Method for undersea in-situ exploitation of natural gas hydrates

Abstract

A method for undersea in-situ exploitation of natural gas hydrates is provided. The method includes: 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 ship configured to deploy deep-water equipment, selecting the well location according to the well pattern layout diagram; lowering a drilling device using a casing drilling technique; lowering a completion device, lowering a completion pipe string into the well, and connecting the completion device to an undersea wellhead to perform completion operation; and lowering the wellhead device, connecting the wellhead device to the completion pipe string, connecting the wellhead device to a production pipeline, and starting a depressurization exploitation process to reduce the pressure at a reservoir to make gas and water flow into the production pipeline to be extracted.

What is claimed is: 
     
         1 . 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.   
     
     
         2 . The method for the undersea in-situ exploitation of the natural gas hydrates according to  claim 1 , wherein the step of obtaining the well pattern layout diagram of the natural gas hydrate exploitation block based on the natural gas hydrate exploitation model comprises:
 obtaining geological data and geophysical and geochemical parameters of a target natural gas hydrate block to establish the natural gas hydrate exploitation model; and conducting exploitation simulation experiments under a plurality of well pattern layout schemes, and meanwhile, repeatedly deducing production data under the plurality of well pattern layout schemes using a numerical simulation method to select an optimal well pattern layout scheme to design a well pattern layout diagram of a target natural gas hydrate exploitation block; wherein the natural gas hydrate exploitation model simulates real conditions on a laboratory scale.   
     
     
         3 . The method for the undersea in-situ exploitation of the natural gas hydrates according to  claim 1 , wherein the well pattern layout diagram indicates a location, a depth, and a dip angle of individual well, as well as a direction and a distance between different wells. 
     
     
         4 . The method for the undersea in-situ exploitation of the natural gas hydrates according to  claim 1 , wherein the step of lowering the drilling device from the sea surface to drill the well using the casing drilling technique comprises:
 completing a lowering of an undersea drilling rig at a target location firstly, and completing a deployment of the undersea wellhead and a stable placement of the undersea drilling rig under a control of a mechanical mechanism; and   after confirming that a location of the undersea wellhead is correct and the undersea drilling rig is placed stably, starting to drill the well using the casing drilling technique, completing a drilling process by repeating the drilling process following steps of pipe string accessing and connecting, rotating and drilling, pipe string accessing and connecting, rotating and drilling, and meanwhile, completing a circulation and discharge of drilling mud, wherein a whole process is completed by a remote operation and control on the sea surface.   
     
     
         5 . The method for the undersea in-situ exploitation of the natural gas hydrates according to  claim 1 , wherein the step of lowering the completion device from the sea surface, lowering the completion pipe string into the well, and connecting the completion device to the undersea wellhead at the undersea wellhead to conduct the completion operation comprises:
 lowering the completion device from the sea surface; connecting the completion device to the undersea wellhead at the undersea wellhead, injecting cement slurry between a casing and a borehole of the well; after the cement slurry solidifies to complete a well cementing operation and a connection between the reservoir and a wellbore space is completed, lowering the completion pipe string carried by the completion device into the well; and then lowering the wellhead device from the sea surface, and connecting the wellhead device to the completion pipe string, wherein a whole process is completed by a remote operation and control on the sea surface.   
     
     
         6 . The method for the undersea in-situ exploitation of the natural gas hydrates according to  claim 1 , wherein in a 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 system provides 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 in an emergency. 
     
     
         7 . The method for the undersea in-situ exploitation of the natural gas hydrates according to  claim 2 , wherein the well pattern layout diagram indicates a location, a depth, and a dip angle of individual well, as well as a direction and a distance between different wells.