Method for Undersea In-Situ Exploitation of Natural Gas Hydrates

This application is a continuation-in-part application of International Application No. PCT/CN2024/094199, filed on May 20, 2024, which is based upon and claims priority to Chinese Patent Application No. 202410568685.3, filed on May 9, 2024, the entire contents of which are incorporated herein by reference.

The present invention relates to natural gas hydrate exploitation technology, and more particularly to a method for undersea in-situ exploitation of natural gas hydrates.

At present, worldwide trial exploitation plans for natural gas hydrates still follow conventional offshore oil and gas exploitation methods, using conventional offshore platforms and drilling techniques for trial exploitation. The long drilling cycle and the huge platform operating cost are the most important factors that lead to high cost of hydrate exploitation. Therefore, it is urgent to break through traditional oil and gas exploitation ideas according to geological conditions and characteristics of occurrence of submarine natural gas hydrates, and develop new exploitation technology and equipment through innovation, to reduce exploitation costs and increase exploitation output.

The characteristic of low permeability of natural gas hydrate reservoirs leads to the need to establish well groups with well spacing far smaller than that of conventional oil and gas reservoirs to achieve commercial production. At the same time, the reservoir characteristics of shallow burial of natural gas hydrates and weak cementation of reservoirs lead to smaller well construction depths (about 200 meters) and lower well construction difficulty. Therefore, conventional offshore ultra-deep drilling techniques based on offshore drilling platforms are not suitable for exploitation of natural gas hydrates.

At present, worldwide trial exploitation plans for natural gas hydrates still follow conventional offshore oil and gas exploitation methods, using conventional offshore platforms and drilling techniques for trial exploitation. The long drilling cycle and the huge platform operating cost are the most important factors that lead to high cost of hydrate exploitation.

Both the gas production rate and the production cost of worldwide natural gas hydrate trial production are far from meeting the requirements of commercial production. This is mainly because the decomposition rate of the natural gas hydrates is limited by heat transfer and seepage rate. Hydrate reservoirs in the South China Sea exhibit low permeability before hydrate decomposition, resulting in a slow heat transfer rate, so the effective mining radius of a single well is much smaller than that of traditional oil and gas reservoirs. However, the cost of using a traditional offshore drilling platform to drill a high-density well patterns is too high.

The purpose of the present invention is to overcome the above deficiencies of the prior art and provide a method for undersea in-situ exploitation of natural gas hydrates to increase the exploitation efficiency and reduce the exploitation cost.

To realize the above purpose, the technical solutions of the present invention are as follows. A method for undersea in-situ exploitation of natural gas hydrates includes:

The step of obtaining the well pattern layout diagram of the natural gas hydrate exploitation block based on the natural gas hydrate exploitation model includes:

Obtaining geological data and geophysical and geochemical parameters of the target natural gas hydrate block to establish a natural gas hydrate exploitation model which simulates real conditions on a laboratory scale; and conducting exploitation simulation experiments under different well pattern layout schemes, and at the same time, repeatedly deducing production data under the different well pattern layout schemes using numerical simulation method to select the optimal well pattern layout scheme to design the well pattern layout diagram of the target natural gas hydrate exploitation block.

Further, the well pattern layout diagram indicates the location, depth and dip angle of individual well, as well as direction and distance between different wells.

Further, the step of lowering the drilling device from the sea surface to drill the well using the casing drilling technique includes:

Further, the step including lowering the completion device from the sea surface, lowering the completion pipe string into the well, and connecting the completion device to undersea wellhead at the wellhead to conduct completion operation, including:

Further, in the whole process of steps of drilling, completion and extraction, a support vessel provides energy and electricity, an underwater hydraulic system provides hydraulic power for each mechanical structure, an undersea monitoring system provides operating data and monitoring images, and an electrical and automatic controlling system provides electricity transmission, voltage change, signal communication and transmission, operation and control, and provides emergency disposal signals in an emergency.

Compared with the prior, the present invention has the following beneficial effects:

Technical solutions of the present invention are further described hereinafter with reference to the accompanying drawings and embodiments.

Referring to, the method for undersea in-situ exploitation of the natural gas hydrates according to the embodiment includes the following steps:

S: obtaining a well network layout diagram of a natural gas hydrate exploitation block based on a natural gas hydrate simulation exploitation model.

In this step, a natural gas hydrate exploitation model that simulates real conditions is established by analyzing basic data of address data and geophysical and geochemical parameters of a target natural gas hydrate block to obtain an optimal well pattern layout scheme to design a well pattern layout of the natural gas hydrate target block to improve the efficiency of natural gas hydrate exploitation and reduce the exploitation cost.

S: carrying an undersea in-situ exploitation system to a target sea area using a vessel with the ability to deploy deep-water equipment, and selecting the well location according to the well pattern layout diagram, wherein the undersea in-situ exploitation system includes a drilling device(with a conduit below, which is responsible for wellhead positioning), a completion device (including a completion pipe string), and a wellhead device.

In this step, compared with a conventional offshore oil and gas ultra-deep drilling technique based on an offshore drilling platform, the method adopts undersea in-situ drilling and completion techniques and equipment without a drilling platform to extract the natural gas hydrates, and transfers the drilling equipment from a platform on the sea to the undersea, which can greatly increase the well construction speed and reduce the exploitation cost. Meanwhile, undersea in-situ exploitation is affected less by oceanic climatic conditions, thereby having obvious advantages and good feasibility.

S: lowering the drilling devicefrom the sea surface to drill a well using a casing drilling technique;

In this step, casing is used instead of a traditional drill pipe to complete drilling.

S: lowering the completion device from the sea surface, lowering a completion pipe stringinto the well, and connecting the completion device to an undersea wellhead at the wellhead to perform completion operation; and

S: lowering the wellhead devicefrom the sea surface, connecting the wellhead deviceto the completion pipe string, connecting the wellhead deviceto a production pipeline, and opening a valve of the wellhead deviceto start a depressurization exploitation process to reduce the pressure at the reservoir depth to decompose natural gas hydrates in the reservoir into gas and water which flow into the production pipelineto be extracted.

It can be seen that the method transfers exploitation equipment, especially drilling equipment, from a platform on the sea to the undersea based on the characteristics of deep water, shallow burial and weak cementation of the natural gas hydrate reservoirs, which improving the exploitation efficiency, greatly reducing the exploitation cost, and reducing potential risks of exploitation on the sea. At the same time, the exploitation output and exploitation economical efficiency can be effectively improved through a denser well pattern layout.

In a specific embodiment, Sspecifically includes:

In a special embodiment, Sspecifically includes:

Thus, owing to the casing drilling technique, steps of well construction steps are reduced, the efficiency of undersea drilling is increased, and well wall risks in a well construction process are lowered.

In a special embodiment, Sspecifically includes:

Further, in the whole process of drilling, completion and exploitation, a support vessel provides energy and electricity, an underwater hydraulic system provides hydraulic power for each mechanical structure, an undersea monitoring systemprovides operating data and monitoring images, and an electrical and automatic control system provides electricity transmission, voltage change, signal communication transmission, operation and control, and provides emergency disposal signals and the like in an emergency.

Specifically, as shown in, the drilling deviceincludes an undersea drilling rig; the undersea drilling rig includes a frame; a gantry guide railand a manipulatorare installed in the frame; a lower hydraulic tongis installed at the bottom of the frame; an upper hydraulic tongis arranged coaxially above the lower hydraulic tong; and a coaxial section between the upper hydraulic tongand the lower hydraulic tongis a drill string setting position, that is, the central position of a wellhead. The upper hydraulic tongis connected with a central pipe of a top driveand rotates with the central pipe; and the top driveis assembled in the gantry guide railto move up and down in the height direction of the gantry guide rail. A pipe rackis arranged within the rotation range of the manipulator; the pipe rackis used for storing and holding the pipe string; and the manipulatoris used for grabbing the pipe stringand placing the pipe stringbetween the upper hydraulic tongand the lower hydraulic tong. In this way, the drill string is assembled and disassembled through the top drive, the upper hydraulic tongand the lower hydraulic tong, the manipulatoris used to pass the single drill string between the center of the wellhead and the pipe rack, the manipulatoris arranged between the center of the wellhead and the pipe rack, little equipment is arranged at the wellhead, and the space occupied is small, so that the space of the whole undersea in-situ drilling and completion device is compact. The undersea drilling rig repeats the drilling process according to the steps of string access connection, rotary drilling, string access connection and rotary drilling.

The above embodiments are only intended to illustrate the technical idea and characteristics of the present invention, aim to enable persons of ordinary skill in the art to understand the content of the present invention and implement it accordingly, and are not intended to limit the protection scope of the present invention. All equivalent changes or modifications made in accordance with the substance of the content of the present invention are within the protection scope of the present invention.