The Eagle Ford shale play in South Texas emerged as a leading unconventional prospect even as oil and gas prices crumbled, making it one of the few promising U.S. production prospects in today’s price environment.
Despite its history of development, the play remains something of a puzzle even to key operators.
Pioneer Natural Resources Co. of Irving, Texas, is a major player in the Eagle Ford. Its roots in the area go back to the 1990s, well before horizontal drilling and hydraulic fracturing unlocked shale production.
“One thing we’re trying to understand is the geomechanics of the Eagle Ford. What people don’t know is what’s the geometry of fracture systems,” said Beth McDonald, Pioneer’s vice president of subsurface for South Texas.
Getting the stimulation and completion right is still part-science, part-art, and the company uses all the data it can grab.
“We are trying to use every piece of empirical data plus modeling to help us understand the height and length of these induced fractures and how that changes the mechanics of the system to impact the next well’s stimulation,” McDonald said.
To get a clearer picture, Pioneer has utilized pressure gauges and interference testing during stimulation, according to Doug Portis, Pioneer’s senior geoscience coordinator for South Texas.
“Primarily, I think the answer lies in making a marriage from pressure data back to production. One of the biggest difficulties is that conventional-style modeling does not do a nice job of explaining what’s going on in the subsurface,” Portis said.
Portis co-authored a chapter for the new AAPG Memoir 110, “The Eagle Ford Shale: A Renaissance in U.S. Oil Production,” edited by John A. Breyer, a senior technical consultant in technology application at Marathon Oil in Houston and an emeritus professor of geology at Texas Christian University.
To Portis, one aspect of the Eagle Ford that stands out is the remarkable preservation of the reservoir.
“It’s a combination of the flooding and collapse of the shelf margin and the change in global and local sea level,” he said. “From a regional viewpoint there’s a really nice interplay that sets up the deposition as well as the preservation of the Eagle Ford – it’s quite an elegant story.”
He said the setting was affected by volcanism and mountain-building tectonism to the west, the open Tethys Sea and gentle shelf to the north, and the Woodbine Delta and San Marcos Arch to the east.
Today, pressure in the play area is a primary performance driver.
“When you look at it regionally, the pressure is the first-order driver. Everything else falls back to clay content, total organic carbon (TOC), porosity. The TOC we have really helps drive the sweet spot,” he said.
Thermal maturity windows vary across the Eagle Ford and include dry gas, liquids-rich gas and oil production. Beginning just north of Laredo, the play extends in a dry-gas swoosh to the northeast.
Immediately north of that, in less than a county width, lies a swoosh of wet gas/condensate production. A somewhat wider band of oily production lies to the north and extends several counties farther to the northeast than the other two areas.
Not only does the Eagle Ford contain multiple generation widows, the play is still creating hydrocarbons.
“It’s still generating – it is at or above maximum pressure and temperature for generation. It’s a strange concept to think about: If you don’t drill it, you’re going to get more,” Portis observed.
Wait just a few thousand years, and the Eagle Ford would be an even more prolific play.
Producing the Eagle Ford
Pioneer entered into the basin in the ‘90s in Pawnee Field, “a legacy Edwards Reef play,” McDonald said.
Petrohawk Energy first announced the opening of the Eagle Ford unconventional play in October 2008, and Pioneer completed its first horizontal well in the shale in June 2009.
“While Petrohawk started horizontals in Hawkville in the south, we drilled a few horizontal wells in the northeast part of the play and established what turned into the sweet spot of Karnes and Dewitt counties, in our opinion,” McDonald said.
“We see some variation along our acreage with different performance drivers of varying TOC, pressure, clay content and porosity. Even in our dry gas, we have prolific wells since there is a ‘mini-basin’ between the margins thickening the Eagle Ford section,” she added.
Over time, the company has made some substantial changes to drilling and completions techniques that reduced costs and gained efficiencies, Portis said.
“We leverage our 3-D seismic very heavily,” he said. “One other thing that’s moved the needle for us is the employment of X-ray fluorescence.”
Pressure, organic content, good clays, reservoir integrity and favorable oil-liquids generation combined to make the Eagle Ford a prolific and desirable prospect, but the play does present some special challenges.
“Our major limiting factor in the Eagle Ford is the temperature. We’re trying to push the limit and use the temperature to our advantage by using dissolvable plug technology, diversion technology,” McDonald said.
“The uniqueness of the Eagle Ford is that we’re limited in the number of tools we can run because of the temperature – perhaps only the Haynesville shares that limiting factor with the Eagle Ford,” she noted.
To deal with the temperature challenges, “the industry is going to have to innovate,” McDonald said.
In stimulation, Pioneer has followed the current trend of other resource play operators in using higher volumes of water and, especially, more proppant.
“We definitely have seen that if we pump more in a tighter cluster spacing, the wells have an uplift associated with that change,” McDonald said.
And in drilling, “we’ve done a great job here recently with our rotary steerables. We set three world records for the longest single run,” she noted. “We have some higher horsepower rigs that we could combine with the rotary steerables to see even better results.”
The Eagle Ford hasn’t escaped the effects of the industry downturn. According to the Texas Railroad Commission, 5,613 drilling permits were issued for the play area in 2014 and 2,315 in 2015.
Oil, gas and liquids production have all declined. Eagle Ford oil production reached a high of 1,174,931 barrels per day in 2015 and fell to about 950,000 barrels per day this year.
Operators are still actively experimenting in the Eagle Ford, trying to unlock the play’s full potential.
But “the land grab is all but gone at this point, and more open data and knowledge sharing will benefit the development of the entire basin for all operators,” Portis said.