American Oil and Gas Reporter - April 2016 - 68

SpecialReport: Drilling Technology
existing wellbore into a new reservoir in
the Clinton formation. We did two of
these wells and saw 90-95 percent of
what we believe was the original reservoir
pressure. Even though the existing wells
were stimulated, we are not seeing much
depletion 600 feet from the wellbore.
Drilling those deviated holes gave us
confidence that the Ohio Geological Survey was right."

An Ohio Geological Survey study estimates only 7 percent of the 1.5 billion barrels of
original oil in place has been recovered from the East Canton Field's Clinton reservoir.
EnerVest says it aims to extract an additional 7 percent from conventional geology in
the 115,000 acres the company holds in the field.

mates only 7 percent of the estimated 1.5
billion barrels of original oil in place has
been recovered from the Clinton reservoir
in the East Canton Field. "Hopefully, we
can recover an additional 7 percent," Lay
offers, adding that EnerVest holds 115,000
acres in the field and operates 1,700 wells
producing a combined 1,100 barrels a
day.
The Silurian-aged Clinton consists of
interbedded sandstones, siltstones and
shales with permeabilities ranging from
less than 0.01 to 0.07 millidarcies, Lay
details. The Clinton formation is comprised
of the Stray, Red and White sandstone
intervals, and is overlain by the Packer

Shell-a dense carbonate-and is underlain
by the Cabothead Shale.
"By its nature, secondary recovery is
not effective on a large scale in this field,
so how do you increase recovery and tap
into the massive amounts of oil remaining
in this mature conventional asset? We
believe the answer to that is horizontal
drilling," he states.
EnerVest began by drilling infill sidetracks to test the Clinton for depletion.
"The vertical wells typically were drilled
on 40-acre spacing," he relates. "Most
were drilled about 1,200 feet apart. We
re-entered a marginal well and drilled a
directional wellbore 600 feet from the

Full-Scale Program
Encouraged by the results, Lay continues, EnerVest elected to initiate a fullscale program to drill horizontally through
the Clinton between the existing vertical
wells. The laterals are completed with
the same stimulation techniques the company used previously in the vertical wells,
but EnerVest completes 15-20 fracture
stages instead of only one, Lay says.
"At current pricing, a Clinton horizontal
well with a 2,000-foot lateral costs $1.8
million. A Clinton vertical well costs
$400,000," Lay reveals. "But even though
a horizontal costs 4.5 times more, it yields
6.5 times the production. On a reserves
basis, our data show a net 140,000 barrels
of oil and 160 million cubic feet of gas
reserves for Clinton horizontals, compared
with a net 22,000 barrels of oil and 24
MMcf of gas reserves for verticals. Finding
costs are $10/bbl for a Clinton horizontal
versus $16/bbl for a Clinton vertical."
The company has drilled seven horizontal wells, Lay reports, adding that the
Canton wells' true vertical depth is slightly
greater than 5,000 feet. "There was a
huge learning curve during the sevenwell program," he recounts. "In the first
well, we were able to get only 1,250 feet
of horizontal penetration in 33 drilling
days from start to finish. We made significant strides, however, and the last

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American Oil and Gas Reporter - April 2016

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