The San Juan Basin - the largest producer
of natural gas in the Rocky Mountain region and the proving ground
for coalbed methane production in the United States - is once again
a prolific source of unconventional natural gas resources.
Today operators in the basin are in the early stages
of expanding production from the fractured Lewis Shale, a zone long
overlooked in favor of the conventional reservoirs that lie above
and below it.
Historically, that Upper Cretaceous target was rarely
completed in the San Juan Basin. From 1950 through 1990 only 16
wells that encountered extensive Lewis natural fracture systems
while drilling for deeper Mesaverde and Dakota objectives that have
produced from the shale.
Production rates from those 16 wells ranged from
one to 10 million cubic feet of gas a day per well, and ultimate
recoveries ranged from five to 70 billion cubic feet.
In 1991 Burlington Resources, one of the dominant
players in the San Juan Basin operating over 6,500 of the basin’s
18,255 active wells, began adding the Lewis to existing Mesaverde
completions in specific areas, said Hans G. Dube, Lewis Shale project
coordinator and reservoir engineer with Burlington.
Through 1997 approximately 101 Lewis completions
had been made in existing and new wells, commingled with Mesaverde
or Dakota production. By year-end 2000 the firm estimates it will
have completed 556 wells in the Lewis Shale.
Burlington estimates it has as many as 1,000 remaining
Lewis Shale wells as recompletion candidates.
“Since the mid-1980s, development of the prolific
Fruitland Coal has dominated activity in the basin,” Dube said,
“but Fruitland production hit a plateau at about 2.8 billion cubic
feet of gas per day and has now begun to decline.
“In the last several years,” Dube said, “the focus
of San Juan Basin activity has returned to reservoirs such at the
Dakota, Mesaverde and Pictured Cliffs sandstones, as well as the
Mancos and Lewis shale intervals, which lie between these more conventional
reservoirs.”
An Economic Play
The Lewis Shale play is a large, basin-centered, continuous
type natural gas accumulation that covers 1,100 square miles and
contains an estimated 96.8 trillion cubic feet of gas-in-place distributed
over a gross thickness interval of 1,200 to 1,500 feet.
The key difference between the Lewis Shale and those
in other basins, according to David G. Hill, manager, emerging resources,
with the Gas Technology Research, and Charles R. Nelson, a principal
project manager with GTI, is that operators are not developing the
Lewis as a stand-alone play. The zone is completed as either a secondary
completion zone in new wellbores targeting deeper conventional sandstone
reservoirs, or as a recompletion target in existing wells, where
gas from the Lewis is commingled with production from deeper zones.
Hill and Nelson recently reported on fractured shales
in GTI’s Gas Tips.
Production from Lewis Shale completions averages
about 100 to 200 thousand cubic feet of gas a day, and exhibit very
shallow, stabilized annual decline rates of about 6 percent, they
said. Lewis Shale completions produce very little water or condensate
and provide incremental projected economic recoverable reserves
of .05 to 2.0 billion cubic feet of gas per well.
While this commingling strategy makes the Lewis Shale
extremely economic - an incremental cost of only about 30 cents
per thousand cubic feet - it also makes it difficult for operators
to quantify the incremental production rates, reserves and corresponding
value of the Lewis. So, in 1998 Burlington Resources initiated a
study to characterize the Lewis Shale gas potential in the San Juan
Basin in an effort to optimize exploitation, Dube said.
(The study was the subject of a paper recently presented
by Dube and other participants in the program at the Society of
Petroleum Engineers annual meeting.)
“The program encompassed geological, petrophysical,
reservoir stimulation and production data analysis,” Dube said.
“From this data reservoir characterization, completion optimization
and forecasting models were developed that indicate commercial Lewis
potential through much of the San Juan Basin in both new and existing
wells.”
Geology Lessons
The Campanian-age Lewis Shale is 1,000 to 1,500 feet
thick and lies above the Mesaverde Formation and below the Pictured
Cliffs Formation - both gas prone reservoirs. The Lewis was deposited
as a lower shoreface to offshore, open-marine sediment during a
major transgression-regression cycle of the Western Cretaceous Interior
Seaway, according to Glen Christinsen, Burlington’s Lewis Shale
geologist.
The Lewis is informally divided into four members.
The lower three are capped by a regional flooding surface, while
the Ute member transitions upward to the Pictured Cliffs.
In descending order the four intervals are:
- The Ute.
- The Navajo City.
- The First and Second Benches of the Otero.
The Navajo City and the First and Second Otero intervals
have been Burlington’s focus in the Lewis.
What has Burlington learned as a result of its Lewis
Shale study?
In the area of reservoir characterization the program
indicated that average matrix gas porosity and permeability are
1.72 percent and 0.0001 millidarcies respectively, making the zone’s
natural fractures a necessity for commercial production. The good
news is that based on bulk permeability measurements, core data
and FMI log data, most of the Lewis Shale appears to contain some
natural fractures with local variations in intensity.
Two types of natural fractures were observed in the
Lewis: Macro-fractures, which are larger, more conventional natural
fractures, and micro-fractures, which are very small, hairline fractures
within the matrix.
Based on tests in some of the better producing areas,
Burlington estimates that average daily Lewis production from these
areas will likely range between 100 and 130 thousand cubic feet
per well initially, with average estimated ultimate recoveries between
300 and 500 million cubic feet of gas from each well.