BrineâSand System Using Microfocused X-ray CT - ACS Publications

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In situ local contact angle measurement in a CO2– brine–sand system using microfocused X-ray CT Pengfei Lv, Yu Liu, Zhe Wang, Shuyang Liu, Lanlan Jiang, Junlin Chen, and Yongchen Song Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04533 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 24, 2017

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In situ local contact angle measurement in a CO2–brine–sand system using microfocused X-ray CT Pengfei Lv1, Yu Liu1,*, Zhe Wang1, Shuyang Liu1, Lanlan Jiang1,2, Junlin Chen3,4 and Yongchen Song1,* 1

Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of

Education, Dalian University of Technology, Dalian 116024, China. 2

Research Institute of Innovative Technology for the Earth, Kizugawa City, Kyoto

619-0292, Japan. 3

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing

100190, China. 4

University of Chinese Academy of Sciences, Beijing 100049, China.

Corresponding

author:

Yu

Liu

([email protected]),

Yongchen

Song

([email protected])

Abstract: The wettability of porous media is of major interest in a broad range of natural and engineering applications. The wettability of a fluid on a solid surface is usually evaluated by the contact angle between them. While in situ local contact angle measurements are complicated by the topology of porous media, which can make it difficult to use traditional methods, recent advances in microfocused X-ray computed tomography (micro-CT) and image processing techniques have made it possible to measure contact angles on the scale of the pore sizes in such media. However, The effects of ionic strength, CO2 phase, and flow pattern (drainage or imbibition) on 1

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pore-scale contact angle distribution still not clear and has not been reported in detail in previous study. In this study, we employed a micro-CT scanner for in situ investigation of local contact angles in a CO2–brine–sand system under various conditions. The effects of ionic strength, CO2 phase, and flow pattern on the local contact-angle distribution were examined in detail. The results showed that the local contact angles vary over a wide range as a result of the interaction of surface contaminants, roughness, pore topology and capillarity. The wettability of a porous surface could thus slowly weaken with increasing ionic strength, and the average contact angle could significantly increase when gaseous CO2 (gCO2) turns into supercritical CO2 (scCO2). Contact angle hysteresis also occurred between drainage and imbibition procedures, and the hysteresis was more significant under gCO2 condition. Keywords: wettability; contact angle; pore scale; hysteresis; ionic strength; X-ray computed tomography

Introduction The surface wettability of porous structures is of great importance in a broad range of natural and engineering applications in energy, materials, geophysics, and chemistry science1, 2, 3, such as oil recovery4, 5, 6, filtration membranes7, contaminant transport and remediation8, 9, fuel cells10, 11, and CO2 geo-sequestration12, 13, 14, 15, 16. Understanding the wettability of porous surfaces is a significant step towards understanding multiphase flow or migration processes, phase saturation, capillary 2


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pressure, and relative permeability variations17, 18, 19 in these areas. However, relevant characterization of wetting phenomena in real porous media, especially on the scale of pore sizes, has not been carried out in detail20. Wettability describes the tendency of a solid surface to adhere to one particular fluid rather than other immiscible fluids or gases21. The wetting and non-wetting phases are defined based on the adhesive preference of the solid surface. Wettability is usually determined by the contact angle at the three-phase contact line22, and the contact angle may range from 0° to 180°, which correspond to high and low wettability of the surface. Traditionally, measurements of a contact angle are performed by directly photographing a sessile drop23, 24 of a fluid on a polished and uncontaminated mineral surface mounted in a test cell. The shape of the sessile drop is recorded and then evaluated by different shape analysis techniques25, 26, 27, 28. In a modified sessile-drop method reported by Leach et al.29 and Treiber et al.30, two parallel mineral crystal plates are introduced. To represent pore surfaces, the flats have a crystal composition similar to that of reservoir rocks, and the advancing and receding contact angles are measured by moving the plates. It is easy to test various chemical agents and has a low costs. The captive-drop technique is another sessile-drop method that allows the measurement of both advancing and receding contact angles; it provides more reproducible values than conventional (static) sessile-drop experiments, in which any angle between the advancing and receding angles may be observed31, 32. This method can reflect the complexities of solid-liquid interaction. The Wilhelmy plate method is a widely used sessile-drop method for 3


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determining static contact angles: a plate, whose thickness is much smaller than the other two dimensions33, 34, is partially immersed in the test fluid along one of the larger dimensions; the plate is then hung on a balance and the contact angle is measured. It is a technique that can avoid the line tension effects on contact angle. Gu et al.35 proposed a novel sessile-drop method that analyzes the profile of capillary rise in a cylinder, which can be a powerful alternative tool for accurate measurements of contact angles on curved surfaces; the contact angle is determined by numerically minimizing the discrepancy between the physically observed liquid–vapor interface and the theoretically predicted profile of capillary rise. Rao et al.36 developed a dual-drop dual-crystal (DDDC) technique, in which one drop of a liquid phase and a separate drop of another liquid phase are placed on each of two rock crystals that were previously aged and equilibrated. After a predetermined period of aging, the lower crystal is turned around and the drops are merged together, and the advancing and receding contact angles are measured by shifting the lower crystal sideway37. Among these methods, Wilhelmy plate method and the technique that analyzes the profile of capillary rise in a cylinder have highest accuracy but usually cannot be used in high pressure condition. However, some drawbacks were also involved in these methods. The contact angles need a significant length of time to reach equilibrium. Since the roughness, heterogeneity, and complex geometry of a porous medium, which may strongly affect the wettability of its porous surface, are not taken into account in these methods, there are some difficulties in applying the contact-angle measurements to a real porous medium. As a result, different or even contradicting results can be 4


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obtained by these methods38. Moreover, these techniques can hardly reach the scale of individual pores for measurement of the local contact angles. Recent advances in microfocused X-ray computed tomography (micro-CT) and image-processing

techniques

make

micro-CT

a

powerful

tool

for

microcharacterization of porous media. Excellent reviews of these techniques have been published by Dorthe et al.39 and Steffen et al.40. A large volume of micro-CT-based research has been conducted on pore-scale events in a porous matrix, such as local capillary pressure measurements41, 42, curvature determination43, and investigations on flow mechanism44, 45. However, to the extent of our knowledge, only four studies used micro-CT to measure pore-scale contact angles in porous media. In one such study, Andrew et al.20 firstly determined the local contact angles in an imbibition process in Ketton limestone; they found that the distribution of contact angles resulted from multiple contributing factors of hysteresis and surface heterogeneity on various length scales. In the other study, Lv et al.46 measured pore-scale contact angles using a glass bead pack, and the results showed a wide distribution rather than fixed values. However, in what extend will the parameters, such as ionic strength and phase state which may be confronted in many circumstance47,

48, 49

, and flow pattern (drainage, imbibition), effect the pore-scale

wettability of porous surface still little known currently. In the study reported here, we used a micro-CT scanner and a pore-scale method for in situ determination of local contact angles in a CO2–brine–sand system under various conditions. Two different sand packs were employed as the porous matrix. 5


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The effects of different ionic strengths, CO2 phase state (gaseous or supercritical), and displacement pattern (drainage or imbibition) on the local contact angle distribution were investigated in detail.

Materials and methods Experimental apparatus and materials. A pore-scale imaging system was developed in this study for the investigation of transport phenomena in porous media (Figure 1). The main components include a micro-CT scanner (InspeXio SMX-225CT, Shimadzu, Japan), three syringe pumps (260D, Teledyne Isco, USA), a core holder, an electrical heater (including a carbon film heater and a controller), a heating circulator (F250, JULABO, Germany), and a vacuum pump (RV3, Edwards, UK). The micro-CT scanner captured 1200 projections of the sample at regular angular intervals. It needed 320 s to finish one scan circle, and its resolution mainly depended on the geometric magnification, namely, the source-to-image distance (SID) and the source-to-object distance (SOD). To achieve pore-scale imaging, the SID and SOD were set to 800 and 40 mm, respectively. Accordingly, a 7-µm resolution and a 7.1-mm field of view (FOV) were realized. The local contact angles were measured by using many smaller sub-volumes of FOV. Syringe pumps A and B were used for the injection of CO2 and brine, respectively, while syringe pump C controlled the backpressure of the entire system. The heating circulator was used to control the temperature of the three syringe pumps. The core holder was made of polyether ether ketone (PEEK), which offers both high X-ray penetrability and resistance to pressure; 6


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the inner and outer diameters were 6 and 12 mm, respectively. The holder was heated using a carbon film heater, and the temperature was controlled using an electrical controller. The pumps, pipelines, and core holder were all perfectly covered by cotton insulation. Two unconsolidated sand packs were employed as the porous matrix in this study. One was packed with 0.6-mm-diameter glass beads (BZ06) and 0.4-mm-diameter glass beads (BZ04) with the same mass fractions. The other sand pack was filled with 0.4-mm-diameter quartz sand. The glass beads were nearly spherical, while the quartz sand had almost irregular grains. The glass beads and quartz sands were purchased from Asone, Inc., Japan and a sandpit in Hebei province, China, respectively. They have never been used before. To study the effect of ionic strength on the pore-scale wettability, different concentrations of potassium iodide (KI) solution was used in this study. However, a trade off must be made between salinity range and CT image contrast. In natural formations, the variation of salinities may be confronted in different saline aquifers or same aquifer in different depth in CO2– brine–sand system50. The salinity of saline quifer can vary widely in subsurface reservoirs and can be up to full halite condition51, 52. While in this study, the brine was doped with 3 or 6 wt% KI in the flow experiments in the glass bead pack, while a 0 wt%, 3 wt%, or 6 wt% KI solution was introduced into the quartz sand pack to ensure the image contrast. The detailed measurement conditions are given in Table 1.

Table 1. Conditions for the measurement of local contact angles. Type of sand

CO2 phase

KI concentration (wt%) 7


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pack Glass beads

Supercritical

–

3

6

Gaseous

–

3

–

Supercritical

0

3

6

Quartz sand

Figure 1. Schematic of micro-CT experimental apparatus for in situ measurement of local contact angles in the CO2–brine–sand-rock system.

Flow strategy and image processing. The displacement experiments were conducted as follows: (1) Pre-injection: Glass beads and quartz sands were cleaned with deionized water using a supersonic cleaner. Then the sands were saturated with deionized water for more than 3 days. Before the experiments, the sands were re-saturated by brine. 60 ml of brine (30 PVs) were injected at a flow rate of 0.1 ml/min. The core holder was packed with glass beads or quartz sand. The working stage of the micro-CT scanner was moved to locate a suitable imaging area. The entire experimental system was then evacuated for 2 h. 8


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(2) Injection: The KI solution (60 mL, about the equivalent of 30 pore volumes) was injected into the core holder at 0.1 mL/min to saturate the sand pack. Next, CO2 (9 mL, about 4.5 pore volumes) was injected into the brine-saturated sand pack at a constant flow rate of 0.1 mL/min during drainage under 40 °C, 8 MPa (supercritical state) or ambient condition (gaseous state). Finally, another 9 mL of the KI solution was injected into the sand pack at the same flow rate during imbibition. (3) Post-injection: X-ray scans were continuously conducted during drainage and imbibition. Three successive FOVs were captured in the middle of the sand pack by moving the stage upward to enlarge the investigation area in each procedure. After imaging of the displacement experiments, the grayscale images were processed as follows (Figure 2): (a) The effects of beam hardening on the original grayscale image stacks were corrected by using a surface-fitting algorithm53. Each image stack was then cropped into a small sub-volume for measurement of the local contact angle, and a non-local means filter was applied to remove the salt-and-pepper noise in the image. Finally, edge enhancement was applied to the image stack by using an unsharp masking algorithm. All noise reduction processes were conducted with a three-dimensional (3D) model. (b) The grayscale image stack was turned into a multi-binary image stack by using the watershed algorithm, a local thresholding method that has been proved to work well in multiphase segmentation of images of porous media40. (c) All three-phase interfaces of the CO2–brine–sand-rock system were determined 9


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and indexed based on the multi-binary images after segmentation. (d) The contact points of three-phase interfaces were randomly selected and labeled. The image slice was then rotated to find the normal plane perpendicular to the three-phase interface. The local contact angle at the labeled three-phase contact points was measured using the normal plane. (e) The local contact angles were all determined using the original slices to eliminate thresholding errors. The measurement results were came from various subvolumes of the sand packs and various contact points from abundant three-phase contact line. To ensure representativeness of our measurement results, At least 5 subvolumes were chosen from different location of the sand pack in each condition. A total of more than 3000 contact angles (≥250 under each measurement condition) were measured in this study. All the contact angle images were evaluated by both a computer and a person to ensure the measurement accuracy. Before selecting the images, the length of three-phase interfaces which was smaller than 200 pixels will be filtered by a computer, because small three-phase interfaces may introduce some errors in both segmentation procedure and finding the images perpendicular to the three phase contact point. Then the images, which were ambiguous caused by reslicing procedure, were removed manually.

10


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Figure 2. Procedures of extraction and measurement of a local contact angle: (a) de-noising and enhancing the cropped image stack by using a non-local means filter and an unsharp masking algorithm; (b) segmenting the gray images by using the watershed algorithm; (c) finding the three-phase interface based on the multi-binary images; (d) labeling the three-phase contact points and finding the normal slice that is perpendicular to the three-phase interface; (e) measuring the contact angle by using 11


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the original gray images.

Results and discussion Local contact angle distribution analysis. The local contact-angle distributions after the scCO2–6wt%KI-brine drainage procedure in the glass-bead pack and the quartz-sand pack are shown in Figure 3a and b, respectively. The measured local contact angles had approximately normal distributions in both cases. The results obtained under other measurement conditions also had similar normal distributions (Supplementary Information, Figures S1 and S2). In particular, the local contact angles ranged from 30° to 150° (mainly from 60° to 120°) in the glass-bead system, while they varied from 20° to 130° (mainly from 40° to 90°) in the quartz-sand system. This distribution of measured angles represents a significant difference from the single contact angles obtained by traditional methods under ideal conditions, which were measured using smooth material with a long equilibrium time13. Similarly, the local contact angle in other pore-scale studies also shows a wide distribution20, 54, 55, 56

. In their study, the local contact angles also had approximately normal

distributions. Especially, this phenomenon is very clear in uniform wetting or single mineral crystal condition, while the environment of mix wetting or real rock cores containing various mineral crystals will destroy the normal distribution. We suggest that this difference is related to the attached surface contaminants, surface roughness, the pore topologies and capillarity, which will be explained in detail in a following section. 12


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Figure 3. Distribution of local contact angles after drainage procedure in (a) scCO2– 6wt%KI-brine—glass-bead system and (b) scCO2–6wt%KI-brine—quartz-sand system.

The experiments were performed in unconsolidated sand packs, which mainly consisted of single mineral crystals. However, there were some contaminants among the glass beads or quartz sand (indicated by white dots in Figure S3). Since the wettability of contaminants is usually different from that of glass beads or quartz sand, the surface contaminants inevitably affected the results of local contact-angle measurements. Moreover, though the single mineral crystals can be considered as chemical homogeneity in most cases, the porous surface may not be perfectly homogenous on the microscale because of the inherent vice of sand packs. The local contact angles were sensitive to the distribution of chemical elements on the surface. The distribution of local contact angles may be likely affected as a result. The roughness of the porous surface roughness was another critical factor of the distribution of local contact angles. Surface roughness could affect the interaction of the liquid with the solid surface57. Surface roughness also impact contact angles and 13


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contact angle hysteresis58, 59. As shown in Figure 4, the microscale roughness could result in measurement errors if it was beyond the resolution of the micro-CT scanner. Figure 5 shows scanning electron microscope images of the surface roughness of the sand packs. The surface of the quartz sand was very coarse on the microscale while that of the glass beads was much smoother. So the microscale roughness will impact the wetting properties of quartz sands more significantly than glass beads. Specially, since quartz crystal usually is hydrophilic, the roughness may decrease the contact angle according to the previous study60, 61. The roughness also enlarged the interface between the porous surface and the brine, which made the movement of the brine more difficult during different displacement procedures. Therefore, the range of local contact angles on the glass beads was wider than that on the quartz sand even at a low injection rate because the glass beads had a smoother surface. The difference in roughness between glass beads and quartz sands may also give rise to different hysteresis effects between drainage and imbibition procedure.

Figure 4. The surface roughness enlarged the contact interface and led to measurement errors.

14


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Figure 5. Surface roughness of different porous media, as illustrated by scanning electron microscope images with three different resolutions: (a) quartz sand with significant microscale roughness; (b) glass beads with relatively smooth surface.

The pore topology (pore size, structures, pore body orientation, etc.) and capillarity could also be another cause of the wide distribution in local contact angles (Figure S4). Since contact angles are very sensitive to pressure change, variations in pore topology will result in different local capillary pressure, and therefore lead to the variations in local

contact

angles.

Moreover,

our

measurements

were

based

on

vertical-displacement experiments, in which the pressure dropped along the direction of displacements. The pore topology affected the local distribution of the pressure field through the sand pack, thus also contributing to the wide distribution of local contact angles.

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Effect of ionic strength on distribution of local contact angles. The effect of ionic strength on the wettability of a gas–water–rock system has been studied by traditional methods in a number of studies. However, the results reported by different laboratories are scattered and even conflicting. For example, some studies showed that the contact angle of rocks increases with ionic strength while others reported that the contact angle decreases with ionic strength62, 63. Unlike these previous studies, we measured more than 200 local contact angles under each condition in different areas of porous surfaces to precisely determine the effect of salinity on the wettability of these surfaces. We then calculated the mean value of these local contact angles, and we are confident that the average value represents the general wettability of a porous matrix. To construct the histogram of grayscale contrast in the CT images, only brine solutions containing 0–6 wt% KI were used for the measurements. Figure 6 shows the correlation between average contact angle and the salinity of the KI solution (represented by the KI concentration). Each point in the image is the average value of more than 250 data. And 100 contact angle images were selected to repeat the measurements by 10 times. It is found that the error range of each data is less than ±2.7°. In our study, the average contact angle increased with increasing salinity in all cases; however, the increase was not high as. The increase in contact angle was less than 10° when the KI concentration in the brine solution was increased from 0 to 6 wt%, indicating that the wettability of the porous surfaces in the glass-bead pack and quartz-sand pack weakened with increasing ionic strength during both drainage and imbibition procedures. However, the effect of ionic strength on the wettability was 16


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very mild.

Figure 6. Correlation between the average contact angle and salinity of the KI solution.

Effects of phase of CO2 on distribution of local contact angles. The local contact angles were determined in sand pack systems containing CO2 in different phases—gCO2 and scCO2—where scCO2 has higher density and higher viscosity than gCO2. The phase change had great impact on the wettability of the CO2–brine–rock system, which further affected the transport procedure in the porous medium. Figure 7 shows the distribution of local contact angles during drainage and imbibition procedures under gCO2 and scCO2 conditions in the quartz-sand pack. Local contact angles below 10° were ignored because the brine phase was too close to the pore surface and could introduce significant errors in the measurements. The results showed that the average local contact angle during drainage and imbibition was 33.76° and 43.06°, respectively, under gCO2 condition and increased to 62.02° and 66.54°, respectively, under scCO2 condition. It was also observed that the range of local 17


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contact angles in scCO2 was much wider than that in gCO2. In particular, the local contact angles ranged from 10° to 100° in gCO2 and from 10° to 150° in scCO2, indicating contact angle hysteresis tends to be enhanced with pressure increasing. Therefore, the wettability of the pore surface would be subdued and become more complicated as gCO2 turned into scCO2. Jung and Wang introduced a sessile-drop method to investigate CO2 pressure effects on wettability of silica surface62. They concluded that the wettability of silica surface become weak as gCO2 turned into scCO2, which was consistent with our finding. CO2 will shrink when its phase changes from gCO2 to scCO2. As they reported, the shrinking may occur in two different ways alternatively. One way is in a constant radius of the droplet-base but decreased CO2 contact angle, and the other is in a constant contact angle but shrinking droplet-base. Similarly, CO2 shrinking will also happen in porous media as gCO2 turns into scCO2. However, CO2 shrinking seems more complex influenced by the pore topology. In our study, CO2 shrinking leads to contact angle changed, which can be an explanation of the wide local contact angle distribution under scCO2 condition.

18


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Figure 7. Distribution of local contact angles in various CO2–brine–quartz-sand systems: (a) drainage under gCO2 condition; (b) imbibition under gCO2 condition; (c) drainage under scCO2 condition; (d) imbibition under scCO2 condition.

Effects of different displacement procedures on advancing and receding contact angles. The advancing contact angle (ACA) is generally defined as the contact angle when the liquid boundary advances on a surface, while the receding contact angle (RCA) is defined as the contact angle when the liquid boundary recedes on a surface. The difference between ACA and RCA is defined as the contact-angle hysteresis64. Similarly, for a local contact angle in a porous medium, we define the contact angle during drainage (CO2 displacing brine) as the RCA and the contact angle during imbibition (brine displacing CO2) as the ACA, and the difference between the contact angles during drainage and imbibition is defined as the contact-angle hysteresis. Figure 8 shows the average contact angle obtained under all conditions. The contact angles during imbibition were slightly higher than those during drainage under scCO2 condition, and the contact angle hysteresis phenomenon was more obvious under gCO2 condition. Overall, however, the contact angle hysteresis was not very significant when compared with to results obtained with traditional methods. The insignificant hysteresis is mainly ascribed to the following reasons: (1) A low injection rate was chosen in our experiments. (2) Unlike traditional methods in which a liquid can be moved freely on the surface, 19


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the movement of CO2–brine was restricted by the pore architecture and capillary effects. (3) In the gCO2 displacement experiments, there were large differences in the density and viscosity of CO2 and brine. Thus, phase movement proceeded more easily than the displacement in the scCO2–brine system.

Figure 8. Distribution of average contact angles during drainage and imbibition under various conditions.

Conclusion and perspective In this study, CO2 drainage and brine imbibition experiments were conducted in a glass-bead pack and a quartz-sand pack. A micro-CT scanner was employed for in situ contact-angle measurements in the porous media and the local wettability was studied on the pore scale. The distribution of local contact angles was measured and analyzed in detail under different ionic strengths, CO2 phase (gaseous or supercritical), and displacement procedures (drainage or imbibition). Based on the experimental results 20


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and analysis, the following conclusions can be drawn: (1) There was a wide range of local contact angles in the CO2–brine systems in the glass-bead pack and quartz-sand pack, which can be explained by the interactions of surface contaminants, roughness, pore topology and capillarity. The results indicate that pore-scale wettability is very complicated even in a porous medium of single mineral crystals. Reservoir rocks are usually composed of various mineral types, which vary in wettability. Therefore, a more complicated distribution of local contact angles can be expected. (2) The average local contact angle was used to characterize the effects of ionic strength, CO2 phase, and flow pattern. It was observed that the wettability weakened with ionic strength, but its effect was not very strong. The average contact angle also increased significantly as gCO2 turned into scCO2. Contact angle hysteresis was also found between drainage and imbibition procedures, and the hysteresis was enhanced under gCO2 condition. This can be another explanation of the capillary hysteresis phenomenon. Some issues have not been addressed to in this study. Future studies will consider the effect of injection rate on the wetting characteristics, modeling based on pore-scale wetting characteristics, and the effect of pore-scale wetting on the displacement process.

Acknowledgments This study was supported by the National Natural Science Foundation of China 21


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