( )Pore-scale dissolution mechanisms in calcite-CO2-brine systems: The impact of non-linear reaction kinetics and coupled ion transport

We simulate two sets of dissolution experiments in which CO2-saturated solutions are injected into calcite formations. We explore the impact of non-linear reaction kinetics and charge-coupled ion transport in systems representing different levels of flow and mineralogical complexity. First, we flow CO2-saturated water and brine through cylindrical channels drilled through solid calcite cores and compare directly with experimental dissolution rates. We find that simulations using a linear saturation model match experimental results much better than the batch-reactor-derived non-linear saturation model. The use of a coupled diffusion model causes only a very small increase in the overall dissolution rate compared to a single diffusion coefficient, due to the increase in transport rates of reaction products, particularly the highly charged Ca2+ ion. We also determine the relative importance of the two calcite dissolution pathways, with H+ and H2CO3, and conclude that the H2CO3 – calcite reaction is by far the more dominant, in contrast with common assumptions in the literature. Then, we compare to the experiments of Menke et al. (2015) in which CO2-saturated brine was injected into a microporous Ketton carbonate, and compare dissolution rates over time. We find that including non-linear saturation behaviour markedly changes the simulated dissolution rate, by up to a factor of 0.7 in the case of the experimentally derived saturation model of Anabaraonye (2017), however neither case matches the experimental result which is several times slower than the simulation. Including the effects of coupled ion transport lead to virtually no change in overall dissolution rate due to the convection dominated behaviour. The model also shows differences in the trend of the dissolution rate over time observed in Menke et al, with an approximately linear relationship with time compared to the experimental square-root dependence on time. We conclude that the geochemical model may need to include other effects such as dissolution inside microporous regions

Insights into non-Fickian solute transport in carbonates

[1] We study and explain the origin of early breakthrough and long tailing plume behavior by simulating solute transport through 3‐D X‐ray images of six different carbonate rock samples, representing geological media with a high degree of pore‐scale complexity. A Stokes solver is employed to compute the flow field, and the particles are then transported along streamlines to represent advection, while the random walk method is used to model diffusion. We compute the propagators (concentration versus displacement) for a range of Peclet numbers (Pe ) and relate it to the velocity distribution obtained directly on the images. There is a very wide distribution of velocity that quantifies the impact of pore structure on transport. In samples with a relatively narrow spread of velocities, transport is characterized by a small immobile concentration peak, representing essentially stagnant portions of the pore space, and a dominant secondary peak of mobile solute moving at approximately the average flow speed. On the other hand, in carbonates with a wider velocity distribution, there is a significant immobile peak concentration and an elongated tail of moving fluid. An increase in Pe , decreasing the relative impact of diffusion, leads to the faster formation of secondary mobile peak(s). This behavior indicates highly anomalous transport. The implications for modeling field‐scale transport are discussed

A Particle Model for Prediction of Cement Infiltration of Cancellous Bone in Osteoporotic Bone Augmentation.

PMC3693961Femoroplasty is a potential preventive treatment for osteoporotic hip fractures. It involves augmenting mechanical properties of the femur by injecting Polymethylmethacrylate (PMMA) bone cement. To reduce the risks involved and maximize the outcome, however, the procedure needs to be carefully planned and executed. An important part of the planning system is predicting infiltration of cement into the porous medium of cancellous bone. We used the method of Smoothed Particle Hydrodynamics (SPH) to model the flow of PMMA inside porous media. We modified the standard formulation of SPH to incorporate the extreme viscosities associated with bone cement. Darcy creeping flow of fluids through isotropic porous media was simulated and the results were compared with those reported in the literature. Further validation involved injecting PMMA cement inside porous foam blocks - osteoporotic cancellous bone surrogates - and simulating the injections using our proposed SPH model. Millimeter accuracy was obtained in comparing the simulated and actual cement shapes. Also, strong correlations were found between the simulated and the experimental data of spreading distance (R2 = 0.86) and normalized pressure (R2 = 0.90). Results suggest that the proposed model is suitable for use in an osteoporotic femoral augmentation planning framework.JH Libraries Open Access Fun