Nissa Darbonne March 1, 2012
Knock, knock. Who's there? Buda. Under the world-class Eagle Ford oil window is yet more—and much easier—oil in the Buda limestone. The formation is naturally fractured, so stimulation isn't necessary; early horizontal attempts have tested as much as 1,635 barrels of oil equivalent (BOE); and more plentifully available, ordinary-horsepower rigs can do the job.
But Houston-based Goodrich Petroleum Corp., which began looking at tapping this hydrocarbons-rich reservoir in early 2010, finds that it's still early in determining whether the additional play is highly repeatable or wildly statistical in nature.
"We're encouraged, but it's a totally different play than what the Eagle Ford is," says Rob Turnham, Goodrich president and chief operating officer.
While Eagle Ford oil-window wells have become fairly predictable in the past few years in terms of initial production (IP), decline curve, estimated ultimate recovery, and best drilling and completion practices, Buda Lime results are still all over the spreadsheet.
"The results vary from wells in areas that are highly naturally fractured to those that don't have as many fractures. And, you're relying on those fractures for productivity," Turnham says.
In Frio County, Texas, where Goodrich and privately held, San Antonio-based Blackbrush Oil & Gas LP are focusing their Buda work, their Carnes 7H tested 1,167 BOE from an unstimulated, 4,215-foot lateral and their Carnes 6H IP'ed 1,635 BOE (89% oil). Meanwhile, their Lancaster C 1H came in at 512 BOE, Pals Ranch #9H at 530, and Burns Ranch 30H at 500.
Also, among the two highest IP'ing wells, Carnes 7H has held up better over time, with a first-30-day-average of 871 BOE per day or a 25% decline, while Carnes 6H made an average 1,000 BOE per day in its first 30 days online or 39% less.
"You're looking at $3.5- to $4 million per well and your threshold to get a 30% internal rate of return is very low—call it 50,000 or 60,000 barrels at current pricing. So, the hurdle rate is very low, but it's a play where you might get 40,000 barrels on one well and 500,000 barrels on another."
Buda Lime has long been produced northeast of the Eagle Ford play and it is known to be present beneath the Eagle Ford in South Texas, updip of the Edwards Trend. "But no one has really, until recently, drilled it horizontally."
Vertical tests of the past produced spotty results—again , due to the hit-or-miss natural fracturing. The hydrocarbons have varied as well, from sour to sweet. "It's been very erratic—what you find and where you find it."
Over the Eagle Ford, Goodrich holds some 55,000 gross and 40,000 net acres in Frio and La Salle counties, including two old Buda vertical wells. "That led us to try the Buda horizontally. We weren't stepping out on a frontier—not knowing we even had something that would flow oil. We had vertical wells that had produced in the area."
It is currently focusing its Buda work on the northern 40% of its block, entirely in Frio County, and on the Eagle Ford in the southern 60%. "More wells have tested the Buda in Frio County, therefore we have a little bit more well control there."
The key is discovering where the vertical fractures exist. "The Buda on a log looks very similar no matter what well you look at," says Turnham. "The problem isn't finding where the Buda has a consistent log caricature; it's knowing, when you go in that well, whether it is naturally fractured. On a log, they all look basically the same."
The Buda Lime has long been produced in Texas northeast of the Eagle Ford play and is known to be present beneath Eagle Ford in South Texas, updip of the Edwards Trend.
Learnings to date
Goodrich has gathered a great deal of data already on the Buda and continues to experiment, as production is some two-thirds oil and one-third gas.
• In Frio County, there has been good production from the Austin Chalk, which is a limestone and sits above the Eagle Ford. A theory within Goodrich is that the Buda is better where there is also good Austin Chalk production. "But, that's not totally conclusive. We have spread wells out initially to try to get some idea of the variability."
• To date, analysis of past Buda penetrations and of the new horizontal wells suggest the Buda is not a blanket formation in Frio County. "We may have an area here that is a little more unique—much like a field or structural situation—that has caused the Buda to be fractured, so we are hesitant to call it a 'resource play.'"
• Tapping both Eagle Ford and Buda via a lateral in the Buda isn't likely yet. Goodrich tried this with its Burns 30H well last year with a 5,060-foot lateral in the Buda and 19 frac stages. "We were thinking the frac would grow up and stimulate the Eagle Ford, so we would get what you might call 'the Budaford.'"
Instead, "we likely just propagated Buda fractures and lost a good bit of the frac down—instead of up. It felt like the 500 barrels a day it made wasn't a bad rate, but when you look at what an Eagle Ford well in the area typically gets, that is more like 900 or even 1,000 barrels. We just didn't get the rate we would have had we properly stimulated the Eagle Ford."
• Its newest science project that was under way at press time was to drill laterally at the bottom of the Eagle Ford and see if some of the frac grows down into the Buda. "Yet most of your frac typically goes up, so maybe you can only adequately stimulate the Eagle Ford and perhaps get a little bit of the Buda to contribute."
• Buda Lime doesn't produce water, but Buda wells many times may be drilled with an oil-based, and not water-based, fluid, as getting the limestone wet can cause it to swell.
• Also, Goodrich finds that drilling the Buda while balanced is best and while underbalanced is better, meaning the pressure downhole is greater than uphole. "You'll want to burn a gas flare as you're drilling it, such that the gas and fluids are flowing on you rather than losing the drilling fluids into the formation and causing it to close up and not produce."
• Also, while a Buda well doesn't require fracture stimulation—thus $5 million or more in well-completion cost savings in comparison with an Eagle Ford well—the rig needed is the less costly and more readily available 1,000-horsepower type.
But, Goodrich does prefer using a slightly bigger rig in the Buda, and a rig with a top drive. "You can spin out of the hole in case you get stuck versus just pulling the drill pipe out. That's a big advantage, especially if you're drilling in a formation where you're worrying about staying in balance or slightly underbalanced. What you don't want to do, obviously, is get stuck."
• Also, it has been holding Buda laterals to between 3,000 and 4,000 feet. "The longer the lateral the better, but as you hit these fractures, it can be difficult to keep from getting stuck. You have a big pressure differential that might cause you to get either stuck or lose a lot of your drilling fluids into the formation. So, it is a bit difficult to keep the well either in balance or slightly underbalanced, as we're trying to do."
Next for the play
Goodrich has drilled three additional Buda wells—Carnes 8H, Shiner Ranch G 4H and Shiner Ranch G 1H—and results were expected at press time. Its goal is to drill 10 or so more science wells and then develop a decline-rate type curve. For 2012, the company plans five Buda wells, bringing it to a total of 11 of these wells by year-end, and to try longer laterals.
Their steep initial decline rate to date doesn't suit Goodrich's internal goal of sequentially growing reliable volumes of liquids-rich production. "You're draining these fractures instead of matrix porosity and permeability within the rock and having that rock contribute over time.
"The treadmill is fast. You're having to run more rigs, drill more wells and grow the volumes as fast as you can because the production volume is coming off faster than an Eagle Ford well would.
"But they do make a turn and produce for long life, just at a lower daily volume than what you see in an Eagle Ford well."
What is baiting Goodrich's continued work on the formation is the super-low well cost. "At $100 oil, to achieve a sufficient internal rate of return is fairly easy."
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