Direct Pore-Level Visualization of Methane Hydrate Growth in an Authentic Sandstone Replicate Stian Almenningen1*, Josef Flatlandsmo1, Martin A. Fernø1, Geir Ersland1 1 University of Bergen, Norway *Corresponding author:
[email protected]
Abstract Direct pore-level visualization of sedimentary methane hydrate growth is presented in this paper. A silicon micromodel was used as porous media with pores and grains replicating the pore network of a thin section of natural sandstone. A transparent glass wafer on top and vertical pore walls with constant height of 25 µm allowed for direct pore-scale visualization under controlled pressures and temperatures. Initial growth occurred at the interface between liquid water and methane gas and continued into the gas phase. Methane hydrate displayed different growth patterns and final textures for different p-T conditions. Low driving forces led to homogeneous hydrate layers on the original gas/water interface whereas increased driving forces resulted in dendritic hydrates with sharper edges. Flow tests confirmed gas permeability in hydrate saturated pores showing that multiple phases could exist simultaneously through the depth of the model. The degree of cementation between hydrate and solid grains seemed to be independent of driving forces. The visualizations of sedimentary hydrate growth for different driving forces presented in this study offer new insight on growth patterns, wetting properties and relative permeability in porous media. Pore-scale behavior of methane hydrate is of great importance when assessing the original state and nature of hydrate-bearing sediments. In turn, phase distributions on pore-level will govern the sediments’ response and gas/water mobilization in all proposed production schemes of methane gas from hydrate deposits.
Introduction Research on flow and phase behavior in porous media, including studies of sedimentary methane hydrates, have mostly relied on measurements on core samples without direct information of the pore space. Improved in situ information has followed from advancements of medical equipment like CT PET and MR imaging, but these measurement techniques are costly to operate and are usually limited by a spatial resolution that cannot identify individual pores. However, pore-scale observations are made possible by microscope examining of glass micromodels. Direct monitoring of single pores enables valuable information regarding hydrate growth pattern and morphology. Knowledge of hydrate distribution within pores are vital to address important issues related to exploration and production of hydrates in nature, such as permeability and flow control, slope stability and interpretation of seismic data. The first depiction of hydrate growth in synthetic porous media was presented by Tohidi et al. [1]. They demonstrated that hydrates can form in systems without the presence of a free gas phase (CO2) and that methane hydrate growth initiated at the gas/water interface. Further crystallization encapsulated the gas bubbles and because methane was the limiting component, all gas was converted to hydrate. Methane hydrate growth from water-filled pores was not observed, nor in the pore-level hydrate formations conducted by Hauge et al. [2]. No cementing of pore walls was detected as the hydrate tended to concentrate in the center of pores with water films wetting the solid surface. Katsuki et al. [3] used straight rectangular microchannels with a top width of 100 µm to investigate methane hydrate growth at different degrees of subcooling. The channels were carved into a quartz glass plate and arranged in a grid pattern at regular intervals. Hydrate was formed from presaturated water and gaseous methane by cooling at constant pressure (101 bar). Low degree of subcooling led to growth of faceted hydrate initiating at the interface between
liquid water and gaseous methane. Further growth continued into the water phase and some of the faceted crystals were observed bridging the microchannels with no visual (resolution down to 1 µm) water between the hydrate and solid surface. Increased subcooling resulted in initial hydrate growth in the water phase and formation of dendritic hydrate crystals turning eventually into a faceted morphology. The dendritic crystals did not bridge the channels. Variations of initial hydrate morphology with different degrees of subcooling was explained by the difference in methane concentration in liquid water with temperature. Increased subcooling enhanced the mass transfer of methane from the bulk of liquid to the existing hydrate surface, and has also been observed in bulk hydrate experiments [4,5]. This work is a continuation of the hydrate formation experiments presented in Hauge et al. [2] and gives visual evidence of hydrate morphology at different driving forces. Exact replication of sandstone material and possibilities for high pressures and low temperatures, make the experimental conditions close to natural hydrate environments. Hydrate has been formed from free gas and unsaturated water, as in contrast to Katsuki et al. [3], and approximates one proposed method in which hydrate formation takes place in earth: formation from free or recycled gas travelling upward into the hydrate stability zone [6].
Materials and Methods High pressure pumps (Quizix Q5200), one filled with methane gas (>99.5%) and one filled with distilled water, were connected to opposite corners of the micromodel through PEEK (polyetheretherketone) capillary tubing (Fig. 1). The micromodel consisted of a glass plate glued together with a silicon wafer through anodic bonding. An authentic pore network (replicate of thin section of natural sandstone) was carved into the silicon wafer through DRIE (deep reactive ion etching) and had an average pore size in the order of 100 µm with a constant height of 25 µm (Fig. 2). The anodic bonding technique resulted in a water-wet model [7]. A steel casing was used as confinement (Fig. 2) and the mounted model was submerged in still water for temperature control. Constant temperature was applied by circulation of antifreeze through a separate outer chamber adjacent to the still water. The model was cleaned with water prior to each experiment and then saturated with equal parts of water and methane gas. The water pump was shut off and the pore pressure was maintained at the desired level by the gas pump. Hydrate formation was initiated by decreasing the temperature and hydrate growth commenced with constant pressure and temperature. Still pictures and video were recorded during growth by a camera (Nikon D7100) connected to a microscope (Nikon SMZ 1500) situated above the micromodel. The pore pressure was logged at the gas pump and a thermocouple (Omega) located directly below the model in the still water provided temperature readings.
Figure 1: Experimental setup
Figure 2: (Left) Part of the micromodel filled with water and methane gas. The width of the pore network in this figure is 850 µm. (Middle) Bottom-side of the micromodel steel cover with connection ports and (right) top-side of the micromodel mounted in the steel cover. The complete micromodel has a width of 3.6 cm
Results and Discussion Hydrate formation started at the gas/water interface for all experiments conducted in this study. Homogeneous hydrate growth from dissolved methane in the water phase was not observed, neither as an initial growth mechanism nor as secondary formation after complete consumption of free gas. The degree of subcooling was in fact higher for some of the formations performed in this study (p = 145 bar and T = 0.6°C), compared to Katsuki et al. [3] (p = 125 bar and T = 1.8°C) who observed growth in the water phase. This clearly shows the importance of presaturating the liquid water with methane in order to get hydrate growth from dissolved methane. After the initial growth at the fluid interface, further growth proceeded into the gas phase and encapsulated the whole gas bubble (Fig. 3). Note that sometimes the gas bubbles were completely consumed whereas other times it remained a gaseous core surrounded by the hydrate layer. The different configurations were determined from the color of the hydrate layer.
Black hydrate layers (Fig. 3) contained free gas encapsulated inside the hydrate structure and brighter hydrate layers (Fig. 4) contained no free gas. Different coloring resulted from the difference in refractive index between gas, water and hydrate, as discussed in Almenningen et al. [8]. This phenomenon was also verified by injecting gas into parts of the micromodel filled with transparent hydrate layers surrounded by water; the displacement of water by gas led to an immediate change of color of the hydrate layer from pale grey to almost black.
Figure 3: Sequence of pictures showing hydrate formation at low driving forces (p ≈ 50 bar and T ≈ 4°C). The growth ceased to continue after approximately five minutes and free gas was encapsulated inside the hydrate layers giving the hydrate phase a dark color. The growth occurred rapidly despite the low driving forces because it was a secondary formation and the memory effect was present
Figure 4: Hydrate formation at low driving forces (p = 41 bar and T = 3.9°C). The formation was finished after one day and the transparent hydrate layer indicates complete consumption of gas in the field of view
Low driving forces (p = 41 bar and T = 3.9°C) led to homogeneous hydrate layers with a grainy texture (Fig. 4). The consumption of gas was slow, it could often last for several days, and the initial hydrate growth followed the gas/water interface. Some gas bubbles retracted as a consequence of gas consumption but generally the phase interfaces remained static throughout the growth process (Fig. 4). The hydrate morphology became columnar and dendritic (Fig. 5) when the driving forces were higher (p = 145 bar and T = 0.6°C), just as Katsuki et al. [3] observed in their work. The onset of growth was associated with water and gas displacing each other and the original fluid interfaces were seldom preserved. The growth process was usually completed within three hours. Also the growth mechanism was changing for increasing driving forces. Growth at low driving forces started at the vertical gas/water interface and continued inward to the gas along the fluid interface (Fig. 3). At higher driving forces, the growth started simultaneously at all gas/water interfaces; not only at the vertical interface but also at the horizontal interface between free gas and water films wetting the top glass plate and bottom silicon wafer. These water films have lower chemical potential than the bulk water occupying entire pores and will influence the growth process more profoundly at high driving forces. Water transport along the water-wet solid surface did thus increase the overall growth rate at elevated driving forces. Isolated gas bubbles were usually not entirely consumed since the pressure inside the encapsulated gas decreased rapidly as the hydrate layer continued to grow, and the pressure may have been lowered to the stability pressure. Parts of bigger gas clusters could however be totally consumed as the pressure was maintained by inflow of nearby gas. The amount of gas that was consumed during growth depended on the degree of subcooling and for how long the hydrate was allowed to grow. Hydrate formation at high driving forces could crystallize entire gas bubbles as shown by the particulate hydrate in Fig. 5. It was not observed any free water between grains and hydrate at vertical contacts and potential water films had to be thinner than 4 µm (lowest thickness visible at given resolution). Water films between grains and free gas were not observed neither although they had to exist because of the water-wet solid surface. The interface between hydrate and grains was probably separated by similar water films but this could not be verified experimentally. The degree to which the hydrate was cementing the solid surface is therefore unclear. However, the hydrate did usually grow to fill entire pores horizontally, single hydrate crystals floating around in water were never observed.
Figure 5: Comparison of hydrate morphology made at low (left) and high (right) driving forces at the same field of view. The hydrate formation at low driving forces (p=41 bar and T=3.9°C) took one day whereas the hydrate formation at high driving forces (p=145 bar and T=0.6°C) was finished after three hours. The picture to the right shows that the hydrate morphology is characterized by straight lines and sharp edges, but particulate hydrate crystals (shown by three red arrows at the top) are also found
Conclusions Methane hydrate was successfully formed inside a replicate of porous material from liquid water and gaseous methane. The pore-level hydrate growth was visualized and showed that the initial growth started at the gas/water interface. Further growth continued at the fluid interface and resulted in encapsulation of gas bubbles; sometimes the gas was locally consumed whereas other times a core of free gas was preserved inside the hydrate. Growth at low driving forces (p = 41 bar and T = 3.9°C) led to homogenous hydrate layers with little disturbance in the original fluid interface. High driving forces (p = 145 bar and T = 0.6°C) resulted in faster growth and hydrate textures characterized by straight lines and sharp edges. Water films between hydrate and solid grains were not observed (fluid phases could be differentiated from each other down to a horizontal length of 4 µm), but did probably exist because of the water-wet nature of the solid surface.
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