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.