Methods and systems for determining whether conditions exist for presence of clathrates are disclosed. One method includes determining a thickness of a clathrate stability zone based, at least in part, on a depth at which a temperature reaches a three-phase equilibrium temperature of the clathrates. The method also includes calculating a temperature and a three-phase equilibrium temperature for a range of depths in the clathrate stability zone, and determining a minimum pore size at each of the depths in the range of depths, the minimum pore size permitting a predetermined saturation level of clathrates and based at least in part on the temperature and three-phase equilibrium temperature. The method further includes converting the minimum pore size at each of the depths to a minimum porosity.

The invention discloses a suction cylinder exploitation device and method for marine natural gas hydrates. The exploitation device comprises an exploitation cylinder, a water pump, a sand control device, a liquid-gas filling system and the like. Through the specially-designed exploitation cylinder and mating devices thereof, the exploitation cylinder can sink below a seabed surface to exploit natural gas hydrates deep below the seabed surface and can be withdrawn. A series of problems such as high well drilling and completion cost of traditional deep-sea drilling exploitation methods, and damage, collapses and sand generation of plain concrete wellbores under the effect of formation pressure are solved, and the limitations that traditional capping depressurization methods can only exploit submarine superficial hydrates and are low in exploitation efficiency are overcome. The invention can greatly reduce the exploitation cost of natural gas hydrates deep below the seabed surface and is of great significance for commercial exploitation of marine natural gas hydrates.

A methane production assembly comprising a subsea well (3) extending from the seabed to a methane hydrate formation (5). The assembly comprises a well casing (7) extending into the subsea well (3), a subsea well control assembly (9), a submersible pump (17) in fluid communication with the methane hydrate formation, and a methane-water separator (29) having a water outlet (31) and a methane outlet (32). The submersible pump is arranged above the subsea well.

A method and equipment for harvesting natural gas from seabed hydrates are disclosed. The preferred equipment includes a mobile riser, a water injection nozzle, a gas collector, a gas separator, a gas compressor, a water pump, and a water boiler. A fraction of produced gas is used to heat water which is in turn injected to seafloor for dissociating gas hydrates. The preferred method of the invention comprises producing natural gas from seabed hydrates using a production ship with a moving riser installed. This method eliminates the need of drilling wells and thus cuts cost of gas production tremendously.

A method for recovering gas in natural gas hydrate exploitation is disclosed, in which a gas-water mixture at a bottom of a exploitation well is delivered to an ocean surface platform through a marine riser, by adopting the gas-lift effect of methane gas derived from the dissociation of natural gas hydrate, so as to achieve a controllable flowing production of marine natural gas hydrate. In the startup stage, the pressure in the bottom of the well is decreased by the gas-lift effect of the injected gas to allow dissociation of the hydrate. In the flowing production stage, the flowing production is achieved by the gas-lift effect of the gas derived from the dissociation of the natural gas hydrate, wherein a seafloor gas tank is employed to control the flowing rate and replenish the consumed gas.

The example of implementation of the invention provides a method used for exploiting a natural gas hydrate reservoir and belongs to the field of natural gas hydrate reservoir exploitation. The method mainly comprises a heat-injecting vertical well and a production horizontal well. The production horizontal well is opened in a segmental manner. In combination with natural gas hydrate heat exploitation, heat flow injected into the vertical well is enabled to heat different positions of the natural gas hydrate reservoir to increase a recovery ratio of the natural gas hydrate reservoir. The method specifically comprises the steps of injecting a heat flow into the natural gas hydrate reservoir by utilizing the vertical well to promote decomposition of the hydrate; firstly opening a horizontal branch of the horizontal well fully and keeping depressurization exploitation. When gas production amount is reduced to a certain degree, dividing a segmentation number based on permeability of the natural gas hydrate reservoir and performing segmental exploitation on the horizontal branch from a toe end to a heel end to realize heat exploitation of the heterogeneous natural gas hydrate reservoir. The method makes full use of a horizontal well structure, reduces exploitation cost and provides effective measures for efficient exploitation of the heterogeneous natural gas hydrate reservoirs.

A method for mechanically stabilizing deep sea sediments, marine raw material deposits and/or submarine slope and/or to a control/conditioning method for the hydraulic properties of deep sea sediments. A gas hydrate-forming substance is injected into marine or submarine sediments, and gas hydrate sediment composites are formed.

A system for exploiting natural gas hydrate with downhole gas-liquid synergic depressurization includes a casing configured to form an exploitation well. An upper end of the exploitation well is connected to a produced gas collection pipeline, and the produced gas collection pipeline is configured to be connected to a produced gas recovery system. A perforated channel is distributed in a section of the casing located in a natural gas hydrate reservoir. A tubular string component assembly is mounted in the exploitation well, and includes an outer string, a production tubular string and an auxiliary riser. A first check valve is mounted at the bottom of the outer string, a gas supply pipeline is connected into an upper portion of the outer string, and a flow controller is mounted in the gas supply pipeline. The production tubular string is mounted in the outer string.

A system for exploiting natural gas hydrate with downhole gas-liquid synergic depressurization includes a casing configured to form an exploitation well. An upper end of the exploitation well is connected to a produced gas collection pipeline, and the produced gas collection pipeline is configured to be connected to a produced gas recovery system. A perforated channel is distributed in a section of the casing located in a natural gas hydrate reservoir. A tubular string component assembly is mounted in the exploitation well, and includes an outer string, a production tubular string and an auxiliary riser. A first check valve is mounted at the bottom of the outer string, a gas supply pipeline is connected into an upper portion of the outer string, and a flow controller is mounted in the gas supply pipeline. The production tubular string is mounted in the outer string.

An apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates includes a gas recovery pipe column, a water recovery pipe column, a gas-liquid mixed transportation pipe section, a data collecting and processing unit, and a reaction control apparatus; according to characteristics of different exploitation pipe columns, three injection pipelines and three monitoring points are arranged to predict dynamic changes in a secondary generation risk of the hydrates throughout the wellbore; measures for preventing and controlling the secondary generation of the hydrates are taken at different pipe column positions by the integrated utilization of inhibitor injection, heating for pipe columns, the additional arrangement of electric submersible pumps, and other methods.