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Quantifying "collective permeability" of the Upper Cretaceous Niobrara B2 chalk in the Hereford field of the Denver Basin and implications for a new play concept for basins worldwide
Grimm, Mitchell A.
Grimm, Mitchell A.
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2023
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Abstract
All rocks are fractured. The fracture networks of the “tight” low-porosity low-permeability Niobrara B2 (NIO B2) chalk resource play throughout the Denver Basin is known as an ultra-tight reservoir unit. The NIO B2 has been studied extensively since the play’s initiation in 2009. Scales of study range from the regional structure and stratigraphic history to pore throat sizes in single wells, all with goals toward defining optimal spacing for drilling and achieving the best ultimate hydrocarbon recovery from the NIO B2. Current recovery factors across the Denver Basin for wells drilled in the NIO B2 interval average between 3-15% (Personal comm. Dillewyn, 2023; Baillie, 2022; Sterling, 2021). Since 2009, recovery factors in the NIO B2 have improved slightly but for the most part, remain relatively unchanged.
It is known that the fracture network in the NIO B2 is the driving mechanism behind why the low-porosity and low-permeability reservoir is at all economical. This research focuses on quantifying the fracture apertures in the NIO B2 chalk at the Hereford Field in the greater Denver Basin. The larger goal of this research is to better understand the importance of the collective and total permeability systems in the NIO B2 interval.
Analyses show that fracture frequency is related to applied stresses, bed thickness, and composition, all of which play into the collective permeability and the overall total permeability systems present in the NIO B2 at Hereford Field. Also, image log data reveal that gas shows occur at economical levels even after the near-wellbore permeabilities are partially or completely plugged by drilling muds. These findings suggest that consideration of fractures in the NIO B2 chalk can have greater collective permeability than anticipated. Observations lead us to the conclusion that there is a strong need to examine our drilling and completion practices in NIO B2 wells across Hereford Field and the greater Denver Basin.
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