The Bakken formation is a large, unconventional resource located in North Dakota. Horizontal wells are drilled from pads at tight spacings, and placement within the target formation is critical to ultimate recovery rates. Geologic placement is established by evaluating the natural gamma-ray tool response and comparing it to a representative gamma-ray log from a vertical wellbore. Typically, a bulk gamma-ray detector is used, and small gamma-ray differences are used to infer stratigraphic placement. This paper presents the current geosteering methodology, as well as the use of an azimuthally sensitive gamma-ray sensor, to place the well more accurately in the target formation.
Accurate stratigraphic placement is necessary to achieve optimum production rates and to avoid a shale strike in the formations above and below the middle Bakken member. Shale strikes can result in hole-condition problems and may require sidetracks or the use of oil-based drilling fluid instead of water-based drilling fluid, which increases the cost of the well.
The azimuthally sensitive natural gamma-ray tool transmits a real-time gamma-ray image of the wellbore. The wellbore image can be used to infer relative stratigraphic movement of the drilling assembly through the formation. Additionally, the image can be used to identify shearing or faulting across the wellbore and relative formation dip.
Occupying approximately 200,000 mi2 of the subsurface of the Williston Basin and underlying parts of Montana, North Dakota, Saskatchewan, and Manitoba, The Bakken formation has proven to be one of the most prolific shale plays in the world, with average production rates of 1MM BOPD. The Bakken formation is part of a petroleum system that comprises five distinct stratigraphic units: the overlying Lodgepole Limestone formation, the upper, middle, and lower members of the Bakken formation, and the underlying upper Three Forks-Sanish formation (Meissner 1978). The upper and lower shale members of the Bakken provide the source rocks for the oil contained in all of these reservoirs (Reddy and Pitcher 2012).
Unconventional resources are often costly to produce as a result of the need for hydraulic fracturing. In order for such shale plays to be considered economical, drilling costs are kept to a minimum, and many operators drill and geosteer wells with only average bulk gamma-ray measurements obtained from a logging-while-drilling tool. While the Three Forks formation has sufficient gamma-ray character to effectively geosteer, Middle Bakken wellbore placement can be more challenging owing to little gamma-ray variation. (Carson et al. 2016).