Fluid flow characteristics in porous media using Magnetic Resonance Imaging (MRI) technique: Determining the capillary pressure and relative permeability

Abstract

Conventional laboratory methods for measuring and determining fluid flow parameters often involve complexities and simplifications that can lead to inaccuracies. These limitations hinder a comprehensive understanding of fluid flow behavior in reservoirs. This study reviews the use of Magnetic Resonance Imaging (MRI) as a non-invasive technique to investigate key parameters governing fluid flow in porous media, such as capillary pressure and relative permeability. MRI enables direct measurement of capillary pressure without the need for traditional, routine procedures. By analyzing MRI intensity, saturation levels can be determined across different sections of the rock based on hydrogen content. Additionally, MRI allows for the evaluation of porous structures and pore-throat geometry. This technique is particularly effective and efficient for analyzing cores with high capillary pressures and fully water-saturated conditions. It also facilitates the observation of fingering and channeling mechanisms during water or gas injection processes. Relative permeability can be assessed using MRI by analyzing signal intensity and water/oil saturation at specific points within the porous medium. Furthermore, MRI provides insights into breakthrough events for water and gas, as well as the morphology of the displacing front during fluid injection. Despite its advantages, the MRI technique has certain limitations. These include constraints related to spatial resolution and the performance of MRI radiofrequency coils for long core samples. Additionally, its accuracy diminishes in cases of high water saturation at the outlet face of the core or when fluid flow rates are low. The MRI technique based on hydrogen offers significant advantages over traditional methods for measuring capillary pressure and relative permeability, as it directly determines fluid saturation within porous media. This method is highly accurate and efficient, even for cores with higher capillary pressures or fully water-saturated conditions, without requiring the extraction of water from the core. Additionally, MRI enables precise characterization of pore-throat structures, heterogeneities in reservoir rock pore systems, and the mobility ratios of water and oil, which are critical factors in understanding phenomena like fingering and channeling during CO₂ capillary trapping. Relative permeability can be derived from MRI analysis by examining saturation levels and fluid flow velocities. The displacement processes of fluids (gas or liquid) within porous media are determined using hydrogen-based MRI signals, as fluids lacking hydrogen produce negligible signal responses. This capability allows for detailed observation of the structure and behavior of injected fluids, including the displacing front at various locations in porous media. It is particularly useful in high-permeability zones where breakthrough events occur under different pressures and flow conditions. Unlike traditional methods, MRI accounts for the capillary end effect—a phenomenon that can introduce errors in evaluating oil/water saturation—by considering stationary wetting phase distribution during relative permeability measurements. This makes MRI a more reliable tool for accurately assessing fluid dynamics in porous media. However, limitations such as resolution constraints and challenges with low flow rates or high water saturation at the outlet face still exist, requiring further optimization for broader applicability. This paper highlights the potential of the MRI technique as a highly accurate, fast, and efficient method for studying fluid flow in the porous media of reservoir rocks. Its application is steadily transforming reservoir rock analysis, offering new perspectives and opportunities for laboratories and research centers in this field.