American Oil and Gas Reporter - September 2015 - 74

FIGURE 3A

FIGURE 3B
3-D Structural Model Constructed
From 2-D Geosteering Profile

Hypocenter Locations in 3-D Depth View

be expected, based on the amount of
stress-related frac height growth.
Figures 2A and 2B show per-well and
per-stage production versus stage count,
respectively, after 30, 90, 180 and 365 days
for Bakken wells. The diminished perstage returns of closer spacing are evident
from the start of production, and likely are
the result of the stress interaction between
a growing hydraulic fracture and the previous fractures placed in a lateral. We believe the physics controlling incremental
per-stage production is related to the observed cyclical reduction of the in-zone
fracture area.
Case Study Wells

Case study data from three Hess-operated horizontal wells-two in the Utica play
in Ohio and one in the Middle Bakken in
the Williston Basin-illustrate the results
of analyzing microseismic events to optimize stage spacing and the influence of
stress shadowing on closely spaced fractures in multistage completions.
Well A (referenced in Figure 1) is in the
Utica and has a 5,000-foot lateral section
with 15 stages placed in a plug-and-perf
fashion in a cased and cemented borehole.
Each stage was 290 feet long with four perforation clusters spaced at 70 feet and
about 70 feet from the closest perfs in the
previous stage. Well B also is a Utica well
with a 5,000-foot lateral. It was cased and
cemented with its stages 230 feet apart,
each with four perf clusters spaced 50 feet
apart. Well C is in the Bakken. It was completed with a hybrid design of 19 slidingsleeve stages followed by another 10
plug-and-perf stages, all using tubing
held in the open hole by packers that pressure isolated the successive stages. It had
a 10,000-foot lateral with nominal stage
spacing of 250 feet.
In each case, nearby monitor wells were
instrumented with geophones to record microseismic events and determine hypocen74 THE AMERICAN OIL & GAS REPORTER

ter locations while the completions took
place. Analyzing depth histograms of
points grouped by stage provides a statistically significant measure of the depth
range encountered by the fracturing
process. By creating histograms of the position of microseismic events relative to
a prominent stratigraphic boundary, the
analysis avoids the confusion that results from the effects of dip over the length
of the lateral, or from variations in the
placement of the perforated interval where
the lateral crosses bedding.
This requires building a 3-D structural model using geosteering results from
drilling the lateral to develop a cross-section along the borehole and tops from the
monitor well or nearby wells to determine
structure away from the lateral. Geosteering provides a one-dimensional view extrapolated into two dimensions along the
plane of the lateral, so it does not include
variations in layer thickness. However, in
these examples, our understanding of the

structure is that it is mostly simple, with
planar dip in the regions around the wells.
A small 8- to 10-foot fault is identified
from geosteering in the second stage of
well C, with another possible fault in stage
18.
Depth Histograms

Figures 3A-3C illustrate the process of
constructing microseismic event depth histograms for well A in the Utica. Figure 3A
shows a profile view of the microseismic
hypocenter locations in 3-D depth view
relative to the borehole. Figure 3B shows
the geosteering profile and 3-D structure
near the borehole, with nearby well tops
and one stage of events in the gray cloud.
Figure 3C shows calculated histograms
of relative frequency of points by depth
above the Trenton Limestone (red line) for
each stage (the Trenton formation is a
prominent carbonate platform underlying
the reservoir section). The yellow triangles

FIGURE 3C
Calculated Depth Histograms of Hypocenter Frequencies
Relative Trenton Limestone (Utica Well A)
Stage No. 1
Toe

Triple Combo Log

9 10 11 12 13 14 15

9,100

Top Utica
9,300
Top Pt. Pleasant

9,400

Top Trenton Ls.

9,600

9,700

Histograms of Microseismic Events by Stage Referenced to Stratigraphy

Heel

American Oil and Gas Reporter - September 2015

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