American Oil and Gas Reporter - July 2019 - 61

SpecialReport: Reservoir Characterization
taken at the surface, regardless of whether a well is flowing or
pumping. One common description is that PLT data is akin to a
snapshot of a movie, while chemical tracing data shows the
entire movie.
OST analysis not only aids in understanding total oil recovery
by lateral section, but generates a pseudo-production log over
time. The data provided in the Midland Basin project highlights
the dynamic and transient nature of oil production from different
sections of two laterals. Identifying which productive layers
return strong, but relatively short-lived OST production versus
more consistent OST recovery rates supports type-curve
predictions of estimated ultimate recovery. In addition, the follow-on financial and operational gains generated by analyzing
OST datasets in target-rich zones aids in the full exploitation of
an operator's property.
Midland Basin Project
The two horizontal Wolfcamp wells are spaced 990 feet apart
horizontally, with a difference of 485 feet in true vertical depth
at the respective heels. As noted, both wells were drilled toe-up.
Offset geologic data allowed the mapping of potential landing
zones along with shale barriers in both sections. The wells were
intended to cut different potential productive benches in each
zone. The wellbores were drilled carefully in order to reach all
the potential targets without any significant unwanted curvature.
The completion designs were held constant (gallons of fluid
and pounds of proppant per lateral foot) in each well so every
stage would be treated as similarly as possible. Identical
stimulation designs for both wells were pumped, with the only
variable being the volume of the stage size (gallons/foot of
fluid and pounds/foot of sand) as a function of stage length.
In Well No. 1, 11 seemingly geologically unique zones were
identified over 8,500 feet of lateral. Fifty stages of varying
lengths (a minimum of 165 feet and a maximum of 205 feet)
were completed utilizing a standardized choice of 4 kilograms
of OST per 175 feet of stage length. Standardization resulted in
each stage receiving between 3.77 and 4.69 kg of OST during
FIGURE 2A
Well No. 1 Cumulative OST Recovery
By Stage Grouping

stimulation. The total stimulation treatment for Well 1 was
351,000 barrels of slickwater and 16.4 million pounds of sand.
Another 11 unique zones were identified over 8,500 feet of
lateral in Well No. 2. Forty-nine stages with lengths varying between 140 and 230 feet were completed using the same tracer
volume standardization as in the first well. This resulted in each
stage receiving between 3.2 and 4.2 kg of OST during stimulation.
The total stimulation treatment for this well was 352,000 barrels
of slickwater and 16.6 million pounds of sand.
As shown in Figure 1A, the wellbore trajectory of Well 1
had a toe-up rise of nearly 250 feet. The bar chart under the
wellbore diagram shows adjacent frac stages that were grouped
together based on similar rock types. The same OST chemical
was used in each adjacent stage in the same rock type. The
bars' height represents cumulative OST concentration recoveries
during the first week of production.
Sampling occurred throughout a 435-day period that ended
in February 2017. While only 70 samples were subjected to
laboratory analysis, it is worth noting that when the sampling
program ended, five of the 11 grouped stages still were returning
OST levels greater than 20 parts per billion. The remaining six
stage groupings showed zero tracer recovery at day 435.
Figure 1B shows the same data for Well 2, which had a toeup rise of 150 feet. Samples again were collected during the
same 435-day period, with 70 samples analyzed in the lab.
Unlike Well 1, this well returned no measurable tracer chemical
from any stages on day 435.
OST Sample Recovery
Figures 2A and 2B show the cumulative OST sampling recoveries over time by stage groupings for both wells. For Well
1, two stage groupings (stages 6-11 and stages three-five) were
the two highest cumulative tracer-producing stages. Prior to
production, geological data obtained while drilling Well 1 determined stage groupings 6-11, 17-25 and 43-50 would be the
most productive targets. The bar chart shows OST recovery
from stages three-five to be similar to adjacent stages 6-11.
FIGURE 2B
Well No. 2 Cumulative OST Recovery
By Stage Grouping

1,600

1,600
1,400

1,400

1-3

6-11
3-5

40-41

17-25

1,000

38-42
800

30-31
26-28

600

1-2
43-50

400

21-29

1,200
Tracer (PPB)

Tracer (PPB)

1,200

1,000

45-46

800

30-32
4-8

600

16-20

400

42-44

12-16
200

32-37

33-39
200

47-48

9-15

brl

brl
0

40
Days On Line

JULY 2019 61

American Oil and Gas Reporter - July 2019

Table of Contents for the Digital Edition of American Oil and Gas Reporter - July 2019