CO2 EOR Mechanisms in Bakken Shale Oil Reservoirs

Preliminary studies have been done to characterize rock-fluid properties, and flow mechanisms in the shale reservoirs. Most of these studies, through modifying methods used for conventional reservoirs, fail to capture dynamic features of shale rock and fluids in confined nano-pore space.   In unconventional reservoirs, interactions between the wall of shale and the contained fluid significantly affect phase and flow behaviors. The inability to model capillarity with the consideration of pore size distribution characteristics using commercial software may lead to an inaccurate oil production performance in Bakken. This paper presents a novel formulation that consistently evaluates capillary force and adsorption using pore size distribution (PSD) directly from core measurements. The new findings could better address differences in flow mechanisms in unconventional reservoirs, and thus lead to an optimized IOR practice.

This paper presents a novel formulation that consistently evaluates capillary force with respect to pore size distribution (PSD) directly from Bakken core measurements. We also demonstrate how permeability would change as reservoir depletes, and more importantly CO₂ huff-and-puff, and incorporate the new model to a 3D dynamic simulator.

We further developed adsorption models using a local density optimization algorithm, to better address the incapability of Langmuir model for wet and liquid-rich formations. Our advances were also designed for multi-component interactions at adsorption sites for a full spectrum of reservoir pressures of interests. This new adsorption model, based on exactly the same physical chemistry principles at different pressures, revolved from monolayer at low pressure to more complex multilayer model spontaneously and consistently, imperative to CO₂ cycling.

With our advances in understanding key production mechanisms of capillarity and adsorption, we are able to differentiate production driving mechanisms in unconventional reservoirs vs. conventional ones.  A new compositional simulator was developed that captures those differentiations and results show that should capillarity be consistently formulated with PSD, significant difference in production profile is observed.  As shown for CO₂ huff-and-puff process in unconventional reservoirs, a smaller amount of soaking time and a 30% higher in ultimate recovery was achieved, as compared with the case not considering capillarity and adsorption properly.

1. This paper implements a novel formulation that captures capillarity pressure under pore confinement using a full spectrum of PSD characterization for shale oil core analysis.
2. The new model consistently evaluates capillary adsorption effects for reservoir fluid density evolution using industrial accepted equation-of-state model and reservoir pressure depletion and CO₂-IOR process.
3. A substantial increase in oil production is illustrated when PSD, an important unconventional reservoir characteristics, is properly modeled.
4. The new method may bring additional insight to greatly mitigate uncertainties for IOR potential evaluation, productivity and EUR assessment for unconventional reservoirs.