WO2024042851A1 - Method for underground storage of carbon dioxide, and device for underground storage of carbon dioxide - Google Patents

Abstract

Provided are a method for underground storage and a device for underground storage of carbon dioxide, in which, by ensuring strength of a carbon dioxide blocking layer, carbon dioxide that is injected downward can be reliably sealed in, and a large quantity of carbon dioxide can be efficiently stored.

Description

Carbon dioxide underground storage method and carbon dioxide underground storage device

The present invention relates to a carbon dioxide underground storage method and a carbon dioxide underground storage device.
This application claims priority based on Japanese Patent Application No. 2022-134227 filed in Japan on August 25, 2022, the contents of which are incorporated herein.

Carbon dioxide capture and storage (hereinafter simply referred to as "CCS") is attracting attention as an innovative technology that can suppress carbon dioxide (CO ₂ ) emissions while generating electricity using fossil fuels. . Furthermore, with regard to methods of storing carbon dioxide underground using CCS technology, verification tests and surveys of suitable storage sites are being actively carried out.

Generally, carbon dioxide exists in one of four phases: gas, liquid, solid, and supercritical, depending on pressure and temperature conditions. Also, carbon dioxide becomes solid dry ice under conditions of normal pressure and temperature below 194K (-79.15℃), but when mixed with water, it becomes hydrate (solid) under different temperature and pressure conditions. It is known to do. Note that carbon dioxide _hydrate is produced by mixing liquid carbon dioxide with water at a temperature of 10°C or lower and a pressure of 4.5 MPa or higher. overlaps with a part of the phase region).

Carbon dioxide hydrate as described above is a solid having a crystal structure similar to that of methane hydrate, and has a structure in which gas molecules are trapped within a steric lattice made up of water molecules. Such carbon dioxide hydrate is also generally called gas hydrate, and the following non-patent document 1 describes the first research report that carbon dioxide becomes hydrated and causes clogging in natural gas transfer lines. Confirmed. Furthermore, carbon dioxide hydrate has been confirmed as a clogging substance in plant piping for oil and gas processing, and research is progressing in the scientific field related to the earth and planets.

Additionally, Non-Patent Document 2 below reports that in 1990, natural carbon dioxide hydrate was discovered rising up from the sea floor in a columnar shape.

The density of carbon dioxide changes due to a gas-liquid phase change when the pressure increases, but since it becomes supercritical at a temperature of 31°C or higher and a pressure of 7.4 MPa or higher, the density change due to a further pressure increase is comparatively It's slow. Since the density of carbon dioxide in a liquid or supercritical state is lower than that of seawater, for example, when trying to store carbon dioxide in a geological layer beneath the ocean floor, buoyancy occurs due to the density difference, causing carbon dioxide to float to the surface. Some kind of sealing function is required to contain it.

On the other hand, in Japan, as a method for underground carbon storage, aquifer storage (DSA; Deep Saline Aquifer) has been under consideration since the 1990s under the initiative of the government and administrative agencies. Commercial storage locations and storage capacity have not yet been determined. This is because Japan's lack of oil and natural gas production suggests that Japan's geological formations contain natural caprock (shielding layers such as mudstone) that are commonly found in the Middle East, Europe, America, Asia, etc. (Caprock)) is thought to be a contributing factor.

For this reason, another method for storing carbon dioxide underground is to take advantage of the above-mentioned property of carbon dioxide becoming hydrated and solid, and to use hydrated carbon dioxide as a shielding layer. The practical application of carbon dioxide hydrate storage, which has a sealing function to contain carbon dioxide, is expected to be put into practical use, and active research is underway.

The hydrated carbon dioxide shielding layer as described above has the advantage of not requiring a cap rock such as mudstone. In addition, the underground storage target layer for carbon dioxide hydrate storage is, for example, a Quaternary sedimentary layer (geological age is (approximately 2.58 million years ago), and is widely distributed in the seas surrounding Japan.

Additionally, since methane hydrate exists in the sea areas surrounding Japan, it has become clear that methane gas exists deep underground. On the other hand, this methane gas is not trapped in the cap rocks mentioned above, but is trapped as methane hydrate in the shallow strata several hundred meters below the seafloor, so it is present in large quantities in the seas surrounding Japan. It can be considered that

The basic concept of carbon dioxide hydrate storage as described above is shown in, for example, the following non-patent documents 3 and 4, but the reality is that research on the specific mechanism has not progressed. On the other hand, in the United Kingdom (Scotland), research results from laboratory experiments have been reported as a government-led project, as shown in Non-Patent Document 4 below.

Here, Patent Document 1 proposes a method that achieves both carbon dioxide fixation and methane gas production without relying on a methane hydrate layer. Patent Document 1 below describes a step of forming a methane gas production layer 4 by adding a group of microorganisms containing at least methane-producing bacteria to the interstices of the stratum under temperature and pressure conditions where the methane-producing bacteria produce methane gas; Carbon dioxide hydrate is produced by injecting an emulsion in which fine particles of liquid carbon dioxide smaller than the gaps are dispersed in water into the gaps in the strata that are shallower than the layers and under temperature and pressure conditions where carbon dioxide becomes hydrate. a step of injecting liquid carbon dioxide as a raw material for methane gas generation into the gap between the methane gas production layer and the seal layer; and a step of injecting methane gas 5 generated in the methane gas production layer above ground. A method is described that includes a step of recovering the material.

In addition, as a concern in promoting carbon dioxide hydrate storage, as shown in Non-Patent Document 1, the formation of carbon dioxide into hydrates causes stratum pores to become clogged and clogged, causing damage to the underlying strata. It has been pointed out that it may become difficult to inject carbon dioxide. In order to solve such problems, Patent Document 2 below discloses that a carbon dioxide shielding layer that exists from the ocean floor to a predetermined depth and that satisfies the pressure and temperature conditions that are capable of producing carbon dioxide hydrate. Methods and devices for forming a carbon dioxide reservoir by injecting carbon dioxide have been proposed. According to the technology described in Patent Document 2, clogging when storing carbon dioxide underground can be prevented, and a large amount of carbon dioxide can be efficiently stored.

Japanese Patent Application Publication No. 2010-239962 Japanese Patent Application Publication No. 2019-126787

On the other hand, when storing carbon dioxide underground using carbon dioxide hydrate, in order to efficiently and reliably store carbon dioxide underground, it is necessary to destroy the shielding layer formed by carbon dioxide hydrate. In order to prevent the injected carbon dioxide from floating to the surface and leaking out, it is necessary to further increase the strength of the shielding layer. However, in the past, there was no knowledge regarding the conditions for ensuring the strength of the shielding layer, and the reality was that one had no choice but to rely on trial and error at the storage site.

The present invention was made in view of the above circumstances, and by ensuring the strength of the carbon dioxide shielding layer made of carbon dioxide hydrate, carbon dioxide injected downward can be reliably contained, and a large amount of carbon dioxide can be contained. An object of the present invention is to provide a carbon dioxide underground storage method and a carbon dioxide underground storage device that can efficiently store carbon dioxide.

In order to solve the above problems, the present invention employs the following configuration.
[1] A method of storing carbon dioxide alone or in a liquefied state as a mixed gas mainly composed of carbon dioxide in a sub-seafloor stratum made of sediment on an acoustic substrate or in a stratum on land, comprising: A carbon dioxide reservoir is created by injecting the carbon dioxide below the carbon dioxide seal area, which exists from the seabed or ground to a predetermined depth and consists of a stratum that satisfies the pressure and temperature conditions capable of producing carbon dioxide hydrate. and causing at least a portion of the carbon dioxide injected into the carbon dioxide storage layer to naturally rise to the carbon dioxide sealing area side by the buoyancy of the carbon dioxide to generate carbon dioxide hydrate, A carbon dioxide shielding layer is formed in the carbon dioxide sealing region, and when the carbon dioxide is injected into the sub-seafloor stratum or the land stratum, at least one of seawater and water is used as pore water in the carbon dioxide sealing region. A method for storing carbon dioxide underground, which increases pore pressure in the carbon dioxide sealing region by injecting an artificial water seal.
[2] An underground storage device that stores carbon dioxide alone or in a liquefied state as a mixed gas mainly composed of carbon dioxide in a sub-seafloor stratum made of sediment on an acoustic substrate or in a stratum on land. It penetrates a carbon dioxide seal area consisting of a stratum that exists to a predetermined depth from the seabed or ground and satisfies the pressure and temperature conditions capable of producing carbon dioxide hydrate, and is located below the carbon dioxide seal area. an injection well extending to a carbon dioxide storage layer; a pumping facility for pumping the mixed gas to the injection well; and a pumping equipment disposed on the upper side of the carbon dioxide sealing area, which uses at least one of seawater and water as pore water. An underground storage device for carbon dioxide, comprising: an artificial water sealing facility that increases the pore pressure of the carbon dioxide sealing area by injecting it into the carbon dioxide sealing area.

According to the underground carbon dioxide storage method of the present invention, by adopting the above structure, the strength of the entire carbon dioxide sealing area is increased, so the strength of the carbon dioxide shielding layer formed by hydrated carbon dioxide is increased. can be ensured. This allows the carbon dioxide injected below the carbon dioxide shielding layer to be reliably contained, making it possible to efficiently store a large amount of carbon dioxide.

Further, according to the underground carbon dioxide storage device of the present invention, it can be applied to the above-mentioned underground carbon dioxide storage method with a simple configuration, and it is possible to efficiently store a large amount of carbon dioxide.

FIGS. 1(a) and 1(b) are diagrams illustrating a carbon dioxide underground storage method and a carbon oxide underground storage device, which are one embodiment of the present invention. FIG. 1(a) is a schematic cross-sectional view showing the schematic structure of an underground storage device for storing carbon. FIG. 1(a) is an image diagram showing each layer in a wide area, and FIG. FIG. FIG. 2 is a diagram illustrating an underground storage method for carbon dioxide, which is an embodiment of the present invention, and shows isolines of carbon dioxide density in the temperature-pressure relationship and stable regions of carbon dioxide hydrate. This is a graph showing. FIG. 3 is a diagram illustrating a carbon dioxide underground storage method according to an embodiment of the present invention, and is a graph showing a temperature-pressure phase diagram of carbon dioxide hydrate. FIG. 4 is a diagram illustrating an underground storage method for carbon dioxide, which is an embodiment of the present invention. It is a graph showing pressure. FIG. 5 is a diagram illustrating a carbon dioxide underground storage method and a carbon oxide underground storage device, which are one embodiment of the present invention, and shows a pressure cell used in an indoor experiment to demonstrate the present invention. FIG. 2 is a schematic diagram showing the schematic structure of FIGS. 6(a) to 6(c) are diagrams illustrating an underground storage method for carbon dioxide, which is an embodiment of the present invention. 2 is a graph showing the strength of a film made of carbon dioxide hydrate. FIG. 7 is a diagram illustrating an underground storage method for carbon dioxide, which is an embodiment of the present invention. It is a graph showing the temperature rise due to the heat of formation of carbon dioxide hydrate when it permeates. FIG. 8 is a diagram illustrating a carbon dioxide underground storage method that is an embodiment of the present invention, and the differential pressure obtained in an experiment using the pressure cell shown in FIG. It is a graph showing the relationship between holding time and differential pressure when holding for 17 days under the condition that the pressure is 2 m or less. FIG. 9 is a diagram illustrating a carbon dioxide underground storage method that is an embodiment of the present invention, and the differential pressure obtained in an experiment using the pressure cell shown in FIG. It is a graph showing the strength of a carbon dioxide shielding layer made of a carbon dioxide hydrate film when the pressure difference is maintained at 1 MPa for 24 hours after being maintained for 17 days under the condition that the pressure is 2 m or less. FIG. 10 is a diagram illustrating a carbon dioxide underground storage method according to an embodiment of the present invention, in which the differential pressure obtained in an experiment using the pressure cell shown in FIG. It is a graph showing the strength of a carbon dioxide shielding layer made of a film of carbon dioxide hydrate as the strength increases. FIG. 11 is a diagram illustrating a carbon dioxide underground storage method that is an embodiment of the present invention, and shows the differential pressure in the opposite direction obtained in an experiment using the pressure cell shown in FIG. 2 is a graph showing the strength of a carbon dioxide shielding layer made of a carbon dioxide hydrate film when activated. FIG. 12 is a diagram illustrating a carbon dioxide underground storage method, which is an embodiment of the present invention, and shows isolines of the optimal volume ratio of {carbon dioxide/water} in the temperature-pressure relationship. This is a graph showing. FIG. 13 is a diagram illustrating an underground storage method for carbon dioxide, which is an embodiment of the present invention. Based on the experimental results using the pressure cell shown in FIG. It is a diagram considering the shielding mechanism. FIG. 14 is a diagram illustrating an underground storage method for carbon dioxide, which is an embodiment of the present invention, and shows the temperature conditions at a depth where the density of carbon dioxide and the density of seawater are in equilibrium in the geological layer below the seabed. It is a graph showing the relationship with pressure conditions. FIG. 15 is a diagram illustrating a carbon dioxide underground storage method that is an embodiment of the present invention, and is a diagram schematically showing a storage method assuming an island, a seamount, and a sea plateau. It is a graph showing the relationship between water depth and water depth.

Hereinafter, a carbon dioxide underground storage method and a carbon oxide underground storage device, which are one embodiment of the present invention, will be described in detail using the drawings. Note that the drawings used in the following explanations may show characteristic parts enlarged for convenience in order to make the characteristics easier to understand, and the dimensional ratio of each component may not be the same as the actual one. do not have.

Figures 1(a) and (b) show a carbon dioxide underground storage method and a carbon dioxide underground storage device (hereinafter simply referred to as "storage device" or "storage device"), which are one embodiment of the present invention. Fig. 1(a) is a schematic cross-sectional view showing the schematic structure of a storage device 1 when carbon dioxide is stored in a geological layer U under a seabed surface (seafloor) F. is a diagram showing a wide area of the stratum U including each stratum, and FIG. 1(b) is an enlarged diagram of the main part in FIG. 1(a).

The storage method and storage device 1 of the present embodiment store carbon dioxide (CO ₂ ) alone or in a liquefied state as a mixed gas containing carbon dioxide as a main component, under a seafloor surface F consisting of deposits on an acoustic substrate B. This is a method and apparatus for storing water in a geological stratum U or a stratum in a land area (not shown). That is, in the storage method of the present embodiment, carbon dioxide is present at a predetermined depth from the seafloor surface F or the ground of a land area (not shown), and is composed of a geological stratum that satisfies pressure and temperature conditions capable of producing carbon dioxide hydrate. Carbon dioxide is pressurized below the seal area S to form a carbon dioxide storage layer G. In addition, in the storage method and storage device of the present embodiment, at least a portion of the carbon dioxide injected into the carbon dioxide storage layer G is naturally raised to the carbon dioxide sealing area S side by the buoyancy of carbon dioxide, and carbon dioxide hydrate is formed. A carbon dioxide shielding layer C is formed in the carbon dioxide sealing region S by generating.

In addition, as described above, the underground carbon dioxide storage method and carbon dioxide underground storage device according to the present invention can store either the stratum U below the ocean floor F or the stratum under the ground on land. In this embodiment, a case where carbon dioxide is stored in a stratum U under the seabed F will be described as an example. On the other hand, in this embodiment, if predetermined pressure conditions and temperature conditions are satisfied, it is also possible to use a geological formation under the ground in a land area as a storage location.

Further, the carbon dioxide described in this embodiment includes not only pure carbon dioxide (CO ₂ ) alone but also a mixture containing carbon dioxide as a main component as described above. That is, the carbon dioxide to be stored in this embodiment may contain water by using existing countermeasure techniques. Furthermore, in addition to carbon dioxide itself, the carbon dioxide to be stored in this embodiment includes other components other than carbon dioxide, such as carbon monoxide (CO), hydrogen (H ₂ ), methane (CH ₄ ), It also includes water (H ₂ O), hydrogen sulfide (H ₂ S), and the like.

<Underground carbon dioxide storage device>
First, a carbon dioxide underground storage device, which is an embodiment to which the present invention is applied, will be described.
FIG. 1(a) shows an underground carbon dioxide storage device (hereinafter sometimes simply referred to as “storage device”) 1 of the present embodiment (enlarged main part of FIG. 1(b)). (see also figure).
The illustrated storage device 1 injects and stores carbon dioxide alone or in a liquefied state as a mixed gas containing carbon dioxide as a main component into a geological layer U below a seafloor surface F consisting of sediments on an acoustic substrate B. It is something. The storage device 1 penetrates a carbon dioxide sealing region S that exists from the seabed (seafloor surface) F to a predetermined depth and is made up of a stratum that satisfies pressure and temperature conditions that are capable of producing carbon dioxide hydrate. An injection well 2 extending to a carbon dioxide storage layer G below the seal area S, a carbon dioxide supply source loaded on a ship (not shown), for example on the sea surface M, and a carbon dioxide supply source similar to the carbon dioxide supply source. It is equipped with a pumping facility (not shown) that is loaded on a ship or the like and pumps carbon dioxide generated from a carbon dioxide supply source to the injection well 2 .
Further, the storage device 1 is provided with a supply line (not shown) for supplying carbon dioxide from a pressure feeding facility installed on a ship to an injection well 2 provided starting from the seabed surface F.
In the storage device 1 of this embodiment, when carbon dioxide is injected into the geological formation U below the seabed surface F, an artificial water seal is used to inject at least one of seawater and water into the carbon dioxide seal area S as pore water. , an artificial water sealing facility 6 for increasing the pore pressure of the carbon dioxide sealing region S is provided.

In the schematic cross-sectional view of FIG. 1(a), the stratum U below the seafloor F located below the seawater W is usually at the depth from the sea surface M to the seafloor F, and at any distance from the seafloor F to the stratum U. The pressure changes depending on the depth. The illustrated storage device 1 is located in a carbon dioxide storage layer G below a carbon dioxide sealing area S that exists from a seabed surface F to a predetermined depth and satisfies pressure and temperature conditions capable of generating carbon dioxide hydrate. Carbon dioxide is injected and stored.

The carbon dioxide supply source (not shown) supplies carbon dioxide stored in the carbon dioxide storage layer G in the geological layer U, as described above. Such carbon dioxide sources are not particularly limited, and include, for example, carbon dioxide derived from coal and oil emitted from fossil fuel power generation equipment in various facilities such as offshore plants that extract oil and gas. Examples include devices that capture carbon dioxide, and devices that temporarily receive and store the carbon dioxide recovered by these devices from ships and pipelines that transport it.

In addition, in the storage device 1, the carbon dioxide supply source (not shown) is exposed above the sea level M, depending on the characteristics of the device, but it is recommended that the supply of materials such as fuel and carbon dioxide is improved. This is preferable from the viewpoint of maintainability, device life, etc. In this way, an example of a configuration in which the carbon dioxide supply source is exposed above sea level M is to construct an offshore platform similar to those installed to extract resources such as oil and gas from the seabed. Examples include those with a carbon dioxide supply source installed on top.

Examples of the above-mentioned marine platforms include fixed types and floating types.
Examples of fixed marine platforms include those constructed by directly fixing structures assembled from high-strength steel materials to the seabed surface F, and the like.
Furthermore, examples of the floating marine platform include those consisting of semi-submersible vessels and the like.

Note that the carbon dioxide supply source is not limited to one installed on an offshore platform such as a ship. For example, a configuration may be adopted in which carbon dioxide generated by a carbon dioxide supply source installed on land is transported to the injection well 2 via pressure feeding equipment and piping (pipeline).

The pumping equipment (not shown) pumps carbon dioxide (CO ₂ ) into the injection well 2, and as described above, is installed on an offshore platform such as a ship, for example.
In addition, the storage device 1 is configured such that carbon dioxide is supplied from pressure feeding equipment installed on a ship to an injection well 2 provided starting from the seabed surface F via a supply line (not shown). I can do it.

As the pressure-feeding equipment, any pump or the like for pressure-feeding liquid that has been conventionally used in this field can be employed without any limitations.
Further, as the supply line (not shown) connecting the pressure feeding equipment and the injection well 2, for example, a piping member made of a metal or resin material that can be used in seawater can be used without any restriction.
Note that the pressure feeding equipment is not limited to those installed on a marine platform such as a ship as described above, and it is also possible to install the pressure feeding equipment on land or on the seabed surface F, for example.

As described above, the injection well 2 penetrates the carbon dioxide seal region S that exists from the seabed surface F to a predetermined depth, and extends to the carbon dioxide reservoir G below the carbon dioxide seal region S, and Carbon dioxide is injected into the carbon reservoir G.
The injection well 2 is, for example, a carbon dioxide injection/injection hole made of a well drilled by boring or the like.

The injection well 2 in the illustrated example is provided as a vertical well, but if necessary, it may be an inclined well, or a combination of a vertical well and a horizontal well (for example, an L-shaped cross section), a vertical well, It is also possible to have a structure that combines horizontal wells and inclined wells as appropriate. Moreover, it is also possible to install it as an inclined well directly from land, and in this case, the carbon dioxide supply source and pumping equipment (not shown) will be installed on land.

As described above, the storage device 1 of this embodiment is provided with the artificial water sealing equipment 6 for increasing the pore pressure of the carbon dioxide sealing region S, and in the illustrated example, the artificial water sealing equipment 6 increases the pore pressure of the carbon dioxide sealing region S. It is arranged so that a part thereof is exposed on the upper surface of the carbon seal area S.
Moreover, the artificial water sealing equipment 6 is comprised of multiple pipes provided in an annular shape so as to surround the injection well 2, as shown in the enlarged view of FIG. 1(b). Further, in the illustrated example, the artificial water sealing equipment 6 is composed of multiple pipes having a step-shaped cross section extending outward from the injection well 2 side.

When the artificial water sealing equipment 6 injects carbon dioxide into the stratum U below the seafloor surface F, it injects at least one of seawater and water as pore water into the carbon dioxide sealing region S. As a result, the pore pressure of the carbon dioxide sealing region S increases, so that the strength of the entire carbon dioxide sealing region S including the carbon dioxide shielding layer C can be increased. As a result, the strength of the carbon dioxide shielding layer C, which is made of hydrated carbon dioxide, can be ensured, so the carbon dioxide injected below the carbon dioxide shielding layer C can be reliably contained, and a large amount of carbon dioxide can be can be stored efficiently.

In addition to the above-mentioned configurations, the storage device 1 used in this embodiment also has a carbon dioxide shielding layer C in which the state of carbon dioxide stored in the carbon dioxide storage layer G or carbon dioxide hydrate is generated. Various monitoring devices may be provided for detecting the state of the carbon dioxide seal area S including the carbon dioxide sealing area S.

<Method for storing carbon dioxide underground>
Hereinafter, a carbon dioxide underground storage method according to an embodiment of the present invention will be described in detail with reference to FIGS. 2 to 15 in addition to FIGS. 1(a) and (b) above.
The storage method of this embodiment can be a method using the storage device 1 of this embodiment shown in FIGS. 1(a) and 1(b).

[Artificial water seal in the carbon dioxide seal area]
As described above, in the storage method of this embodiment, carbon dioxide (CO ₂ ) alone or a mixed gas containing carbon dioxide as a main component is liquefied, and carbon dioxide (CO 2 ) is stored on the seafloor surface F made of sediments on the acoustic substrate B. This is a method of storing it in the underlying stratum U or in a stratum on land (not shown). In the present embodiment, a carbon dioxide seal area S exists at a predetermined depth from the ground of the seabed (seafloor surface) F or a land area (not shown), and is composed of a stratum that satisfies pressure and temperature conditions capable of producing carbon dioxide hydrate. Carbon dioxide is injected under pressure to form a carbon dioxide storage layer G. In addition, at least a portion of the carbon dioxide injected into the carbon dioxide storage layer G is naturally raised toward the carbon dioxide sealing area S by the buoyancy of the carbon dioxide to generate carbon dioxide hydrate. A carbon dioxide shielding layer C is formed therein.
In the storage method of the present embodiment, a method is adopted in which the pore pressure of the carbon dioxide seal area S is increased by an artificial water seal in which at least one of seawater W and water is injected into the carbon dioxide seal area S as pore water. ing.

In the storage method of this embodiment, using the storage device 1 shown in FIGS. It can be stored in the stratum U below the seafloor F, which is made up of sediments on the bedrock B.

As equipment for performing the above-described artificial water sealing, an artificial water sealing equipment 6 such as that provided in the storage device 1 can be mentioned. The illustrated artificial water sealing equipment 6 is disposed on the upper surface side of the carbon dioxide sealing area S, and as described above, by injecting at least one of seawater and water as pore water into the carbon dioxide sealing area S, carbon dioxide is removed. It is possible to increase the pore pressure in the seal area S.
By providing such artificial water sealing equipment 6, the pore pressure of the carbon dioxide sealing region S increases, so that the strength of the entire carbon dioxide sealing region S including the carbon dioxide shielding layer C is increased.
Note that the detailed configuration of the artificial water seal equipment 6 is as already described.

Below, details of the case where artificial water sealing is performed in the storage method of this embodiment will be explained.
In the schematic cross-sectional view of FIG. 1(a) described above, the maximum pressure based on the buoyancy of carbon dioxide acts in the vicinity of point (b) located vertically above the carbon dioxide injection point. At this time, if the pressure of the stored carbon dioxide exceeds the time-intensity relationship of the carbon dioxide shielding layer C, there is a risk that the carbon dioxide will leak. It is highly likely that the pressure of carbon dioxide exceeds the strength of the carbon dioxide shielding layer C at the beginning of the production of the carbon dioxide hydrate film that will become the carbon dioxide shielding layer C (for example, within two weeks).

On the other hand, hydrostatic pressure due to seawater W acts on the upper part of the carbon dioxide shielding layer C, while pressure generated by the buoyancy of carbon dioxide acts on the lower part of the carbon dioxide shielding layer C. If the pressure difference generated between the upper and lower sides of the carbon dioxide shielding layer C is lower than the strength of the carbon dioxide shielding layer C, carbon dioxide will not start permeating into the carbon dioxide shielding layer C.
Therefore, in the initial stage of formation of the carbon dioxide hydrate film (for example, 2 weeks after the start of production) when the strength of the carbon dioxide shielding layer C is weak, the differential pressure between the upper and lower sides of the carbon dioxide shielding layer C does not exceed 2 m in water head. Thus, it is conceivable to provide an artificial water sealing facility 6 using a gap structure of multiple pipes (casing annulus), etc., which is installed on the seabed surface F when cutting the injection well 2. By installing such artificial water sealing equipment 6, seawater/water is injected from the upper part of the carbon dioxide sealing area S, thereby reducing the pore pressure (pore pressure) of the carbon dioxide sealing area S including the carbon dioxide shielding layer C. It becomes possible to increase the

Carbon dioxide hydrate storage targets the latest unconsolidated Quaternary sedimentary layer in the geological era, as mentioned above. When considering faults caused by earthquakes, etc., the Quaternary sedimentary layers are unconsolidated, so deformation is allowed, and dilatancy (volume change caused by shear deformation) is thought to occur along with this deformation. It is thought that the porosity of the Quaternary sedimentary layer increases due to the above-mentioned dilatancy, which increases the permeability, and it is necessary to consider how much of an impact this has on the risk of carbon dioxide leakage.

Here, the schematic diagram of FIG. 5 shows a schematic structure of a pressure cell 100 used in an indoor experiment to demonstrate the present invention. The maximum pressure within this pressure cell 100 is 70 MPa, which corresponds to the pressure at a depth of 7000 m underwater.
In this demonstration experiment, No. 7 silica sand (fine sand) or No. 9 silica sand (silt) was set in the pressure cell 100 shown in Figure 5 to reproduce conditions that simulated the pressure and temperature in the strata beneath the ocean floor. After that, an experiment was conducted in which liquid carbon dioxide was permeated from the bottom of the pressure cell 100. The measurement items at this time are the temperature measured by thermometers installed at a pitch of 100 mm from the bottom side, and the pressure at the upper and lower ends of the pressure cell 100.

The present inventors conducted a demonstration experiment using the pressure cell 100 shown in FIG. 5 regarding the time-intensity relationship of the carbon dioxide shielding layer C assuming an artificial water seal, and the results are shown in the graph of FIG. Ta.
Here, FIG. 11 shows the strength of the carbon dioxide shielding layer C made of a carbon dioxide hydrate film when a pressure difference in the opposite direction is applied, which was obtained in an experiment using the pressure cell 100 shown in FIG. This is a graph showing.
In this experiment, the "reverse pressure differential" means that the relationship between the density of carbon dioxide (ρ _CO2 ) and the density of pore water (ρ _sw ), which will be described in detail later, is expressed by the following formula: {ρ _CO2 <ρ _sw } This refers to the case where the relationship is expressed as

As a result of the above experiment, when artificial water sealing was performed in the pressure cell 100 in such a way that water permeated into the pores of the sand infiltrated with carbon dioxide, the effect of reducing the differential pressure acting on the carbon dioxide shielding layer C was found. In addition, when the pressure increase due to the artificial water seal became excessive, the strength of the carbon dioxide shielding layer C was increased in a short period of time (1 to 2 hours in this experiment).

The strength of the carbon dioxide shielding layer C made of carbon dioxide hydrate, which will be explained in this embodiment, is the so-called threshold pressure, which is the pressure at which carbon dioxide starts permeating. That is, the strength of the carbon dioxide shielding layer C needs to be at least a threshold pressure below which carbon dioxide does not permeate (is shielded). The threshold pressure explained in this embodiment is based on the "sub-seafloor disposal of specified carbon dioxide gas" in the DSA demonstration test based on the "Act on the Prevention of Marine Pollution, Etc. and Maritime Disasters" (Act No. 136 of 1971). According to the method described in "Application for Permit" (http://www.env.go.jp/press/102082.html), mercury intrusion is applied as the pressure at which carbon dioxide CO ₂ starts permeating into the carbon dioxide shielding layer C. Measured by law etc. It should be noted that the carbon dioxide shielding layer C is thought to be formed mainly by carbon dioxide hydrate filling the pores of the strata, and the shielding mechanism is thought to be different from the threshold pressure in DSA described above, which is caused by capillary pressure. It will be done.

As mentioned above, carbon dioxide (CO ₂ ) exists in one of four phases: gas, liquid, solid, and supercritical within a predetermined temperature and pressure range. Also, carbon dioxide (CO ₂ ) becomes solid (dry ice) under normal pressure and at temperatures below 194K (-79.15℃), but when mixed with water, it becomes solid (dry ice) under different temperature and pressure conditions. Turn into hydrate (solid).

Furthermore, carbon dioxide (CO ₂ ) becomes supercritical when the temperature is 31° C. or higher and the pressure is 7.4 MPa or higher, and the density change becomes relatively slow. Carbon dioxide (CO ₂ ) has a density lower than that of seawater when the temperature is 35°C or lower and the pressure is 20MPa or lower, and it has a density lower than that of water unless the temperature is around 0°C and the pressure is 14MPa or higher. small. In other words, in a liquid or supercritical state, carbon dioxide (CO ₂ ) has a lower density than seawater, so carbon dioxide (CO ₂ ) is stored in the seabed substratum U (see Figure 1). This requires a sealing function to contain the carbon dioxide that floats up due to the above density difference.

Incidentally, natural gas generally exists in geological formations in the form of conventional natural gas, shale gas, methane hydrate, or the like.
Conventional natural gas is stored in geological formations with highly airtight caprock such as mudstone that acts as a sealing layer.
Analysis of shale gas's DNA has revealed that it is produced from shale, which is considered to be the source rock of petroleum, at the same time as shale oil.
It has been confirmed that methane hydrate exists as a solid in the sub-seafloor layer of the continental margins and in the permafrost layer, and although there is no cap rock like the one mentioned above, it is difficult to prevent methane hydrate from occurring due to the temperature and pressure conditions in which it becomes hydrated. , a sealing layer that performs a so-called sealing function is formed.

The existence of natural gas in its natural state, as described above, can be viewed as a natural analogue (natural analogue phenomenon) of carbon dioxide storage. That is, in the underground storage (also called aquifer storage) of conventional natural gas and general carbon dioxide, the above-mentioned cap rock functions as a sealing layer. Furthermore, methane hydrate and carbon dioxide hydrate can be used as a sealing layer by setting the temperature and pressure conditions for hydration.
Furthermore, shale gas exists when methane is adsorbed on shale or coal seams, and it is known that carbon dioxide is also adsorbed.
As mentioned above, naturally occurring natural gas is confined within geological formations with different sealing functions. By utilizing such a sealing function, that is, natural conditions, it becomes possible to store carbon dioxide underground.

Below, the behavior of carbon dioxide hydrate in geological pores and the image of carbon dioxide hydrate stored in geological formations will be explained in detail.
In the schematic cross-sectional view of FIG. 1(a), the carbon dioxide seal area S below the seafloor surface F is a stratum that satisfies the pressure and temperature conditions that allow carbon dioxide hydrate to be produced, that is, the carbon dioxide hydrate is stable. This is the area where the pressure and temperature exist.
In addition, the carbon dioxide storage layer G located below the carbon dioxide sealing area S is an area with pressure and temperature conditions that allow carbon dioxide to be stored as a liquid (the temperature-pressure phase diagram of carbon dioxide hydrate in Figure 3 is also reference).
In addition, the acoustic base B under the carbon dioxide reservoir G is a base defined by the reflection method used to interpret submarine geological maps, etc., and for example, the existence of basalt, etc., which has a composition similar to that of magma is thought to exist. It is the foundation.
There is a sedimentary layer (Quaternary sedimentary layer) of several hundred meters on acoustic substrate B, and based on the stable area of carbon dioxide hydrate shown in the graph of Figure 3, the carbon dioxide seal area described above is It is divided into S and carbon dioxide reservoir G.

In the carbon dioxide reservoir G shown in FIG. 1(a), when carbon dioxide is injected at the tip 21 of the injection well 2, the carbon dioxide rises due to buoyancy and penetrates to the lower part of the carbon dioxide seal area S . Then, when the temperature of the carbon dioxide that has permeated into the carbon dioxide sealing region S decreases, carbon dioxide hydrate is generated, and a carbon dioxide shielding layer C is formed at a lower position of the carbon dioxide sealing region S. After that, the carbon dioxide injected into the carbon dioxide storage layer G is prevented from floating upward from that position by the carbon dioxide shielding layer C, so the storage area expands horizontally and the carbon dioxide It is considered that a carbon dioxide liquid layer G1 is gradually formed above the carbon reservoir G.

[Relationship between time and intensity after carbon dioxide hydrate formation]
In the carbon dioxide underground storage method of this embodiment, in addition to increasing the pore pressure of the carbon dioxide sealing region S by the artificial water seal as described above, the strength at the initial stage of carbon dioxide hydrate generation is When the thickness of the carbon dioxide liquid layer G1 formed on the upper part of the carbon dioxide storage layer G is 10 m to 40 m, it is also possible to adopt a method of configuring it so that it corresponds to the pressure acting on the carbon dioxide shielding layer C. It is.

As a result of extensive studies, the inventor of the present invention has determined that, in forming the carbon dioxide shielding layer C by generating carbon dioxide hydrate as described above, the strength at the initial stage of carbon dioxide hydrate formation should be adjusted to It has been found that when the carbon dioxide liquid layer G1 formed on the upper part of G has a predetermined layer thickness, it is optimized so as to correspond to the pressure acting on the carbon dioxide shielding layer C. That is, the initial strength of carbon dioxide hydrate generation in the carbon dioxide shielding layer C is determined by determining the strength of the carbon dioxide shielding layer when the thickness of the carbon dioxide liquid layer G1 formed on the upper part of the carbon dioxide storage layer G is 10 m to 40 m. It has been found that by optimizing the carbon dioxide shielding layer C based on the pressure acting on the carbon dioxide shielding layer C, the carbon dioxide shielding layer C becomes a sealing layer with sufficient strength.

Furthermore, by optimizing the relationship between the time after carbon dioxide hydrate generation and strength in the carbon dioxide shielding layer C, the carbon dioxide shielding layer C becomes a sealing layer with sufficient strength. It was discovered that this makes it possible to store large amounts of carbon dioxide more efficiently.

That is, as described above, the present inventors formed the carbon dioxide shielding layer C with sufficient strength by optimizing the relationship between the time from the generation of carbon dioxide hydrate in the carbon dioxide shielding layer C and the strength. It was discovered that by doing so, the carbon dioxide injected into the carbon dioxide reservoir G can be prevented from leaking, and it becomes possible to efficiently store large amounts of carbon dioxide without the need for a cap lock. .

Note that the strength at the initial stage of carbon dioxide hydrate production in the carbon dioxide shielding layer as defined in the present invention is, for example, the strength at a time of 0 to 40 hours from the start of carbon dioxide hydrate production.

Below, a trial calculation of the pressure acting on the carbon dioxide shielding layer C due to the rising pressure of the carbon dioxide liquid layer G1 will be described in detail.
In parallel with a laboratory experiment whose details will be described later, the inventor calculated the pressure acting on the carbon dioxide shielding layer C made of carbon dioxide hydrate shown in FIG. 1(a). This pressure is based on the buoyancy of liquid carbon dioxide in water (seawater), and by integrating the difference between the density of water or seawater and the density of carbon dioxide, the result shown in the graph of Figure 4 is obtained. Calculated.

Here, the carbon dioxide density contour line used for the trial calculation of the pressure acting on the carbon dioxide shielding layer C is shown in the graph of the temperature-pressure phase diagram of carbon dioxide hydrate in FIG. 2. The region indicated by R1 in the graph of FIG. 2 is a region indicating pressure and temperature conditions under which carbon dioxide hydrate exists stably.

Furthermore, in the graph of the temperature-pressure phase diagram of carbon dioxide hydrate in FIG. 3, the stable region of carbon dioxide hydrate is shown with the temperature axis enlarged. In FIG. 3, the above-mentioned isolines of density of carbon dioxide, isolines of seawater density, and isovalues of density difference are also shown superimposed to form a graph from which the buoyancy of carbon dioxide can be read. Note that FIG. 3 shows a curve plotting the relationship between seawater temperature and pressure (water depth) in the Pacific Ocean and the Sea of Japan. Figure 3 also shows the Span & Wagner equation of state (Reference 1: Span, R. and Wagner, W. “A New Equation of State for Carbon Dioxide Covering the Fluid Region from the Triple-Point Temperature to 1100 K at Pressures Also shown are carbon dioxide isopycnal lines calculated using “up to 800 MPa”, Journal of Physical and Chemical Reference Data, 25, 1996, p1509-1596). Also, in Figure 3, EOS80 (Reference 2: Millero, F.J., Poisson, A. “International one-atmosphere equation of state of seawater, Deep Sea Research Part A. Oceanographic Research Papers”, Vol. 28, Issue 6 , 1981, p625-629; Reference 3: Millero, F.J., Poisson, A. “Density of seawater and the new International Equation of State of Seawater 1980”, IMS Newsletter, Number 30, Wright (Ed), Special Issue The equidensity line for 35‰ seawater calculated using 1981-1982, UNESCO, Paris, France, 1981, p.3) is also shown. FIG. 3 also shows isovalue lines of the density difference Δ (carbon dioxide density−seawater density). Furthermore, in FIG. 3, the temperature and pressure of the stratum shown in the schematic cross-sectional view of FIG. )} is also shown.

Here, the region indicated by R2 in the graph of FIG. 3 is the carbon dioxide shielding layer when the thickness of the carbon dioxide liquid layer G1 formed above the carbon dioxide storage layer G is 10 m to 40 m. This is the area where the pressure acting on C can be read. A region R2 shown in FIG. 3 is a region indicating the pressure and temperature of carbon dioxide stored in the carbon dioxide storage layer G. Here, when reading the value of the carbon dioxide density line L1 shown in Figure 3, the density of carbon dioxide is 850 to 970 kg/ ^m3 , and when reading the value of the seawater density line L2, the density of seawater is is 1029 to 1035 kg/m ³ . These densities are necessary for calculating the buoyancy of carbon dioxide, and when the thickness of the carbon dioxide liquid layer G1 formed above the carbon dioxide storage layer G is 10 m to 40 m, the carbon dioxide shielding layer C The pressure acting on can be determined based on the values of each density mentioned above.

The region indicated by R3 in the graph of FIG. 3 indicates the range of the density of carbon dioxide stored by carbon dioxide hydrate storage (when the thickness of the carbon dioxide liquid layer G1 is 50 m). Looking at this region R3, the density of the stored carbon dioxide is about 900 to 950 kg/m ³ , which is equivalent to 90 to 95% of the density of seawater. This shows that when the density of carbon dioxide stored in the carbon dioxide storage layer G (carbon dioxide liquid layer G1) approaches the density of seawater, the pressure generated by the buoyancy acting on the carbon dioxide shielding layer C becomes smaller.

Line segments (a) - (b) - (c) shown in the graph of Fig. 3 represent the temperature and pressure at the bottom of the ocean floor F (a) and the temperature and pressure at the lower end of the carbon dioxide shielding layer C (b), respectively. , and a line segment connecting the plot positions showing the temperature and pressure (c) of the carbon dioxide injection point, respectively, where the temperature of the seafloor surface F is 5°C, 1000m (approximately 1,000 decibar), and the temperature and pressure (c) of the sub-seafloor stratum U. The case where the geothermal gradient is 30°C/km and the temperature at the carbon dioxide injection point is 15°C is shown. Since this temperature and pressure change depending on the water depth and geothermal gradient at the storage point, the line segment (a)-(b)-(c) differs from point to point.

On the other hand, regarding the temperature and pressure of the seafloor surface F, the relationship between seawater temperature and pressure (water depth) in the Pacific Ocean and the Sea of Japan has been confirmed and reported (Reference 4: Yojiro Ikegawa, "About Japan regarding _CO2 hydrate storage"). “Formulation of seawater temperature in the deep sea” Proceedings of the Japan Society of Civil Engineers, 2022.01.14). The relationship between seawater temperature and pressure (water depth) in the Pacific Ocean and the Sea of Japan, as reported in Reference 4, is shown in Figure 3 by two nearly vertical curves on the left side in the X-axis direction of the graph. The one on the lower temperature side shows the relationship in the Sea of Japan, and the other side shows the relationship in the Pacific Ocean. These two curves are both plotted as curves at water depths with a standard deviation of 0.5°C or less; the Sea of Japan has a water depth in the range of 500 to 2000 m, and the Pacific Ocean has a water depth in a range of 1000 to 4000 m. is plotted. Here, in the graph of FIG. 3, the envelopes of the maximum and minimum values are shown as broken lines because the temperature change in the Pacific Ocean is large, which is considered to be the influence of the Kuroshio Current. Furthermore, the above two curves shown in FIG. 3 are the results regarding the range of water depths confirmed in the database covered in Reference Document 4. Regarding the geothermal gradient (GG), it is assumed that the average value of the geothermal gradient in the land area is 30℃/km, and that it is the same in the ocean area, but the geothermal gradient is about 0 to 200℃/km at each point. Varies within the range of .

Based on the relationship between seawater temperature and water depth as described above and the formula for the boundary (three-phase line) of the stable region of CO ₂ hydrate shown in Reference 4, the temperature at the lower end of the carbon dioxide shielding layer C, The pressure (T, P) can be calculated using the following general formula (1) using Tsf, Psf, and GG as known inputs.

On the other hand, at the upper end of the carbon dioxide shielding layer C, there is a phenomenon of supercooling of CO ₂ hydrate, and the molar ratio of CO ₂ and water (1:5.5 to 6) that allows generation of CO ₂ hydrate. There is insufficient data to accurately calculate this, as there are certain conditions, and there are points to keep in mind, such as freezing point depression occurring when pore water in stratum U beneath the ocean floor contains salt like seawater. There is a possibility that Here, the above-mentioned supercooling (SUPER-COOL) is a phenomenon in which the liquid remains in the liquid state even at a temperature and pressure that would cause the phase to change to a solid. , is known as a phenomenon in which the whole turns into a solid at once. Furthermore, freezing point depression is known as the fact that when ice is floated in salt water with a concentration of about 3%, the melting point of this ice drops to about -5°C, and as the concentration of salt increases, the melting point drops even further. is also known.

As mentioned above, since the pore water contained in the underground stratum U is considered to contain salt like seawater, freezing point depression in the production of CO ₂ hydrate will be explained below.
When CO ₂ hydrate is produced, it is produced in a way that removes salt, similar to the production of ice. As a result, a phenomenon occurs in which the salinity of the remaining water increases. Furthermore, since the freezing point depression varies depending on local salinity, it is not uniform and varies by about ±2°C. Furthermore, when measuring the change in calorific value near the freezing point of highly purified water, the freezing point can be determined by the occurrence of a clear exothermic peak at 0°C. However, when measuring the change in calorific value near the freezing point of salt-containing water, no clear peak appears, and the freezing point varies by 1 to 2 degrees Celsius, making it difficult to determine clearly.

As mentioned above, in order to determine the temperature and pressure at the upper end of the carbon dioxide shielding layer C, there are uncertainties from the viewpoints of supercooling, freezing point depression, and the molar ratio of _CO2 and water. The range is within a certain width. For example, in the case of average seawater having a temperature of 35‰, the freezing point depression is 2°C on average, and the range is about ±2°C. In the graph of FIG. 3, this freezing point depression is shown by a plurality of line segments L3 to L8 parallel to the temperature axis (X axis). In the graph of Figure 3, with respect to the boundary (three-phase line) of the stable region of CO ₂ hydrate shown by line segment L9, the above-mentioned line segments L3 to L8 are located on the low temperature side, which lowers the freezing point. is appearing. This data shows the results of a laboratory experiment conducted by the applicant, and line segments L3 to L8 are drawn to connect the maximum and minimum values.

From the above findings, the temperature at the upper end of the carbon dioxide shielding layer C is 2° C. lower on average than the temperature at the lower end, and has a width of approximately ±2° C. Furthermore, when the geothermal gradient is 30°C/km, a temperature change of 2°C is equivalent to a layer thickness of 66.7 m, which has a width of ±66.7 m, but this layer thickness and ± range are close to the average value. There is a high probability that Therefore, it is considered that the carbon dioxide shielding layer C is formed on the lower end side of the carbon dioxide sealing area S, that is, between 0 and 133 m from the boundary with the carbon dioxide storage layer G. Note that this finding is based on a geothermal gradient of 30° C./km and a salinity of 35‰, so if these change, the above-mentioned layer thickness will also change.

In the graph of Figure 4, the right end of the line segment for Case 1-3 shows the water depth (m) when the reservoir thickness is 0 m (top of the storage tank), and the pressure value when the reservoir thickness is 200 m. is shown as positive downward. Moreover, in FIG. 4, the pressure when the reservoir layer thickness is 20 m is about 1/10 of the pressure when the reservoir layer thickness is 200 m. In addition, in Figure 4, Cases 1 and 2 are carbon dioxide hydrate storage, and Case 3 is aquifer storage. In aquifer storage, carbon dioxide becomes supercritical, the carbon dioxide density is small, and the buoyancy is large. It can be seen that the pressure increases.

Each of the graphs in FIGS. 6(a) to (c) shows the strength of a film made of carbon dioxide hydrate obtained in an experiment using the pressure cell 100 shown in FIG. 5.
In each of the graphs in FIGS. 6(a) to (c), the flow rate of carbon dioxide indicated by a solid line in the lower graph is 0.006 mL/min in FIG. 6(a), 0.006 mL/min in FIG. 6(b): By slowing down in the order of 0.003 mL/min and 0.0015 mL/min (FIG. 6(c)), the time interval at which the carbon dioxide hydrate film breaks is set to become longer. If this time interval is short, it can be considered that the carbon dioxide hydrate film is broken before it gains sufficient strength. In Figures 6(a) to (c), as the above time interval becomes longer, a pressure difference (=strength) of about 0.2 MPa is measured after 115 hours, and the strength of the carbon dioxide hydrate membrane increases. An increasing trend is emerging.

The above differential pressure is the pressure acting on the carbon dioxide hydrate membrane. When this pressure difference becomes higher than the strength of the carbon dioxide hydrate membrane, part of the membrane ruptures and carbon dioxide permeates, reducing the pressure difference to 0. After that, the permeation of carbon dioxide stops. , a carbon dioxide hydrate film is repeatedly produced.

From the results shown in the graphs of Figures 6(a) to (c), it is assumed that the initial strength of the carbon dioxide hydrate film is 0.02 MPa, and in the following experiments, the effect on the initial strength of the carbon dioxide hydrate film is A pump (not shown) is controlled so that the differential pressure is 0.02 MPa.

The graph in FIG. 7 shows the temperature rise due to the heat of formation of carbon dioxide hydrate when carbon dioxide permeates into the geological formation, which was obtained in an experiment using the pressure cell 100 shown in FIG. In this experiment, the position of the penetration tip of liquid carbon dioxide into a sample (No. 7 silica sand) saturated with seawater was confirmed by temperature changes.

The graph in FIG. 8 shows the relationship between the holding time and the differential pressure when the differential pressure was held for 17 days under the condition that the pressure cell 100 shown in FIG. It shows a relationship. Furthermore, in this experiment, a sample (No. 9 silica sand) saturated with 40‰ seawater was used.
The initial conditions for the experiment whose results are shown in the graph of FIG. 8 were that carbon dioxide was infiltrated into the pores of the sand saturated with seawater to the position of the thermometer installed 400 to 500 mm from below.
In Figure 8, the relationship between time and differential pressure shown by the solid line in the third graph from the top (third graph from the bottom) is the initial carbon dioxide hydride, which is assumed to be generated at the tip position where carbon dioxide has permeated. Regarding the rate membrane, the differential pressure was maintained at 0.02 MPa for 400 hours (about 17 days).

The graph in FIG. 9 shows the result obtained in an experiment using the pressure cell 100 shown in FIG. 5, which was maintained for 17 days under the condition that the differential pressure was approximately 2 m or less in water head, and then maintained for 24 hours at a differential pressure of 1 MPa. It shows the strength of the carbon dioxide shielding layer C made of a carbon dioxide hydrate film when

In this experiment, a differential pressure of 0.08 MPa was forcibly applied to the carbon dioxide shielding layer C made of carbon dioxide hydrate, which was thought to have increased in strength after 410 hours in the above experiment, and this state was maintained for 24 hours. . After 24 hours in this maintained state, the differential pressure was raised to 1 MPa and maintained for 4 hours. As a result, it was confirmed that the strength of the carbon dioxide shielding layer C increased from 0.02 MPa, which is the initial strength of the carbon dioxide hydrate film, to 1 MPa.

Here, the differential pressure of 0.02 MPa mentioned above is the differential pressure that occurs when carbon dioxide is stored in carbon dioxide hydrate storage so that the thickness of the carbon dioxide storage layer G is about 20 m. In actual carbon dioxide storage, it is assumed that 5 million tons of carbon dioxide per year will be continuously injected over 20 to 30 years, so the thickness of the carbon dioxide reservoir G will gradually increase. It will be done. For example, if the time required for the carbon dioxide storage layer G to reach a thickness of 0 to 20 m is two weeks or more, the amount of storage can be achieved using the effective shielding function of the carbon dioxide shielding layer C with a strength of 1 MPa or more. can be expected.

In this experiment, first, as explained with reference to the graph in Figure 8, the temperature was 2°C, the pressure was 7 MPa, and the differential pressure was maintained at a water head of 2 m or less for 17 days. An experiment was conducted in which the pressure of carbon was increased to a differential pressure of 1 MPa and maintained for about 24 hours. As a result, as mentioned above, it was confirmed that the strength (pressure at which carbon dioxide starts permeating) in the carbon dioxide shielding layer C was 1 MPa or more.

The graph in FIG. 10 shows the strength of the carbon dioxide shielding layer made of a carbon dioxide hydrate film when the differential pressure is increased stepwise, which was obtained in an experiment using the pressure cell 100 shown in FIG. It shows. Here, the strength shown in the graph of FIG. 10 is the strength of the carbon dioxide hydrate membrane (pressure at which permeation of carbon dioxide starts).

The data shown in FIG. 10 is the result after data on the carbon dioxide shielding layer C having a strength of 1 MPa or more was acquired three times in a row over a period of about two weeks.
The reason for conducting this experiment was that based on previous knowledge, it was predicted that the strength of the carbon dioxide shielding layer C would gradually increase over a period of about two weeks, and it was necessary to confirm this. What happened will be mentioned.
In this experiment, first, as an initial condition, sand saturated with seawater was maintained at a temperature of about 4° C. under water pressure (8 MPa) equivalent to a depth of 800 m.
Then, various treatments were performed and progress observations were made at each of the timings shown in (1) to (6) below.

(1) 0 to 7 hours Carbon dioxide was permeated while adjusting the flow rate so that the above differential pressure was about 0.02 MPa (see the second graph from the bottom in the graph of FIG. 10).
During this time, carbon dioxide permeated at a maximum rate of 0.04 mL/min, and the amount of seawater permeated by carbon dioxide was discharged from the upper side of the pressure cell 100 (as shown in the second graph from the bottom in the graph of Figure 10). ).

In addition, as for the differential pressure at this time, a graph with a pressure scale of 0 to 3 MPa is shown in the third graph from the top in Figure 10, and a graph with a pressure scale of 0 to 0.25 MPa is shown in Figure 10. This is shown in the fourth graph from the top. That is, in FIG. 10, a case where the differential pressure is as small as 0.2 MPa or less is shown so that the pressure can be read in the fourth graph from the top.

(2) 7 to 18 hours After 7 hours had passed, the amount of carbon dioxide permeation decreased, so the above differential pressure was increased to 0.025 MPa. At this time, seawater in an amount that had permeated carbon dioxide was discharged from the upper side of the pressure cell 100, and it was confirmed that the pressure cell 100 was not shielded.

(3) For 18 to 42 hours When the above differential pressure was forcibly increased to 0.05 MPa, it appeared that a part of the carbon dioxide hydrate membrane was ruptured, and the permeation amount of carbon dioxide was 0.05 mL/min or more. There was a sudden increase in However, after another 4 hours (22 hours), the amount of carbon dioxide permeation decreased, and the seawater that had been discharged from the upper side of the pressure cell 100 (drainage has a negative flow rate) gradually became injected from the discharge (injection). is a positive flow rate), indicating that the carbon dioxide shielding layer C is gradually being formed.
The reason for this is thought to be that since the production of carbon dioxide hydrate causes a volumetric contraction of about 10%, both carbon dioxide and seawater flow in during this production process.

(4) When the differential pressure was gradually increased to 0.2 MPa for 42 to 50 hours, no penetration through the carbon dioxide shielding layer C was observed, indicating that the strength of the carbon dioxide shielding layer C was 0.2 MPa. It can be confirmed that the pressure is 2 MPa or more.

(5) 50 to 67 hours When the above differential pressure was maintained at 0.5 MPa for 17 hours, it was confirmed that the strength of the carbon dioxide shielding layer C was 0.5 MPa or more.

(6) 67 to 72 hours The differential pressure was increased stepwise, and the experiment was completed after confirming that the strength of the carbon dioxide shielding layer C was 3 MPa or more.

From the above results, in this experiment, although there was a rapid increase in the flow rate of carbon dioxide at 50 hours and from 65 to 72 hours (the third graph from the bottom in Figure 10), this This is the apparent flow rate that occurs due to the compression of carbon dioxide. That is, during the same time period, seawater is not discharged from the upper part of the pressure cell 100, and since both are in an injected state, it is clear that no carbon dioxide permeates through the carbon dioxide shielding layer C. It could be confirmed.

The above results are considered to indicate the time-strength relationship of the carbon dioxide shielding layer C, and it can be seen that the strength increases as time passes. In addition, it is thought that the carbon dioxide hydrate membrane was broken during the 18 to 22 hour period, but the repair function of the carbon dioxide shielding layer C itself acts on the breakage caused by this level of pressure difference, and the pressure of 1 MPa or more It is thought that the time it takes to reach this strength has been shortened to about 67 hours (just under 3 days).

The graph of FIG. 11 described above shows the carbon dioxide shielding made of a carbon dioxide hydrate membrane when a pressure difference in the opposite direction is applied, which was obtained in an experiment using the pressure cell 100 shown in FIG. The strength of layer C is shown.

In this experiment, first, as an initial condition, carbon dioxide is infiltrated into sand saturated with seawater so that the tip of the carbon dioxide infiltration is located in a range of 400 to 500 mm from the bottom of the pressure cell 100. The differential pressure was maintained in the range of 0 to 0.02 MPa for about 15 hours.
At the tip of carbon dioxide permeation in this initial stage, a temperature increase is observed due to the heat of formation of carbon dioxide hydrate. This temperature rise can be confirmed by observing the temperature rise at equal time intervals in the order of 100 mm, 200 mm, and 300 mm from the bottom of the pressure cell 100, thereby confirming the position of the tip of carbon dioxide permeation.

The graph in FIG. 11 shows the results when the pressure of carbon dioxide was suddenly reduced after 15 hours.
Due to this rapid decrease in the pressure of carbon dioxide, a remarkable temperature rise due to the heat of formation of carbon dioxide hydrate was observed at a location 400 mm from the bottom of the pressure cell 100, which indicates that a carbon dioxide shielding layer C was formed at this location. Conceivable.
Moreover, this timing coincides with the timing when the pressure of carbon dioxide decreases from 20 MPa to about 17.5 MPa and then rises again to 20 MPa.

Moreover, while the pressure of carbon dioxide is decreasing from 20 MPa to 17.5 MPa, the carbon dioxide shielding layer C has sufficient strength and is not torn, so a pressure difference is generated. However, when the pressure of carbon dioxide decreased to 17.5 MPa, the carbon dioxide shielding layer C was once broken.
At the timing when the carbon dioxide shielding layer C is once broken, water starts permeating into the vicinity of the carbon dioxide shielding layer C, and this permeation increases the pressure on the carbon dioxide side to 20 MPa. In addition, since the temperature rise due to the generation of heat of carbon dioxide hydrate generation is noticeable at a position 400 mm from the bottom of the pressure cell 100, a carbon dioxide shield made of a stronger carbon dioxide hydrate film is installed at this position. It is thought that layer C has been regenerated.

Also, looking at the flow rate of carbon dioxide, it was 0.04 mL/m at 15 hours, and stopped at 0 mL/m after 15 hours. In addition, as mentioned above, after the carbon dioxide shielding layer C is once ruptured and regenerated, the pressure on the carbon dioxide side gradually decreases, and the differential pressure acting on the carbon dioxide shielding layer C moves in the opposite direction (minus). becomes. This pressure difference shows that the carbon dioxide shielding layer C has a strength equivalent to 10 MPa (1000 m in water head).

Here, in the pressure cell 100 shown in FIG. 5, when an artificial water seal, which will be described in detail later, is performed in a direction in which water permeates into the pores of the sand into which carbon dioxide has permeated (artificial water seal equipment 6 in FIG. ), the effect of reducing the differential pressure acting on the carbon dioxide shielding layer C is expected. In addition, if the pressure increase due to the artificial water seal becomes excessive, the carbon dioxide shielding layer C is expected to have an effect of increasing its strength in a short period of time (1 to 2 hours in this experiment).

The graph in FIG. 12 shows contour lines of the optimal volume ratio of {carbon dioxide/water} in the temperature-pressure relationship.
As shown in Figure 12, as a result of calculating the volume ratio considering the molecular formula of carbon dioxide hydrate crystals (5.75H ₂ O.CO ₂ ), the volume of water is required to be 2.3 times the volume of carbon dioxide. becomes. Therefore, in order for the pores into which carbon dioxide has permeated to become clogged, it is considered that a configuration in which there is insufficient water/seawater is likely to occur. In other words, it is considered that water has been used up (dry up) and carbon dioxide is in surplus.

On the other hand, FIG. 13 shows a schematic diagram in which the shielding mechanism of carbon dioxide hydrate by a film is considered based on the experimental results using the pressure cell 100 shown in FIG. That is, FIG. 13 is a diagram showing a partial cross section of the cylindrical pressure cell 100 shown in FIG. 5. As shown in FIG.
As shown in the schematic diagram of Figure 13, near the interface between liquid carbon dioxide and water/seawater in the sand pores, the saturation level of water gradually decreases from 100%, indicating that the interface is never flat. .
Further, as shown in FIG. 13, near the tip of carbon dioxide permeation, there is a region close to the volume ratio according to the above-mentioned molecular formula.

It is thought that a water film exists on the surface of the sand due to the wettability of water, and the structure is thought to be that a carbon dioxide hydrate film is formed at the interface that intricately penetrates the pores of the sand. Therefore, it is highly likely that the carbon dioxide shielding layer C is not a horizontal one-sided film but a region having a thickness of a certain value or more.

Furthermore, the thickness is about 20 μm in the pores of the sand (Reference 5: Takehiko Yauchi, Yutaka Abe, Akiko Kaneko, Kenji Yamane, "Effect of flow field on CO ₂ hydrate film thickness", Transactions of the Japan Society of Mechanical Engineers, B ed. , Vol. 78, No. 787, 2012) is generated in the shape of a straw, and further assumes that the average pore diameter is 60 μm, the 20 μm remaining in the center of the straw shape It is thought that liquid carbon dioxide exists in the region of . It takes about two weeks for the portion where this liquid carbon dioxide remains to turn into carbon dioxide hydrate, and it is thought that this portion will have a strength of 1 MPa or more.
It is thought that carbon dioxide molecules and water molecules are supplied to the remaining portion of the liquid carbon dioxide by diffusion based on the concentration gradient, and this supply by diffusion takes time for the formation of the carbon dioxide shielding layer C. This is considered to be a factor.

According to the above-mentioned demonstration experiment of the present invention, it is preferable that the carbon dioxide shielding layer C has a strength of 0.01 to 0.04 MPa in the pores of the carbon dioxide shielding layer C at the initial stage of carbon dioxide hydrate generation. More preferably, the average value is about 0.02 MPa. By ensuring the initial strength of the carbon dioxide hydrate forming the carbon dioxide shielding layer C within the above range, the carbon dioxide injected below the carbon dioxide shielding layer C can be more reliably contained, and a large amount of carbon dioxide can be contained. It becomes possible to store carbon dioxide with higher efficiency.
Note that the strength of 0.02 MPa at the initial stage of carbon dioxide hydrate formation is an average value for the particle sizes of the above-mentioned No. 9 silica sand to No. 7 silica sand.

In the storage method of this embodiment, the initial strength of carbon dioxide hydrate generation in the carbon dioxide shielding layer C and the layer thickness of the carbon dioxide liquid layer G1 formed on the upper part of the carbon dioxide storage layer G are 10 m to 40 m. Calculate the pressure acting on the carbon dioxide shielding layer C when A method of controlling at least one of the depths can be adopted. Each of the above calculations can be performed using a calculation program on a computer, tablet terminal, etc., and based on the calculation results, the injection flow rate of carbon dioxide, the number of injection wells 2 used, and the depth. All you have to do is control. In this case, for example, by installing a plurality of injection wells 2 with different inner diameters and injection depths, it becomes easy to appropriately change the flow rate and the number of injection wells 2 used.

According to the storage method of this embodiment, by employing the control method as described above, the carbon dioxide is Since the injection flow rate and injection position of carbon can be adjusted, efficient operation of the carbon dioxide storage facility becomes possible. Further, as described above, it becomes possible to reliably and effectively contain the carbon dioxide injected below the carbon dioxide shielding layer C, and therefore it becomes possible to store a large amount of carbon dioxide with high efficiency.

Furthermore, it is preferable that the carbon dioxide shielding layer C has a strength of 1 MPa or more after 10 to 20 days have passed from the start of carbon dioxide hydrate production. By having the strength within the above range after 10 to 20 days have elapsed from the start of production of carbon dioxide hydrate forming the carbon dioxide shielding layer C, the strength of the carbon dioxide shielding layer C can be ensured more reliably. Thereby, the carbon dioxide injected below the carbon dioxide shielding layer C can be more reliably contained, so that a large amount of carbon dioxide can be stored even more efficiently.

In addition, in the storage method of this embodiment, the strength of the carbon dioxide shielding layer C after 10 to 20 days have passed from the start of carbon dioxide hydrate generation is calculated, and based on the calculated value, the carbon dioxide A method of controlling at least any one of the injection flow rate into the stratum U below the surface F, the number of injection wells 2 used, and the injection depth can be adopted. The above calculated value can be obtained using a calculation program on a computer, tablet terminal, etc., and similarly to the above, based on this calculation result, the injection flow rate of carbon dioxide, the number of injection wells 2 used, and the depth can be determined. It is possible to control.
As a result, as described above, the injection flow rate and injection position of carbon dioxide can be adjusted while monitoring the storage status of carbon dioxide in the carbon dioxide storage layer G and the strength of the carbon dioxide shielding layer C, etc. This will enable even more efficient operation of storage facilities. Further, as described above, it is possible to reliably and effectively contain the carbon dioxide injected below the carbon dioxide shielding layer C, and therefore it is possible to store a large amount of carbon dioxide with even higher efficiency.

In addition, in this embodiment, the stratum thickness of the entire carbon dioxide seal area S is not particularly limited. However, in order to improve the sealing performance of the carbon dioxide seal area S, that is, to reliably prevent carbon dioxide (CO ₂ ) stored in the carbon dioxide reservoir G from leaking into the seawater W, the carbon dioxide seal It is preferable to ensure that the stratum thickness of the region S is at least 100 m or more. In this way, the method of ensuring the desired thickness of the stratum in the carbon dioxide seal area S is, for example, by thoroughly understanding the temperature and pressure situation of the stratum U below the seafloor surface F through a preliminary survey. It is preferable to set the depth of the injection well 2 so that the stratum thickness of the carbon dioxide sealing region S has a predetermined dimension, and to optimize the injection depth of carbon dioxide (injection depth).

[Depth region in the stratum where the density of carbon dioxide and the density of pore water are in equilibrium]
In the underground carbon storage method of the present embodiment, increasing the pore pressure of the carbon dioxide sealing area S by artificial water sealing, as described above, and generating carbon dioxide hydrate forming the carbon dioxide shielding layer C. In addition to optimizing the relationship between time and intensity, we also optimize the density of carbon dioxide (ρ _CO2 ) and the density of pore water (ρ _sw ) in the stratum U below the seafloor F or in the strata on land. It is also possible to adopt a method in which carbon dioxide is stored in a deep region in the geological formations that satisfies the pressure and temperature conditions in which the carbon dioxide is in equilibrium.

That is, in this embodiment, by conducting experiments and field surveys in advance, for example, the density of carbon dioxide (ρ _CO2 ) and the density of pore water (ρ _sw ) in the stratum U below the ocean floor F are determined. After identifying a geological formation that satisfies the pressure and temperature conditions in which the carbon dioxide is in equilibrium, carbon dioxide can be stored in the depth region.

In addition, in this embodiment, the depth of the stratum that satisfies the pressure and temperature conditions in which the density of carbon dioxide ρ _CO2 and the density of pore water ρ _sw are in equilibrium in the stratum U below the seafloor F or in the strata on land is determined. It is also possible to adopt a method of calculating and controlling the injection position of carbon dioxide into the stratum U under the ocean floor F or the strata on land based on the calculated value.

Considering the geothermal gradient in the stratum U below the ocean floor F, there exists a depth at which the density of liquid carbon dioxide (ρ _CO2 ) and the density of pore water such as seawater (ρ _sw ) match (ρ _CO2 = ρ _sw ). Conceivable. In other words, it is thought that carbon dioxide can be stored in the stratum at this depth by taking advantage of the fact that carbon dioxide injected as a liquid remains without sinking or rising.

The graph in Figure 14 shows the calculation results of the relationship between temperature and pressure conditions at the depth where the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) are in equilibrium in the stratum U below the seafloor F. ing.
The calculation of the results shown in the graph of FIG. 14 was performed under the conditions of seawater salinity: 35‰ psu and geothermal gradient: 30° C./km, and for the seawater temperature, an example of Argo data in the Pacific Ocean was used.

In the graph of FIG. 14, (1) is the generation (stable) region of carbon dioxide hydrate, where the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) are expressed by the following formula: {ρ _CO2 <ρ _sw } This is the region where the relationship is expressed as , and the temperature region deeper than this position is where carbon dioxide liquefies.
In addition, (2) in the graph of FIG. 14 is a region where the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) satisfy the relationship expressed by the following formula {ρ _CO2 = ρ _sw }. , the water depth/region where the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) satisfy pressure and temperature conditions in equilibrium.
In addition, (3) in the graph of FIG. 14 is a region where the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) have a relationship expressed by the following formula {ρ _CO2 > ρ _sw }. , Acoustic base: This is the area that appears in the reflection method used in geological surveys.

Then, as a result of calculating the water depth that has the relationship expressed by the following formula {ρ _CO2 > ρ _sw } in the stratum U below the seafloor F, the water depth = 3964.645 m, the water depth on the seabed F = 3600.000 m, and the seafloor The depth from the plane F = 364.645 mbsf was calculated.

Note that the conditions for applying the above calculation are such that the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) are expressed by the following formula {ρ _CO2 >ρ _sw } in the relationship between seawater temperature and pressure (water depth) in the ocean area. This is the stratum U below the seafloor F at a depth of 2,700 m or deeper (Pacific Ocean excluding the Sea of Japan), which has the relationship expressed by .

In the graph of FIG. 3 shown above, the hatched areas on the base points of the X and Y axes correspond to temperature conditions where the density of carbon dioxide (ρ _CO2 ) is greater than the density of seawater (ρ _sw ). Shows pressure conditions.
A method has also been proposed in which carbon dioxide is stored in areas of concave topography on the ocean floor by utilizing the environment where the relationship is expressed by the following equation {ρ _CO2 > ρ _sw } (see reference 5 above). ), but in reality, this is not possible due to the ``Act on the Prevention of Marine Pollution, Etc. and Maritime Disasters'' (Marine Pollution Prevention Act) based on international law. On the other hand, the stratum U below the ocean floor F is not within the scope of the above Marine Pollution Prevention Law. Therefore, the study results of storing carbon in the stratum U below the seafloor F at a depth of more than 3000 m have been reported (Reference 6: Yihua Teng and Dongxiao Zhang., “Long-term viability of carbon sequestration in deep-sea sediments”) , Science Advances, Vol.4, No.7, 04 Jul 2018), geothermal gradients have not been considered so far.

Therefore, in the graph showing the temperature-pressure relationship in Figure 14, as a result of calculations that took into account the geothermal gradient of the stratum U below the seafloor F, we found that the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) It has been found that a Density-Matched Depth (Density-Matched Depth) appears where the following equation (ρ _CO2 =ρ _sw } is satisfied and there is an equilibrium).
The solid line in the graph of FIG. 14 indicates the seawater temperature at a depth of 0 to 3500m, and at a depth of 3500m or deeper, the temperature increases due to the influence of the soil temperature of the stratum U below the seafloor F.
In addition, the chain line in the graph of Figure 14 shows the density of seawater (ρ _sw ) calculated using the existing seawater equation of state, taking into account temperature (seawater/geological formation), pressure, and salinity of 35‰. .
Furthermore, the dashed-dotted line in the graph of FIG. 14 indicates the density of carbon dioxide (ρ _CO2 ) calculated using the existing equation of state for carbon dioxide and taking into account temperature (seawater/geological formation) and pressure.

As shown in the graph of FIG. 14, the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) match (equilibrium) at around 2,700 m depth. On the other hand, at a depth of about 2,700 to 3,500 m, the density of carbon dioxide (ρ _CO2 ) is greater than the density of seawater (ρ _sw ). In addition, at a depth of 3,500 m or deeper, the density of carbon dioxide (ρ _CO2 ) decreases as the temperature rises as it enters the stratum U below the ocean floor F, while at a depth of 4,000 m, the density of carbon dioxide (ρ CO2 ) decreases again. _CO2 ) and the density of seawater (ρ _sw ) match. Further, at a depth of 4000 m or deeper, the density of seawater (ρ _sw ) is greater than the density of carbon dioxide (ρ _CO2 ).

Based on the changes in the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) as described above, in the case of the above calculation conditions, by injecting carbon dioxide into the stratum U below the seafloor F near the depth of 4000 m, It is thought that this carbon dioxide stays at the relevant location without floating or settling, and spreads horizontally. Therefore, it is considered that carbon dioxide can be stored underground at the above-mentioned position in the stratum U below the seafloor surface F.

Here, in Fig. 14, in the stratum U below the seafloor F, the seawater transitional zone, water depth 2500, described in the 2005 IPCC report (15, p. 280, Fig. 6, 9/p. 286) is shown. ~3000m). In addition, in FIG. 14, the water depth has a width as a transition zone because the water depth, which has the relationship expressed by the following equation {ρ _CO2 =ρ _sw }, changes due to changes in seawater temperature. Similarly, in the stratum U below the ocean floor F, the geothermal gradient differs from point to point, so the depth from the ocean floor F changes, with the relationship expressed by the following equation {ρ _CO2 =ρ _sw }.

From the changes in the density of carbon dioxide (ρ _CO2 ) and the density of seawater (ρ _sw ) as described above, in the case of the above calculation conditions, it is possible to inject carbon dioxide into the stratum U below the seafloor F at a depth of about 4000 m. Therefore, this carbon dioxide is thought to remain at the relevant location without floating or settling, and spread horizontally. Therefore, it is considered that carbon dioxide can be stored underground at the above-mentioned position in the stratum U below the seafloor surface F.

As mentioned above, liquid carbon dioxide stored at a depth where the density of carbon dioxide (ρ _CO2 ) in the stratum U below the ocean floor F matches the density of seawater (ρ _sw ) does not float or sink. It stays at that position. Previously, it was thought that carbon dioxide would settle if it was stored at a depth of 3,000 m or deeper, but it was unclear to what depth in the geological formations it would settle, but according to the findings of the present inventors. If carbon dioxide is stored at the above depth, no sedimentation or surfacing of carbon dioxide will occur.

Note that the density of pore water in which carbon dioxide is dissolved (carbon dioxide dissolved water) is greater than the density of pore water, so it settles, and calcium (Ca), etc. When it comes into contact with a bedrock rich in alkali metals, it becomes carbonates such as calcium carbonate (CaCO ₃ ), and becomes the final sequestration state of carbon dioxide (CO ₂ ).

By using a stratum with balanced density, that is, a stratum with the relationship expressed by the following formula {ρ _CO2 = ρ _sw }, even if a fault with high permeability occurs due to an earthquake, etc., the carbon dioxide based on buoyancy will be reduced. The risk of carbon leakage is likely to be reduced, and the cap lock described above is not required.

Here, when a stratum is deformed due to an earthquake or the like, the pore volume generally increases (dilatancy). In this way, the pressure in the area where the pore volume has increased decreases, and carbon dioxide flows in from the surrounding area due to the pressure gradient, but there is a possibility that the pressure will exceed the hydrostatic pressure due to inertia. In this case, there is a risk that an upward flow of carbon dioxide will occur and leak into the ocean, but since a carbon dioxide shielding layer C consisting of a carbon dioxide hydrate film is formed in strata shallower than this depth, the above leakage risk can be reduced. Double defense works. This double protection refers to the use of a geological formation that has the relationship expressed by the above-mentioned following formula {ρ _CO2 =ρ _sw }, and the generation of a carbon dioxide shielding layer C made of a carbon dioxide hydrate film.

Here, at a depth of 3000 m or deeper, the temperature of the seabed surface F is generally less than 2°C. Furthermore, considering the geothermal gradient underground, it is possible to draw an isothermal line parallel to the F plane of the ocean floor.
When these isotherms are taken into account around islands, seamounts, and ocean plateaus, as shown in Figure 15, isopycnic lines that satisfy the relationship expressed by the following equation {ρ _CO2 = ρ _sw } are found on the seafloor surface F. It is thought that it is tilted so as to be in equilibrium with. Here, the graph of FIG. 15 shows a storage method assuming an island, a seamount, and a sea plateau, and shows the relationship between horizontal distance and water depth with an aspect ratio of 1:10.
In areas shallower than the above isopycnal lines, _the relationship is expressed by the following equation {ρ _CO2 > _{ρ sw} }, as shown in FIG. Since the area has _the relationship _expressed by do.

As mentioned above, considering the distribution of isopycnal lines that satisfy the relationship expressed by the following formula {ρ _CO2 = ρ _sw } in the stratum U below the seafloor F at a depth of 3000 m or more, the depth of this isopycnal line can be determined by In order to store carbon dioxide nearby, it is necessary to drill a well below the ocean floor F to a depth of more than 3000 m. As described above, in order to drill a well below the seabed F at a depth of 3000 m or more, it is essential to use the latest drilling technology, and a significant increase in construction costs is expected. Therefore, as shown in Fig. 15, if we consider the isopleth line (ρ _CO2 = ρ _sw ) that is considered to be distributed parallel to the F plane of the seabed, it is possible to drill a well from the F plane of the seabed at a depth of less than 3000 m. There may be a place where you can. By installing injection wells in such locations using more general-purpose, low-cost drilling techniques, it is possible to store carbon dioxide in an area extending to a depth of 3,000 meters or more, and a larger storage capacity can be achieved.

In particular, islands, seamounts, and sea platforms made of volcanic rock are the result of the shape of the airways of volcanic gas emitted from lava spewed out on the ocean floor surface F being rapidly cooled by seawater. In addition, volcanic rocks such as Mt. Fuji have cavities (pores) that are continuous with lava paths (vents) and fine continuous pores (airtight for volcanic gas), and carbon dioxide is stored in these cavities. be done.

The term "island" mentioned above refers to a land area surrounded on all sides by water bodies such as oceans and lakes.
In addition, the above-mentioned "sea mount" is a type of submarine topography, and refers to a relatively isolated high place with a relative height of 1000 m or more above the sea floor, and whose peak diameter is not large. say.
Furthermore, the above-mentioned "sea plateau" is a plateau-like terrain with a relatively flat top on the ocean floor, which is ¹⁰⁰ km2 or more in area, and is raised by 200 m or more from the surrounding sea floor. Say something.
Moreover, "volcanic gas" is a mixed gas such as water vapor, CO ₂ , hydrogen sulfide, and carbon monoxide.

<Effect>
As explained above, according to the carbon dioxide underground storage method of the present embodiment, by adopting the above configuration, the strength of the entire carbon dioxide sealing area is increased, so that carbon dioxide is hydrated. The strength of the carbon dioxide shielding layer C can be ensured. This allows the carbon dioxide injected below the carbon dioxide shielding layer C to be reliably contained, making it possible to efficiently store a large amount of carbon dioxide.

Moreover, according to the underground carbon dioxide storage device of this embodiment, it can be applied to the underground storage method of this embodiment described above with a simple configuration, and it is possible to efficiently store a large amount of carbon dioxide. Become.

<Other forms>
Note that the technical scope of the present invention is not limited to the above-described embodiments, and various changes can be made without departing from the spirit of the present invention.
For example, in the above embodiment, an example is described in which carbon dioxide is stored in a geological layer under the seabed, but the method and underground storage device for carbon dioxide according to the present invention are not limited to this. First, as mentioned above, it can also be applied to the storage of carbon dioxide in geological formations on land.

The underground carbon dioxide storage method and the underground carbon dioxide storage device of the present invention reliably confine carbon dioxide injected downward by ensuring the strength of the carbon dioxide shielding layer formed by hydrated carbon dioxide. This is a method and device that can efficiently store a large amount of carbon dioxide. Therefore, the present invention is very suitable for use in various plants, for example, in which carbon dioxide is recovered while generating electricity using fossil fuels, and this carbon dioxide is stored underground to reduce emissions. .

1... Carbon dioxide underground storage device (storage device)
2...Injection well 21...Tip 6...Artificial water seal equipment 100...Pressure cell M...Sea surface W...Seawater F...Seafloor surface (seafloor)
U... Geological layer S... Carbon dioxide sealing area C... Carbon dioxide shielding layer G... Carbon dioxide storage layer G1... Carbon dioxide liquid layer B... Acoustic base

Claims (2)

1. A method of storing carbon dioxide alone or in a liquefied state as a mixed gas mainly composed of carbon dioxide in a sub-seafloor stratum made of sediment on an acoustic substrate or in a stratum on land, comprising:
   A carbon dioxide reservoir is created by injecting the carbon dioxide below the carbon dioxide seal area, which exists from the seabed or ground to a predetermined depth and consists of a stratum that satisfies the pressure and temperature conditions capable of producing carbon dioxide hydrate. form,
   At least a portion of the carbon dioxide injected into the carbon dioxide storage layer is naturally raised toward the carbon dioxide sealing area by the buoyancy of the carbon dioxide to generate carbon dioxide hydrate, thereby forming the carbon dioxide sealing area. Forms a carbon dioxide shielding layer on
   When the carbon dioxide is injected into the sub-seafloor stratum or the land stratum, an artificial water seal injects at least one of seawater and water as pore water into the carbon dioxide seal area. A method of storing carbon dioxide underground that increases pore pressure.
2. An underground storage device that stores carbon dioxide alone or in a liquefied state of a mixed gas mainly composed of carbon dioxide in a sub-seafloor stratum made of sediment on an acoustic substrate or in a stratum on land,
   Carbon dioxide that penetrates a carbon dioxide seal region that exists to a predetermined depth from the ocean floor or the ground and is composed of a stratum that satisfies pressure and temperature conditions that are capable of producing carbon dioxide hydrate, and that is located below the carbon dioxide seal region. an injection well extending into the reservoir;
   a pressurizing equipment for pressurizing the mixed gas to the injection well;
   an artificial water sealing facility that is disposed on the upper surface side of the carbon dioxide sealing area and increases the pore pressure of the carbon dioxide sealing area by injecting at least one of seawater and water as pore water into the carbon dioxide sealing area; An underground storage device for carbon dioxide.

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