JP2024048816A - Methods for geological storage of carbon dioxide - Google Patents

Abstract

[Problem] To provide a method for underground storage of carbon dioxide that can reliably seal carbon dioxide injected below a carbon dioxide seal layer containing carbon dioxide hydrate and efficiently store large amounts of carbon dioxide. [Solution] The method includes a carbon dioxide hydrate production possible area S2 within a predetermined depth range from the seabed surface F and made up of a geological layer that satisfies pressure and temperature conditions that allow carbon dioxide hydrate to be produced, and includes a step (1) of identifying an unconsolidated sedimentary layer U containing methane hydrate, and a step (2) of injecting carbon dioxide into the unconsolidated sedimentary layer U below the methane hydrate production possible area S1. [Selected Figure] Figure 1

Description

Application for application of Article 30, Paragraph 2 of the Patent Act filed. Publication date: October 25, 2021 Publication: "FY2021 Domestic Oil and Natural Gas Geological Surveys and Methane Hydrate Research and Development Project (Methane Hydrate Research and Development) Public Offering Guidelines for "Study Aiming to Further Expand the Range of Sites Suitable for CO2 Storage"" issued by the National Institute of Advanced Industrial Science and Technology (Website public address: URL: https://www.aist.go.jp/pdf/aist_j/business/itaku/20211025-2_1.pdf) <Materials> Public Offering Guidelines for Commissioned Research

The present invention relates to a method for underground storage of carbon dioxide.

Carbon dioxide capture and storage (hereinafter simply referred to as "CCS") has been attracting attention as an innovative technology that can reduce carbon dioxide ( _CO2 ) emissions while generating electricity using fossil fuels. In addition, demonstration tests and investigations into suitable storage locations are being actively conducted for a method of storing carbon dioxide underground using CCS technology.

Generally, carbon dioxide exists in one of four phases, gas, liquid, solid, and supercritical, depending on the pressure and temperature conditions. Carbon dioxide becomes solid dry ice at normal pressure and at a temperature of 194 K (-79.15°C) or less, but when mixed with water, it is known to become hydrated (solid) under different temperature and pressure conditions. Carbon dioxide hydrate (CO ₂ hydrate) is generated when liquid carbon dioxide is mixed with water and is generated under conditions of a temperature of 10°C or less and a pressure of 4.5 MPa or more, and overlaps with a part of the region of liquid carbon dioxide (liquid phase region).

Carbon dioxide hydrate as described above is a solid with a crystal structure similar to that of methane hydrate, with gas molecules trapped within a three-dimensional lattice of water molecules. Carbon dioxide hydrate is generally called gas hydrate, and the first research report confirmed that carbon dioxide hydrate causes clogging in natural gas transport lines. Carbon dioxide hydrate has also been confirmed as a blocking substance in plant piping for oil and gas processing, and research into it is also progressing in the field of science related to the Earth and planets.

The density of carbon dioxide changes due to a gas-liquid phase change when pressure increases, but because it goes into a supercritical state at temperatures of 31°C or higher and pressures of 7.4 MPa or higher, density changes due to further pressure increases are relatively slow. And because the density of carbon dioxide in a liquid or supercritical state is lower than that of seawater, for example, when attempting to store carbon dioxide in a geological layer below the seabed, buoyancy occurs due to the density difference, and some sort of shielding function is required to contain the rising carbon dioxide.

Here, a method for producing methane gas has been proposed that utilizes a sealing layer (shielding layer) made of carbon dioxide hydrate as described above (see, for example, Patent Document 1). Patent Document 1 describes a method that includes the steps of forming a methane gas production layer by adding a group of microorganisms to the gaps of a stratum under temperature and pressure conditions in which methanogenic bacteria produce methane gas, forming a carbon dioxide hydrate sealing layer by injecting an emulsion in which fine particles of liquid carbon dioxide are dispersed into a stratum shallower than the methane gas production layer and under temperature and pressure conditions in which carbon dioxide becomes hydrate, and injecting liquid carbon dioxide, which is a raw material for producing methane gas, into the stratum between the methane gas production layer and the sealing layer.

In addition, in order to mine methane hydrate, a method has been proposed in which an emulsion of liquid carbon dioxide dispersed as fine particles smaller than the pores in the formation is produced in a well and then injected into the formation, in order to generate and immobilize carbon dioxide hydrate in a formation under temperature and pressure conditions that allow carbon dioxide to become hydrate (see, for example, Patent Document 2).

Also, as a low carbon dioxide emission power generation method and system, a method and system have been proposed that includes the steps of generating electricity using methane gas as fuel, obtaining heated seawater with adsorbed carbon dioxide produced by power generation, injecting the obtained heated seawater into a methane hydrate layer on the seafloor to isolate and fix the carbon dioxide in the heated seawater in the methane hydrate layer, transferring the heat of the heated seawater to methane hydrate in the methane hydrate layer and recovering methane gas dissociated from the methane hydrate by receiving this heat, and supplying the recovered methane gas to a power generation facility (see, for example, Patent Document 3).

As a method for storing carbon dioxide in a geological layer under the seabed, a method and apparatus have been proposed in which carbon dioxide is injected below a carbon dioxide hydrate generation area that exists from the seabed surface to a specified depth and satisfies the pressure and temperature conditions for generating carbon dioxide hydrate to form a carbon dioxide storage layer (see, for example, Patent Document 4). The method and apparatus described in Patent Document 4 can prevent clogging when storing carbon dioxide underground, and can efficiently store large amounts of carbon dioxide.

JP 2010-239962 A JP 2010-284605 A JP 2010-209591 A JP 2019-126787 A

As mentioned above, the conditions for carbon dioxide hydrate formation are a temperature of 10°C or less and a pressure of 4.5 MPa or more, and these conditions are met over a wide area in the ocean areas surrounding Japan. However, identifying locations in strata below the seafloor that meet the geological conditions for carbon dioxide hydrate formation remains a challenge for the future in achieving efficient underground storage of large amounts of carbon dioxide.

Here, it is known that unconsolidated sedimentary layers containing methane hydrate are also suitable as suitable sites for generating carbon dioxide hydrate and storing carbon dioxide underground. Among the unconsolidated sedimentary layers described above, methane hydrate is present at high density and has high potential as a development target for methane gas production. In the methane hydrate concentrated zone, methane gas is produced from the viewpoints of industrial production and effective utilization of resources. However, in places with unconsolidated sedimentary layers, the density of methane hydrate is originally low, or the methane hydrate has already been used for methane gas production in the methane hydrate non-concentrated zone, which includes a methane hydrate non-concentrated layer, is excluded from the production target areas due to its low productivity of methane gas, and the methane hydrate in the layer is not effectively utilized.

On the other hand, the above Patent Documents 1 to 3 are all aimed at mainly promoting the production of methane hydrate, while the above Patent Document 4 is aimed at preventing the pores of the formation from being blocked and clogged due to the hydration of carbon dioxide. In other words, none of Patent Documents 1 to 4 significantly improves the low permeability of the shielding layer, which can be said to be an important condition for efficiently storing a large amount of carbon dioxide.
For this reason, there is a strong demand for a method to efficiently store large amounts of carbon dioxide underground using a shielding layer made of hydrate, which has excellent shielding properties.

The present invention has been made in consideration of the above circumstances, and aims to provide a method for underground storage of carbon dioxide that can reliably contain the carbon dioxide injected below a carbon dioxide sealing layer containing carbon dioxide hydrate, and efficiently store large amounts of carbon dioxide.

In order to solve the above problem, the inventors of the present invention have conducted extensive research. As a result, they have first identified a methane hydrate generation possible area that includes a carbon dioxide hydrate generation possible area and is composed of an unconsolidated sedimentary layer in which methane hydrate is present, and have found that by injecting carbon dioxide into the unconsolidated sedimentary layer below this methane hydrate generation possible area, a portion of the carbon dioxide passes through the lower layer side of the methane hydrate generation possible area, and carbon dioxide hydrate is generated in the carbon dioxide hydrate generation possible area on the upper layer side of the methane hydrate generation possible area. In this way, a layer made of carbon dioxide hydrate is formed in the carbon dioxide hydrate generation possible area on the upper layer side of the methane hydrate generation possible area, and by forming a carbon dioxide seal layer having a two-stage multi-seal function on the lower layer side and the upper layer side of the methane hydrate generation possible area, the carbon dioxide hydrate shielding performance in the carbon dioxide seal layer is improved, and it is possible to efficiently store a large amount of carbon dioxide, and thus the present invention has been completed.

That is, the present invention employs the following configuration.
[1] A method for storing carbon dioxide, either alone or in the form of a mixture mainly composed of carbon dioxide, in a geological layer below the seabed surface made of sediments, the method comprising the steps of: (1) identifying a methane hydrate production possible area within a predetermined depth range from the seabed surface, the methane hydrate production possible area being made of an unconsolidated sedimentary layer in which methane hydrate is present, the methane hydrate production possible area being made of a geological layer satisfying pressure conditions and temperature conditions capable of producing carbon dioxide hydrate; and (2) injecting the carbon dioxide into the unconsolidated sedimentary layer below the methane hydrate production possible area.
[2] The method for underground storage of carbon dioxide described in [1] above, in which the step (2) comprises forming a carbon dioxide reservoir in an unconsolidated sedimentary layer below the region where methane hydrate can be produced, and forming a carbon dioxide hydrate layer by causing at least a part of the carbon dioxide injected into the carbon dioxide reservoir to rise toward the region where methane hydrate can be produced and pass through a lower layer side of the region where methane hydrate can be produced, and generating carbon dioxide hydrate in the region where methane hydrate can be produced on the upper layer side of the region where methane hydrate can be produced, thereby forming a carbon dioxide hydrate layer, in which the lower layer side of the region where methane hydrate can be produced is a primary sealing layer and the carbon dioxide hydrate layer is a secondary sealing layer.
[3] The method for underground storage of carbon dioxide described in the above [1] or [2], wherein the step (2) includes identifying a region that is a non-concentrated zone of the methane hydrate among the unconsolidated sedimentary layer containing the methane hydrate, and injecting carbon dioxide into the unconsolidated sedimentary layer in the non-concentrated zone of the methane hydrate below the region where methane hydrate can form.
[4] The method for underground storage of carbon dioxide according to the above-mentioned [3], wherein the non-concentrated zone of methane hydrate contains methane hydrate derived from methane present in the stratum, and is a region in which the methane hydrate saturation rate has been reduced by the production of methane gas using the methane hydrate.
[5] The method for underground storage of carbon dioxide according to the above-mentioned [3], wherein the methane hydrate non-concentrated zone contains artificially produced methane hydrate and is composed of a layer having a low methane hydrate saturation rate.
[6] The method for underground storage of carbon dioxide according to any one of [1] to [5] above, wherein in the step (2), the carbon dioxide is injected into the unconsolidated sedimentary layer below the methane hydrate production possible area at a pressure of 6 MPa or more.
[7] The method for underground storage of carbon dioxide according to any one of [1] to [6] above, wherein the step (2) comprises injecting the carbon dioxide in a liquid state or a supercritical state into an unconsolidated sedimentary layer below the region where methane hydrate can be produced.
[8] The carbon dioxide hydrate generation possible area is an area where the underground temperature is 10°C or less, and exists from the seabed surface at a depth of 450 m or more to a predetermined depth. The carbon dioxide underground storage method according to any one of [1] to [7].
[9] The method for underground storage of carbon dioxide according to any one of [1] to [8] above, wherein in the step (2), when the underground temperature at a depth of 600 m or more from the sea surface is 20° C. or higher, the carbon dioxide is injected to that depth.

As described above, the carbon dioxide underground storage method of the present invention employs a method including the step (1) of identifying a methane hydrate production potential area that includes a carbon dioxide hydrate production potential area and is composed of an unconsolidated sedimentary layer in which methane hydrate is present, and the step (2) of injecting carbon dioxide into the unconsolidated sedimentary layer below the methane hydrate production potential area.
In this way, by injecting carbon dioxide below the region where methane hydrate can be produced, a portion of the carbon dioxide rises toward the region where methane hydrate can be produced, and carbon dioxide hydrate is produced in the region where carbon dioxide hydrate can be produced on the upper side of the region where methane hydrate can be produced, thereby forming a carbon dioxide hydrate layer. As a result, the carbon dioxide seal layer has a two-stage multi-seal function with the lower layer side in the region where methane hydrate can be produced as the primary seal layer and the carbon dioxide hydrate layer as the secondary seal layer, thereby improving the shielding performance of the carbon dioxide seal layer against the carbon dioxide stored below.
Therefore, the carbon dioxide injected below the carbon dioxide sealing layer can be reliably sealed, making it possible to efficiently store a large amount of carbon dioxide.

FIG. 1 is a diagram for explaining a method for underground storage of carbon dioxide which is one embodiment of the present invention, and is a schematic diagram showing the boundaries of a carbon dioxide storage layer and a carbon dioxide sealing layer which is composed of a primary sealing layer composed of a methane hydrate layer and a secondary sealing layer composed of a carbon dioxide hydrate layer, as well as showing an example of an underground storage device which can be used in the present invention. FIG. 2 is a diagram for explaining a method for underground storage of carbon dioxide according to one embodiment of the present invention, and is a graph showing the temperature and pressure conditions under which methane hydrate and carbon dioxide hydrate are produced. FIG. 3 is a diagram for explaining a method for underground storage of carbon dioxide, which is one embodiment of the present invention, and is a schematic diagram showing the boundaries between the area where methane hydrate can be generated and the area where carbon dioxide hydrate can be generated, as well as the relationship between the depth from the seabed and the temperature. FIG. 4 is a diagram for explaining a method for underground storage of carbon dioxide according to one embodiment of the present invention, and is a graph showing the relationship between the hydrate saturation rate and the permeability of a primary seal layer made of a methane hydrate layer.

The configuration of a method for underground storage of carbon dioxide, which is one embodiment of the present invention, will be described in detail below, along with an example of an underground storage device for carbon dioxide that can be used in the present invention, with appropriate reference to Figures 1 to 4. Note that the drawings used in the following description may show characteristic parts in an enlarged scale for the sake of convenience in order to make the characteristics easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality.

1 is a schematic diagram showing the boundaries of a carbon dioxide storage layer G (a region where carbon dioxide exists in liquid form) and a carbon dioxide sealing layer C consisting of a primary sealing layer C1 consisting of a methane hydrate layer and a secondary sealing layer C2 consisting of a carbon dioxide hydrate layer. FIG. 1 also shows an underground storage device 1 as an example of an apparatus that can be used in the present invention. FIG. 1 also shows a graph showing the temperature at each depth from the seabed surface F in the stratum, with the temperature (°C) on the X-axis and the depth (m) on the Y-axis, showing the relationship when the geothermal gradient is assumed to be 3°C/100°C.
FIG. 2 is a graph showing the temperature and pressure conditions under which methane hydrate and carbon dioxide hydrate are produced, with temperature (°C) on the X-axis and pressure (MPa) on the Y-axis. The graph shown in FIG. 2 is quoted from FIG. 3 in the following reference 1 (Reference 1: Tobase Takaomi, Odomegawa Tsuyoshi, Ikegawa Yojiro, Kimura Haruo, "Proceedings of the 45th Symposium on Rock Mechanics: Proposal for _CO2 Underground Storage Using Hydrates", Japan Society of Civil Engineers, January 2018, Lecture No. 41).
Fig. 3 is a schematic diagram showing the boundaries between the region S1 where methane hydrate can be generated and the region S2 where carbon dioxide hydrate can be generated, and showing the relationship between the depth from the seabed surface and temperature. As in Fig. 1, Fig. 3 also shows a graph showing the temperature at each depth from the seabed surface F in the stratum, with temperature (°C) on the X-axis and depth (m) on the Y-axis, showing the relationship when the geothermal gradient is assumed to be 3°C/100°C.
FIG. 4 is a graph showing the relationship between hydrate saturation and permeability of the primary seal layer C1 consisting of a methane hydrate layer, with the X-axis showing saturation (%) and the Y-axis showing permeability (permeability (md)). The graph shown in FIG. 4 is quoted from Figure 5 in the following reference 2 (Reference 2: Yoshihiro Konno, Jun Yoneda, Kosuke Egawa, Takuma Ito, Yusuke Jin, Masato Kida, Kiyofumi Suzuki, Tetsuya Fujii, Jiro Nagao, “Permeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough”, Marine and Petroleum Geology, Volume 66, Part 2, September 2015, Pages 487-495).

The carbon dioxide underground storage method of this embodiment (hereinafter sometimes simply referred to as the "storage method") is a method for storing carbon dioxide alone or in the form of a mixture mainly composed of carbon dioxide in a geological layer below the seabed surface F made of sediments. That is, the storage method of this embodiment includes a step (1) of identifying a methane hydrate generation possible area S1 that is within a predetermined depth range from the seabed surface F and includes a carbon dioxide hydrate generation possible area S2 made of a geological layer that satisfies pressure and temperature conditions capable of generating carbon dioxide hydrate, and is made of an unconsolidated sedimentary layer U in which methane hydrate is present. Furthermore, the storage method of this embodiment includes a step (2) of injecting carbon dioxide into the unconsolidated sedimentary layer U below the above-mentioned methane hydrate generation possible area S1.

In addition, in the storage method of the example described in this embodiment, the above step (2) forms a carbon dioxide reservoir G in the unconsolidated sedimentary layer U below the methane hydrate generation possible region S1, and at least a portion of the carbon dioxide injected into the carbon dioxide reservoir G rises to the methane hydrate generation possible region S1 side and passes through the lower layer side of the methane hydrate generation possible region S1. As a result, carbon dioxide hydrate is generated in the carbon dioxide hydrate generation possible region S2 on the upper side of the methane hydrate generation possible region S1, forming a secondary seal layer (carbon dioxide hydrate layer) C2, and the lower layer side of the methane hydrate generation possible region S1 is the primary seal layer C1, and the carbon dioxide hydrate layer is the secondary seal layer C2, forming a carbon dioxide seal layer C.

The carbon dioxide described in this embodiment includes not only pure carbon dioxide (CO ₂ ) alone, but also the above-mentioned state of a mixed gas containing carbon dioxide as the main component. That is, the carbon dioxide to be stored in this embodiment may contain water by using existing countermeasures technology in combination. Furthermore, the carbon dioxide to be stored in this embodiment may contain, in addition to carbon dioxide itself, other components other than carbon dioxide, such as carbon monoxide (CO), hydrogen (H ₂ ), methane (CH ₄ ), water (H ₂ O), and hydrogen sulfide (H ₂ S).

The definition of each layer in the stratum shown in FIG. 1 will be explained below.
First, the unconsolidated sedimentary layer U described in this embodiment refers to a low-consolidated sedimentary layer that exists in a relatively shallow region of the seabed substratum.
The methane hydrate production possible region S1 refers to a stratum that satisfies pressure and temperature conditions that allow methane hydrate to be produced. However, in this embodiment, it does not matter whether methane hydrate is actually produced in the methane hydrate production possible region S1.
The carbon dioxide hydrate formable region S2 refers to a geological layer that satisfies pressure and temperature conditions that allow carbon dioxide hydrate to be formed. However, in this embodiment, it does not matter whether carbon dioxide hydrate is actually formed in the carbon dioxide hydrate formable region S2.

In addition, taking into consideration the relationship between the depth from the sea surface M and the temperature in the sea, and the relationship between the depth from the seabed surface F and the temperature underground, it is believed that carbon dioxide does not hydrate in strata deeper than the carbon dioxide hydrate generation possible region S2. Also, it is believed that methane does not hydrate in strata deeper than the methane hydrate generation possible region S1. And, as shown in FIG. 1, the lower limit of the methane hydrate generation possible region S1 is deeper than the lower limit of the carbon dioxide hydrate generation possible region S2.

Further, the primary sealing layer (methane hydrate layer) C1 is a layer in which methane hydrate is actually produced and has a sealing effect against the carbon dioxide stored in the carbon dioxide reservoir G below.
The secondary sealing layer (carbon dioxide hydrate layer) C2 is a layer in which carbon dioxide hydrate is actually produced and has a sealing effect against the carbon dioxide stored in the carbon dioxide reservoir layer G below.

<Carbon dioxide underground storage device>
First, an example of a geological storage device that can be used in a method for underground storage of carbon dioxide, which is one embodiment of the present invention, will be described with reference to FIG. 1 (hereinafter, sometimes simply referred to as a "storage device").

As shown in Fig. 1, a storage device 1 that can be used in the storage method of this embodiment is a device that stores carbon dioxide (CO ₂ ) in a carbon dioxide reservoir (carbon dioxide storage region) G in a geological stratum. The storage device 1 is generally configured to include an injection well 4 that extends through a methane hydrate formable region S1, including a carbon dioxide hydrate formable region S2 (details of which will be described later) that exists from the seabed surface (seabed) F to a predetermined depth, and to a carbon dioxide reservoir G below the methane hydrate formable region S1, a carbon dioxide supply source 2 that generates carbon dioxide (CO ₂ ), and a pumping facility 3 that pumps the carbon dioxide (CO ₂ ) generated in the carbon dioxide supply source 2 to the injection well 4.

In the schematic diagram of Fig. 1, the pressure of a stratum (unconsolidated sedimentary layer U) below the seabed surface F located below seawater W usually changes depending on the depth from the sea surface M to the seabed surface F and on any depth of the stratum from the seabed surface F. The storage device 1 of this embodiment includes a carbon dioxide hydrate formable region S2 that exists from the seabed surface F to a predetermined depth and satisfies pressure and temperature conditions capable of forming carbon dioxide hydrate, and stores carbon dioxide (CO ₂ ) alone or in the form of a mixture mainly composed of carbon dioxide in a carbon dioxide reservoir G below the methane hydrate formable region S1 consisting of an unconsolidated sedimentary layer in which methane hydrate is present.

As described above, the carbon dioxide supply source 2 supplies carbon dioxide (CO ₂ ) to be stored in the carbon dioxide reservoir G in the unconsolidated sedimentary layer U. Such carbon dioxide supply source 2 is not particularly limited, and examples thereof include devices for recovering carbon dioxide derived from coal, oil, etc. discharged from power generation facilities using fossil fuels in various facilities such as offshore plants for extracting oil and gas, and devices for temporarily receiving and storing the carbon dioxide recovered by these devices from ships or pipelines transporting them.

Furthermore, in the storage device 1, it is preferable that the carbon dioxide supply source 2 is exposed above the sea level M, from the viewpoints of supplyability and maintainability of materials such as fuel and carbon dioxide, device life, etc., although this depends on the characteristics of the device. As such, an example of a method for exposing the carbon dioxide supply source 2 above the sea level M is to construct an offshore platform similar to those installed for extracting resources such as oil and gas from the seabed, and install the carbon dioxide supply source 2 on the platform.
Offshore platforms may be, for example, fixed or floating.
An example of a fixed offshore platform is one constructed by directly fixing a structure assembled from high-strength steel or the like to the seabed F.
Furthermore, examples of floating offshore platforms include semi-submersible vessels.
In the storage device 1 of the example shown in FIG. 1, a ship 5 is floated on seawater W (ocean surface M), and a carbon dioxide supply source 2 is installed on the ship 5.

The carbon dioxide supply source 2 is not limited to being installed on an offshore platform such as a ship 5. For example, although not shown, a configuration may be adopted in which carbon dioxide generated in the carbon dioxide supply source 2 installed on land is transported to the injection well 4 via a pumping facility 3 and piping (pipeline) described below.

The pumping facility 3 pumps carbon dioxide (CO ₂ ) to an injection well 4 , and in the example shown in FIG. 1 , is installed on an offshore platform consisting of a ship 5 .
In addition, in the illustrated example, carbon dioxide (CO ₂ ) is supplied from a pressure pumping facility 3 installed on a ship 5 via a supply line L to an injection well 4 provided with its origin at the seabed surface F.

As the pressure-feeding equipment 3, a pump for pressure-feeding liquid, which has been conventionally used in this field, can be used without any restrictions.
Furthermore, as the supply line L connecting the pumping facility 3 and the injection well 4, for example, piping members made of metal or resin materials that can be used in seawater can be used without any restrictions.
In addition, the pumping equipment 3 is not limited to being installed on an offshore platform such as a ship 5 as described above, and it is also possible to install the pumping equipment on land or on the seabed F, for example.

As described above, the injection well 4 penetrates the methane hydrate production potential area S1, which exists from the seabed surface F to a predetermined depth, including the carbon dioxide hydrate production potential area S2, and extends to the carbon dioxide reservoir G below the methane hydrate production potential area S1, and injects carbon dioxide (CO ₂ ) into the carbon dioxide reservoir G.
The injection well 4 is, for example, a hole formed by drilling or the like, into which carbon dioxide (CO ₂ ) is injected and injected.

In the illustrated example, the injection well 4 is provided as a vertical well, but it can be an inclined well if necessary, or a combination of a vertical well and a horizontal well (e.g., L-shaped cross section), or a structure in which a vertical well, horizontal well, and inclined well are appropriately combined. It can also be installed as an inclined well directly from land, in which case the carbon dioxide supply source 2 and pumping equipment 3 will be installed on land.

In addition to the above-mentioned configurations, the storage device 1 of this embodiment may further be provided with various monitoring devices for detecting the state of carbon dioxide (CO ₂ ) stored in the carbon dioxide storage layer G, or the state of the entire methane hydrate production possible region S1 including the carbon dioxide sealing layer C consisting of the primary sealing layer C1 and the secondary sealing layer C2, details of which will be described later.

<Method of underground storage of carbon dioxide>
Hereinafter, the method for underground storage of carbon dioxide according to this embodiment will be described in detail with reference to FIGS. 1 to 4 as appropriate.

As described above, the storage method of this embodiment is a method for storing carbon dioxide (CO ₂ ) in a geological layer below the seabed surface F. This storage method is a method for forming a carbon dioxide reservoir G by injecting carbon dioxide (CO 2 ) below a methane hydrate production possible region S1 consisting of an unconsolidated sedimentary layer containing methane hydrate, the region including a carbon dioxide hydrate production possible region _S2 consisting of a geological layer that exists from the seabed surface F to a predetermined depth and satisfies pressure and temperature conditions capable of producing carbon dioxide hydrate.

That is, as described above, the storage method of this embodiment includes a step (1) of identifying a methane hydrate generation potential area S1 that includes a carbon dioxide hydrate generation potential area S2 and is composed of an unconsolidated sedimentary layer U in which methane hydrate is present. Furthermore, the storage method of this embodiment includes a step (2) of injecting carbon dioxide into the unconsolidated sedimentary layer U below the above-mentioned methane hydrate generation potential area S1.

As described above, in the storage method of the example described in this embodiment, in step (2), a carbon dioxide reservoir G is formed in the unconsolidated sediment layer U below the methane hydrate generation possible region S1. Furthermore, at least a part of the carbon dioxide injected into the carbon dioxide reservoir G rises to the methane hydrate generation possible region S1 side and passes through the lower layer side of the methane hydrate generation possible region S1. Then, the carbon dioxide that has passed through the lower layer side of the methane hydrate generation possible region S1 generates carbon dioxide hydrate in the carbon dioxide hydrate generation possible region S2 on the upper side of the methane hydrate generation possible region S1, forming a secondary seal layer (carbon dioxide hydrate layer) C2. As a result, the lower layer side of the methane hydrate generation possible region S1 is the primary seal layer C1, and the carbon dioxide hydrate layer is the secondary seal layer C2 to form a carbon dioxide seal layer C. The carbon dioxide seal layer C illustrated in FIG. 1 is formed so that the upper end side is at a depth slightly deeper than the seabed surface F in the methane hydrate generation possible region S1.

Furthermore, in the storage method of this embodiment, in step (2), a method can be adopted in which an area of the unconsolidated sedimentary layer U containing methane hydrate is identified as a non-concentrated zone of methane hydrate, and carbon dioxide is injected into the unconsolidated sedimentary layer U below the area S1 where methane hydrate can be generated in the non-concentrated zone of methane hydrate.

In the storage method of this embodiment, the storage device 1 illustrated in FIG. 1 is used to inject carbon dioxide (CO ₂ ) below the methane hydrate production potential area S1, which includes the carbon dioxide hydrate production potential area S2, thereby allowing the carbon dioxide (CO ₂ ) to be stored in a carbon dioxide reservoir G in a stratum below the seabed surface F.

As described above, carbon dioxide (CO ₂ ) exists in any one of four phases, gas, liquid, solid, or supercritical, in a given temperature and pressure range. Carbon dioxide (CO ₂ ) becomes solid (dry ice) at a temperature of 194 K (−79.15° C.) or less under normal pressure, but when mixed with water, it becomes hydrated (solidified) under different temperature and pressure conditions. As shown in FIG. 2, for example, when the state of carbon dioxide is represented by a graph consisting of a temperature-pressure gradient curve, the region on the graph is divided into a stable region of carbon dioxide hydrate (CO ₂ hydrate stable zone) and a region where carbon dioxide exists in a liquid state (liquid zone) with the temperature-pressure gradient curve as a boundary. The above-mentioned stable region is a temperature and pressure range in which carbon dioxide (CO ₂ ) exists stably in a hydrated state when mixed with water. In addition, the stable region of carbon dioxide hydrate overlaps with a part of the region of liquid carbon dioxide (liquid phase region).

Carbon dioxide (CO ₂ ) also becomes supercritical when the temperature is 31° C. or higher and the pressure is 7.4 MPa or higher, and the density change is relatively slow. Carbon dioxide (CO ₂ ) also has a smaller density than seawater when the temperature is 35° C. or lower and the pressure is 20 MPa or lower, and a smaller density than water except when the temperature is around 0° C. and the pressure is 14 MPa or higher. That is, since carbon dioxide (CO ₂ ) has a smaller density than seawater when in a liquid or supercritical state, in order to store carbon dioxide (CO ₂ ) in a stratum below the seabed surface F, a sealing function is required to contain the carbon dioxide that floats up due to the 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 due to the cap rock, such as highly airtight mudstone, acting as a sealing layer.
DNA analysis has revealed that shale gas is produced simultaneously with shale oil from shale, which is considered to be the source rock of petroleum.
In contrast, methane hydrate has been confirmed to exist as a solid in strata beneath the seafloor of continental margins and in permafrost. Although there is no cap rock as described above, a seal layer that performs a so-called sealing function is formed depending on the temperature and pressure conditions for hydration.

The existence of natural gas in a natural state as described above can be considered as a natural analogue of carbon dioxide storage. That is, in the underground storage of conventional natural gas and general carbon dioxide, the above-mentioned cap rock functions as a sealing layer. In addition, methane hydrate and carbon dioxide hydrate can be used as a sealing layer for storing carbon dioxide by setting the temperature and pressure conditions for hydration.
Shale gas exists when methane is adsorbed into shale or coal layers, and is known to also adsorb carbon dioxide.
As mentioned above, naturally occurring natural gas is trapped in the earth's layers by different sealing mechanisms. By utilizing these sealing mechanisms, i.e., natural conditions, carbon dioxide can also be stored underground.

As a result of intensive research, the inventors adopted a method of first identifying the methane hydrate generation possible area S1, which includes the carbon dioxide hydrate generation possible area S2 and is made of an unconsolidated sedimentary layer containing methane hydrate, as described above, and then injecting carbon dioxide into the unconsolidated sedimentary layer U below the methane hydrate generation possible area S1. As a result, as described in detail below, the inventors discovered that the methane hydrate layer derived from methane naturally present in the stratum is used as the primary seal layer C1, and further, a secondary seal layer C2 made of carbon dioxide hydrate is formed on top of the primary seal layer C1, thereby forming the carbon dioxide seal layer C into a two-stage multi-seal structure. In other words, the inventors discovered that an unprecedentedly strong shielding performance can be obtained by the primary seal layer C1 made of methane hydrate having low permeability and the secondary seal layer C2 formed on the primary seal layer C1 by the rise of at least a part of the carbon dioxide injected into the carbon dioxide reservoir G.

As shown in the graph in Figure 2, the temperature and pressure conditions under which carbon dioxide hydrate and methane hydrate form are slightly different. On the other hand, the geological conditions under which carbon dioxide hydrate and methane hydrate form are thought to be similar, and the geological conditions under which methane hydrate exists (unconsolidated sedimentary layers) are also suitable for the formation of carbon dioxide hydrate. The permeability of the unconsolidated sedimentary layers described above is generally about 1000 (md), but as shown in the graph in Figure 4, the permeability of the layer consisting of methane hydrate (primary seal layer C1) is about 1 to 100 md regardless of the saturation rate of methane hydrate, and has excellent low permeability.

The graph in FIG. 4 has the following meaning.
In places where the original stratum permeability is high, the pores of the stratum are large and hydrate is easily generated, and the hydrate saturation rate is high and the rate of decrease in permeability is large. On the other hand, in places where the original stratum permeability is low, the pores of the stratum are small and hydrate is difficult to generate, and the hydrate saturation rate is low and the rate of decrease in permeability is small. Therefore, regardless of the hydrate saturation rate, the permeability falls within a certain range.
In addition, the hydrate saturation rate is close to 0% in regions where the permeability is 1 md or less, which indicates that hydrate cannot be generated because the pores in the original strata are extremely small.

Based on the above findings, unconsolidated sedimentary layers containing methane hydrate are suitable sites for producing carbon dioxide hydrate and storing carbon dioxide underground.
As a result of further investigations, the inventors have clarified that, as shown in the graph of Fig. 4, the non-concentrated zone of methane hydrate, the details of which will be described later, is considered to have a significantly low permeability. As shown in Fig. 1, the inventors have found that a two-stage sealing mechanism can be expected, consisting of a primary seal layer C1 made of a methane hydrate layer naturally existing in an unconsolidated sedimentary layer U in the stratum, and a secondary seal layer C2 made of at least a part of the carbon dioxide injected into the unconsolidated sedimentary layer U in the stratum. Such a two-stage multi-seal structure made of the primary seal layer C1 and the secondary seal layer C2 can be expected to have stronger shielding performance against the carbon dioxide stored below, compared to a single seal structure made of only a seal layer made of carbon dioxide hydrate.

In addition, the methane hydrate concentrated zone refers to an area that has an original resource amount (total amount of methane present) of 10 billion ^m3 or more from the viewpoint of economic efficiency, and can produce gas at a rate of ^50,000 m3 or more per well per day (Reference 3: Koji Yamamoto, Norio Tenma, "Methane Hydrate Development - Issues and Current Research and Development for Commercial Production", Journal of the Japan Society of Marine Engineering, Vol. 56, No. 2 (2021)). On the other hand, the "non-concentrated zone of methane hydrate" described in this specification is an area that does not meet the above-mentioned methane gas production conditions. As shown in the graph in FIG. 4, regardless of the saturation rate of the hydrate, the permeability of the methane hydrate layer is in the range of 1 to 100 md, and both the concentrated zone and non-concentrated zone of methane hydrate function as a primary seal layer.

Here, the non-concentrated zone of methane hydrate in this embodiment may be, for example, a layer that contains methane hydrate derived from methane naturally present in the strata, and in which the methane hydrate saturation rate has been reduced by the production of methane gas using this methane hydrate. In addition, the non-concentrated zone of methane hydrate may be a layer in an area where the density of methane hydrate is originally low, among the unconsolidated sedimentary layer U in which methane hydrate exists.

As mentioned above, non-concentrated zones of methane hydrate are excluded from production areas due to their low productivity of methane gas, so no inconvenience is caused to those producing methane hydrate. Furthermore, using non-concentrated zones of methane hydrate for underground storage of carbon dioxide can lead to effective use of areas that are not suitable for methane hydrate production.

According to the storage method of this embodiment as described above, first, the residual methane hydrate after methane hydrate production in the stratum or the non-concentrated zone of methane hydrate is effectively utilized as a low-permeability zone as the primary seal layer C1. Then, as carbon dioxide hydrate is produced on the primary seal layer C1, the secondary seal layer C2 is formed by clogging the pores, and by utilizing the shielding effect, the shielding performance of the carbon dioxide seal layer C consisting of the primary seal layer C1 and the secondary seal layer C2 is enhanced. This allows liquid carbon dioxide to be efficiently stored below the carbon dioxide seal layer C.

Here, although detailed illustration is omitted, when the relationship between the depth from the sea surface M and the temperature in the sea and the relationship between the depth from the seabed surface F and the temperature in the ground are graphed, it can be seen that carbon dioxide does not hydrate in a geological layer deeper than the methane hydrate generation possible region S1 including the carbon dioxide hydrate generation possible region S2. Therefore, by injecting carbon dioxide (CO ₂ ) below the methane hydrate generation possible region S1, it becomes possible to treat this injection region as a carbon dioxide storage layer (Liquid CO ₂ injection/storage layer) without clogging. In addition, for example, when carbon dioxide (CO ₂ ) is injected into the ground in a liquid or supercritical state, a device for generating dissolved water of carbon dioxide (CO ₂ ) is not required, so that the storage device 1 as described above can be configured simply.

The carbon dioxide (CO ₂ ) in a liquid or supercritical state injected and stored in the carbon dioxide reservoir G rises due to buoyancy caused by the density difference with the seawater W, and reaches the primary seal layer C1 present at the lower layer (lower end side) of the methane hydrate production possible region S1, and when it permeates this primary seal layer C1, the carbon dioxide (CO ₂ ) self-hydrates and stabilizes. As a result, a secondary seal layer C2 made of carbon dioxide hydrate is formed on top of the primary seal layer C1, and the shielding effect of the carbon dioxide seal layer C made of the primary seal layer C1 and secondary seal layer C2 reliably prevents carbon dioxide (CO ₂ ) from leaking into the seawater W.

In the storage method of this embodiment, it is preferable to inject carbon dioxide (CO ₂ ) in either a liquid state or a supercritical state into the carbon dioxide reservoir G. Here, for example, even if carbon dioxide (CO ₂ ) is supplied in a gaseous state, carbon dioxide (CO _{2 ) will be in a liquid or supercritical state under the pressure and temperature conditions in the carbon dioxide reservoir G. On the other hand, when injecting carbon dioxide (CO 2 )} into the carbon dioxide reservoir G through the injection well 4, there is an advantage in that the transport efficiency is superior to that of supplying carbon dioxide (CO 2 ) in a liquid or supercritical state rather than in a gaseous state.

As described above, the temperature at which carbon dioxide hydrates is 10° C. or lower, and the pressure is 4.5 MPa or higher. For this reason, in the storage method of this embodiment, it is preferable to adjust the depth at which carbon dioxide (CO ₂ ) is injected so that the carbon dioxide hydrate generation possible region S2 in which the secondary seal layer C2 is formed is a region in which the underground temperature is 10° C. or lower, and exists to a predetermined depth from the seabed surface F at a depth of 450 m or deeper. For example, as shown in the schematic diagram of FIG. 3, assuming that the water depth is 1000 m in the Pacific Ocean off the coast of Japan and the water temperature near the seabed surface F is 5° C., the underground temperature in the stratum below the seabed surface F increases with depth, so that carbon dioxide hydrates when the underground temperature is in the range of 10° C. or lower, and the secondary seal layer C2 made of stable carbon dioxide hydrate is secured.

Furthermore, in the storage method of this embodiment, when the underground temperature at a depth of 600 m or more from the sea level M is 20° C. or higher, it is preferable to inject carbon dioxide (CO ₂ ) to that depth. In this manner, by injecting carbon dioxide (CO ₂ ) below the methane hydrate production possible region S1 in the stratum at a depth of 600 m or more from the sea level M, that is, by injecting carbon dioxide (CO ₂ ) at a depth where the temperature and pressure conditions at the injection position are such that carbon dioxide is in a liquid phase state, it is possible to obtain an effect of preventing carbon dioxide from hydrating and preventing clogging of the outlet of the injection well 4.

The conditions for injecting carbon dioxide (CO ₂ ) below the carbon dioxide sealing layer C (primary sealing layer C1) in the above step (2) are not particularly limited. For example, when the underground temperature is 20° C., it is preferable to inject/pressure carbon dioxide (CO ₂ ) so that the pressure in the carbon dioxide reservoir G is 6 MPa or more.

In addition, regarding the injection of carbon dioxide (CO ₂ ), by adjusting the depth at which carbon dioxide (CO ₂ ) is injected based on the underground temperature, it becomes possible to inject carbon dioxide (CO ₂ ) in a stable state, for example, in a liquid or supercritical state.

In this embodiment, the total layer thickness of the methane hydrate formable region S1 including the carbon dioxide seal layer C consisting of the primary seal layer C1 and the secondary seal layer C2 is not particularly limited. However, in order to improve the sealing property of the entire methane hydrate formable region S1 including the carbon dioxide seal layer C, that is, in order to reliably prevent the carbon dioxide (CO ₂ ) stored in the carbon dioxide reservoir G from leaking into the seawater W, it is preferable to secure the methane hydrate formable region S1 at least 100 m. In this way, as a method for securing the layer thickness of the methane hydrate formable region S1 at a desired thickness, for example, the injection depth of carbon dioxide (CO ₂ ) by the injection well 4 can be optimized after fully understanding the temperature and pressure conditions in the layer below the seabed surface F and the position of methane hydrate by a preliminary survey.

The carbon dioxide that forms the secondary sealing layer C2 made of carbon dioxide hydrate is carbon dioxide that rises after being injected into the carbon dioxide reservoir G and permeates the primary sealing layer C1, as described above. On the other hand, the secondary sealing layer C2 may be a layer that is derived from, for example, the carbon dioxide injected into the primary sealing layer C1 replacing part of the methane hydrate that constitutes the primary sealing layer C1 with carbon dioxide hydrate.

In addition, in the storage method of this embodiment, the primary seal layer C1 is basically a layer consisting of methane hydrate derived from methane present in the stratum, and uses methane hydrate originally present in the stratum, but is not limited to this. For example, the primary seal layer C1 consisting of methane hydrate may be a layer containing artificially generated methane hydrate and having a low methane hydrate saturation rate. In other words, it is also possible to use various artificial hydrates, such as artificially generated hydrates having low permeability similar to that of the carbon dioxide hydrate constituting the secondary seal layer C2, specifically artificial methane hydrate or nitrogen hydrate, or artificial TBAB (tetrabutylammonium bromide)-based semiclathrate hydrate, in the primary seal layer C1.

<Action and effect>
As described above, the method for underground storage of carbon dioxide in this embodiment employs a method including the step (1) of identifying a methane hydrate production potential region S1 that includes a carbon dioxide hydrate production potential region S2 and is made up of an unconsolidated sedimentary layer in which methane hydrate is present, and the step (2) of injecting carbon dioxide into the unconsolidated sedimentary layer U below the methane hydrate production potential region S1.
In this way, by injecting carbon dioxide below the methane hydrate generation possible region S1, even if a part of the carbon dioxide rises to the methane hydrate generation possible region S1 side, the methane hydrate generation possible region S1 has a low permeability, so it functions as a primary seal layer C1 and traps the carbon dioxide. Although the above-mentioned primary seal layer C1 has a low permeability, a small percentage of permeation occurs, so there is a possibility that a part of the carbon dioxide passes through the primary seal layer C1. On the other hand, when the carbon dioxide that has passed through the primary seal layer C1 reaches the carbon dioxide hydrate generation possible region S2 on the upper side of the methane hydrate generation possible region S1, carbon dioxide hydrate is generated to form a carbon dioxide hydrate layer. As a result, the carbon dioxide seal layer C has a two-stage multi-seal function in which the lower layer side in the methane hydrate generation possible region S1 is the primary seal layer C1 and the carbon dioxide hydrate layer is the secondary seal layer C2, so that the shielding performance of the carbon dioxide seal layer against the carbon dioxide stored below is improved.
Therefore, the carbon dioxide injected below the carbon dioxide sealing layer C can be reliably sealed, making it possible to efficiently store a large amount of carbon dioxide.

<Other forms>
The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

The underground carbon dioxide storage method of the present invention can reliably seal the carbon dioxide injected below the carbon dioxide sealing layer containing carbon dioxide hydrate, and can efficiently store large amounts of carbon dioxide. Therefore, the underground storage method of the present invention is highly suitable for applications such as capturing carbon dioxide while generating power using fossil fuels in various plants and storing this carbon dioxide underground to reduce emissions.

1... Underground storage device (storage device)
2... Carbon dioxide supply source 3... Pressure pumping equipment 4... Injection well 5... Ship L... Supply line W... Seawater M... Sea surface F... Seabed (ocean floor)
U: Unconsolidated sedimentary layer S1: Possible methane hydrate generation area S2: Possible carbon dioxide hydrate generation area C: Carbon dioxide sealed layer C1: Primary sealed layer (methane hydrate layer)
C2: Secondary sealing layer (carbon dioxide hydrate layer)
G: Carbon dioxide reservoir

Claims (9)

A method for storing carbon dioxide alone or in a state of a mixture mainly composed of carbon dioxide in a sub-sea bottom layer made of sediments, comprising the steps of:
A step (1) of identifying a methane hydrate production possible region, the methane hydrate production possible region including a carbon dioxide hydrate production possible region made of a geological layer satisfying pressure and temperature conditions capable of producing carbon dioxide hydrate within a range of a predetermined depth from the seabed surface and made of an unconsolidated sedimentary layer containing methane hydrate;
and (2) injecting the carbon dioxide into an unconsolidated sediment layer below the area where methane hydrate can be produced. The carbon dioxide underground storage method according to claim 1, wherein the step (2) forms a carbon dioxide reservoir in an unconsolidated sedimentary layer below the region where methane hydrate can be produced, and at least a portion of the carbon dioxide injected into the carbon dioxide reservoir rises toward the region where methane hydrate can be produced and passes through the lower layer of the region where methane hydrate can be produced, thereby forming a carbon dioxide hydrate layer in the region where carbon dioxide hydrate can be produced on the upper side of the region where methane hydrate can be produced, thereby forming a carbon dioxide hydrate layer, with the lower layer of the region where methane hydrate can be produced being a primary seal layer and the carbon dioxide hydrate layer being a secondary seal layer. The carbon dioxide underground storage method according to claim 1 or 2, wherein the step (2) involves identifying an area of the unconsolidated sedimentary layer containing the methane hydrate that is a non-concentrated zone of the methane hydrate, and injecting carbon dioxide into the unconsolidated sedimentary layer in the non-concentrated zone of the methane hydrate that is below the area where methane hydrate can form. The method for underground storage of carbon dioxide according to claim 3, wherein the non-concentrated zone of methane hydrate comprises a region that contains methane hydrate derived from methane present in the stratum and in which the methane hydrate saturation rate has decreased due to the production of methane gas using the methane hydrate. The method for underground storage of carbon dioxide according to claim 3, wherein the non-concentrated zone of methane hydrate contains artificially produced methane hydrate and is composed of a layer with a low methane hydrate saturation rate. The method for underground storage of carbon dioxide according to claim 1 or 2, wherein in step (2), the pressure at which the carbon dioxide is injected into the unconsolidated sediment layer below the area where methane hydrate can form is 6 MPa or more. The method for underground storage of carbon dioxide according to claim 1 or 2, wherein the step (2) comprises injecting the carbon dioxide in a liquid state or a supercritical state into an unconsolidated sediment layer below the area where methane hydrate can be generated. The method for underground storage of carbon dioxide according to claim 1 or 2, wherein the region where carbon dioxide hydrate can be generated is a region where the underground temperature is 10°C or less, and exists from the seabed surface to a predetermined depth at a depth of 450 m or more. The method for underground storage of carbon dioxide according to claim 1 or 2, wherein in step (2), the carbon dioxide is injected to a depth of 600 m or more below sea level when the underground temperature at that depth is 20°C or higher.

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