AGS / Energy / Natural Gas / Main
Natural gas consists of combustible hydrocarbons which are gaseous at ordinary temperatures and pressures, and have essentially the same origin as fluid hydrocarbons. Methane (also called marsh gas) and ethane are commonly the chief constituents. Most natural gases usually contain small and variable quantities of carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen sulfide, nitrogen, hydrogen, and oxygen. In the absence of sulfurous compounds, natural gas is colorless, nearly odorless, and, when mixed in certain proportions with air, is highly explosive. An odorant is added before gas is sold to the public to aid in detection of gas leaks.
Natural gas resources can be categorized into two main types on the basis of producing rock characters: conventional natural gas and unconventional natural gas. Conventional natural gas is produced by a well drilled into a geologic formation in which the reservoir and fluid characteristics permit the natural gas to readily flow to the wellbore. Unconventional natural gas does not exist in these conventional reservoirs - rather, this natural gas takes another form, or is present in a peculiar formation that makes its extraction quite different from conventional resources. The major unconventional gas resources in U.S. include tight gas, shale gas, and coalbed methane (CBM). The latter two types are present in Arkansas.
Arkansas Natural Gas
Natural gas was first discovered in 1887 at Fort Smith, but commercial development did not begin until 1902 when two gas wells were completed near Mansfield in Sebastian County. Gas was first discovered in southern Arkansas on April 22, 1920, when the Constantin Oil Company completed a gas well near El Dorado in Union County. The heating value of gas varies from about 700 to 1,200 British thermal units (Btu) per cubic foot. Dry natural gas from the Arkoma basin fields has a heating value of 986 to 1,016 Btu per cubic foot, and is used principally as fuel.
Major accumulations of natural gas are present in two areas in Arkansas – the Arkoma basin and the southern Arkansas oil fields in the West Gulf Coastal Plain. Natural gas is commonly discussed as either "wet" or "dry" gas. Wet gas contains some of the heavier fluid hydrocarbons as vapor, is commonly associated with petroleum, and is valuable because of the extractable hydrocarbon liquids it contains. Most gas from oil fields in southern Arkansas is of this type. Dry gas differs from wet gas in that it does not carry appreciable amounts of the heavier hydrocarbons as vapor. The gas of the Arkoma basin in west-central Arkansas is of this type.
The Arkoma basin is an elongate sedimentary basin extending from east-central Oklahoma into Arkansas. It includes, but is not restricted to, the Arkansas River Valley physiographic region of western Arkansas. Due to the extremely high thermal maturation process over geological time, only dry gas is produced in the conventional sandstone reservoirs of this area.
The Atoka Formation of Pennsylvanian age contains the principal gas-producing units in the Arkoma basin. The Atoka Formation is a succession of alternating beds of sandstone and shale with a maximum subsurface thickness of approximately 15,000 - 20,000 feet in this region. Some dry gas has also been produced from the Morrowan Series (Bloyd and Hale formations), which underlies the Atoka Formation. Atokan and Morrowan beds have been folded into numerous east-west trending open folds which trap the gas within porous beds. Pre-Pennsylvanian reservoirs including Ordovician-age rocks in the Arkoma basin have been sporadically tested to determine their potential for natural gas. As of January of 2007, there are 145 gas fields that have been identified, and 6,223,259,026 Mcf of gas produced in the Arkansas portion of the Arkoma basin. Annual 2006 gas production data is estimated at 181,381,915 Mcf.
During the past 45 years, at least a dozen scattered, small gas fields have been discovered in Washington, Madison, and Benton Counties in northwestern Arkansas. Production in this region has come from five formations of Late Mississippian to Middle Ordovician age.
Natural gas has been recovered, commonly with oil, in the southern Arkansas oil and gas fields in Ashley, Bradley, Calhoun, Columbia, Hempstead, Lafayette, Miller, Nevada, Ouachita, and Union Counties. Annual gas production for 2006 is compiled from 192 fields in south Arkansas and is estimated at 7,902,865 Mcf of gas.
Unconventional gas has grown in importance as a complement to conventional fossil fuel as world demand continues to increase. The Energy Information Administration (EIA) expected U.S. production from unconventional gas sources to increase more rapidly than conventional gas production. In its long-term forecast, the lower 48 states unconventional gas production grows from 6.6 Tcf in 2003 to 8.6 Tcf in 2025, and from 35% of total production in 2003 to 44% in 2025.
Coalbed Natural Gas Methane
Coalbed natural gas or coalbed methane (CBM) is the methane gas contained in coal seams. The coalification process, begins with plant material that is progressively converted to coal and this results in large quantities of methane-rich gas that is generated and stored within the coal. The presence of this gas has been long-recognized due to explosive outbursts associated with underground coal mining. Only recently has coal been recognized as a reservoir rock and a source rock, thus representing an enormous undeveloped "unconventional" energy resource. Because of its large internal surface area, coal stores between six and seven times more gas than the equivalent rock volume of a conventional gas reservoir. The United States is expected to have 700 Tcf of CBM, compared with its conventional gas reserves of 187 TCF.
The development of Arkansas’s coalbed natural gas resources began in 2001 and has yielded an approximate cumulative production of 10 Bcf. Estimated 2007 annual production of CBM is approximately 3 Bcf. CDX Gas LLC, a Texas based energy company, is currently the only producer of this resource and has drilled approximately 37 Z-pinnate horizontal wells and 15 vertical wells in Sebastian County, Arkansas. The wells are completed in the Pennsylvanian Lower Hartshorne coal and approximately 564,238 feet of horizontal pinnate lateral has been drilled in Arkansas. On average, approximately 15,000 feet of horizontal lateral is drilled for each of CDX’s Z-pinnate wells in the Lower Hartshorne coal.
Fayetteville Shale Gas
The Fayetteville Shale Formation (Upper Mississippi) is the current focus of a regional shale-gas exploration and development program within the eastern Arkoma Basin of Arkansas. Approximately 2.5 million acres have been leased in the Fayetteville Shale gas play with a cumulative production of 106 BCF since drilling began in 2004. Annual 2007 production from the Fayetteville Shale B-43 producing region is reported as 89,138,371 Mcf by the Arkansas Oil and Gas Commission. The thickness of the producing zone ranges from 50 to 550 feet and wells range from 1,500 to 6,500 feet deep.
As of September 17, 2007, approximately 436 wells have been drilled and completed in the Fayetteville Shale and petroleum services companies are relocating to Arkansas in order to capitalize on the economic boon that is occurring in the state. Southwestern Energy and Chesapeake Energy are the major players in this emerging gas play and the two companies report a combined expenditure of > $1.5 billion on Fayetteville Shale development for 2007. As of February 19, 2008 there are 561 producing gas wells; however, many additional wells have been drilled and completed and operators await construction of gas gathering systems before these wells begin production. Southwestern Energy reports that average completed well costs for horizontal wells with multistage slick water fracture completions is approximately $2.9 Million. The average horizontal lateral length for Southwestern wells drilled in 2007 is 3,120 feet and average time to drill to total depth is 15 days. Estimated ultimate recoveries (EUR) for horizontal wells with laterals greater than 3,000 feet range from 2.0 to 2.5 Bcf per well. Approximately 1700 governmental sections are available for Fayetteville Shale development within the core region of the gas play. Schlumberger has built a 31,000 sq ft facility in Conway, Arkansas to provide operators with well services for Fayetteville Shale development and currently touts a staff of 202 employees. Southwestern Energy and Chesapeake Energy have opened offices in Conway County and White County respectively to facilitate Fayetteville development. Texas Gas Transmission plans to build a 167-mile gas transmission line from Conway County, Arkansas to Coahoma County, Mississippi to transport Fayetteville Shale gas. This proposed pipeline is referred to as the “Fayetteville Lateral” and has an estimated ultimate capacity of 1.1 BCF/day. Other industry partners involved with Fayetteville Shale exploration and development ventures include: Hallwood Energy, KCS Resources, Tepee Petroleum, Edge Petroleum, Alta Operating, Aspect Energy, XTO Energy and 14 other companies.
|Southwestern Energy drilling operation in Van Buren County, Arkansas. Horizontal drilling and hydraulic stimulation techniques are utilized to tap natural gas resources from the Mississippian Fayetteville Shale. (Photo taken in 2007)|
The Arkansas Geological Survey has completed and published an extensive geochemical research project on the Fayetteville Shale and has provided this information to the oil and gas industry and the public to assist with exploration and development projects. The study is available at the Arkansas Geological Survey as Information Circular 37 (Ratchford, et. al, 2006) and integrates surface and subsurface geologic information with organic geochemistry and thermal maturity data. The Arkansas Geological Survey continues to partner with the petroleum industry to pursue additional Fayetteville Shale related research. Ongoing research is focused on the chemistry and isotopic character of produced gases, mineralogy of the reservoir, and outcrop to basin modeling.