American Oil and Gas Reporter - December 2019 - 68

SpecialReport: Stimulation & Completion Technology
field, which plot well below the GMWL
and yield generally enriched δ18OH2O
and δDH2O values. Importantly, the timelapse hydrogen and oxygen isotope data
from the new wells co-vary and fall on
a linear trend between the frac water
and legacy-well produced water endmember populations.
Given the mixing trend evident in
Figure 2, the isotopic composition of
each discrete, time-lapse produced water
sample can be understood in terms of a
simple two-component mixing model in
which the produced or flowback water
at any given time is interpreted as a
mixture of the source water injected
during hydraulic fracturing (frac water)
and the long-term produced Niobrara or
Codell formation water in the reservoir,
base-lined from the legacy EOG wells
at Hereford Field.
Combining this mixing model with
the produced and flowback water volumes,
HighPoint and DIG were able to calculate
the relative contributions of each water
source with time, as shown in Figures
3A and 3B. These plots show time-lapse
data-shown as actual water volume in
Figure 3A and as a percentage of produced
water volume in Figure 3B-collected

from a single Niobrara horizontal well
that was hydraulically fractured in March
2017, sampled twice daily for the first 10
days of flowback and then daily through
the first five months of production.
Key Observations
By calculating the fraction of frac
water present in each time-lapse sample
collected from this and 21 other new
wells with time series data, several key
observations were made about the dynamic
nature of the produced water and its
sources:
· Produced water collected at the
very beginning of flowback is a mixture
of frac water and native Niobrara/Codell
formation water. The initial breakdown
of frac versus formation water varies well
to well, but samples collected at the start
of flowback on average were 46% returned
frac water (maximum 68%, minimum
31%).
· The contribution of frac water to
the overall produced water stream decreases sharply once flowback begins,
indicating that these wells clean up quickly
after stimulation, and formation water
begins to dominate the production stream.
Within three-four weeks, almost all sam-

FIGURE 4
100
90
80

% Frac Water

70
60
50
40
30
20
10
0

1/
2

4/
20
17
2/
13
/2
01
7
3/
5/
20
17
3/
25
/2
01
7
4/
14
/2
01
7
5/
4/
20
17
5/
24
/2
01
7
6/
13
/2
01
7
7/
3/
20
17
7/
23
/2
01
7
8/
12
/2
01
7
9/
1/
20
17

-10

Date
Based on the isotopic mixing model, the percentage of frac water in each time lapse
sample is calculated and plotted with time. Trend A is typical flowback behavior and
shows a decreasing contribution of frac water with time once flowback begins. Event B
shows a dramatic increase in frac water because of a frac hit from a nearby well sited
heel to heel on the same pad. Event C shows a smaller frac hit from a toe-toe offset well.

68 THE AMERICAN OIL & GAS REPORTER

pled wells were producing water with
less than a 10% frac water contribution.
· The isotopic composition of the
produced water stabilizes with time. This
equilibrated composition is consistent for
Niobrara and Codell wells in the field
and overlaps with many of the legacy
wells that were sampled after seven-eight
years of production. Consequently, this
signature is hypothesized to be representative of predominantly Niobrara/Codell
formation water or a mixture of connate
and frac water that equilibrates during
the imbibition process, but more work is
being done to test this theory.
· Finally, the amount of frac water
that ultimately is returned to the surface
during flowback and production is relatively low in these wells. An average of
3.5% of the frac water injected returned
during flowback and production (maximum 6.7%, minimum 1.9%).
Quantifying Frac Hits
The individual well data shown in
Figures 3A and 3B was collected from
the last well completed in that area, and
thus does not show any shut-in periods
during the hydraulic fracturing of nearby
offset wells. It also shows a predictable
decrease in the frac water contribution
through time. This is in contrast to many
of the other wells selected for time lapse
sampling, which were subject to shut-ins
while nearby wells were being completed.
Sampling these wells immediately before
shut-in and immediately after provides a
unique opportunity to study the produced
water chemistry response during wellto-well fracture interactions.
Common industry practice is to monitor
offset wells for pressure responses during
nearby completions to describe and quantify frac hits. Time-lapse water stable
isotope data provides an additional level
of detail; this data can show the physical
transport of frac water from a nearby
completion into the existing fracture network of a parent well.
These types of responses in the water
stable isotope data during the nearby
completions were seen in at least six of
the sampled wells. An example well
where interwell fracture communication
is evident during the hydraulic fracturing
of offset wells is shown in Figure 4.
The well shown in Figure 4 initially
shows typical flowback behavior with
the frac water contribution decreasing
sharply once flowback begins, followed
by a large increase in frac water contri-
American Oil and Gas Reporter - December 2019

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