Title: Method for introducing CO2 into the ground

The invention relates to a method for introducing CO2 into the ground and to use of this method for underground storage of CO2.

The increase of CO2 in the atmosphere is supposed to have a major effect on global climate. It is therefore of great importance to reduce the emission of anthropogenic CO2 into the atmosphere in order to control climate change. In addition to the development of low -emission energy plants, energy -saving automobiles and forms of energy that are CO2 neutral, the durable storage of CO2 in the ground is an important means of achieving the Kyoto protocol targets of reducing the average CO2 emission. There are various methods known for storing CO2. A known method is to store CO2 underground in (exhausted) natural gas fields. However, such a method has a limited range of application, since storage depends on the presence of such a field.

Storage of supercritical CO2 in an aquifer also requires a closed geological structure which retains the supercritical CO2 , which is lighter than the groundwater. Such a structure is sometimes referred to as "closure".

It has also been proposed to dissolve CO2 in water and then to store it in the ground or to inject supercritical CO2 directly into a geothermal reservoir ('Feasibility of CO2 storage in geothermal reservoirs, example of the Paris Basin (France). Final report', Bonijoly. D, et al., 2003, Paris). A drawback of this method is that the requisite CO2 compression entails large material investments and has a high energy consumption during the operation itself, so that moreover the intended environmental effect of the CO2 storage is at least partly undone again. The high costs and the great technological efforts of introducing CO2 into the ground and storing it durably there, have an inhibitory effect on the realization of this kind of

projects and moreover are prejudicial to social acceptance of this kind of projects.

EP-A 1 571 105 describes a method for underground CO2 storage, in which CO2 is added at the beginning of a water stream going from the earth's surface into the ground. This publication describes a method of fixing CO2 in the ground through chemical reactions with a mineral-forming agent (sulfate /base); this needs to be added separately, unless use is made of particular geological structures, in particular those structures that by nature contain these agents in large quantities. Without being bound by theory, it is suspected that mineralization of CO2, in particular through addition of substances, can lead to reduced permeability or even clogging of the geological structure. As a result, the subterranean hydrology may be disturbed.

JP 06 170215 describes a method of introducing a mixture of water and CO2 into the ground. To this end, the CO2 is mixed with the water, aboveground, after which the mixture is introduced into the ground under pressure. The measures mentioned for carrying out this method, such as the supply of liquid CO2, a booster pump, a heat exchanger and a pump to obtain the requisite pressure, seem to render the process complicated and energy-intensive.

An object of the present invention is to provide a new method of introducing CO2 into the ground, which can serve as an alternative to known methods of introducing CO2 into the ground.

Further, it is an object of the present invention to accomplish a new method for the storage of CO2 which entails less material investments and/or has a lower energy consumption during practice of the method.

One or more other objects that can be realized through the invention will appear from the description below.

It has now been found that one or more objects are realized by utilizing the principle that the (liquid) pressure in a liquid column

increases, under the influence of gravity, with the depth in that column. Technology utilizing this principle is also known as 'gravity pressure vessel technology ¹ , abbreviated GPVT.

Accordingly, the present invention relates to a method for introducing CO2 into the ground, wherein gaseous CO2 is passed via a shaft with a downwardly flowing liquid into a layer of the ground and is compressed under the influence of the pressure of the liquid column in the shaft.

A method according to the invention is highly suitable for storing CO ₂ .

It is an advantage of the invention that CO2 can be effectively stored in a geological structure which cannot be qualified as a closure.

It is moreover an advantage that a solution of CO2 in general has a higher density than the solvent (such as groundwater), so that this solution tends to flow further downwards. Thus, the invention provides an inherently safe method of storing CO2.

The inventors have realized that it is not necessary first to dissolve the CO2 in the liquid under high pressure, supplied by a compressor, prior to introduction into the ground, or to inject supercritical CO2 directly into an underground layer. Through the invention, the CO2 is mixed as a gas with the liquid and, with the downwardly flowing liquid, passed (deeper) into the ground and compressed there under the influence of the ambient pressure, and wholly or at least substantially dissolved. Dissolution of the CO2 usually take place wholly, or at least substantially, before it reaches the depth where it is stored, as in an aquifer. This is regarded as an important advantage, especially with regard to the efficiency with which the CO2 is entrained in the downward liquid flow and can be stored.

As use is made of the (subterranean) ambient pressure, the CO2 does not need to be compressed, or at least needs to be compressed to a lesser extent, by means of a compressor. As a result, the energy

consumption for the compression of the CO2 is smaller. Also, optionally, use can be made of a simpler compressor (having for instance a lower maximum compressor output).

In a method according to the invention, the speed at which Cθ2-containing gas bubbles move up - relative to the liquid surrounding the bubbles - is exceeded by the speed at which the gas-liquid mixture as a whole moves downwards in a shaft. Under the influence of the pressure increase ('gravity pressure') that accompanies the downward movement of the gas-liquid mixture, gaseous CO2 is compressed. An advantage of the invention is also that the CO2 that is already dissolved does not need compression anymore, so that the necessary compression decreases compared with CO2 introduction without dissolution of CO2 in water in the shaft with downward liquid. This has important advantages in investments and exploitation costs for the compression. In principle, a method according to the invention can be applied using a mineral-forming additive or a geological structure by nature containing such an agent. In a preferred embodiment, a method according to the invention is used without adding an effective amount of a mineral-forming agent to the CO2, the liquid or the CCVliquid mixture. Also, the invention is eminently suitable to introduce CO2 into (and store it in) a geological structure containing no or insufficient mineral-forming agent to realize essential mineralization of CO2.

Thus the invention provides a method which is also suitable for storing CO2 in a geological structure in which no or little mineral-forming agent is present to chemically bind the CO2 at least for the most part.

Advantageously, the invention provides a method which is applicable for CO ₂ storage in various geological structures without addition of mineral- forming additives, and/or a method in which the permeability of the respective geological layer is not reduced essentially, or at any rate is reduced to a minor extent only, as a result of the CO2 introduction. Use of

the invention can even provide improved porosity around the injection point, under conditions where earth material dissolves under the influence of the relatively low acidity of the infiltration water (carbon dioxide -containing water). In a preferred embodiment of the invention, CO2 and the liquid are brought together as a gas-liquid mixture, after which the dissolution of CO2 in the liquid takes place wholly or partly under the influence of the gravity pressure vessel effect. Typically, the liquid used is water or an aqueous liquid. In the context of the invention, water is understood to mean not only pure water, but also liquids consisting at least substantially of water. Examples of the latter category are groundwater, seawater, river water, tap water, waste water, and the like.

The initial gas/liquid volume ratio, that is, the gas/liquid volume ratio at mixing of gaseous CO2 and liquid, can be chosen within wide limits. Preferably, a mixture is prepared having an initial gas/liquid volume ratio of at least 1/12, more preferably at least 1/10, most preferably at least 1/4. Usually, the initial volume ratio is 1/1 at a maximum. The initial gas/liquid mass ratio representing this depends on the liquid pressure on the injection point. As will be understood by those skilled in the art, the volume ratio is lower according as the pressure rises and/or more gas dissolves in the liquid. In a preferred embodiment of the invention, CO2 is introduced into the liquid, in particular water, in a (mass/volume) ratio of at least 5 kg of CO ₂ per m ³ of liquid, more preferably in a ratio of at least 10 kg of CO2 per m ³ of liquid. Typically, CO2 is mixed with the liquid in a ratio such that the CO2 can dissolve wholly in the liquid under the conditions prevailing in the earth layer to which the CO2 is passed and where it is stored, respectively. From practical considerations, the proportion of CO2 in the liquid is preferably at most 75 kg of CO2 per m ³ of liquid, in particular at

most 50 kg of CO2 per m ³ of liquid, more particularly at most 40 kg of CO2 per m ³ of liquid.

The amount of CO2 that can be stored per m ³ of liquid depends on the liquid that is used for mixing, and on what temperature and salt concentration this liquid has. Further, the amount of CO2 that can be stored per m ³ depends on the pressure, temperature and on the concentration of salt, in particular sodium chloride (NaCl), in the earth layer where the CO2 is passed to and stored, respectively.

For a good solubility of CO2 in the liquid, the temperature of the liquid at mixing is preferably at most 70 ⁰ C, more preferably at most 50 ⁰ C, in particular at most 30 ⁰ C. In addition, a relatively low temperature is advantageous in that per meter of liquid column a higher pressure is realized.

The minimum temperature corresponds to the melting temperature of the liquid (0 ⁰ C for pure water). From practical considerations, the temperature is usually at least 10 ⁰ C, in particular at least 20 ⁰ C, more particularly at least 25 ⁰ C.

As the skilled person knows, the salt concentration also affects the solubility of CO2: the higher the salt concentration, the lower the solubility. For that reason, it is preferred to make use of a liquid such as water having a salt concentration at which CO2 solubility is good, for instance of at most 4 mol NaCl per liter of water (4M), in particular of at most 3 M, more particularly of at most 2 M.

Preferably, the pressure at which CO2 and the liquid are brought together is at least 5 bar, at least 10 bar or at least 15 bar. Preferably, this pressure is at most 70 bar, at most 60 bar, at most 45 bar, or at most 35 bar. This is the pressure prevailing under a liquid column having as height the vertical distance from the liquid surface to the point of mixing.

Preferably, the CO2 is brought together with the liquid below ground level, while mixing of CO2 with the liquid takes place at a pressure

corresponding to the pressure prevailing at the depth at which mixing takes place.

The CO2 is adjusted with high pressure compressors to the pressure which is somewhat higher than the pressure prevailing at the depth at which mixing is taking place. CO2 may be introduced into the liquid in the form of bubbles, for instance having a number-average diameter of less than 10 mm. To promote the dissolving rate and/or to realize a high effective downward speed, the CO2 may be introduced as fine bubbles. CO2 bubbles are preferably introduced having a number-average diameter of about 5 mm or less, in particular of about 2 mm or less. The lower limit of the average bubble diameter may for instance be 0.1 mm, 0.3 mm or 0.5 mm.

Upon further downward movement of the gas-liquid mixture from the point of mixing of CO2 with the liquid, further compression of CO2 will take place under the influence of the ambient pressure, so that the use of a high pressure compressor which can realize such a pressure increase is not necessary.

It is particularly preferred to bring CO2 and the liquid together under a liquid column of 120 to 990 m. This depth can be optimized on the basis of factors such as the desired amount of CO2 which is to be stored per m ³ of liquid, the desired deployment of high pressure compressors, the local pressure at the depth at which CO2 and the liquid are brought together and the temperature and the salt concentration of the liquid with which CO2 is mixed. By way of indication, starting from 10 to 50 kg of CO2 per m ³ of liquid (water) and an initial gas/water volume ratio of between 1/4 (20% of gas, 80% of liquid) and 2/3 (40% of gas), the desired density of the CO2 is between 25 kg/m ³ (10 kg per 0.4 m ³ ) and 250 kg/m ³ (50 kg per 0.2 m ³ ). At a temperature of 25 ⁰ C of the liquid with which mixing is done, the corresponding pressures are 13 and 65 bar. At a temperature of 70 ⁰ C of the

liquid with which mixing is done, the corresponding pressures are 18 and 144 bar. The above-mentioned pressures in the temperature range of 25-70 ⁰ C normally correspond to a liquid column of 120 to 1430 m. At a temperature of 50 ⁰ C of the liquid with which mixing is done, the pressure is between 15 and 85 bar (under a liquid column of 140 to 840m).

The depth which the CCtø-liquid mixture reaches via the shaft is typically at least 300 m, preferably 500-4,000 m, and more preferably 1,000-3,000 m, with the understanding that this depth is usually at least the depth at which CO2 is mixed with the liquid. A greater depth has the advantage of high pressure prevailing there, so that more CO2 can be dissolved. On the other hand, an earth layer of lesser depth is generally simpler to strike by drilling. Further, an earth layer at a relatively low depth may be advantageous because of the lower temperature and an — at least usually - lower salt concentration, which is advantageous for the solubility of CO2.

Preferably, for CO2 introduction an earth layer is chosen that has a relatively high permeability to CO2 so that per unit time much of the Cθ2-liquid mixture can be introduced into it. The permeability is preferably greater than 0.2 D. The permeability is preferably greater than 10 Dm. The permeability is typically 1,000 Dm or lower.

On the basis of the information described herein, common general knowledge in the art and routine experiments, the skilled person himself can determine suitable conditions for use of the invention and for a high CO2 storage capacity. The shaft may be provided with a casing to stabilize the shaft.

Suitable materials for this are metal and so-called construction plastics, in particular thermoplastic materials. Suitable plastics are described on www.airborne.nl. Suitable in particular are one or more plastics chosen from the group of polyolefins, in particular PE (polyethylene) or PP (polypropylene); PPS (plasma-polymerized styrene); PEKK (poly-(ether

ketone ketone)); PEEK (poly-(ether ether ketone)); PA (polyamide), PET (polyethylene terephthalate), PBT (polybutylene terephthalate), PEI (polyethyleneimine) and the like.

In an embodiment of the invention, the shaft is provided, at least on the interior wall, with a plastic, in particular a thermoplastic material as mentioned above. This has as an advantage that the casing has a higher corrosion resistance than a shaft having a metal interior wall which is in contact with the Cθ2-liquid mixture.

In an embodiment of the invention, the Cθ2-water mixture can be introduced into an aquifer.

In a preferred embodiment of the invention, the method is combined with another process, such as a geothermy process. This is advantageous inter alia in that the introduction and storage of a CCVliquid mixture in the ground is linked to an installation (already existing or not) whereby a liquid is introduced into the ground and which is used for another purpose, or at any rate is advantageous in that the introduction and storage of a Cθ2-liquid mixture in the ground has been carried out with a system having another function as well. Such a system is an installation for geothermy. In the context of the invention, geothermy is understood to mean the extraction of energy using the temperature difference between the surface of the earth and heat reservoirs deep into the earth (www . geothermie . nl) . Water here serves as a means of transport of the geothermal energy; after pumping up hot water from the crust of the earth, the energy can be extracted using a heat exchange installation.

In an embodiment of the invention where geothermy plays a role, CO2 is mixed with water that has been used for geothermy. The thus obtained Cθ2-water mixture is used in the method of the invention to introduce CO2 into the ground.

In a special embodiment of the invention, the CCVliquid mixture is introduced into the ground at such a distance from the extraction area of the water that has been used for the geothermy that essentially an open system is involved, rather than a closed system (where at least a substantial part of the water is pumped up and recycled several times), or at least a system in which the introduced CCVliquid mixture does not reach the extraction area within a foreseeable time, in particular not within 25, 50 or 75 years.

The distance between the extraction area and the area of CO2 introduction is therefore in a preferred embodiment typically at least 2000 m, and preferably 2500-5000 m for a high CO2 storage capacity and long-term possibility to carry out the energy extraction using geothermy.

In an embodiment of the invention where geothermy plays a role, the earth layer prior to CO2 introduction is relatively thick, in particular at least 20 m thick, in particular at most 200 m thick, so that the water takes a long time to reach the extraction area of the water that is used for the geothermy.

On the basis of the information described herein and common knowledge in the art and, possibly, routine experiments, the skilled person himself can determine the suitable depth of CO2 introduction and the suitable distance between the extraction area and the area of CO2 introduction.

, In a preferred embodiment of the invention, the direction of the casing for the introduction of CO2 is at least substantially parallel to the radius of the earth, in particular the angle with the radius of the earth is 0-10°.

This is advantageous with a view to the preservation of a gas-liquid mixture having relatively many small bubbles, which may be advantageous to the rate of dissolution of CO2 in the liquid.