PBIOS - October 2018 - 157

one-mile lateral) are projecting over
300,000 barrels for their estimated ultimate
recoveries. Western Yoakum County has
a few wells that project EURs in excess
of 400,000 barrels.
In checking with the most active companies, it is very clear that well completion
strategies still are developing. Operators
are evolving the approaches they are
using to select landing depths, and design
well completions in terms of number of
stages, stimulation job size, and whether
to use scale inhibitor flushes with each
stage.
Hydraulic fracturing treatments all use
lower pump rates and volumes than the
shales, with many using rates as low as
10-20 barrels a minute. There also seems
to be a growing consensus that the vertical
profile of porosity (and permeability) is
important to the size and rate of the fracturing stimulation treatments used with
the landing depth biased to the upper,
gassier intervals, as noted from mud logs.
Open-hole completions have been used
successfully in analog plays such as the
Hunton carbonate in Oklahoma and are
mechanically possible in the San Andres,
but have not proven commercially successful in the San Andres as of yet.
The composition of the produced gas
can vary considerably. It is always sour
(along with the oil), but the hydrogen
sulfide composition ranges widely with
several wells reporting in excess of 1,000
parts per million H2S. Carbon dioxide
also vary widely from less than 3 percent
to greater than 10 percent. Methane composition also varies dramatically, with
many wells reporting less than the ethane
percentages. Btu content is very high,
often in spite of the low methane and
high CO2 content, and trends upward
with increasing time and decreasing reservoir pressure.
This also was quite true in the Hunton
so-called dewatering play where some
late stage (more than 10 year-old) wells
reported Btu contents in excess of 1,600.
The delayed onset of oil production; increasing Btu content; changing gas composition with increasing yields of ethane,
propane and higher natural gas liquid
hydrocarbons; and low crude oil decline
rates are consistent with the concept of
the play being an oil-wet conventional
reservoir play, but with residual oil left
behind after the natural, paleo oil water-

flood.
Water salinities also vary widely, but
are almost always less than standard main
pay zone connate waters. The degree of
natural waterflood flushing is believed
to be responsible for the low and regionally
variable water salinities, as well as the
retained gas and composition. The waters
are saturated with sulfates, causing high
sulfate scaling tendencies.
Residual Oil Target
There is still much confusion regarding
the nature of the horizontal San Andres
play. Many operators still profess to be
producing mobile oil. However, the horizontal San Andres play is developing in
many of the areas recently mapped as
being a huge paleo oil trap that was
flushed of its mobile oil in the geological
past. But, like man's waterfloods, Mother
Nature bypassed a percentage of the oil,
leaving a residual oil target that has never
been depressured.
The "greenfield" areas can have thin
shingles of "attic" mobile oil that, by
themselves, are not commercial, but can
provide excellent economics when commingled with the upper ROZ intervals.
The natural waterflood sweep can be areally and vertically variable, which adds
risk to the ultimate oil cuts and GORs,
but appears to correlate with the carbonate
facies, water salinities and the degree of
diagenesis the facies have undergone during the sweep.
A 12-18 percent oil cut appears to be
the P-50 value thus far. Lateral landing
depths and completion strategies are beginning to take those variables into account, making the play a regional one,
but one needing some regional fine-tuning
considering the aforementioned factors.
The play is dramatically overshadowed
by the multiple Wolfcamp bench shale
plays in the Delaware and Midland subbasins, but has grown to produce over
37,000 barrels of oil in six counties on
the Central Basin Platform and Northwest
Shelf areas. The average daily oil production growth rate has been more than
130 percent in the past 18 months. To
date, 30 different operators are involved
in the play, mostly small independents
funded by private equity financing.
The wells target the upper sections
of a conventional reservoir dubbed the
residual oil zone. They are located where

the entire reservoir-quality rock has
been naturally waterflooded. Where
there was closure atop the ROZ interval
providing retained mobile oil, 16 main
pay zone CO2 floods have been deepened
into the ROZ intervals below the oil/water
contacts-some by as much as 300 feet.
One CO2 enhanced oil recovery project
is under way in a greenfield fairway of
the ROZ.
All of these projects are proving the viability of EOR in the ROZ. The applicability
of converting the horizontally developed,
depressured areas to an EOR project will
likely be tested in the coming years.
r

STEVE
MELZER

Steve Melzer is a consulting engineer
in Midland, Tx., specializing in CO2
injection, EOR projects and the science
of residual oil zones (ROZs). With
three decades of experience, he formed
Melzer Consulting in 1997 after previously leading a research organization
as part of the University of Texas
system and operating a small independent oil and gas company. Melzer
is director of the annual CO2 Flooding
and ROZ Conference, co-director of
the annual EOR Carbon Management
Workshop, and has been active in the
formulation of policy and legislative
initiatives centered on carbon capture
storage and utilization.

Doubling Down
The U.S. Energy Information Administration reports that Permian natural gas production has doubled since
2016, and expected continued growth
in natural gas and gas liquids output
is driving multiple large-scale pipeline
projects to expand Permian takeaway capacity for gas and NGLs.
OCTOBER PBIOS 2018 157

PBIOS - October 2018

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