Low primary recovery factors in ultratight unconventional reservoirs such as the Bakken, usually in the range of 5-15%, present a potential opportunity for developing effective and economic Enhanced Oil Recovery (EOR) methods. Identified as one of the leading EOR methods for the Bakken, surfactant injection has shown good potential in the laboratory due to its capability to alter wettability, lower interfacial tension (IFT), and reduce residual oil saturation. However, successful implementation of surfactant EOR requires in-depth characterization of the rock-fluid system at multiple scales, ranging from pore scale, to core scale, to field scale. This paper presents a comprehensive study that systematically characterizes Bakken rock properties at the pore scale and the core scale, with the goal of understanding the impact of the pore structure and static and dynamic rock properties on the EOR potential for surfactant injection.
At the pore scale, high-resolution 3D X-ray microtomography (microCT) imaging was acquired to evaluate microstructure, interconnected porosity, and fluid distribution. Fluid saturation mapping, before and after surfactant Spontaneous Imbibition (SI), provides fundamental insights into pore-scale recovery efficiency.
At the core scale, techniques for measuring static rock properties, including porosity, permeability, and saturation in the Middle Bakken, are reviewed, with a particular focus on measuring matrix permeability across a broad range of values. Fluid properties critical to surfactant injection, such as water salinity and oil polarity, are also discussed. Results from SI experiments are presented with the goal of evaluating imbibition rate and oil recovery with proper scaling. To address the challenges of measuring dynamic rock properties such as imbibition capillary pressure and relative permeability, a plug-scale simulation model was used to history match the laboratory experiment and derive dynamic rock properties that govern the recovery process. The calibrated model was used to address plug-scale recovery efficiency and evaluate recovery uncertainty range, providing fundamental flow characteristics that can be incorporated into upscaled field-scale studies.