Dan Sharp for the Tribune
The success of horizontal drilling and hydraulic fracturing in the Bakken play has come to bear on not only how an oil-bearing unit is exploited but also how oil and gas wells are spaced. During recent years, the landscape in North Dakota’s oil patch is taking on a much different look from the one that started in the spring of 1951 when Amerada Hess struck oil on Clarence Iverson’s farm just south of Tioga.
Matching drilling to infrastructure
Prior to 1987, all wells drilled in the Williston Basin were vertical with the well bore penetrating one or more producing zones. With those conventional wells, a producing unit’s pressure forces oil and gas to the well bore. Eventually, however, the flow of oil and gas declines and must be accomplished with mechanical pumps. With vertical well drilling, typical North Dakota well spacing was one well per 40-80 acres, in some areas one well per 160-320 acres. The spacing depended on the expected drainage of the producing zones.
In order to space wells for maximum production and the most efficient use of resources, operators and regulators normally employ accepted formulas that take into consideration a reservoir’s permeability, depth, number of pay zones and other data. The calculation methods have improved over time with technological improvements and as petroleum engineers gained a better understanding of individual reservoir characteristics.
Historically, only about 1-3 of every 10 exploratory vertical wells produced commercial quantities of oil and/or natural gas. By contrast, well over 95 percent of Bakken horizontal wells drilled become producers.
“Because horizontal drilling exposes a much longer portion of the well bore, it has the potential for much greater – and much faster – production from a single well,” says Bruce Hicks, assistant director of the North Dakota Oil and Gas Division. “So, operators must focus more intensively on their long-term production goals – everything must be taken into consideration in order to get oil to market.”
Hicks says being able to connect to existing infrastructure is especially critical in an operator’s planning. “Having gathering facilities, storage units, processing plants, pipelines and rail shipment capacity in place has to be planned well ahead of drilling,” he says. “Otherwise, infrastructure can be overwhelmed and result in serious delays and bottlenecks.”
Energy corridors, multi-well pads, and multiple layer drilling
North Dakota’s daily oil production now tops 900,000 barrels and is forecast to continue to increase to more than 1 million barrels per day in the coming months. Some forecasters believe production could reach 1.5 to 2 million bopd by 2025. Current changes in well spacing could make those forecasts achievable.
“During the past few years, the Oil and Gas Division has worked with operators to position and space drilling sites along ‘energy corridors,’ especially in the mature Bakken play, which underlies parts of nine western North Dakota counties – primarily Williams, Mountrail, Dunn and McKenzie,” Hicks continues. “The corridors provide orderly development on 1,280-acre (one mile by two mile) spacing units. Using this configuration, well pads are positioned in-line parallel to east-west access roads (corridors). Horizontal laterals (up to 10,000 feet long and perhaps 10,000 feet deep) then extend in a parallel direction to the foot of the spacing unit.”
While the use of energy corridors is having a marked landscape impact, subsurface changes are occurring as well. Operators are now tapping multiple layers in the Bakken and underlying Three Forks formations from single well pads. “Some operators are even using a single well bore to drill two laterals, which makes more efficient use of drilling rigs,” Hicks says. Nearly three-fourths of all Bakken wells are now drilled from pads. About four wells per pad is the norm.
Energy corridors also allow drillers to exploit multiple producing zones of the Bakken and Three Forks (see diagram). Using this strategy, drillers might access as many as 15-20 wells from a single pad allowing a spacing unit to be drained of oil and gas as efficiently as possible. Many operators are also employing a technique called “batch drilling,” where several adjacent wells are drilled in alternating segments. First the well bores are started, then the vertical bores drilled, and finally the laterals are completed. The efficiency gains here come from less down time due to cleaning tanks and changing and cleaning drilling equipment from one operation to another.
More wells drilled per rig
Hicks underscores the benefits of energy corridors, multi-pad drilling, and multi-zone drilling. “Drilling four wells from a single pad saves eight acres of surface area over single well spacing,” he says. “In addition, it reduces truck traffic, allows for the common use of tanks and other on-site facilities, and improves the planning and timing of pipeline construction.”
He says the major savings in these strategies is the reduced time it takes to move equipment and facilities from one well site to another. “The drilling rig is the most expensive piece of equipment in drilling a well. So, reducing the number of rig days can be a substantial money saver,” he explains. “For example, just using batch drilling can save an operator 10-15 percent on drilling costs – $400,000 to $500,000 per well. By working more efficiently, operators can actually drill more wells (and thereby increase total production) even when fewer rigs are operating across the basin.”
Hicks notes that reducing costs helps keep Bakken oil competitive with other tight shale oil plays – most notably Texas’ Eagle Ford shale. “Companies that have assets in several plays will invest where they get the best return – it simply makes good business sense and it’s what investors expect.”
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