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1. Augmenting X-ray micro-CT data with MICP data for high resolution pore-scale simulations of flow properties of carbonate rocks

   https://doi.org/10.1016/j.geoen.2024.212982  

   Ishola, Olubukola; Vilcáez, Javier (August 2024, Geoenergy Science and Engineering)

   Pore-scale modeling is essential in understanding and predicting flow and transport properties of rocks. Generally, pore-scale modeling is dependent on imaging technologies such as Micro Computed Tomography (micro-CT), which provides visual confirmation into the pore microstructures of rocks at a representative scale. However, this technique is limited in the ability to provide high resolution images showing the pore-throats connecting pore bodies. Pore scale simulations of flow and transport properties of rocks are generally done on a single 3D pore microstructure image. As such, the simulated properties are only representative of the simulated pore-scale rock volume. These are the technological and computational limitations which we address here by using a stochastic pore-scale simulation approach. This approach consists of constructing hundreds of 3D pore microstructures of the same pore size distribution and overall porosity but different pore connectivity. The construction of the 3D pore microstructures incorporates the use of Mercury Injection Capillary Pressure (MICP) data to account for pore throat size distribution, and micro-CT images to account for pore body size distribution. The approach requires a small micro-CT image volume (7–19 mm3) to reveal key pore microstructural features that control flow and transport properties of highly heterogeneous rocks at the core-scale. Four carbonate rock samples were used to test the proposed approach. Permeability calculations from the introduced approach were validated by comparing laboratory measured permeability of rock cores and permeability estimated using five well-known core-scale empirical model equations. The results show that accounting for the stochastic connectivity of pores results in a probabilistic distribution of flow properties which can be used to upscale pore-scale simulated flow properties to the core-scale. The use of the introduced stochastic pore-scale simulation approach is more beneficial when there is a higher degree of heterogeneity in pore size distribution. This is shown to be the case with permeability and hydraulic tortuosity which are key controls of flow and transport processes in rocks. 

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2. Estimating Permeability of Partially Frozen Soil Using Floating Random Walks

   https://doi.org/10.1029/2021WR030598  

   Chen, Jiangzhi; Mei, Shenghua; Rempel, Alan_W (December 2021, Water Resources Research)

   Abstract Flow through partially frozen pores in granular media containing ice or gas hydrate plays an essential role in diverse phenomena including methane migration and frost heave. As freezing progresses, the frozen phase grows in the pore space and constricts flow paths so that the permeability decreases. Previous works have measured the relationship between permeability and volumetric fraction of the frozen phase, and various correlations have been proposed to predict permeability change in hydrology and the oil industry. However, predictions from different formulae can differ by orders of magnitude, causing great uncertainty in modeling results. We present a floating random walk method to approximate the porous flow field and estimate the effective permeability in isotropic granular media with specified particle size distributions, without solving for the entire flow field in the pore space. In packed spherical particles, the method compares favorably with the Kozeny‐Carman formula. We further extend this method with a probabilistic interpretation of the volumetric fraction of the frozen phase, simulate the effect of freezing in irregular pores, and predict the evolution of permeability. Employing no adjustable parameters, our results can provide insight into the coupling between phase transitions and permeability change, which plays important roles in hydrate formation and dissociation, as well as in the thawing and freezing of permafrost and ice‐bed coupling beneath glaciers. 

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3. An Experimental Investigation of Asphaltene Aggregation Under Carbon Dioxide Injection Flow in Ultra-Low-Permeability Pore Structure

   https://doi.org/10.2118/208950-MS  

   Elturki, Mukhtar; Imqam, Abdulmohsin (March 2022, SPE Canadian Energy Technology Conference, Calgary, Alberta)

   Abstract One of the major problems during gas injection in unconventional reservoirs is asphaltene precipitation and deposition. Asphaltenes can reduce the pore throat in the reservoir and plug the surface and subsurface equipment during the production process, thus, result in oil production reduction with significant financial consequences. The impact of carbon dioxide (CO2) gas injection on asphaltene deposition in unconventional reservoirs still poorly investigated. This research investigates the impact of CO2 gas injection on asphaltene aggregation in ultra-low-permeability pore structures, mainly present in unconventional shale resources. First, the minimum miscibility pressure (MMP) of crude oil with CO2 was determined using the slim tube technique. Then, several CO2 injection pressures were selected to conduct the filtration experiments using a specially designed filtration apparatus. All pressures selected were below the MMP. Various sizes of filter paper membranes were used to study the effect of pore structure on asphaltene deposition. The results showed that asphaltene weight percent was increased by increasing the pressure and a significant asphaltene weight percentage was observed on smaller pore size structures of the filter membranes. The visualization tests revealed the process of asphaltene precipitation and deposition and showed that asphaltene particles and clusters were precipitated after one hour and fully deposited in the bottom of the test tube after 12 hours. High-resolution photos of filter paper membranes were presented using microscopy imaging and scanning electron microscopy (SEM) analysis; these photos highlighted the asphaltene particles inside the filter paper membranes and pore plugging was observed. The study's findings will contribute to a better understanding of the main factors influencing the stability of asphaltene particles in crude oil under immiscible CO2 injection pressure, particularly in nano pores, which are predominant in shale unconventional resources. 

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4. Reduced Model for Properties of Multiscale Porous Media with Changing Geometry

   https://doi.org/10.3390/computation9030028  

   Peszynska, Malgorzata; Umhoefer, Joseph; Shin, Choah (March 2021, Computation)

   null (Ed.)

   In this paper, we consider an important problem for modeling complex coupled phenomena in porous media at multiple scales. In particular, we consider flow and transport in the void space between the pores when the pore space is altered by new solid obstructions formed by microbial growth or reactive transport, and we are mostly interested in pore-coating and pore-filling type obstructions, observed in applications to biofilm in porous media and hydrate crystal formation, respectively. We consider the impact of these obstructions on the macroscopic properties of the porous medium, such as porosity, permeability and tortuosity, for which we build an experimental probability distribution with reduced models, which involves three steps: (1) generation of independent realizations of obstructions, followed by, (2) flow and transport simulations at pore-scale, and (3) upscaling. For the first step, we consider three approaches: (1A) direct numerical simulations (DNS) of the PDE model of the actual physical process called BN which forms the obstructions, and two non-DNS methods, which we call (1B) CLPS and (1C) LP. LP is a lattice Ising-type model, and CLPS is a constrained version of an Allen–Cahn model for phase separation with a localization term. Both LP and CLPS are model approximations of BN, and they seek local minima of some nonconvex energy functional, which provide plausible realizations of the obstructed geometry and are tuned heuristically to deliver either pore-coating or pore-filling obstructions. Our methods work with rock-void geometries obtained by imaging, but bypass the need for imaging in real-time, are fairly inexpensive, and can be tailored to other applications. The reduced models LP and CLPS are less computationally expensive than DNS, and can be tuned to the desired fidelity of the probability distributions of upscaled quantities. 

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5. Persistent Homology as a Heterogeneity Metric for Predicting Pore Size Change in Dissolving Carbonates

   https://doi.org/10.1029/2023WR034559  

   Thompson, E. P.; Ellis, B. R. (September 2023, Water Resources Research)

   Abstract Accurate prediction of physical alterations in carbonate reservoirs under dissolution is critical for development of subsurface energy technologies. The impact of mineral dissolution on flow characteristics depends on the connectivity and tortuosity of the pore network. Persistent homology is a tool from algebraic topology that describes the size and connectivity of topological features. When applied to 3D X‐ray computed tomography (XCT) imagery of rock cores, it provides a novel metric of pore network heterogeneity. Prior works have demonstrated the efficacy of persistent homology in predicting flow properties in numerical simulations of flow through porous media. Its ability to combine size, spatial distribution, and connectivity information make it a promising tool for understanding reactive transport in complex pore networks, yet limited work has been done to apply persistence analysis to experimental studies on natural rocks. In this study, three limestone cores were imaged by XCT before and after acid‐driven dissolution flow‐through experiments. Each XCT scan was analyzed using persistent homology. In all three rocks, permeability increase was driven by the growth of large, connected pore bodies. The two most homogenous samples saw an increased effect nearer to the flow inlet, suggesting emerging preferential flow paths as the reaction front progresses. The most heterogeneous sample showed an increase in along‐core homogeneity during reaction. Variability of persistence showed moderate positive correlation with pore body size increase. Persistence heterogeneity analysis could be used to anticipate where greatest pore size evolution may occur in a reservoir targeted for subsurface development, improving confidence in project viability. 

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