Bakken and Three Forks Oil: The Water and Proppant Future Wells Would Need

The U.S. Geological Survey has estimated the water and proppant that producing the undiscovered oil and gas of the Bakken and Three Forks Formations might require, and the water that production would bring to the surface. The two formations, of Late Devonian to Early Mississippian age, lie in the Williston Basin Province of Montana and North Dakota. The estimate builds directly on the Survey's 2013 geology-based assessment of the undiscovered, technically recoverable oil and gas in continuous accumulations there, and it was the first assessment of its kind the USGS had carried out.

The Williston Basin Province, the Bakken Total Petroleum System, and its six continuous assessment units: five in the Bakken Formation (Elm Coulee-Billings Nose, Central Basin, Nesson-Little Knife, Eastern Transitional and Northwest Transitional) and one in the Three Forks Formation. USGS map, adapted from Gaswirth and others (2013), on a National Park Service base.

Where the water goes, and where it comes from

Oil and gas in continuous accumulations such as these are usually produced with the help of hydraulic fracturing: fluids made of water and proppant are pumped into the reservoir to make it easier for oil and gas to flow. Proppant is a granular material, often sand, meant to hold the new fractures open.

Water is also used to drill a production well and to cement its casing in place. The horizontal wells often, though not always, used in continuous reservoirs usually need more water for drilling and cement than comparable vertical wells.

Once a well starts producing, water generally comes up with the oil and gas. Some of it is the fracturing water returning early in the well's life, called flowback water. The rest is formation water, which sits in or next to the reservoir rock and keeps coming up for as long as the well produces.

The scale is already large. Between the 2013 petroleum assessment and this one — made in 2016 with data up to August 2016 — more than 6,000 more production wells were drilled into the two formations, using at least 22 billion gallons of water and 12 million tons of proppant for fracturing. From January 2013 to August 2016, the formations produced about 42 billion gallons of water along with 1.3 billion barrels of oil.

How the estimate was made

The method, described by Haines (2015), borrows much of the USGS approach to continuous petroleum resources: the estimated ultimate recovery of oil per well and the calculation of how many wells producing each assessment unit's oil could take. To those it adds:

• the water per well for drilling and cement, and for fracturing;
• the ratio of proppant to water in fracturing;
• the share of fracturing water that returns as flowback;
• the long-term ratio of water to oil (or gas) the well produces.

That long-term ratio stands for formation water; any water above it in the first few months after fracturing counts as flowback. Every input is a range of possible values rather than a single number, so that the uncertainty can be carried through, and a Monte Carlo method turns the ranges into ranges of results.

The assessment keeps the geology of the 2013 assessment and its figures for recovery per well and for well counts. That assessment defined six continuous assessment units, five in the Bakken and one in the Three Forks, all within the Bakken Total Petroleum System. It also included hypothetical conventional units in other parts of the two formations, but those are left out here because the method applies only to continuous resources.

The inputs came from analysis of water use, proppant use and water production in the two formations, using commercial well data (IHS Markit, 2016) and data from the North Dakota Industrial Commission.

The inputs

Some quantities depend on the technical choices of the production companies. Since similar technology is used across the whole area, these were given the same values in every unit:

• Water for fracturing: 2.5 to 6.0 million gallons a well, most likely 4.0, mean 4.17.
• Proppant: 1.0 to 1.5 pounds per gallon of water, mean 1.2.
• Water for drilling and cement: 0.10 to 0.20 million gallons a well, mean 0.15 million (150,000 gallons). Data on the water actually used to drill and cement wells here are limited, so this rests on trends reported for similar wells.
• Flowback: 1 to 12 percent of the fracturing water returns, mean 6 percent, based on all available monthly production data.

The long-term ratio of water to oil was set separately for each unit, and for its geological "sweet spots" and the rest, because it depends closely on the reservoir, which varies a great deal across the province. The means range from about half a barrel of water per barrel of oil in the sweet spots of the Elm Coulee-Billings Nose and Nesson-Little Knife units to 4.0 barrels outside the sweet spots of the Northwest Transitional unit.

The results

The results are ranges, summarised by the 95th, 50th (median) and 5th fractiles and the mean. Across all six units, the mean estimates for developing the whole undiscovered resource are:

• Water for drilling and cement: 5.9 billion gallons.
• Water for fracturing: 164.3 billion gallons.
• Proppant: 101.3 million tons.
• Flowback water: 9.9 billion gallons.
• Total produced water: 414.5 billion gallons.

The Three Forks unit is the largest by far. Its mean fracturing water is 88,208 million gallons, and its range is wide: a 95 percent chance of at least 41,248 million gallons, a median of 82,537, and a 5 percent chance of at least 153,924. Its mean produced water is 207,572 million gallons.

Per year, and in context

These totals cover all the undiscovered oil. If it is ever produced, when and how fast is unknown, and most water planning works year by year. So the fact sheet also gives:

• Wells drilled each year across the six units: 1,273 in 2011, 1,978 in 2012, 2,109 in 2013, 2,254 in 2014 and 1,546 in 2015.
• What a year's drilling would take in one unit, using mean inputs. For 100 wells: 432 million gallons of water and 257 thousand tons of proppant, with 25 million gallons of flowback. For 1,000 wells: 4,317 million gallons, 2,569 thousand tons and 250 million gallons. For 5,000 wells: 21,583 million gallons, 12,844 thousand tons and 1,250 million gallons.

For comparison it lists other water in each unit's area. In the Three Forks unit's area — which takes in the areas of all the others, since the Three Forks lies beneath the Bakken — oil and gas wells produced a mean of 14,687 million gallons of water a year in 2011–2015. In 2010, surface-water withdrawals there were 485,359 million gallons and groundwater withdrawals 29,525 million; water use was 182,259 million gallons for agriculture, 11,580 million for industry, 10,109 million for municipal supply and 310,937 million for thermoelectric power.

Finally, to allow comparison between units and with areas the USGS plans to assess later, the fact sheet expresses the results per unit of oil. Weighted across all six units by their undiscovered oil, each million barrels would need about 23 million gallons of water and 14 thousand tons of proppant, and would bring up 1.3 million gallons of flowback and 56 million gallons of produced water in total. Per million barrels, produced water ranges from 31 million gallons in the Nesson-Little Knife unit to 143 million in the Northwest Transitional.

Sources

• Seth S. Haines, Brian A. Varela, Sarah J. Hawkins, Nicholas J. Gianoutsos, Joanna N. Thamke, Mark A. Engle, Marilyn E. Tennyson, Christopher J. Schenk, Stephanie B. Gaswirth, Kristen R. Marra, Scott A. Kinney, Tracey J. Mercier and Cericia D. Martinez, Assessment of Water and Proppant Quantities Associated with Petroleum Production from the Bakken and Three Forks Formations, Williston Basin Province, Montana and North Dakota, 2016, USGS Fact Sheet 2017–3044, June 2017. https://doi.org/10.3133/fs20173044
• The method: Haines, USGS Open-File Report 2015–1117. https://doi.org/10.3133/ofr20151117
• The 2013 oil assessment: Gaswirth and others, USGS Fact Sheet 2013–3013. https://pubs.usgs.gov/fs/2013/3013/