American Oil and Gas Reporter - January 2017 - 69

Shale Resource Science
3 identifies this comparison for all three
data bins.
During a shorter time span (i.e., 30
days), the cumulative gas between the
toe-up/toe-down wells is nearly identical,
but as time goes on, separation becomes
readily visible. Figures 4A, 4B and 4C
are cross plots of lateral TVD gain versus
production metrics. These illustrations
are especially useful for determining how
much lateral TVD gain is beneficial or
harmful. Negative lateral TVD gain is
indicative of a toe-up well, positive lateral
TVD gain is toe-down, and zero lateral
TVD gain is a flat well.
It should be noted that the middle
TVD data bin has more pronounced separation over time than the other two data
bins. This is because of the lack of wells
with TVD gain ranging from positive 50
feet to negative 50 feet (Figure 4B).
The data imply drilling toe-up wells
is better in the Cana Woodford play.
Figure 5 is a normalized plot to indicate
the impact of production variance between
a toe-up and a toe-down well with the
same TVD change of 250 feet. These are
the two extreme scenarios that can occur
in the Cana Woodford.
As observed from the plot, the pro-

duction loss in a significantly toe-down
well can be as high as 30 percent of the
EUR at the economic limit. In turn, the
production loss can result in significant
NPV attrition. While detailed economic
analysis was beyond the scope of this
study, engineers can take these results
into consideration and account for loss
of NPV while performing analyses for
different plays.
Overall, the impact that drilling toe
up versus toe down will have on a well's
production performance is highly dependent on reservoir conditions and properties. There are various physics-based
considerations, including that toe up
allows the gravity drive to force fluid to
the heel so less of the internal reservoir
pressure is expended in moving fluid, allowing for longer production without artificial lift.
Two-phase fluid flow effects also
need to considered. In general, a toedown configuration lends itself to high
liquid holdup, requiring higher superficial
gas velocity to unload the lateral. If the
liquid holdup is significant and cannot
attain critical velocity, then production
will decrease or cease. In the case of
infill drilling or field development, off-

setting well fracture interference will
increase liquid holdup, requiring more
energy to move the fluid through the
lateral.
Wellbore orientation, in reference to
hydraulically stimulated fractures, could
further compound the issue of "toe-downess," depending on fluid saturations in
the fracture and the necessary pressure
difference to start unloading the fracture
into the wellbore.
r

SAM
BROWNING

Sam Browning is a reservoir engineer at Devon Energy Corp. in Oklahoma City. He joined Devon in 2014.
Prior to that, he worked as a completions engineer at Apache Corp. Browning holds a B.S. in petroleum engineering from the University of Tulsa.

FIGURE 5
Normalized Production Variance
100%

% of 250' Toe-Up Well

90%

RAM
JAYAKUMAR

80%
70%
60%
50%
40%

-250

-200

-150

-100
Shallow TVD

-50

Lat TVD Change Ft
Middle TVD

100
Deep TVD

150

200

250

Ram Jayakumar is a senior reservoir engineer at Devon Energy Corp.
His core work areas include reservoir
modeling, reservoir simulation, completion optimization, production analysis and forecasting, economic modeling,
sensitivity analysis, well testing, and
nodal system analysis. Jayakumar
holds a bachelor's in chemical engineering from Anna University in India
and an M.S in petroleum engineering
from Texas A&M University.
JANUARY 2017 69

American Oil and Gas Reporter - January 2017

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