Method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention

Abstract

Disclosed is a method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention, belonging to the technical field of natural gas hydrates mining. A grouting layer is constructed between a natural gas hydrates reservoir layer and an upper overburden layer by adopting a multi-branch horizontal well process, a vertical shaft is drilled further into the natural gas hydrates reservoir layer after the grouting layer is stabilized, and a mining device and a bottom plugging device are installed in the vertical shaft according to a thickness of the reservoir.

What is claimed is: 
     
         1 . A method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention, comprising:
 S 1 , determining an area where natural gas hydrates are located, and analyzing the area for geological parameters, comprising a stratum permeability coefficient, a stratum temperature and grain composition;   S 2 , determining a drilling position in the area and setting up an offshore production platform; and constructing a vertical shaft on the offshore production platform to penetrate an overburden layer and extend to a boundary position between the overburden layer and a natural gas hydrates reservoir layer;   S 3 , around the vertical shaft, arranging and opening horizontal wells at the boundary position between the overburden layer and the natural gas hydrates reservoir layer with the vertical shaft as a center;   S 4 , grouting slurry in a horizontal direction of the horizontal wells to form a grouting layer;   S 5 , installing a plugging device at a junction of the horizontal wells and the vertical shaft after the slurry is solidified;   S 6 , extending the vertical shaft to the natural gas hydrates reservoir layer in the area, and installing a mining device and a drilling hole bottom plugging device on the vertical shaft; and   S 7 , mining the natural gas hydrates.   
     
     
         2 . The method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention according to  claim 1 , wherein the overburden layer in the S 2  is below a marine layer; the vertical shaft penetrates through the overburden layer and is located at a boundary between the natural gas hydrates reservoir layer and the overburden layer. 
     
     
         3 . The method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention according to  claim 2 , wherein the vertical shaft in the S 3  is located by a bottom in the boundary between the natural gas hydrates reservoir layer and the overburden layer, and the horizontal wells are constructed in the boundary between the natural gas hydrates reservoir layer and the overburden layer. 
     
     
         4 . The method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention according to  claim 1 , wherein the grouting in the horizontal wells in the S 4  comprises determining a grouting method according to spacings and bearing capacities of the horizontal wells, where the grouting method comprises compaction grouting, split grouting, and combining grouting of the split grouting and the compaction grouting. 
     
     
         5 . The method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention according to  claim 4 , wherein the grouting is carried out at a pressure of 3-5 times of a pore water pressure in the horizontal wells. 
     
     
         6 . The method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention according to  claim 1 , wherein the slurry in the grouting layer is water-based liquid retarding materials. 
     
     
         7 . The method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention according to  claim 1 , wherein the plugging device is used for sealing the horizontal wells at wellheads.