American Oil and Gas Reporter - December 2017 - 54

challenge in this case because of a lack
of modern open-hole wireline logs within
the target area. However, extensive experience in the surrounding area and other
resources provided a general-type log of
the Woodford Shale.
The Woodford can be classified into
four rock types: siliceous mud rock,
clayey siliceous mud rock, clayey mud
rock, and organic-poor clayey mud rock.
Experience indicates that stimulation success is more likely in the siliceous mud
rock and failed fracturing treatments are
most likely to occur in the clayey and organic-poor clayey mud rocks.
Optimally, the goal is to target the
Woodford "B" interval, which is characterized by high resistivity and neutron and
density porosity coming together or crossing
over, and lower total gamma ray (GR)
than other Woodford intervals. Additionally,
the photoelectric (PE) factor deflects toward
2.0 barns/electron (b/e), and a nuclear
magnetic resonance log shows a stronger
signal at longer relaxation times.
Interpretation of the B interval shows
higher effective porosity, lower clay volume,
lower total organic carbon content, and
improved pore size compared with the
"C" interval directly below it. This zone
comprises a siliceous mud rock that can
be cherty in some areas. It is also likely to

54 THE AMERICAN OIL & GAS REPORTER

contain natural fractures. Figure 1 shows
an example of the interpreted rock type
for the case study well. In contrast, the
overlying Woodford "A" can have much
higher clay content, lower effective porosity,
and higher neutron-density separation,
while the underlying "C" is a high-organic
black shale that also can have higher clay
content than the siliceous mud rock above
it. This interval is softer, with lower effective
porosity and higher TOC content.
Analysis And Lessons Learned
The first engineered completions design
along the lateral used open-hole conventional logs (gamma, resistivity, density
and neutron), a dipole sonic log, and a
resistivity-based imager. Most of the 21
stages had three perforation clusters. As
noted, less than 10 percent of the designed
proppant volume was placed in five of
the stages (17-21).
In analyzing the initial results prior to
drilling the sidetrack, it appears porosity
was extremely spikey and variable in the
initial interpretation, even at smaller
scales. This is usually a strong indication
of unresolved volumes in the interpretation,
causing erratic results. One of the most
difficult interpretations to resolve, without
the aid of an NMR porosity log, is the
quantity of kerogen versus porosity.

In evaluating the raw data, the first
concern was the log data quality of the
challenging borehole and horizontal logging operation. Attempts were made to
compare cross-plots from vertical offset
logs, and later the sidetrack well. These
well logs showed a significant departure
from comparison wells. It was determined
that the initial lateral was drilled through
three major rock types corresponding
with the characteristics of the A, B and C
intervals, as shown in Figure 2A.
The conclusion was that the petrophysical model was insufficiently calibrated,
and therefore, unable to accurately determine the petrophysical properties necessary
to match stage and perforation placement
with ideal near-well bore locations for
improved well stimulation and productivity.
A better understanding of vertical well
petrophysical models and results, along
with careful calibration to offset data, was
necessary to develop a proper engineered
completion design for the sidetrack.
Figure 2B shows a closeup of stages
17-21. All the stages in the upper Woodford
section had experienced high pressures
that forced stage placement to end prematurely. This was an immediate indicator
that the engineered completions design
for the second lateral should account for
any sections of the lateral that move into
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American Oil and Gas Reporter - December 2017

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