A T-type well group and a method for exploiting non-diagenetic natural gas hydrate

Abstract

The present invention proposes a T-type well group and a method for exploiting non-diagenetic natural gas hydrates, including: a vertical well and a horizontal well located on one side of the vertical well, the vertical well and the horizontal well share a wellhead; the vertical well target is located in a stable formation below the reservoir; the deflection section, the deflection increase section, the deflection stabilization section and the horizontal section of the horizontal well are all located in a stable formation above the reservoir, and the horizontal well target is located in the reservoir or a stable formation above the reservoir; the vertical well and the horizontal well are both provided with a fracturing sand control layer, and the vertical well fracturing is a horizontal fracture; the horizontal well fracturing is a vertical fracture, the lower end of which extends into the reservoir, and the number of the horizontal well fracturing is not less than 2 sections. The beneficial effects of the present invention are as follows: by setting the vertical well target, the deflection section, the deflection increase section, the deflection stabilization section and the horizontal section of the horizontal well in the T-type well group in a stable formation underlying or overlying the reservoir, the stability and well control safety of the entire well group wellbore can be increased, and the contact area between the wellbore and the reservoir can be increased.

Description

Technical Field

[0001] The invention relates to the technical field of natural gas hydrate drilling and exploitation, in particular to a T-shaped well group and a method for exploiting non-diagenetic natural gas hydrate. Background Art

[0002] Non-diagenetic gas hydrates are mainly buried in shallow seabeds, and their reservoirs are not hard or are not diagenetic but are in the form of quicksand. Therefore, wellbore instability is common in the exploitation of non-diagenetic gas hydrates. If quicksand is produced in large quantities and the reservoir is mined out, the overlying strata will become unstable and collapse, and even induce catastrophic accidents such as submarine landslides or tsunamis.

[0003] Non-diagenetic natural gas hydrates are solid under low temperature and high pressure conditions. During the mining process, as the temperature and pressure around them change, their phase will change from solid to gas, and water will be produced at the same time. If the phase transition is uncontrollable, the volume of the hydrate will expand rapidly. In addition, non-diagenetic natural gas hydrates in the ocean are shallowly buried and the cementing is not strong, which can easily cause accidents such as well wall instability, uncontrolled blowouts, and casualties.

[0004] In summary, in the process of non-diagenetic natural gas hydrate exploitation, safe and stable wellbore conditions are conducive to the effective resolution of production control, environmental risks and other issues. At present, large wellhead vertical wells are commonly used at home and abroad to conduct trial production of non-diagenetic natural gas hydrates in the ocean. Since the above trial production work is mainly to obtain formation information and related data, the methane production is low, so there is no need to consider too much about various out-of-control and safety accidents that may be caused by wellbore instability. However, during the industrial exploitation of non-diagenetic natural gas hydrates, with the large-scale decomposition of hydrates, methane, water, and reservoir sand will be produced at a high speed. This process is very likely to cause catastrophic accidents such as reservoir depletion, instability, wellbore collapse, and uncontrolled blowout. Summary of the invention

[0005] The present invention provides a T-shaped well group and a method for exploiting non-diagenetic natural gas hydrate, which solves the problems of poor well wall stability, unstable wellbore and high risk in the exploitation process in the prior art.

[0006] The technical solution of the present invention is achieved in this way:

[0007] A T-shaped well group comprises: a vertical well and a first horizontal well located on one side of the vertical well, wherein the vertical well and the first horizontal well share a wellhead; a vertical well target of the vertical well is located in a second stable stratum below a reservoir, and a distance from the vertical well target to the reservoir is greater than or equal to 1 / 2 of a distance from the wellhead to the reservoir; a deflection section, an inclination increase section, a deflection stabilization section and a horizontal section of the first horizontal well are all located in a first stable stratum above the reservoir, and a horizontal well target of the first horizontal well is located in the first stable stratum or the reservoir; the vertical well and the first horizontal well are both provided with a fracturing sand control layer, the vertical well fracturing of the vertical well is a horizontal fracturing, and the half-length of the vertical well fracturing is not less than 50 meters; a first horizontal well fracturing of the first horizontal well is a vertical fracturing, the lower end of the first horizontal well fracturing extends into the reservoir, and the number of the first horizontal well fracturing is not less than 2.

[0008] Preferably, a second horizontal well is provided on the other side of the vertical well, and the second horizontal well shares a wellhead with the vertical well; the horizontal well target point, the inclination section, the inclination increase section, the inclination stabilization section and the horizontal section of the second horizontal well are all located in the first stable formation above the reservoir; the second horizontal well is provided with a fracturing sand control layer, the second horizontal well hydraulic fracture of the second horizontal well is a vertical fracture, the lower end of the second horizontal well hydraulic fracture extends into the reservoir, and the number of the second horizontal well hydraulic fractures is not less than 2.

[0009] Preferably, the length of the horizontal section is not less than 200 meters.

[0010] A method for exploiting non-diagenetic natural gas hydrate using a T-shaped well group, the steps of which are as follows:

[0011] (1) Open the vertical well and inject chemical reagents and hot water into the vertical well to start the decomposition of natural gas hydrates in the reservoir;

[0012] (2) When the natural gas hydrate in the reservoir reaches a certain decomposition rate, the methane gas will be separated under the action of gravity, that is, the methane gas will be located above the water layer and the hydrate layer, forming a gas cap. When the gas cap is detected in the reservoir, the vertical well is closed;

[0013] (3) Open the horizontal well and inject chemical reagents as needed, then reduce the pressure and control the production rate according to the actual situation to produce methane gas in the gas cap of the reservoir to ensure that most of the water and sand remain in the reservoir.

[0014] Preferably, the chemical reagent includes but is not limited to carbon dioxide and hot water.

[0015] The construction method of the T-shaped well group of the present invention is as follows:

[0016] A vertical well is drilled first, with the target point of the vertical well located in the second stable formation below the reservoir. After casing completion, perforation, fracturing and sand control operations are carried out before shutting down the well. A horizontal well is drilled using the wellhead of the vertical well. The inclination section, inclination increase section, inclination stabilization section and horizontal section of the horizontal well are all located in the first stable formation overlying the non-diagenetic natural gas hydrate reservoir (not necessarily adjacent to the reservoir). The target point of the horizontal well may be in the overlying stable formation or in the reservoir. After casing completion, perforation, fracturing and sand control operations are carried out before shutting down the well.

[0017] The horizontal wells in the T-shaped well group described in the present invention are production wells, which are mainly used to extract methane in the reservoir. At the same time, the wells help to improve the stability of the well group and enhance the well control effect; the vertical wells can be used as process wells or production wells. In addition to enhancing the stability and well control effect of the well group, they can also be used to inject required reagents or substances including but not limited to CO2, hot water, etc. into the reservoir.

[0018] The beneficial effects of the present invention are:

[0019] The industrial exploitation of non-diagenetic natural gas hydrates using T-type well groups and anti-sand fracturing + gravity differentiation methods is beneficial to the stability of marine formations and ensures the long-term and efficient development of hydrates and the safety of the marine environment.

[0020] By setting the vertical well target points, horizontal well inclination sections, inclination increase sections, inclination stabilization sections and horizontal sections in the T-type well group in stable formations underlying or overlying the reservoir, the stability and well control safety of the entire well group can be increased, and the contact area between the wellbore and the reservoir can be increased.

[0021] The fracturing sand control layer after casing completion slows down the sand production rate to a certain extent and increases the decomposition, diffusion and seepage rate of hydrates.

[0022] By setting the length of the horizontal section to no less than 200 meters, it is beneficial to stabilize the wellbore and increase methane production.

[0023] When there is a gas cap in the reservoir, only the methane in the gas cap is extracted, while most of the water and sand remain in the reservoir. On the one hand, it can effectively avoid catastrophic accidents caused by sand production, reservoir collapse, uncontrollable phase transition of natural gas hydrates and wellbore instability. On the other hand, it can also effectively increase the single-well production and economic benefits of natural gas hydrates. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. 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 paying creative work.

[0025] Figure 1 This is a schematic diagram of the structure of the T-shaped well group described in Example 1 of the present invention;

[0026] Figure 2 This is a schematic diagram of the structure of the T-shaped well group described in Example 2 of the present invention.

[0027] In the figure:

[0028] 1. Vertical well, 2. First horizontal well, 3. Vertical well target, 4. Horizontal well target, 5. Fracturing sand control layer, 6. Vertical well fracturing, 7. First horizontal well fracturing, 8. First stable formation, 9. Second stable formation, 10. Reservoir, 11. Second horizontal well, 12. Second horizontal well fracturing. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] Example 1

[0031] like Figure 1 As shown, the T-type well group includes a vertical well 1 and a first horizontal well 2 that share a wellhead, both of which are completed with casing. The vertical well target 3 of the vertical well 1 is located in the second stable formation 9 below the reservoir 10, and the distance from the vertical well target 3 to the reservoir 10 is 260 meters (the distance from the seabed to the reservoir 10 is 500 meters); the horizontal well target 4, the deflection section, the deflection increase section, the deflection stabilization section and the horizontal section of the first horizontal well 2 are all located in the first stable formation 8 above the reservoir 10, and the length of the horizontal section is 400 meters; the vertical well 1 and the first horizontal well 2 are both provided with a fracturing sand control layer 5, and the vertical well fracturing 6 of the vertical well 1 is a horizontal fracture, and the half length of the vertical well fracturing 6 is 80 meters; the first horizontal well fracturing 7 of the first horizontal well 2 is a vertical fracture, and the lower end of the first horizontal well fracturing 7 extends into the bottom of the reservoir 10, and the number of the first horizontal well fracturing 7 is 3 sections.

[0032] The well construction method of the above T-type well group is as follows:

[0033] First, a vertical well 1 is drilled, and after casing completion, perforation, fracturing and sand control operations are performed before shutting down the well; the wellhead of the vertical well 1 is used to drill a first horizontal well 2, so that the inclination section, inclination increase section, inclination stabilization section and horizontal section of the first horizontal well 2 are all located in the first stable formation 8 overlying the non-diagenetic natural gas hydrate reservoir 10 (not necessarily adjacent to the reservoir), and the horizontal well target 4 can be in the stable formation or in the reservoir. After casing completion, perforations are performed in the direction of the reservoir 10, fracturing and sand control operations are performed, and then the well is shut down.

[0034] The method for exploiting non-diagenetic natural gas hydrate using the above-mentioned T-type well group comprises the following steps:

[0035] (1) opening the vertical well 1 and injecting chemical reagents and hot water into the vertical well 1 to cause the natural gas hydrate in the reservoir 10 to begin to decompose;

[0036] (2) When the natural gas hydrate in the reservoir 10 reaches a certain decomposition rate, the methane gas will be separated under the action of gravity, that is, the methane gas will be located above the water layer and the hydrate layer to form a gas cap. When the gas cap is detected in the reservoir 10, the vertical well 1 is closed;

[0037] (3) Open the first horizontal well 2 and inject chemical reagents as needed, then reduce the pressure and control the production speed according to the actual situation to produce the methane gas in the gas cap of the reservoir 10 to ensure that most of the water and sand continue to remain in the reservoir 10.

[0038] The above-mentioned chemical reagents include but are not limited to carbon dioxide and hot water.

[0039] Example 2

[0040] like Figure 2 As shown, the T-type well group includes a vertical well 1, a first horizontal well 2, and a second horizontal well 11 that share a wellhead, all of which are completed with casing. The vertical well target 3 of the vertical well 1 is located in the second stable formation 9 below the reservoir 10, and the distance from the vertical well target 3 to the reservoir 10 is 480 meters (the distance from the seabed to the reservoir 10 is 800 meters); the horizontal well target 4, the deflection section, the inclination increase section, the inclination stabilization section, and the horizontal section of the first horizontal well 2 are all located in the first stable formation 8 above the reservoir 10, and the length of the horizontal section is 1000 meters; the horizontal well target, the deflection section, the inclination increase section, the inclination stabilization section, and the horizontal section of the second horizontal well 11 are all located in the first stable formation 8 above the reservoir 10, and the length of the horizontal section is 800 meters; the vertical well 1 and the first horizontal well 2 are both provided with a fracturing sand control layer 5, the vertical well fracturing 6 of the vertical well 1 is a horizontal fracture, and the half-length of the vertical well fracturing 6 is 100 meters; the first horizontal well fracturing 7 of the first horizontal well 2 is a vertical fracture, and the lower end of the first horizontal well fracturing 7 extends into the bottom of the reservoir 10, and the number of the first horizontal well fracturing 7 is 5; the second horizontal well fracturing 12 of the second horizontal well 11 is a vertical fracture, and the lower end of the second horizontal well fracturing 12 extends into the top of the reservoir 10, and the number of the second horizontal well fracturing 12 is 3.

[0041] The well construction method of the above T-type well group is as follows:

[0042] First, a vertical well 1 is drilled, and after casing completion, perforation, fracturing and sand control operations are performed before shutting down the well; the wellhead of the vertical well 1 is used to drill the first horizontal well 2 and the second horizontal well 11, so that the inclination section, inclination increase section, inclination stabilization section and horizontal section of the first horizontal well 2 and the second horizontal well 11 are all located in the first stable formation 8 overlying the non-diagenetic natural gas hydrate reservoir 10 (not necessarily adjacent to the reservoir), and the horizontal well target 4 can be in the stable formation or in the reservoir. After casing completion, perforation and fracturing and sand control operations are performed in the direction of the reservoir 10 before shutting down the well.

[0043] The method for exploiting non-diagenetic natural gas hydrate using the above-mentioned T-type well group comprises the following steps:

[0044] (1) opening the vertical well 1 and injecting chemical reagents and hot water into the vertical well 1 to cause the natural gas hydrate in the reservoir 10 to begin to decompose;

[0045] (2) When the natural gas hydrate in the reservoir 10 reaches a certain decomposition rate, the methane gas will be separated under the action of gravity, that is, the methane gas will be located above the water layer and the hydrate layer to form a gas cap. When the gas cap is detected in the reservoir 10, the vertical well 1 is closed;

[0046] (3) Open the first horizontal well 2 or the second horizontal well 11 or both at the same time, and inject chemical reagents as needed, then reduce the pressure, and control the production speed according to actual conditions to produce methane gas in the gas cap of the reservoir 10 to ensure that most of the water and sand continue to remain in the reservoir 10.

[0047] The above-mentioned chemical reagents include but are not limited to carbon dioxide and hot water.

[0048] In summary, the present invention utilizes T-shaped wells to industrially exploit non-diagenetic natural gas hydrates using anti-sand fracturing + gravity differentiation methods, which is beneficial to the stability of marine formations and ensures long-term and efficient development of hydrates and marine environmental safety.

[0049] By setting the vertical well target points, horizontal well inclination sections, inclination increase sections, inclination stabilization sections and horizontal sections in the T-type well group in stable formations underlying or overlying the reservoir, the stability and well control safety of the entire well group can be increased, and the contact area between the wellbore and the reservoir can be increased.

[0050] The fracturing sand control layer after casing completion slows down the sand production rate to a certain extent and increases the decomposition, diffusion and seepage rate of hydrates.

[0051] When there is a gas cap in the reservoir, only the methane in the gas cap is extracted, while most of the water and sand remain in the reservoir. On the one hand, it can effectively avoid catastrophic accidents caused by sand production, reservoir collapse, uncontrollable phase transition of natural gas hydrates and wellbore instability. On the other hand, it can also effectively increase the single-well production and economic benefits of methane.

[0052] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for exploiting non-diagenetic natural gas hydrate using a T-shaped well group, the T-shaped well group comprising: A vertical well (1) and a first horizontal well (2) located on one side of the vertical well (1), wherein the vertical well (1) and the first horizontal well (2) share a wellhead; a vertical well target point (3) of the vertical well (1) is located in a second stable stratum (9) below a reservoir (10), and a distance from the vertical well target point (3) to the reservoir (10) is greater than or equal to 1 / 2 of a distance from the wellhead to the reservoir (10); a deflection section, an increase in deflection section, a stabilization section and a horizontal section of the first horizontal well (2) are all located in a first stable stratum (8) above the reservoir (10), and a horizontal well target point (4) of the first horizontal well (2) is located in a second stable stratum (9) below a reservoir (10). In the first stable formation (8) or the reservoir (10); the vertical well (1) and the first horizontal well (2) are both provided with a fracturing sand control layer (5); the vertical well fracturing fracture (6) of the vertical well (1) is a horizontal fracture, and the half-length of the vertical well fracturing fracture (6) is not less than 50 meters; the first horizontal well fracturing fracture (7) of the first horizontal well (2) is a vertical fracture, the lower end of the first horizontal well fracturing fracture (7) extends into the reservoir (10), and the number of the first horizontal well fracturing fractures (7) is not less than 2; it is characterized in that the method steps for exploiting non-diagenetic natural gas hydrates using the T-type well group are as follows: (A) opening a vertical well and injecting chemical reagents and hot water into the vertical well to cause the natural gas hydrate in the reservoir to begin to decompose; (B) When the natural gas hydrate in the reservoir reaches a certain decomposition rate, the methane gas will be separated under the action of gravity, that is, the methane gas will be located above the water layer and the hydrate layer, forming a gas cap. When the gas cap is detected in the reservoir, the vertical well is closed; (C) Open the horizontal well and inject chemical reagents as needed, then reduce the pressure and control the production rate according to actual conditions to produce methane gas in the gas cap of the reservoir to ensure that most of the water and sand remain in the reservoir.

2. The method for exploiting non-diagenetic natural gas hydrate using a T-shaped well group according to claim 1, characterized in that: The chemical reagents include, but are not limited to, carbon dioxide and hot water.

3. The method for exploiting non-diagenetic natural gas hydrate using a T-shaped well group according to claim 1, characterized in that: A second horizontal well (11) is also provided on the other side of the vertical well (1), and the second horizontal well (11) and the vertical well (1) share a wellhead; the horizontal well target point, the inclination section, the inclination increasing section, the inclination stabilizing section and the horizontal section of the second horizontal well (11) are all located in the first stable stratum (8) above the reservoir (10); the second horizontal well (11) is provided with a fracturing sand control layer, the second horizontal well hydraulic fracture (12) of the second horizontal well (11) is a vertical fracture, the lower end of the second horizontal well hydraulic fracture (12) extends into the reservoir (10), and the number of the second horizontal well hydraulic fracture (12) is not less than 2.

4. The method for exploiting non-diagenetic natural gas hydrate using a T-shaped well group according to claim 1, characterized in that: The length of the horizontal section is not less than 200 meters.

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