SPE Abstract. Introduction

Transcription

1 SPE Optic imaging of Oil/Water flow behavior in nano-scale channels Songyuan Liu, Qihua Wu, Baojun Bai, Yinfa Ma, Mingzhen Wei, Missouri University of Science and Technology, Xiaolong Yin, Keith Neeves, Colorado School of Mines Copyright 2014, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Improved Oil Recovery Symposium held in Tulsa, Oklahoma, USA, April This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Enginee rs is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright. Abstract Unconventional oil reservoir has presented the potential to become a significant source of hydrocarbon in the future productions. Many shale oil systems consist of nano-scale pores and micro-scale fractures that are significantly smaller than those from conventional reservoirs. Such small size of the pore throats in shale may differ from the size of the saturating fluid molecules by only slightly more than one order of magnitude. This difference will result in different wettability, fluid flow mechanisms and storage capabilities in unconventional shale oil systems. However, the fluid flow behavior in the micro-scale or even nano-scale pores and channels is poorly investigated and understood. Therefore, it is increasingly important to investigate fluid flow behaviors and the fluids residue saturation in the nano-scale channels. In this work, a lab-on-chip approach for direct visualization of the water/oil flow in nano-scale channels is developed by using advanced confocal microscopy system combined with nano-fluidic chip. A comprehensive study of water/oil flow behaviors is presented. During drainage process, liquids tend to be piston like flow under micro; both residual phase saturation and configuration affect the flow behavior under smaller scale. During imbibition process, a fading out phenomenon was observed which. For residual phase saturation, confocal system can give a precise description within 3-dimensional scale. Introduction Oil and water multiphase flow behavior has been studied for a long time. Variety of methods and equipment has been developed to study these two most important fluids in petroleum industry. Flow condition before, flow pattern during, residual phase saturation after the flow were the major focus of research. While transparent pipelines and tubes on large scale of millimeter and centimeter are more common subjects, in this study the micro- and nano-scale fluidic chips with the application of visualization and three-dimensional reconstruction of confocal microscope were used to overcome the limitation of traditional methods and to describe the flow behavior of oil and water in smaller scale by giving an optical image and even 3- dimensional reconstruction.

2 2 Charles et al. (1961) conducted the first flow pattern and pressure gradient study using 1-inch diameter laboratory pipeline. Trallero et al. (1996) used transparent test section with cm inside diameter and conductivity probes to identify six flow patterns. The flow of oil and water in a horizontal pipe with an inner diameter of 59 mm was investigated by Nädler and Mewes (1997) using high frequency impedance probes to present the effect of emulsification and phase inversion on the pressure drop for different flow regimes. Angeli and Hewitt (1998) measured the pressure gradients during concurrent flow of low viscosity oil and water in two 1-inch nominal bore horizontal test sections made from steel and acrylic resin respectively to show the flow pattern difference between materials. Elseth (2001) studied the behavior of the simultaneous flow of oil and water in horizontal pipes with inner diameter of 2 inches. Later oil-water two phase flow experiments were conducted by Rodriguez and Oliemans (2006) in 8.28 cm-diameter steel pipe to obtain pressure gradient and holdup over pipe inclinations. Yusuf et al. (2011) showed the efficiency of drag reducing polymer to horizontal oil-water flow under pipes with different diameter. Recently, X-ray computed micro-tomography (CT) techniques were used to visualize and monitor two phase flow processes in 3D during in-situ core flooding experiments at pore-scale resolution by Pak et al. (2013). Though the majority of publications about oil and water two phase flow were conducted under larger scale using pipelines or core sample in microscale, our research focused on the smaller scale two phase flow behavior by using fluidic chips with nanochannels of 500 nm depth. Confocal microscopy is an optical imaging technique and its principle was patented early in 1957 by Marvin Minsky (1988). The technique was first brought to petroleum industry by Li and Wan (1995, 1996) to investigate the equilibrium asphaltene particle shape, size and size distribution and then to describe the characteristics of brine-in-bitumen emulsions. In a case study of shale reservoirs, Hampton et al. (1996) identified micro fractures and gas desorption by using confocal microscopy. Ibrahim et al. (2012) showed the fast and accurate method with confocal microscopy to measure pore-body and pore-throat size distributions then simulated capillary pressure curves. Shah et al. (2013) used confocal laser scanning microscopy as one of the tools to predict porosity and permeability of carbonate rocks. These studies mainly focused on a certain object, while our study used confocal microscopy to visualize the flow behavior of oil and water as well as the 3- dimensional reconstruction of residual phase saturation distribution. For flow behavior under nano-scale, most of laboratory experiments were conducted by using shale samples where fluids flow mechanisms and residual saturation were difficult to measure during the experiments. In this study, the flow behavior and residual saturation of water/oil in nano-scale channels were investigated in semi-transparent nano-fluidic channels. Flow patterns of water displacing oil and oil displacing water in 500 nm depth channels were characterized and compared with results in conventional-sized channels. Residual saturation of oil was obtained by the imaging data. This work demonstrated a novel visualization method for the characterization of water/oil flow patterns in nano-scale channels of oil shale formations by using confocal microscopy system combined with nano-fluidic chips. The results of the fluid transportations and residual oil/water saturations in nano-scale channels will be helpful for researchers to understand the mechanisms of fluids transportation in unconventional shale oil system. This technique has the potential to provide an important tool to enhance the oil production and evaluate the residual saturation in unconventional shale oil reservoirs.

3 3 Experimental methods Materials: Deionized water was obtained from the Milli-Q Ultra-pure water system. ACS grade (99.9%) decane was purchased from Fisher Scientific. All solutions were filtered by a 0.22 μm pore size Nylon membrane prior to use. Nile Red was purchased from Invitrogen (Grand Island, NY) to serve as fluorescent dye at the final concentration of 100 mg/l in decane. Nile red has very low water solubility and no fluorescence in the aqueous solution. The emission spectra of Nile red in different solvents is showing in Figure 1. In non-polar solvent, such as Hexane and Decane, the emission is around 525 nm. Figure 1: Emission spectrum of Nile Red in different solvents: (a) n-hexane; (b) Ethyl acetate; (c) Methanol; (d) Water; Experimental apparatus and test section: The experimental system of confocal microscopy combined with the nano-fluidic chip was similar to the previous setup in our published work (Wu et al., 2013). Figure 2 shows the schematic of experimental apparatus. Water and oil were injected from two syringe pumps (Pico plus Elite) from Harvard Apparatus (Holliston, MA) with a controllable injection rate ranging from pl/min to 11.7 ml/min. The injection pressures of both oil and water were measured by pressure sensors. Figure 2: Schematic of the experimental apparatus

4 4 As mentioned above, epi-fluorescence microscopy method was used for images collection. The wavelength of the excitation light is at 488 nm and the emission wavelength is at 525 nm. FITC dichroic mirror was used to filter out the excitation light (incident light) to reduce the background noises. All images were captured by a Nikon TiE motorized microscope equipped with Yokagowa X1 confocal scan head and 4 lasers with AOTF control. The exposure time was set at 500 ms and data were acquired through the Element software provided by Nikon. The schematic of the entire nano-fluidic chip was shown in Figure 3. The nanochannels array consisted of 20 nanochannels with a dimension of 500 nm (depth) 25 µm (width) 365 µm (length) built into the nano-fluidic chip. Two micro channels with a dimension of 25 µm (depth) 20 µm (width) 30 mm (length) for fluid introduction were connected to the nanochannels. The micro channels were perpendicularly connected to the nanochannels to avoid direct injection and each of them had equal lengths in order to balance the pressure drop. Figure 3: Top view of nano-fluidic chip; Nano-fluidic model fabrication The nano-fluidic chip consisting of 2 micro- and nanochannels with the given dimensions were formed in doublesided polished <100> silicon wafers (thickness = 250 µm) with low-stress silicon nitride (~100 nm) on both sides. The fabrication process on the cross sectional view was demonstrated in Figure 4. First, an array of 20 nanochannels spaced 10 µm edge-to-edge was defined by a deep reactive ion etches through the back side of the wafer. Second, two micro channels were defined at either end of the nanochannel array. Third, the inlet and outlet pores were defined at both ends of the micro channels. Each device was removed from the wafer by cleavage along crystal planes. Finally the front side of each device (40 mm 20 mm 0.25 mm) was anodically bonded (330 C, 1 kv, 1 hour) to a thin Pyrex cover slip (Pyrex 7740, 40 mm 20 mm 0.3 mm, Newport Industrial Glass, Inc, Stanton, CA). In order to maintain the surface conditions of the nano-scale channels, the whole nano-fluidic chip was rinsed by lab reagent water, methanol and nitrogen gas prior to use. Experimental procedure Before each set of experiments, preliminary tests were conducted with the nano-fluidic chip; no leakage was observed, and the flow was stable. The nanochip was cleaned by using 0.1mole/L HCl, isopropanol and deionized water prior to use. All experiments were conducted at room temperature.

5 5 Define nanochannels (Width 25 µm, length 365 µm, depth 500 nm) Define Micro channels (Width 20 µm, depth 25µ m) Make inlet and outlet holes (Diameter 1mm) Seal the device by anodic bonding Figure 4: Cross-sectional schematic of Si-Pyrex micro- and nano-channel fabrication process Oil displacing water: The nanochannels were 100% saturated first with water by forced imbibition into the pre-vacuumed nano-fluidic chip; then decane was pressure-injected to displace water. Water displacing oil: For the water displacing oil experiments, the nanochannels were saturated with decane. Decane was then displaced by water at a constant flow rate until a constant oil/water ratio was obtained. Oil saturation was obtained from the image data. Data processing flow velocity Flow velocity was obtained by processing images shown in Figure 5.. Figure 5: image processing using intensity profile axis First, an intensity profile axis was drawn on the nanochannel to be measured. Set the start and end point and adjusted the dip angle of the axis to make sure that it was on the center axis of flow direction.

6 6 Second, read the intensity profile after adding the distance scale and matching the frontier position. Combined the distance data L 1, L 2, L 3... with the time data T 1, T 2, T 3... provided by the image capturing software EIS-Elements, the superficial velocity of displacing phase is calculated as: Eq.1 Data processing saturation determination The phase saturation in the nanochannels was determined by calculation after processing the images. The procedure was demonstrated in Figure 6. Figure 6: Saturation measurements using intensity thresholding method First, the area of each nanochannel on the X-Y axis (A t ) was calculated by the original parameters that had been confirmed by the image capturing software: A t =25*365= µm 2 Second, set the intensity thresholding to select all oil phase and then manually added some weak or abnormal region that had been left out and eliminated the interference of some false signal and micro channel of inlet and outlet side of nanochannel. Then the area data of oil phase in a specific nanochannel (A o ) could be conducted by the software and automatically matched with the images. Thus, the oil saturation (S o ) for each nanochannel was determined by the ratio of area of oil phase to the area of the nanochannel on X-Y axis: Eq.2 After oil saturation is determined, water phase saturation (S w ) was calculated by (1-S o ).

7 7 Results and discussion The study of oil and water two-phase flow pattern started from Charles et al. (1961) with the material of 1-inch pipeline. Most flow pattern study since then focused on either transparent centimeter-scale pipeline or the influence of different transportation media material and inclinations (Angeli and Hewitt, 1998) (Rodriguez and Oliemans, 2006). Due to the larger space of the transportation media, stratified and dispersed flow pattern were observed. In addition, when Reynolds number was larger than 4000, turbulent flow occurred. Flow patterns in rectangular micro channel with width of 300 µm and depth of 600 µm were obtained using CCD camera by Zhao et al. (2006) To study the formation mechanism of slug, monodispersed droplet and droplet populations, Weber numbers of water and kerosene were calculated to predict the flow regime transition and the flow patterns map. In this work, the Reynolds number varies from 1.2 *10-6 to 8.9 *10-5, only piston-like displacing occurred due to the extreme small media size. Weber number was not applicable here because of the lack of discrete phases such as slug or monodispersed droplets. Under this circumstance, velocity profile by demonstrating frontier velocity at the specific position of the nanochannel was used to determine flow pattern together with visualized image of the flow behavior. Flow patterns of both drainage and imbibition processes were detected in water-wet fluidic chip. Oil displacing water The drainage flow behavior study was conducted in nanochannels. After the fluidic chip was cleaned and saturated with water, oil was injected into the model from one inlet with a constant flow rate of 0.05 ml/hr. Because of the limitation of injection pump and the extremely small capacity of the nanochannels, even under a very flow rate of 0.05 ml/hr, directed to go through nanochannels, break through by the oil would take place within less than several milliseconds. Thus for a better observation and control of the flow process, other three inlet and outlet pores were kept open to avoid the sudden built-up of the pressure. In this condition, the pressure of injection kept increasing with at a rate about 0.06 psi/min. The first nanochannel started to flow at 42.7 psi and the break through time for each nanochannel ranged from 14min to 20min. By calculation, for a single nanochannel the pressure change during the entire oil break through time range from 0.84psi to 1.2psi. The deviation comparing to the original pressure was from 1.97% to 2.81%. Therefore we assumed that the pressure didn t change during each nanochannel s breaking through process. During the injection, photos were taken by camera every a few seconds to output the images and data to the software. After gathering all data of oil phase frontier position and time, velocity was calculated using equation 1. Six channels which had the most representative velocity curves were obtained as shown in Figure 7. Note there are 6 different velocity curves in Figure 7. All channels were observed to have an increasing velocity of oil phase frontier except channel e. The reason is that decane has a smaller viscosity than water, therefore the further the frontier, the more saturation the decane, the less the mixture viscosity. In order to explain the differences among the velocity profiles, the time sequence of each nanochannel from a to f listed in Figure 7 was shown in Figure 8. The flow pattern was clear with the help of confocal scanning microscope technique. For the two-dimensional perspective, the focal plane was set at middle layer of nanochannel area which had the strongest signal intensity. As shown below, all the displacing processes were piston like, which means there is only one interface between oil and water and after oil break through the residual water saturation hardly changes

8 Time sequence Time sequence Time sequence Time sequence Time sequence Time sequence Velocity (µm/s) Oil Frontier Distance (µm) a b c d e f Figure 7: Velocity of oil phase frontier as function of oil phase frontier position from nanochannel entrance (a) (b) (c) (d) (e) (f) Figure 8: Optical flow pattern under time sequence for six most representative velocity profile curve: (a) Normal piston-like flow; (b) Residual water drop (c) Linking flow (d) Larger residual water drop (e) Residual water bubble (f) Entrance residual water and multiple residual water drop

9 Velocity (µm/s) 9 The optical images of the time sequence of six channels were displayed in Figure 8. Figure 8 (a) showed a perfect piston-like displacing which leads to a relative stable speed of oil frontier and a smooth acceleration at last. For channel b, it is hard to observe the change because of the improper scale when it is listed with other velocity profiles. Figure 9 plotted velocity as a function of oil frontier distance for channel b where there was a slight and sudden acceleration at 120 µm. Comparing to Figure 8 (b), the optical image for channel b showed a residual water drop as arrowed stick to channel side wall which leadto the shrink of flow area and the enlargement of the flow superficial velocity. In Figure 8 (c), before the oil-water interface proceeded to the end, an oil slug appeared from the outlet side of nanochannel as arrowed. The oil slug didn t change or move until major oil phase reach the distance of 300 µm and suddenly merged with the oil slug and broke through like a linking procedure. The oil slug might be caused by the back flew of the oil coming from other nanochannels. Besides, we noticed that during the merging process the oil phase left a strange shaped residual water area which can also be called unswept zone. Unlike other residual water, this one in channel c might be caused by the increasing superficial velocity when it was unsteady and after some time when it had been swept, the residual water disappeared Oil frontier distance (µm) Figure 9: Velocity of oil frontier as function of oil phase frontier position from nanochannel entrance of channel b In Figure 8 (d), a huge residual water drop occurred at 150 µm. The flow path for oil phase became very small which made oil phase reached a very high superficial velocity to get to the end. There were a few differences between channel d and c including the shape of residual water drop and the stability of the residual water. For channel d the water drop appeared to be sharp at the forward and narrowing at the back. According to Newton s second law of motion, the net force acting on the fluid particle, which was the pressure multiplying the area in this case must equal its mass times its acceleration. Therefore, assumed having a steady flow beyond the point of the smallest flow area which means the pressure does not change with time at given location, each particle slides along its path and its velocity vector is everywhere tangent to the path. The lines that are tangent to the velocity vectors throughout the flow field will form so called streamlines which shown as the narrowing back of residual water. Hence channel c might be the condition that velocity changing caused lower swept efficiency and remaining water, however, channel d might be the condition that velocity changing was caused by flow area reduction as a result of residual water. Velocity profile of channel e in Figure 7 is different from other profiles. Velocity started to increase at 200 µm then drop down at 300 µm. According to Figure 8 (e) oil phase frontier encountered a medium water drop which caused the

10 Time sequence Time sequence 10 increasing of velocity. Then after 50 µm oil phase went around the residual water and cut off the connection between residual water and side wall to make residual water a slender shaped bubble. Due to the inclination which was along the direction of liquid movement and reduction of size, water bubble could hardly serve as an effective resistance. The velocity went down so that the velocity profile completed a peak value. The curve indicates that fluid flow under micro or nano-scale can be affected by residual phase saturation as well as residual phase configuration. In figure 8 (f), it shows the residual water exists at the entrance of channel f which leads to the higher starting velocity. Velocity profile curve increased twice as the oil phase frontier encountered small and large residual water, respectively. Water displacing oil After all nanochannels broke through, oil phase kept injecting until residual water saturation stopped changing. Then water injection began from the same inlet of oil injection. As the material of the fluidic chip was mainly silica which is strongly water wet, the injection rate of water was reduced to 0.02ml/hr. The velocity profile was created but was not showed here due to the fading out phenomenon which will be introduced later. Two major flow patterns were showed in Figure 10. (a) (b) Figure 10: Optical flow pattern under time sequence for water displacing oil: (a) Typical displacing; (b) Fading out Figure 10(a) shows a typical flow behavior of water displacing oil except the velocity did not have much regularity due to the spontaneous flow of the wetting phase water and the heterogeneous of inner surface of channels after the oil soaking and asymmetric distribution of residual water. Figure 10 (b) provides some new information about the fading out of nonwetting phase which is oil. First the water frontier started to move to the distance of about 50 µm; after that oil phase started to fade out while water frontier remained at the same position until all the oil phase disappeared from channel. A possible explanation is that for the imbibition process the wetting phase might form an extra thin layer along the wall that will break through before the frontier. Therefore after forming the water layer, oil layer got thinner and thinner which led to the weakening of the signal strength and then the fading out. After the displacing process, water injection continued until the residual oil saturation stopped changing. Threedimensional reconstruction was used to calculate the residual oil saturation. Due to the 0.2 µm resolution of the focus of microscope system, only three layers of nanochannel area was scanned and shown as Figure 11.The saturation for three layers are 11.3%, 11.2% and 11.2% respectively, calculated using the output data of the software and a total area of μm 2. The

11 11 largest derivation is 1.6%, so it is reasonable to assume there is no significant difference among the layers in terms of residual oil saturation. Figure 11: Residual oil saturation measurements on three layers Conclusion Oil-water liquid-liquid two phase flow behavior through well-defined nanochannels with depth of 500 nm has been demonstrated using confocal microscope imaging system. Velocity profile curve and time sequenced optical image of oilwater flow pattern were developed. During drainage process, unlike fluid flow pattern in larger scale such as pipelines, liquids tend to be piston like flow under micro and nano-scale. Under the experimental conditions, encounter of residual water during the displacing process will increase the superficial velocity while velocity increasing caused by other situations such as linking flow may generate remaining water. Both residual phase saturation and configuration may affect the flow behavior under smaller scale. During imbibition process, a fading out phenomenon was observed, indicating the wetting phase might form an extra thin layer along the wall that will break through before the interface. For residual phase saturation, confocal microscope imaging system can give a precise description within 3-dimensional scale. Acknowledgements Funding for this project was provided by the Partnership to Secure Energy for America (RPSEA) through the Ultra- Deep Water and Unconventional Natural Gas and Other Petroleum Resources program authorized by the US Energy Policy Act of A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. The authors also appreciate the consulting support from Dr. Qu Qi in Baker-Hudges and Leonard Kalfayan from Hess Company.

12 12 References Al Ibrahim, M. A., Hurley, N. F., Zhao, W., & Acero-Allard, D An Automated Petrographic Image Analysis System: Capillary Pressure Curves Using Confocal Microscopy. Society of Petroleum Engineers. doi: / ms Angeli P., Hewitt G.H Flow Structure in Horizontal Oil-Water Flow, International Journal of Multiphase Flow 26, pp , Charles M.E., Govier G.W Hodgson G.W., The Horizontal Pipeline Flow of Equal Density Oil-Water Mixtures, The Canadian Journal of Chemical Engineering Elseth, G An experimental study of oil water flow in horizontal pipes. Ph.D. thesis. The Norwegian University of Science and Technology, Porsgrunn. Guet, S., Rodriguez, O. M. H., Oliemans, R. V. A., & Brauner, N An inverse dispersed multiphase flow model for liquid production rate determination. International journal of multiphase flow, 32(5), Hampton, Thomas J., Thomas J. Hampton Querin, E. Mark, E. Mark Querin McCabe, William D., William D. McCabe Case History: 31S C/D Shale Reservoirs, Monterey Formation, Elk Hills Field, California MS SPE Conference Paper Li, H., Wan, W. K., & Yuan, J Characterization of Bitumen Emulsions By Confocal Scanning Laser Microscopy. Petroleum Society of Canada. doi: /96-29 Li, H., & Wan, W. K Investigation of the Asphaltene Precipitation Process from Cold Lake Bitumen by Confocal Scanning Laser Microscopy. Society of Petroleum Engineers. doi: /30321-ms Minsky, Marvin. "Microscopy apparatus." U.S. Patent No. 3,013, Dec Nädler M., Mewes D Flow Induced Emulsification in the Flow of Two Immiscible Liquids in Horizontal Pipes, International. Journal Multiphase Flow, Vol. 23, No.1, pp Ok, Joeng, Xiaolong Yin, Qihua Wu, Baojun Bai, Yinfa Ma, and Keith Neeves Optic Imaging of Two-phase Flow Behavior in Nano-scale Fractures. Paper presented in SPE Unconventional Resources Conference-USA. Qihua Wu, Jeong Tae Ok, Yongpeng Sun, S. T. Retterer, Keith B. Neeves, Xiaolong Yin, Baojun Bai, and Yinfa Ma, Optic imaging of single and two-phase pressure-driven flows in nano-scale channels, Lab on a Chip, pp ( ) doi: /C2LC41259D Shah, S. M. K., Crawshaw, J. P., Gharbi, O., Boek, E. S., & Yang, J Predicting Porosity and Permeability of Carbonate Rocks From Core-Scale to Pore-Scale Using Medical CT, Confocal Laser Scanning Microscopy and Micro CT. Society of Petroleum Engineers. doi: / ms Trallero J.L., Cem Sarica, Brill J.P A Study of Oil/Water Flow Patterns in HorizontalPipes, SPE Production & Facilities Yusuf, N., Al-Wahaibi, T., Al-Wahaibi, Y., Al-Ajmi, A., Al-Hashmi, A., Olawale, A.., & Mohammed, I. A Effect of Pipe Diameter on the Efficiency of Drag Reducing Polymer in Horizontal Oil-Water Flows. BHR Group. Zhao, Y., Chen, G. and Yuan, Q Liquid-liquid two-phase flow patterns in a rectangular microchannel. AIChE J., 52: doi: /aic.11029