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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No. 2022101859356, filed on Feb. 28, 2022, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present application relates to a deep-sea mining method of hydrates, and in particular to a method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention.

BACKGROUND

As one of the potential energy sources with large amount of reserves and high energy density, natural gas hydrates are cage crystals formed by natural gas and water under high pressure and low temperatures, widely distributed in deep-water strata such as high latitude polar tundra, oceans and lakes. Among the components of natural gas, methane has a high energy density (volume of methane per unit of rock volume at standard conditions), which is 10 times that of coal and black shale and 2.5 times that of natural gas.

Areas where natural gas hydrates are widely distributed include the slopes of continents and islands, uplifts on the edges of active and passive continental margins, and the deep-water environments of polar continental shelves, oceans and some inland lakes; and the conditions for the formation of natural gas hydrates include: low temperature, generally below 10 degrees Celsius (° C.); high pressure, generally above 10 megapascals (MPa); sufficient natural gas sources (hydrocarbons, mainly methane); and favorable hydrate occurrence space.

The rock matrix of the hydrate reservoir layer is usually unconsolidated, weakly consolidated or fractured layers, which are susceptible to changes in the cementation strength, porosity, effective stress and permeability coefficient of the layer as a result of hydrate extraction; moreover, as hydrate is layered in the subsea strata, phenomena of water and sand production from the layers are prone to occur due to the difference in permeability between the hydrate layer and the overburden layer during the mining process, in addition to the disturbance of the original seepage equilibrium caused by hydrate decomposition; as a result, the stability of the well wall is deteriorated, leading to well collapse, which seriously affects the effective exploitation of hydrate resources. Thus, layer deformation and sand control become the key issues in the hydrate decomposition process, and an effective mining method that can control subsea layer deformation while slowing down sand emergence from the well is urgently needed.

SUMMARY

One of the objectives of the present application is to provide a method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention, so as to solve the problems of layer deformation and sand production in the process of mining natural gas hydrates in marine areas, and to improve the mining of deep-sea natural gas hydrates with safety and efficiency.

In order to achieve the above objectives, the present application provides a technical scheme as follows:

a method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention, including:

S1, determining an area where natural gas hydrates are located, and analyzing the area for geological parameters, including stratum permeability coefficient, stratum temperature and grain composition;

S2, 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;

S3, 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;

S4, grouting slurry in a horizontal direction of the horizontal wells to form a grouting layer;

S5, installing a plugging device at a junction of the horizontal wells and the vertical shaft after the slurry is solidified;

S6, 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

S7, mining the natural gas hydrates.

The overburden layer in the S2 is below a marine layer; the vertical shaft penetrates the overburden layer and is located at a boundary between the natural gas hydrates reservoir layer and the overburden layer.

The vertical shaft in the S3 is located by its 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.

The grouting in the horizontal wells in the S4 includes determining a grouting method according to spacings and bearing capacities of the horizontal wells 5, and the grouting method includes compaction grouting, split grouting, and combining grouting of split grouting and compaction grouting.

The grouting is carried out at a pressure of 3-5 times that of a pore water pressure in the horizontal wells.

The slurry in the grouting layer is water-based liquid retarding materials.

The plugging device is used for sealing the horizontal wells at their wellheads.

The present application has the following technical effects: a grouting layer is constructed between the natural gas hydrates reservoir layer and the upper overburden layer by adopting a multi-branch horizontal well process, so that the permeability of the upper part of the natural gas hydrates reservoir layer is reduced, the movement of water from the upper overburden layer to the natural gas hydrates reservoir layer is reduced, and the upper part of the natural gas hydrates is therefore improved in terms of mechanical strength, with mitigated deformation and subsidence of subsea stratum and sand emergence from the wellbore caused by extracting natural gas hydrates. The present application has great value for promotion and application in the technical field of deep-sea mining; it is designed with a clear concept and a simple and operable construction method, so as to effectively improve the stability of the subsea formation during the extraction of deep-sea natural gas hydrates and reduce the accidents of sand production from the wellbore caused by the water produced during extraction.

BRIEF DESCRIPTION OF THE DRAWINGS

For a clearer illustration of the technical schemes in the embodiments of the present application or in the prior art, a brief description of the accompanying drawings to be used in the embodiments is provided below. It is apparent that the accompanying drawings in the following description are only some embodiments of the present application and that other drawings are available on the basis of these drawings to a person of ordinary skill in the field without any effort of creative labor.

FIG. 1 shows a schematic diagram of horizontal wells construction of the present application.

FIG. 2 is a schematic diagram of grouting slurry in the horizontal wells according to the present application.

FIG. 3 shows a schematic plan view of grouting slurry in the horizontals according to the present application.

FIG. 4 is a schematic diagram illustrating hydrates exploitation of the present application.

FIG. 5 illustrates a schematic diagram of seabed stratum subsidence without grouting layer.

FIG. 6 illustrates a schematic diagram of seabed stratum subsidence after being reinforced by the grouting layer of the present application.

FIG.7 is a process illustrating a method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention according to one embodiment of the present application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The technical schemes in the embodiments of the present application are described clearly and comprehensively below in conjunction with the accompanying drawings in the embodiments of the present application. It is clear that the embodiments described are only a part of the embodiments of the present application and not all of them. Based on the embodiments in the present application, all other embodiments obtained without creative labor by a person of ordinary skill in the art fall within the scope of protection of the present application.

In order to make the above-mentioned objectives, features and advantages of the present application more obvious and understandable, the present application is described in further detail below in conjunction with the accompanying drawings and specific embodiments.

The present application provides a method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention as shown in FIG. 7, including:

S1, determining an area where natural gas hydrates are located, and analyzing the area for geological parameters, including permeability coefficient of natural gas hydrates stratum, and grain composition;

S2, determining a drilling position in the area and setting up an offshore production platform 1; and constructing a vertical shaft 3 on the offshore production platform 1 to penetrate an overburden layer 4 and extend into the area;

S3, arranging and opening horizontal wells 5 in the area with the vertical shaft 3 as a center;

S4, grouting slurry in a horizontal direction of the horizontal wells 5 to form a grouting layer 8;

S5, installing a plugging device 9 at a junction of the horizontal wells 5 and the vertical shaft 3 after the slurry is solidified;

S6, extending the vertical shaft 3 to a natural gas hydrates reservoir layer 6 in the area, and installing a mining device 10 and a drilling hole bottom plugging device 11 on the vertical shaft 3; and

S7, mining the natural gas hydrates.

The overburden layer 4 in the S2 is below a marine layer 2; the vertical shaft 3 penetrates through the overburden layer 4 and is located at a boundary between the natural gas hydrates reservoir layer 6 and the overburden layer 4.

The vertical shaft 3 in the S3 is located by its bottom in the boundary between the natural gas hydrates reservoir layer 6 and the overburden layer 4, and the horizontal wells 5 are constructed in the boundary between the natural gas hydrates reservoir layer 6 and the overburden layer 4.

The grouting in the horizontal wells 5 in the S4 includes determining a grouting method according to spacings and bearing capacities of the horizontal wells 5, where the grouting method includes compaction grouting, split grouting, and combining grouting of split grouting and compaction grouting.

The grouting is carried out at a pressure of 3-5 times that of a pore water pressure in the horizontal wells 5.

The slurry in the grouting layer 8 is water-based liquid retarding materials.

In one embodiment of the present application, the slurry is injected into the boundary between the overburden layer 4 and the natural gas hydrates reservoir layer 6, so as to fill up pore particles in the boundary and displace the seawater therein and solidify and cement the surrounding particles.

The plugging device 9 is used for sealing the horizontal wells 5 at their wellheads.

In one embodiment of the present application, the area where natural gas hydrates are located is generally found below a subsea sediment layer, with the overburden layer 4 of seawater mixed with particulate pores between the marine layer 2 and the natural gas hydrates reservoir layer 6, and a deeper lower layer 7 at the bottom of the natural gas hydrates reservoir layer 6;

according to the present application, as shown in FIG. 1 to FIG. 3, the vertical shaft 3 is firstly constructed to a designed horizon located in the boundary between the overburden layer 4 and the natural gas hydrates reservoir layer 6, and the grouting layer 8 is formed in the horizontal wells 5 by using the stratum grouting process, where the grouting layer 8 of a plate structure is designed to lower the permeability at the top of the natural gas hydrates reservoir layer 6, and reduce the flow of water in the overburden layer 4 to the natural gas hydrates reservoir layer 6, so as to improve the mechanical strength of the upper part of the natural gas hydrates and mitigate the deformation and subsidence of the seabed stratum caused by natural gas hydrates mining.

In the prior art, there is generally no protective setup in the process of extraction, with only one vertical shaft 3 opened directly into the natural gas hydrates reservoir layer 6; or, the horizontal wells 5 are arranged and the extraction is performed directly downwards; as comparing to the technical schemes of the present application, the seawater and sand in the overburden layer 4 applied with prior art are prone to subside, thus affecting the mining device and causing accidents.

In a further optimized technical scheme, the vertical shaft 3 is designed to be constructed in sections, which is different from a vertical shaft 3 directly connected to the natural gas hydrates reservoir layer 6 in the prior art. In the present application, it is possible to construct two vertical shafts 3 at the same position, with one being constructed to the designed horizon to build the grouting layer; while the other vertical shaft 3 is extended to the natural gas hydrates reservoir layer 6 by a same construction mode, then sealed by the drilling hole bottom plugging device 11, and the natural gas hydrates are mined by the mining device.

In a further optimized technical scheme, the pressure of grouting and the spacings between the horizontal wells 5 are designed and determined according to the geological parameters including permeability coefficient of natural gas hydrates stratum, stratum temperature and grain composition in S1, and the specific grouting method is determined by a ratio of the grouting pressure to the pore water pressure in horizontal wells 5.

Embodiment 1

Deep-sea drilling data is utilized to analyze the geological parameters such as stratum permeability coefficient, stratum temperature and grain composition in the area where natural gas hydrates are located, and the position of vertical shaft 3 and the thickness of overburden layer 4 are determined, then the stratum pressure of regional strata is calculated with reference to the depth of marine layer; then the location of the boundary between overburden layer 4 and natural gas hydrates reservoir layer 6 is determined according to logging curves and geological radar data, and the diameter and spacings of horizontal wells 5 are determined according to the mining area; taking the occurrence of natural gas hydrates reservoir layer in a certain sea area as an example, the specific data are as follows: seawater depth of 800 meters (m), thickness of overburden layer of 200 m, stratum permeability coefficient of 1.2×10⁻⁶ centimeters per second (cm/s), stratum pressure of 12 megapascals (A/Pa), stratum temperature of 4 degree Celsius (° C.), and average particle size of stratum particles of 100 micrometers (μm), diameter of the vertical shaft of 2 m, diameter of the horizontal well 5 of 0.3 m, where 6 horizontal wells at 60° intervals centered on the vertical shaft 3 are arranged with a drilling depth of 10 m;

the offshore production platform 1 and the vertical shaft are then constructed on the basis of the above parameters, where the vertical shaft 3 penetrates through the overburden layer 4 to the designed position of the horizontal wells 5; considering the extremely fine average particle size of the stratum particles, the method of split grouting is adopted to grout slurry in the horizontal wells 5, with a grouting pressure of 40 MPa; then the grouting layer 8 is developed and the plugging device 9 is installed to separate the horizontal wells 5 from the passage of the vertical shaft 3; then the construction is extended to the natural gas hydrates reservoir layer 6;

Further, the natural gas hydrates are mined by the mining device 10 in the vertical shaft 3, as shown in FIG. 4.

Further, in the present embodiment, the grouting layer 8 is formed to stably isolate the overburden layer 4 form the natural gas hydrates reservoir layer 6; the permeability at the top of the natural gas hydrates reservoir layer 6 and the flow of water in the overburden layer 4 to the natural gas hydrates reservoir layer 6 are reduced accordingly, and the mechanical strength of the upper part of the gas hydrates is improved so as to mitigate the deformation and subsidence of the seabed stratum caused by the gas hydrates exploitation, see FIG. 5 and FIG. 6 for comparison.

Embodiment 2

The present embodiment is different form Embodiment 1 in that the grouting method of compaction grouting, or combining grouting of split grouting and compaction grouting is adopted when the stratum permeability coefficient is larger than 3×10⁻⁵ cm/s, and the average particle size of stratum particles is greater than 1 millimeter (mm) in the horizontal wells 5.

In the description of the present application, it is to be understood that the terms “longitudinal”, “transverse”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, etc. indicate an orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings and are intended only to facilitate the description of the invention and do not indicate or imply that the device or element referred to must have a particular orientation, or be constructed and operated in a particular orientation, and are therefore not to be construed as limiting the present application.

The above-mentioned embodiments only describe the preferred mode of the present application, and do not limit the scope of the present application. Under the premise of not departing from the design spirit of the present application, various modifications and improvements made by ordinary technicians in the field to the technical schemes of the present application shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method for deep-sea extraction of natural gas hydrates with reservoir top control and sand prevention, comprising:

S1, 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;

S2, 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;

S3, 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;

S4, grouting slurry in a horizontal direction of the horizontal wells to form a grouting layer;

S5, installing a plugging device at a junction of the horizontal wells and the vertical shaft after the slurry is solidified;

S6, 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

S7, 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 S2 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 S3 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 S4 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.