Environmental concerns are a major factor in all U.S. exploration, but it is especially true in the West.
That includes Wyoming’s Pinedale Field, a tight gas sand giant in the northwest part of the Greater Green River Basin.
Ultra Petroleum, relying on cutting-edge geophysical and geological techniques, was able to unlock the natural gas potential of Pinedale, the country’s fourth largest field.
But they also relied on environmentally acceptable practices.
“Reasonable recovery will require wells drilled on equivalent to 10-acre density (5,000 wells) with wildlife and archaeological restrictions, water disposal and air quality issues, and permitting constraints,” said Steve Kneller, Ultra Petroleum vice president of domestic exploration.
Ultra also relies on hydraulic frac technology.
“We use pretty extensive frac treatments,” said Sally Zinke, Ultra’s Denver-based geoscience manager. “That’s been utilized for about a decade in this area. That’s what makes Pinedale economic.”
Although Ultra is headquartered in Houston, the Pinedale Field work is conducted from the company’s Denver office or from Wyoming, Zinke said. The Pinedale Field area covers a 90-square-mile region.
“We have 5,700 feet of an alluvial section that’s called Lance Pool, comprised of highly discontinuous sands and overpressured,” she said. “We view it as a stratigraphic play with a structural overprint.”
In the reservoir itself, the sands are typically nine to 15 feet thick.
“They typically come stacked in packages in excess of 50 feet,” she said. “We describe it like it’s a bowl of potato chips. Some chips are bigger than others when we drill into the pile.”
Keeping It Fresh
Ultra acquired 450 square miles of 3-D seismic and has used it extensively in defining the area and establishing the initial reserve.
“We have nearly a 100 percent success rate,” Zinke said earlier this year when speaking before the annual 3-D Seismic Symposium in Denver.
The company acquired its first 3-D seismic survey of the area in 1999. Seismic data has been used for location planning, reserve validation, lateral and vertical field expansion, drilling risk assessment, completion monitoring and resource recovery planning.
Geophysically, the company has used crosswell seismic and achieved five-foot horizontal and five-foot vertical resolution between wells to help verify the discontinuity in the reservoir and fracture efficiency, she said.
“We monitored an 18-stage frac job on one of the wells and saw the behavior of the fracs relative to the reservoir discontinuities,” she said. “It was a very powerful thing for us. We saw what types of barriers exist on that scale.”
Extensive core work also has been particularly significant in the Pinedale Field project, she said.
Geologic technology has helped assess the resources in this high pressure, low porosity and permeability interval. Extensive core work also has been significant in the Pinedale project, she said. Ultra initiated a 21-interval, 10-well coring program that evaluated over 900 feet of core across the entire field.
“We’ve discovered that it is extremely important to have fresh state cores,” she said. “The core information has really helped our understanding of reservoir fluids.”
Zinke said this marked the first time the company has handled cores this way.
Conventionally, cores are dried “but we have a very small clay content in them,” she added. “Our typical porosity is 7 percent. Drying the core plugs changes the fabric of the clays. If you shrivel up all the clay, you do not understand the rocks as well.
“We try to preserve them as close to the reservoir condition as we can because conventional analysis changes the rock structure,” she continued. “That’s really changed some of our understanding. We have almost no mobile water.
“Our formation water salinity is higher than the water we’re producing. Using correct data gives us a thicker productive section and we end up with a petrophysical model that has significantly more gas in place than what was originally perceived,” she said.
The petrophysical model was applied to 283 wells to develop gas in place estimates. Pinedale Field has in excess of 44 tcf of original gas in place.
“That gives us about 25.8 tcf recoverable,” she said, “and that makes it a giant field.
A Sensitive Play
Meanwhile, because the field is located in an environmentally sensitive area, the company has limited surface disturbances by opting for multi-well pads.
“Typically we’re drilling 16 well pads, all directional,” she said.
The average well cost in the field is $5.7 million and typical reserves per well are 7.65 bcfe.