initial
sweet spot map can be depleted by horizontal well production for any point in
time (Figure 5B). TVD contours from
surface to the top of the Wolfcamp A are
again overlain in white.
What is important to understand is
that these maps are not simple estimates
of original oil in place. Such a model
would only have a geologic focus, and
include estimates of thickness, porosity
and oil saturation. Rather, these sweet
spot maps are estimates of how much oil
will be produced from the Wolfcamp B
over six months for a typical wellbore of
average horizontal length, frac intensity,
well spacing, etc.
The dynamic sweet spot map then depletes the initial Wolfcamp B sweet spot
map over time. The resulting stipple-like
pattern is an estimate of well and pad re-

FIGURE 5A
Wolfcamp B
Sweet Spot Map

FIGURE 5B
Wolfcamp B Sweet Spot Map with
Depletion from Producing Wells

Customized Sweet Spot Maps

gions of oil depletion, with a nominal
wellbore drainage radius. As these models
are updated and wells produce, a clear
picture is maintained of remaining development opportunities.
As the Midland Basin moves into
"factory" infill development mode, wells
may target thousands of feet of prospective reservoir. Optimizing spacing patterns, both laterally and vertically, means
understanding the multiple Wolfcamp
benches, as well as shallower Upper/Middle/Lower Spraberry/Clear Fork/Jo Mill
and deeper Barnett/Woodford targets.
What is critical, however, is comprehending the dynamic effects of nearby
horizontal wellbores, whether during
fracturing or in the production phase.
The initial and current sweet spot
maps in Figures 5A and 5B were modeled
for the Wolfcamp B formation only. In
the emerging full-field development of
the Midland Basin, a volumetric perspective spanning multiple geologic benches
is required. Multivariate analytics can
reconcile geologic properties and engineering parameters for any granularity
of reservoir thickness, and the results can
then be combined into a full 3-D model.

FIGURE 6
Multi-Formation Wolfcamp Sweet
Spot 3-D Model with
Wolfcamp A Stripped Off to Show
Wellbores Landing in the
Formation

86 THE AMERICAN OIL & GAS REPORTER

American Oil and Gas Reporter - September 2018

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