Reverse three-horizontal well pattern structure and method for exploiting natural gas hydrates

Abstract

The present invention discloses a reverse three-horizontal well network structure and method for exploiting natural gas hydrates, which belongs to the technical field of natural gas hydrate exploitation. The reverse three-horizontal well network structure includes a plurality of three-horizontal well groups uniformly arranged radially along the injection wells, each of which is composed of a production well, a heat injection branch well, and a sand drainage well, wherein the heat injection branch well is connected to the injection well; on the well wall of the horizontal section of each well located in the natural gas hydrate reservoir, a plurality of insulation pipe groups are evenly spaced along the length direction, each insulation pipe group is composed of insulation pipes respectively arranged on the horizontal section of each well, and the insulation pipes in the same insulation pipe group are aligned up and down; openings are provided on the parts of the well wall of the production horizontal section, the heat injection branch horizontal section, and the sand drainage horizontal section where insulation pipes are not provided. The well network structure described in the present invention can effectively discharge sand and water, avoid wellbore blockage, and prevent reservoir collapse.

Description

Technical Field

[0001] The present invention belongs to the technical field of natural gas hydrate exploitation, and in particular relates to a reverse three-horizontal well pattern structure and method for exploiting natural gas hydrates. Background Art

[0002] Natural Gas Hydrate is a kind of cage-shaped crystal substance formed by natural gas and water under specific temperature and pressure (low temperature and high pressure). It is similar to ice and snow. Because it can be ignited, it is commonly known as "combustible ice". Natural Gas Hydrate is mainly stored in solid form in permafrost and deep sea. The estimated global reserves of natural gas hydrate are 10 15 ~10 18 STm 3 , exceeding all proven conventional oil and gas reserves. Therefore, natural gas hydrates are considered the most promising alternative energy source, capable of meeting global energy needs and addressing future climate change. Over the past few decades, natural gas hydrates have become a hot topic of research, with most research focused on extracting natural gas from them.

[0003] Natural gas extraction from hydrate reservoirs is a complex process involving multiphase flow (gas, liquid, ice, and hydrates), heat transfer, and endothermic reactions. Currently, the main methods for extracting natural gas hydrates include heat injection, pressure reduction, chemical treatment, and carbon dioxide replacement. Heat injection involves injecting a hot fluid at a certain temperature into the natural gas hydrate reservoir, or using downhole heating methods such as fire flooding to raise the system temperature within the reservoir and promote the decomposition of natural gas hydrates. Pressure reduction reduces the reservoir system pressure, altering the hydrate phase equilibrium to a certain extent and promoting decomposition. Chemical treatment involves injecting methanol-based chemical reagents to promote decomposition. Carbon dioxide replacement involves injecting carbon dioxide at a certain pressure and temperature into the reservoir to displace methane from the natural gas hydrates and promote decomposition. While all of these methods are capable of extracting natural gas hydrates, the low heat transfer efficiency and low conductivity of hydrate reservoirs remain two major challenges in extracting natural gas hydrates.

[0004] The invention patent with publication number CN108915643A discloses a dual-connected well structure for the exploitation of marine hydrates. By setting two connected wells spaced apart in the hydrate layer, a pressure differential can be generated between the two connected wells. Under the action of the pressure differential, a corresponding flow channel will be formed in the hydrate layer to facilitate the flow of the injected fluid and the fluid in the hydrate layer, increasing the contact area between the injected fluid and the hydrate layer. In addition, the flow of the fluid can also drive heat exchange, thereby improving the heat transfer efficiency and flow conductivity of the hydrate layer and enhancing the extraction efficiency of natural gas hydrates. However, this invention adopts both heat injection and displacement methods, providing two injection channels but only one extraction channel. If the gas, liquid, and sand particles after the decomposition of natural gas hydrates cannot be discharged in time, it will cause the reservoir pressure to increase, and the sand particles will block the extraction channel, affecting the extraction effect. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a reverse three-horizontal well network structure and method for exploiting natural gas hydrates, which is centered on the injection well and radially arranged in a cluster manner. This structure can effectively discharge sand and water, avoid wellbore blockage, and prevent reservoir collapse.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A reverse three-horizontal well pattern structure for exploiting natural gas hydrates includes multiple three-horizontal well groups, each of which is evenly arranged radially along the injection well; each three-horizontal well group is composed of production wells, heat injection branch wells, and sand drainage wells, wherein the production well includes a production vertical section and a production horizontal section, the heat injection branch well includes a heat injection deflection section and a heat injection branch horizontal section, and the sand drainage well includes a sand drainage vertical section and a sand drainage horizontal section, the production horizontal section, the heat injection branch horizontal section, and the sand drainage horizontal section are all located in the natural gas hydrate reservoir, the production horizontal section is close to the overburden, the sand drainage horizontal section is close to the underburden, and the heat injection branch horizontal section is located between the production horizontal section and the sand drainage horizontal section; the heat injection branch wells in the three-horizontal well group are all connected to the injection well;

[0008] In each three-horizontal well group, several insulation pipe groups are evenly spaced along the length direction on the well walls of the production horizontal section, the heat injection branch horizontal section and the sand drainage horizontal section. Each insulation pipe group consists of an insulation pipe arranged on the production horizontal section, an insulation pipe arranged on the heat injection branch horizontal section and an insulation pipe arranged on the sand drainage horizontal section. The insulation pipes in the same insulation pipe group are aligned up and down; openings are provided on the parts of the well walls of the production horizontal section, the heat injection branch horizontal section and the sand drainage horizontal section where no insulation pipes are provided.

[0009] Furthermore, in each three-horizontal well group structure, it is preferred that the extension direction of the production horizontal section is the same as the extension direction of the sand discharge horizontal section, and the extension direction of the heat injection branch horizontal section is opposite to it, which is more conducive to the layout of the three-horizontal well group.

[0010] Furthermore, in each three-horizontal well structure, the openings are spaced apart along the length of the wellbore in the production, heat injection, or sand removal sections, and evenly distributed along the circumference of the wellbore. Furthermore, the diameter of the openings is preferably at least twice the radius of the largest sand grain in the natural gas hydrate reservoir.

[0011] The present invention also includes a method for exploiting natural gas hydrates using the above-mentioned reverse three-horizontal well pattern structure, comprising the following steps:

[0012] 1) injecting thermal fluid into the production wells, heat injection branch wells, and sand drainage wells in each three-horizontal well group, respectively, and performing thermal circulation inside the production wells, heat injection branch wells, and sand drainage wells in the corresponding three-horizontal well group. The thermal fluid diffuses outward through the openings on the production wells, heat injection branch wells, and sand drainage wells. Through continuous thermal circulation, a vertically connected thermal cavity is formed in the same vertical area including the corresponding opening positions on the production horizontal section, the heat injection branch horizontal section, and the sand drainage horizontal section;

[0013] 2) After forming a hot cavity interconnected with the upper and lower parts, stop injecting hot fluid into the production well and the sand drainage well, and continue injecting hot fluid into the branch heat injection well to dissolve the natural gas hydrate. The dissolved natural gas is produced through the production well, while the water and sand produced by the dissolution of the natural gas hydrate are discharged through the sand drainage well. The flow rate of the output in the production well is controlled by controlling the temperature and pressure of the hot fluid injected into the branch heat injection well.

[0014] 3) When the natural gas hydrate is mined to the expected level or exhausted, water is first injected into the production wells, heat injection branch wells and sand drainage wells in each of the three horizontal well groups. After the water is filled, cement or proppant is injected into the production wells, heat injection branch wells and sand drainage wells to complete the plugging of the well network.

[0015] In the above method, if the blocked well pattern is reactivated as needed to further produce natural gas hydrates, the above method further includes the following step 4):

[0016] 4) Reopen the plugged well pattern and perform perforation at corresponding positions of the insulation pipe group set on the production horizontal section, the heat injection branch horizontal section, and the sand drainage horizontal section of each three-horizontal well group. After completion, continue steps 1) to 3) until the natural gas hydrate is fully mined.

[0017] Compared with the prior art, the present invention is characterized in that:

[0018] 1. A reverse three-horizontal well pattern structure with the injection well as the center and cluster wells radially arranged in the surrounding areas is adopted to achieve unified heat injection in each three-horizontal well group, realize directional heat transfer, and improve heat utilization efficiency; it also eliminates the need to drill multiple heat injection channels, saving development costs.

[0019] 2. The specially designed three-horizontal well group can effectively discharge sand and water, avoid wellbore blockage, and prevent reservoir collapse. Specifically, a sand drainage well is added near the underlying rock formation. Combined with the setting of the insulation pipe group and the opening, through thermal circulation, a vertically connected thermal cavity is formed in the same vertical area corresponding to the opening position on the horizontal section of the three wells. While providing the required heat for the decomposition of natural gas hydrates, the water and sand produced by the dissolution of natural gas hydrates are discharged through the sand drainage well, effectively avoiding wellbore blockage. At the same time, the natural gas hydrates above and below the insulation pipe group are reserved as supports to prevent natural gas leakage caused by reservoir collapse. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is an implementation of the reverse three-horizontal well pattern structure for exploiting natural gas hydrates described in the present invention.

[0021] Figure 2 for Figure 1 A schematic cross-sectional view of the injection well and the heat injection branch well in the embodiment.

[0022] Figure 3 for Figure 1 A schematic cross-sectional view of a three-horizontal well group in the embodiment.

[0023] The numbers in the figure are:

[0024] 1. Sand drainage well; 101. Vertical sand drainage section; 102. Horizontal sand drainage section; 2. Heat injection branch well; 201. Heat injection deflection section; 202. Heat injection branch horizontal section; 3. Production well; 301. Vertical production section; 302. Horizontal production section; 4. Overburden; 5. Natural gas hydrate reservoir; 6. Underburden; 7. Insulated pipe; 8. Heat cavity; 9. Opening; 10. Injection well. DETAILED DESCRIPTION

[0025] The present invention will be described in further detail below with reference to the accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0026] See also Figures 1 to 3The reverse three-horizontal well network structure for exploiting natural gas hydrates described in the present invention includes multiple three-horizontal well groups, and each three-horizontal well group is evenly arranged radially along the injection well 10. The injection well 10 is a drilled well extending from the ground or sea surface to the natural gas hydrate reservoir 5. Each three-horizontal well group consists of a production well 3 extending from the ground or sea surface to the natural gas hydrate reservoir 5, a heat injection branch well 2 located in the natural gas hydrate reservoir 5, and a sand drainage well 1 extending from the ground or sea surface to the natural gas hydrate reservoir 5, wherein the production well 3 includes a production vertical section 301 from the wellhead downward and a production horizontal section 302 located in the natural gas hydrate reservoir 5, the heat injection branch well 2 includes a heat injection deflection section 201 and a heat injection branch horizontal section 202, and the sand drainage well 1 includes a sand drainage vertical section 101 from the wellhead downward and a sand drainage horizontal section 102 located in the natural gas hydrate reservoir 5; the production horizontal section 302 is close to the overburden 4, the sand drainage horizontal section 102 is close to the underburden 6, and the depth of the heat injection branch horizontal section 202 is between the production horizontal section 302 and the sand drainage horizontal section 102. The heat injection branch wells 2 in each three-horizontal well group are connected to the injection well 10, specifically, the heat injection deflection section 201 of the heat injection branch well 2 is connected to the injection well 10, that is, the heat injection deflection section 201 and the heat injection branch horizontal section 202 are connected in sequence from the wellhead of the injection well 10 downwards, and the injection well 10 and the heat injection branch well 2 jointly complete the heat supply to the entire well network structure, thereby injecting heat into the well network. The specific number of three-horizontal well groups can be determined based on relevant parameters such as the reservoir thickness and storage characteristics of the target reservoir. Under normal circumstances, the number of three-horizontal well groups arranged around an injection well 10 is usually 2 to 6. Figure 1 In the embodiment, the number of three horizontal well groups arranged in a cluster well manner around an injection well 10 is 6.

[0027] In the same three-horizontal well group, the extension directions of the production horizontal section 302, the heat injection branch horizontal section 202 and the sand drainage horizontal section 102 can be in the same direction or in opposite directions. It is preferably designed that the extension direction of the production horizontal section 302 is the same as the extension direction of the sand drainage horizontal section 102, and the extension direction of the heat injection branch horizontal section 202 is opposite to the extension directions of both the production horizontal section 302 and the sand drainage horizontal section 102. This is more conducive to the layout of the three-horizontal well group.

[0028] In each three-horizontal well group, several insulation pipe groups are evenly spaced along the length direction on the well walls of the production horizontal section 302, the heat injection branch horizontal section 202 and the sand drainage horizontal section 102. Each insulation pipe group consists of an insulation pipe 7 arranged on the production horizontal section 302, an insulation pipe 7 arranged on the heat injection branch horizontal section 202 and an insulation pipe arranged on the sand drainage horizontal section 102. The insulation pipes 7 in the same insulation pipe group are aligned up and down; openings 9 are provided on the parts of the well walls of the production horizontal section 302, the heat injection branch horizontal section 202 and the sand drainage horizontal section 102 where no insulation pipe 7 is provided.

[0029] In the same three-horizontal well group, the distance between two adjacent insulated pipe groups is determined as needed, and is preferably set to 10 to 30 meters. Preferably, the lengths of the insulated pipes 7 are equal. It is further preferred to wrap the insulated pipes 7 with insulating materials to isolate the heat exchange between the wellbore of each well and the natural gas hydrate reservoir 5, so that the natural gas hydrates in the upper and lower corresponding parts of the insulated pipes 7 are not mined, so as to support the natural gas hydrate reservoir 5 and thus maintain the stability of the natural gas hydrate reservoir 5. The insulated pipes 7 in the same insulated pipe group are aligned up and down, which means that the starting ends of the insulated pipes 7 on the production horizontal section 302, the insulated pipes 7 on the heat injection branch horizontal section 202, and the insulated pipes 7 on the sand discharge horizontal section 102 in the same insulated pipe group are aligned, or the absolute value of the maximum distance between the starting ends of the three in the vertical plane is less than or equal to 2 meters.

[0030] In the same three-horizontal well group, the openings 9 are spaced apart along the length of the well wall in the production horizontal section 302, the heat injection branch horizontal section 202, or the sand removal horizontal section 102, and are evenly distributed along the circumference of the well wall. The shape of the openings 9 can be regular shapes such as circular, elliptical, rectangular, or square, or irregular shapes. Figure 1 In an embodiment, these openings 9 are rectangular. Furthermore, the diameter of the openings 9 is preferably more than twice the radius of the largest sand grain in the natural gas hydrate reservoir 5. Furthermore, in order to facilitate the formation of a vertically connected heat cavity 8 within the same vertical region corresponding to the positions of the openings 9 on the horizontal sections of each well, the starting ends of the first opening 9 on the production horizontal section 302, the first opening 9 on the heat injection branch horizontal section 202, and the first opening 9 on the sand drainage horizontal section 102 in the same vertical region are aligned, or the absolute value of the maximum distance between the starting ends of the three in the vertical plane is less than or equal to 2 meters.

[0031] The present invention also includes a method for exploiting natural gas hydrates using the above-mentioned reverse three-horizontal well pattern structure, comprising the following steps:

[0032] 1) After the well group structure is arranged and the well completion operation is completed, steam or thermal fluid is injected into the production well 3, the heat injection branch well 2 and the sand drainage well 1 in each three-horizontal well group, and thermal circulation is carried out inside the production well 3, the heat injection branch well 2 and the sand drainage well 1 in the corresponding three-horizontal well group. The steam or thermal fluid diffuses outward through the openings 9 on the production well 3, the heat injection branch well 2 and the sand drainage well 1. Through continuous thermal circulation, a thermal cavity 8 connected vertically is formed in the same vertical area including the corresponding positions of the openings 9 on the production horizontal section 302, the heat injection branch horizontal section 202 and the sand drainage horizontal section 102.

[0033] Before starting step 1), first, based on geological data, a natural gas hydrate reservoir 5 with reservoir thickness, storage characteristics and other relevant parameters that meet the requirements is selected as the target reservoir; then, existing conventional technologies are used to complete the layout of the well pattern structure and completion operations on the target reservoir.

[0034] In this step, thermal circulation is simultaneously performed in the production well 3, the heat injection branch well 2, and the sand drainage well 1 in the same three-horizontal well group. When pressure linkage occurs simultaneously in the three wells (pressure linkage occurs when a pressure change in one well causes a corresponding change in the other two wells), a vertically connected thermal cavity 8 is formed. The thermal fluid can be hot water, hot brine, or other thermal fluids commonly used in the prior art.

[0035] Within the same vertical area corresponding to the positions of the openings 9 in the production horizontal section 302, the heat injection branch horizontal section 202 and the sand discharge horizontal section 102, it refers to a space with a certain horizontal distance and a certain vertical distance formed by being connected together directly below and directly below the position of the opening 9 in the production horizontal section 302, directly below and directly below the position of the opening 9 in the heat injection branch horizontal section 202, and directly below and directly below the position of the opening 9 in the sand discharge horizontal section 102.

[0036] 2) After forming the upper and lower connected thermal cavity 8, stop injecting thermal fluid into the production well 3 and the sand drainage well 1, and continue injecting steam or thermal fluid into the heat injection branch well 2 to dissolve the natural gas hydrate. The dissolved natural gas is produced through the production well 3, and the water and sand particles produced by the dissolution of the natural gas hydrate are discharged through the sand drainage well 1; the flow rate of the output in the production well 3 is controlled by controlling the temperature and pressure of the steam or thermal fluid injected into the heat injection branch well 2.

[0037] After forming the vertically connected thermal cavity 8, the flow ratio of the sand drainage well 1 to the heat injection branch well 2 is controlled to be greater than or equal to 1 to prevent gas leakage from the sand drainage well 1 (after forming the vertically connected thermal cavity 8, the production well 3 and the sand drainage well 1 stop injecting thermal fluid, while the heat injection well continues to inject thermal fluid. In order to prevent the thermal fluid from accumulating and absorbing heat in the reservoir and avoid energy waste, the thermal fluid that has released heat and cooled should be discharged through the sand drainage well 1).

[0038] After forming the upper and lower connected thermal cavity 8, by controlling the temperature and pressure of the hot fluid injected into the heat injection branch well 2, the volume and shape of the thermal cavity 8 after the natural gas hydrate is mined can also be controlled.

[0039] After forming the upper and lower connected heat cavity 8, the temperature of the output of the sand drainage well 1 (including condensed water, dissolved water of natural gas hydrate, sand particles in the reservoir, etc.) is preferably not more than 40°C and not less than 0°C, and is adjusted by controlling the temperature and pressure of the hot fluid injected into the heat injection well.

[0040] In the above method, when natural gas or steam is produced in the sand drainage well 1, the injection of hot fluid into the heat injection branch well 2 is stopped, or the temperature and pressure of the hot fluid injected into the heat injection branch well 2 are reduced, and the temperature difference between the injected hot fluid and the produced liquid is kept within the required range to prevent the sand drainage well 1 from producing natural gas or steam.

[0041] 3) When the natural gas hydrate is mined to the expected level or exhausted, water is first injected into the production well 3, the heat injection branch well 2 and the sand drainage well 1 in each of the three horizontal well groups. After the water is fully filled, cement or proppant is injected into the production well 3, the heat injection branch well 2 and the sand drainage well 1 to complete the plugging of the well pattern.

[0042] In this step, the operation of achieving plugging is the same as that in the prior art. The proppant may usually be quartz sand, ceramsite, etc. The amount of cement or proppant injected is slightly larger than the cumulative production volume from the natural gas hydrate reservoir 5.

[0043] When the well pattern is plugged after mining to a certain extent but not after mining is completed in step 3), it is necessary to reactivate the plugged well pattern to further mine natural gas hydrates according to specific circumstances. In this case, the method of the present invention further includes the following step 4):

[0044] 4) Use existing conventional technology to reopen the blocked well network, and perform perforation (i.e., opening 9) at the corresponding positions of the insulation pipe group set on the production horizontal section 302, the heat injection branch horizontal section 202, and the sand discharge horizontal section 102 of each three-horizontal well group. After completion, continue with steps 1) to 3) until the natural gas hydrate is fully mined.

Claims

1. A reverse three-horizontal well pattern structure for exploiting natural gas hydrates, characterized by: The invention comprises a plurality of three-horizontal well groups, each of which is evenly arranged radially along an injection well (10); each three-horizontal well group is composed of a production well (3), a heat injection branch well (2) and a sand drainage well (1), wherein the production well (3) comprises a production vertical section (301) and a production horizontal section (302), the heat injection branch well (2) comprises a heat injection deflection section (201) and a heat injection branch horizontal section (202), and the sand drainage well (1) comprises a sand drainage vertical section (101) and a sand drainage horizontal section (102). ), the production horizontal section (302), the heat injection branch horizontal section (202) and the sand removal horizontal section (102) are all located in the natural gas hydrate reservoir (5), the production horizontal section (302) is close to the overburden (4), the sand removal horizontal section (102) is close to the underburden (6), and the heat injection branch horizontal section (202) is located between the production horizontal section (302) and the sand removal horizontal section (102); the heat injection branch wells (2) in the three-horizontal well group are all connected to the injection well (10); In each three-horizontal well group, a plurality of insulation pipe groups are evenly spaced along the length direction on the well walls of the production horizontal section (302), the heat injection branch horizontal section (202) and the sand discharge horizontal section (102), each insulation pipe group consists of an insulation pipe (7) arranged on the production horizontal section (302), an insulation pipe (7) arranged on the heat injection branch horizontal section (202) and an insulation pipe arranged on the sand discharge horizontal section (102), and the insulation pipes (7) in the same insulation pipe group are aligned vertically; openings (9) are provided on the parts of the well walls of the production horizontal section (302), the heat injection branch horizontal section (202) and the sand discharge horizontal section (102) where no insulation pipe (7) is provided.

2. The reverse three-horizontal well pattern structure for exploiting natural gas hydrates according to claim 1 is characterized in that: The extension direction of the production horizontal section (302) is the same as the extension direction of the sand discharge horizontal section (102), and the extension direction of the heat injection branch horizontal section (202) is opposite to the extension direction.

3. The reverse three-horizontal well pattern structure for exploiting natural gas hydrates according to claim 1 or 2, characterized in that: The openings (9) are distributed at intervals along the length direction of the well wall on the well wall of the production horizontal section (302), the heat injection branch horizontal section (202) or the sand discharge horizontal section (102), and are evenly distributed along the circumference of the well wall.

4. The reverse three-horizontal well pattern structure for exploiting natural gas hydrates according to claim 1 or 2, characterized in that: The diameter of the opening (9) is more than twice the radius of the largest sand grain in the natural gas hydrate reservoir (5).

5. A method for extracting natural gas hydrates using the reverse three-horizontal well pattern structure according to any one of claims 1 to 4, comprising the following steps: 1) injecting hot fluid into the production well (3), the heat injection branch well (2) and the sand drainage well (1) in each three-horizontal well group, performing heat circulation inside the production well (3), the heat injection branch well (2) and the sand drainage well (1) in the corresponding three-horizontal well group, and the hot fluid diffuses outward through the openings (9) on the production well (3), the heat injection branch well (2) and the sand drainage well (1). Through continuous heat circulation, a heat cavity (8) connected vertically is formed in the same vertical area including the corresponding positions of the openings (9) on the production horizontal section (302), the heat injection branch horizontal section (202) and the sand drainage horizontal section (102); 2) After the upper and lower connected heat cavity (8) is formed, the injection of hot fluid into the production well (3) and the sand drainage well (1) is stopped, and steam or hot fluid is continuously injected into the heat injection branch well (2) to dissolve the natural gas hydrate. The dissolved natural gas is produced through the production well (3), and the water and sand particles produced by the dissolution of the natural gas hydrate are discharged through the sand drainage well (1); The flow rate of the output in the production well (3) is controlled by controlling the temperature and pressure of the hot fluid injected into the heat injection branch well (2); 3) When the natural gas hydrate is mined to the expected level or has been mined out, water is first injected into the production wells (3), the heat injection branch wells (2) and the sand drainage wells (1) in each of the three horizontal well groups. After the water is fully injected, cement or proppant is injected into the production wells (3), the heat injection branch wells (2) and the sand drainage wells (1) to complete the plugging of the well network.

6. The method for extracting natural gas hydrates using the reverse three-horizontal well pattern structure of claim 5 further comprises the following steps: 4) The blocked well pattern is reopened, and perforations are performed at corresponding positions of the insulation pipe groups set on the production horizontal section (302), the heat injection branch horizontal section (202), and the sand discharge horizontal section (102) of each three-horizontal well group. After completion, steps 1) to 3) are continued until the natural gas hydrate is completely mined.

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