SUBTERRANEAN STORAGE OF CO ₂ OR ITS DERIVATIVES

The present invention relates to the securing of underground sequestration of CO ₂ , and its subsequent in situ mineralization in particular, the invention provides means of preventing CO ₂ leaks from underground reservoirs and optionally after injection of converting it at least partially into carbonate precipitates and/or crystals. The invention can also provide means for the indirect disposals of CO ₂ in geological structures by injecting into a geological structure one or more chemical species containing organic carbon and, optionally oxygen if the later in not locally available in the structure, e.g. as in nitrates, and injecting one or more selected microorganisms and a nutrient to convert the carbon and oxygen to carbonate.

Underground geological disposals are being considered for waste CO ₂ disposal from CO ₂ generating activities such as power plants and other industrial facilities. Examples of reservoirs that have been proposed are depleted oil or gas reserves and deep saline aquifers. The concern with such disposal technologies is that the walls of the depositions may not be integral and CO ₂ might ultimately seep out through cracks or voids perhaps returning to the surface.

This invention is concerned with converting the CO ₂ or the carbon and oxygen in molecules directly or indirectly derived from CO ₂ to carbonate to avoid such problems and optionally for locking cracks in the reservoir. The invention employs a tailored form of microbial biocementation for the conversion of the CO ₂ or the molecules. Appropriately microbial composition in combination with ionic solution and nutrients, can be used to both plug voids or cracks in the geological structures, and also absorb at least a portion of the CO ₂ in the carbonate precipitated by the operation. The conversion of CO ₂ to carbonate for environmental benefits is known from for example United States Patent 7,132,090 and 6,890,497. These patents are not, however, concerned with the sequestration of CO ₂ in subterranean deposits.

The present invention therefore provides a process for the disposal of CO ₂ in geological structures comprising injecting CO ₂ into the geological structure and injecting one or more selected carbonatogenic micro-organisms and a nutrient for the micro-organism into the structure.

In a further embodiment the invention provides a process for the indirect disposal of CO ₂ in geological structures comprising injecting into them micro-organism nutrients containing one or more chemical species containing organic Carbon and, in the absence of a local metabolic source of it in the injected structure, Oxygen, in combination or separate and injecting one or more selected micro-organisms including at least one carbonatogenic one and a nutrient for the micro-organism into the structure to convert the combined Carbon and Oxygen into carbonate.

In a preferred embodiment calcium and/or magnesium ions preferably in solution such as a solution of calcium chloride or calcium nitrate is also injected into the geological structure concurrently or sequentially to the injection of the carbonatogenic microorganism and/or nutrient. In this embodiment the calcium ions can be precipitated perhaps as calcium carbonate to solidify the CO ₂ to prevent escape and also to plug cracks in the structure. The requirement to add calcium and/or magnesium ions depends upon the nature of the geological structure, the nature of the CO ₂ stream or the organic molecule and the nature of the natural reservoir.

In this preferred embodiment the invention has the benefit that the materials can be tailored according to the CO ₂ stream or the organic molecule and the strength required for the structure. Depleted oil and gas reservoirs and deep saline aquifer used as geological CO ₂ reservoirs will have their own needs and require their own tailoring. The nature of the bacteria, the nutrient and the amount of calcium and/or magnesium ions supplied can therefore be tailored according to the needs of the system.

In a further preferred embodiment the invention provides a process for the indirect disposal of CO ₂ in geological structures comprising injecting, as part of the bacterial nutrient mix one or several organic Carbon containing molecules , as well insuring by injection or other means the presence of oxygen containing molecules selected from inorganic phosphates, sulfates, nitrates and injecting one or more selected micro- organisms including at least one carbonatogenic one into the structure to convert the combined Carbon and Oxygen into carbonate.

Several bacterial biomineralisation processes are known and any of these may be employed in the process of the present invention. Three possible pathways are described in the publication Bacterial Roles in the Precipitation of Carbonate Minerals by Castanier et al 2000 Microbiol Sediments which describes various metabolic pathways of bacterial calcium carbonate formation as autotrophic pathways, and heterotrophic pathways and it also describes the relationship between bacteria minerals and the environmental conditions. The following may be derived from this publication.

Autotrophic Pathways In autotrophy, three metabolic pathways are involved: non-methylotrophic methanogenesis, anoxygenic photosynthesis and oxygenic photosynthesis. All three pathways use CO ₂ . Thus, they induce CO ₂ depletion of the medium or of the immediate environment of the bacteria. When calcium ions are present in the medium, such a depletion favours calcium-carbonate precipitation as depicted in Figure 1.

Heterotrophic Pathways

Heterotrophic pathways are those which cannot synthesize organic compounds directly from CO ₂ . Organic compounds is their starting point. Hence, in heterotrophy two bacterial processes may occur, one, autotrophic, providing the organic source for the second one, which is heterotrophic, sometimes concurrently. Alternatively, a nonbacterial source of organic compound can be provided to the sole heterotrophic pathway.

Passive Precipitation Passive precipitation or passive carbonatogenesis operates by producing carbonate and bicarbonate ions and inducing various chemical modifications in the medium that lead to the precipitation of calcium carbonate. Two metabolic cycles can be involved: the nitrogen cycle and the sulphur cycle.

In the nitrogen cycle, passive bacterial precipitation follows three different pathways: (i) the ammonification of amino-acids in aerobiosis (i.e. in the presence of gaseous or dissolved oxygen), in the presence of organic matter and calcium); (ii) the dissimilatory reduction of nitrate (in anaerobiosis (i.e. in the absence of oxygen) or microaerophily (i.e. in the presence of very low amounts of oxygen), in the presence of organic matter,

calcium and nitrate); and (iii) the degradation of urea or uric acid (in aerobiosis, in the presence of organic matter, calcium, and urea or uric acid). Both urea and uric acid result from eukaryotic activity, notably that of vertebrates. These three pathways induce production of carbonate and bicarbonate ions and, as a metabolic end-product, ammonia, which induces pH increase as depicted in Figure 2. When the H+ concentration decreases, the carbonate-bicarbonate equilibria are shifted towards the production of CO3 2- ions.

If calcium ions are present, calcium-carbonate precipitation occurs. If Ca2+ (and/or divalent cations) are lacking in the medium, carbonate and bicarbonate ions accumulate, and the pH increase and bacterial activity may favour zeolite formation.

In the sulphur cycle, bacteria use a single metabolic pathway: the dissimilatory reduction of sulphate which is depicted in Figure 3. The environment must be anoxic, and rich in organic matter, calcium and sulphate. Using this pathway, bacteria produce carbonate, bicarbonate ions and hydrogen sulphide. If calcium ions are present, the precipitation of Ca-carbonates depends on the hydrogen sulphide behaviour. If the hydrogen sulphide degasses, this induces pH increase and, calcium bicarbonate precipitation. On the other hand, hydrogen sulphide may be used by other bacteria. If anoxygenogenic sulphide phototrophic bacteria are involved, the hydrogen sulphide is oxidised into sulphur which forms intra-cellular or extra-cellular deposits. Hydrogen sulphide up-take induces pH increase favouring calcium-carbonate precipitation. If autotrophic sulphide-oxidising aerobic bacteria are involved, they produce sulphate ions. Together with hydrogen ions from water this gives sulphuric acid, the pH decreases and no solid Ca-carbonate appears. If hydrogen sulphide is neither used by bacteria nor discharged, pH decreases and Ca-carbonates cannot precipitate.

Active Precipitation

Active precipitation or active carbonatogenesis is another means for the bacterial production of carbonate ions and is independent of the other previously mentioned metabolic pathways. The carbonate particles are produced by ionic exchanges through the cell membrane by activation of calcium and/or magnesium ionic pumps or channels, probably coupled with carbonate ion production. Numerous bacterial groups are able to operate such processes. Carbonatogenesis appears to be the response of heterotrophic

bacterial communities to an enrichment of the milieu in organic matter. After a phase of latency, there is an exponential increase of bacterial strengths together with the accumulation of metabolic end products. These induce an accumulation of carbonate and hydrogenocarbonate ions in the medium and, by different ways, a pH increase that favours carbonate precipitation. This phase ends into a steady state when the majority of the initial enrichment is consumed. Particulate carbonatogenesis occurs during the exponential phase and ends more or less after the beginning of the steady state. In most cases, the active carbonatogenesis seems to start first and to be followed by the passive one which induces the growth and shape modifications of initially produced particles.

The process of the present invention can employ any of these pathways and tailor the CO ₂ sequestration and geological structure sealing according to the pathway that is best suited to the requirements according to the nature of the CO ₂ stream and the geology of the structure. For example, the micro-organism and the nutrient are selected to ensure that a gel is developed rapidly or slowly as is required and the gel formed may have sufficient strength to provide the desired strength to the geological structure. Furthermore the source and the quantity of calcium and/or magnesium ions, when used, may be selected according to the rate of CO ₂ injection o enable a control on the rate of precipitation of the carbonate and the quantity of carbonate material that is precipitated.

In some instances the natural microbial biocementation process may be sufficient to convert the CO ₂ to carbonate and seal the geological structure. In other instances it may be desirable to increase the cementitious content of the structure to prevent CO ₂ leakage. This may be accomplished by the provision of a solution of calcium and/or magnesium ions as well as the microbial and nutrient materials. In this instance it is preferred that the microbial and/or nutrient materials contain nitrogen. In this way the natural processes will liberate alkaline materials leading to the precipitation of calcium and/or magnesium carbonates. For example, this may be accomplished by the presence of amino-acids, urea and/or uric acid whereby the production of ammonium ions increase the pH which stimulates the initial precipitation of carbonates which in turn provides the nucleation sites for further precipitation. The calcium and/or magnesium ions can be provided from solutions of calcium or magnesium salts such as chlorides, sulphates and nitrates and mixtures thereof may be used as may be desired to provide the carbonate mix appropriate for the cementatious material that is being repaired.

The invention therefore provides an effective, environmentally friendly and simple method for the more effective sequestration of CO ₂ in underground structures.

Examples of microbes and nutrients that may be used in the present invention are summarized in the following table. The combination of microbe and nutrient is selected according to the required mineralizing and filming.

Vibrio strains produce magnesium calcite, whereas H. eurihalina produces spherical bioliths of 50 μm of magnesium

For the Halomonas, calcite, aragonite and Marinomonas cultures, 1% monohydrocalcite in yeast extract (Difco) ; 0.5 %

Variety of variable proportions, and

Moderately halophilic proteose-peptone no.3 the Halomonas &

Calcium bacteria like Vibrio, (Difco); 0.1% glucose; Marinomonas produce Halomonas supplemented with a and calcite, magnesium calcite, eurihalina, 14 other balanced mixture of sea aragonite, dolomite,

Magnesium species of salts to final concentrations monohydrocalcite,

Halomonas and 2 of of 2.5 %, 7.5 % and 20 % carbonates hydromagnesite and Marinomonas, and (w/v). The medium is struvite in variable

(see also amended with 0.4 % proportions. Aragonite,

Chromohalobacter calcium acetate and the pH comments) magnesium calcite, vaterite, marismortui. adjusted to 7.2 with 1 M monohydrocalcite, struvite, KOH. To obtain solid kutnahorite and huntite can media, 20 g/l "Bacto-Agar" be produced by (Difco) can be added.

Chromohalobacter marismortui. These species are adapted to high salinities, and hence, may be used in saline aquifers.

First developing competency to degrade a surfactant by growing the bacteria with the surfactant present as the sole organic nutrient; then starving the resulting competent

Water-retaining structures, bacteria until the cells reach will benefit from the bacteria a diameter less than about

Enterobacter, based precipitation of 0.4 μm. "Starvation" is

Film Serratia, Bacillus, calcium carbonate. Bacillus achieved by placement of forming Klebsiella, or firmus and Bacillus the bacteria in a carbon-

Pseudomonas sphaericus and Bacillus free environment, such as a amyloliquefaciens can be phosphate buffer salts triggered to the same use. solution (PBS), for at least two weeks. Nutrient solution provided to "revive" the microbes is trisodium citrate, and will allow the digestion of the surfactant before plugging.