The mere mention of the Bakken Formation conjures up thoughts of a seemingly unlimited oil supply.
The now-famous upper Devonian-lower Mississippian Bakken shale oil play in Montana and North Dakota is indisputably a world class petroleum system.
The prolifically productive play no doubt is a long-term hydrocarbon supply source, given that the just-released 2013 U.S. Geological Survey resource assessment estimates a mean oil resource of 3.65 Bbo for the Bakken Formation, along with an additional 3.73 Bbo for the underlying Three Forks.
Reportedly, 450 million barrels have been produced in this area since 2008.
Like any play, some wells are better than others, for any number of reasons.
Factors influencing productivity in the Bakken play are the focus of a doctoral dissertation being prepared by AAPG member Cosima Theloy, a doctorate candidate in geology at the Colorado School of Mines.
The Bakken is a technology-driven play showing a clear trend of increasing production rates over time, as
drilling techniques and well completion designs have become more sophisticated.
The formation is comprised of an upper and lower shale member and a mixed siliciclastic middle member, which is usually referred to as a dolomite sand or a sandy dolomite.
The shale zones source the hydrocarbons for the fractured dolomite and for the Three Forks formation below.
Theloy compiled a lengthy array of objectives to tackle for her research, including:
By her own account, she worked with a vast amount of data. These data included:
When it comes to well completions, optimal completion design depends on the area and field maturity.
“Since 2010, the majority of operators have employed massive hydraulic fracturing treatments (in the Bakken) with up to 40 stages while pumping millions of pounds of proppant,” Theloy said.
“But numerous older wells outperform younger wells despite technological advancements,” she noted, “suggesting that geological factors have a larger impact on production than the completion design.”
Theloy pointed to such fields as the giant Elm Coulee and Parshall, where wells ordinarily underwent maybe five-stage fracturing early on. These wells are still out-performing new ones in fields such as Rough Rider, where high-end completions using 40-stage fracturing are the norm.
The Rough Rider field lies to the east of Elm Coulee field and west of Parshall.
“Rough Rider is not the best geological area, but the aggressive completions work,” Theloy said. “But they produce a lot of water, and a barrel of oil there probably costs a lot more than a barrel at Elm Coulee.”
She did emphasize that Elm Coulee is a geological sweet spot with enhanced reservoir properties. This is most assuredly a big plus for productivity/economics.
Theloy summarized some of the geological factors that can influence productivity:
“The interplay of hydrocarbon generation potential and maturity results in tremendous over-pressuring and creation of fracture permeability and secondary porosity,” she said.
“A combination of overpressure and buoyancy-driven migration of hydrocarbons into updip traps can result in large scale accumulations, such as Sanish-Parshall and Elm Coulee,” she continued.
There’s much ado about hydraulic fracturing these days – particularly the chemicals being used. The proppants used to hold the cracks open to allow for flow have escaped attention, in large part, during the conversations/confrontations.
They were on Theloy’s radar screen.
“The main type (in the Bakken) is sand, but there’s been more of a shift to ceramic,” she said. “I looked at the effect of proppants on production and separated it into sub-areas, so the geology was fairly the same.
“In all three areas, it showed that a mixture of two-thirds sand and one-third ceramics works best.”
Some of her other conclusions include:
AAPG member Cosima Theloy, a doctorate candidate in geology at the Colorado School of Mines, will present the paper “Factors Influencing Productivity in the Bakken Play, Williston Basin,” at the inaugural Unconventional Resources Technology Conference, which will be held Aug. 12-14 in Denver.
Theloy’s talk will be presented at 2:20 p.m. Tuesday, Aug. 13. Her co-author is past AAPG president and Honorary member Steve Sonnenberg.
The paper is part of a session on “Unconventional Tight Oil and Tight Gas- Carbonates.”