The Bakken is one of a growing number of shale formation success stories, thanks to new, innovative technologies that make unconventional plays possible, as well as to the constant quest of the oil and gas companies to find new reserves. The increase of Bakken oil production in North Dakota has come within the past five years. In the beginning of 2007, North Dakota had 303 wells producing 12,000 bopd. By early 2009, that number had risen to 904 wells producing 106,000 bopd. Jump to November of 2011 (the most recent date published for North Dakota) where 3,118 wells were producing 443,425 bopd. The North Dakota Department of Mineral Resources predicts that oil production from the Bakken and Three Forks Formations will exceed 700,000 bpd in the next four to seven years.
North Dakota is not the only area currently with Bakken production. Neighbouring Montana, where the present oil boom originated (see GEO ExPro Vol. 7, No. 2), has been producing a steady 64,000 boepd. Further north in Canada, production is 75,000 boepd. In total, there are around 5,700 producing wells, with about 2,000 new wells to be drilled this year.
Huge Potential
The Three Forks, Bakken, and Lodgepole formations all pinch out on the flanks of the Williston Basin. Source: EERCA recent report prepared for the U.S. Department of Energy by the Energy and Environmental Research Center (EERC) at the University of North Dakota in Grand Forks, North Dakota, had this to say about the Bakken: “It is expected that the play will continue to expand to the south-west (into South Dakota), newly develop in the north-eastern and north-western corners of the basin in North Dakota, and fully develop in between… Currently, only about 15% of the play has been drilled, and recovery rates are less than 5%, providing a significant future of wells to be drilled and untouched hydrocarbons to be pursued through improved stimulation practices or enhanced oil recovery.”
This potential has not been missed by companies operating in the area. At the last count there were at least 82 operators, and new ones are continually added, including Norwegian giant, Statoil, which entered the region in 2011. One of the most aggressive is the play’s largest lease holder and Bakken oil producer, Continental Resources. Their spokesman, Brian Engel, states, “The field could have up to 24 billion barrels of technically recoverable oil… the [2008] USGS estimate was fair and reasonable given the data available at the time of its report. Like Continental, the USGS utilised existing producing Bakken wells to estimate ultimate oil recoveries per well and the effective drainage area. The difference between the estimates is that recoveries on a per-well basis have increased substantially since June 2007. Since that time, approximately 1,680 new horizontal producing Bakken wells have been drilled, and these wells have been completed using almost exclusively single leg horizontal and multi-staged fracture stimulation technology.”
The USGS has certainly recognised this fact, saying that, “the new scientific information presented to us from technical experts clearly warrants a new resource assessment of the Bakken”. To this end, a new assessment got underway in October 2011 and should be out in September 2013.
Source and Reservoir
Diagram of the sleeve/packer completion system commonly used for multi-stage stimulations in the Bakken. Ball-actuated sliding sleeves are run inside a liner or in openhole. Ported sleeves are installed between isolation packers on the liner string. A ball pumped down the sleeve will seat in the mechanical sleeve exposing the ports and diverting frac fluids into the formation. Progressively larger balls are pumped down to activate the sleeves from the toe of the well back to the heel (closest to the well curve). Flow back to the surface cleans out the well. This technology allows for quick and efficient fracture stimulation by minimising fluid use and downhole trips. Source: EERCThe age of the Bakken Formation straddles the Devonian/Mississippian boundary and it was deposited approximately 360 million years ago. It is contained in the Williston Basin where stratigrapher Lawrence Sloss first proposed his cratonic sequence concept (GEO ExPro Vol. 7, No. 2).
The thickest portion of the Bakken (46m) is in north-western North Dakota and it thins evenly south-eastwards toward the margins of the Williston Basin. The upper and lower members consist of hard, siliceous, black organic-rich shales. These form effective seals for the middle member, which consists of five highly variable lithologies, from several argillaceous siltstones to fine-grained sandstone and limestone, all with low primary permeability (0.04 mD average) and porosity (5% average). The other important reservoir target, the Three Forks Formation, consists of shales, dolostones, siltstones, and sandstones with a maximum thickness of 76m.
Depositionally, the Three Forks Formation consists of shallow marine to terrestrial sediments that transition from highly oxidising conditions to the anoxic conditions of the lower Bakken. The middle Bakken sedimentation is associated with the drop in sea level and influx of sedimentary material into near-shore tidal basins and lower shoreface environments. Anoxic conditions resumed during the deposition of the upper shale member, followed by oxidising water conditions during the deposition of the overlying Lodgepole Formation.
According to federal 2009 testimony from the Director of the North Dakota Department of Mineral Resources Lynn Helms, “Hydraulic fracturing is a critical component of developing the Bakken Formation, indeed every shale play throughout the U.S. and Canada. Without hydraulic fracturing, under regulation of the states, this resource could not be produced.”
Completion and Stimulation
Water and sand make up most of the frac treatments with additives to give the fluids special properties, like cross-linked gels to better transport proppants into the formation. Proppants, typically sand or ceramics, are used to hold the fractures open and allow reservoir fluids to flow to the wellbore. Source: EERCBakken Formation completion and stimulation has come a long way since its first oil production in 1955. The upper Bakken shale member was first targeted for production at the Elkhorn Ranch Field in 1976. Vertical wells were fracture-stimulated with sand and oil. The first horizontal wells were completed in 1987, however the play ended in the 1990s because of low oil prices. The real breakthrough came in 2000 when Richard Findley approached Lyco Energy Corporation to drill horizontal wells in Montana’s Elm Coulee Field (see the complete story in GEO ExPro Vol. 7, No. 2), targeting the middle Bakken member. The drilled program was a huge success with ultimate recoveries approaching 750,000 barrels of oil.
In 2005, EOG Resources utilised horizontal drilling combined with hydraulic fracturing to recover significant oil from the middle Bakken interval at the Ross Field in North Dakota. At that time the average rig count was 25 and production under 100,000 bpd. EOG quickly expanded into adjacent areas utilising multistage fracturing of horizontal wells and obtained rates exceeding 500 bpd per well. Completion technology has kept improving and in 2011, Baker Hughes completed a successful 40-stage stimulation (the largest to date) using a fracturing sleeve/packer completion system. Now the average rig count is 200 and production is approaching 500,000 bpd.
The North Dakota wells are completed on 640 acre (2.6 km2) spacing for 5,000 ft (1,524m) horizontal wellbores to 1,280 acre (5.2 km2) spacing for 10,000 ft (3,048m) horizontal wellbores. Most wells are oriented in a northsouth or a north-west to south-east direction to take advantage of the induced fracture propagation. A cemented liner is required from the surface to the base of the curve to horizontal, to ensure the Bakken Formation is isolated from the overlying formations. From this point a number of different completion methods have been employed. These methods range from a cemented liner throughout the horizontal well bore to openhole completions and combinations of both. The cemented liner completions utilise a ‘plug and perf’ stimulation that require time to trip perforation guns, but allow specific frac placement in the horizontal wellbore. The openhole completions only allow for single-stage fracturing which provide little control over fracture initiation and propagation. To gain more control in the fraccing process, uncemented, preperforated liners can be used with or without positive annular isolation. The use of swell packers set at various intervals along uncemented, preperforated liners allows for multi-stage fraccing. It is the most commonly used completion method allowing a high degree of fracture control and success in stimulation long-term production.
Fracturing fluids and the size of treatments have varied widely over time. The amount of fluids used in one treatment can be staggering as seen from a 2010 example, where 10,000 horizontal feet (3,048m) were treated with 3.5 million gallons (13.3 million cubic litres) of fluid to place 4.5 million pounds (2 million kilograms) of proppant.
Future Growth
Along with more effective completion technology, companies like Continental Resources are increasing production efficiency by drilling and completing four wells on one pad. With their ECO-Pad© concept, they can develop two separate formations on two separate spacing units simultaneously while reducing environmental impact on the surface of the land. Source: David McNeeseLynn Helms reflects on the oil boom affecting North Dakota, “We’re estimating now about 18,000 mi² (46,600 m²) in western North Dakota, another 6,000 mi² (15,500 m²) in Montana, Saskatchewan and Manitoba that is mature oilsource rock (in the Bakken Formation). It can be drilled up almost (like) an oil-producing factory. We did not drill a single dry hole in the last year-and-a-half.” Later, he went on to say, “We’re very confident that we’ve got a 20-year oil boom ahead of us.”
It has been the hard work of a few visionary people and oil companies with new technologies that have made this all possible. Research by organisations like the EERC hopes to help refine exploration and make production of this resource increasingly efficient in the future. New areas of shale oil exploration in source rocks like the Eagle Ford in Texas and at Repsol’s Vaca Muerta Basin shale oil discovery in Argentina are being opened up thanks to the pioneering work on the Bakken. This is exactly what Richard Findley had in mind after the success at Elm Coulee and ultimate expansion of this type of play “beyond Elm Coulee and the Williston Basin”.
