American Oil and Gas Reporter - July 2015 - 90

SpecialReport: New Products/Technology
while tripping out of hole.
However, before engineers pump out
the tool string, it could suffer damage
from shock and vibration inside the drill
collar. Without deploying the tool string
on wireline after the bit has reached TD
and verifying that it is fully operational
prior to logging operations, engineers
would have to wait until the entire assembly returned to surface before they
could determine whether the tools recorded
any data whatsoever.
Dipole Sonic
Until recently, through-the-bit logging
offered only a monopole sonic tool, which
provided a single shear measurement.
However, optimally engineered completions require compressional slowness as
well as fast- and slow-shear slowness-
two independent shear measurements-to

fully quantify variations in anisotropy,
stress, and completion quality along a
horizontal wellbore.
Engineers and geoscientists also need
low-frequency Stoneley wave measurements and cross-dipole measurements to
accurately detect and quantify the orientation and permeability of natural fractures.
By combining dipole with Stoneley measurements, they can develop a full 3-D
acoustic characterization of the formation,
which makes it possible to identify fractures and borehole stress regimes. A fitfor-purpose cross-dipole sonic tool provides this data.
In the absence of a small-diameter dipole sonic tool, operators have had to
base engineered completion designs on
ad hoc methods of estimating shear, or
use synthetic and offset well data to approximate missing acoustic measurements,

FIGURE 4

These comparisons of uniformly spaced (geometric) stages in a Wolfcamp lateral (track 1,
left) with engineered completion stages with similarly stressed rock (track 2), are based on
modeling reservoir, completion and composite quality (right).

90 THE AMERICAN OIL & GAS REPORTER

which has not been terribly accurate.
Otherwise, they had to run a standard
wireline dipole sonic tool separately from
the through-the-bit logging tool string,
which incurred additional risks, time and
costs.
Nevertheless, shale operators are proving that acquiring dipole sonic data is
well worth the effort. Some time ago, for
example, four Eagle Ford operators and
Schlumberger formed a completion optimization consortium to determine how
best to improve multistage completion
designs in horizontal wells using log
data. The operators were particularly interested in accounting for the impact on
perforation efficiency of variations in
stress along the laterals.
A study of production log data in 17
wells over two years showed that, on average, only 64 percent of the perforation
clusters in conventional, geometrically
completed wells were contributing to
total production. The consortium predicted
that grouping reservoir rock of similar
high quality into fracture stages, and positioning perforation clusters within those
stages to minimize differences in stress,
would achieve more efficient stimulation.

Proof Of Theory
To provide the petrophysical and geomechanical parameters needed to model
both reservoir and completion quality,
each of the four operators acquired openhole logs in three wells. To minimize
risks and accelerate data acquisition, the
consortium ran slim, through-the-bit quadcombo tools in all 12 wells. Using tractor
conveyance, each operator subsequently
ran a standard-size dipole sonic tool to
obtain full 3-D mechanical properties in
selected wells.
Petrotechnical experts input the results
from detailed petrophysical and geomechanical analyses into automated stimulation design software to generate custom-engineered completions based on intervals of similar stress. Accurately determining stress variations along the laterals ultimately depended on the availability of advanced dipole sonic measurements.
Completion engineers stimulated each
study well according to its uniquely engineered design, then ran production logs
to evaluate the efficiency of individual
perforation clusters. At the time the consortium published its study in October
2013, operators had assessed six of the

American Oil and Gas Reporter - July 2015

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