Pore network model simulation (PNM) is an important method to simulate reactive transport processes in porous media and to investigate constitutive relationships between permeability and porosity that can be implemented in continuum-scale reactive-transport modeling.The existing reactive transport pore network models (rtPNMs) assume that the initially cylindrical pore throats maintain their shape and pore throat conductance is updated using a form of Hagen-Poiseuille relation.However, in the context of calcite dissolution, earlier studies have shown that during dissolution, pore throats can attain a spectrum of shapes, depending upon the imposed reactive-flow conditions (Agrawal et al., 2020).In the current study, we derived new constitutive relations for the calculation of conductance as a function of pore throat volume and shape evolution for a range of imposed flow and reaction conditions.These relations were used to build animproved new reactive pore network model (nrtPNM).Using the new model, the porosity-permeability changes were simulated and compared against the existing pore network models.In order to validate the reactive transport pore network model, we conducted two sets of flow-through experiments on two Ketton limestone samples.Acidic solutions (pH 3.0) were injected at two Darcy velocities i.e., 7.3 × 10 -4 and 1.5 × 10 -4 m. s -1 while performing X-ray micro-CT scanning.Experimental values of the changes in sample permeability were estimated in two independent ways: through PNM flow simulation and through Direct Numerical Simulation.Both approaches used images of the samples from the beginning and the end of experiments.Extracted pore networks, obtained from the micro-CT images of the sample from the beginning of the experiment, were used for reactive transport PNMs (rtPNM and nrtPNM).We observed that for the experimental conditions, most of the pore throats maintained the initially prescribed cylindrical shape such that both rtPNM and nrtPNM showed a similar evolution of porosity and permeability.This was found to be in reasonable agreement with the porosity and permeability changes observed in the experiment.Next, we have applied a range of flow and reaction regimes to compare permeability evolutions between rtPNM and nrtPNM.We found that for certain dissolution regimes, neglecting the evolution of the pore throat shape in the pore network can lead to an overestimation of up to 27% in the predicted permeability values and an overestimation of over 50% in the fitted exponent for the porosity-permeability relations.In summary, this study showed that while under high flow rate conditions the rtPNM model is accurate enough, it overestimates permeability under lower flow rates.
Abstract Multiphase flow is important for many natural and engineered processes in subsurface geoscience. Pore‐scale multiphase flow dynamics are commonly characterized by an average balance of driving forces. However, significant local variability in this balance may exist inside natural, heterogeneous porous materials, such as rocks and soils. Here, we investigate multiphase flow in heterogeneous rocks with different wetting properties using fast laboratory‐based 4D X‐ray imaging. The mixed‐wet dynamics were characterized by displacement rates that differed over orders of magnitude between directly neighboring pores. While conventional understanding predicted strongly capillary‐dominated conditions, our analysis suggests that viscous forces played a key role in these dynamics, facilitated by a complex interplay between the mixed‐wettability and the pore structure. These dynamics highlight the need for further studies on the fundamental controls on multiphase flow in geomaterials, which is crucial to design, for example, groundwater remediation and subsurface CO 2 storage operations.
Abstract Heterogeneous fracture aperture distribution, dictated by surface roughness, mechanical rock and fracture properties, and effective stress, limits the predictive capabilities of many reservoir‐scale models that commonly assume smooth fracture walls. Numerous experimental studies have probed key hydromechanical responses in single fractures; however, many are constrained by difficulties associated with sample preparation and quantitative roughness characterization. Here, we systematically examine the effect of roughness on fluid flow properties by 3D printing seven self‐affine fractures, each with controlled roughness distributions akin to those observed in nature. Photogrammetric microscopy was employed to validate the 3D topology of each printed fracture surface, enabling quantification using traditional roughness metrics, namely the Joint Roughness Coefficient ( JRC ). Core‐flooding experiments performed on each fracture across eight incremental confining pressure increases (11–25 bar), shows smoother fractures ( JRC < 5.5) exhibit minor permeability variation, whilst rougher fractures ( JRC > 7) show as much as a 219% permeability increase. Micro‐computed tomography imaging of the roughest fracture under varying effective stresses (5–13.8 bar), coupled with inspection into the degree of similarity between fracture closure behavior in 3D‐printed and natural rock fractures, highlight the capabilities of 3D‐printed materials to act as useful analogs to natural rocks. Comparison of experimental data to existing empirical aperture‐permeability models demonstrates that fracture contact area is a better permeability predictor than roughness when the mechanical aperture is below ∼20 μm. Such findings are relevant for models incorporating the effects of heterogeneous aperture structures and applied stress to predict fracture flow in the deep subsurface.
Abstract Lab‐based high‐resolution Computed Tomography ( μ CT) is an important tool to investigate multiphase fluid flow through porous media. Rapid and continuous μ CT acquisition can be used to resolve the dynamics of the 3D fluid distribution in the pores over time while the flow processes occur. However, the temporal resolution of this technique remains limited due to a trade‐off between acquisition time and image quality. This work presents a method to improve the temporal resolution of multiphase flow imaging experiments by limiting the angular range of radiographs used to reconstruct each time step, while compensating for this loss of information by including a temporal total variation term in the reconstruction algorithm. This addition penalizes improper temporal fluctuations in the reconstructed images, but not those dynamic events that are consistent with the radiographs. We perform a thorough evaluation of the resulting gain in temporal resolution at the single‐pore level. The method is validated on both simulated and experimental data representing multiphase flow in porous media. We find that this method improved the temporal resolution up to a factor 3 compared to reconstructions that use full 360° rotations for each time step.
This study investigates the impact of brine composition—specifically calcium ions and NaCl-based salinity—on the development of dissolution features in Ketton, a porous calcium carbonate rock. Utilizing a laboratory XMT (X-ray microtomography) scanner, we captured time-lapse in situ images of Ketton samples throughout various dissolution experiments, conducting four distinct flow-through experiments with differing brine solutions at a flow rate of 0.26 ml min⁻1. The scans yielded a voxel size of 6 μm, enabling the assessment of the temporal evolution of porosity and pore structure through image analysis and permeability evaluations via single-phase fluid flow simulations employing direct numerical solutions and network modeling, as opposed to direct measurement. Time-lapse imaging technique has delineated the extent to which the concentrations of CaCl₂ and NaCl in the injecting solution control the structural evolution of dissolution patterns, subsequently triggering the development of characteristic dissolution pattern. The inflow solution with no Ca2+ ions and with the minimal salt content manifested maximum dissolution near the sample inlet, coupled with the formation of numerous dissolution channels, i.e., wormholes. Conversely, solutions with a trace amount of Ca2⁺ ions induced focused dissolution, resulting in the formation of sparsely located channels. Inflow solutions with high concentrations of both Ca2⁺ ions and salt facilitated uniformly dispersed dissolution, primarily within microporous domains, initiating particle detachment and displacement and leading to localized pore-clogging. The relative increase in permeability, in each experiment, was correlated with the developed dissolution pattern. It was discerned that varying ratios of salt and calcium concentrations in the injected solution systematically influenced image-based permeability simulations and porosity, allowing for the depiction of an empirical porosity-permeability relationship.
Abstract The wetting properties of pore walls have a strong effect on multiphase flow through porous media. However, the fluid flow behavior in porous materials with both complex pore structures and non‐uniform wettability are still unclear. Here, we performed unsteady‐state quasi‐static oil‐ and water‐flooding experiments to study multiphase flow in two sister heterogeneous sandstones with variable wettability conditions (i.e., one natively water‐wet and one chemically treated to be mixed‐wet). The pore‐scale fluid distributions during this process were imaged by laboratory‐based X‐ray micro‐computed tomography (micro‐CT). In the mixed‐wet case, we observed pore filling events where the fluid interface appeared to be at quasi‐equilibrium at every position along the pore body (13% by volume), in contrast to capillary instabilities typically associated with slow drainage or imbibition. These events corresponded to slow displacements previously observed in unsteady‐state experiments, explaining the wide range of displacement time scales in mixed‐wet samples. Our new data allowed us to quantify the fluid saturations below the image resolution, indicating that slow events were caused by the presence of microporosity and the wetting heterogeneity. Finally, we investigated the sensitivity of the multi‐phase flow properties to the slow filling events using a state‐of‐the‐art multi‐scale pore network model. This indicated that pores where such events took place contributed up to 19% of the sample's total absolute permeability, but that the impact on the relative permeability may be smaller. Our study sheds new light on poorly understood multiphase fluid dynamics in complex rocks, of interest to for example, groundwater remediation and subsurface CO 2 storage.
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