The renowned Permian Basin has long been an oil producer’s dream, always rising anew after one of those nasty downturns that come about when commodity prices tumble into the dumpster.
A raft of formation names are in the limelight lately, including Bone Spring, Wolfcamp, Spraberry, Strawn and even Strawberry, which actually is a commingling of production from the latter two.
Much of the basin is a rough-and-tumble, ragged landscape running the gamut from nearly flat terrain to near vertical crags.
Even so, the wide-open, sparsely populated parts of the region offer opportunities for relatively unhindered application of innovative technologies – particularly in the seismic business.
When one considers the intense competition in the basin overall, it’s no surprise that operators are digging deep into their technology stash to use only the best available, particularly when it comes to acquiring seismic data.
A Challenge Accepted
The basal Permian-age packed-limestone Wolfcamp was the target of a fairly recent 3-D seismic shoot dubbed “The University Lands” survey, located on the northern flank of the Ozona Arch, which separates the Midland and Val Verde basins.
The Wolfcamp sits 6,000 to 8,000 feet deep in the survey area.
Global Geophysical Services implemented the multi-client survey, which was a very demanding undertaking.
“The client emphasized that this was a very difficult area and they didn’t want a normal speculative survey, but one with different parameters,” said Tom Fleure, senior vice president of geophysical technology at Global. “They wanted substantially higher fold and a more detailed survey.
“This was a bad data quality area,” he emphasized, “and typical parameters wouldn’t suffice to get the data.
“Geologically, there are a bunch of carbonates on the surface (of the formation) and they trap a lot of energy on the surface,” Fleure noted. “That’s not data; it’s just noise to us.
“The energy from the vibrators rings in that top layer of carbonates and tends to obscure the reflection energy that’s coming up that you want,” he said. “So that makes it pretty bad.”
Push-Pull Shooting
To accommodate the expressed needs of the client in a cost-efficient and timely manner necessitated going wireless.
The equipment used was the Autoseis® HDR-1 autonomous nodal recording system. The system has no interconnecting cable and no complex radio links to or between the nodes, according to Fleure.
“We could have gotten increased fold and tighter receiver spacing with cable, but the costs would go way up,” Fleure said. “For example, it would be very expensive to fly 26,000 cables by helicopter, but you can fly 26,000 channels of Autoseis around far less expensively because they weigh a lot less.”
A typical survey design might entail a 165-foot or even a 220-foot receiver point interval, but this program went much tighter, using a 55-foot interval.
“We shot on both sides of the spread rather than the middle, so it’s called push-pull,” Fleure said. “Push-pull shooting reduced the number of live channels by about half, which in turn reduces the capital costs for the recording equipment.
“For this survey, a 26,000-channel recording system was deployed,” he said. “Using the equivalent template-centered patch would have required 52,000 channels and would have taken far more time to record the same density of traces.”
What’s Your Hurry?
Push-pull aside, it also was hurry-hurry.
Each Autoseis unit has about three weeks of battery life, meaning it was imperative to roll through the spread quickly.
“Because we used offset-separated slip sweep, it allowed us to shoot a lot of shot points faster,” Fleure noted. “By shooting on both sides of the spread, we shoot every point twice and roll through quickly, so we could pick up those (recording) units in two, maybe two-and-a-half weeks.
“If we shot in the center and put out twice as many lines, then that spread would have run about twice as long, taking four or five weeks and requiring us to go in and change batteries,” he said.
The offset-separated slip sweep was one of the innovative aspects of the program.
“When you slip sweep, you start one vibrator fleet (two vibrators), and while it’s still shaking, you start another fleet,” Fleure noted. “Before the first fleet finishes, you start the second fleet.
“To avoid some of the complications, some of the noise generated by slipping, we’re offset-separating that,” he said, “so that only the vibrators that were further apart than 15,000 feet would actually start right after each other.
“One of the tricky parts of the design is we have six fleets spread out, and because we have them on both sides of the spread and all up and down the survey, they’re separated typically by 10 to 15 feet,” Fleure noted. “But some of them are 30,000 to 40,000 feet apart, and that way we can essentially vibrate them at the same time without interfering with each other.”
This combo of lightweight nodal recording systems and offset-separated slip sweep vibrator methodology is the first survey of its kind in the Permian Basin.
When all was said and done, over two billion traces were recorded over 355 square miles in about five months.
Fleure said the increased acquisition effort clearly improved the quality of the prestack time migrated results.
An eight-fold increase in traces per bin was achieved – with only a 58 percent increase in acquisition cost.