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A novel high-pressure apparatus to study hydrate–sediment interactions
Journal of Petroleum Science and Engineering 56 (2007) 101 – 107 www.elsevier.com/locate/petrol
A novel high-pressure apparatus to study hydrate–sediment interactions Michael Eaton a , Devinder Mahajan a,b,⁎, Roger Flood c b
a Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York 11794 USA Energy Sciences and Technology Department, Building 815, Brookhaven National Laboratory, Upton, New York 11973 USA c Marine Sciences Research Center, Stony Brook University, Stony Brook, New York 11794 USA
Received 4 June 2005; accepted 15 September 2005
Abstract Hydrates formed from methane and water over thousands of years under both gas-lean (single phase) and gas-rich (two-phase) conditions are commonly present in marine sediments. Several factors such as dissolved minerals in seawater, mineral content, and pore size of sediments are thought to affect hydrate growth. There is much interest in exploiting this energy source, but there are many unknown aspects that need to be addressed. In order to develop or improve methane recovery methods, it is important to be able to mimic natural conditions in a laboratory and study dynamics of methane hydrates in host sediments. To date, a large data set from laboratory studies is available for pure methane hydrates for which kinetic models have been proposed but reproducible data collection in the presence of sediments has proved challenging. We describe herein a new experimental apparatus named FISH (Flexible Integrated Study of Hydrates) that has been designed to confine artificial and natural sediments in a pressure vessel and mimic oceanic conditions in order to study kinetics of methane hydrate formation/ decomposition in these sediments. The unit: 1) consists of a pressure vessel equipped with a first-of-its-kind viewport that is large enough to observe macroscopic hydrate behavior, 2) configuration allows convenient interchangeability of different volume pressure vessels, 3) can accept acoustic probes, and 4) holds multiple sensors for operation under precise pressure and temperature conditions. The unit set up, operation, and preliminary results for experiments with a pressure vessel in which the effective gas to liquid volume (Vg/Vl) ratio was 1.86, are described. The availability of accurate data on the formation/ decomposition cycle and acoustic properties of hydrates will aid in developing a much sought after economical method to extract methane from this vast resource. Published by Elsevier B.V. Keywords: Methane; Methane hydrate; Gas hydrate; Clathrate; Hydrate kinetics; Host sediments; Acoustic properties
1. Introduction Clathrate hydrates are crystalline solids formed from mixtures of water and low molecular weight compounds, ⁎ Corresponding author. Energy Sciences and Technology Department, Building 815, Brookhaven National Laboratory, Upton, New York 11973 USA. Tel.: +1 631 344 4985. E-mail address: [email protected] (D. Mahajan). 0920-4105/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.petrol.2005.09.006
referred to as hydrate formers or “guests,” which are almost exclusively gases at ambient conditions (Sloan, 1998). Often, hydrates in the laboratory are formed in a sealed vessel by contacting the hydrate former (either in the gas or liquid phase) and liquid water, and then increasing the pressure until crystalline hydrates form. Alternatively, a technique developed by Stern et al. (2000) is used in which hydrates are formed from powdered water ice and a pressurized hydrate guest, and the temperature is allowed to
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increase until hydrate conversion begins. While both methods have advantages, they are not without their disadvantages. The second method, beginning from ice, allows for rapid water-to-hydrate conversion, but begins with a substrate (ice) not known to exist in oceanic hydrate conditions. The first case, while accurately mimicking gas-
rich hydrate formation conditions (Suess et al., 2001), neglects the gas-lean case where hydrate formation from a single-phase solution using a dissolved hydrate guest occurs. Finally, most laboratory hydrates are formed in sealed or limited-sight vessels. These include two otherwise custom hydrate study units: 1) the Seafloor Process
Fig. 1. (Top) Schematic of the hydrate-forming unit. The heart of the unit is a high-pressure vessel with 12-in. vertical windows in which hydrates formation/decomposition can be viewed under a variety of simulated conditions including those encountered in nature. (Bottom) Actual view of the pressure vessel through a window in the constant-temperature bath.
M. Eaton et al. / Journal of Petroleum Science and Engineering 56 (2007) 101–107
Simulator (SPS) at Oak Ridge National Laboratory (ORNL) consisting of a 70-L pressure vessel that is ideal for scale up but too cumbersome for routine kinetic studies and 2) Gas Hydrate and Sediment Test Laboratory Instrument (GHASTLI), a unit at the United States Geological Survey (USGS), Woods Hole consisting of a 0.5 L pressure vessel. The ability to observe hydrate formation is particularly useful when sediments are involved because the morphology of the resulting hydrated sediment is important, and there can be unexpected variability within the pressure vessel. The ability to visualize the hydrating sediment gives additional insights into the processes involved and allows for modification of experiments to achieve accurate results. In this paper, we describe a customized unit named FISH (Flexible Integrated Study of Hydrates) that was designed and constructed at Brookhaven National Laboratory (BNL) and suitable for forming hydrates while allowing visual observations and accurate measurements of temperature and pressure conditions that accompany phase changes during the entire hydrate forming process. This experimental unit allows us to study the sensitivity of hydrate formation in fine-grained sediments to inlet flow rate and to describe a novel method of hydrate decomposition to study kinetics. Initial results show that the unit can accurately reproduce in situ oceanic hydrate conditions, creating hydrates on a laboratory time scale with properties (formation pressure and temperature) very close (<6%) to those predicted by CSMGem, Colorado School of Mines' hydrate prediction program (Ballard et al., 2002). 2. Experimental 2.1. The Flexible Integrated Study of Hydrates (FISH) unit The FISH unit is shown in Fig. 1. The main component of the hydrate unit is a high-pressure vessel fabricated from 316 stainless steel. The unit is based on a Jerguson liquid-level gage that has been retrofitted with several precision sensors for measurement of temperatures and pressures at various points along the unit and an inlet gas sparger. The availability of different volume liquid-level gauges from the manufacturer allows us to construct pressure vessels of different volumes: this gives an unprecedented flexibility to the unit. The physical wetted (internal) dimensions of the pressure vessel (410 mm L × 17 mm W × 28 mm D) yields an internal volume of 198 mL (includes correction for all associated fittings) and gives an effective gas to liquid volume (Vg/Vl) ratio of ∼2. The Vg/Vl ratio can be changed by changing to a pressure
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vessel of a different volume. The vessel has rectangular viewing windows on two opposite sides constructed from borosilicate (rated to 20 MPa) and is immersed in a temperature-controlled bath consisting of an equal volume mixture of ethylene glycol and water. Gas and water are brought into contact in the vessel in a countercurrent fashion. Water enters from the top, while the gas enters from the bottom in order to achieve a uniform gas–sediment–water mixture. Before coming into contact with the sediment and water, the gas passes through a homemade sparger of glass wool sandwiched between two 50 μm stainless-steel sieves. The sparger serves several purposes. First, it reduces or eliminates channeling in the sediment. Without proper attention to inlet conditions, large gas bubbles travel along the “path of least resistance” through the vessel, which is usually between the wall or glass viewport and the outside of the sediment sample. While some hydrates will form under these conditions, this type of hydrate formation is undesirable as only a fraction of the sediment contributes to hydrate formation. As the goal of the project is to simulate in situ conditions, a finer, more homogeneous packed-bed type flow was desired. Second, the sparger acts as a one-way valve, eliminating back flow of the water and sediment into the gas lines and averting a potential unwanted plugging condition. The experimental temperatures of both the gas and liquid phases are measured with the aid of type K thermocouples. Two thermocouples are located inside the pressure vessel: one near the top and other near the bottom to measure and establish any temperature gradient along the length of the vessel during the hydrate formation/decomposition event. The pressure inside the vessel is measured by differential pressure transducers, and both the temperature and pressure readings are continuously recorded using the LabVIEW software suite. The pressure transducers measure bulk pressure, not pore pressure. Recent work by Turner and Sloan (2002), suggests no difference, within experimental accuracy, between hydrate formation temperature and pressure in tetrahydrofuran (THF) hydrates formed with and without porous media. There is certainly a difference between methane gas hydrates and miscible THF hydrates, and further experiments are planned to investigate pore pressure effects. The experimental methane gas flow rate into the system is measured and regulated by a Brooks mass flow controller with a range of 70–2000 mL/min. The system is fitted with a calibrated needle valve to explore lower flowrates (<70 mL/min), although results are not yet available. A backpressure regulator (BPR) is used to ensure constant pressure during both formation and decomposition experiments. A dry test meter is used downstream of the BPR to
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measure the amount of gas exiting the high-pressure vessel during the formation or decomposition process. Trace gases may be injected through a special 300 mL Hoke bomb to determine sediment stability during the hydrate formation/ decomposition cycle if so desired. Obtaining accurate measurements of the sediment acoustic properties before, during, and after hydrate formation is important because seismic techniques are being used to locate and characterize sediment properties (e.g., bottom simulating reflectors-BSRs). The acoustic properties of hydrate-bearing sediments differ dramatically from those of gassy sediments or saturated sediments without hydrates. The glass windows of the FISH vessel create an opportunity to measure the acoustic velocity of hydrated sediments in a setting where the morphology of the hydrate-bearing sediments can be observed. Initial attempts to measure p-wave sound velocities in the vessel used two transducers attached to the glass windows: one to transmit a 200 kHz pulse and one to measure the arrival of the pulse after it transited the vessel, including the sediment sample. The received signal was digitized with a digital oscilloscope and transferred to a computer for analysis. This transmitted pulse technique, which is routinely used to measure sediment velocity in sediment cores, has provided a few reasonable measurements. However it is often difficult to analyze the results of this experiment because the pulse transmitted through the vessel arrives before the pulse transmitted through the sediment sample and signal processing techniques are being modified to identify which part of the recorded signal has transited the vessel. We are now testing higher-frequency transducers operating in a pulse-echo mode, a technique that is being used to measure the acoustic properties of minerals in high-pressure vessels (Li et al., 2004). It is to be mentioned that the inclusion of “Integrated” in the acronym FISH derives from the fact that the proximity of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) allows us to focus on an integrated approach to study hydrates: characterization of host or depleted sediments by Computed Microtomography (CMT) followed by a designed kinetic study in the FISH unit. The use of the CMT technique for sediment characterization is the subject of a companion paper in this issue by Jones et al. (2007-this volume). 2.2. Sediment sample Three sediment samples, originally mined from Blake Ridge (Cruise: ODP leg 164; Latitude: 31° 48.210′ N; Longitude: 75° 31.343′ W; Hole/core: 995A-80X-1; Water Depth: 2278.5 m) were obtained through the Unites States Geological Survey (USGS), Woods Hole, Massachusetts.
Fig. 2. Experimental hydrate formation conditions compared to CSMGem.
Previous measurements of stress history and geotechnical properties have been reported by Winters et al. (2004) (water content: 39.3% dry weight; porosity: 51.0%; maximum past stress: 2730 kPa). Three selected samples ranged in sub-seafloor depths from 1 m to 667 m. All sediment-based works presented here were performed on the 667 m sample. All sediment samples used in these experiments were characterized at NSLS, BNL. 3. Experimental results 3.1. Baseline methane–water run Our initial work focused on obtaining repeatable formation trends. We first verified that the vessel could be used to obtain accurate thermodynamic results with the baseline methane/water system, also known as laboratory-prepared pure hydrates. The vessel was charged with 80 mL of deionized water and sufficient methane to obtain the desired pressure. Temperature of the cooling bath and, consequently the vessel were lowered and the hydrate formation process was followed in the vessel. A pressure versus temperature plot of such a run is shown in Fig. 2. Results of the experiment in Fig. 2 were compared to CSMGem (Ballard et al., 2002), the Colorado School of Mines' de facto standard for hydrate prediction. Results compare favorably: formation conditions deviated only 6.35% from theoretical, with maximum deviation at 1500 psi, the upper limit of the pressure transducers used in the experiment. Of note are the 0.5 K constant errors in the thermocouple measurements (given by the manufacturer). This error is well above the noted temperature effect of porous media on hydrate formation temperatures, further justifying the use of bulk pressure measurement in lieu of, or addition to, pore pressure measurements. It is
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also important to note that these hydrates were formed from a gas–liquid water mixture. The hydrates formation/ decomposition cycle was repeated several times with minimum deviation in temperature/pressure regimes. The reproducibility of results for the pure hydrates experiments in the FISH unit is noteworthy and led us to conduct more complex experiments. 3.2. Customized methods for methane hydrate formation and decomposition The flexibility of the unit allows us to conduct several probing runs related to methane hydrate formation/ decomposition cycle. Below we describe two hydrate formation methods and a decomposition method to collect kinetic data in the FISH unit. 3.2.1. Hydrate formation measurement method 3.2.1.1. Method 1 – dynamic mode. In this method, CH4 gas (typically at 1200 psi) continuously flows from the bottom of the vessel in the pressure vessel during the hydrate formation event. The sparger creates tiny bubbles and as these bubbles travel upwards they continuously mix sediment, gas, and water. The bubble size is controlled by the gas flow rate that itself can be precisely controlled by a needle valve as well as a mass flow meter. Hydrates are formed as the temperature of the vessel is decreased. 3.2.1.2. Method 2 – static mode. This method is similar to the dynamic method except CH4 gas is added slowly and intermittently to the pressure vessel through the sparger. This allows make up for the gas absorbed from the gas phase in the vessel. This method is more representative of the seafloor conditions where methane gas/water does not agitate and the methane hydrate formation phenomenon probably occurs mostly by diffusion. To our knowledge, this method has never been used before to make hydrates in the laboratory. 3.2.1.3. Hydrate decomposition measurement method. We have devised a step-down pressure method to quantitatively measure CH4 gas evolution during hydrate decomposition under isothermal conditions at a temperature at which hydrates were initially formed. In the stepdown pressure method, the original pressure (typically between 8–10 MPa) is decreased in ∼1.4 MPa (200 psi) increments and the evolved gas is recorded. The process is continued till the vessel is at an ambient pressure. Both, the quantitative gas release at a given pressure as well as the total gas evolution are recorded.
Fig. 3. Hydrate gas consumption versus inlet gas flowrate.
3.3. Methane hydrate formation run – effect of inlet gas flow rate The next experiment undertaken was investigating the effect that inlet gas flow rate had on hydrate formation. In nature, hydrates have much time to form, and often do so with limited resources – gas formation by microbes/ percolation through the seafloor occurs slowly. This achieves almost 100% conversion of the water/gas to hydrates, but does so over a period of thousands of years. However, due to the time scale of laboratory experiments, a compromise between accurate reproduction of in situ conditions and production of any quantity of data was necessary. Thus, a sensitivity study was initiated to achieve maximum hydrate production with maximum inlet gas flowrate (obtaining accurate results with the least amount of time). A mixture of 20 mL of water and 60 g of sediment was loaded into the pressure vessel, and the apparatus was cooled down to 4 °C. At this loading, the effective gas to liquid volume (Vg/Vl) ratio was kept constant at 1.86. The flexibility of the unit allows variability of the Vg/Vl parameter. Methane, at flow rates between 70 mL/min and 2000 mL/min was added to the cell until the desired pressure was achieved. No channeling was observed in the system at any flowrate; at high flow rates the sediment column was agitated to the point of fluidity, and at low flow rates the effect of the sparger was apparent – methane appeared to permeate the column uniformly. Results of the sensitivity study are shown in Fig 3. It is important to note that thermodynamics are the “final” determining factor in hydrate formation. Given enough time, a certain quantity of water/sediment will hydrate a constant volume of gas. However, on the laboratory time frame, kinetics is the deciding factor. There are several notable features of Fig. 3. First, at high inlet flowrates (>1000 mL/min), the asymptotic decay suggests very little, if any, hydrate formation – most gas ‘consumption’
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was probably due to simply filling the cell with gas. This is an expected behavior, as at high flowrates (flow velocities), there is very little gas–sediment–water interaction, and therefore the only hydrate-forming surface becomes the few square centimeters of the water–gas interface at the top of the cell. Second, the occurrence of another apparent asymptote at low flow rates (<150 mL/ min). This too is expected, because there is a physical limit to the amount of hydrate that can form and therefore a limit to the amount of gas consumed, based upon the total available volume of water in the system. This result is encouraging, as it suggests that laboratory-scale experiments can approach the conditions and hydrate saturation level found in nature, but in a much shorter time. Further experiments are being carried out at flowrates lower than 70 mL/min to ensure that the asymptotic behavior at low flowrates is not anomalous. Of special importance to the low-flow condition described above is that hydrates were formed from liquid water in a gas-rich (two-phase) condition. Huo (2003), and more recently Winters et al. (2004) have cited this formation condition difference for discrepancies between laboratory and natural hydrates. An explanation of such a discrepancy could possibly be the changes in small cage occupancy seen with varying levels of the methane/water ratio at the time of hydrate formation. Of course, cage occupancy will alter lattice size, and hence the properties of the macroscopic hydrate formations in sediment. However, in most laboratory experiments, hydrates are formed from finely powdered ice, and the temperature is increased to stimulate formation. Because this does not accurately reflect oceanic conditions, we have chosen to use liquid water rather than ice as our starting water phase that interacts with methane in the FISH unit. Cage occupancy analysis has not yet been performed to determine if such a change will affect the laboratory/ oceanic hydrate discrepancy, but lattice size is being investigated at NSLS/BNL.
(volume = 198 mL; Vg/Vl ratio = 1.86) has been shown to accurately reproduce in situ conditions of temperature, pressure, and gas availability. Hydrates have been repeatedly formed in the vessel from pure water and methane gas, verifying the reproducibility of hydrate thermodynamic measurements within a 6.4% experimental error on pressure. Early kinetic work has shown that, at gas flow rates <150 mL/min, the sediment/water in the vessel becomes saturated with hydrates suggesting accurate reproduction of natural time scales using laboratory equipment over the course of several hours. Initial results of the acoustic measurements suggest that useful physical properties can be determined from hydrate-bearing sediments. The configuration and flexibility of the newly commissioned FISH unit is allowing us to collect data under varying conditions in the presence of sediments and the initial results are promising. The availability of different volume pressure-vessels allows us to study the effect of varying volume and the Vg/Vl ratio on hydrate formation. Currently only distilled water and high-purity methane gas are used. Later studies will use simulated seawater and possibly a less pure natural gas mixture (higher percentages of higher hydrocarbon) to more closely approximate oceanic conditions. Future kinetic work will be performed using a first-ofits-kind step-down depressurization method while hydrates will be formed from either a one- or a twophase mixture of water and gas and allowed to equilibrate. The integrated approach involving characterization of depleted or host sediments by CMT at NSLS and the utilization of the FISH unit for kinetic study in these sediments will allow for accurate acquisition of rate constants over a whole range of the hydrate stability zone and correlate these values with sediment properties such as porosity and tortuosity. These data will further our understanding of an economical approach to hydrate recovery, the ultimate goal of this research.
3.4. Acoustic measurements Acknowledgements The transmitted pulse technique has been used to estimate the p-wave velocity of hydrated water, resulting in an estimated velocity of about 4 km/s. Similar measurements of hydrated sediments are in progress. 4. Concluding remarks Work thus far has been encouraging. A high-pressure vessel, based on a Jerguson liquid-level gage with a first-of-its-kind viewport, has been designed and constructed for hydrate formation. The pressure vessel
This work was supported by the Laboratory Directed Research and Development (LDRD) program at Brookhaven National Laboratory and the U. S. Department of Energy under Contract No. DE-AC02-98CH10886 through National Energy Technology Laboratory. DM also thanks Stony Brook University for a start-up grant. The authors wish to thank Dr. William Winters, United States Geological Survey, Woods Hole, Massachusetts, for providing sediment samples from the Ocean Drilling Program upon which this study was based.
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References Ballard, A., 2002. A non-ideal hydrate solid solution model for a multi-phase equilibria program. Ph. D. Thesis, Colorado School of Mines, Golden, CO, USA. Huo, Z., 2003. Hydrate Phase Equilibria Measurements by X-ray Diffraction and Raman Spectroscopy. Ph. D. Thesis, Colorado School of Mines, Golden, CO, USA. Jones, K.W., Feng, H., Tomov, S., Winters, W.J., Prodanović, M., Mahajan, D., 2007. Characterization of methane hydrate host sediments using synchrotron-computed microtomography (CMT). J. Pet. Sci. Eng. 56, 136–145 (this volume). Li, B., Kung, J., Liebermann, R.C., 2004. Modern techniques in measuring elasticity of Earth materials at high-pressure and high temperature using ultrasonic interferometry in conjunction with synchrotron X-radiation in multi-anvil apparatus. Phys. Earth Planet. Inter. 143–144, 559–574. Sloan Jr., E.D., 1998. Clathrate Hydrates of Natural Gas, 2nd ed. Marcel Dekker, New York. Stern, L.A., Kirby, S.H., Durham, W.B., Circone, S., Waite, W.F., 2000. Laboratory synthesis of pure methane hydrate suitable for
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measurement of physical properties and decomposition behaviour. In: Max, M.D. (Ed.), Natural gas hydrate in oceanic and permafrost environments. Kluwer Academic Publishers, Dordrecht, pp. 323–348. Suess, E., Torres, M.E., Bohrmann, G., Collier, R.W., Rickert, D., Goldfinger, C., Linke, P., Heuser, A., Sahling, H., Heeschen, K., Jung, C., Nakamura, K., Greinert, J., Pfannkuche, O., Trehu, A., Klinkhammer, G., Whiticar, M.J., Eisenhauer, A., Teichert, B., Elvert, M., 2001. Sea floor methane hydrates at Hydrate Ridge, Cascadia margin. In: Paull, C.K., Dillon, W.P. (Eds.), Natural Gas Hydrates: Occurrence, Distribution, and Detection. Am. Geophys. Union, Geophys. Monogr. Ser., vol. 124, pp. 87–98. Turner, D.J., Sloan, E.D., 2002. Hydrate phase equilibria measurements and prediction in sediments. Proceedings of the 4th ICGH, Yokohama, pp. 327–330. Winters, W.J., Waite, W.F., Pecher, I.A., et al., 2004. Comparison of methane gas hydrate formation on physical properties of fine- and coarse-grained sediments. AAPG Hedberg Conference, Sept. 12–16, 2004, Vancouver, BC, Canada.
A novel high-pressure apparatus to study hydrate–sediment interactions Michael Eaton a , Devinder Mahajan a,b,⁎, Roger Flood c b
a Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York 11794 USA Energy Sciences and Technology Department, Building 815, Brookhaven National Laboratory, Upton, New York 11973 USA c Marine Sciences Research Center, Stony Brook University, Stony Brook, New York 11794 USA
Received 4 June 2005; accepted 15 September 2005
Abstract Hydrates formed from methane and water over thousands of years under both gas-lean (single phase) and gas-rich (two-phase) conditions are commonly present in marine sediments. Several factors such as dissolved minerals in seawater, mineral content, and pore size of sediments are thought to affect hydrate growth. There is much interest in exploiting this energy source, but there are many unknown aspects that need to be addressed. In order to develop or improve methane recovery methods, it is important to be able to mimic natural conditions in a laboratory and study dynamics of methane hydrates in host sediments. To date, a large data set from laboratory studies is available for pure methane hydrates for which kinetic models have been proposed but reproducible data collection in the presence of sediments has proved challenging. We describe herein a new experimental apparatus named FISH (Flexible Integrated Study of Hydrates) that has been designed to confine artificial and natural sediments in a pressure vessel and mimic oceanic conditions in order to study kinetics of methane hydrate formation/ decomposition in these sediments. The unit: 1) consists of a pressure vessel equipped with a first-of-its-kind viewport that is large enough to observe macroscopic hydrate behavior, 2) configuration allows convenient interchangeability of different volume pressure vessels, 3) can accept acoustic probes, and 4) holds multiple sensors for operation under precise pressure and temperature conditions. The unit set up, operation, and preliminary results for experiments with a pressure vessel in which the effective gas to liquid volume (Vg/Vl) ratio was 1.86, are described. The availability of accurate data on the formation/ decomposition cycle and acoustic properties of hydrates will aid in developing a much sought after economical method to extract methane from this vast resource. Published by Elsevier B.V. Keywords: Methane; Methane hydrate; Gas hydrate; Clathrate; Hydrate kinetics; Host sediments; Acoustic properties
1. Introduction Clathrate hydrates are crystalline solids formed from mixtures of water and low molecular weight compounds, ⁎ Corresponding author. Energy Sciences and Technology Department, Building 815, Brookhaven National Laboratory, Upton, New York 11973 USA. Tel.: +1 631 344 4985. E-mail address: [email protected] (D. Mahajan). 0920-4105/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.petrol.2005.09.006
referred to as hydrate formers or “guests,” which are almost exclusively gases at ambient conditions (Sloan, 1998). Often, hydrates in the laboratory are formed in a sealed vessel by contacting the hydrate former (either in the gas or liquid phase) and liquid water, and then increasing the pressure until crystalline hydrates form. Alternatively, a technique developed by Stern et al. (2000) is used in which hydrates are formed from powdered water ice and a pressurized hydrate guest, and the temperature is allowed to
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increase until hydrate conversion begins. While both methods have advantages, they are not without their disadvantages. The second method, beginning from ice, allows for rapid water-to-hydrate conversion, but begins with a substrate (ice) not known to exist in oceanic hydrate conditions. The first case, while accurately mimicking gas-
rich hydrate formation conditions (Suess et al., 2001), neglects the gas-lean case where hydrate formation from a single-phase solution using a dissolved hydrate guest occurs. Finally, most laboratory hydrates are formed in sealed or limited-sight vessels. These include two otherwise custom hydrate study units: 1) the Seafloor Process
Fig. 1. (Top) Schematic of the hydrate-forming unit. The heart of the unit is a high-pressure vessel with 12-in. vertical windows in which hydrates formation/decomposition can be viewed under a variety of simulated conditions including those encountered in nature. (Bottom) Actual view of the pressure vessel through a window in the constant-temperature bath.
M. Eaton et al. / Journal of Petroleum Science and Engineering 56 (2007) 101–107
Simulator (SPS) at Oak Ridge National Laboratory (ORNL) consisting of a 70-L pressure vessel that is ideal for scale up but too cumbersome for routine kinetic studies and 2) Gas Hydrate and Sediment Test Laboratory Instrument (GHASTLI), a unit at the United States Geological Survey (USGS), Woods Hole consisting of a 0.5 L pressure vessel. The ability to observe hydrate formation is particularly useful when sediments are involved because the morphology of the resulting hydrated sediment is important, and there can be unexpected variability within the pressure vessel. The ability to visualize the hydrating sediment gives additional insights into the processes involved and allows for modification of experiments to achieve accurate results. In this paper, we describe a customized unit named FISH (Flexible Integrated Study of Hydrates) that was designed and constructed at Brookhaven National Laboratory (BNL) and suitable for forming hydrates while allowing visual observations and accurate measurements of temperature and pressure conditions that accompany phase changes during the entire hydrate forming process. This experimental unit allows us to study the sensitivity of hydrate formation in fine-grained sediments to inlet flow rate and to describe a novel method of hydrate decomposition to study kinetics. Initial results show that the unit can accurately reproduce in situ oceanic hydrate conditions, creating hydrates on a laboratory time scale with properties (formation pressure and temperature) very close (<6%) to those predicted by CSMGem, Colorado School of Mines' hydrate prediction program (Ballard et al., 2002). 2. Experimental 2.1. The Flexible Integrated Study of Hydrates (FISH) unit The FISH unit is shown in Fig. 1. The main component of the hydrate unit is a high-pressure vessel fabricated from 316 stainless steel. The unit is based on a Jerguson liquid-level gage that has been retrofitted with several precision sensors for measurement of temperatures and pressures at various points along the unit and an inlet gas sparger. The availability of different volume liquid-level gauges from the manufacturer allows us to construct pressure vessels of different volumes: this gives an unprecedented flexibility to the unit. The physical wetted (internal) dimensions of the pressure vessel (410 mm L × 17 mm W × 28 mm D) yields an internal volume of 198 mL (includes correction for all associated fittings) and gives an effective gas to liquid volume (Vg/Vl) ratio of ∼2. The Vg/Vl ratio can be changed by changing to a pressure
103
vessel of a different volume. The vessel has rectangular viewing windows on two opposite sides constructed from borosilicate (rated to 20 MPa) and is immersed in a temperature-controlled bath consisting of an equal volume mixture of ethylene glycol and water. Gas and water are brought into contact in the vessel in a countercurrent fashion. Water enters from the top, while the gas enters from the bottom in order to achieve a uniform gas–sediment–water mixture. Before coming into contact with the sediment and water, the gas passes through a homemade sparger of glass wool sandwiched between two 50 μm stainless-steel sieves. The sparger serves several purposes. First, it reduces or eliminates channeling in the sediment. Without proper attention to inlet conditions, large gas bubbles travel along the “path of least resistance” through the vessel, which is usually between the wall or glass viewport and the outside of the sediment sample. While some hydrates will form under these conditions, this type of hydrate formation is undesirable as only a fraction of the sediment contributes to hydrate formation. As the goal of the project is to simulate in situ conditions, a finer, more homogeneous packed-bed type flow was desired. Second, the sparger acts as a one-way valve, eliminating back flow of the water and sediment into the gas lines and averting a potential unwanted plugging condition. The experimental temperatures of both the gas and liquid phases are measured with the aid of type K thermocouples. Two thermocouples are located inside the pressure vessel: one near the top and other near the bottom to measure and establish any temperature gradient along the length of the vessel during the hydrate formation/decomposition event. The pressure inside the vessel is measured by differential pressure transducers, and both the temperature and pressure readings are continuously recorded using the LabVIEW software suite. The pressure transducers measure bulk pressure, not pore pressure. Recent work by Turner and Sloan (2002), suggests no difference, within experimental accuracy, between hydrate formation temperature and pressure in tetrahydrofuran (THF) hydrates formed with and without porous media. There is certainly a difference between methane gas hydrates and miscible THF hydrates, and further experiments are planned to investigate pore pressure effects. The experimental methane gas flow rate into the system is measured and regulated by a Brooks mass flow controller with a range of 70–2000 mL/min. The system is fitted with a calibrated needle valve to explore lower flowrates (<70 mL/min), although results are not yet available. A backpressure regulator (BPR) is used to ensure constant pressure during both formation and decomposition experiments. A dry test meter is used downstream of the BPR to
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measure the amount of gas exiting the high-pressure vessel during the formation or decomposition process. Trace gases may be injected through a special 300 mL Hoke bomb to determine sediment stability during the hydrate formation/ decomposition cycle if so desired. Obtaining accurate measurements of the sediment acoustic properties before, during, and after hydrate formation is important because seismic techniques are being used to locate and characterize sediment properties (e.g., bottom simulating reflectors-BSRs). The acoustic properties of hydrate-bearing sediments differ dramatically from those of gassy sediments or saturated sediments without hydrates. The glass windows of the FISH vessel create an opportunity to measure the acoustic velocity of hydrated sediments in a setting where the morphology of the hydrate-bearing sediments can be observed. Initial attempts to measure p-wave sound velocities in the vessel used two transducers attached to the glass windows: one to transmit a 200 kHz pulse and one to measure the arrival of the pulse after it transited the vessel, including the sediment sample. The received signal was digitized with a digital oscilloscope and transferred to a computer for analysis. This transmitted pulse technique, which is routinely used to measure sediment velocity in sediment cores, has provided a few reasonable measurements. However it is often difficult to analyze the results of this experiment because the pulse transmitted through the vessel arrives before the pulse transmitted through the sediment sample and signal processing techniques are being modified to identify which part of the recorded signal has transited the vessel. We are now testing higher-frequency transducers operating in a pulse-echo mode, a technique that is being used to measure the acoustic properties of minerals in high-pressure vessels (Li et al., 2004). It is to be mentioned that the inclusion of “Integrated” in the acronym FISH derives from the fact that the proximity of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) allows us to focus on an integrated approach to study hydrates: characterization of host or depleted sediments by Computed Microtomography (CMT) followed by a designed kinetic study in the FISH unit. The use of the CMT technique for sediment characterization is the subject of a companion paper in this issue by Jones et al. (2007-this volume). 2.2. Sediment sample Three sediment samples, originally mined from Blake Ridge (Cruise: ODP leg 164; Latitude: 31° 48.210′ N; Longitude: 75° 31.343′ W; Hole/core: 995A-80X-1; Water Depth: 2278.5 m) were obtained through the Unites States Geological Survey (USGS), Woods Hole, Massachusetts.
Fig. 2. Experimental hydrate formation conditions compared to CSMGem.
Previous measurements of stress history and geotechnical properties have been reported by Winters et al. (2004) (water content: 39.3% dry weight; porosity: 51.0%; maximum past stress: 2730 kPa). Three selected samples ranged in sub-seafloor depths from 1 m to 667 m. All sediment-based works presented here were performed on the 667 m sample. All sediment samples used in these experiments were characterized at NSLS, BNL. 3. Experimental results 3.1. Baseline methane–water run Our initial work focused on obtaining repeatable formation trends. We first verified that the vessel could be used to obtain accurate thermodynamic results with the baseline methane/water system, also known as laboratory-prepared pure hydrates. The vessel was charged with 80 mL of deionized water and sufficient methane to obtain the desired pressure. Temperature of the cooling bath and, consequently the vessel were lowered and the hydrate formation process was followed in the vessel. A pressure versus temperature plot of such a run is shown in Fig. 2. Results of the experiment in Fig. 2 were compared to CSMGem (Ballard et al., 2002), the Colorado School of Mines' de facto standard for hydrate prediction. Results compare favorably: formation conditions deviated only 6.35% from theoretical, with maximum deviation at 1500 psi, the upper limit of the pressure transducers used in the experiment. Of note are the 0.5 K constant errors in the thermocouple measurements (given by the manufacturer). This error is well above the noted temperature effect of porous media on hydrate formation temperatures, further justifying the use of bulk pressure measurement in lieu of, or addition to, pore pressure measurements. It is
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also important to note that these hydrates were formed from a gas–liquid water mixture. The hydrates formation/ decomposition cycle was repeated several times with minimum deviation in temperature/pressure regimes. The reproducibility of results for the pure hydrates experiments in the FISH unit is noteworthy and led us to conduct more complex experiments. 3.2. Customized methods for methane hydrate formation and decomposition The flexibility of the unit allows us to conduct several probing runs related to methane hydrate formation/ decomposition cycle. Below we describe two hydrate formation methods and a decomposition method to collect kinetic data in the FISH unit. 3.2.1. Hydrate formation measurement method 3.2.1.1. Method 1 – dynamic mode. In this method, CH4 gas (typically at 1200 psi) continuously flows from the bottom of the vessel in the pressure vessel during the hydrate formation event. The sparger creates tiny bubbles and as these bubbles travel upwards they continuously mix sediment, gas, and water. The bubble size is controlled by the gas flow rate that itself can be precisely controlled by a needle valve as well as a mass flow meter. Hydrates are formed as the temperature of the vessel is decreased. 3.2.1.2. Method 2 – static mode. This method is similar to the dynamic method except CH4 gas is added slowly and intermittently to the pressure vessel through the sparger. This allows make up for the gas absorbed from the gas phase in the vessel. This method is more representative of the seafloor conditions where methane gas/water does not agitate and the methane hydrate formation phenomenon probably occurs mostly by diffusion. To our knowledge, this method has never been used before to make hydrates in the laboratory. 3.2.1.3. Hydrate decomposition measurement method. We have devised a step-down pressure method to quantitatively measure CH4 gas evolution during hydrate decomposition under isothermal conditions at a temperature at which hydrates were initially formed. In the stepdown pressure method, the original pressure (typically between 8–10 MPa) is decreased in ∼1.4 MPa (200 psi) increments and the evolved gas is recorded. The process is continued till the vessel is at an ambient pressure. Both, the quantitative gas release at a given pressure as well as the total gas evolution are recorded.
Fig. 3. Hydrate gas consumption versus inlet gas flowrate.
3.3. Methane hydrate formation run – effect of inlet gas flow rate The next experiment undertaken was investigating the effect that inlet gas flow rate had on hydrate formation. In nature, hydrates have much time to form, and often do so with limited resources – gas formation by microbes/ percolation through the seafloor occurs slowly. This achieves almost 100% conversion of the water/gas to hydrates, but does so over a period of thousands of years. However, due to the time scale of laboratory experiments, a compromise between accurate reproduction of in situ conditions and production of any quantity of data was necessary. Thus, a sensitivity study was initiated to achieve maximum hydrate production with maximum inlet gas flowrate (obtaining accurate results with the least amount of time). A mixture of 20 mL of water and 60 g of sediment was loaded into the pressure vessel, and the apparatus was cooled down to 4 °C. At this loading, the effective gas to liquid volume (Vg/Vl) ratio was kept constant at 1.86. The flexibility of the unit allows variability of the Vg/Vl parameter. Methane, at flow rates between 70 mL/min and 2000 mL/min was added to the cell until the desired pressure was achieved. No channeling was observed in the system at any flowrate; at high flow rates the sediment column was agitated to the point of fluidity, and at low flow rates the effect of the sparger was apparent – methane appeared to permeate the column uniformly. Results of the sensitivity study are shown in Fig 3. It is important to note that thermodynamics are the “final” determining factor in hydrate formation. Given enough time, a certain quantity of water/sediment will hydrate a constant volume of gas. However, on the laboratory time frame, kinetics is the deciding factor. There are several notable features of Fig. 3. First, at high inlet flowrates (>1000 mL/min), the asymptotic decay suggests very little, if any, hydrate formation – most gas ‘consumption’
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was probably due to simply filling the cell with gas. This is an expected behavior, as at high flowrates (flow velocities), there is very little gas–sediment–water interaction, and therefore the only hydrate-forming surface becomes the few square centimeters of the water–gas interface at the top of the cell. Second, the occurrence of another apparent asymptote at low flow rates (<150 mL/ min). This too is expected, because there is a physical limit to the amount of hydrate that can form and therefore a limit to the amount of gas consumed, based upon the total available volume of water in the system. This result is encouraging, as it suggests that laboratory-scale experiments can approach the conditions and hydrate saturation level found in nature, but in a much shorter time. Further experiments are being carried out at flowrates lower than 70 mL/min to ensure that the asymptotic behavior at low flowrates is not anomalous. Of special importance to the low-flow condition described above is that hydrates were formed from liquid water in a gas-rich (two-phase) condition. Huo (2003), and more recently Winters et al. (2004) have cited this formation condition difference for discrepancies between laboratory and natural hydrates. An explanation of such a discrepancy could possibly be the changes in small cage occupancy seen with varying levels of the methane/water ratio at the time of hydrate formation. Of course, cage occupancy will alter lattice size, and hence the properties of the macroscopic hydrate formations in sediment. However, in most laboratory experiments, hydrates are formed from finely powdered ice, and the temperature is increased to stimulate formation. Because this does not accurately reflect oceanic conditions, we have chosen to use liquid water rather than ice as our starting water phase that interacts with methane in the FISH unit. Cage occupancy analysis has not yet been performed to determine if such a change will affect the laboratory/ oceanic hydrate discrepancy, but lattice size is being investigated at NSLS/BNL.
(volume = 198 mL; Vg/Vl ratio = 1.86) has been shown to accurately reproduce in situ conditions of temperature, pressure, and gas availability. Hydrates have been repeatedly formed in the vessel from pure water and methane gas, verifying the reproducibility of hydrate thermodynamic measurements within a 6.4% experimental error on pressure. Early kinetic work has shown that, at gas flow rates <150 mL/min, the sediment/water in the vessel becomes saturated with hydrates suggesting accurate reproduction of natural time scales using laboratory equipment over the course of several hours. Initial results of the acoustic measurements suggest that useful physical properties can be determined from hydrate-bearing sediments. The configuration and flexibility of the newly commissioned FISH unit is allowing us to collect data under varying conditions in the presence of sediments and the initial results are promising. The availability of different volume pressure-vessels allows us to study the effect of varying volume and the Vg/Vl ratio on hydrate formation. Currently only distilled water and high-purity methane gas are used. Later studies will use simulated seawater and possibly a less pure natural gas mixture (higher percentages of higher hydrocarbon) to more closely approximate oceanic conditions. Future kinetic work will be performed using a first-ofits-kind step-down depressurization method while hydrates will be formed from either a one- or a twophase mixture of water and gas and allowed to equilibrate. The integrated approach involving characterization of depleted or host sediments by CMT at NSLS and the utilization of the FISH unit for kinetic study in these sediments will allow for accurate acquisition of rate constants over a whole range of the hydrate stability zone and correlate these values with sediment properties such as porosity and tortuosity. These data will further our understanding of an economical approach to hydrate recovery, the ultimate goal of this research.
3.4. Acoustic measurements Acknowledgements The transmitted pulse technique has been used to estimate the p-wave velocity of hydrated water, resulting in an estimated velocity of about 4 km/s. Similar measurements of hydrated sediments are in progress. 4. Concluding remarks Work thus far has been encouraging. A high-pressure vessel, based on a Jerguson liquid-level gage with a first-of-its-kind viewport, has been designed and constructed for hydrate formation. The pressure vessel
This work was supported by the Laboratory Directed Research and Development (LDRD) program at Brookhaven National Laboratory and the U. S. Department of Energy under Contract No. DE-AC02-98CH10886 through National Energy Technology Laboratory. DM also thanks Stony Brook University for a start-up grant. The authors wish to thank Dr. William Winters, United States Geological Survey, Woods Hole, Massachusetts, for providing sediment samples from the Ocean Drilling Program upon which this study was based.
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