The Upper Devonian-Lower Mississippian age Bakken shale play in the Williston Basin in North Dakota and Montana has attained crude oil production levels that could surprise even the most jaded operators in the industry.
Think one million bopd.
That’s the number set forth by the U.S. Energy Information Administration (EIA) in November, when it estimated the total wellhead output from the area in December would actually top this projection.
The rich Bakken petroleum system had been subject to a lot of poking and prodding by explorers over the years. Its ascendancy into the spotlight is said to date back to 1995 when explorer and AAPG member Richard “Dick” Findley determined there was good porosity and a likely oil zone in the fractured dolomitic middle section of the shale.
This “A-ha” moment ultimately led to development of the giant Elm Coulee field in the Bakken in eastern Montana. In the midst of the excitement that ensued, Findley was tapped to receive the AAPG Outstanding Explorer of the Year award.
But even giant discoveries can pose giant challenges when it comes to determining how best to efficiently drill and economically produce the trapped hydrocarbons.
To help to reach these goals, lengthy laterals and multi-stage hydraulic fracturing quickly became de rigueur.
Unfortunately, there’s no one-size-fits-all when it comes to these applications.
For example, the now-common and seemingly straightforward practice of geosteering the drillbit along the lateral to stay in zone can be highly challenging.
We’re talking about guiding that bit deep beneath the surface along, say, a 10-foot lateral for perhaps a one-mile journey.
Finding the Right Path
New tools to help with this are being used, yielding improved geosteering performance, which has been hailed publicly by Bakken player WPX Energy management as a key driver in achieving lower well costs in the Williston Basin.
“Geosteering errors in the basin were reduced by 90 percent from mid 2012 to early 2013,” said AAPG member Angie Southcott, geology team lead at WPX in Golden, Colo. “This impressive improvement is in part attributable to the use of advanced processing of 3-D seismic data.”
Southcott noted how “using 3-D seismic data for geosteering applications requires:
- Resolving the Bakken interval, which is a well documented challenge in the Williston Basin.
- Accurately converting seismic surfaces to depth – “an easy task filled with hidden problems.”
Southcott emphasized the Bakken ordinarily is a very difficult section to properly image because it’s so thin and because of the velocity contrasts that are there.
“A shop in town did some frequency enhancement processing on the data for us,” she said. “With that high frequency extender (HFE) processing they were able to bring out frequencies such that it’s almost like we gained a whole other octave level in frequency content of the data.
“With that, we were able to image and map the Upper and Lower Bakken shales,” she added.
With regard to HFE processing, Southcott noted that even though the workflow falls within the normal spectrum of 3-D seismic processing, there is a need for more focus on conditioning the gathers carefully for a cleaner stack. This results in geologically meaningful frequency enhanced volume.
They are using a combo of approaches to get a better handle on the wells.
“We’re using the 3-D to help predict the structure but also running the gamma ray while drilling the lateral and making correlations as we go along,” Southcott said.
“The 3-D helps to predict what’s up ahead of the bit, and we try to position ourselves optimally to reduce dog legs,” she emphasized. “Resolving the Bakken interval goes back to the frequency enhancement processing done for us.
“We started using the data and questioned how good it was until we started crossing faults we were drilling that we were able to map in that frequency enhanced volume, and we began to say ‘this is actually really good.’
“Also, when we cored a well last year, we cut a small fault that was imaged with the 3-D seismic volume,” she added.
“With those couple of things,” Southcott emphasized, “we were really able to hang our hats on this, knowing it was a really good product and not some black box voodoo where it looked pretty, but maybe you can’t trust it.”
Where Did It Go?
Converting seismic surfaces to depth with HFE pinpointed one of the several challenges in creating accurate geosteering surfaces: well tops and their accuracy.
“Examples can be shown that this issue is more the culprit for depth conversion inaccuracies than uncertainties in seismic velocities,” Southcott said.
Imagine having to take the time required to examine every log in your project in order to re-evaluate them on an individual basis.
“Everyone is always using the gamma ray in the Bakken to stay in zone and sometimes resistivity tools as well,” Southcott said. “We’re building on that and enhancing with 3-D.”
She noted the Bakken presents unusual issues for geosteering.
“Everyone says it’s just flat and not changing often or suddenly, but we can see dramatic thinning across the course of a lateral,” she said. “Ten to 15 feet of Bakken mid-section can suddenly be gone.
“The fault may not be huge, but it could fault enough feet so you’re confused,” Southcott noted. “If you see the fault on the seismic – and you do – your correlation is exact.
“Those are the sort of things that have stepped up our game to fix our geosteering,” she emphasized.