CN118855440A - A dual-level multi-branch well type and construction method for seabed hydrate development - Google Patents

Abstract

The invention relates to the technical field of submarine natural gas hydrate exploitation and well construction in the technical field of petroleum and natural gas engineering, in particular to a double-horizontal multi-branch well type and a construction method for submarine hydrate exploitation. The top of the suction anchor is provided with an integrated double-wellhead head, a vertical guide pipe fixedly arranged in one wellhead of the double-wellhead and a pre-bent inclined guide pipe fixedly arranged in the other wellhead of the double-wellhead, and the pre-bent inclined guide pipe can greatly reduce the deflecting guide difficulty of the upper horizontal well on the soft shallow layer of the seabed. The shallow horizontal well and the deep horizontal well do not penetrate through the hydrate reservoir, so that the hydrate decomposition risk caused by drilling disturbance can be avoided, and the well wall instability risk of a long horizontal section is reduced. The multi-branch well can greatly improve the contact area between the well shaft and the hydrate reservoir and improve the recovery ratio of the hydrate reservoir. The size of the guide pipe, the technical sleeve, the production sleeve and the sand prevention screen pipe of the branch well is smaller than that of a conventional deep well, so that the stability of the well wall of a soft stratum at the shallow part of the seabed is improved by a small-well construction technology, and the cost of the sleeve and the well cementation material is reduced.

Description

A dual-level multi-branch well type and construction method for seabed hydrate development

Technical Field

The invention relates to the technical field of seabed natural gas hydrate exploitation and well construction in the technical field of petroleum and natural gas engineering, in particular to a double-level multi-branch well type and a construction method for seabed hydrate development.

Background Art

The reserves of natural gas hydrate (combustible ice) in the sea are huge and are internationally recognized as an important alternative energy source to traditional oil and natural gas. Seabed hydrate resources are often distributed in shallow seabed sediments with a water depth of more than 300 meters and within 1,000 meters below the mud line. The methods for exploiting seabed hydrates include thermal shock mining, decompression mining, chemical reagent injection mining, solid fluidization mining, _CO2 replacement mining, etc.

Due to the weak diagenesis of shallow seabed strata and low sediment strength, it is extremely difficult to maintain the stability of the wellbore when establishing a wellbore in the shallow seabed strata. In addition, the hydrate reservoir is disturbed by factors such as drill string vibration, drill bit rock breaking and heating, and drilling fluid flow during the drilling process, which makes it easy to decompose. Sand production is serious and difficult to effectively control. Therefore, the development and exploitation technology of seabed hydrate resources is still in the early research and experimental exploration stage, and mature technology for large-scale and commercial development has not yet been formed at home and abroad.

my country completed two hydrate production trials in the Shenhu area of the South China Sea in 2017 and 2020, using submarine vertical wellbore and single horizontal wellbore construction modes respectively. The contact area between the vertical wellbore and the hydrate reservoir in the first trial production was very small, making it difficult to increase the hydrate development output of a single well; the horizontal wellbore in the second trial production passed through the hydrate reservoir, which could increase the contact area of the wellbore in the reservoir compared to the vertical wellbore. However, the hydrate reservoir is shallowly buried below the seabed mudline, and it is difficult to construct a surface well when constructing a horizontal well with a high water-to-vertical ratio using conventional methods. In addition, when a long horizontal section passes through the hydrate reservoir, it is easy to cause the hydrate reservoir to decompose, and it is difficult to maintain the stability of the well wall and the integrity of the wellbore.

In recent years, the continuous development and application of complex structure well technologies represented by multi-branch wells, small-hole drilling technology, suction anchor surface well construction technology, etc. have provided new ideas for solving the well construction problems in seabed hydrate exploitation.

Summary of the invention

In view of the above problems existing in the prior art, the present invention is proposed.

Therefore, an object of the present invention is to provide a dual-level multi-branch well type for seabed hydrate development.

In order to solve the above technical problems, the present invention provides the following technical solutions: a dual-level multi-branch well type for seabed hydrate development, comprising:

Suction anchors, shallow horizontal wells, deep horizontal wells and multi-branch wells, wherein the shallow horizontal wells have multi-branch well sidetracking points;

The top cover of the suction anchor is equipped with a double wellhead, a vertical conduit fixedly hung in one wellhead of the double wellhead, and a pre-bent inclined conduit fixedly hung in the other wellhead of the double wellhead.

As a preferred embodiment of the dual-horizontal multi-branch well type for seabed hydrate development of the present invention, the horizontal section of the shallow horizontal well is located in the shallow seabed above the hydrate reservoir, and the vertical depth of the horizontal section of the shallow horizontal well is 10 to 30 meters from the top boundary of the hydrate reservoir; the horizontal section of the deep horizontal well is located in the deep stratum below the hydrate reservoir, and the vertical depth of the horizontal section of the deep horizontal well is 10 to 30 meters from the bottom boundary of the hydrate reservoir.

As a preferred solution of the dual-level multi-branch well type for seabed hydrate development of the present invention, wherein: in the horizontal well section of the shallow horizontal well, the branch well side-drilling point is side-drilled by opening a window downward, and the branch well is connected to the horizontal well section of the shallow horizontal well and the deep horizontal well through the branch well side-drilling point.

As a preferred solution of the dual-level multi-branch well type for seabed hydrate development of the present invention, the suction anchor is composed of an anchor barrel and an anchor cover as the main structure, and is guided into the deep horizontal well by the vertical guide tube hung inside the anchor barrel; and is guided into the shallow horizontal well by the pre-bent inclined guide tube hung inside the anchor barrel.

As a preferred solution of the dual-level multi-branch well type for the development of submarine hydrates of the present invention, the outer diameter of the vertical conduit and the pre-bent inclined conduit is 13-3/8 inches or 20 inches, and the number of each is one. Two sets of technical casings with an outer diameter of 9-5/8 inches are hung at the bottom of the vertical conduit and the pre-bent inclined conduit respectively, and then 7-inch production casings are hung in the two conduit wellheads on the suction anchor cover, and the casing is lowered by floating to the bottom of the shallow horizontal well and the deep horizontal well. The multiple branch wells are drilled by continuous pipe jet drilling and completed by flexible sand control screen pipes, and the screen pipe size can be selected from 4 to 5 inches.

The invention also provides a construction method.

In order to solve the above technical problems, the present invention also provides the following technical solutions: a construction method, including the dual-level multi-branch well type for seabed hydrate development, and a construction method, the construction method comprising the following steps:

S1: Preparation process;

S2: suction anchor installation process;

S3: deep horizontal well construction process;

S4: shallow horizontal well construction process;

S5: Multi-branch well construction process;

The preparation process includes:

Seabed survey and seabed leveling: lowering ROV to detect the levelness of the mud surface at the planned drilling location on the seabed, lowering drilling samplers to explore the sedimentary conditions of the shallow strata 100 meters below the mud surface, and using special equipment to level the seabed;

Operation vessels are in place: engineering vessels, floating drilling platforms or drilling ships are in place, equipped with all drilling and completion equipment, tools and materials, and are ready for drilling and completion.

As a preferred solution of the construction method of the present invention, the suction anchor installation stage includes:

Lowering the suction anchor: The suction anchor is transported to the top of the wellhead to be drilled on the seabed by an engineering vessel or a drilling platform (ship), and slowly lowered to the seabed, using its own weight to make it sink to the soft seabed.

Suction anchor in place: Use a suction pump to pump out the seawater in the anchor barrel, so that the suction anchor sinks further to the predetermined mud penetration depth.

Installing the underwater blowout preventer: Use the drilling riser system to install the underwater blowout preventer on the top of the casing in the lower horizontal well.

In particular, during the installation of the suction anchor, real-time monitoring is required to ensure the verticality of the suction anchor barrel and the horizontality of the anchor cover.

In particular, the double wellhead device is integrated on the suction anchor cover. The 13-3/8-inch or 20-inch vertical well guide tube is pre-hanged and locked in the suction anchor barrel, and is lowered to the designed mud penetration depth along with the suction anchor.

As a preferred embodiment of the construction method of the present invention, the construction process of the lower horizontal well includes:

Drilling: According to the conventional deepwater horizontal well drilling method, the guide drill is lowered from the drilling platform through the watertight pipe into the vertical casing, the vertical well section and the inclined well section are drilled respectively, the 9-5/8-inch technical casing is lowered and hung at the bottom of the casing and cemented, and the drilling is continued to the bottom of the long horizontal well, and the 7-inch production casing is lowered to the bottom of the horizontal well and cemented to above the casing shoe of the technical casing.

Completion: According to the conventional completion method of deepwater wells, the integrated high-pressure wellhead, subsea Christmas tree and completion string are lowered. In particular, since double wellheads need to be arranged in the suction anchor cap, the hydrate reservoir is shallowly buried, and the wellbore is rarely opened, a special compact and simple underwater wellhead suitable for hydrate drilling and production is required.

As a preferred embodiment of the construction method of the present invention, the construction process of the upper horizontal well includes:

Install the pre-bent inclined conduit: Use the jetting method to jet down a 13-3/8-inch or 20-inch pre-bent inclined conduit into the suction anchor barrel to the target depth of mud entry. Check the verticality of the suction anchor barrel and the horizontality of the anchor cover again. If necessary, use a suction pump to pump out the seawater in the anchor barrel again.

Install the underwater blowout preventer: transfer the underwater blowout preventer through the deepwater riser system and install it on the suction anchor wellhead corresponding to the upper horizontal well.

Drilling: According to the conventional deepwater horizontal well drilling method, the guide drill bit is lowered from the drilling platform through the watertight pipe into the pre-bent inclined casing, drilled to the bottom of the deflection section, and the 9-5/8-inch technical casing is lowered and cemented. Then, the well is drilled to the bottom of the horizontal well section, and the 7-inch production casing is lowered and cemented. The cement slurry returns to above the casing shoe of the technical casing.

Completion: Follow the conventional completion method for deepwater wells, lower the integrated high-pressure wellhead, subsea Christmas tree and completion string, etc. Keep the drilling riser system and underwater blowout preventer system connected.

As a preferred embodiment of the construction method of the present invention, the multi-branch well construction process includes:

Casing window sidetracking: lower the casing window sidetracking tool, start from the bottom in the upper horizontal well, open windows and sidetrack at the sidetracking points of each branch well, and then wash the well.

Drilling multi-branch wells: Use continuous tubing drilling, slim hole drilling, water jet drilling, flexible screen drilling, etc. to drill multiple small boreholes from the side boreholes of the upper horizontal well to the lower horizontal well. Drilling small boreholes helps to improve the stability of the open hole wellbore. During the drilling process, strictly monitor and control the wellbore trajectory. When approaching the lower horizontal well, use special electromagnetic detection equipment to navigate to the top of the lower horizontal well. Drill through the lower casing of each branch. If necessary, first remove the multi-branch slim hole drilling tool, replace the casing and grind the drill bit, and drill through the casing of the lower horizontal well one by one.

Completion: Clean the wellbore, check the wellbore integrity, complete the multi-branch well with sand control, and then test and put it into production.

Monitor and inhibit hydrate decomposition: During the drilling of multi-branch wells, monitor the properties of the returned drilling fluid and cuttings in real time, monitor and warn of gas invasion, and prepare a well control and well killing plan. It is recommended to add hydrate inhibitors to the drilling and completion fluid when drilling in hydrate reservoirs, and cool the drilling fluid in the circulation process on the drilling platform (ship).

Beneficial effects of the present invention:

(1) By not passing through the hydrate reservoir through shallow horizontal wells and deep horizontal wells, it is possible to avoid disturbance of the hydrate reservoir during drilling, avoid the adverse effects of hydrate reservoir decomposition on the drilling process, and better maintain the stability of the wellbore.

(2) The technical casing is directly hung at the bottom of the guide tube, and the technical concept of small-hole drilling is adopted. The suction anchor, guide tube, technical casing, production casing and multi-branch sand control pipe all have an outer diameter that is one to two levels smaller than that of conventional deepwater wells, thereby reducing the risk of wellbore instability in shallow soft formations on the seabed and reducing the cost of casing and cementing materials.

(3) The lower horizontal well can be used for heat injection or injection of other fluids, and the upper horizontal well can be used as a production well. By crossing the hydrate reservoir multiple times through multi-branch wells, the contact area between the hydrate reservoir and the wellbore is increased, which can improve the extraction efficiency and recovery rate of hydrates.

(4) The use of double wellhead suction anchors can improve the vertical bearing capacity and horizontal stability of the submarine wellhead. The vertical guide tube is lowered first with the suction anchor, which can save one drilling process and improve drilling efficiency. The pre-bent inclined guide tube can better guide the inclined drilling tool to drill in the shallow seabed, solving the problem of drilling ultra-short radius horizontal wells in soft shallow seabed layers.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. Among them:

FIG1 is an overall schematic diagram of a dual-level multi-branch well of the present invention.

FIG. 2 is an overall schematic diagram of the double wellhead pre-bent inclined guide tube suction anchor of the present invention.

DETAILED DESCRIPTION

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

Example 1

1 and 2 , which are the first embodiment of the present invention, provide a dual-level multi-branch well type for seabed hydrate development, including:

Suction anchor 100, shallow horizontal well 201, deep horizontal well 202 and multi-branch well 203, the shallow horizontal well 201 has multiple branch well sidetracking points 204;

The top of the suction anchor 100 is equipped with a double wellhead 101, a vertical guide tube 102 fixedly hung in one wellhead of the double wellhead 101, and a pre-bent inclined guide tube 103 fixedly hung in the other wellhead of the double wellhead 101.

Specifically, the horizontal section of the shallow horizontal well 201 is located in the shallow seabed B above the hydrate reservoir C, and the vertical depth of the horizontal section of the shallow horizontal well 201 is 10 to 30 meters from the top boundary of the hydrate reservoir. The horizontal section of the deep horizontal well 202 is located in the deep formation D below the hydrate reservoir C, and the vertical depth of the horizontal section of the deep horizontal well 202 is 10 to 30 meters from the bottom boundary of the hydrate reservoir C.

Furthermore, in the horizontal well section of the shallow horizontal well 201 , a branch well side drilling point 204 is side drilled by opening a window downward, and the branch well side drilling point 204 is used to connect the horizontal well sections of the shallow horizontal well 201 and the deep horizontal well 202 to the branch well 203 .

Preferably, the suction anchor 100 is composed of an anchor barrel and an anchor cover as the main structure, and is used to drill into a deep horizontal well 202 through a vertical guide tube 102 installed inside the anchor barrel; and is used to drill into a shallow horizontal well 201 through a pre-bent inclined guide tube 103 installed inside the anchor barrel.

It should be noted that the outer diameter of the vertical guide tube 102 and the pre-bent inclined guide tube 103 is 13-3/8 inches or 20 inches, and the number of each is one; two sets of technical casings with an outer diameter of 9-5/8 inches are hung at the bottom of the vertical guide tube 102 and the pre-bent inclined guide tube 103 respectively, and then 7-inch production casings are hung in the two guide tube wellheads 101 on the anchor cover of the suction anchor 100, and the casing is floated to the bottom of the shallow horizontal well 201 and the deep horizontal well 202; multiple branch wells 203 are drilled by continuous pipe jet drilling and flexible sand control screens are sent in for completion, and the screen size can be selected from 4 to 5 inches.

By not passing through the hydrate reservoir C through the shallow horizontal well 201 and the deep horizontal well 202, it is possible to avoid disturbance of the hydrate reservoir C during drilling, avoid adverse effects of decomposition of the hydrate reservoir C on the drilling process, and better maintain the stability of the wellbore. By injecting heat or fluid into the deep horizontal well 202 and extracting hydrates from the shallow horizontal well 201, the dual horizontal well development mode of "one injection and one extraction" can be used to improve the recovery efficiency of the hydrate reservoir.

The method of using small-bore multi-branch wells 203 to cross the hydrate reservoir C can greatly increase the contact area between the wellbore and the hydrate reservoir C, improve the efficiency of hydrate extraction, and reduce the number of drilling wells; using small-bore drilling technology to cross the hydrate reservoir C is conducive to improving drilling efficiency and better maintaining the wellbore stability of the branch wells 203 inside the hydrate reservoir C.

Example 2

1 and 2 , a third embodiment of the present invention is shown, based on the first embodiment:

In the shallow horizontal well 201, multiple branch well side drilling points 204 are drilled in batches, and the drilling of the branch wells 203 is completed one by one after the casing window side drilling 204, which can reduce the drilling period and save drilling costs. The suction anchor 100 of the pre-bent inclined double wellhead 101 is used for surface well construction. The suction anchor 100 can improve the vertical bearing capacity and horizontal stability of the submarine wellhead. The vertical guide tube is first lowered with the suction anchor to save drilling period. The pre-bent inclined guide tube 103 can better guide the drilling of the shallow horizontal well 201 in the shallow seabed B, reducing the difficulty of inclination guidance of ultra-short radius horizontal wells in soft seabed soil.

Example 3

Referring to FIG. 1 and FIG. 2 , a second embodiment of the present invention is shown. Different from the previous embodiment, this embodiment provides a construction method, including a dual-level multi-branch well type for seabed hydrate development, and a construction method. The construction method includes the following steps:

S1: Preparation process;

S2: suction anchor installation process;

S3: deep horizontal well construction process;

S4: shallow horizontal well construction process;

S5: Multi-branch well construction process;

The preparation process includes:

Seabed survey and seabed leveling: lowering ROV to detect the levelness of the mud surface at the planned drilling location on the seabed, lowering drilling samplers to explore the sedimentary conditions of the shallow strata 100 meters below the mud surface, and using special equipment to level the seabed;

Operation vessels are in place: engineering vessels, floating drilling platforms or drilling ships are in place, equipped with all drilling and completion equipment, tools and materials, and are ready for drilling and completion.

As a preferred solution of the construction method of the present invention, the suction anchor installation stage includes:

Lowering the suction anchor: The suction anchor is transported to the top of the wellhead to be drilled on the seabed by an engineering vessel or a drilling platform (ship), and slowly lowered to the seabed, using its own weight to make it sink to the soft seabed.

Suction anchor in place: Use a suction pump to pump out the seawater in the anchor barrel, so that the suction anchor sinks further to the predetermined mud penetration depth.

Installing the underwater blowout preventer: Use the drilling riser system to install the underwater blowout preventer on the top of the casing in the lower horizontal well.

In particular, during the installation of the suction anchor, real-time monitoring is required to ensure the verticality of the suction anchor barrel and the horizontality of the anchor cover.

In particular, the double wellhead device is integrated on the suction anchor cover. The 13-3/8-inch or 20-inch vertical well guide tube is pre-hanged and locked in the suction anchor barrel, and is lowered to the designed mud penetration depth along with the suction anchor.

As a preferred embodiment of the construction method of the present invention, the construction process of the lower horizontal well includes:

Drilling: According to the conventional deepwater horizontal well drilling method, the guide drill is lowered from the drilling platform through the watertight pipe into the vertical casing, the vertical well section and the inclined well section are drilled respectively, the 9-5/8-inch technical casing is lowered and hung at the bottom of the casing and cemented, and the drilling is continued to the bottom of the long horizontal well, and the 7-inch production casing is lowered to the bottom of the horizontal well and cemented to above the casing shoe of the technical casing.

Completion: According to the conventional completion method of deepwater wells, the integrated high-pressure wellhead, subsea Christmas tree and completion string are lowered. In particular, since double wellheads need to be arranged in the suction anchor cap, the hydrate reservoir is shallowly buried, and the wellbore is rarely opened, a special compact and simple underwater wellhead suitable for hydrate drilling and production is required.

As a preferred embodiment of the construction method of the present invention, the construction process of the upper horizontal well includes:

Install the pre-bent inclined guide tube: Use the jetting method to jet down a 13-3/8-inch or 20-inch pre-bent inclined guide tube into the suction anchor barrel to the target depth of mud entry. Check the verticality of the suction anchor barrel and the horizontality of the anchor cover again. If necessary, use a suction pump to pump out the seawater in the anchor barrel again.

Install the underwater blowout preventer: transfer the underwater blowout preventer through the deepwater riser system and install it on the suction anchor wellhead corresponding to the upper horizontal well.

Drilling: According to the conventional deepwater horizontal well drilling method, the guide drill bit is lowered from the drilling platform through the watertight pipe into the pre-bent inclined casing, drilled to the bottom of the deflection section, and the 9-5/8-inch technical casing is lowered and cemented. Then, the well is drilled to the bottom of the horizontal well section, and the 7-inch production casing is lowered and cemented. The cement slurry returns to above the casing shoe of the technical casing.

Completion: Follow the conventional completion method for deepwater wells, lower the integrated high-pressure wellhead, subsea Christmas tree and completion string, etc. Keep the drilling riser system and underwater blowout preventer system connected.

As a preferred embodiment of the construction method of the present invention, the multi-branch well construction process includes:

Casing window sidetracking: lower the casing window sidetracking tool, start from the bottom in the upper horizontal well, open windows and sidetrack at the sidetracking points of each branch well, and then wash the well.

Drilling multi-branch wells: Use continuous tubing drilling, slim hole drilling, water jet drilling, flexible screen drilling, etc. to drill multiple small boreholes from the side boreholes of the upper horizontal well to the lower horizontal well. Drilling small boreholes helps to improve the stability of the open hole wellbore. During the drilling process, strictly monitor and control the wellbore trajectory. When approaching the lower horizontal well, use special electromagnetic detection equipment to navigate to the top of the lower horizontal well. Drill through the lower casing of each branch. If necessary, first remove the multi-branch slim hole drilling tool, replace the casing and grind the drill bit, and drill through the casing of the lower horizontal well one by one.

Completion: Clean the wellbore, check the wellbore integrity, complete the multi-branch well with sand control, and then test and put it into production.

Monitor and inhibit hydrate decomposition: During the drilling of multi-branch wells, monitor the properties of the returned drilling fluid and cuttings in real time, monitor and warn of gas invasion, and prepare a well control and well killing plan. It is recommended to add hydrate inhibitors to the drilling and completion fluid when drilling in hydrate reservoirs, and cool the drilling fluid in the circulation process on the drilling platform (ship).

In summary, compared with the construction process of traditional submarine hydrate exploitation methods, this solution reduces drilling procedures, saves casing and cementing materials, uses pre-bent inclined guide pipes to reduce the difficulty of guiding and deflecting the seabed surface, and uses small hole technology to provide well wall stability, thereby reducing well construction costs while improving drilling safety in submarine hydrate development.

Importantly, it should be noted that the construction and arrangement of the present application shown in a plurality of different exemplary embodiments are only exemplary. Although only a few embodiments are described in detail in this disclosure, it should be readily understood by those who refer to this disclosure that many modifications are possible (e.g., the size, scale, structure, shape and proportion of various elements, and parameter values (e.g., temperature, pressure, etc.), installation arrangement, use of materials, color, directional changes, etc.) without substantially departing from the novel teachings and advantages of the subject matter described in the application. For example, the element shown as integrally formed can be composed of multiple parts or elements, the position of the element can be inverted or otherwise changed, and the nature or number or position of the discrete element can be changed or changed. Therefore, all such modifications are intended to be included in the scope of the present invention. The order or sequence of any process or method steps can be changed or reordered according to alternative embodiments. In the claims, any "device plus function" clause is intended to cover the structure of performing the function described herein, and is not only structurally equivalent but also equivalent structure. Without departing from the scope of the present invention, other replacements, modifications, changes and omissions can be made in the design, operating conditions and arrangement of the exemplary embodiments. Therefore, the invention is not limited to a specific embodiment, but extends to numerous modifications still falling within the scope of the appended claims.

Additionally, in order to provide a concise description of exemplary embodiments, all features of an actual embodiment (ie, those features that are not relevant to the best mode presently contemplated for carrying out the invention or those that are not relevant to implementing the invention) may not be described.

It will be appreciated that in the development of any actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made. Such a development effort may be complex and time-consuming, but will be a routine task of design, fabrication, and production for those of ordinary skill having the benefit of this disclosure without undue experimentation.

It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims (10)

1. A dual-level multi-branch well type for seabed hydrate development, characterized in that: it includes: A suction anchor (100), a shallow horizontal well (201), a deep horizontal well (202), and a plurality of branch wells (203), wherein the shallow horizontal well (201) has a plurality of branch well sidetracking points (204); A double wellhead (101) is installed at the top of the suction anchor (100), a vertical guide tube (102) fixedly hung in one wellhead of the double wellhead (101), and a pre-bent inclined guide tube (103) fixedly hung in the other wellhead of the double wellhead (101). 2. The dual-horizontal multi-branch well type for seabed hydrate development as described in claim 1 is characterized in that: the horizontal section of the shallow horizontal well (201) is located in the shallow seabed (B) above the hydrate reservoir (C), and the vertical depth of the horizontal well section of the shallow horizontal well (201) is 10 to 30 meters from the top boundary of the hydrate reservoir; the horizontal section of the deep horizontal well (202) is located in the deep stratum (D) below the hydrate reservoir (C), and the vertical depth of the horizontal well section of the deep horizontal well (202) is 10 to 30 meters from the bottom boundary of the hydrate reservoir (C). 3. The dual-level multi-branch well type for seabed hydrate development as described in claim 2 is characterized in that: in the horizontal well section of the shallow horizontal well (201), the branch well side drilling point (204) is side drilled by opening a window downward, and the branch well (203) is connected to the horizontal well section of the shallow horizontal well (201) and the deep horizontal well (202) through the branch well side drilling point (204). 4. The dual-level multi-branch well type for seabed hydrate development as described in claim 3 is characterized in that: the suction anchor (100) is composed of an anchor barrel and an anchor cover as the main structure, and is guided to drill into the deep horizontal well (202) through the vertical guide tube (102) arranged inside the anchor barrel; and is guided to drill into the shallow horizontal well (201) through the pre-bent inclined guide tube (103) arranged inside the anchor barrel. 5. The double-horizontal multi-branch well type for the development of submarine hydrates as claimed in claim 4 is characterized in that: a slim-hole technology design concept is adopted, the outer diameter of the vertical guide tube (102) and the pre-bent inclined guide tube (103) is 13-3/8 inches or 20 inches, and the bottom ends thereof are respectively hung with 9-5/8-inch technical casing to the bottom of the shallow horizontal well (201) and the deep horizontal well (202) inclination section, and the horizontal sections of the shallow horizontal well (201) and the deep horizontal well (202) are respectively inserted with 7-inch production casing by floating casing technology, and multiple branch wells (203) are drilled by continuous pipe jet drilling and completed with flexible sand-proof screen pipes, and the screen pipe size can be selected from 4 to 5 inches. 6. A construction method, characterized in that it comprises the dual-level multi-branch well type for seabed hydrate development as claimed in any one of claims 1 to 5, and a construction method, the construction method comprising the following steps: S1: Preparation process; S2: suction anchor installation process; S3: deep horizontal well construction process; S4: shallow horizontal well construction process; S5: Multi-branch well construction process; The preparation process includes: Seabed survey and seabed leveling: lowering ROV to detect the levelness of the mud surface at the planned drilling location on the seabed, lowering drilling samplers to explore the sedimentary conditions of the shallow strata 100 meters below the mud surface, and using special equipment to level the seabed; Operation vessels are in place: engineering vessels, floating drilling platforms or drilling ships are in place, equipped with all drilling and completion equipment, tools and materials, and are ready for drilling and completion. 7. The construction method according to claim 6, characterized in that the suction anchor installation stage comprises: Lowering the suction anchor: The suction anchor is transported to the top of the wellhead to be drilled on the seabed by an engineering ship or a drilling platform, and slowly lowered to the seabed, and the suction anchor is used to sink to the soft seabed by its own weight; Suction anchor in place: Use a suction pump to pump out the seawater in the anchor barrel, so that the suction anchor sinks further to the predetermined mud depth; Install the underwater blowout preventer: Use the drilling riser system to install the underwater blowout preventer on the top of the guide tube in the lower horizontal well; In particular, during the installation of the suction anchor, real-time monitoring is required to ensure the verticality of the suction anchor barrel and the horizontality of the anchor cover; In particular, the double wellhead device is integrated on the suction anchor cover, and the guide tube of the vertical well is pre-hung and locked in the suction anchor barrel, and is lowered to the designed mud penetration depth along with the suction anchor. 8. The construction method according to claim 7, characterized in that the construction process of the deep horizontal well includes: Drilling: According to the conventional deepwater horizontal well drilling method, the guide drilling tool is lowered from the drilling platform through the watertight pipe into the vertical guide pipe, and the vertical well section and the deflection well section are drilled respectively, the technical casing is lowered for cementing, and the long horizontal well section is continued to be drilled, the production casing is lowered and the cementing is carried out; Completion: According to the conventional completion method of deepwater wells, the integrated high-pressure wellhead, subsea Christmas tree and completion string are lowered; In particular, since dual wellheads need to be arranged in the suction anchor cap, the hydrate reservoir is shallowly buried, and the wellbore is rarely opened, a special compact and simple underwater wellhead suitable for hydrate drilling and production is required. 9. The construction method according to claim 8, characterized in that the shallow horizontal well construction process includes: Install the pre-bent oblique guide tube: Use the jetting method to jet the pre-bent oblique guide tube into the suction anchor cylinder to the target depth of mud entry, check the verticality of the suction anchor cylinder and the horizontality of the anchor cover again, and use the suction pump to pump out the seawater in the anchor cylinder again; Install the underwater blowout preventer: transfer the underwater blowout preventer through the deepwater riser system and install it on the suction anchor wellhead corresponding to the upper horizontal well; Drilling: According to the conventional deepwater horizontal well drilling method, the guide drilling tool is lowered from the drilling platform through the watertight pipe into the pre-bent inclined guide pipe, and the shallow well section and the deflection well section are drilled, the technical casing is lowered and cemented, the long horizontal well section is drilled, the production casing is lowered and cemented; Completion: According to the conventional completion method of deepwater wells, the integrated high-pressure wellhead, subsea Christmas tree and completion string are lowered, and the drilling riser system and underwater blowout preventer system are kept connected. 10. The construction method according to claim 9, characterized in that: the multi-branch well construction process includes: Casing window sidetracking: Run the casing window sidetracking tool, start from the bottom in the upper horizontal well, open windows and sidetrack at the sidetracking points of each branch well, and then wash the well; Drilling multi-branch wells: Use continuous tubing drilling, slim hole drilling, water jet drilling, and screen drilling to drill multiple small holes from the side boreholes of the upper horizontal well to the lower horizontal well. Drilling small holes helps to improve the stability of the open hole wellbore. During the drilling process, strictly monitor and control the wellbore trajectory. When approaching the lower horizontal well, use special electromagnetic detection equipment to navigate to the upper part of the lower horizontal well and drill through the lower casing of each branch. First, take out the multi-branch slim hole drilling tool, replace the casing, grind the drill bit, and drill through the casing of the lower horizontal well one by one; Completion: cleaning the wellbore, checking the wellbore integrity, completing the multi-branch well with sand control, and then testing and putting it into production; Monitor and inhibit hydrate decomposition: During the drilling of multi-branch wells, monitor the properties of the returned drilling fluid and cuttings in real time, monitor and warn of gas invasion, prepare well control and well killing plans, and when drilling in hydrate reservoirs, add hydrate inhibitors to the drilling and completion fluids, and cool the drilling fluids in the circulation process on the drilling platform.