American Oil and Gas Reporter - January 2019 - 77

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Near-Wellbore Complexity
The added pressures and stresses increase formation stress
past the critically stressed failure point, allowing fractures to
fail as the completions progress. In Figure 4, the propped SRV
is increasingly compact from Well 1 to Well 4, with smaller
height and lateral extents, suggesting increased near-wellbore
complexity as tank development advances. The increased
propped volume and near-wellbore complexity seen in microseismic data are corroborated by production diagnostics. A diagnostic plot of the reciprocal productivity index of DSU-B indicates that tank development created a larger fracture network
surface area and greater fracture complexity than the nontank
developed DSU-A.
Completion datasets contain trends that imply the process
creating this complexity. Pressure data from DSU-C exhibit a
rising trend in prefrac pressures when wells are completed in
close proximity to one another. Figure 5 shows initial shut-in
pressures rising between stages toward the end of a sequence of
tank developed wells on the west side of DSU-C. The pressures
for the first two pairs of wells did not increase dramatically,
perhaps because of the greater spatial extent of their positions,
having been spread over two formations, but well pairs three
and four in the sequence exhibit increasing initial pressure
profiles. This suggests that the latter wells were influenced by
energy from the preceding completions.
The fluid placed by the first two pairs elevated reservoir
pressures, increased local pore pressures and diminished the
system's normal effective stress, moving the system toward its
shear failure envelope. Completion pairs three and four then
were completed in the same reservoir and at tighter spacing
than the first two pairs, into a system for which energy had been
elevated by the first two completions. This region of heightened
pressure and stress above well pairs three and four appears to
have acted as a partial hydraulic seal for the last two completion
pairs, leading to greater localization of completion energy and

greater elevation in initial pressures.
The relationship between rising initial pressures during the
course of a tank developed package of wells is significant because
it indicates tank development is supercharging the reservoir,
adding energy to the system faster than it can be dissipated
through fracture propagation and leak-off. It is apparent in the microseismic data that the increasing residual energy correlates with
accelerating fracture event generation and cumulative moment.
The connection between the rising prefrac pressures and accelerating event generation is shown schematically in Figure 6.
Starting points A and B are higher than the first well's initial
prefrac shut-in pressure, translating into more fracture initiations,
greater near-wellbore complexity and greater event generation.
Because the prefrac shut-in pressure increases for each subsequent
well in a tank development sequence, the net energy needed to
return the system to a critical state diminishes with each additional
completion. The end result is that subsequent completions in tank
development sequences spend more completion energy generating
fractures, and less time, energy and capital are spent elevating the
system energy to the point of formation breakdown.
The correlation between initial shut-in pressure and cumulative
moment magnitude is tank development's value creation mechanism: High density tank development results in more fracture
network surface area per unit volume of reservoir than conventional
sequential development. As a result, the same volume of rock
can support a greater number of NPV-optimal wells, achieve
greater cumulative asset value and recover more resource.
FIGURE 6
Adding Energy to Reservoir System
With Tank Development
Formation
breakdown

Well 1
Stage 1

Well 2
Stage 1

ISIP

Well 3
Stage 1
Shmin

Fracture
propagation

Pressure

tank development advanced and wells were stimulated continually,
both measured event count and magnitude of each well increased
(Figure 4). This is evidence of breaking more rock by supercharging the reservoir and is consistent with Mohr-Coulomb
failure criteria (as fluid injection increases pore pressure, normal
effective stress diminishes, moving the system into a state of
shear and causing those fractures optimally oriented to regional
stresses to fail first).

Leak-off
Point

Bleed-off
Pshut-in(Pre-frac)
A

B

Time

FIGURE 7
Average EUR and PV10 Trends of Spraberry Wells (Tank versus Non-Tank Development)
DSU BFIT PV10

EUR (Mboe)

BFIT PV10 (M$)

Single Well EUR

Density (Wells/Mile)
Nontank EUR
Tank EUR

Density (Wells/Mile)
Tank Cum PV10

Nontank Cum PV10

Actual Nontank EURs

p Actual Tank EURs

JANUARY 2019 77
American Oil and Gas Reporter - January 2019

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