The United States’ Antrim shale play cuts a swath across the northern lower peninsula of Michigan with initial production and focus on Otsego County and additional production from all of Antrim and Montmorency counties and northern Kalkaska, Crawford and Oscoda counties.

Production from the Antrim has been reported as far as Alpena and Alcona counties in the east, according to “Modern Shale Gas Development in the United States: A Primer,:” prepared by the Ground Water Protection Council and ALL Consulting for the National Energy Technology Laboratory and the Office of Fossil Energy.

Production

Total cumulative gas production reached 2.917 Tcf by the end of April 2010. In Michigan, Antrim production is reported by project, and the state had some 780 projects.

State records show that Antrim production declined from 158 Bcf in 2008 to 149 Bcf in 2009. Daily production in the first four months of 2010 was some 330 million cubic feet per day, and more than 30 operators were active in the play.

As of April 2010, the Antrim shale had 9,784 producing wells, according to state records. A total of 11,375 producing wells had been drilled in the play, and the number of producing wells continues to decline.

Drilling permits for reservoir peaked at 1,200 in 1993 but fell to less than 200 by 2009. Of that total, 111 new wells were drilled.

A 2008 report by the Energy Information Administration put recoverable resources from the Antrim Shale at about 10.6 Tcf. It ranked the Antrim play as the 13th largest U.S. gas field in 2009.

Geology

The formation spreads across 12,000 square miles with a total organic content ranging from 1% to 20% or 40 to 100 cubic feet per ton. That territory contains some 76 Tcf of natural gas in place, but only up to 20 Tcf may be recoverable with existing technology. Gas in place ranges between 6 Bcf and 15 Bcf per section.

Although the late Devonian-age Antrim formed about 350 million years ago, its shallow depth and fresh water incursion allowed microbes to begin producing the gas no more than 22,000 years ago. The biogenic gas creation continues but at a much slower rate than production.

Natural fractures collected gas from the tight Lachine and Norwood sections of the Antrim at depths from 600 to 2,200 feet deep. The 70- to 120-foot-thick production zone offers a porosity of about 9% and a reservoir pressure of some 400 psi, according to “Gas Productive Fractured Shales: An Overview and Update,” published by David G. Hill and Charles R. Nelson in the Summer 2000 “Gas Tips” newsletter.

In some areas, production from the Lachine and Norwood zones are combined with the Lower Mississippian Sunbury Shale and Upper Devonian Ellsworth Shale on twin-well locations.

Drilling and Completion

Operators first started pulling gas from the Antrim in the 1930s, but development was sporadic. According to the Oil-Gas-News.com Web site, operators drilled eight Antrim wells south of Gaylord in central Otsego County and five of those wells still are producing.

Technology caught up with the play's potential in the 1980s and continued into the present time.

Wells produce from depths as shallow as 350 to just over 3,000 feet, although the vast majority of wells are completed from 1,000 to 2,500 feet deep. Wells are fraced with water and sand. Some wells are fraced using nitrogen or foam. Horizontal completions are a new trend, but currently are only a small percentage of total well count.

A vertical well in the Antrim currently costs between $180,000 and $250,000 to drill and another $25,000 to $50,000 to complete. When completed, a typical well will produce from 30 Mcf/d to 500 Mcf/d of gas and 5 b/d and 500 b/d of water. After six months to a year on line an average well will produce between 125 Mcf/d and 200 Mcf/d of gas and will produce between 400 MMcf and 800 MMcf over 20 years or more. Some wells will produce up to 1.2 Bcf of gas.

CO2 is an issue as Antrim production ages. Wells begin production with low levels of CO2 in the gas mix, but that percentage rises over time. Projects with long production histories can make as much as 30% CO2.

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