Method for undersea in-situ exploitation of natural gas hydrates

A method for undersea in-situ exploitation of natural gas hydrates, comprising:

• obtaining a well pattern layout diagram of a natural gas hydrate exploitation block based on a natural gas hydrate simulation exploitation model;
• carrying an undersea in-situ exploitation system to a target sea area using a vessel configured to deploy a deep-water equipment, and selecting a well location according to the well pattern layout diagram, wherein the undersea in-situ exploitation system comprises a drilling device, a completion device, and a wellhead device;
• lowering the drilling device from a sea surface, and drilling a well using a casing drilling technique;
• lowering the completion device from the sea surface, lowering a completion pipe string into the well, and connecting the completion device to an undersea wellhead at the undersea wellhead to perform a completion operation;
• and
• lowering the wellhead device from the sea surface, connecting the wellhead device to the completion pipe string, connecting the wellhead device to a production pipeline, and opening a valve of the wellhead device to start a depressurization exploitation process to reduce a pressure at a reservoir depth to decompose the natural gas hydrates in a reservoir into a gas and water, wherein the gas and the water flow into the production pipeline to be extracted.