American Oil and Gas Reporter - September 2015 - 76

SpecialReport: Reservoir Characterization
it is a reasonable way to track bounces.
Faulting and pump pressures may affect the
top of the histogram depth range in any given stage. Following the bottom of the histogram seems best for tracking cycles related to stress shadowing.
The green line in Figure 3C (representing the average value of the deepest 15 percent of points from each histogram) has a
loopy, two-bounce structure within the
length of the lateral. The stresses were
building in the zone of interest as initial
stages were pumped, so that at some
point, the stresses accumulating in the
reservoir zone diminished the stress contrast at the top of the fractures, allowing
them to propagate upward. As stress accumulated higher in the section, new
fractures were able to reform close to the
borehole, and the process repeated.
As shown in Figures 5A and 5B, using
hydraulic fracturing data from the Bakken
formation, the induced, incremental minimum horizontal stress increases as fracture height and injection pressure increase. In this case, treatment pressure was
11,500 psi and in-situ minimum horizontal
stress (σh) was 7,860 psi, for a net pressure
of 3,640 psi. The figures plot the relationship of the induced stresses versus the distance away from the center of a fracture

at fracture heights of 300 and 100 feet, respectively.
The treatment pressure exerts a compressive stress in the direction normal to
the fracture on top of the minimum in-situ
stress that is equal to the net pressure at the
fracture face, but quickly falls with distance from the fracture. At a distance beyond one fracture height, the induced stress
is only a small fraction of the net pressure.
The stress shadow describes the increase of stress in the direct vicinity of this
fracture. If a second hydraulic fracture is
created parallel to the existing open fracture within the stress shadow, it will
have a closure stress greater than the original in-situ stress, requiring a higher fracture initiation pressure.
This modeling indicates that a typical
hydraulic fracture with a height of 300
feet (Figure 5A) causes a significant increase in the minimum horizontal stresses for distances less than 300 feet from
the fracture. This implies that if the
spacing of the two hydraulic fractures is
less than 300 feet, strong stress shadowing should be expected.
Our observations show that a greater
fracture height has a larger impact area of
the stress, and that smaller hydraulic
fracture spacing creates a stronger stress

FIGURE 6
Conceptual Model of Changes in Minimum Horizontal Stress
Resulting from Closely Spaced Fractures (Bakken Well)
10,400

6,000

Minimum Horizontal Stress (psi)
7,000
8,000
9,000
z

Depth (ft)

10,500

10,550
U B Shl
10,600

10,650

psi

10,450

Middle
Bakken

Diminishing rσ, frac
height barriers
disappear, and frac
grows up.

L B Shl

76 THE AMERICAN OIL & GAS REPORTER

Side View
x

Contributions to
minimum stress
from previous fracs
at next frac.

shadow and stress impact. In fact, the induced stresses may change the in-situ
stress state, stress direction, and stress
regimes in the interference area, causing
subsequent fractures to grow in an irregular fashion.
Minimum Horizontal Stress

Fracture initiation pressure (pb) is
much more dependent on minimum horizontal stress than the other in-situ stresses. If a change in minimum horizontal
stress is large enough, it will alter the fracture initiation position and propagation direction. Our observations indicate stress
shadowing increases minimum horizontal stress, increases pb, and makes the formation harder to fracture.
When the stress increase is significant,
particularly in the case of closely spaced hydraulic fractures, it will cause new hydraulic
fractures to redirect. This often results in
fracture growth and propagation preferentially upward, if the minimum horizontal
stress is lower in the shallower formation.
Figure 6 shows the original in-situ
stress estimated from logs in a vertical
Bakken well. The right side of the figure
illustrates how stress shadowing increased
the minimum horizontal stress after three
parallel fractures were generated. The
dashed lines show the sequentially elevated minimum stress (stress shadow) as fractures were added.
After three fractures were generated,
the minimum stress at the location of the
fourth frac was highly altered, and the
stress barrier above the reservoir almost
disappeared as the stress shadow increased the minimum horizontal stress in
the planned fracture zone, and reduced the
stress contrast with the overburden. The
result was that subsequent hydraulic fractures grew upward because of the reduced
stress contrast between the target reservoir
and the overburden.
'Self-Correcting' Process

Figures 7 and 8 show the histogram results from well B (Utica) and well C
(Bakken), verifying the observations in
well A. The 19-stage well B (Figure 7) has
230 feet of spacing between stages (compared with 290 feet in well A), and more

American Oil and Gas Reporter - September 2015

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