Azimuthal gamma-ray images uncover stratigraphic features in a seemingly featureless unconventional reservoir
Challenge – Unconventional resources are often costly to produce as a result of the need for hydraulic fracturing. In order for these shale plays to be considered economical, drilling costs must be kept to a minimum, and many operators drill and geosteer wells with only average gamma-ray measurements obtained from a logging-while-drilling (LWD) tool. While the Three Forks formation has sufficient gamma-ray character to effectively geosteer, the wellbore placement in the Middle Bakken formation can be more challenging because of the limited gamma-ray variation, making it difficult to interpret structural changes along the wellbore and to refine the structural map.
Solution – At the recommendation of Sperry Drilling, the operator drilled two wells using the Radian™ azimuthal gamma and inclination tool. The Radian tool features four scintillation detectors spaced 90° apart that allowed for gamma-ray imaging while rotating or sliding, along with an inclinometer that provided continuous inclination measurements for real-time monitoring of the well trajectory. Even with low gamma-ray variations in the formation, the target zone can be precisely identified based on the superior gamma-ray images provided by the Radian service. In addition, the Radian tool allowed for an enhanced survey based on the continuous inclination data for a more accurate structural map. A stratigraphic correlation between average (non-azimuthal) gamma-ray measurements and the enhanced survey was performed on one of the two wells in real time. Real-time interpretation of Radian data made it possible to detect the stratigraphic movement of the wellbore, where bulk gamma-ray measurement was featureless.
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