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.
Not all areas are created equal: Subdivision of the Bakken play into 10 areas across North Dakota and Montana, based on township boundaries, proves the point: There are diverse factors and significant differences in productivity across basin. Graphics, photos courtesy of Cosima Theloy
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
Examples of natural fractures found in the Bakken Formation.
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:
- Identify sweet spot and low productivity areas and analyze the cause.
- Evaluate the effect of improving technology.
- Develop a method to distinguish completion-related production enhancement versus geology-induced productivity variations.
- Study the relationship between hydrocarbon generation and observe pore-overpressure in both middle Bakken and Three Forks.
- Create pressure map for middle Bakken without using older, unreliable DST data.
- Determine role of natural fractures in Bakken play.
- Impact of facies variations on rock mechanical properties and fracturing action.
- Whether hydrocarbon migration is significant in Bakken petroleum system.
- Importance of traps and their impact on presence or absence of hydrocarbons.
By her own account, she worked with a vast amount of data. These data included:
- Production and completion.
- EUR data.
- Reservoir rock.
- Rock mechanic.
- Source rock.
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.
Factors for Productivity
Theloy summarized some of the geological factors that can influence productivity:
- Reservoir quality and thickness.
- Rock mechanical properties.
- Natural fractures.
- Pore-overpressure distribution.
- Organic geochemical parameters.
“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:
- Natural fractures play a significant role for production but don’t define sweet spots.
- There is good correlation between hydrocarbon generation, pore-overpressure, inferred oil saturations and productivity.
- Low productivity areas probably are a result of migration, e.g., flanks of Nesson anticline.
- Bakken and Three Forks pressures indicate an inverted continuous system with pressure leaking off at the top, apart from Parshall pressure cell.
- Parshall area has a mix of locally generated and migrated oils.