This paper describes the use of reservoir simulation to refine reservoir descriptions of the Bakken Shale for predictions of single well and multiple well performance. The descriptions were achieved by matching drawdown, buildup, and interference data from horizontal wells in the Buckhorn Field, McKenzie County, North Dakota, Figure 1.
Reservoir descriptions — matrix and fracture storativities, Phi CtH, and matrix and fracture permeability-thickness products, KmH and KfH — were developed for both single well and multi-well interference data. More than one description could be found to match single well data; however, only one description was found which matched all the data. The best match of long term production interference data was obtained with a dual porosity description that included anisotropic, stress-sensitive permeability.
In the dual porosity model, both microgeneration fractures and enhanced or extension fractures were modeled with a lumped description. The permeability anisotropy for this lumped fracture system was approximately 4 to 1 with the maximum permeability direction in the northern Buckhorn area being east-to-west.
Bakken Shale porosity was determined to be 2 to 3 % with one-tenth of that volume or 0.2% being in microfractures The ratio of fracture to matrix permeability was 100 to 1 with an effective permeability for the fracture system of 0.6 md at maximum net confining stress. It is believed that siltstones and dolomites adjacent to the Bakken Shales make a significant contribution to the oil production through the extension fracture system.
The Bakken Formation is the basal unit of the upper Devonian through Mississippian transgressive sequence, Figure 2. The Bakken Shale is an organic-rich shale which is believed to have sourced large volumes of oil in the Williston Basin. In the thermally mature areas, the Bakken is overpressured and matrix porosity is oil saturated. The Bakken ranges in thickness from 150 feet near the center of the Basin to 0 feet at the depositional limits along the Basin flanks. In the central Basin, the Bakken is comprised of three overlapping units — the upper shale member, a middle siltstone member, and a lower shale member, Figure 1. In the 3 well study area discussed in this paper, the Bakken formation is approximately 15 to 25 feet thick, and the lower shale member is absent.
Production from the Bakken dates back to 1953 when Antelope Field was discovered. Production at Antelope is related to natural fractures which have been enhanced by structural deformation, i.e. extensional fractures. Prior to initiation of horizontal drilling, the Bakken produced approximately 19 MMSTB from 194 vertical wells. In September 1987, Meridian Oil Inc. completed the first horizontal Bakken well, the MOI 33-11 in section 11–143N-102W, Elkhorn Ranch Field. Initial production was 258 BOPD. The well has produced 271,929 barrels of oil through December, 1990. Subsequent to the Meridian well, approximately 135 Bakken horizontal wells have been drilled, 15 of which have been reported as dry holes.
Volumetric oil-in-place calculations have historically been difficult in the Bakken because of a lack of knowledge of the distribution of effective oil-in-place within the Bakken interval and the surrounding Lodgepole and Three Forks Formations.