Coal-on-a-Chip: Visualizing Flow in Coal Fractures - Energy & Fuels

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Coal-On-a-Chip: visualizing flow in coal fractures Alireza Gerami, Ryan Troy Armstrong, Benjamin Johnston, Majid Ebrahimi Warkiani, Nader Mosavat, and Peyman Mostaghimi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01046 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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Coal-On-a-Chip: visualizing flow in coal fractures

Alireza Gerami1, Ryan T. Armstrong1, Benjamin Johnston2, Majid Ebrahimi Warkaini3, Nader Mosavat4, Peyman Mostaghimi1*.

1

School of Petroleum Engineering, The University of New South Wales, NSW 2052, Sydney,

Australia. 2

Centre for Lasers and Applications, Division of Information and Communication Sciences

Macquarie University, NSW 2109, Sydney, Australia. 3

School of Mechanical and Manufacturing Engineering, Australian Centre for Nanomedicine,

The University of New South Wales, NSW 2052, Sydney, Australia. 4

Oil and Gas Research Centre, Sultan Qaboos University, Muscat, Sultanate of Oman

* Corresponding author: Peyman Mostaghimi ([email protected])

School of Petroleum Engineering Tyree Energy Technologies Building, UNSW Kensington Campus, NSW 2052, Australia 1 ACS Paragon Plus Environment

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Abstract

Geomaterial microfluidics are the next generation of tools necessary for studying fluid flows related to subsurface engineering technologies. Traditional microfluidic devices do not capture surface wettability and roughness parameters that can have a significant influence on porous media flows. This is particularly important for coal seam gas reservoirs in which methane gas is transported through a well-developed system of natural fractures that display unique wettability and roughness characteristics. A coal geomaterial microfluidic device can be generated by etching a fracture pattern on a coal surface by using three-dimensional laser micromachining; however, it is unclear if the resulting surface properties are representative of real coal. In an effort to generate a realistic coal microfluidic device, we characterize coal surface roughness properties from real coal cleats. We then, compare these results to the roughness of the patterns, generated from laser etching. Roughness measurements in real coal fractures show that cleats and microfractures are mostly oriented parallel to the coal beddings rather than perpendicular to the bedding, which is important when selecting coal for fabrication of a microfluidic device since we find that the natural microfractures influence the resulting roughness of etched fractures. We also compare resulting coal/brine/gas contact angles under static and dynamics conditions. The contact angle for coal is highly heterogeneous. Surface roughness and pore pressure may influence the contact angle. With the aid of the coal geomaterial device, the effect of these parameters on coal wettability can be explored and a range of possible coal contact angles can be visualized and represented. The geomaterial fabrication, as outlined herein, provides a tool to study more realistic coal surface properties in microfluidics experiments.

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Keywords: Geomaterial microfluidic chip, Coal seam gas reservoirs, Coal wettability, Micro-CT imaging 1. Introduction Gas production from coal seams is being commercially conducted in many countries such as Australia, USA, Canada, China, and India1. The rate of recovery is highly dependent on reservoir characteristics, the specific pore scale structure, and the multiphase fluid flow mechanisms2-3. Typical techniques for collecting such data include laboratory core-flooding experiments and/or history matching to production data4. However, Zhang, et al. 5 highlighted that the flow behavior and gas/water transport parameters for coal are still far from understood. Microfluidics is a methodology for visualizing multiphase flow in pore structures and obtaining experimental data6-9. For instance, permeability of porous media and the relationship between permeability and effective porosity have been studied via this method7, 10

. Gunda, et al.

11

fabricated a microfluidic chip with a designed network of pore structures

derived from scanning electron microscopy (SEM) images of a rock sample. They measured oil production from the chip, which mimics the standard water flooding process. Polydimethylsiloxane (PDMS), glass, and silicon based devices were previously considered for fabricating the chip7-8, 12-13. The use of a geomaterials in microfluidic chip fabrication is an advanced platform for studying flow within real rock porous media. Traditional microfluidic chips such as glass or PDMS micromodels do not capture the effect of surface wettability and roughness properties on porous media flows. Porter, et al.

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implemented a thin cut of shale rock for fabricating

their microfluidic device. They visualized flow transport and interactions between the fracture wall surface and fluid during multiphase flow displacement. Bowden, et al. 3 ACS Paragon Plus Environment

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developed an efficient technique for the fabrication of mineralogically realistic micromodels. They studied displacement and oil recovery within packed beds of mineral grains. Acid injection process, which is a secondary and tertiary recovery method, was simulated by Song, et al.

16

by employing a calcite rock sample for the microfluidic chip fabrication. They

visualized interactions between the pore network matrix and acidified fluid in these experiments. Meanwhile, there are only a few studies focusing on coal seam gas reservoirs and its specific pore structure and flow properties17-21. In our previous work, images of a real coal cleat geometry was captured and relative permeability data was measured in a (PDMS) device17. Even though this attempt was a step forward in predicting fluid flow in coal seams, it lacked the effect of real coal physical properties. Mahoney, et al.

19

studied a set of methods for

creating micro-channels on a coal surface. They concluded that reactive ion etching (RIE) and laser etching could generate cleat geometries that mimic coal cleats shape and size. Multiphase flow transport in a single straight channel etched on the coal surface was explored and visualized. In another study, the effect of coal rank on wettability was investigated for both drainage and imbibition experiments18. Additional studies have further highlighted that the pore scale displacement processes are highly influenced by wettability22-25. One important parameter, which influences the wettability, is surface roughness26. Solid surface heterogeneously and roughness affect the contact angle value by deforming the shape of the contact line for a multiphase flow transport27. Among the various studies on coal wettability, there are only a few studies that examine dynamic two-phase flow in coal22-23,

28

.

Understanding and studying this key parameter will improve the analysis of gas migration in coal and its importance in the optimization of methane gas recovery from coal seams.


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Herein, X-ray micro-Computed Tomography (micro-CT) imaging is used to obtain a pattern of the microfractures and cleat structure in coal. A laser etching technique is then used to transfer the cleat structure onto a coal surface to generate a geomaterial microfluidic device; the overall workflow is presented in Figure 1. The fabrication process is introduced and important considerations in terms of coal wettability and surface roughness are discussed. Surface roughness for both fabricated channels and real coal cleats is measured and compared. Dynamic contact angles are measured in the fabricated microfluidic device and compared to static data. We find that surface roughness and wettability of coal are highly heterogeneous, which can only be captured by using a geomaterial microfluidic device. This work is a step forward in developing microfluidics with realistic surface roughness and wettability properties, which is essential for studying two-phase flow in coal cleats.


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Figure 1.

Microfluidic workflow for the Coal-On-a-Chip design and fabrication process:

Coal sample is prepared17, fractures (cleats) are captured via micro-CT imaging17, a network of the cleat structure is derived, patterns are laser etched on coal sample, and finally a microfluidic chip (COC) is fabricated by bonding a cover slide to the etched coal substrate and attaching the inlet and outlet ports.

2. Materials and methods 2.1. Coal sample preparation The coal sample is selected from a medium volatile bituminous coal extracted from Moura mine in Queensland, Australia. The coal sample contains both bright and dull bands with porosity of 8% and permeability of 13 mD. Details of the coal properties are explained elsewhere29. Eight blocks with dimensions of 20×15×10mm are cut from the coal sample, and 6 ACS Paragon Plus Environment

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then sliced into 2-3mm thick wafers to be used as substrates. To avoid the influence of existing natural fractures on the designed pattern, the substrates are selected from regions of the coal sample where no natural fracture exists. A drop saw with a sintered wafer blade is used for the slicing. The flat faces of the slices are finely ground to a less than 5µm roughness. Four of the eight blocks are cut parallel to the bedding plane while the other four blocks are cut perpendicular to the bedding plane (Figure 2).

A

B Cross Bedding Cut

Through Bedding Cut

Figure 2.

Coal sample prepared to fabricate the COC device. Schematic view of

perpendicular to the bedding cut (A) and parallel to bedding cut (B).

2.2. Cleat structure design A 25 mm in diameter disk is cut from the same coal sample, which the substrates are obtained from. The natural cleat images of the coal disk are captured using a combined X-ray microComputed Tomography (micro-CT) and Scanning Electron Microscopy (SEM) technique. Details of the imaging techniques have been explained elsewhere29. The network is designed 7 ACS Paragon Plus Environment

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based on a set of face and butt cleats obtained from a fully connected section of coal microCT images (Figure 3). To fabricate a network of fractures connected to the inlet and outlet, the length and direction of fractures adjacent to the boundaries are slightly modified. The entire porous structure is divided into five regions (Figure 3), in which each region includes one inlet fracture adjacent to the reservoir. This will provide categorized areas for data measurement when characterizing the fabricated chip.

Figure 3.

The developed pattern from the coal fracture images. Pattern area has been

arbitrary divided and numbered into five regions. Each region includes a fracture with a specified entrance width.

2.3. Fractures laser etching The instrument used for laser etching is a microSTRUCT-C by 3DMicromac (Chemnitz Germany), located at the OptoFab node of the Australian National Fabrication Facility (ANFF), Macquarie University. This facility houses a super rapid-he picosecond laser and a 3-axis positioning system, which allows machining of feature sizes on the order of 10µm, with sub-µm placement accuracy. The CAD drawing of the coal cleat structure is easily imported into the laser machine and a computer visual interface allows positioning the samples and monitoring the laser machining process.


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The fracture patterns are etched onto the coal with a 5µm-pitch raster fill of the design, using the 532 nm second harmonic generation (SHG) output from the laser, which is delivered by a scanlab galvanometer scanner and a telocentric optic with the focal length of 100mm. Further information on laser etching process can be found in the literature

30-31

. Other parameters of

the laser beam employed to etch the coal are nominally 10 picosecond pulses with a 25 kHz pulse frequency and average power of 1.2 W. These settings result in inferred pulse energy of 48 µJ and a 100mm/s laser spot scan speed. The substrates are located in the laser chamber and the laser target area is controlled and visualized via a camera inside the chamber. Since coal is a combustible material, care is taken during the laser etching process, in particular the short pulse nature of the laser system proved suitable for this task with no adverse secondary burning of the coal. Patterns with visible depth are observed with a single iteration of the etch pattern, and samples with three iterations are also prepared in order to increase the depth of fractures and to see how surface roughness varies with fracture depth. The properties of the etched channels including average height and roughness are measured using a 3D laser microscope (VK-X200, Keyence). The microscope has a resolution of 0.5 nm in the z direction and 1 nm in the X-Y plane, and transmit violet laser lights with 408 nm wavelength. The magnification of the microscope is set on 400x for the measurements. The roughness of all etched coal surfaces are studied and compared for samples cut parallel and perpendicular to the bedding plane. When dealing with roughness data, the measured roughness is the arithmetic mean roughness and indicates the average of the absolute value along the reference length. Average elevation (Zc) indicates the average values of the elevation of each curve element (Zti) along the reference length. In X-Y plane (Eq. (2)) Zxi refers to the elevation in X direction and Zyj indicates the elevation in Y direction.


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1 =

(1)

1 = ( ,)

(2)

Where m and n indicate the total number of curved elements in a rough curve in X and Y

direction respectively. Figure 4 illustrates the fracture height measurement procedure through application of microscope observation and optical profilometry. The average height is calculated by subtracting the etched fracture elevation from the coal surface elevation. In addition to the coal substrates, several coal samples with natural fractures were selected from a similar coal type, and employed for the roughness and height measurements. The fracture width changes along the fracture length according to the micro-CT images and the design. However, variations in fracture width exist due to the fabrication process.


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Figure 4.

Measuring of fracture dimensions. The image relates to the measurements at

the inlet of region 2 with the width obtained from micro-CT image (refer to Figure 3): (A) light blue (bottom of fracture) and yellow lines (top surface) of the etched coal surface before bonding, (B) Absolute profile of etched fracture elevation and the coal surface elevation from a nominated reference line. The fracture height is calculated by subtracting the two profiles, and (C) Fracture depth profile in the traverse direction. A non-uniform profile on the sidewalls is observed.


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2.4. Fabrication of Coal-On-a-Chip A slab of PDMS is used for sealing the etched fractures along the top surface of the geomaterial chip. The PDMS pre-polymer is first mixed with curing agent at a 1:10 ratio, followed by degassing in a desiccator, and further curing in an oven at 80 °C for 2h. It is then, cut to size to cover the entire coal sample and fluid access holes (one-inlet and one-outlet) are punched at a distance of 10.7 mm from each other. 20 µm-thick transparent double-sided adhesive (Research adhesive, USA) is used to attach the PDMS cover to the coal substrate. For achieving a strong bond, PDMS was plasma treated for 4 minutes in an oxygen plasma machine (Harrick Plasma Cleaner, NY, USA) with the power of 200 w. Inlet/outlet holes are punched through the adhesive and then the surfaces of PDMS, adhesive and coal are immediately brought into contact with each other and cured for 2 hours to complete the bonding processes. A thick layer of PDMS is used as a cover to reduce the possibility of damages and air diffusion from the inlet/outlet holes. In a separate process, the designed pattern is transferred on a PDMS based chip. For the PDMS chip fabrication, standard lithography technique is employed to transfer the pattern on a silicon mold, using deep reactive ion etching (DRIE). PDMS is poured onto the mold and cured for 2 h inside an oven at 80°C. The PDMS is then, cut from the mold, drilled at the inlet and outlet holes, Oxygen plasma treated, and bonded to a thick layer of PDMS cover as mentioned earlier. The details of fabricating PDMS based chips have been described elsewhere17. The fluid flow in both the coal and PDMS material is then, compared and studied. 2.5. Experimental setup Deionized (DI) water, mixed with 1.0 wt.% red food color (AmeriColor, Placentia, CA, USA) is used for the fluid injection experiment. The fluid is passed through a 0.45µm filter for eliminating residual particles. To obtain a smoothly flowing liquid, dyed water is stored in 12 ACS Paragon Plus Environment

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a falcon tube initially (50 ml falcon tube, Elvesys, Paris, France) and pressurized to the injection pressure. The flow is injected into the chip at a constant rate of 1 µl min-1 with a capillary number of Ca=2.2×10-6. Capillary number is obtained from Ca=µV/σ, where µ=0.9 cp indicates the viscosity of dyed water, V=12.1 mm min-1 refers to the flow velocity which is the mean value of fluid velocity inside the fractures, and σ=70.4 mN m-1 is the interfacial tension of air and dyed water, which is measured by using the pendant drop method in ImageJ. The interfacial tension of air and dyed water is assumed to be same as the interfacial tension of air and water. The flow rate is controlled via a micro-scale high-accuracy flow sensor (MFS1, Elveflow, Elvesys, France) with the accuracy of 20 nl min-1 for 1 µl min-1 flow rate. Also the pressure variations are monitored via a high-precision pressure pump (AF1 Dual, Elveflow, Elvesys, France), which can generate pressures up to 1000 mbar with a sensor resolution of 122 µbar. An automated zoom Microscope (Axio Zoom.V15, Zeiss, Germany) with a resolution of 1 µm is employed for the flow visualization. High quality images were captured by setting the microscope magnification on 112x and 12x at separate experiments. Fluid is injected in both the COC and PDMS chips at ambient conditions and the recovery process is visualized. Dynamic contact angles of the water/air/coal contact lines are measured from images collected during the injection experiment. Static contact angles are obtained in-situ, by stopping flow injection prematurely. An equilibration time of is allowed to ensure that flow has ceased and near mechanical equilibrium. Stopping flow at several saturations before breakthrough allows for the collects images of interfaces within different spatial regions. The static contact angles are then, compared to dynamic contact angles obtained at the same location within the pattern. The macroscopic contact angles are measured using a modified captive bubble method22, 32. The flat surface of the coal sample is fixed in a holder, which is located in a chamber of water. The sample holder is secured by water and gas-leak-proof fittings and by two nuts on the top wall of the chamber. The


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chamber is then, filled with water, which is also pressurizing the sample. The water pressure is slightly set below the injected gas pressure, so that bubbles can be created. Pressurized gas is released from a nozzle placed below the coal surface and the bubbles are formed, rise up and come to contact with coal surface. A camera is employed to capture images of the gas bubbles on the coal surface, and image analysis is performed using ImageJ software with the drop analysis plugin and the drop snake method to determine the contact angles. 3. Results and discussion 3.1. Coal-On-a-Chip properties We present the coal roughness data for samples cut parallel and perpendicular to the bedding in Table 1. The coal surface roughness column refers to the roughness of the samples before etching while the other columns provide roughness data after the etching process. We assume that the roughness variations due to the fabrication process are similar for samples cut parallel and perpendicular to the bedding. However, the main factor that influence the variation of roughness observed between these samples (parallel cut and perpendicular cut) could be due to the existence of microfractures and fissures. For example, in Table 1 the values obtained for surface roughness indicate a constant value for all regions of the samples cut perpendicular to the bedding. On the other hand, the roughness values are higher with frequent changes among all regions for samples cut parallel to the coal bedding. The average roughness for samples cut parallel to the bedding is 1.7x more than the average roughness for samples cut perpendicular to beds. This can demonstrate that the microfractures are mostly laid on the beddings rather than on the surfaces perpendicular to the beds.


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Table 1.

Measurement results of roughness variations and fracture heights in single-

pass and triple-pass laser cuts single-pass laser cut (µm)

triple-pass laser cut (µm)

Sample Cuttings

Region

Coal Surface Roughness (µm)

Average fracture height

Fracture Surface Roughness

Average fracture height

Fracture Surface Roughness

Samples cut perpendicular to the bedding

1 2 3 4 5

1.1 1.1 1.1 1.0 1.1

112 125 123 127 157

6.3 4.9 8.9 6.7 6.5

330 335 294 355 340

23.2 18.3 33.6 19.5 21.7

Samples cut parallel to the bedding

1 2 3 4 5

1.6 1.1 1.4 1.8 1.5

106 122 139 137 155

12.7 11.2 8.9 10.7 11.5

320 366 389 354 381

87.5 93.1 74 78.8 61.8

Fracture heights are also indicated in Table 1, by calculating the average values in each region. For fractures made with the triple-pass laser cutting (laser was applied three times for etching the fracture), the effect of laser etching non-uniformity is more obvious on the fracture roughness. Specifically, the etched fractures with triple-pass laser cut have an average surface roughness of 23.3 µm, which are 3.5x times more than the single pass laser cut. However, roughness of the etched samples prepared by cutting along the bedding is significantly higher. For example, the roughness for the triple-passes cutting was found to be 7.2x greater than the single-pass cutting (Table 1). Another parameter that could influences fracture roughness is the aspect ratio of the etched fracture. Aspect ratio is defined as the ratio of fracture width to height. In general, the fracture roughness will depend on the resolution of the laser engraving machine, the material to be etched and the aspect ratio of the features30-31. When machine and material parameters are fixed, fracture aspect ratio could result in roughness variations. However, for our measurements, only a weak trend between roughness and lower aspect ratio is found. Alternatively, roughness measurements indicate higher roughness values for greater fracture height, as indicated in Table 1. For large fracture height the laser must pass over the fracture 15 ACS Paragon Plus Environment

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many times to etch the necessary depth and each sequential pass appears to add a greater degree of roughness to the surface. According to Table 1 the etching depth varies between the regions. This is because the laseretching machine is limited to a certain aspect ratio. Choosing appropriate resolution that is relevant to the fracture aspect ratio is the key criterion in selecting the laser-engraving machine. Depth variations in the region 2 inlet channel and in a selected area in region 1 are illustrated in Figure 5 (A, D). Roughness of the etched reservoir is also visualized in Figure 5 (C). In addition, minor fracture depth variation and surface roughness on the coal sample due to device-related limitations can lead to surface roughness variation on the etched surface, as visualized in Figure 5.


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Figure 5.

Characteristics of the etched pattern on coal: (A) depth variation in the inlet of

etched fracture with single-pass laser cutting (located in region 2 of the pattern), (B) Entire pattern etched on coal sample with a representation of the chip and pattern scale (C) Roughness profile of the etched reservoir, (D) fracture depth variation of a selected etched area.

To minimize the experimental error introduced by etching, samples with single-pass laser cut and prepared perpendicular to the bedding plane are considered for further assessment. In addition, the microfracture orientation with regards to the bedding is found to be a crucial parameter in COC fabrication and can be investigated in the future for different coal types. 17 ACS Paragon Plus Environment

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Coal samples with natural fractures are selected from a similar coal type and the arithmetic mean roughness and fracture height are measured. Figure 6 compares fracture roughness versus fracture height for natural coal fractures and fractures generated from laser etching. Natural fracture roughness ranges from 3.9-39.0 µm, with a mean value of 15.4 µm and standard deviation of 10.2. Many of the etched coal samples provide surface roughness values that are within this range. However, we measure the highest degree of surface roughness when coal samples cut parallel to the bedding plane are subjected to a triple-pass of the laser (Figure 6). This is due to the existence of microfractures in samples cut parallel to the bedding and sequential passes of the laser, which provides additional roughness to the cut regions. For all other samples, we find that the roughness of the fabricated fractures is similar to natural coal fracture roughness with an average value of 13.6 µm. Therefore the difference in average roughness between etched fractures and natural coal fractures is 11.60%. In Figure 6 the fractures are categorized according to their orientation to the bedding. As discussed, the results indicate higher roughness value for the fractures oriented parallel to the bedding. The minimum value of roughness measured for fractures etched parallel to the bedding is 8.9 µm. However, according to inhomogeneity of the fractures, a range of roughness values is measured for each particular sample type.


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100

Natural fracture, Prependicular to Bedding Natural fracture, Parallel to Bedding COC, Prependicular, single-pass COC, Parallel, single-pass COC, Prependicular, triple-pass COC, Parallel, triple-pass

90

Surface Roughness (µm)

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80 70 60 50 40 30 20 10 0 0.5

100.5

200.5

300.5

400.5

500.5

Fracture Height (µm) Figure 6.

Surface roughness measurements for selected coal natural fractures. The

fractures are categorized based on the orientation to the beddings. COC fracture roughness is compared to the roughness of coal fractures. 3.2. Fluid flow analysis 3.2.1. Coal-On-a-Chip wettability DI water at a constant flow rate of 1 µl min-1 (Ca=2.2×10-6) is injected into the COC initially saturated with air. As can be observed towards the bottom of Figure 7, fluid enters the widest inlet with the throat size of 64.6 µm. This drainage sequence is expected by the YoungLaplace capillary pressure equation33.


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Injection port

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Coal matrix

Coal cleat

Production port

Air 600 µm Flow direction

Figure 7.

Water

Flow visualization in a coal-based microfluidic chip (COC). DI water is

injected in COC that was initially filled with air.

The dynamic contact angle of this system is determined by performing image processing on high-quality images during fluid flow. The microscope magnification is set on 112x for obtaining snapshots of selected fractures with the resolution of 1 µm. It is known that contact angles vary due to surface roughness and the mineral composition of the surface18,

25, 34

.

Typical apparent contact angles observed during COC drainage are displayed in Figure 8. It is shown that contact angle in the coal cleat structure is not a constant value. The variation of apparent contact angles measured in the COC could be contributed to the nature of the coal surface comprising of different minerals and fractures having roughness variations. On a PDMS chip with the same fracture design and same experiment, a uniform contact angle of around 87°±5° is found for all brine/gas/coal contacts (Figure 9). Contact angle variations due to the intrinsic property of PDMS are considered within the measurement errors. This 20 ACS Paragon Plus Environment

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shows that the conventional PDMS chip fails in capturing the heterogeneous nature of real coal whereas the developed COC provides more heterogeneous surface wettability.

Figure 8.

Magnified image of COC during the displacement test showing contact angle

measurement at different locations in the coal cleats. Variations in the contact angle values are perhaps due to surface roughness, local capillarity, and existence of potential minerals in coal.


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Figure 9.

Uniform contact angle obtained from displacement in PDMS chip with the

same cleat pattern as COC.

A statistical measurement is performed on the dynamic contact angles, visualized based on the flow displacement in COC. A total number of 204 images are selected throughout the experiment as the water-air interface moves through the cleats. Microscope field of view is adjusted to cover the entire pattern (12x magnification), which causes lower resolution images at this set of measurements. Hence, a higher error of 10° is considered and images with errors more than this are excluded. The measured contact angles are categorized based on the regions of the pattern where the data are obtained. The measurements indicate a mean value of 67.5° with a standard deviation of 20.1 for the contact angles visualized in the entire pattern. The data in region 2 have the highest mean value of 78.2° with the standard deviation of 17.7. The lowest contact angle mean value is obtained 61.5° for the flow displacement in 22 ACS Paragon Plus Environment

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region 5 with the standard deviation of 23.1. Based on the categorized measurements, no correlations between the contact angle mean values and fracture surface roughness or average fracture height are observed. The local velocity of the water-air interface is calculated by dividing the flow displacement by the time difference of two sequential images. The highest velocities are obtained in regions 4 and 5, where the main longitudinal displacement occurs. The maximum velocity of 1790 µm s-1 is calculated in fractures of region 4. However, no correlation exists between the interface local velocity and corresponding contact angles. Static contact angles are measured from the three-phase contact point after the fluid 15 minutes of equilibration time to allow for mechanical equilibrium. The data are compared to the corresponding contact angles obtained during the displacement experiment at the same locations. The results indicate higher contact angle values, when the fluid is in a static condition. A mean value of 107.1° is obtained for the static contact angles with all of the data presenting values greater than 90°. It is observed that fluid/fluid interface inverts over the three-phase contact line during dynamic conditions. This results in a dynamic contact angle measurement that suggests gas-wet conditions. This can be due to the stick and slip instabilities of the three-phase contact point as it moves along rough coal surface. We observe very abrupt movements of the interfaces during flow, which could be caused by either mineral difference or roughness along the coal surface that causes pinning of the three-phase contact point. Captive bubble measurement The macroscopic contact angle of the coal sample is measured by calculating the contact angle values of air bubbles produced in a water chamber (Figure 10). The values are obtained as a function of pressure and measurements are taken (1) immediately after the gas bubble comes into contact with the coal surface and (2) after a period of time necessary to achieve an


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equilibrium contact angle. Measurements are taken at 0.3, 2, 4.4, and 6.4 MPa and the temperature is kept constant at 22°C. The data are obtained from two run of experiment by gradually increasing the pressure and gradually decreasing the pressure. For each pressure, snapshots are taken from the system and the average contact angle of the data is presented. The error bar in Figure 10 indicates the maximum variations of the measured data at each pressure level. It is observed that the contact angle reduces as the pressure increases. The highest contact angle is 122°, which indicates a water-wet system for the water/air/coal combination. However, the wettability of the macroscopic measurements can also be influenced by the roughness of the coal surface, coal composition, and the location of the bubbles. For instance, at Pressure 6.34 MPa at the left side of the bubble a contact angle of 159° is measured while the right contact angle value is obtained 152°. The higher variations of contact angle values, found in the dynamic measurement, are mainly due to the following reasons: (1) greater roughness values of the COC patterns (4.9-33.6 µm), in comparison to the flat surface roughness (1.1-1.8 µm) (refer to Table 1) and (2) measurements taken from different regions in the COC where different surface wettability could exist. By increasing the pressure towards reservoir conditions, it is visualized that the contact angle is changing to a stronger water-wet system (Figure 10). Hence, according to the YoungLaplace capillary equation, at high capillary pressure the wetting phase will have a greater tendency to reside in the narrowest patterns, corners, and on the coal surface roughness. At pressures greater than 4.4 MPa, we observe that the initial contact angle measurement is greater than the equilibrium measurement. For the lower pressures the equilibration time required for the captive bubble to equilibrate in the chamber took about 30 min while for the higher pressures the time was much longer (2 h). For example, we measure an equilibrium contact angle of 163° while the initial measurement before equilibrium was 159° for reservoir pore pressure of 6.4 MPa. Surprisingly the equilibrium contact angle changes from 122° to 24 ACS Paragon Plus Environment

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163° when pore pressure is changed from 0.34 to 6.4 MPa. This type of process would suggest that equilibrium contact angles might not be observed in a reservoir near the wellbore where there is a strong pressure gradient.

A

B

180 Immediate Measurement Equilibrium Measurement

170 160

Contact Angle (°)

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150 140 130 120 110 100 90 0.00

2.00

4.00

6.00

8.00

Pressure (MPa)

Figure 10.

Macroscopic contact angle measurement of COC: (A) captured image from an

air bubble contacting the coal surface in a chamber full of water, (B) contact angle measurements as a function of pressure. The values are obtained at start of the experiment (Immediate Measurement), and after a time period for reaching an equilibrium system (Equilibrium Measurement). Error bars indicated on the graph applies for all the measurements.

3.2.2 Fluid flow in COC versus PDMS


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As visualized in Figure 8, water which is the non-wetting fluid (refer to Figure 8) displaces gas (wetting fluid) at a rate of 1 µl min-1 in the COC. The interfacial tension of methane and water is around 73 mN m-1 35, which is similar to the interfacial tension of air and water, which is 72.8 mN m-1 36. Hence, this experiment is a reasonable simulation of methane flow through coal fractures. The main flow occurs in region 5 (as depicted in Figure 3) where the widest fractures with the maximum height exist (Figure 8). In regions 3, 2 and 1, the flow ceases for a few seconds until the pressure increases enough to push the water through the narrower channels. Meanwhile, water will tend to prefer other paths toward the outlet with lower capillary entry pressures. Trapped gas in both dead-end and open channels can be observed in the images. In a repeated experiment, the flow displacement is obtained similar to Figure 11 (A) with the only variations in the total saturation (~3%). The flow paths in the PDMS chip are different from the COC, as displayed in Figure 11. The main longitudinal flow however, still occurs in region 5 with the largest fracture. Trapped gas can be observed in the PDMS chip as well as COC.


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Figure 11.

Comparison of fluid flow in coal cleat structure at water breakthrough: (A) in

COC and (B) in PDMS chip.

Coal surface roughness and the existence of microfractures within the coal surface cause extra resistance to flow in the coal chip, in comparison with the flow in the PDMS chip. In order to overcome the flow resistance due to coal surface roughness, a higher-pressure gradient is obtained in the experiment made on the COC (322 mbar), compared to the backpressure in PDMS chip (250 mbar). The inlet flow rate is always set to be constant during the experiment; however, the pump control unit will adjust pressure at the inlet to keep


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flow rate constant. This higher backpressure causes the water to penetrate into new channels within the structure that are not accessible for the lower pressures applied to the PDMS chip. This results in a higher fraction of pore saturation in COC at the breakthrough (56.4%) and afterwards at steady state condition (63.7%). This is in the range of recovery factors, obtained from literature for methane production37. The total fraction of gas recovery in the PDMS chip is 46.3%, which is considerably less than that of the COC. The 17.4% difference in the saturation of the PDMS chip and COC is due to the wettability variations and surface heterogeneously in the coal chip. The use of geomaterial device can be served as a tool of linking the pore scale properties to fractional production at the reservoir scale.

4. Conclusions Coal cleat fractures in the size range of 14-80 µm were etched into a geomaterial coal surface using laser micromachining. The etching properties were explored and the roughness was compared to the roughness of selected natural coal fractures. The measurements indicate higher roughness when the surface is prepared by cutting the coal sample parallel to the bedding plane. Contact angle measurements indicate a gas-wet system in coal seams that becomes less gas-wet as pore pressure is decreased. The contact angle is also influenced by coal roughness and surface composition that are naturally captured by the COC design. Flow transport in the coal microfluidic chip was visualized and the resulting dynamic contact angles were measured. We found that the COC provides heterogeneous contact angles as expected for real coal systems and that fracture roughness results in higher pressure drops over the COC than the PDMS chip for the same flow rate. It is visualized that there is no correlation between dynamic contact angle and fracture geometry or velocity. Multiphase flow in real coal fractures and the coal surface wettability was studied, using the COC device.


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However, assessments for different coal lithotypes, and the effect of coal compositions and minerals on contact angle measurements are not included in this study, and are addressed for future work. In addition, resolving the entire 3D geometry of a meniscus can be considered for further studies. The COC platform provides an exciting opportunity for studying pore scale processes in coal seams that are highly relevant to gas recovery, such as, swelling and shrinkage, matrix wettability and gas exchange between cleats and matrix. These studies will help us understand the many interesting aspects of complex fluid displacement mechanisms that occur in coal cleats. Acknowledgements This work was performed (in part) at the OptoFab node of the Australian National Fabrication Facility (ANFF), Macquarie University, and the authors acknowledge the collaborations of this organization. References 1.

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