Natural gas production has declined, but operations continue in this gas- and liquid-rich area.
by Lori K. Ditoro
April 30, 2012

Brittle, naturally-fractured rock 
makes up the Woodford Shale, which covers sections of the state of Oklahoma. The first commercial oil well was drilled there in 1939, and only the Barnett Shale, covered in the Fall 2011 issue of Upstream Pumping Solutions, has experienced production longer than the Woodford. Natural gas production in the area has decreased due to declining prices. However, because of liquid reserve projections and successes, operations have continued in the play.

History

The Woodford Shale is made up of four major basins:

  • South Oklahoma—also called the Ardmore
  • Anadarko—also called Cana Woodford—(central to western Oklahoma)
  • Arkoma (southeastern Oklahoma)
  • Chautauqua Platform

Although drilling began at the end of the 1930s and oil was extracted before 2008, drilling exploded in the area after the successes of the Barnett Shale and—as has occurred in many shale plays across the U.S.—because of advances in horizontal drilling and hydraulic fracturing. More than 1,500 wells have been drilled in the play.
From the first use of unconventional methods up to 2011, the focus of drilling in the Woodford was in the natural gas regions. With a drop in natural gas prices, a natural gas surplus that is becoming a storage issue and the rise in crude oil prices, the focus has shifted toward the liquid rich areas of the shale. This has meant that the drilling focus has moved from the Arkoma area—primarily dry gas—to the Anadarko region, which has both dry and liquid reserves.

The Geology

This Devonian/early Mississippian formation was deposited approximately 350 to 400 million years ago. The Woodford area is made up of brittle, silica-rich rock. Because of its brittle nature, it contains abundant natural fractures. However, hydraulic fracturing is still used to connect these natural fractures, and the rock responds well to fracturing. This shale is typically found at depths between 7,500 to 8,500 feet. Depending on the location, it can be anywhere from 50 to 300 feet thick.

Completion and Production

While the Woodford has been primarily considered more of a natural gas play, due to its level of thermal maturity, liquid hydrocarbons are also possible in some areas. The Andarko Basin produces oil and condensate, as do certain regions in the other basins. Some estimates indicate that the Woodford holds 4 trillion cubic feet of natural gas. Each well drilled in liquid-producing areas may deliver 500 to 1,000 barrels of liquid hydrocarbons per day. The Anadarko (which extends into Colorado, Kansas and Texas) alone may produce more than 5 billion barrels of oil.

Hydraulic fracturing is employed in this shale to connect the existing natural fractures. Specifically, slick water fracturing is often used. In slick water fracturing, water is predominantly used, pumped at high pressure, with less amounts of sand and diluted concentrations of additives and chemicals designed to stimulate the well, enhance flow back and increase the production from the reservoir. The chemistry of the fluid may be different for each well, and each fracturing stage may require 500,000 to 1 million gallons of water. In this shale, multiple fracturing stages may be needed.

The Future

With both liquid and gas producing areas, operators continue to operate in the Woodford Shale. While not experiencing the same type boom as the Bakken in North Dakota, the Woodford is still seeing new wells drilled and new operators coming to the area. Future success seems to be in the Ardmore and Anadarko Basins. However, as exploration and production continues, new potential may be discovered, and if natural gas prices rebound, interest in dry gas regions could be renewed.