Gildehauser Sandstone

• https://doi.org/10.1002/2015GL064007
• doi:10.1002/2015GL064007
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Abstract — During imbibition, initially connected oil is displaced until it is trapped as immobile clusters. While initial and final states have been well described before, here we image the dynamic transient process in a sandstone rock using fast synchrotron-based X-ray computed microtomography. Wetting film swelling and subsequent snap off, at unusually high saturation, decreases nonwetting phase connectivity, which leads to nonwetting phase fragmentation into mobile ganglia, i.e., ganglion dynamics regime. We find that in addition to pressure-driven connected pathway flow, mass transfer in the oil phase also occurs by a sequence of correlated breakup and coalescence processes. For example, meniscus oscillations caused by snap-off events trigger coalescence of adjacent clusters. The ganglion dynamics occurs at the length scale of oil clusters and thus represents an intermediate flow regime between pore and Darcy scale that is so far dismissed in most upscaling attempts.

• https://doi.org/10.1016/j.advwatres.2016.01.010
• 10.1016/j.advwatres.2016.01.010
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Abstract — Pore-scale images obtained from a synchrotron-based X-ray computed micro-tomography ( μCT) imbibi- tion experiment in sandstone rock were used to conduct Navier–Stokes flow simulations on the con- nected pathways of water and oil phases. The resulting relative permeability was compared with steady- state Darcy-scale imbibition experiments on 5 cm large twin samples from the same outcrop sandstone material. While the relative permeability curves display a large degree of similarity, the endpoint sat- urations for the μCT data are 10% in saturation units higher than the experimental data. However, the two datasets match well when normalizing to the mobile saturation range. The agreement is particularly good at low water saturations, where the oil is predominantly connected. Apart from different satura- tion endpoints, in this particular experiment where connected pathway flow dominates, the discrepan- cies between pore-scale connected pathway flow simulations and Darcy-scale steady-state data are minor overall and have very little impact on fractional flow. The results also indicate that if the pore-scale fluid distributions are available and the amount of disconnected non-wetting phase is low, quasi-static flow simulations may be sufficient to compute relative permeability. When pore-scale fluid distributions are not available, fluid distributions can be obtained from a morphological approach, which approximates capillary-dominated displacement. The relative permeability obtained from the morphological approach compare well to drainage steady state whereas major discrepancies to the imbibition steady-state exper- imental data are observed. The morphological approach does not represent the imbibition process very well and experimental data for the spatial arrangement of the phases are required. Presumably for mod- eling imbibition relative permeability an approach is needed that captures moving liquid-liquid interfaces, which requires viscous and capillary forces simultaneously.

• https://doi.org/10.1016/j.advwatres.2018.09.001
• doi: 10.1016/j.advwatres.2018.09.001

Abstract — Pore-to-Darcy scale upscaling of multiphase flow is one of the major unresolved problems in many fields of porous media research. While this problem involves very fundamental aspects, there are many practical and application-driven challenges as well, such as the accurate prediction of Darcy-scale multiphase effective properties, e.g., relative permeability by pore-scale flow simulation on the basis of the imaged pore geometry, e.g., via X-ray computed micro-tomography. Validation of pore-scale modeling methods against experimental data by comparison of measured against simulated relative permeability curves has proven to be insufficient. Comparison of the fluid topology, in particular, the non-wetting phase topology, is a much more reliable criteria as relative permeability shows a very strong correlation with connectivity. While percolation-based quasi-static modeling approaches operating in the capillary limit have proven moderately successful in drainage, they largely fail to predict fluid connectivity in imbibition. We show that a fully coupled visco-capillary simulation, using a free-energy based lattice Boltzmann approach, correctly predicts fluid connectivity in imbibition, which was not predicted correctly by a quasi-static approach in a previous study on the same dataset. The respective relative permeability data shows a close match with results from Darcy-scale core flooding experiments while there is a major mismatch from quasi-static approach. The results once more highlight the close connection between relative permeability and fluid topology and suggest that topological measures are much more meaningful validation criteria for pore-scale simulation. The results also show that the direct simulation approach described in this paper, using appropriate initial and boundary conditions, correctly predicts Darcy-scale effective properties.