American Oil and Gas Reporter - July 2015 - 87

SpecialReport: New Products/Technology

Through-Bit Logging Pays Dividends
By Robin Slocombe,
John Walsh
and Sachin Bammi

HOUSTON-After drilling a number
of horizontal wells several years ago to
exploit a narrow oil window in the Eagle
Ford play, engineers noticed that production varied erratically, even though
they had completed all laterals using
roughly the same geometrically spaced
perforation clusters. Some wells produced
high water volumes; apparently, hydraulic
fractures had propagated into underlying
formations.
The operator had acquired only measurement-while-drilling gamma ray logs
to guide geosteering. However, this information was inadequate to properly
characterize the reservoir properties that
impacted production. Geoscientists and
engineers needed additional log data to
design more efficient completions, increase
production, and reduce costs.
Traditional wireline logging methods
face greater risks and logistical challenges
in horizontal wells, especially in shale
reservoirs. Typically, these boreholes are
too small to easily accommodate standard
logging tools. Also, running wireline
technology in and out of hole without
gravity assistance requires specialized
conveyance methods-drill pipe or tractors,
for example. These tend to be more complex, time-consuming, and expensive.
Therefore, many shale operators forego
petrophysical logging altogether. This is
why they typically position fracture stages
at arbitrary intervals along the lateral.
There is insufficient geomechanical data
to optimize completions according to
varying rock and fracture properties.
After weighing available conveyance
options, the Eagle Ford operator decided
to deploy a small-diameter, through-thebit logging suite for his next horizontal
drilling campaign. Once drillers tripped
the directional bottom-hole assembly,
they ran a special portal bit to condition
the hole to total depth (Figure 1).
They then pumped down a 21⁄8-inch
tool string on wireline inside the drill
pipe and out through a portal in the center
of the bit. After verifying the tool string
was operational, the logging engineer
disconnected it from the wireline, which
he reeled back to the surface.
The quad-combo suite recorded density,

neutron, resistivity and sonic data in
memory mode as drillers pulled the pipe
out of hole. When the tool string reached
the casing shoe, the logging engineer retrieved it on wireline and downloaded
all the data. By combining the logging
run with a conditioning trip, the company
saved a full 24 hours of rig time, compared
with traditional pipe conveyance.
Using through-the-bit sonic, density
and other log data, the operator's team
was able to infer anisotropy caused by
natural fractures, derive elastic and mechanical properties, and group hydraulic
fractures into optimal stages with similar
rock properties. These "engineered completions" successfully confined fractures
to the target reservoir, while eliminating
water production, reducing completion
costs by roughly 60 percent per stage,
and increasing the number of stages per
lateral. Compared with previous wells,
the company doubled its average 30-day

oil production.

Through-The-Bit Logging
Small-diameter, through-the-bit logging
technology is only about 10 years old,
and its unique capabilities continue to
evolve. Today's advanced through-thebit logging tool kit provides a full wireline
measurement suite for difficult boreholes
such as extended-reach wells, horizontal
wells in unconventional reservoirs, and
even challenging vertical wellbores.
One independent operator adopted
through-the-bit deployment to log vertical
wells in an unconventional shale play as
early as 2009. Because of unstable openhole conditions that occurred using water-based mud (WBM), engineers often
were unable to reach bottom with conventional wireline tools. They were forced,
therefore, to drill and log wells using oilbased mud, or drill with WBM and log
with tubing-conveyed tools-both of which

FIGURE 1

The typical through-the-bit logging sequence, as shown here, is to 1) drill to total depth; 2)
pull the bit off bottom and lower the tool string on wireline, pumping the tool string out
through a portal in the bit; 3) disconnect and remove wireline from the drill pipe; 4) log the
hole while pulling the drill string out of hole; 5) draw the tool string back into the casing; 6)
retrieve tool string on wireline; and 7) continue drilling or conditioning as needed.

JULY 2015 87

American Oil and Gas Reporter - July 2015

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