American Oil and Gas Reporter - July 2019 - 58

SpecialReport: Reservoir Characterization
Many operators in the Wolfcamp and other plays are finding
that strict adherence to well paths dictated by lithology and
other drilling-derived data limits wastes completion dollars.
The costs of wasted fluid and stranded proppant in pressure-depleted or nonproductive sections can be mitigated, thereby improving return on capital. Moreover, drilling and completion
precision can help keep the stimulated rock volume within the
confines of the planned well spacing and reduce reserves lost as
a result of parent/child well interference issues.
A two-well project in the Midland Basin used multiple
unique solid oil-soluble tracers (OSTs) as a reservoir characterization aid to determine the best landing zone and most economic
well placement strategy in the Wolfcamp formation. This was
accomplished by monitoring and analyzing the OST datasets
produced from different frac stages and reservoir zones during
a 435-day sampling period. The two wells were intentionally
drilled highly toe-up in order to cross all the potentially
productive benches in two distinct Wolfcamp intervals.
The dynamics of the OST recovery over time provided
insights into the uniqueness of reservoir layering within the
Wolfcamp. This conclusion was supported strongly by the fact
that differing sectional lithologies were stimulated with the
same completion design, yet tracer recovery was vastly different
for many stages.
Some productive layers in the two wells exhibited initially
high, but transient OST recoveries, while other zones produced
OST data longer and much more consistently. By using granularlevel tracer data in conjunction with other geoscience information,
the operator was able to identify the formation layers with the
highest potential for optimal production economics. This new
methodology not only provided single-well placement optimization,
but gave important insights for future completions in the area.

Solid particulate OSTs can be used for both qualitative and
quantitative analysis, and can be employed in conjunction with

more traditional tools to observe fracture energy movements. In
fact, data from fiber optics, microseismic arrays, downhole
pressure gauges, etc., complement tracer data very well. When
viewed as a tapestry of data, and not a single stand-alone point,
the understanding of how fractures propagate and what lithologies
will provide good and long-lasting conductivity is greatly improved, as demonstrated in the case study wells.
OST chemicals are extremely hydrophobic, meaning they
have a great affinity for liquid hydrocarbon relative to water,
and are almost completely insoluble in water. As such, the oil
tracer chemicals readily partition into reservoir oil in situ, and
not into formation or frac fluids. Laboratory analysis of oil
samples taken at the surface measure the concentration of the
OST chemicals in the oil. Because petrophysical and geological
properties affect hydrocarbon production, they affect total tracer
recoveries and can give strong evidence as to each stage's oil
production potential.
Solid particulate OSTs differ from liquid OSTs in that the
particulates remain locked in situ in the proppant pack after the
frac pressure wave has subsided. For OST to be released from
the carrier media, oil must flow over the trapped particulate in
the reservoir. Geopressure will not force the solid tracers back
into the wellbore after fracturing, as often occurs with liquid
tracers, eliminating the chance that "false positives" will be
generated in the data. In the case study wells, the different longterm behaviors of the tracer datasets highlight the uniqueness
of the geological and petrophysical makeups of the rock types
penetrated by each well.
Although relatively new, particulate OST technology rapidly
has gained acceptance as a cost-effective way to provide longerterm downhole flow datasets than what can be achieved with
conventional production logging tools (PLTs). With PLTs, the
well must be flowing with no obstructions in the casing
(including tubing, gas lift values or downhole pumps), which
limits the amount of data that can be acquired. In contrast,
chemical OST data can be gathered anytime a fluid sample is

FIGURE 1A
Well No. 1 Wellbore Path and First
Five-Day OST Recovery by Stage Grouping

FIGURE 1B
Well No. 2 Wellbore Path and First
Five-Day OST Recovery by Stage Grouping

Tapestry Of Data

5,000

5,000

8,250

4,500
4,000

8,300

8,350

Total OST Recovery (PPB)

Total OST Recovery (PPB)

3,500

3,000

8,400

2,000
1,500

9,800

10,800

11,800

12,800

13,800

14,800

15,800

16,800

17,800

4,000

3,500

2,500

8,800
8,450

4,500

8,450

8,500

3,000
2,500

8,550

2,000
1,500
8,600

1,000

1,000
8,500

58 THE AMERICAN OIL & GAS REPORTER

OST 3,300
OST 3,300
OST 3,300
OST 3,300
OST 3,300

OST 2,500
OST 2,500
OST 2,500
OST 2,500
OST 2,500
OST 2,500
OST 2,500

OST 3,200
OST 3,200
OST 3,200
OST 3,200
OST 3,200

OST 4,300
OST 4,300
OST 4,300

2 1

OST 2,100
OST 2,100
OST 2,100
OST 2,100
OST 2,100
OST 2,100
OST 2,100
OST 2,100
OST 2,100

5 4 3

OST 3,100
OST 3,100
OST 2,200
OST 2,200
OST 2,200
OST 2,200
OST 2,200
OST 2,200
OST 2,200
OST 6,000
OST 6,000
OST 6,000

11 10 9 8 7 6

OST 2,600
OST 2,600
OST 2,600

25 24 23 22 21 20 19 18 17 16 15 14 13 12

OST 5,500
OST 5,500

28 27 26

8,650
OST 2,000
OST 2,000

29

OST 2,400
OST 2,400

37 36 35 34 33 32 31 30

OST 4,100
OST 4,100
OST 4,100

OST 3,400
OST 3,400
OST 3,400
OST 3,400
OST 3,400
OST 3,400
OST 3,400
OST 3,400
OST 3,400

42 41 40 39 38

OST 3,000
OST 3,000
OST 3,000
OST 3,000
OST 3,000
OST 3,000

OST 2,900
OST 2,900
OST 2,900

50 49 48 47 46 45 44 43

OST 5,200
OST 5,200
OST 5,200
OST 5,200
OST 5,200

OST 4,200
OST 4,200

OST 3,500

OST 2,300
OST 2,300
OST 2,300
OST 2,300
OST 2,300
OST 2,300

0

OST 5,100
OST 5,100
OST 5,100
OST 5,100
OST 5,100

500

0

OST 2,700
OST 2,700
OST 2,700
OST 2,700
OST 2,700
OST 2,700
OST 2,700
OST 2,700

500

48 47

46 45

44 43 42

41 40 39 38 37 36 35 34 33 32 31 30

29 28 27 26 25 24 23 22 21

20 19 18 17 16

15 14 13 12 11 10 9

8 7 6 5 4

3 2 1
American Oil and Gas Reporter - July 2019

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