The present invention relates to a process for the recovery of organic gases from ground (earth strata), bed¬ rock or lake bottom sediments containing organic material.

The energy value of ground or bedrock containing organic matter is usually exploited in the following way: First the whole raw material is collected, e.g. by mining or other mechanical means. The material is then either burnt directly or subjected to pyrolysis for the recovery of gaseous and liquid hydrocarbons and coke. However, if the content of organic matter in the ground or bedrock is not sufficiently high these methods are uneconomical.

According to the present invention there is provid¬ ed an economical process for the recovery of valuable gaseous products in situ from ground or bedrock containing organic matter. The process is hi-ghly worthwhile economi¬ cally even if the content of organic matter is relatively low. Another advantage of the process is that it will not affect the natural environment to any major extent; the only noticeable event affecting the natural surroundings will consist in the boring of a number of wells. In the process according to the invention microbiological process¬ es are utilized for the production of organic gases, main¬ ly methane; a microbiological process is initiated or a natural microbiological activity is stimulated.

The novel process is applicable also to the recovery of organic gases from bottom sediments in lakes. By this it will be possible to restore contaminated or overgrown lakes or meres and at the same time recover a valuable product, preferably methane or ethylene.

Thus, the invention relates to a process for the recovery of organic gases from ground, bedrock or lake sediments containing organic material, and the process is characterized in that (a) microorganisms capable of fer¬ menting at least a part of the organic material to thus form organic gases, and/or substances promoting the growth of such microorganisms are added to water; that (b) the water thus treated is pumped down through one or more wells, pipes, drain pipes, radial screen pipes and/or ditches in the ground, bedrock or lake sediment; that (c) water con¬ taining the organic gases thus produced, said gases being present therein in gaseous form and/or dissolved form, is pumped up through one or more wells, pipes, drain pipes, radial screen pipes and/or ditches located at a suitable distance from said first-mentioned wells, pipes, drain pipes, radial screen pipes and/or ditches; and that (d) the pressure of the water pumped up is reduced, in a separating chamber, to a pressure lower than that existing down in said ground, bedrock or lake sediment so that the greatest part of the organic gases is released.

Preferably a cyclic process is used, that is the water is pumped all the way round so as to circulate in a closed circuit or cycle. Microorganisms and/or growth pro¬ moting substances are added to the water as it is coming forth from the separating chamber; this water has a low con¬ tent of dissolved organic gases. Next, the water is pumped again down into the ground, bedrock or lake sediment.

A preferred embodiment of the inventive process is the recovery of methane, e.g. from oil shale, and in order to simplify the subsequent description this preferred embo¬ diment will be dealt with in the first place.

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When methane is to be recovered so called methane bacteria are added to the water. Examples of such bacteria are Sarcina methanica, Pseudosarcina, Methanobacterium formi- cium, M. omelianskii, M. propionicum, M. sδhngenii, M. sub- oxydans, Methanococcus mazei, M. vannelii, Methanosarcina methanica and M. barkerii.

It is usually also necessary to add substances pro¬ moting the growth and development of the microorganisms and thus promoting the production of methane. These substances may be nutrients, such as compounds containing nitrogen, phosphorus or potassium. Furthermore, trace elements are usually necessary, such as one or more of the elements iron, manganese, magnesium, calcium, nickel, cobalt, copper, zinc and molybdenum.

As their carbon source for the production of methane the bacteria are to utilize the organic substances present in the ground, bedrock or lake sediment. Even if the bacte¬ ria cannot utilize all of the said organic substances as a substrate the process is nevertheless useful and economically profitable as long as the amount of methane recovered per volume of water circulated is sufficiently high (of the order of about 20 mg of methane per litre of water and higher).

The optimum pH for the growth of methane bacteria is between 5 and 8. The pH-value should suitably be between β and 8 and preferably between 7.2 and 8.0. If the ground water in the ground or bedrock or the water in the lake is too acidic or too alkaline the pH value should therefore be ad¬ justed to a desired value . This is conveniently done by an addition of suitable pH regulating and/or buffering substan¬ ces to the water to be pumped down. For instance sodium hyd¬ rogen carbonate, calcium oxide and/or calcium hydroxide may be used to increase the pH value.

In order to ensure that the microbiological process and concomitantly the degradation of organic material into methane will proceed at a sufficient rate (velocity), it is usually necessary to add both additives to the water, that

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is, the microorganisms as well as the substances promoting the growth of microorganisms. The best efficiency is attain¬ ed if the water which is pumped up for the release of methane is saturated with methane under the pressure and temperature conditions prevailing. However, in certain cases it may be sufficient to add only nutrients and/or trace elements, name¬ ly if the naturally occuring microbiological flora is suffi¬ cient. In other cases an addition of only microorganisms may be sufficient.

The particular microorganisms and chemicals to be add ed to the water have to be chosen specifically in each indi¬ vidual case depending upon the conditions prevailing in the ground, bedrock or bottom sediment wherein methane is to be produced. The same applies to the ehoosing of the amounts of microorganisms and chemicals to be added.

In the process according to the invention water con¬ taining microorganisms and/or chemicals is pumped down throug one or more wells, drain pipes, radial screen pipes and/or ditches in the ground or bedrock. The water is pumped down to a suitable depth which depends upon the ground water level and upon the location and thickness of the layer containing the organic material. The water pumped down will flow towards the extraction well or wells, at a velocity depending on various factors, such as for instance the permeability of the ground or bedrock. Partly as a function of said velocity the distance between the injection wells and the extraction wells is determined. This distance may amount to between a few metres and over 100 metres, usually between 10 and 100 metres. It is often desirable to use a system of a plurality of wells so that the microbiological process will then take place over a large area, to thus produce a large total volume of methane. It is for instance possible to bore two parallel rows of wells, the wells in one row being injection wells and the wells in the other row being extrastion wells. Alterna¬ tively, a number of injection wells may be arranged around a central extraction well.

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In the recovery of methane from bottom sediments in lakes water containing microorganisms and/or chemicals is pumped down into the bottom sediment through one. or more pipes . The water is pumped down to a suitable depth in the bottom sediment containing the organic material. The water flowing through the sediment is then pumped up through one or more extraction pipes. The injection pipes and the ex¬ traction pipes are arranged in the same manner as stated above for the wells.

In the ground or bedrock the microorganisms will degrade organic material to form methane and the methane thus formed will dissolve in the flowing water. When reach¬ ing the extraction well the water should suitably be satu¬ rated with methane, the saturation concentration of course depending upon the pressure and temperature prevailing. Therefore, for a given ground or rock permeability the suit¬ able distance between the injection and extraction wells will depend upon the time it takes for the water to become saturated with methane.

In the recovery of methane from lake sediments the methane concentration should not exceed the saturation con¬ centration when the water reaches the extraction pipe. If the saturation concentration is exceeded gaseous methane will be released which results in methane losses. The cir¬ culation velocity and the distance between the injection pipes and the extraction pipes should therefore be chosen or adjusted in such a way that the methane concentration in the water will notrise to the saturation point before the water is pumped up for releasing methane.

The methane-containing water is pumped up through the extraction well or pipe and introduced into a separat¬ ing chamber wherein the pressure is lower than the pressure down in the well or the lake sediment. As a result, the water in said separating chamber releases that amount of gas which exceeds its saturation concentration at the conditions existing in the separating chamber. The gas thus released is

passed to a storage or consumption site. The pressure in the separating chamber is preferably equal to the atmospheric • pressure or lower. Suitably the water is injected into the separating chamber in such a manner that it will be atomized into drops. This will accelerate the release of gas.

Necessary microorganisms and chemicals (nutrients, trace elements, pH regulating substances) are added to the water coming from the separating chamber, and then the water again is pumped down into the ground, bedrock or lake sedi¬ ment through an injection well or an injection pipe. The addi tion may be made either directly in the pipe through which the water is pumped, or in a special mixing container. A con¬ tinuous closed circuit is created in this manner and the pro¬ cess may be containued as long as the ground, bedrock or lake sediment contains organic substances which can be utilized by the bacteria for methane production. The flow rate of the water circulating in the closed circuit of a given plant may for instance be between 1 litre/sec. and up to more than 1000 litres/sec.

The production of methane by means of methane bacteri is an anaerobic process but since the water above the ground level is passed through a closed system, so that no oxygen can dissolve in the water, no special stripping of dissolved oxygen will be required. If desired, it is possible to degasi fy that water which is initially pumped down into the ground, bedrock or lake sediment to start the closed circuit process . Such degasification may be carried out in any suitable way.

According to an embodiment of the inventive process aerobic and anaerobic conditions are applied alternatingly in order to utilize the existing organic material more completel During the aerobic operation periods water containing oxygen or oxygen releasing compounds is pumped down; this has the effect that those or some of those organic substances that are not "utilizable", that is, cannot be utilized by the microorganisms employed, are transformed into "utilizable" organic substances. In this way e.g. saturated fatty acids

can be transformed into unsaturated fatty acids which can be utilized by methane bacteria. The water is usually aera¬ ted in a suitable manner before being pumped down. During the anaerobic operation periods oxygen-free water containing microorganisms and/or chemicals is pumped down and methane is produced and recovered as described above. Such an alter¬ nating operation has been found to be a very efficient way of exploiting the organic material maximally for methane produc¬ tion.

In certain cases, e.g. in the recovery of methane from peat moors, it may even be advisable continuously to pump down water containing oxygen or oxygen releasing com¬ pounds as well as microorganisms and/or growth promoting sub¬ stances. This is the case if the prevailing conditions are such that the oxygen in the water pumped down is consumed relatively quickly so that anaerobic conditions arise before the water reaches the extraction well.

By the recovery of methane from shales in accordance with the present invention it is possible to utilize the energy value of these shales economically. Methane gas has many different fields of practical application or use, and the utilization of shales in situ for the production of methane is therefore a very advantageous manner of exploit¬ ing this source of energy.

In addition to being applicable to the recovery of methane from shales, the process according to the invention can be used also for the recovery of methane or other orga¬ nic gases (e.g. ethylene) from other rocks or earth forma¬ tions, such as swamp and marsh areas or organic sediments, e.g. coal bearing formations, oil deposits or gas deposits. The gas recovered in the separating chamber may in addition to the desired organic gas possibly contain also various inorganic gases such as hydrogen sulphide, nitrogen, ammonia, hydrogen, etc., normally in minor concentration. The content of organic gas is greater than 90 % in most cases. If desired, the inorganic gases may be separated

_f rom the organic gas if this is necessary for the intended use of the organic gas .