FR2983898A1 - Method for monitoring geological site of carbon dioxide gas storage, involves determining change in pH value of carbon dioxide gas within overlying storage tank by detecting variations of emission spectrums of fluorescent markers - Google Patents

Abstract

The method involves providing a porous environment that contains carbon dioxide gas. A set of fluorescent markers is fixed at an end of an optical fiber, where a set of emission spectrums is determined based on a pH value of carbon dioxide gas for a given wavelength of the fluorescent markers. The optical fiber is introduced from a surface of an overlying storage tank or aquifer (AQ). A change in the pH value is determined within the overlying storage tank by detecting variations of the set of emission spectrums of the fluorescent markers. An independent claim is also included for a device for measuring variation of pH of an underground medium.

Description

The invention relates to the technical field of geological storage of gas, such as carbon dioxide (CO2), and in particular the monitoring of geological site of storage of CO2. The present invention proposes in particular a method for determining the pH evolution within an aquifer overlying the storage area. The scenarios developed by the IPCC (Intergovernmental Panel on Climate Change) show that the concentration of CO2 in the atmosphere, in the absence of any corrective action, will evolve from a current concentration of 360 ppm to more 1000 ppm at the end of the 21st century with important consequences for climate change. Capturing CO2 emissions from high-flow sources (eg thermal power plants), transporting this CO2 and storing it in suitable underground formations is one of the solutions among others to reduce greenhouse gas emissions. . Pilot projects for geological storage of CO2 already exist, but the continued deployment of this technology requires high quality technologies to meet the requirements of the regulations being implemented and to meet the expectations of the public. The volume of gas injected into an underground geological formation is easily known by measuring the flow of gas at the wellhead. However, the fate of the gas once injected is much more difficult to control: this gas can migrate vertically out of the storage formation (in more superficial geological layers, or even to the surface) or laterally in the host formation in zones not originally planned. In addition, the injected gas may experience bio-physico-chemical changes over time that may modify the pressure in the reservoir. These processes can be solubilization, precipitation, biological metabolism and chemical reactivity. The "injected gas" is the molecular species to be stored as well as the impurities it contains, in their gaseous form during the injection. The "species to be stored" is the molecular species that it is desired to store in the geological reservoir, excluding the impurities contained in the injected gas. The most complete possible follow-up of the fate of the injected gas must be carried out to comply with the regulations in force, as well as to support the societal acceptance of this technology. This monitoring must at least involve the detection of leaks out of the geological storage formation. Monitoring techniques must therefore be put in place to control the behavior of the injected CO2 in the different compartments (reservoir rock, cover, sub-surface, surface and atmosphere). This monitoring should make it possible to predict the evolution of the CO2 plume in the short, medium and long term, with the aim of detecting a possible leak towards the surface and the atmosphere. These techniques are intended to serve as an alarm signal in the event of poor containment of the site and therefore aim to ensure the security of it. They are the starting point for possible remedial actions in case of CO2 leakage from the storage tank. To follow the evolution of fluids injected into a porous medium, many techniques have been developed by manufacturers. Among these techniques, repetitive seismic, known as 4D seismic, is used in industry (petroleum or environmental). Such a technique consists in carrying out different seismic campaigns at different times (generally the campaigns are spaced apart by at least one year). Thus, the specialist can follow the evolution of the gaseous fluids of the geological storage site. These techniques have been exploited in the environmental field to estimate, from the seismic data, the total volume and the total mass of gaseous CO2 in place in the subsoil. Patent FR 2 827 960 also discloses a method of quantitatively monitoring a gas injected into a reservoir that is capable of chemically reacting with the injection medium. This method consists in injecting into the reservoir a mixture of the potentially reactive gas to be quantified with a small proportion of a tracer gas whose total chemical inertness, and determining the variation over time of the initial proportion of the reactive gas which was able to disappear by transformation, this by a measurement of the concentration variation of the tracer gas in the mixture. Methods based on the measurement of pH are also known. Indeed, the geological formations of CO2 storage (underground reservoir) are generally overcome (thus to a lesser depth than the reservoir) by an aquifer. However, when CO2 enters an aquifer, it dissolves and causes acidification of the aquifer. This acidification is therefore sought to determine a possible CO2 leak from the reservoir to the upper aquifer. To measure pH, it is known to take samples in the aquifer for subsequent measurements. This implies two major problems: the raising of the sample is difficult and the bottom conditions may not be preserved which implies a modification of the equilibrium of the sample and thus a variation of the physicochemical parameters such as the pH; the need to put a whole bottom levy system that is expensive, with a monitoring / sampling well which limits the number of measurements. Currently, all pH measurements are done in very near surface or after sampling for greater depths. The subject of the invention relates to a method of monitoring an underground CO2 storage site, by means of a pH measurement in an aquifer overhanging the CO2 reservoir. Depending on the process, these pH measurements can be performed at significant depths, in situ, and without sampling. The method is based on the use of fluorescent markers at high local concentration, whose fluorescence emission spectrum is pH dependent for at least one wavelength.

The method and the device according to the invention In general, the invention relates to a method for monitoring an underground CO2 storage site, said site comprising a porous medium containing stored CO2 surmounted by an aquifer. The method comprises the following steps: one sets at one end of an optical fiber a set of fluorescent markers whose fluorescence emission spectrum is pH dependent for at least one wavelength; the optical fiber is introduced from the surface to said aquifer, said end being at the level of the aquifer; said fluorescent markers are excited with at least said wavelength; a pH evolution is determined in said aquifer by detecting a variation of the emission spectrum of said markers. According to the invention, the fluorescent marker is preferably an impermeant molecule.

According to one embodiment, the markers are fixed on the fiber, by positioning the markers in a chamber adapted to let water in and out of the aquifer but to maintain the markers within the enclosure, the enclosure being bound or attached to the end of the optical fiber. According to one embodiment, the markers are fixed on the fiber, by fixing a plurality of markers on particles, and by fixing a plurality of particles on the optical fiber. According to this mode, a plurality of SNARF-COOH molecules can be fixed as a fluorescent marker on gold nanoparticles.

According to the invention, a SNARF-COOH molecule is preferably used as a fluorescent marker. The optical fiber can be introduced into the aquifer by means of a monitoring well reaching the aquifer, and preferably horizontal while remaining in the aquifer, and the end of the optical fiber having markers at an unslotted portion of the well in the aquifer. Finally, according to the invention, it is possible to perform a remediation action when the variation in pH indicates a presence of CO2 in the aquifer. The invention also relates to a device for measuring a pH variation of an underground medium, of the type comprising a light source for exciting a fluorescent marker whose fluorescence emission spectrum is pH-dependent for at least one length of time. wave, at least one transmission means of said light source to said fluorescent marker, at least one means for collecting the fluorescence emitted by said fluorescent marker. The means for collecting the fluorescence emitted comprises, according to the invention, at least one optical fiber at one end of which is bound or fixed a set of fluorescent markers. An impervious molecule may be used as a fluorescent label, such as a SNARF-COOH molecule. According to one embodiment, the fluorescent markers are positioned in an enclosure adapted to let in and out fluids, while maintaining the fluorescent markers within the enclosure, said enclosure being bonded or fixed to said end of the optical fiber . According to another embodiment a set of fluorescent markers is attached to a particle, and a set of particles is attached to said optical fiber. According to this mode, the fluorescent label can be a SNARF-COOH molecule and the particle, a gold particle. Other features and advantages of the method and device according to the invention will become apparent on reading the following description of nonlimiting examples of embodiments, with reference to the appended figures and described hereinafter.

Brief presentation of the figures Figure 1 shows an explanatory diagram of the implementation at the level of the monitoring well. Figure 2 shows magnification on the monitoring well (vertical section).

Figure 3 illustrates the percentage of signal transmission of an optical fiber as a function of distance. Figure 4 illustrates the fluorescence spectra of Snarf-COOH in distilled water at a concentration of 2.10-4 mol / L at different pHs. Figure 5 illustrates a fluorescence calibration curve of Snarf-COOH in distilled water at a concentration of 2x10-4 mol / L. Figure 6 is a diagram of the optical fiber head with the enclosure enclosing the pH sensitive fluorescent particles. FIG. 7 is a schematic representation of a sensitive pH particle. Detailed description of the method and the device For the application of this method for an underground CO2 storage site, the site comprising a porous medium containing stored CO 2 topped with an aquifer (FIG. 1), the monitoring method essentially comprises the following steps: a set of fluorescent markers whose fluorescence emission spectrum is pH-dependent for at least one length of time is fixed at one end of an optical fiber 'wave ; the optical fiber is introduced from the surface to the aquifer, the end being at the level of the aquifer; the fluorescent markers are excited with at least the wavelength; the evolution of the pH within the aquifer is determined by detecting a variation of the emission spectrum of the markers. 1. Fixation of pH-dependent fluorescent markers at an end of an optical fiber According to the invention, the presence of a CO2 leak in the aquifer above the reservoir in which the CO2 is stored is determined by detecting a variation pH within this aquifer. It is therefore a question of being able to measure the pH of a deep underground environment, in situ, and without sampling.

According to the invention, the pH is measured by a fluorescence measurement of the medium by means of a fluorescent marker whose fluorescence emission spectrum is pH-dependent for at least one wavelength. The problem with this deep fluorescence measurement is the loss of signal strength with the feedback of information (signal loss as a function of distance), as shown in Figure 3. Figure 3 illustrates the percentage signal transmission (Tr (%)) of an optical fiber according to the distance (D) traveled by the signal (optical fiber length in meters). To overcome this problem, a large number of fluorescent markers are attached to one end of an optical fiber. This makes it possible to considerably increase the local concentration of the pH-sensitive markers, without these markers being dispersed within the aquifer. This makes it possible to increase the intensity of the signal raised to the surface via the optical fiber, and thus, to obtain a signal strong enough to be detected and interpreted on the surface after the ascent of the latter.

Thus, during this first step, a set of fluorescent markers is fixed at one end of an optical fiber whose fluorescence emission spectrum is pH dependent for at least one wavelength. Choice of fluorescent markers sensitive to pH Two large families of fluorophores are most often used to measure pH variations via fluorescence: the Fluorescein family, which is permeant (entering the biological cell membranes of bacteria / microorganisms ); the family of SNARF, which is waterproof.

These fluorophores can be used in water. As with all fluorophores, techniques such as spectrophotometry are used to monitor fluorescence. The advantage of spectrophotometry is to be able to record in a given volume fluorescence spectra. To meet the objective of measuring the pH within an aquifer, we prefer the impermeable molecules, because they do not penetrate the microorganisms possibly present in saline aquifers (deep biosphere), monitoring or drinking. We also prefer molecules whose fluorescence translating the pH is independent of the temperature (in particular between 25 and 37 ° C, between 100 and 300 m of depth if we look at the natural geothermal gradient on Earth).

For these reasons, fluorescent molecules of the SNARF type are preferably used, in particular the carboxy form SNARF-1, denoted SNARF-COOH, (carboxyseminaphthorhodafluor-1-acetoxymethyl ester). This molecule is described in: S. Chu and M. H. Montrose, 1995. Proc. Natl. Acad. Sci. 92: 3303 There is a large number of molecules, corresponding to isomer mixtures and belonging to the SNARF family, sold by Molecular Probes® (www.molecularprobes.com). These fluorescent molecules are adapted to the detection of a pH variation within an aquifer due to an arrival of CO2 in this aquifer, because they have a pH sensitivity between pH = 6 and pH = 9 corresponding to pH values. underground / environmental. The chemical formula of the Snarf-COOH molecule is as follows: Snarf-COOH fluorescence study. The fluorescence measurement of this molecule (for a stock solution of concentration of 2x10-4 mol / l.) At different pH was carried out using a Raman spectrometer of the LabRam HR HORIBA-Jobin Yvon type in fluorescence mode with an Arlon laser. exciter wavelength equal to 514 nm. The results are shown in Figure 4. Figure 4 shows the fluorescence intensity (IF) versus wavelength (X).

Fluorescence spectra evolve as a function of pH. At pH = 6 the fluorescence spectrum is characterized by a main peak at 580 nm and a secondary peak at 640 nm; at pH = 7, the main peak is at 640 nm and the secondary peak at 580 nm; finally, at pH 8 and 9, the spectrum has only one peak at 640 nm. There is therefore a variation of fluorescence as a function of the pH. The ratio of peaks at 580nm and 640nm known to be characteristic of acid species and basic species respectively can be calculated. This ratio is even more important that the pH is acidic. The ratio of the peaks as a function of the pH is then reported in the following table: ## EQU1 ## Ratio of the peaks R = 1518 nm / 1640 nm 1.448 0.890 0.391 0.282 Fluorescence calibration of Snarf-COOH in distilled water. With this table, a fluorescence calibration curve of Snarf-COOH in distilled water as a function of pH could be established (FIG. 5). According to FIG. 5, the fluorescence ratio (R) of Snarf-COOH in distilled water calculated from the ratios of the fluorescence peaks obtained at 580 nm and 640 nm decreases when the pH increases exponentially. . The triangles represent the measurements, and the curve represents the modelization: R = 45.211 * e-0.57325 * pH Determination of the fluorescence detection threshold of Snarf-COOH.

Further studies were performed to determine up to what Snarf-COOH concentration the fluorescence could be detected. From the stock solution of Snarf-COOH in distilled water, several successive dilutions were carried out and for each diluted solution obtained, the fluorescence was measured. It appears that the fluorescence can be detected up to a concentration of 10-7M Snarf-COOH. Indeed, below this concentration, it can be considered that the fluorescence is no longer detectable insofar as the intensity of the peak is less than the product of the width of the peak and, r2,. Influence of temperature on pH measurements.

Other complementary studies show that the pH measured by fluorescence does not change as the temperature increases, unlike the pH measurements made by electrodes. Means for Fixing Fluorescent Markers According to one embodiment (FIG. 6), to multiply the number of fluorescent markers at the end of the optical fiber, and thus to overcome the loss of signal as a function of distance, a enclosure (E) attached to the optical fiber (FO). Figure 6 illustrates this embodiment, the right part. According to this embodiment, the closed enclosure attached to the head of the optical fiber contains a set (several thousand) of fluorescent markers (Ma). This chamber allows the passage of the fluid in the presence in the basement without leaving out the fluorescent markers (shown schematically by arrows in Figure 6). For this, it suffices that the enclosure is porous with a pore diameter smaller than the diameters of the fluorescent markers. According to another embodiment (FIG. 7), a plurality of markers (Ma) are fixed on particles (Pa), and a plurality of particles is fixed on the fiber. The SNARF-COOH molecule has a carboxylic function which can be chemically modified in order to graft a ligand (Lig) having a thiol function having a high affinity with gold, thus making it possible to graft the whole (Snarf + ligand pH-sensitive fluorescent molecule ) on the surface of gold particles. This molecule can be bonded to a sulfur molecule via its carboxylic function to graft the whole to the surface of gold particles. Thus, with several thousand particles on which about ten / hundred pH-sensitive and fluorescent molecules are grafted, the fluorescence signal emitted is much greater than that obtained in the case of a simple measurement in a diffuse medium without this system. There is therefore an increase in sensitivity and signal exacerbation, thus allowing, despite the loss of transmission along the optical fiber, to be able to determine the pH in situ even for large depths (fiber lengths). 2. Introduction of the optical fiber from the surface to the aquifer Figure 1 illustrates an exemplary embodiment, where the CO2 is injected into the subsoil (geological storage) via an injector well connecting the surface to the ground. So-called "reservoir" (RR) rock, and monitoring (MpH) measurements are performed within an aquifer (AQ) located above the cover rock (RC). The soil in contact with the surface is noted S.

To carry out the pH measurements, one or more optical fibers, equipped according to step 1, are introduced into a monitoring well (monitoring well which has a standard diameter of 6 inches, knowing that an optical fiber is about 1 cm of diameter). A horizontal well is recommended because it allows monitoring over a much greater distance. Up to a maximum of ten optical fibers can therefore be introduced into a monitoring well and thus monitor the pH in situ up to 10 different places. At each place where the pH measurement is carried out, the well is said to be uncracked (thus allowing the aquifer to pass there). Each compartment is isolated from each other by isolation packer or cementation (hatched rectangles), allowing to have accurate measurements well targeted at the spatial level. Figure 2 illustrates this device: three zones are isolated. The gray rectangles indicate the location of the pH measurement. The vertical straight lines represent the optical fibers within the well. 3. Excitation of Fluorescent Markers During this step, the fluorescent markers are excited with at least one wavelength. To excite the fluorescent markers is introduced a light source, an optical fiber for example, for emitting light of wavelength equal to the wavelength for which the fluorescent markers have a fluorescence emission spectrum dependent on the pH. For example, in the case of SNARF-COOH molecules, a light source of length less than 580 nm (excitation wavelength less than emission wavelength) is used. According to another embodiment, different wavelengths are used. For example, in the case of SNARF-COOH molecules, a light source emitting on several wavelengths, chosen to determine the height of the fluorescence peaks at 580 and 640 nm, is used. 4. Determination of the evolution of the pH within the aquifer The evolution of the pH within the aquifer is determined by detecting a variation of the emission spectrum of the markers.

According to the embodiment, where the markers are excited only by a single wavelength, a relative variation of the pH is determined. For example, by using a SNARF-COOH molecule, excited at a wavelength close to 540 nm, a drop in fluorescence will indicate a drop in pH (FIG. 1), and therefore a CO2 leak in the aquifer. According to the embodiment, where the markers are excited by a range of wavelengths, the pH is determined. For example, using a SNARF-COOH molecule, excited by wavelengths between 540 and 680 nm, the ratio of the peaks R = 158onm / 164onm can be measured and the pH can be deduced using the following table: pH 6 7 8 9 Peak ratio R = I / I 1,448 0,890 0,391 0,282 580nm- -640nm Fluorescence calibration of Snarf-COOH in distilled water.30 5. Remediation If a pH change is detected, indicating the presence of CO2 in the aquifer, we put in place techniques known as "remediation", to reduce or stop the leakage of CO2. At first, stopping the CO2 injection is recommended. Then, an injection of divalent cations into the aquifer can be considered; these cations may react with dissolved CO2 ions to form carbonates. Injections of microorganisms using CO in their metabolism may also be considered. The invention also relates to a device for measuring a pH variation of an underground medium, of the type comprising a light source for exciting a fluorescent marker whose fluorescence emission spectrum is pH dependent for at least one wavelength. , and means for detecting the fluorescence emitted by said fluorescent marker, at least one means for transmitting said light source to said fluorescent marker, at least one means for collecting the fluorescence emitted by said fluorescent marker. The means for collecting the emitted fluorescence comprises at least one optical fiber at one end of which is bound or fixed a set of fluorescent markers whose fluorescence emission spectrum is pH dependent for at least one wavelength.

Claims (3)

REVENDICATIONS1. A method for monitoring an underground CO2 storage site, said site comprising a porous medium containing stored CO2 surmounted by an aquifer, characterized in that one end of an optical fiber is fixed with a set of fluorescent markers whose Fluorescence emission spectrum is pH dependent for at least one wavelength; the optical fiber is introduced from the surface to said aquifer, said end being at the level of the aquifer; said fluorescent markers are excited with at least said wavelength; a pH evolution is determined in said aquifer by detecting a variation of the emission spectrum of said markers. The method of claim 1, wherein said fluorescent label is an impermeant molecule. Method according to one of the preceding claims, wherein said markers are fixed on said fiber, positioning said markers in a chamber adapted to let water in and out of the aquifer but to maintain said markers within the enclosure said enclosure being bonded or attached to said end of the optical fiber. Method according to one of claims 1 and 2, wherein said markers are fixed on said fiber, fixing a plurality of markers on particles, and fixing a plurality of particles on said optical fiber. The method of claim 4 wherein said markers are attached to said optical fiber by attaching a plurality of SNARF-COOH molecules as a fluorescent label on gold nanoparticles. Process according to one of the preceding claims, in which a SNARF-COOH molecule is used as a fluorescent marker. Method according to one of the preceding claims, in which said optical fiber is introduced into said aquifer by means of a monitoring well reaching said aquifer, and preferably horizontal while remaining in said aquifer, and the end is positioned. said optical fiber having the markers at a non-pinned portion of the well located in said aquifer. 10 2. 15 3. 204. 5. 25 6. 7.8. Method according to one of the preceding claims, in which a remediation action is carried out when the variation in pH indicates a presence of CO2 in the aquifer, to reduce or stop the leakage of CO2. 9. A device for measuring a pH variation of an underground medium, of the type comprising a light source for exciting a fluorescent marker whose fluorescence emission spectrum is pH dependent for at least one wavelength, at least one means for transmitting said light source to said fluorescent marker, at least one means for collecting the fluorescence emitted by said fluorescent marker, characterized in that said means for collecting the fluorescence emitted comprises at least one optical fiber at one end of which is bound or fixed a set of fluorescent markers. The device of claim 9, wherein said fluorescent label is an impermeant molecule. 11. Device according to one of claims 9 or 10, wherein the fluorescent marker is a molecule SNARF-COOH. 12. Device according to one of claims 9 to 11, wherein said fluorescent markers are positioned in an enclosure adapted to let in and out fluids, while maintaining the fluorescent markers within the enclosure, said enclosure being bonded or attached to said end of the optical fiber. 13. Device according to one of claims 9 to 11, wherein a set of fluorescent markers is attached to a particle, and a set of particles is attached to said optical fiber. The device of claim 13, wherein the fluorescent label is a SNARF-COOH molecule and the particle is a gold particle.