METHOD OF TREATING THE AREA SURROUNDING ACID GAS STORAGE WELLS

Abstract

Method of treating the area surrounding an acid gas storage well (1) using a reactive solution, wherein the following stages are carried out: a) injecting from the well a wash fluid for washing the rock of said surrounding area so that it no longer contains products reactive with said solution,b) injecting into the rock thus washed a predetermined volume of reactive solution suited to react with acid gases.

Description

FIELD OF THE INVENTION

The present invention relates to the sphere of geological storage of acid gases, including CO₂. In particular, the invention relates to the closing of wells giving access to geological formations wherein storage is achieved through injection. One of the goals is to prevent leaking of acid gas, CO₂ for example, through said wells or the surrounding area.

BACKGROUND OF THE INVENTION

Well abandonment procedures wherein plugs of various qualities: mechanical plugs made of an expandable material, cement plugs, resin plugs, are fed into well casings are known. However, the durability of these materials and of the casing pipe subjected to corrosion is not sufficient in the case of acid gas storage, notably CO₂.

Forced injection (squeeze) of specific plugging products through well perforations in order to seal the porous and permeable formation is also known. However, setting of the products injected is difficult to control, which makes the plugging efficiency unpredictable.

SUMMARY OF THE INVENTION

The present invention thus relates to a method of treating the area surrounding an acid gas storage well using a reactive solution, wherein the following stages are carried out:

a) injecting from said well a wash fluid for washing the rock of said surrounding area so that it no longer contains products reactive with said solution,b) injecting into the rock thus washed a predetermined volume of reactive solution suited to react with acid gases, said solution comprising basic oxides so as to precipitate minerals into the rock, in contact with the acid gases.

According to the method, acid gas can be stored prior to stages a) and b).

The reactive solution can be suited to precipitate carbonates, hydrogen carbonates or sulfides.

The basic oxides can be selected from the following group: alkaline or alkaline-earth oxides, their hydrated forms, Zn, Ti, Mn, Fe, Zr oxides or their admixtures.

The basic oxides can come from the following minerals: basic and ultrabasic rocks, such as basalts, serpentinites, peridotites, magnesite, possibly calcined, dolomite, albite, cements, hydrated or not, ultrafine cements, blast furnace slag, geopolymers, alkaline silicates, wollastonite, pouzzolanic materials, plaster, clinker, talc, kaolin, other clays, possibly calcined, silica fumes, fly ashes, zeolites.

The well can be plugged after injecting the reactive solution.

Stages a) and b) can be carried out after drilling into the geological overburden zone overlying said gas storage site so as to reduce or to plug the possible acid gas leaks in said zone.

The reactive solution can comprise oxides of predetermined grain size depending on the nature of the porous medium injected.

The reactive solution can comprise rheological property control agents such as hydrosoluble polymers, associative polymers, clays.

The basic oxides can be colloidal particles.

The wash solution can be aqueous.

The density and the flow properties of the wash solution can be determined for optimized displacement of the acid gases.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the present invention will be clear from reading the description hereafter of embodiments given by way of non limitative examples, with reference to the accompanying figures wherein:

FIG. 1 diagrammatically shows an embodiment of the invention, and

FIG. 2 also diagrammatically shows another embodiment of the invention.

DETAILED DESCRIPTION

The principle of the invention is based on the setting of a material reactive upon contact with acid gas in the area surrounding a well giving access between the surface of the ground and the geological acid gas storage reservoir. The result of the reaction is to greatly limit the permeability of the reservoir rock and even to plug it in said area surrounding the well.

FIG. 1 shows a wellbore 1 drilled through cap rock 2 that overlies reservoir rock 3. Wellbore 1 is cased by a tube 4 cemented in borehole 1 by a cementing material 5. Access to the reservoir is obtained through a drilled drain 6. An injection tube 7 ended by a strainer 8 is lowered into the well and the annulus between said injection tube and casing 4 is sealed by seal means of packer 9 type or equivalents, well known in the trade.

The diagram of FIG. 1 shows a non-limitative well equipment example, other variants being applicable to the present invention, in particular completions or well equipments for horizontal wells.

Acid gas injection is carried out by means of tube 7. Once filling of the reservoir is completed, a washing operation is carried out so as to drive the CO₂ or the H₂S out of zone 10 to be treated. Water is preferably used, but other fluids can also be used for washing insofar as they fulfil equivalent functions. For example, viscosifying additives can be added to improve washing.

The volume of wash fluid must be sufficient to drive the acid gas by a radial distance of at least some meters away from the area surrounding the well. This flushing, preferably with water, ensures thereafter good injectivity of the formulation comprising the reactive material in the area surrounding the well. In the absence of flushing, the method could be ineffective because there is a risk of fast formation of superficial mineral compounds (carbonates and/or sulfides), which may locally cause pore clogging, thus limiting encroachment of the well surroundings by the reactive formulation.

After washing, a formulation comprising the products reactive to acid gases is then injected into the area surrounding the well and within a radius of some meters. Arrows 11 schematize said injection.

In case of circulation of CO₂ (either in supercritical form or in aqueous solution) or of acid gas in the area surrounding the well, the goal of the reactive material is to mineralize this acid gas by inducing a great permeability reduction of the surrounding area, which notably allows to protect the well against an acid gas attack.

This reactive material for the present invention is selected from among the basic oxides of generic formula MxOy, with M an element selected from among alkaline, alkaline-earth or other elements, characterized in that, in the presence of water, they dissociate at least partly and form hydroxyl ions, and in that they react in the presence of acid gases such as CO₂ or H₂S, by forming respectively weakly water-soluble carbonates or sulfides.

The alkaline or alkaline-earth oxides are preferably selected notably from among sodium, potassium, calcium, barium and magnesium oxides, or their hydrated form (LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2,Sr(OH)2, Ba(OH)2, Mg(OH)2), as well as Zn, Ti, Mn, Fe, Zr oxides.

These oxides can come in form of mineral particles, the minerals being selected from among basic and ultrabasic rocks (such as basalts, serpentinites, peridotites, known to be rich in magnesium), magnesite, possibly calcined, dolomite, albite, cements (hydrated or not), ultrafine cements, blast furnace slag, geopolymers, alkaline silicates, wollastonite, pouzzolanic materials and other binders in general, plaster, clinker, talc, kaolin, other clays, possibly calcined, silica fumes, fly ashes, zeolites.

Materials with large proportions of magnesium and/or calcium oxides that form very stable carbonates in the presence of CO₂ are preferably selected.

Examples of carbonation reactions of various calcium and magnesium oxides are given hereafter:

$\mathrm{Calcium}\mathrm{oxide}\text{:}\mathrm{CaO}+{\mathrm{CO}}_2\to {\mathrm{CaCO}}_3$ $\mathrm{Magnesium}\mathrm{oxide}\text{:}\mathrm{MgO}+{\mathrm{CO}}_2\to {\mathrm{MgCO}}_3$ $\mathrm{Calcium}\mathrm{hydroxide}\left(\mathrm{lime}\right)\text{:}{\mathrm{Ca}\left(\mathrm{OH}\right)}_2+{\mathrm{CO}}_2\to {\mathrm{CaCO}}_3+H_2O$ $\mathrm{Magnesium}\mathrm{hydroxide}\text{:}{\mathrm{Mg}\left(\mathrm{OH}\right)}_2+{\mathrm{CO}}_2\to {\mathrm{MgCO}}_3+H_2O$ $\mathrm{Talc}\text{:}\frac{1}{3}{\mathrm{Mg}}_3{\mathrm{Si}}_4{O_{10}\left(\mathrm{OH}\right)}_2+{\mathrm{CO}}_2\to {\mathrm{MgCO}}_3+\frac{4}{3}{\mathrm{SiO}}_2+\frac{1}{3}H_2O$ $\mathrm{Forsterite}\text{:}\frac{1}{2}{\mathrm{Mg}}_2{\mathrm{SiO}}_4+{\mathrm{CO}}_2\to {\mathrm{MgCO}}_3+\frac{1}{2}{\mathrm{SiO}}_2$ $\mathrm{Wollastonite}\text{:}{\mathrm{CaSiO}}_3+{\mathrm{CO}}_2\to {\mathrm{CaCO}}_3+{\left(S\mathrm{iO}\right)}_2$

When the acid gas tends to return to the well, these bases react with the acid gas coming from the storage reservoir and mineralize it in the pores of the porous medium surrounding the well in form of carbonates or hydrogen carbonates in the case of CO₂, and in form of sulfides in the case of H₂S, thus reducing the porosity and the permeability in the surrounding area, and therefore decreasing the potential leak rate of acid gas to the surface through the well. The precipitated minerals also form a protective layer for the well equipments against the acid gases stored.

These bases are brought into aqueous solution, or even oversaturated solution, i.e. in suspension. In the case of suspensions, the grain size has to be adjusted and controlled in relation to the porosity of the medium, because the size of the particles must be small enough to enter the porous medium. This is provided through rigorous selection of the oxides (or oxide mixtures) that make up the formulation, and good dispersion thereof. The D50 of the particles selected preferably ranges between 0.2 and 200 μm, the D90 must be less than ⅓ of the mean diameter of the pores of the formation. A formulation comprising a mixture of grain sizes can be preferably used.

The specific surface area of the oxides selected is maximized so as to have a maximum amount of reactivity.

The rheology of the suspension thus formed is also adjusted in order to ensure high-performance injection while avoiding fracturation. According to the characteristics of the rock formation into which injection is performed, the viscosity of the suspension is adjusted so that the injection pressures range between the pore pressure and the fracture pressure of the medium. The viscosity is also adjusted so as to maintain said particles in suspension throughout injection. Viscosities between 1 mPas and 1 Pa·s are sought.

Rheology modifying additives are therefore advantageously added, notably hydrosoluble polymers stable in basic media, of sufficiently high molar mass to guarantee a viscosity effect with a low volume fraction of polymeric additive in the auto-blocking formulation.

Vinyl polymers carrying carboxylate, sulfonate or phosphate groups can be used, for example polyacrylates of molar mass above 10⁶ g/mol, or sulfonated polymers such as polystyrene sulfonate, polynaphthalene sulfonate.

It is also possible to add colloidal particles, for example swelling clays such as bentonites, smectites, whose sheets disperse in basic aqueous phase and whose function is to confer on the fluid a yield point that is useful for the efficient suspension of mineral particles (such as basic oxides under oversaturation).

Furthermore, the density of the formulation is adjusted according to the characteristics of the medium (rock fracture pressure), with respect to the hydrostatic pressure. The oxides are therefore selected, or their mixtures are adjusted, according to their respective density and volume fraction. The aqueous phase can contain dissolved salts to increase the density of the water, as it is commonplace in conventional drilling fluids (CaCl2, NaCl, . . . ).

The formulation can comprise weighting agents such as salts (barium sulfate or others), weakly soluble and maintained in the dispersed state. Their grain size must be adjusted and controlled, with respect to the porosity of the medium, and the size of the particles must be small enough to enter the porous medium.

The state of dispersion of the formulation can be controlled by adding dispersing agents such as surfactants, preferably anionic, such as alkyl sulfonates, alkyl phosphates, alkyl carboxylates, or non ionic, such as alkylated polyoxyethylenes.

The invention first applies to the closing of an injector well that is no longer going to be used, which can be likened to the abandonment of an oil well. The invention can however also be used during the construction of a well specifically drilled for geological acid gas storage, whether in reservoir rocks, aquifers or coal veins.

After acid gas injection, water, or more generally the wash fluid, is injected through acid gas injection tube 7 so as to drive the stored acid gas far away from the area surrounding the well. In case of acid gas storage in a saline aquifer, the water used as the flush fluid can come from this aquifer, and it may have been produced during injection of the acid gases. Once this operation achieved, the reactive formulation is injected through injection tube 7 and squeezed in the rock formation. The volume injected (squeezed) is suited to invade some meters in the area surrounding the well.

The operation is ended by injecting a cement plug, or any other plugging formulation, so as to maintain the reactive material in place. When this first operation is completed, it is possible to carry out the same operation in other zones, in particular the top of the reservoir, after perforating the casing and the primary cementing so as to allow injection.

The invention also applies when drilling a new well in order to inject acid gases to be stored therein. In this case, once the cap rock of the reservoir reached and traversed by the wellbore, the aforementioned method is applied. FIG. 2 shows a wellbore 21 that has reached cap rock 20 overlying storage reservoir 22. A water flush operation, then injection, just below the level of the overburden (top of the reservoir), of a reactive formulation is carried out. A string of tubes 23 and an annulus insulating packer 24 are therefore used. Zone 25 is thus invaded by a reactive material that will react in case of acid gas leaking in said zone. Drilling is continued with the drilling fluid to the desired depth, then casing and cementing is carried out, prior to performing the perforations required for subsequent injection of the acid gas(es). This preventive method, carried out upstream from the CO₂ injection, allows to limit leak risks at the level of the overburden, where the CO₂ plume might accumulate during and after injection, thus greatly reducing the permeability of the reservoir rock just below the clay layer. During well closing for abandonment, the procedure described above is applied.

Claims

1) A method of treating the area surrounding an acid gas storage well using a reactive solution, wherein the following stages are carried out: a) injecting from said well a wash fluid for washing the rock of said surrounding area so that it no longer contains products reactive with said solution,b) injecting into the rock thus washed a predetermined volume of reactive solution suited to react with acid gases, said solution comprising basic oxides so as to precipitate minerals into the rock, in contact with the acid gases.

2) A method as claimed in claim 1, wherein acid gas has been stored prior to stages a) and b).

3) A method as claimed in claim 1, wherein the reactive solution is suited to precipitate carbonates, hydrogen carbonates or sulfides.

4) A method as claimed in claim 1, wherein the basic oxides are selected from the following group: alkaline or alkaline-earth oxides, their hydrated forms, Zn, Ti, Mn, Fe, Zr oxides or their admixtures.

5) A method as claimed in claim 1, wherein the basic oxides come from the following minerals: basic and ultrabasic rocks, such as basalts, serpentinites, peridotites, magnesite, possibly calcined, dolomite, albite, cements, hydrated or not, ultrafine cements, blast furnace slag, geopolymers, alkaline silicates, wollastonite, pouzzolanic materials, plaster, clinker, talc, kaolin, other clays, possibly calcined, silica fumes, fly ashes, zeolites.

6) A method as claimed in claim 1, wherein the well is plugged after injecting the reactive solution.

7) A method as claimed in claim 1, wherein stages a) and b) are carried out after drilling into the geological overburden zone overlying said gas storage site so as to reduce or to plug the possible acid gas leaks in said zone.

8) A method as claimed in claim 1, wherein the reactive solution comprises oxides of predetermined grain size depending on the nature of the porous medium injected.

9) A method as claimed in claim 1, wherein the reactive solution comprises rheological property control agents such as hydrosoluble polymers, associative polymers, clays.

10) A method as claimed in claim 1, wherein the basic oxides are colloidal particles.

11) A method as claimed in claim 1, wherein the wash solution is aqueous.

12) A method as claimed in claim 1, wherein the density and the flow properties of the wash solution are determined for optimized displacement of the acid gases.