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polar components able to alter the rock wettability. 20. Also, to increase the image constrast for effective image segmentation, a brine solution was prepared ...
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Supplementary Information for

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Wettability in complex porous materials: the mixed-wet state and its relationship to surface roughness

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Ahmed AlRatrout, Martin J. Blunt and Branko Bijeljic

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Ahmed AlRatrout. E-mail: [email protected]

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This PDF file includes:

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Supplementary text Figs. S1 to S3 Tables S1 to S2 References for SI reference citations

Ahmed AlRatrout, Martin J. Blunt and Branko Bijeljic

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Supporting Information Text

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1. Experiment and Materials

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We briefly describe altered wettability experiments (1) which are used to test our method to measure contact angle, oil/brine interface curvature and rock surface roughness.

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A. Brine and crude Oil properties. Two crude oils were used, crude oil A (a light oil from the same reservoir as the rock samples)

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and crude oil B (Arabian Medium) which is heavier: see Table S1 for an analysis of their composition performed by Weatherford Labs to measure the content of the heavier fraction contents in each crude oil, including resins and asphaltenes that contain polar components able to alter the rock wettability. Also, to increase the image constrast for effective image segmentation, a brine solution was prepared using 7 weight percent of potassium iodide (KI) (purity 99.0%, Sigma-Aldarich, UK) mixed with deionized water. As a result, the brine density was 1052.1 ± 2.2 kg/m32 and a pH value of 6.91 ± 0.2 measured with FiveGo pH meter, Mettler Toledo.

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B. Aging protocols. For altering the wettability in the used reservoir samples and investigating its impact on the remaining oil

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saturation, three aging protocols were employed using raw crude oil and crude oil-heptane mixtures at different conditions. Heptane (purity 99%, Sigma-Aldarich, UK) was used to induce asphaltene precipitation. The viscosities of the oil phase at experimental temperature and ambient pressure are shown in Table S2. The viscosity was measured using an Anton Paar MCR301 rheometer.

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C. Image processing. We apply our contact angle, oil/brine interface curvature and roughness measurement method to the

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images after waterflooding, Figure S1. The size of the segmented images in voxels is 435 × 106 for all samples, for a part of the rock samples with a diameter of 1.9 mm and length of 1.2 mm (volume of approximately 3.4 mm 3 ); the total sample size in the experiments was a length of between 13 and 16 mm and a diameter of 4.8 mm. We could not look at the entire rock sample volume due to image artefacts.

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D. Additional Data. Figure S2 shows the spatial correlation of contact angle and interfacial curvature. As seen for roughness,

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the contact angle and interfacial curvature are correlated over approximately a pore diameter. Figure S3 shows the pore-by-pore plots of roughness, contact angle and interfacial curvature variations as a function of pore diameter. Small pores appear to be rougher, simply because they tend to be more curved; beyond a diameter of around 20 µm, however, the roughness is fixed around 0.2 µm, representing the local topography of the surface which is largely independent of pore size. The standard deviation of contact angle reaches an approximately constant value for large pores, principally because we are averaging over many individual values. The variation in interfacial curvature within a pore is small for the WW sample, as oil resides as quasi-spherical trapped ganglia; for the MW and OW samples, the remaining oil resides principally in layers that follow the surface roughness and display a greater local variability, as shown in Figure 1 in the main text.

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Ahmed AlRatrout, Martin J. Blunt and Branko Bijeljic

Table S1. Crude oil A and B properties, modified from (1)

Density at 21◦ C, kg/m3 Saturates, wt% Aromatics, wt% Resins, wt% Asphaltenes, wt%

Ahmed AlRatrout, Martin J. Blunt and Branko Bijeljic

Crude oil A

Crude oil B

830 ± 5 55.25 38.07 6.22 0.46

870 ± 5 33.54 52.88 9.3 4.28

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Table S2. Viscosity of oil phases used in the three experiments, modified from (1) Experiment 1. WW (10% crude oil A & 90% heptane) 2. MW (28% crude oil A & 72% heptane) 3. OW (100% crude oil A)

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Viscosity, mPa.s

Temperature, ◦ C

0.135 0.390 2.02

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Ahmed AlRatrout, Martin J. Blunt and Branko Bijeljic

Fig. S1. Image segmentation. (A-C) Three-dimensional rendering of the raw tomographic image with a voxel size of 2 × 2 × 2 µm for the WW, MW and OW samples, respectively, after waterflooding. (A1-C1) Two-dimensional horizontal cross-sections of the raw tomographic image. The phases are represented as oil (black), brine (dark grey) and rock (light grey). (D-F) Three-dimensional rendering of the segmented images. (D1-F1) Two-dimensional horizontal cross-sections of the segmented images. The segmented phases are represented here as oil (red), brine (blue) and rock (white). (A1-C1) and (D1-F1) are sub-volumes cropped out of raw tomographic images and segmented images, respectively. Modified from (1).

Ahmed AlRatrout, Martin J. Blunt and Branko Bijeljic

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Fig. S2. Spatial correlations for contact angle and interfacial curvature (A-C) The spatial correlation of contact angle measurements. (D-F) The spatial correlation of oil/brine interface measurements. We see a correlation length of approximately a pore size. The vertical lines indicate the minimum pore diameter (dotted), average pore diameter (solid) and maximum pore diameter (dashed). Modified from (2).

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Ahmed AlRatrout, Martin J. Blunt and Branko Bijeljic

Fig. S3. Pore-by-pore plots of roughness, contact angle and interfacial curvature variations as a function of pore diameter. The curvature-based roughness (Ra ) is shown in the top row (A-C), the contact angle standard deviation (stdCA) in the second row (D-F) and the oil/brine interface curvature standard deviation in the third row (G-I). The first, second and third columns represent WW, MW and OW samples, respectively.

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References 1. Alhammadi AM, AlRatrout A, Singh K, Bijeljic B, Blunt MJ (2017) In situ characterization of mixed-wettability in a reservoir rock at subsurface conditions. Scientific Reports 7:10753. 2. AlRatrout AAM, Blunt MJ, Bijeljic B (2018) Spatial correlation of contact angle and curvature in pore-space images. Water Resource Research In press.

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Ahmed AlRatrout, Martin J. Blunt and Branko Bijeljic