US20120181041A1

US20120181041A1 - Gas Hydrate Harvesting

Info

Publication number: US20120181041A1
Application number: US13/008,016
Authority: US
Inventors: Todd Jennings Willman
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-01-18
Filing date: 2011-01-18
Publication date: 2012-07-19

Abstract

A harvester (10) with warm water spray jets (11) that fluidizes and releases gas hydrates from seafloor sand and soil. The released gas hydrates float into the harvester (10) and are then transferred through a hydrate slurry pipe (14) to a separation vessel (18). Dissociation of the gas hydrates into methane gas occurs in the separation vessel (18) from its lower pressure than the seafloor and the optional use of heat. The liberated methane gas flows upward in a gas vent pipe (27) to a topside vessel (30) for storage from the pressure of the submerged separation vessel (18). Alternatively, the vent pipe can be fed directly into an available natural gas pipeline. Crude oil collected in the separation vessel (18) can be separated and pumped to a topside vessel (30) for storage and use in a refinery. No other processing is required to produce commercial quality natural gas harvested directly from gas hydrates on or near the seafloor.

Description

BACKGROUND OF INVENTION

1. Field of the Invention
This invention relates to the harvesting of solid gas hydrates within marine sand and soil on or near the surface of the seafloor. More specifically, a method and apparatus to release and capture solid gas hydrates by fluidizing marine sand and soil materials on the seafloor using high pressure warm water spray jets. The gas hydrates are then transported in a liquid slurry to a separation vessel for dissociated into methane gas for commercial use as natural gas.
2. Prior Art
It is widely agreed that harvesting of gas hydrates from the seafloor would solve the world's energy problems. In a recent article by Ray Boswell of the National Energy Technology Laboratory (Science Magazine, August 2009), he stated that the global resource of methane in gas hydrate deposits is commonly cited as 20,000 trillion m³. However, the challenge of reaching and working at the depth of these deposits on the seafloor has limited progress towards harvesting this vast energy source. Recently, the US Department of Energy has been focusing research and development effort on applying established drilling technology to the harvesting of gas hydrates. However, the harvesting of gas hydrates using current drilling technology is limited to deposits of gas hydrates that lie several hundred feet below rock or shale type formations on the seafloor.
Considering the enormous potential of harvesting gas hydrates to meet the world's energy needs, no prior art satisfactorily addresses the significant challenge of efficiently removing the vast deposits of gas hydrates buried within seafloor sediment consisting of sand, soil, decayed marine life, etc. With seafloor sand and soil having a very low thermal conductivity, it is also not practical or efficient to heat the seafloor or seawater near the seafloor since no significant heat transfer into gas hydrates buried within seafloor sand and soil would occur.
Elliot et al. (U.S. Pat. No. 4,376,462) discloses pumping relatively warm brine water down to hydrates under a body of land or water through a conduit, allowing the brine to circulate through the hydrates to melt and produce gaseous hydrocarbons, and then separating gaseous hydrocarbons from the spent brine. This discloses how to harvest gas hydrates from solid concentrations of gas hydrates, but does not disclose how to harvest gas hydrates from sand and soil on the seafloor where the most significant amounts of gas hydrates reside.
Agee, et al. (U.S. Pat. No 5,950,732) discloses a means to harvest gas hydrates by use of heat or depressurization immediately above the surface of the gas hydrates. However, this patent does not disclose a means to fluidize the seafloor sand and soil to release gas hydrates from their pore spaces to allow efficient heat transfer into the gas hydrates.
Nohmura (U.S. Pat. No. 6,192,691) discloses a flexible sheet which is sunk to the sea floor to trap methane hydrate gas which is filled up by the buoyancy of the gasified methane. However, this patent does not provide a means to fluidize the seafloor sand and soil to release the gas hydrates from their pore spaces. The presence of hot water above the seafloor sand and soil without fluidization of the sand and soil will not provide sufficient heat transfer to dissociate the gas hydrates into methane gas. Additionally, this patent does not disclose any means of movement on the bottom of the seafloor to allow harvesting of gas hydrates over wide areas and instead is anchored in one position.
Borns et al. (U.S. Pat. No. 6.209,965) discloses mining of hydrocarbons from gas hydrates by first disrupting gas hydrate containing strata using continuous mining means such as a rotary filling drum, a fluid injector, or a drill. The solids are carried away to relatively lower pressure and higher temperature for dissociation into methane gas. However, this patent does not provide a means to release gas hydrates from the pore spaces of seafloor sand and soil with no moving parts such as high pressure warm water spray jets and instead relies on the impractical use of mechanically driven mining equipment at great depths on the seafloor.
Wyatt (U.S. Pat. No. 6,299,256) discloses a flexible cover with steerable pods with a mining module connected to an inside surface of the cover to dislodge deposits by mechanically agitating and/or heating and thawing. However, this patent relies on a flexible cover to capture the released methane gas without any structural strength to avoid collapse of the flexible cover from variations of up and down currents on the seafloor either naturally or artificially occurring. Additionally, this patent relies on mechanical equipment being operated up to a mile below the sea surface to move the cover and mine the gas hydrates which is not practical or reliable.
Yemington (U.S. Pat. No. 6,978,837) discloses a method and apparatus for producing methane gas from a gas hydrate formation through a wellbore using a modified material that can transmit heat. However, this patent relies on the use of an existing well that contains gas hydrates and relies on existing drilling technology. Additionally, it does not address how to harvest gas hydrates safely and economically from seafloor sand and soil.
Wendland (U.S. Pat. No. 6,994,159) discloses a method to extract natural gas from gas hydrates using two conduits with heating fluid being added. However, this patent addresses gas hydrates in strata reservoirs below the surface of the sea floor and does not provide a means to harvest gas hydrates from the seafloor sand and soil.
Ayoub, Jardine and Ramakrishnan (U.S. Pat No. 7,165,621) discloses the use of at least one wellbore to produce gas from a hydrate subterranean formation with a liquid aquifer that can be heated to produce methane gas. However, this patent does not disclose how to harvest the abundant source of gas hydrates from seafloor sand and soil on the seafloor.
Wittle and Bell (U.S. Pat. No. 7,322,409) discloses a system of two electrodes to produce gas hydrates in a drilled well that is extended into an aquifer to heat the gas hydrates. However, this patent is only applicable to concentrations of gas hydrates in reservoirs that can achieve an electro-conductive path and are only accessible using conventional drilling technology in reservoirs under the sea surface. This patent does not provide a means to harvest gas hydrates from marine sand and soil on the seafloor.
Gullapalli, Jones, and Moridis (U.S. Pat. No. 7,537,058 discloses a depressurization method to recover gas hydrates from existing wells relying on the use of existing drilling technology and practices, but does not address a way to recover gas hydrates from marine sand and soil on the surface of the seafloor.
Zhang, Brill and Sarica (U.S. Pat. No. 7,546,880) discloses a process for extracting gas hydrates from sediments below the seafloor by drilling a hole and inserting electrical probes to decompose the gas hydrates that are captured in a balloon type container just above the hole where the gas hydrates are reformed due to the cold temperature near the seafloor. The container is then raised to a higher elevation to decompose the gas hydrates again so that the methane gas can be collected. Only gas hydrates immediately adjacent to the electrical probes would be dissociated into methane gas due to the low thermal conductivity of the seafloor sand and soil, therefore this patent does not provide a practical, economical means to harvest gas hydrates from marine sand and soil on the seafloor.
In summary, no prior art discloses the use of fluidization of marine sand and soil on the seafloor as an efficient means of releasing, and collecting gas hydrate crystals from within the seafloor sand and soil for efficient harvesting and dissociation into methane gas.

OBJECTS AND ADVANTAGES

Large quantities of gas hydrates form in depths of 500 meters to 1000 meters in the Gulf of Mexico. In the US, the Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE) estimated that more than 190 trillion m³of gas exist in marine sand reservoirs on or near the seafloor in the northern Gulf of Mexico. A 21-day expedition to the Gulf of Mexico in April 2009, discovered highly saturated gas hydrates in sand reservoirs at the majority of sites drilled. With the annual natural gas use in the United States of just over 600 billion m³, this local and abundant energy resource could supply the US demand for natural gas for at least the next 200 years.
Hydrocarbons captured in gas hydrates come from various sources. Methane hydrates are the most abundant type in the earth's ocean. They occur in deep water and on land in polar areas. A methane molecule only contains one carbon atom and four hydrogen atoms (CH₄). It is a small molecule, and by itself can form one simple type of hydrate, which is known as structure I. One cubic foot of solid gas hydrate at reservoir temperature and pressure yields approximately 160 cubic feet of methane gas at atmospheric standard conditions. The dissociation of solid gas hydrates requires approximately 10% of the energy of the methane gas produced and is represented by: (CH4.6H2O) solid→(CH4) gas+6(H2O) liquid.
There are significant amounts of gas hydrates predicted to be available in the Gulf of Mexico within close proximity to existing oil and gas production facilities and infrastructure, which will make transportation and distribution very cost effective for harvesting of gas hydrates. Roger Sassen, a gas hydrate geochemist at Texas A&M University, sees methane hydrates as the United States' best way of meeting our country's growing need for energy, especially with our diminishing supply of oil and natural gas in the near future. “Methane hydrates represent the largest most environmentally clean usable fuel resource in the world,” says Dr. Sassen, who has played a pioneering role in identifying and analyzing the fuel source in the Gulf. Dr. Sassen also has pointed out that exploration in the gulf is attractive due to the abundance of oil and gas infrastructure available to assist in the harvesting of gas hydrates. Dr. Sassen also believes that the continual production of methane gas from volcanic activity seeping up through the seafloor surface in the Gulf of Mexico would replenish any gas hydrates harvested on or near the surface of the seafloor in 6-12 months. This means that the safe and economical harvesting of gas hydrates as claimed would be a renewable energy source that could be managed much like harvesting of agricultural food crops in the US.
It is no accident that large amounts of gas hydrates were discovered near Jolliet Field, a site now occupied by a huge oil platform. Oil and gas continually migrate from great depths in the earth's crust toward the gulf floor. Some of the oil and gas is trapped below the sea bottom, but much migrates toward the seafloor where hydrates form. For this reason, natural oil and gas seeps and their associated hydrates are studied as a guide to the presence of subsurface fields and are naturally occurring close-by existing offshore platforms.
Bacteria in marine sediments naturally produce enormous volumes of methane when they feed on plant debris washed into the gulf from rivers and swamps. This “biogenic” methane is often trapped in layers of hydrate that simulate the contours of the seafloor, and can be detected by various seismic techniques commonly used in oil and gas exploration.
From proprietary laboratory experimentation, it has been confirmed that the marine sand and soil found on the seafloor can be very efficiently fluidized using high pressure warm water spray jets in the invention as claimed. Fluidization is a process where a granular material is converted from a static solid-like state to a dynamic fluid-like state. This process occurs when a liquid or gas fluid is passed through the granular material. Once the high pressure warm water spray jets fluidize the seafloor sand and soil, there is no longer any significant barrier to allow efficient heat transfer between the warm water used for the fluidization and the gas hydrate crystals now fluidized with the warm water and seafloor sand and soil. Only slight heating is required to quickly free the gas hydrate crystals from any seafloor sand or soil particles they are attached to. Once this is accomplished, the significantly higher buoyant forces of the solid gas hydrate crystals results in their upward movement and collection beneath a gas hydrate harvester. Buoyancy is an upward acting force exerted by a fluid that opposes the weight of a solid object. If the object is less dense than the liquid, as is the case with the gas hydrates versus seawater, the buoyancy forces the object to float towards the surface and remain afloat.
The use of a gas hydrate harvester provides a mobile artificial reservoir above the seafloor surface to capture gas hydrates. This avoids the requirement to drill into a subsea reservoir with the challenges of cementing the piping in place to prevent blowouts that could result in significant, negative environmental impacts. The use of only existing drilling technology to harvest gas hydrates, as is now being promoted by the US Department of Energy, severely limits the potential of harvesting gas hydrates and significantly increases the cost and complexity of harvesting gas hydrates.
Efficient dissociation of gas hydrate crystals can be done with the addition of heat, the lowering of pressure and both. This is accomplished by transporting the solid gas hydrate crystals in a slurry to a separation vessel at a much lower pressure submerged between the seafloor and sea surface in the invention as claimed. For example, at a depth of 1000 feet (304 meters), the hydrostatic pressure surrounding a submerged separation vessel would be approximately 500 psig. A significant amount of the energy required for the dissociation of the gas hydrates into methane gas is provided by the lower pressure separation vessel at no cost versus the use of heating.
The harvester's gradual movement and small footprint on the seafloor removes solid gas hydrates in an ecologically safe way on the top most surface of the seafloor. There is no harm to marine life that is repelled by a boundary layer surrounding the high pressure warm water spray jets. The remaining sand and soil material being fluidized are evenly distributed back on the surface of the seafloor after the harvester passes. This restores the seafloor to near original condition once the sand and soil has settled on the seafloor.
Producing gas hydrates would redirect methane away from the atmosphere. Using hydrate methane industrially would convert it to carbon dioxide, actually decreasing the short-term effect on atmospheric chemistry and global change. In addition, methane is an environmentally cleaner fuel than oil, coal, or oil shale which all have an immense environmental impact during production and combustion. And no further processing is required for use of the methane gas that is produced in the invention as claimed. This is in stark contrast to other energy sources such as crude oil that requires refining and associated additional energy losses and the additional, negative impacts on green house gases.
The environmental impact of gas hydrate harvesting on marine life is significantly reduced due to the boundary layer of steadily decreasing pressures around the high pressure warm water spray jets. This boundary layer effectively repels any marine life with no noticeable impact as confirmed in laboratory experiments. Marine life is moved away from the edge of the boundary layer surrounding the high pressure, warm water spray jet comes in contact with it. No physical harm occurs to the marine life since the slow moving boundary layer pressure increases as the high pressure warm water spray jets get closer that safely lifts and conveys the marine life out of the way. This boundary layer effect is similar to how a leaf blower moves leaves away from the highest velocity stream without causing any physical damage to the leaves. The harvester would be raised sufficiently above the seafloor to insure the boundary layer extends beyond the harvester perimeter to effectively repel any marine life as the harvester approaches.
The pressure and flowrate of the high pressure warm water spray jets can be controlled according to each specific type of gas hydrate harvesting location to avoid any negative environmental impact on marine life. For applications of dispersed gas hydrate crystals in seafloor sand and soil with lower concentrations of marine life, the use of higher flows and pressures may be safely utilized, as well. Ongoing monitoring of marine life during the harvesting operation can be done to insure that the marine life is not being negatively impacted by the harvesting of gas hydrates.
The most exciting aspect of this invention as claimed is that the natural gas collected can be directly used as a clean energy source without any further refining or processing required as is now required with crude oil collected from offshore rigs. Additionally, there is significantly less negative environmental impacts on the water and air quality due to the less intrusive, surface harvesting approach of the invention as claimed.

BRIEF SUMMARY OF THE INVENTION

A method and apparatus for fluidizing seafloor sand and soil on the seafloor using high pressure warm water spray jets to release gas hydrate crystals that rise beneath a harvester due to buoyant forces. The concentrated gas hydrate crystals collected beneath the harvester are then transferred as a hydrate slurry from the harvester through a hydrate slurry pipe to a separation vessel located between the seafloor and the sea surface. Dissociation of the gas hydrates into methane gas is accomplished in the separation vessel from the lower pressure and the optional use of heat. The methane gas that is liberated from the dissociation of the solid gas hydrate crystals flows upward to a topside vessel on the sea surface from the pressure of the separation vessel and is stored under pressure for transportation to commercial users or alternatively fed directly into an available natural gas pipeline. Any crude oil that is collected in the separation vessel can be separated as a separate liquid phase and pumped to a topside vessel for storage and use in a refinery. Typically, no other processing would be required to produce clean burning natural gas product harvested directly from gas hydrates on or near the seafloor in the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is one embodiment of a gas hydrate harvester process diagram

FIG. 2 is one embodiment of a gas hydrate harvester design

FIG. 3 is one embodiment of a gas hydrate harvester fan spray header design

FIG. 4 is one embodiment of a gas hydrate harvester flowsheet

REFERENCE NUMERALS IN DRAWINGS

10 harvester
11 high pressure warm water spray jets
12 horizontally-directed, high pressure, warm water spray jets
14 hydrate slurry pipe
15 camera
16 solids analyzer
17 hydrate slurry pump (optional)
18 separation vessel
19 demister pad
20 vapor space
21 crude oil phase
22 aqueous phase
23 level control valve
24 crude oil pump
25 crude oil pipe
26 warm water pipe
27 gas vent pipe
28 fixed plate
29 heating supply pipe
30 topside vessel
31 gas storage vessel
32 warm water supply pump
33 warm water heater
34 demineralizer supply pump
35 seawater demineralizer
36 boiler
40 winged formation harvester
41 modular harvesting unit
42 fan spray jet nozzle header

DETAILED DESCRIPTION OF THE INVENTION

The invention as claimed is shown in FIG. 1 that utilizes a harvester 10 with a sloping, pyramid shaped roof that is open on the bottom. The harvester 10 fluidizes the seafloor sand and soil using high pressure warm water spray jets 11 that releases solid gas hydrate crystals from the pore spaces of the seafloor sand and soil. The warm water beneath the harvester 10 releases the gas hydrate crystals from any seafloor sand and soil. The lower density and resulting higher buoyant forces of the solid gas hydrate crystals causes them to float upward towards the harvester 10. The solid gas hydrate crystals that accumulate and concentrate beneath the harvester 10 then enter a hydrate slurry pipe 14 leading to a hydrate slurry pump 17 discharging flow into a separation vessel 18. When the warm spray water, seawater and solid gas hydrate crystals enter the separation vessel 18, the lower pressure and optional use of steam or hot water heating causes the dissociation of the gas hydrate crystals into methane gas. The methane gas produced forms a vapor space 20 in the separation vessel 18. If any crude oil is present on the seafloor, it can also be collected by the harvester 10 due to the lower gravity of the crude oil than the surrounding seawater and transferred with the gas hydrate crystals into the separation vessel 18. The crude oil will separate as the top liquid phase 21 in the separation vessel 18 and is transported by a crude oil pump 24 through a crude oil pipe 25 to the topside vessel 30 for storage and transportation to a refining operation. The aqueous phase 22 will form a discrete liquid phase below the crude oil phase 21. Liberated methane gas that is produced will flow upward out of the gas vent pipe 27 on the top of the separation vessel 18. The liquid level in the separation vessel 18 can be controlled by removing the aqueous phase 22 containing the seawater and warm spray water. A level control valve 23 can be used to release flow of the bottom aqueous phase 22 including any dissolved methane gas to a lower depth. The higher pressure at the lower depth than the separation vessel 18 will keep any residual methane gas dissolved in the liquid phase. The crude oil phase 21, if present, would require a crude oil pump 24 to transport any crude oil collected to the topside vessel 30. A hydrate slurry pump 17 with a variable speed control can be used to control flow of the concentrated solid gas hydrate crystals, warm spray water and seawater from the harvester 10 to the separation vessel 18. Alternatively, the hydrate slung would naturally flow upward into the separation vessel due to its lower, decreasing density without the need for a slurry pump 17. The concentration of gas hydrate solids in the hydrate slurry flow to the separation vessel 18 can be closely monitored with a solids analyzer 16 to achieve optimum hydrate slurry flow and forward speed of the harvester 10. The upward flow energy of the hydrate slurry would be directed into a fixed plate 28 inside the separation vessel 18 to further reduce the size of the gas hydrate crystals to increase the speed of their dissociation into methane gas.
The use of a separation vessel 18 at a higher elevation and lower pressure with the optional addition of steam or hot water through a heating supply pipe 29 provides an efficient, controlled environment for complete dissociation of the gas hydrate crystals. Methane gas liberated from the dissociation of the gas hydrate crystals in the separation vessel 18 then travels up towards the surface and topside vessel 30 under the operating pressure of the separation vessel 18. The pressure of the methane gas is established by the depth of submersion of the separation vessel 18. A methane gas pressure of approximately 500 psi would result from a depth of 1000 ft of the separation vessel 18 below the sea surface assuming the internal and external pressures are approximately equalized to lower the construction cost of the separation vessel 18. The methane gas pressure in the separation vessel 18 transports this gas stream upward to a gas storage vessel 31 on a topside vessel 30 through a gas vent pipe 27. Optionally, a demineralizer supply pump 34 and seawater demineralizer 35 can be used to provide purified water for use in creating high pressure steam or hot water in a boiler 36. Alternative ways to depressurize, heat, and operate the separation vessel to dissociate gas hydrates from an incoming hydrate slurry to produce natural gas and remove excess seawater and any collected crude oil are also considered alternative embodiments of the invention as claimed.
High pressure warm water spray jets 11 are pointed vertically downward to fluidize the seafloor sand and soil and to provide a lift force to reduce drag forces so that the harvester 10 can slowly travel forward on the seafloor with minimal force required. Horizontally directed, high pressure warm water spray jets 12 on each side of the harvester 10 can be used to propel it forward in the direction and speed that is desired by opening and closing valves to these horizontally directed, high pressure warm water spray jets 12. Alternatively, other propulsion means such as used on remotely operated vehicles (ROV's) can be utilized. Cameras 15 will assist in the operation and movement of the harvester 10 on the seafloor by remote control to avoid any obstacles or significant depressions on the seafloor. Naturally occurring warmer seawater near the sea surface is pumped under high pressure by a warm water supply pump 32 through an optional warm water heater 33 to the high pressure warm water spray jets 11 and the horizontally direct, high pressure warm water spray jets 12 through a warm water pipe 26. The mechanical design of the harvester 10 will provide the ability to conform closely to changes in terrain and elevation of the seafloor. This will help insure the harvester 10 is in close contact with the seafloor for efficient fluidization of the seafloor sand and soil to a depth of 6 inches or greater.
Many variations of the invention as claimed are possible. This includes processing of the gas hydrates into methane gas in separate processing vessels located anywhere from the seafloor to the surface. The methane gas that is produced can be stored on separate, adjacent vessels or fed into available distribution piping systems for transportation to consumers. Any means of forward movement of the harvester can be used including the use of cables and winches from the topside vessel 30 or a subsea location with the harvester 10 using sled runners and additional weight to insure its close contact to the seafloor. Land-based, robotic, remote control of any part of the operation of the harvester 10, separation vessel 18, or topside vessel 30 are additional variations of the invention as claimed.
The size and speed of movement of the harvester 10 determines the production rate for the harvesting of gas hydrates. A winged formation harvester 40 of 1000 ft wide and 100,000 ft²surface area exposed to the seafloor using ten, 100 ft wide×100 ft long modular harvester units 41 connected together has been used as one embodiment of a design basis as shown in FIG. 2. An efficient design of a fan spray jet nozzle header 42 as shown in FIG. 3 needs to take into consideration the forward movement of the winged formation harvester 40 on the seafloor to efficiently provide surface coverage for fluidization of the seafloor sand and soil. An efficient approach for effective fluidization of the seafloor sand and soil is to utilize a fan spray jet nozzle header 42 positioned perpendicular to the forward movement of the winged formation harvester 40 to provide both high impact energy and wide coverage. Two rows of a fan spray jet nozzle header 42 provide effective fluidization for each 100 ft wide×100 ft long modular harvester unit 41 to enable a forward movement of approximately 3 inches/sec. The design parameters of a fan spray jet nozzle header 42 for each modular harvester unit 41 including the number of high pressure warm water spray jets 11, their positions and the forward movement of the harvester 10 would be optimized for each type of seafloor environment being harvested.
With a designed distance of 12 inches from the discharge tip of the high pressure warm water spray jets 11 to the seafloor and a typical 40 degree fan spray pattern, a horizontal spacing of 18 inches would provide 100% coverage on the seafloor. With two rows of a fan spray jet nozzle header 42 for each 100 ft wide×100 ft long modular harvester unit 41, this would require 133 vertical spray jet nozzles for each modular harvester unit 41 and 1330 spray jet nozzles total for a winged formation harvester 40.
With a design flow of 7.1 gal/min per nozzle, a total flow of 9443 gal/min would be required. Sizing and design of a fan spray jet nozzle header 42 in each modular harvester unit 41 requires two 12 inch diameter supply pipes to supply warm water to a winged formation harvester 40 of this size. Hydrate slurry pipes 14 collect the gas hydrates that form at the top of each modular harvester unit 41 and transport the solid gas hydrate crystals, warm water and seawater to the separation vessel 18. The weight of the harvester 10 to overcome buoyant forces for the chosen depths of gas hydrate harvesting needs to be considered in the design flow and pressure output to the high pressure warm water spray jets 11. The objective of the high pressure warm water spray jets when harvesting from the seafloor sand and soil is to achieve the desired fluidization of the seafloor sand and soil and to provide lift of the harvester 10 to allow forward movement by use of horizontally-directed, high pressure warm water spray jets 12 from the rear of the harvester 10.
The connection between modular harvesting units 41 would be designed to provide flexibility in the overall structure to allow it to conform to variations in the seafloor topography. An outer skirt of a flexible seal material on the bottom perimeter of each modular unit 41 can also be used to minimize loss of any solid gas hydrate crystals to the outside seawater. No significant dissociation of gas hydrates from the high pressure warm water spray jets 11 is anticipated in the gas hydrate harvester 10. The desired surface dissociation of the gas hydrates from the warm water will assist in the separation of the gas hydrates from the seafloor sand and soil particles that is necessary to allow them to float upward into the harvester 10. The design of the harvester 10 would be optimized for each specific seafloor condition and location being harvested.
Movement of the harvester 10 on the seafloor can be facilitated by the lift of the high pressure warm water spray jets 11 so that the harvester 10 would be suspended above the seafloor by approx. 12 inches. Horizontally-directed, warm water spray jets 12 would be positioned on all sides of the harvester 10 with flow control valves to move the harvester 10 in any direction to achieve the desired production rates. Forward speed would be controlled by the flow of the horizontally-directed, warm water spray jets 12 in the rear of the harvester 10. Braking would be accomplished by the flow of the horizontally-directed, warm water spray jets 12 in the front of the harvester 10. Horizontally-directed, warm water spray jets 12 on either side of the harvester 10 would be used for steering and changing-direction. Alternatively, the harvester 10 could be provided with sufficient, additional weight and sled runners on the bottom to provide close contact with the seafloor. Under this approach, the topside vessel 30 can provide movement using cables and winches between the topside vessel 30 and the harvester 10.
Furthermore, the invention as claimed could be used to help recover crude oil on the seafloor that would float upward into the harvester 10 and be pulled into the hydrate slurry pipe 14 leading into the separation vessel 18 that would be separated into a crude oil phase 21.
Location of gas hydrate deposits on the seafloor can be identified by sonar, modified DART, ultrasound, and other similar, proven technologies. Monitoring devices can be utilized at various locations on the harvester 10, within the separation vessel 18 and the hydrate slurry pipe 14 to monitor production and operations.
Methane gas is produced from the dissociation of gas hydrates with approx. 160 cubic feet of methane gas at atmospheric pressure and temperature being produced for every cubic feet of solid gas hydrate that is dissociated. For ten modular harvester units 41 of 100 ft wide×100 ft long with a fluidization depth of 6 inches, the total volume of seafloor sands and gas hydrates that would be fluidized under the harvester 10 would be 50,000 cubic feet (10×100 ft×100 ft×0.5 ft). Assuming 30% by volume of the seafloor sand and soil contain gas hydrates, the total volume of gas hydrates released would be 15,000 cubic feet (50,000 cubic feet×0.3). Assuming complete dissociation of these gas hydrates into methane gas, a volume of 2.4 million cubic feet of methane gas at standard atmospheric conditions would be produced. With the speed of the harvester 10 traveling at 0.25 ft/sec, it would take approx. 6.7 minutes to produce this 2.4 million cubic feet of methane gas which translates into a production rate of approximately 360,000 standard cubic feet per minute (SCFM) of methane gas.
Based on a depth from the sea surface of the separation vessel 18 of 1000 ft, the pressure of the methane gas delivered to the topside vessel 30 would be approx. 500 psig. Any significant, additional processing for removal of water and contaminants is not anticipated due to the purification and concentration of methane gas in hydrates with increasing depth of their formations, the separation efficiency of the separation vessel 18, and the assistance of a demister pad 19 to remove any entrained water. However, additional separation vessels and processing equipment can be added as required to meet required specifications for use of the produced methane gas as natural gas which are considered part of the invention as claimed. Any small percentages of propane, ethane, carbon dioxide that are contained in the gas hydrates will typically be within the acceptable range for use as commercial natural gas. Therefore, the methane dominant gas produced from this invention as claimed would provide a commercial quality product ready for immediate use in the natural gas distribution grid.
With an energy value of 1000 Btu per cubic feet of methane gas and market price of $4 per million Btu, the total value of this production rate would be $722,000,000 per year for a 350 day, 24 hour per day operation for the winged formation harvester 41 as shown: 2.400,000 ft3/min×1000 Btu/ft3×$4/million Btu/6.7 min×60 min/hr×24 hr/day×350 day/yr=$722,000,000/yr
The economic benefits of this invention as claimed are quite significant as shown from the above sample design case based on a single winged formation harvester 40, and many could be in operation at the same time in the Gulf of Mexico. The production rates of methane gas produced are directly influenced by the efficiency and depth of penetration of the high pressure warm water spray jets 11 to fluidize the gas hydrates trapped in the pore spaces of the seafloor sand and soil. Optimization of the penetration depth and fluidization of the gas hydrates will further increase the production rates and economic benefits.
Storage of the methane gas on the topside vessel 30 would be at high pressure for transportation to commercial and industrial users or directly fed into an available natural gas distribution pipeline. Alternatively, the methane gas could be liquefied for storage at atmospheric pressure to increase the transportation capacity of the topside vessel 30. The topside vessel 30 can be located directly above the harvester 10 on the sea surface or alternatively submerged below the sea surface. The topside vessel 30 could operate from a remote location, and multiple instances of a topside vessel 30 could be utilized. Additional storage vessels could be docked nearby a topside vessel 30 for receiving and transporting the natural gas produced back to the mainland. If an available natural gas distribution pipeline is readily available nearby, the produced methane gas could be fed directly into the natural gas distribution pipeline and a topside vessel 30 would not be required. The high pressure warm water supply pump 32 could be located adjacent to the separation vessel 18 with an inlet suction pipe located near the sea surface if a topside vessel 30 is not being utilized. FIG. 4 shows a flow diagram of gas hydrate harvesting.
The size of the gas hydrate harvesting operation including the harvester 10, separation vessel 18, and topside vessel 30 can be from very small to very large depending upon the desired business investment and return on investment that is desired. Multiple instances of a topside vessel 30 and harvester 10 could be used in a number of different configuration and in adjacent paths to increase the overall production rates as variations of the invention as claimed.

Claims

1. A method of recovering a methane gas from a solid gas hydrate in a seafloor sand or soil comprising:

a. placing a high pressure warm water spray jet at a specific location on the seafloor to fluidize said seafloor sand and soil;

b. placing a harvester above said high pressure warm water spray jet to capture said solid gas hydrate floating upward due to buoyant forces from the fluidization of said seafloor sand and soil;

c. transferring. said solid gas hydrate from said harvester through a hydrate slurry pipe to a separation vessel to dissociate said solid gas hydrate into said methane gas from localized depressurization of said separation vessel.

2. The method of claim 1 wherein said methane gas from said separation vessel is vented to a topside vessel through a gas vent pipe for storage and use as a natural gas.

3. The method of claim 1 wherein said methane gas is vented directly into a natural gas pipeline.

4. The method of claim 1 wherein said solid gas hydrate is transported to said separation vessel using a hydrate slurry pump.

5. The method of claim 1 wherein the separation vessel is heated.

6. The method of claim 1 wherein a crude oil is separated as a separate liquid phase in said separation vessel and pumped to said topside vessel through a crude oil pipe for storage and transportation to a crude oil refinery.

7. An apparatus for recovering said methane gas from said solid gas hydrate in said seafloor sand or soil comprising:

a. placing said high pressure warm water spray jet at a specific location on said seafloor to fluidize said seafloor sand and soil;

b. placing said harvester above said high pressure warm water spray jet to capture said solid gas hydrate floating upward due to buoyant forces from the fluidization of said seafloor sand and soil;

c. transferring said solid gas hydrate from said harvester through said hydrate slurry pipe to said separation vessel to dissociate said solid gas hydrate into said methane gas from localized depressurization of said separation vessel.

8. The apparatus of claim 1 wherein said methane gas from said separation vessel is vented to said topside vessel through said gas vent pipe for storage and use as said natural gas.

9. The apparatus of claim 1 wherein said methane gas is vented directly into said natural gas pipeline.

10. The apparatus of claim 1 wherein said solid gas hydrate is transported to said separation vessel using said hydrate slurry pump.

11. The apparatus of claim 1 wherein said separation vessel is heated.

12. The apparatus of claim 1 wherein said crude oil is separated as said separate liquid phase in said separation vessel and pumped to said topside vessel through said crude oil pipe for storage and transportation to said crude oil refinery.