Barnett Shale Showcases Tight-Gas Development

Abstract

Horizontal drilling and continuing advances in hydraulic fracturing have made the Barnett Shale formation of north Texas one of the great recent success stories in gas production and a showcase for tight-reservoir development technologies. Yearly production from the north-Texas Barnett Shale, officially called the Newark East field by the Texas Railroad Commission, grew to 1.1 Tcf of gas equivalent in 2007, making it second in size only to the Panhandle-Hugoton field among US producing gas fields. Cumulative Barnett production from 2000 onward now exceeds 4 Tcf. The north-Texas Barnett Shale extends over 5,000 square miles and at least 17 counties, with the core areas lying within Denton, Tarrant, and Wise counties. While the formation can be found at depths as shallow as 3,000 ft in some areas, it primarily appears between 7,000- and 9,000-ft depths. Pay-zone thickness ranges from 100 to 1,000 ft and averages 300–500 ft. Notably, shales like the Barnett once were seen mainly for their role as barriers that trapped hydrocarbons in other rock or were useful for containing secondary-recovery repressurization or fracturing operations. They were seldom considered producible formations because of shale's low permeability, making it difficult for fluids to move within the rock and, thus, for hydrocarbons to flow to the wellbore. Matrix permeability in the Barnett is extremely low, ranging generally between 10–7 and 10–9 darcies. Partially improving the permeability is the presence of interbedded silt- and sand-sized particles. The Barnett Shale was believed to be hydrocarbon-rich even before a discovery well was drilled by Mitchell Energy in 1981. Nonetheless, development activity had to wait another decade or more and did not attain any momentum until the late 1990s. With the extremely tight formation, the use of fracturing to break up as much reservoir rock as possible was a necessity. At first, Mitchell's development strategy consisted of drilling vertical wells completed with massive hydraulic fracturing treatments, using crosslinked gels to transport from 1 to 1.5 million lbm of sand proppant in the formation. In 1997, with the success that another operator had experienced with slickwater fracturing (water-frac) technology in the east-Texas Cotton Valley formation, Mitchell began to apply slickwater fracs in the Barnett, with impressive results.

Demographic response of Louisiana Waterthrush, a stream obligate songbird of conservation concern, to shale gas development

“…It has been a showcase for modern tight-reservoir development typical of gas shales in the U.S. 133 . The development of the Barnett Shale has been a proving ground for combining the technologies of horizontal drilling and large-volume hydraulic fracture treatments.…”

“…Alternatively, six to eight horizontal wells (potentially more), drilled from only one well pad, could access the same reservoir volume, or even more 235 . The low natural permeability of shale requires vertical wells to be developed at closer spacing intervals than conventional gas reservoirs in order to effectively manage the resource.…”

“…Horizontal wells have also been used in many areas of the country to remotely access natural gas resources beneath existing infrastructure, buildings, environmentally sensitive areas, or other features that would prevent the use of vertical wells. The development of the Barnett Shale near Dallas-Fort Worth International Airport is a prime example of how development of urban areas is possible with horizontal wellbores 246 .…”

“…the target formation and into adjacent rock strata 274 . Allowing the fractures to extend beyond the target formation would be a waste of materials, time, and money.…”

Inventory of Shale Formations in the US, Including Geologic, Hydrological, and Mechanical Characteristics

“…Parshall (2008) cites a range of 10 -19 m 2 to 10 -21 m 2 for Barnett Shale permeability. The correlation is on the low end of this range with 6.0 x 10 -22 m 2 for normal to bedding and 1.8 x 10 -21 m 2 parallel to bedding.…”