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We observed features of pore scale fluid distributions during oil-brine displacement in a mixed-wet sandstone rock sample. High-resolution X-ray imaging was used in combination with differential pressure measurements to measure relative permeability and capillary pressure simultaneously during a steady-state waterflood experiment on a sample of Bentheimer sandstone 51.6 mm long and 6.1 mm in diameter. After prolonged contact with crude oil to alter the surface wettability, a refined oil and formation brine were injected through the sample at a fixed total flow rate but in a sequence of increasing brine fractional flows. When the pressure across the system stabilized, X-ray tomographic images were taken. The images were used to compute saturation, interfacial area, curvature and contact angle. From this information relative permeability and capillary pressure were determined as functions of saturation. We compare our results with a previously published experiment with strongly water-wet conditions. The oil relative permeability was lower than in the water-wet case, although a smaller residual oil saturation, of approximately 0.11, was obtained, since the oil remained connected in layers in the altered wettability rock. The capillary pressure was slightly negative and ten times smaller in magnitude than a similar water-wet rock, and approximately constant over a wide range of intermediate saturation. The oil-brine interfacial area was also largely constant in this saturation range. The measured static contact angles had an average of $80^{\circ}$ with a standard deviation of $17^{\circ}$.We observed that the oil-brine interfaces were not flat, as may be expected for a very low mean curvature, but had two approximately equal, but opposite, curvatures in orthogonal directions. These interfaces were approximately minimal surfaces which allow efficient displacement and imply well-connected phases. Saddle-shaped menisci swept through the pore space at a constant capillary pressure and with an almost fixed area, removing most of the oil.

We study the impact of physical and chemical heterogeneity on reaction rates in multimineral porous media. We selected two pairs of carbonate samples of different physical heterogeneity in accordance with their initial computed velocity distributions and then injected CO 2 saturated brine at reservoir conditions at two flow rates. We periodically imaged the samples using X-ray microtomography. The mineralogical composition was similar (a ratio of dolomite to calcite of 8:1), but the intrinsic reaction rates and mineral spatial distribution were profoundly different. Visualizations of velocity fields and reacted mineral distributions revealed that a dominant flow channel formed in all cases. The more physically homogeneous samples had a narrower velocity distribution and more preexisting fast channels, which promoted dominant channel formation in their proximity. In contrast, the heterogeneous samples exhibit a broader distribution of velocities and fewer fast channels, which accentuated nonuniform calcite distribution and favored calcite dissolution away from the initially fast pathways. We quantify the impact of physical and chemical heterogeneity by computing the proximity of reacted minerals to the fast flow pathways. The average reaction rates were an order of magnitude lower than the intrinsic ones due to mass transfer limitations. The effective reaction rate of calcite decreased by an order of magnitude, in both fast channels and slow regions. After channel formation calcite was shielded by dolomite whose effective rate in slow regions could even increase. Overall, the preferential channeling effect, as opposed to uniform dissolution, was promoted by a higher degree of physical and/or chemical heterogeneity.

The experimental determination of capillary pressure drainage curves at the pore scale is of vital importance for the mapping of reservoir fluid distribution. To fully characterize capillary drainage in a complex pore space, we design a differential imaging-based porous plate (DIPP) method using X-ray micro- tomography. For an exemplar mm-scale laminated sandstone microcore with a porous plate, we quantify the displacement from resolvable macropores and subresolution micropores. Nitrogen (N 2 ) was injected as the nonwetting phase at a constant pressure while the porous plate prevented its escape. The measured porosity and capillary pressure at the imaged saturations agree well with helium measurements and experi- ments on larger core samples, while providing a pore-scale explanation of the fluid distribution. We observed that the majority of the brine was displaced by N 2 in macropores at low capillary pressures, fol- lowed by a further brine displacement in micropores when capillary pressure increases. Furthermore, we were able to discern that brine predominantly remained within the subresolution micropores, such as regions of fine lamination. The capillary pressure curve for pressures ranging from 0 to 1151 kPa is provided from the image analysis compares well with the conventional porous plate method for a cm-scale core but was conducted over a period of 10 days rather than up to few months with the conventional porous plate method. Overall, we demonstrate the capability of our method to provide quantitative information on two- phase saturation in heterogeneous core samples for a wide range of capillary pressures even at scales smaller than the micro-CT resolution

[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. Citation: Bijeljic, B., P. Mostaghimi, and M. J. Blunt (2013), Insights into non-Fickian solute transport in carbonates, Water Resour. Res., 49, 2714–2728, doi:10.1002/wrcr.20238.

We study the influence of the pore-space geometry on sample-averaged dissolution rates in millimeter-scale carbonate samples undergoing reaction-controlled mineral dissolution upon the injection of a CO2 -saturated brine. The representation of the pore space is obtained directly from micro-CT images with a resolution of a few microns. Simulations are performed with a particle tracking approach on images of three porous rocks of increasing pore-space complexity: a bead pack, a Ketton oolite, and an Estaillades limestone. Reactive transport is simulated with a hybrid approach that combines a Lagrangian method for transport and reaction with the Eulerian flow field obtained by solving the incompressible Navier-Stokes equations directly on the voxels of three-dimensional images. Particle advection is performed with a semianalytical streamline method and diffusion is simulated via a random walk. Mineral dissolution is defined in terms of the particle flux through the pore-solid interface, which can be related analytically to the batch (intrinsic) reaction rate. The impact of the flow heterogeneity on reactive transport is illustrated in a series of simulations performed at different flow rates. The average dissolution rates depend on both the heterogeneity of the sample and on the flow rate. The most heterogeneous rock may exhibit a decrease of up to two orders of magnitude in the sample-averaged reaction rates in comparison with the batch rate. Furthermore, we provide new insights for the dissolution regime that would be traditionally characterized as uniform. In most cases, at the pore-scale, dissolution preferentially enlarges fast-flow channels which greatly restricts the effective surface available for reaction.

© 2016. The Authors.The structure and connectivity of the pore space during the pyrolysis of oil shales determines hydrocarbon flow behavior and ultimate recovery. We image the time evolution of the pore and microfracture networks during oil shale pyrolysis using synchrotron X-ray microtomography. Immature Green River (Mahogany Zone) shale samples were thermally matured under vacuum conditions at temperatures up to 500°C while being periodically imaged with a 2μmvoxel size. The structural transformation of both organic-rich and organic-lean layers within the shale was quantified. The images reveal a dramatic change in porosity accompanying pyrolysis between 390 and 400°C with the formation of micron-scale heterogeneous pores. With a further increase in temperature, the pores steadily expand resulting in connected microfracture networks that predominantly develop along the kerogen-rich laminations.

This document is a supporting document to deliverable MS6.1 UK Storage Appraisal Final Report. This is Appendix A5.4 - Storage Capacity in Large Open Aquifers Examplar : Base Case Simulation and Sensitivities to Top Surface and HeterogeneityOne of the three main types of saline aquifer store relevant to UK CO2 storage capacity that this project identified were “open aquifers”. In order to obtain more reliable storage capacityestimates for open aquifers, dynamic modelling was performed by numerical simulation using two classes of model: simplified generic models, termed “Representative Structures”, and more detailed models of selected regions of actual UKCS aquifer units, termed “Exemplars”.The Forties sandstone of the Seles Formation off North East Scotland was modelled in detail as an exemplar of a large open aquifer.Overall, this work has presented a quantitative framework for assessing storage si tes based on different constraints – pressure, migration distance and migration speed and placed different cases into three storage regimes. This work has shown that open aquifers of modest permeability and dip can prove to be favourable storage sites with large storage capacities.These aquifers limit the speed with which the CO2 migrates while the extensive open pore volume can help dissipate pressure, avoiding pressure problems associated with other types of storage site.This suggests that with careful selection and design, large open aquifers are promising sites for CO2 storage

This document is a supporting document to deliverable MS6.1 UK Storage Appraisal Final Report. It is Appendix A5.1:Summary of Dynamic Modelling Scoping Studies.The purpose of this preliminary work was to define a common approach for the dynamic modelling including the physical processes to be represented, modelling tools to be used, the definition of common/standardised parameters and a basis for the recommendations. This was achieved through an extensive literature review, investigation of modelling software and modelling assessments. The following main recommendations were made.Modelling gravity effects with a sufficiently fine grid where needed is important.The solubility of CO2 in brine and the effect of capillary pressure should normally be included in dynamic models, but the effect of diffusion is not likely to be significant.The effect of hysteresis on relative permeabilities will be required to model residual trapping as it may be an important trapping mechanism after injection has ceased for poorly confined structures.It was concluded that the bulk of the dynamic modelling could be performed isothermally with sufficient accuracy using the industry standard finite difference ‘black-oil’ simulator ECLIPSE100™, and appropriate PVT data input, which was defined. This solution has the advantage of speed over the ECLIPSE300™/CO2STORE module compositional combination. It was proposed that a streamline simulator, such as 3DSL™, be considered for simulation of fine scale models of Exemplar open aquifer units as this would enable greater detail to be modelled due to faster run speeds. Streamline simulation is particularly effective where modelling displacement is more important than pressure changes, as for open aquifers.It was also proposed that a single simulator, GEM™, be used for well injectivity and associated thermal and geomechanical sensitivity calculations.It was recommended that the most comprehensive set of consistent CO2/brine relative permeability and capillary pressure data available from a Canadian dataset be used for the modelling.