DEGRADATION RELATED TO C02 EXPOSURE

BACKGROUND OF THE INVENTION

[0001] The present invention broadly relates to well cementing. More particularly the

invention relates to a cement composition and related method of cementing for carbon dioxide environment, such as for instance a reservoir for storage of carbon dioxide gas.

[0002] In the construction of wells, cement is often used to secure and support casing

inside the well and prevent fluid communication between the various underground fluid- containing layers or the production of unwanted fluids into the well. Long-term isolation and integrity of C0 ₂ injection wells clearly needs to be improved to ensure long-term environmental safety. Failure of the cement in the injection interval and above it may create preferential channels for carbon dioxide migration to the surface. This may occur on a much faster timescale than geological leakage. CO ₂ injection well construction starts with drilling followed by well completion before starting C0 ₂ injection operations. In the framework of well completion, the cementation phase guarantees well isolation from the reservoir to the surface and isolation between geological formations. A crucial technical problem in well cementing operation or C0 ₂ sequestration is the chemical resistance of cement to CO2 over time.

[0003] In addition, the environment of an underground reservoir might have high

temperatures or pressures or have other hazards such as hydrogen sulfide, which might provide challenges to the material selection, well construction and/or placement process. Indeed, C0 ₂ combined with water can form carbonic acid, which could damage some conventional materials that might be used to transport, place or contain the CO2, so material selection is important.

[0004] In the CO ₂ sequestration, for example, one may want to prepare surface of the

sequestration location in such a way that any leakage of C0 ₂ to the surface after placement is easily detectable and/or mitigated. It is desirable that the site be properly decommissioned after a preferably optimum amount of C0 ₂ is in place and the site monitored for sometime afterwards to ensure that the C0 ₂ placement is stable. It is desirable to ascertain the amount, location and state of the CO ₂ during the placement process, after the CO ₂ has been placed and for some period thereafter to ensure that the C0 ₂ stays where it has been placed and does not migrate to the surface or into drinking water aquifers. Post-decommissioning monitoring may be required by regulatory agencies.

[0005] Conventional Portland cement based systems, used during the well cementation phase, are known not to be thermodynamically unstable under C0 ₂ rich environments. This type of cement tends to degrade once exposed to such acid gases. Optimization of advanced systems allowing long-term well isolation is critical to allow safe and efficient underground activities. Indeed, any cracks or hole in the cement sheath might lead to safety problem such as "blow-out".

[0006] But C0 ₂ environment present new challenges. While facing C0 ₂ underground, being for carbon sequestration application or well cementing for isolation, leakage behind the wall of an injection well casing may provide a pathway for CO ₂ migration into unplanned zones (other formations, adjacent reservoir zones, and other areas) potentially leading to economic loss and/or safety problems. Such CO ₂ leakage through the annulus may occur much more rapidly than geologic leakage through the formation rock. The possibility of such leaks raises considerable concern about the long-term wellbore isolation and the durability of hydrated cement that is used to isolate the annulus across the producing and injection intervals in C0 ₂ -related wells.

[0007] The long-term C0 ₂ sequestration is a more recent concept but also requires long- term wellbore integrity. Indeed, the major risk associated by the public with C0 ₂ injection is a wellbore failure, which may result in escape of C0 ₂ that will migrate upwards; most cement being not thermodynamically stable under carbon dioxide environment.

[0008] The present invention provides methods, apparatuses and systems for more

effectively and efficiently, directly and indirectly monitoring cement degradation related to carbon dioxide exposure by measuring one or more electrical properties related to resistivity of cement behind a casing in a well. SUMMARY OF THE INVENTION

[0009] The present invention also relates to the detection of any change in the cement sheath integrity in any well application. This might be, indeed, for oil-well, water storage, carbon sequestration or any gas containment.

[00010] The present invention relates to a method for measuring the integrity of the cement sheath involving resistivity measurement in situ. Such measurement would allow detecting any changes in the cement integrity especially when it reacts with C0 ₂ .

[00011] An object of the present invention is to provide methods, apparatuses and systems for transporting, measuring, sequestering carbon, as well as site preparation and design, decommissioning a carbon sequestration site and providing post- decommissioning monitoring of the sequestration site, while eliminating or minimizing the impact of the problems and limitations described.

[00012] A further object of the invention concerns methods of sequestering C02

comprising:

a. collecting carbon dioxide and preparing the carbon dioxide for sequestration; b. selecting a storage site;

c. obtaining permits for using the storage site for sequestration;

d. preparing the storage site, including drilling at least one injection well having a casing cemented in place so that a cement sheath is in place between the final casing and a borehole wall of the well;

e. transporting the carbon dioxide to the storage site, measuring the carbon dioxide as desirable;

f. placing and monitoring C0 ₂ , until sequestration is complete, including checking for problems and addressing the problems as needed, wherein the monitoring of C0 ₂ includes taking one or more measurements of an electrical property of the cement sheath related to resistivity of the cement sheath; g. decommission sequestration site; and

h. monitoring the decommissioned site for leaks.

[00013] Other objects, features and advantages of the present invention will become

apparent to those of skill in art by reference to the figures, the description that follows and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[00014] FIG. 1 is a flowchart for a generalized CO ₂ sequestration process.

[00015] FIG. 2 is a flowchart for a CO2 sequestration process, including an embodiment of the instant invention.

[00016] FIG. 3 is a simplified depiction of a well

[00017] FIG. 4 is a graph depicting evolution of Portland cement's resistivity with time in C0 ₂ fluids at 90°C temperature and 280 bars pressure.

[00018] FIG. 5 is a graph plotting the imaginary part of conductivity of a Portland cement before and after CO2 exposure, both to wet supercritical C0 ₂ and to C0 ₂ saturated water.

DETAILED DESCRIPTION OF THE DRAWINGS

[00019] In the following detailed description of a preferred embodiment and other

embodiments of the invention, reference is made to the accompanying drawings. It is to be understood that those of skill in the art will readily see other embodiments and changes may be made without departing from the scope of the invention.

[00020] Chemical reactions between cement and carbon dioxide occur as the following:

• C0 ₂ + H ₂ 0 <→ H ₂ C0 ₃ <→ H ⁺ + HCO3 ^" <→ 2H ⁺ + C0 ₃ ^2" which corresponds to the dissociation of carbon dioxide in water formation

• the following reactions correspond to the reaction between cement and

carbon dioxide as dissolution and carbonation

o Ca(OH) ₂ (s) + 2H ⁺ + CO3 ^2" → CaC0 ₃ (s) + 2H ₂ 0 (where 2H ₂ 0 is free water)

o C ₃ .4-S ₂ -H ₈ (s) + 2H ⁺ + C0 ₃ ^2" → CaC0 ₃ (s) + SiO _x OH _x (s) (where

SiO _x OH _x (s) is silica gel)

o Ca(OH) ₂ (s) + H ⁺ + HCO3 ^" → CaC0 ₃ (s) + 2H ₂ 0

o C ₃ .4-S2-H ₈ (s) + H ⁺ + HCO3 ^" → CaC0 ₃ (s) + SiO _x OH _x (s)

• Calcite dissolution under acidic condition

o C0 ₂ + H ₂ 0 + CaC0 ₃ (s) <→ Ca ²⁺ + 2HC0 ^3"

o 2H ⁺ + CaC0 ₃ (s)→ C0 ₂ + Ca ²⁺ + H ₂ 0

[00021] Cement alteration may be characterized in laboratory by a complex series of

concentric carbonation fronts after CO2 attack. This chemical alteration is a very effective process with a significant pH decrease of interstitial fluid triggering possibly casing corrosion. An initial sealing by carbonation is followed by a dissolution stage, which starts earlier in C0 ₂ -saturated water than in wet supercritical CO2. After six months, the degradation is very high and shows a "spalling effect" (cement material starts to peel out). The evaluation of this alteration may be performed with different measurements on set cement samples as mechanical properties evolution but also as pH, porosity, permeability and resistivity before and after CO2 exposure.

[00022] The cement alteration firstly controlled by the C0 ₂ diffusion through the cement matrix is followed by a mechanical alteration resulting from the carbonation and the dissolution of calcite, creating micro-cracks in the cement. The micro cracks could accelerate C0 ₂ leakage up to the surface as a preferential pathway for C0 ₂ .

[00023] As the carbon sequestration is probably the conditions where the cement is likely to be in contact with long period and at high concentration, this will be used as a preferred embodiment in the following disclosure; however, the method according to the present invention is applicable to any cemented well in contact with CO2.

[00024] The concept of C0 ₂ generation and sequestration are nowadays pretty well

understood. Large-scale CO2 sources, for example, ethanol plants, cement factories, steel factories, refineries, electricity generation plants, coal and biomass operations generate C0 ₂ and usually vent it as atmospheric C0 ₂ . Some large scale C0 ₂ sources may transport the C0 ₂ to factories where the C0 ₂ can be used for industrial processes or for food and drink products. Some CO2 is sequestered through natural processes such as from trees "inhaling" C0 ₂ and "exhaling" 0 ₂ (oxygen). But C0 ₂ may be taken to a C0 ₂ capture facility, where the C0 ₂ is processed and then geologically sequestered. In geologic sequestration, the C0 ₂ is injected down one or more wells and into an appropriate formation, such as a coal seam where the C0 ₂ can displace methane to a production well by which the methane can be produced. Another appropriate formation might be a suitable depleted oil and/or gas reservoir. Another appropriate formation might be a suitable reservoir with trapped oil that the C0 ₂ may be used to displace to a production well by which the oil may be produced. Another appropriate formation might be a saline reservoir with a suitable trapping structure. [00025] FIG. 1 is a depiction of an overview of a C0 ₂ sequestration process, which may be used with or without embodiments of the instant invention. One selects and prepares 12 a sequestration site. This may be a lengthy and involved process involving many steps and considerations. If the C0 ₂ is being produced with hydrocarbons, it can be separated underground and be sequestered without reaching the surface. If the CO2 is being collected at the surface, a determination would be made as to whether there was a place to sequester the CO2 onsite or whether the CO2 might have to be transported to another site. If transportation is necessary, the possible sites available would be considered along with their relative advantages and disadvantages.

[00026] Referring again to FIG. 1 , C0 ₂ is collected and prepared 15 for storage. How the C0 ₂ is collected and prepared depends in part on the source of the C0 ₂ . The source of the CO2 may be a power plant, refinery, factory or other industrial site or the source of the C0 ₂ may be from hydrocarbon production. There are countless sources of C0 ₂ , including every human being and animal on the planet and most automobiles, but carbon sequestration is currently most practical where the CO ₂ is produced in large amounts and can be easily collected. The source of the C0 ₂ helps to determine whether the C0 ₂ requires processing, such as removal of moisture or other

contaminants before transportation and/or sequestration. The C0 ₂ may be measured 20 as and/or after the C0 ₂ is collected and/or processed.

[00027] Referring again to FIG. 1 , the process may be different, depending on whether the CO2 is separated at the surface 25 and whether the sequestration is co-located with the C0 ₂ production site 30. If the C0 ₂ is not separated at the surface but rather

underground as it is produced, for example with hydrocarbons, it might be possible to have the C0 ₂ placed and monitored 45 underground. In the more typical case of collecting and separating the CO2 at the surface, then a determination is made as to whether 30 the selected sequestration site is co-located with C0 ₂ production site. If the C0 ₂ is co-located with the sequestration site the CO2 may still need to be transported a short distance and measured as part of being placed and monitored 45, but if the sequestration site is not co-located with the CO2 production site substantial

transportation system 35, such as through a pipeline system, may be necessary.

Measurements 40 may have to be taken at several points to ascertain that there is no CO2 leakage occurring or to determine a location of any CO2 leakage or simply to determine the amount of CO ₂ at various points in the process.

[00028] Continuing to refer to FIG. 1 , at the sequestration site, the C0 ₂ is placed generally through injection wells and monitored 45. Buffering may be provided to store CO2 in case the sequestration site has to be temporarily shut down. As monitored

sequestration progresses, if problems are indicated 50, they are evaluated and ameliorated or if possible, completely fixed 55. Monitoring may be used to determine whether the sequestered C0 ₂ is stable or whether the sequestered C0 ₂ is becoming permanently affixed in the storage formation. Sequestration continues until complete 60. Completion might occur for example, if the source of the C0 ₂ permanently shuts down or if the storage formation cannot accept any more C0 ₂ . The sequestration site would then be properly decommissioned 65 and post-decommissioning monitoring 70 might begin. If the post-decommissioning monitoring detects 75 a problem, the problem situation may have to be improved or fixed 80.

[00029] FIG. 2 is a flowchart for a CO2 sequestration process, including an embodiment of the instant invention. As with the general process presented in FIG. 1 , one selects and prepares 1 10 a sequestration site. This may be a lengthy and involved process involving many steps and considerations. If the C0 ₂ is being produced with

hydrocarbons, the CO2 might be separated underground and be sequestered without reaching the surface. If the C0 ₂ is being collected at the surface, a determination would be made as to whether there was a place to sequester the C0 ₂ onsite or whether the CO ₂ might have to be transported to another site. If transportation is necessary, the possible sites available would be considered along with their relative advantages and disadvantages.

[00030] Referring again to FIG. 2, CO2 is collected and prepared 115 for storage. How the C0 ₂ is collected and prepared depends in part on the source of the C0 ₂ . The source of the CO2 may be a power plant, refinery, factory or other industrial site or the source of the C0 ₂ may be from hydrocarbon production. The source of the CO2 helps to determine whether the CO2 requires processing, such as removal of moisture or other contaminants before transportation and/or sequestration. The CO2 may be measured 120 as and/or after the C0 ₂ is collected and/or processed. [00031] Referring again to FIG. 2, the process may be different, depending on whether the CO ₂ is separated at the surface 125 and whether the sequestration is co-located with the C0 ₂ production site 130. If the C0 ₂ is not separated at the surface but rather underground as it is produced, for example with hydrocarbons, it might be possible to have the CO ₂ placed and monitored 145 underground. In the more typical case of collecting and separating the C0 ₂ at the surface, then a determination is made as to whether 130 the selected sequestration site is co-located with CO ₂ production site. If the CO ₂ is co-located with the sequestration site the CO ₂ may still need to be

transported a short distance and measured as part of being placed and monitored 145, but if the sequestration site is not co-located with the CO2 production site substantial transportation system 135, such as through a pipeline system, may be necessary.

Measurements 140 may have to be taken at several points to ascertain that there is no CO ₂ leakage occurring or to determine a location of any CO ₂ leakage or simply to determine the amount of C0 ₂ at various points in the process.

[00032] Continuing to refer to FIG. 2, at the sequestration site, the C0 ₂ is placed through one or more injection wells and monitored 145. The injection wells have a final string of casing which has been cemented into place. In accordance with an embodiment of the present invention, the monitoring process includes measurement of one or more electrical properties related to resistivity of the cement used for the final string of casing. The measurement of the one or more electrical properties related to resistivity may be performed one different ways, some of which are further discussed herein. Buffering may be provided to store CO ₂ in case the sequestration site has to be temporarily shut down. As monitored sequestration progresses, if problems are indicated 150, they are evaluated and ameliorated or if possible, completely fixed 155. Monitoring may be used to determine whether the sequestered CO ₂ is stable or whether the sequestered CO ₂ is becoming permanently affixed in the storage formation. Sequestration continues until complete 160. Completion might occur for example, if the source of the C0 ₂

permanently shuts down or if the storage formation cannot accept any more CO ₂ . The sequestration site would then be properly decommissioned 65 and post- decommissioning monitoring 170 might begin. If the post-decommissioning monitoring detects 175 a problem, the problem situation may have to be improved or fixed 180. [00033] In an embodiment of the present invention, the injection well is drilled using conventional methods. FIG. 3 is a simplified depiction of a well (not to scale). The well is begun by having conductor casing 210 placed in the ground. Sometimes the well is drilled to the depth selected for conductor casing 210 and sometimes conductor casing 210 is pounded into the ground. Conductor casing 210 is placed to provide support for the injection well and is often placed at shallow depths, such as 18 to 35 meters or so. Using a bit smaller than the conductor casing 210, drilling of the well continues, usually through aquifers 215 until a predetermined depth for surface casing 220 is reached. The hole that is created while the well is being drilled is called the borehole 225. Surface casing 220 is set and cemented 235 into place, with the cement being pumped down the inside of the casing, through a casing shoe 265 on the bottom of the surface casing and then filling in the space ("annulus") between the borehole 225 and outside surface of the surface casing 220 wall. Generally, for the surface casing, sufficient cement is used to bring the cement all the way back to the surface 200. The surface casing is designed to provide support for further drilling of the injection well and to protect for example the aquifers 215, preferably all aquifers 215 the injection well would traverse.

[00034] Continuing to refer to FIG. 3, using a bit smaller than the inside diameter of the

surface casing, drilling of the well continues. Sometimes setting and cementing one or more intermediate casings 230 may be necessary, depending on the formations traversed by the injection well, their anticipated pressures and other hazards such as swelling clays and the total depth 270 of the injection well. If an intermediate casing is set, the bits used to drill below the intermediate casing are smaller than the internal diameter of the intermediate casing. In this case, the well is drilled through a cap rock 245, which forms a barrier to CO2 migration, and through a formation to be used for CO ₂ sequestration 255 to total depth 270.

[00035] At total depth 270, after evaluating the formations traversed by the injection well, a final casing 240 is set and cemented into place. In FIG. 3, the total depth 270 is deeper than the formation to be used for sequestration 255, shown along with its cap rock 245. After the cement 235 is put into place and sets properly, the final casing 240 is perforated, so that perforations 260 provide access for CO ₂ injection from the inside of the final casing of the injection well through the cement and into the formation to be used for C0 ₂ sequestration 255. Although a full string of final casing extending from the total depth of the well to the surface is depicted in FIG. 3, sometimes liners that extend from the total depth only up to a hanger set near the bottom of in the last string of intermediate casing may be used, as is well known in the art. Similarly, although one particular well configuration is depicted in FIG. 3, many other configurations are possible as is well known in the art, including but not limited to different types of directional wells, horizontal wells, multiple completions of various kinds.

[00036] As mentioned earlier, the present invention is applicable to all cementing operations downhole; a type of cement commonly used for cementing for example casings in place is Portland cement. As Portland cement is exposed to C0 ₂ , the Portland cement generally becomes more porous and may allow the CO ₂ to leak through the Portland cement and migrate to the surface. Another situation might be a carbonation identified below a cap rock; this would usually be an indication of the formation of a micro-annulus allowing C0 ₂ circulation along the well. In the context of the present invention, by cap rock, it is to be understood an impermeable layer which is above the reservoir where C0 ₂ will be stored. In the petroleum industry, caprock is generally referred to as any not permeable formation that may trap oil, gas or water, preventing it from migrating to the surface. This caprock or trap can create a reservoir of oil, gas or water beneath it and is a primary target for the petroleum industry.

[00037] Measurements of the resistivity of the cement sheath in situ, according to the

present invention, would allow detecting changes in cement integrity when it reacts with carbon dioxide. In fact, the variation of the real part of cement's conductivity (which is the inverse of resistivity) is indicating any changes in the cement sheath during it carbonation and/or dissolution process with C0 ₂ . The cement resistivity measurement would allow an "early warning" of C0 ₂ leakage through the monitoring of the

degradation of the cement behind casing during the life of a well but also after its abandonment.

[00038] Different methods for measuring the resistivity of the cement sheath below or above the cap rock, might be used by the one skilled in the art; examples are cased holed measurements (through the casing) or resistivity in cement involving instrumented casing or sensors outside casing. [00039] The electrical resistivity of sound Portland cement is generally around 10-20 Ohm.m. The evolution of cement resistivity after its exposure to C0 ₂ fluids (e.g. wet supercritical C0 ₂ fluid or CO2 dissolved in water or in brine) has been studied and basically it evolves and increases significantly with the carbonation rate depending to the CO2 exposure duration. The inverse of cement resistivity being cement conductivity, it is thus possible to measure the ability of cement to conduct electricity. Mathematically speaking, conductivity calculations may include the square root of a negative number, which means conductivity may have a real part and an imaginary part. Analysis of the imaginary part of cement's conductivity shows that frequency dependency decreases in direct proportion along with cement's carbonation. This change is correlated to porosity evolution of cement when it reacts with carbon dioxide present or stored in the reservoir. In other words, one can tell whether cement is becoming damaged by the C0 ₂ , if one can measure changes in the cements electrical properties, like resistivity and/or conductivity.

[00040] Porosity of a Portland cement after C02-attack at several durations (0 hour, 44 hours, 1 , 3, 6 weeks and 6 months) in wet supercritical C0 ₂ and in C0 ₂ dissolved in water fluids have been determined through mercury intrusion porosimetry, at 90°C under 280 bars. Results are reproducible, and the trend observed is similar to the one obtained by back-scattered electron imaging. In both fluids, it has been shown that, in a first period, which lasts about 3 weeks, the initial porosity (35%) of Portland cement decreases up to 9% in C0 ₂ dissolved in water and 17% in wet supercritical C0 ₂ , before starting to increase after 6 weeks (14 % and 20% respectively) . This is due to carbonation, followed by a dissolution process. The threshold diameter, which is correlated to the permeability, tends to show the same behaviour: an opening of the pore entrance. These porosity results confirm the same trend as that observed with other physical parameters such as weight, density, pH, compressive strength, chemical analysis, and/or microstructural characterisations, i.e. that Portland cement continuously evolves during the carbonation process. It is noteworthy that pH of interstitial water strongly decreases from 13 initially to 6 during the carbonation process. It has to be noted that Portland cement, being widely used in the oilfield industry, is predominantly cited in the present application but the present invention is also applicable to C0 ₂ resistant cement such as Evercrete™ available from Schlumberger.

[00041] The pH of water in equilibrium with Portland cement cores initially equals to 13.

After C0 ₂ attack, core samples are stored in water and the pH is measured at equilibrium. After each CO2 attack, the new equilibrium pH is around 7 up to 3 weeks and decreases again up to 6 after six weeks and 3 months of experiment. Such CO2 attack-related decrease of pH has already been reported in the literature (Van Gerven et al., 2004a; 2004b). This decrease results from the reaction between C0 ₂ and calcium from the calcium silicate phases or portlandite coming from Portland cement hydration.

[00042] Fig. 4 is a graph depicting evolution of Portland cement's resistivity with time in C0 ₂ fluids at 90 °C temperature and 280 bars pressure. Fig. 4 indicates the resistivity of the cement exposed to C0 ₂ increasing over time, from about 20 ohm-meters at the beginning of the experiment to about 100 ohm-meters after about 3 weeks and to about 1 000 ohm-meters after more than 150 days, in the experiment. In a Portland cement, as for any porous medium, the porosity variation and the composition of the interstitial water inside this porosity will affect its resistivity. The measurements of electrical resistivity of the cements included the use of the Impedance Phase/Gain Analyser SOLARTRON™ 1260 which have been used in the frequency band from 0.1 Hz to 0.3 MHz. As for conventional method of electrical impedance measurement, a 2-electrodes scheme was implemented to provide a homogeneous electric field in cement samples to be tested. Before measurements, each device had been calibrated to the open and closed electrical circuits, the resistance and capacitance of the measuring cell had been automatically compensated from the measured values. The electrical resistivity had been obtained from electrical impedance measurements Re[Zp] and lm[Zp] and samples'geometrical factor. A Portland cement with 35% porosity has a low resistivity around 20 Ohms.m [measured at a frequency of 40 kHz]. As the porosity decreases with a factor of 4 after 3 weeks (21 days) in CO2 dissolved in water, a high variation of resistivity occurs (Fig. 4): the resistivity increases from 20 Ohms.m to 100 Ohms.m after 3 weeks and to 1000 Ohms.m after 6 months (168 days). In other words, as cement started to react with C0 ₂ , the ability of the cement to conduct electricity is also damaged, with resistivity increasing and conductivity decreasing. Thus a cement resistivity measurement or conductivity measurement provides a method of "early warning" for the monitoring of C0 ₂ leaks through the monitoring of the degradation of the cement behind casing (sometimes called a cement sheath) during the life of the well but also after its abandonment. By monitoring the resistivity of cement sheath during the life of the well and even after its abandonment, cement degradation under carbon dioxide may be tracked over with time, yielding a good indication of the level of the carbonation.

[00043] A method to calculate the resistivity of the cement according to the present

invention is described below:

Impedance spectroscopy (IS) is a relatively new and powerful method of characterizing many of the electrical properties of materials and their interfaces using electronically conducting electrodes. The impedance is a complex number that can be written as follows:

Z(co) = Z' + j Z"

with Z' being the real part of Ζ(ω), Re(Z) and Z" the imaginary part of Ζ(ω), lm(Z). The modulus of the impedance equals:

The conductivity spectrum can be obtained from impedance spectrum by using the following equation:

with σ being the con the absolute value of impedance, I the spacing between electrodes (meters) and A _e the electrode crossectional area (in meters squared, m ² ).

[00044] As conductivity is the inverse of resistivity, at each frequency o, the resistivity in Ohm-meters is calculated from the conductivity spectrum. Moreover, the complex dielectric constant ε is also derived from impedance spectrum measurements, with the following equation: 1

ε =

ϊωΖ{ώ)ε ₀ Α _ε Ι

with ε ₀ being the permittivity of free space (= 8,884.10 ^"12 F/m).

[00045] Fig. 5 is a graph plotting the imaginary part of conductivity of a Portland cement before and after CO2 exposure, both to wet supercritical CO2 and to CO2 saturated water. Fig. 5 indicates a decrease in the imaginary part of the conductivity with increased times of C0 ₂ exposure. Moreover, the frequency dependence of the

"imaginaire" part of conductivity (which is the inverse of resistivity) disappears after carbon dioxide exposure: this signal can be also used as an indicator of the cement reaction with carbon dioxide.

[00046] Different methods are available to measure electrical properties of the cement

placed behind or inside the casing, below or above the caprock, in accordance with embodiments of the invention. The measurement may be of resistivity, conductivity or any other property of the cement from which resistivity or conductivity may be derived. The measurements may be made once or repeatedly over time, as a time lapse monitoring. Measurements may include, for example, cased-holed measurements (from the inside of the casing) which might be performed using cased-hole logging tools, or the measurements could be made using instrumented casing or sensors placed outside the casing. With the logging tools, a log may be produced and might evidence, for example cement resistivity of the cement sheath as a function of depth. While casing can interfere with resistivity measurements, resistivity of the cement sheath could be determined, behind the casing.

[00047] Sensors on the outside of the casing could be placed at one or at more than one different depths for measurements or placed in one or more configurations around the casing for measurements of the cement sheath in different directions. Useful set up can be found in US Patent No. 5,642,051 , particularly Fig 2A of that patent and the discussion of Fig. 2A therein. The sensors on the outside of the casing could have electrodes as described in US Patent No. 5,642,051 and instead of measuring properties of the fluids in a reservoir, could measure resistivity of the casing sheath. Signals could be sent from the sensors on the outside of the casing to the surface.

When the measurements indicate cement degradation, methods to repair the cement or other proactive measures may be used to reduce or eliminate the chance of CO2 migration towards the surface.

] Although the foregoing is provided for purposes of illustrating, explaining and describing certain embodiments of the invention in particular detail, modifications and adaptations to the described methods, systems and other embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of the invention.