To study physical properties of methane gas hydrate-bearing sediments, it is necessary to synthesize laboratory samples due to the limited availability of cores from natural deposits. X-ray computed tomography (CT) and other observations have shown gas hydrate to occur in a number of morphologies over a variety of sediment types. To aid in understanding formation and growth patterns of hydrate in sediments, methane hydrate was repeatedly formed in laboratory-packed sand samples and in a natural sediment core from the Mount Elbert Stratigraphic Test Well. CT scanning was performed during hydrate formation and decomposition steps, and periodically while the hydrate samples remained under stable conditions for up to 60 days. The investigation revealed the impact of water saturation on location and morphology of hydrate in both laboratory and natural sediments during repeated hydrate formations. Significant redistribution of hydrate and water in the samples was observed over both the short and long term.
Gas hydrates (herein called “hydrate” or “hydrates”) are non-stoichiometric inclusion compounds formed from a network of hydrogen bonded water molecules encapsulating small gas molecules [
Hydrate redistribution in porous media following initial hydrate formation has been observed at the grain scale in a number of visualization experiments [
Repeated hydrate formation has often been reported to yield a “memory effect.” This is a phenomenon whereby water molecules retain a “memory” of their previous clathrate structure after dissociation at moderate temperatures. This retention of structure means that hydrate crystals can nucleate more easily from the same water body than when it was originally formed [
To understand the formation and growth patterns of methane hydrate in natural sediments, a number of experiments have been performed using a core sample from the Mount Elbert Stratigraphic Test Well located in the Alaskan permafrost. In addition, hydrates were also formed in packed sand samples to allow comparison of hydrate formation in natural and reformed sediments. The aim of the experiments on the two sediment samples was to observe repeated hydrate formation and dissociation using X-ray computed tomography (CT) scanning, investigating the evolving distribution patterns of methane hydrate and also the behavior of hydrate during prolonged time inside the hydrate stability zone. The tests also provided a unique opportunity to investigate whether the memory effect can control the location of hydrate within sediments.
The natural sediment used in the test sequence was recovered from the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well. The Mount Elbert well is located in the Milne Point oil field, on the north slope of the Brooks Range in Northern Alaska. The core used in this series of tests came from 660 m below surface level, part of Unit “C” of the reservoirs sands prevalent in the Milne Point region (Sample HYPV3 in Kneafsey et al. [
After collection, the hydrate-bearing sample was placed in a pressure vessel and pressurized with methane to maintain hydrate stability. The vessel was stored under pressure outdoors in the arctic winter and later moved to a freezer. Prior to shipment, the pressure vessel was rapidly vented and the sample was removed and placed in liquid nitrogen (LN: normal boiling point −196°C). Once received at the laboratory, the core was removed from LN and the ends of the core were rapidly cut flat. A belt sander was used to remove the outer layer of the sample in many steps, with sequential cooling by submersion in LN to keep the sample cold. Similarly, holes were drilled in the ends of the sample to accommodate thermocouples.
The core sample was encased in a custom-manufactured nitrile butadiene rubber sleeve. Poly-vinyl chloride caps were placed on the ends of the sample inside the sleeve and secured with wire to ensure a snug fit. Thermocouples were inserted into the sample from each end through the end caps to obtain temperature readings from locations inside the sample. A thermocouple was attached to the outside of the rubber sleeve to monitor confining fluid temperature. The core was then placed inside the pressure vessel, and confining pressure was applied.
The pressure vessel used was an X-ray transparent 8.9 cm outside diameter aluminum tube with threaded stainless steel end caps (Figure
Schematic of the pressure vessel used in the Mount Elbert core tests.
CT scanning was performed using a modified Siemens Somatom HiQ medical X-ray CT scanner. The X-ray CT scanning produced axial (
CT scanning image presentation for Mount Elbert core section. 54
Prior to the tests described here, the original natural hydrate in the sample had been dissociated by depressurization (the results of this dissociation are described in Kneafsey and Moridis [
Grain-size and XRD analysis for Parts A and B of the Mount Elbert core.
The test sequence consisted of two phases, each with two tests. In the first phase, the aim was to examine changes in the distribution of methane hydrate in the Mount Elbert core during repeated formation of the hydrate after only a couple of days in the stability zone. Prior to this first test, the Mount Elbert core had been out of the hydrate stability zone for over 24 hrs. The first hydrate formation (Test ME1.1) was initiated by increasing the pore pressure to 4.4 MPa (confining pressure to 5.4 MPa). Hydrate formation began after 13 minutes in the stability zone, marked by an increase in temperature inside the sample. Temperature returned to the set value after ~15 hours, and hydrate formation was assumed to be complete. The sample was kept in the hydrate stability zone for 2 days before dissociation by increasing the temperature to 10°C. The sample was then held out of the stability field for another 24 hours before hydrate was reformed by decreasing the temperature to 4°C (Test ME1.2). Once inside the stability zone, hydrate formation did not occur for 80 minutes, with temperature and pressure stabilizing after 11 hours. The sample was held in the stability zone for 3 days before dissociation by allowing temperatures and pressures to be equilibrated to ambient conditions.
The second phase of the Mount Elbert core test sequence aimed to maintain hydrate in the core under stable conditions for extended periods of time. Hydrate was first made via pressurization at a cool temperature. Initially, the temperature of the core was lowered from laboratory ambient temperature to 4°C, with the pore pressure of the system at 2.9 MPa (confining fluid pressure at 4 MPa). These initial conditions were outside the methane hydrate stability zone. Hydrate formation was initiated by increasing the pore pressure to 4.8 MPa (confining pressure 5.5 MPa). Hydrate formation began after only 9 minutes inside the stability zone, indicated by an increase in internal temperature of more than 1°C. After 10 hours, the temperature inside the core returned to the applied temperature (~4°C), and hydrate formation was considered complete. The core sample remained in the hydrate stability zone for 60 days, during which pressure and temperature were monitored and X-ray CT scans were conducted at intervals of 3 to 10 days (Test ME2.1).
After ~60 days, the hydrate was dissociated. The pressure inside the vessel remained at ~4.8 MPa, since the produced methane vented to a large, pressurized bottle. These conditions are outside of the stability zone for methane hydrate. After several days, the temperature of the system returned to 4°C, thereby reforming methane gas hydrate in the core via cooling. X-ray CT scanning revealed that hydrate had formed inside the core, and the sample was allowed to remain under hydrate stable conditions for 41 days for continued observation (Test ME2.2). Table
Summary of tests conducted on the Mount Elbert core.
Sample phase no. | Test no. | Formation method | Pressure start (MPa) | Pressure end (MPa) | Temp | Time in hydrate stability zone | Time to hydrate formation | Hydrate saturation |
---|---|---|---|---|---|---|---|---|
1 | ME1.1 | Pressurization | 4.36 | 4.14 | 3.34 | 2 days | 13 minutes | 60.3% |
ME1.2 | Cooling | 4.41 | 4.16 | 3.50 | 3 days | 95 minutes | 59.3% | |
2 | ME2.1 | Pressurization | 4.83 | 4.58 | 3.99 | 60 days | 9 minutes | 59.5% |
ME2.2 | Cooling | *a | *a | ~4 | >42 days | *a | 60% *b |
*Equilibrium temperature for methane hydrate at given pressure (°C).
*a A power failure temporarily stopped data logger.
*b Estimated from CT scans.
During each test the system remained isolated so that the amount of methane consumed by hydrate formation could be calculated from the pressure drop inside the sample and methane bottle. Knowing the temperature of the bottle and sample, the measured pressure drop could be converted to moles of methane consumed by hydrate formation, assuming a constant water to methane molecular ratio of 6 and the methane hydrate density to be 0.917 g cm−3 [
The spatial distribution of hydrate saturation was calculated through analysis of the CT data combined with the mass balance [
A predetermined volume of water and fine silica sand (F-110: 99.8% silica, rounded to subangular grains, 120
Experiments were performed to observe hydrate distribution during formation and dissociation within two sand packs (SP1 and SP2). The first tests examined the impact of differing time periods between the end of dissociation for preexisting hydrate and the start of new hydrate formation (Tests SP1). The time periods were varied from 19 hours to 48 hours. The second tests with SP2 were performed to check the repeatability of hydrate distribution patterns (Tests SP2). The time gap between the first dissociation and the second formation was 15.3 hrs. SP1 and SP2 had similar initial water saturations (37% and 26%, resp.) and porosity (34%), which was uniform throughout the cores (Table
Measured properties of packed sand cores.
Parameter | SP1 | SP2 |
---|---|---|
Cylinder dimensions: | ||
Diameter, | 51 | 51 |
Length, | 188 | 188 |
Pressure (MPa) | 8.3 | 8.3 |
Temperature (°C) | 8 | 8 |
Porosity, | 0.34 | 0.34 |
Initial water saturation, | 0.37 | 0.26 |
*Hydrate saturation, | 0.38 | 0.28 |
*100% water-to-hydrate conversion is assumed.
As described in Section
Hydrate began to form 13 minutes after pressurizing the core in Test ME1.1 (Table
Methane hydrate formation at location 111 mm in the Mount Elbert core. (a) Test ME1.1—pressurization. (b) Test ME1.2—cooling. Slices in both figures were taken at 10-minute intervals after the initial onset of hydrate formation was observed in the pressure/temperature data for each test.
In the packed sand sample SP1, hydrate started forming within 60 minutes after application of pressure and temperature (8.3 MPa and 8°C) in each of the formation events. It was found, however, that hydrate formation initiated faster after hydrate had once been formed in the core. Subsequent hydrate formation began almost instantaneously as the pressure inside the core reached stability conditions, regardless of the time period left between consecutive dissociation and reformation events. Because of this, no relationship could be determined between induction time and the time period between hydrate dissociation and reformation in the same core. Figure
Hydrate formation distribution patterns on four consecutive formation events in packed sand sample SP1 at 8.3 MPa and 8°C. The time periods between dissociation and consecutive formation were varied: 26 hrs between 1 and 2; 19 hrs between 2 and 3; 48 hrs between 3 and 4.
Two additional hydrate formation events were conducted in SP2. Figure
Methane hydrate formation in sample SP2. (a) First hydrate formation in SP2. (b) Second hydrate formation in SP2 after previous hydrate dissociation from (a).
Figure
The first phase of this test sequence (Test ME2.1) showed hydrate formation initiating after only 9 minutes in the stability zone. A scan was taken 2.5 hours after the onset of hydrate formation and showed hydrate forming in localized regions throughout the core. Another scan performed after 24 hours in the stability zone, showed that hydrate formation had evened out across the core. These first scans showed that, when hydrate formed at specific locations, other areas were depleted of water, lowering the density locally in those areas. After 24 hours, hydrate formation had spread throughout the core. Figure
Profile of the average change in density of each slice at various times, from slices 5 to 48.
Over the course of Test ME2.1, temperature fluctuations ~0.5°C consistently occurred due to the daily ambient temperature changes in the laboratory. These fluctuations were well within the hydrate stability zone and were not accompanied by unexpected increases or decreases in pressure, which would indicate hydrate dissociation or formation, respectively. To investigate this further, the temperature in the control bath was dropped by 0.9°C, corresponding to a drop in pore pressure of 0.07 MPa on day 33 of the test sequence. The temperature was maintained at this lower value for the remainder of the test, placing the hydrate under more stable conditions. The scans from before and after this change (days 30 and 41 in Figure
The first CT scan in Test ME2.2 was taken ~9 days after the core returned to hydrate-stable conditions. After 9 days, methane hydrate was clearly present in distinct patches across the core. In this test, hydrate appeared to form in a more heterogeneous manner in Part A of the core than in Part B, as well as forming concentrated hydrate masses, in contrast to the relatively dispersed nature in Test ME2.1. Over the 41 day duration of Test ME2.2, systematic CT scans were taken to monitor any changes in distribution. Over the first 28 days, several hydrate masses appeared to increase in density into distinct “ring” structures (Figure
(a)
The first phase of testing was conducted to investigate repeated hydrate formation in a natural core sample and in a sand pack. Figure
One factor possibly controlling the varying hydrate morphology in the repeated tests could be the water distribution following hydrate dissociation. To examine this effect, water saturation in the cores was analyzed before and after hydrate formation. In both the natural sediment core and the hand-packed sample, water was more concentrated in certain areas after hydrate dissociation. When this water saturation variation was compared to subsequent hydrate formation in each sample, no correlation was observed in the Mount Elbert core from the end of test ME1.1 to the hydrate formation in ME1.2. However, the comparison in SP2 showed that hydrate formed more predominantly in areas where there was lower water saturation following hydrate dissociation. Figure
A comparison of hydrate nucleation locations with water saturation; (a) hydrate saturation distribution after the second hydrate formation in SP2 with black dots for the center of high hydrate saturation area indicating hydrate nucleation locations and (b) water distribution before the second hydrate formation with the black dots of hydrate nucleation superimposed.
Hydrate nucleation is thought to be a process with a significant random component [
When water is in contact with a hydrophilic surface such as silica, water molecules will orientate in response to the charge on the surface of the mineral [
Although the hand-packed sand samples appear to show a correlation between water saturation and the location of reformed hydrate, the natural sediment samples did not show the same relationship. Distribution of water before and after each hydrate formation event was analyzed, with no correspondence seen throughout the core. Other factors affecting the nucleation probability in the Mount Elbert core must therefore be considered, such as salinity variations, grain distribution, and thermal history.
The original salinity of the pore water in the Mount Elbert core was ~4 ppt [
It is also possible that thermal history may play a role in influencing the nucleation of hydrate across a sediment core, in addition to salinity variability and saturation variations. Tests ME1.1 and ME1.2 do not share the same thermal history with regard to applied conditions for methane hydrate stability. Test ME1.1 began by cooling the sediment to 4°C at a pressure outside the stability zone and once temperature had stabilized, increasing the pressure to within the stability zone (4.4 MPa). Alternatively, Test ME1.2 started with the core at 4.4 MPa pressure but at 10°C, before instigating hydrate formation by cooling the core to 4°C. The induction times for these two tests may indicate the difference in thermal history experienced in the core. Hydrate began forming 13 minutes after application of stability conditions in Test ME1.1 but took 80 minutes to begin forming in Test ME1.2. In addition, in Test ME1.2, the outer regions of the core would have been in the hydrate stability zone longer than the inner regions because of heat transfer.
When considering whether the thermal gradient inside the core could be a factor, examination of the temperature data from the outside and inside the core shows that it takes ~14 minutes for temperatures on the outside of the core to be transmitted to the center. Temperature data from Test ME1.2 also shows that internal temperatures had equilibrated with the outer edge of the core prior to hydrate formation, so it would seem that temperature gradients do not directly affect formation of hydrate here. However, they may contribute to the water distribution. Analysis of the CT scans gives no indication of thermal history control on distribution, with hydrate forming both at the edges and inside of the core in equal measure.
One observation that can be made from the repeated tests on the Mount Elbert core is the significance of the random aspect of hydrate nucleation. Sloan and Koh [
The memory effect can have a strong influence on the induction time of hydrate nucleation [
The memory effect has been linked to the timing of hydrate formation, but not to its location. The memory effect does not affect the location of hydrate formation in repeated tests. Water distribution is shown to have the most control over hydrate location in the packed sand samples. In the Mount Elbert core, no correlation can be seen between old and new hydrate formation in the cores, indicating that prior hydrate formation has no effect on location of hydrate nucleation.
Because the sand in our samples was held in place by a combination of cementation and effective stress, density changes were caused by (1) incorporation of methane into water, resulting in hydrate increasing the mass in a voxel of fixed volume or (2) the change in location of water or hydrate within the pore space. Density redistribution was seen to occur over two timescales: (1) short term, over the hours it takes for hydrate to fully form (Tests ME1.1, 1.2, 2.1, and SP2), and (2) long term, over, several weeks (Tests ME2.1 and ME2.2).
In Test ME2.1 (pressurization), hydrate formation during the initial 2.5 hours produced patchy and localized regions of elevated hydrate saturation in the sediment, which became more homogeneous over 24 hours and with full hydrate formation. Early and more localized density redistribution was also seen in the hydrate formed in Tests ME1.1 and ME1.2 and in the packed sand sample SP2. Two processes need to be considered with regard to the density redistribution observed across these tests: capillarity and recrystallization.
Capillarity will cause water to be drawn towards the hydrate formation front. The physical presence of water-wetting hydrate in the pore space (1) reduces pore sizes, thus strengthening capillary suction, and (2) changes surface energy, increasing the capillary suction locally [
Figure
Cross-sectional profiles of density in the core (I to I′ at 32 mm from the injection point) showing the evolution of density distribution patterns.
The observations of Servio and Englezos [
The long-term tests conducted with the Mount Elbert Core sample, ME2.1 and ME2.2, show density redistribution over several weeks. In Test ME2.1, density was redistributed over the scale of several centimeters, with a large increase in density occurring near the interface between the two regions in the sample, primarily on the higher porosity and larger grain size side. This redistribution is particularly apparent with the changing peak in density between 50 mm and 70 mm in Figure
Hydrate dissociation and reformation in equal amounts could occur if either water or gas was available in a limiting quantity. Since gas was present in excess (macroscopically), water can be considered as the limiting reagent. Under equilibrium conditions, some hydrate is dissociating while forming in equal amounts in other areas. Over long timescales, more hydrate would dissociate in less energetically favorable locations and more would form in more favorable locations (much like ice in a non-frost-free freezer sublimates from the ice cube tray and precipitates on the freezer wall).
The second process, water migration caused by hydrate formation and configuration changes, could also be responsible for the density changes observed. Calculations of initial water content and gas uptake during hydrate formation indicate only a 70% conversion of total water to hydrate in Tests ME2.1 and ME2.2. The remaining water could be considered to be bound to grain surfaces and unavailable for hydrate formation due to its low chemical potential [
Over the first 57 days in Test ME2.1 and 9–28 days in Test ME2.2, the pressure and temperature measurements inside the sample showed no anomalous decreases or spikes, respectively, indicating that neither of the processes described above caused additional hydrate formation in the core. Note that slight thermal gradients occurred in the sample from daily temperature changes in the laboratory impacting the temperature-controlled bath, tubing, and jacket. In addition, slight pressure fluctuations occurred from room temperature changes affecting the external gas bottle, causing the pressure to increase when the room temperature increased. In spite of these oscillations in pressure and temperature, the sample was maintained well within the stability zone for the test duration.
Density redistribution from the above processes can be seen in Test ME2.1 (Figure
Density changes seen in the first 28 days of Test ME2.2 could also be caused by the movement of free water due to capillary pressure gradients. A number of hydrate patches formed predominantly in Part A of the core during Test ME2.2. Within these masses, slight variations in hydrate content would draw water towards certain areas, increasing the density locally. In adjacent areas, the density would decrease where water was depleted. These regions can be seen as ring structures in Figure
A hydrate formation event occurred on the 33rd day of Test ME2.2, resulting in a drop in pressure (0.07 MPa) and spike in temperature (0.9°C). The temperature spike was observed in Part A of the core 10 minutes before the Part B side, indicating that the location of hydrate formation was closer to the A side thermocouple. CT scans show that density increased in both parts, with the largest increase occurring between 80 and 110 mm (Part B, see Figure
Density change along the length of the core between day 28 and day 35 in Test ME2.2. New hydrate formation was indicated by a temperature spike on day 33 of the test.
The most likely explanation for additional hydrate forming in Test ME2.2 after 33 days in the stability zone is that water that had been isolated from the methane gas in the pore space during initial hydrate formation became exposed. Instances of exposure of isolated water have been indicated in previous hydrate formation tests [
A punch-through occurrence during Test ME2.2 would be a localized event. However, the disappearance of the ring structures and redistribution of density across the core occur sample wide. There are a number of possibilities for this global redistribution of density. We assume that the change in density seen in Figure
First, water could have been redistributed by the expansion of trapped gas inside hydrate masses. This would account for a reduction in density in the rings (as seen in Figure
The second hypothesis accounts for the global change in density observed across the core by considering the change in temperature that resulted from the new hydrate formation. If liquid water was accumulating in regions of higher hydrate saturation due to capillary pressure gradients, the rings would have higher unconverted water saturations than the adjacent areas. The 0.9°C increase in temperature caused by hydrate formation (temperature in the sample was 0.9°C above bath temperature for 60 minutes) may have caused more methane to dissolve into solution in the residual water in the rings, as methane solubility increases with temperature when in the presence of hydrate [
The third hypothesis assumes that the new hydrate formation that occurred on day 33 of the test altered the capillary pressures in the core and caused moisture redistribution as a result. New hydrate formation in certain pores would increase capillary suction by reducing pore size locally. The increased capillary suction in these areas would draw residual water away from previous accumulations (rings) and redistribute density in the core. Interpretation of the results discussed in previous sections agrees with this hypothesis; however it would occur locally to where hydrate formed, and density redistribution was seen globally in each ring structure not just in isolated regions.
It is possible that a combination of processes is responsible for the density changes seen throughout the core between days 28 and 35 of test ME2.2. The global nature of the redistribution indicates that a response to the temperature and/or pressure changes during new hydrate formation may be responsible; however, the results from tests ME1.1, 1.2, and 2.1 suggest that the capillary pressure alterations caused by additional hydrate presence should not be discounted.
Between 35 and 41 days in Test ME2.2, the ring structures began to appear once again in the hydrate masses. The higher local capillary suction that caused the rings to occur between days 9 and 28 would not have changed significantly during the hydrate formation event at 33 days. Even if new hydrate formed elsewhere, the larger accumulations of hydrate in the masses in Part A would still impart capillary pressure gradients that would draw the water into ring-like structures again. In addition, more hydrate could form around the new gas bubbles, increasing capillary suction further and drawing water into those regions once again.
This study has a number of implications regarding forming hydrate in the laboratory, as well as our understanding of natural hydrate deposits. Previous assumptions that methane hydrate is an immobile mass in porous media must be reconsidered. Short-term redistribution of methane hydrate as it grows in a sediment must be accounted for by those making physical measurements because the changing morphology will impact physical characteristics until complete hydrate formation has been achieved. In addition, long-term observation of hydrate-bearing sediments has shown that the capillary suction differences imposed in a sediment due to hydrate formation can cause moisture and/or hydrate to redistribute weeks after initial hydrate formation has been completed.
Methane hydrate was repeatedly formed and dissociated in a natural sediment core from the Mount Elbert Stratigraphic Test Well, as well as in laboratory sand packs, to investigate hydrate redistribution in sediments over time and the growth patterns of hydrate when made repeatedly in the same material. CT scanning was employed in both sets of experiments to directly observe hydrate structure within the cores.
Results from repeated hydrate formation experiments showed that hydrate formed randomly across the cores indicating that numerous factors affect hydrate formation. Hydrate nucleation is a random process, with salinity, saturation of the pore water, grain size distribution, and mineralogy each altering the probability of nucleation under the low driving force formation conditions. In the samples with no pore water salinity and uniform grain-size, water saturation variation was the dominant control on the location of reformed hydrate, with hydrate forming preferentially in areas of lower water saturation adjacent to regions of higher water saturation. In the natural sample with varying pore water salinity and grain size distribution, no correlation between previous hydrate formation and water saturation, salinity or mineralogy changes could be found to affect the location of hydrate formation in the core. The random nature of hydrate formation dominates nucleation location in this case. The memory effect did not control the location of nucleation events in reforming hydrate in these samples in spite of reducing the time needed for hydrate to begin forming.
Density redistribution was observed in both short- and long-term tests. Over the time period of hydrate formation (hours), density variations occurred whereby hydrate masses went from being concentrated to disperse. This change is attributed to early rapid hydrate formation in dendritic, high surface energy morphologies, then recrystallizing as hydrate formation progressed into more energy-efficient crystal configurations. Over the long-term tests, density redistributions were observed over periods of up to 60 days, which we attribute to mobile free water in the samples being drawn to areas of high capillary suction causing areas of higher density. We believe this moisture redistribution also caused renewed hydrate formation in one test by exposing previously trapped water so that it became available for more hydrate formation.
The results from the tests described in this paper suggest that in natural hydrate-bearing deposits, hydrate and residual moisture may not be stagnant phases and are likely to redistribute in response to stimuli. These stimuli include the thermal gradient changes that could occur from wells or pipelines carrying fluid at a different temperature than that of the ambient conditions.
This work was supported by the Assistant Secretary for Fossil Energy, Office of Oil and Natural Gas, Gas Hydrate Program, through the National Energy Technology Laboratory of the US Department of Energy under Contract no. DE-AC02-05CH11231. The authors would also like to thank Kelly Rose and Kyle Littlefield of NETL for providing the grain size distributions and mineralogy of the Mount Elbert core sample.