American Oil and Gas Reporter - December 2015 - 55

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SpecialReport: Well Stimulation & Completion Technology
bore. This allows for fracture propagation
and proppant embedment into the relatively ductile flooding surface above, which
typically has more oil saturation and
kerogen content.
Unlike the relatively ductile flooding
surface, the carbonate bench is more
fracture-prone with higher Vcal, which
correlates strongly to a dynamic Young's
modulus (the rock's ability to maintain a
fracture). Poisson's ratio also is a key mechanical property that quantifies the rock's
ability to fail under stress. When combined, these variables define the rock's
"fracability," in this case, expressed as a
brittleness index calculated using the psonic and dipole logs from the Neal 307
offset pilot hole.
Clay volume is crucial in brittleness index correlations. Brittleness decreases as
calcite is subtracted and replaced by
Vclay, increasing ductility. A general
rule of thumb in shale plays is that the lower the Poisson's ratio and the higher the
Young's modulus, the more brittle and
fracture competent the rock becomes.
Poisson's ratio and quartz (which is brittle and prone to fracturing) both show minimal variability throughout the Midland
Basin, making them poor proxies for
characterizing Wolfcamp brittleness.
As shown in the cross plots in Figure
2, however, strong correlations do exist
basinwide between Vclay and Young's
modulus, in addition to acoustic p-impedance. Specifically, as Vclay decreases and
is replaced by Vcal, Young's modulus increases linearly with increasing values of
p-impedance rock property measurements
estimated directly from inverted 3-D surface seismic.
The analysis shows a good correlation
between initial 120-day cumulative production from adjacent laterals in the field
and average p-impedance extrapolated
lengthwise along each of the well trajectories. Higher producing wells correlate to
higher magnitudes of p-impedance, where
Vcal is interpreted to exist to create higher brittleness values and more efficient
fracture stimulation.
Better producing wells are located in
the extreme northwest quadrant of the field
(Figure 3), where a carbonate-rich (detrital) lobe has been confirmed by petrophysical logs in vertical wellbores.
Consequently, a high degree of vertical and lateral heterogeneity exists within the Wolfcamp that is geology-driven
and associated with the irregular stacking
of discrete depositional carbonate units.

FIGURE 2
Regional Well Cross-Plots of P-Impedance and B Bench Vclay (A)
And Young's Modulus for Wolfcamp A and B (B)
Well
A
B

60%

Midland

E

Glasscock

B
Upton

C

Study
area

40%

Reagan

D

H

E

F

H

I

G

C
20%
I
G
0%
25,000

30,000

35,000

40,000
45,000
P-Impedance (ft/sec*g/cc)

50,000

55,000

Young's
Modulus
(Gpa)

A
B

60%

60,000

Midland

Glasscock
1.27e+007

Upton

C
9.96e+006

Study
area

40%

Reagan

D
E

F

H
G

I

7.20e+006

20%

4.44e+006

1.68e+006

0%
25,000

30,000

35,000

40,000
45,000
P-Impedance (ft/sec*g/cc)

The result is varying mineralogy compositions that ultimately define and correlate
with geomechanical properties characterized by relative fracability and brittleness.
These mechanical properties ultimately influence fracture stimulation geometries,
including hydraulic fracture height and
half-length, which require calibration to
microseismic and completion data.
Inverted Seismic Impedance
The normal-incidence, post-stack 3-D
surface seismic data were acquired with
82.5-foot bin spacing, and were processed
for zero phase to preserve relative amplitude. The 3-D data were inverted to p-impedance, resulting in seismic-derived
rock property units that could be integrated and correlated with petrophysical and
geomechanical dipole logs as well as microseismic data.
The seismic inversion process included
deterministic wavelet estimation to ensure
zero-phase data for peak-energy seismic re-

50,000

55,000

60,000

sponse at acoustic interfaces to extrapolate
accurate p-impedance maps. Mixed- or rotated-phase data would have resulted in
varying energy distributions across amplitude events, leading to inaccurate results not
characteristic of true stratigraphic and
mineralogical variabilities.
Because of the high volume of calcite,
the seismic response to bench horizons is
represented by a p-impedance amplitude
peak converse to clay-rich flooding surfaces
(low impedance amplitude troughs). The relative thickness of the Wolfcamp A and B
bench and flooding surface are well within the resolvable limits of the seismic data,
as is lateral resolution (Fresnel-zone),
which was essential when extrapolating
pseudo-lateral logs from the seismic.
To capture local lateral variability in
rock properties, zone-wise cross plots of
p-impedance versus Vclay, Vcal, Young's
modulus and closure stress were created
for the Wolfcamp A and B from dipole and
p-sonic logs from the Neal 307 offset verDECEMBER 2015 55
American Oil and Gas Reporter - December 2015

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