SPE 139713

The Effect of CO2-Saturated Brine on the Conductivity of Wellbore-Cement Fractures Tevfik Yalcinkaya, SPE, Mileva Radonjic, Richard G. Hughes, SPE, Clinton S.Willson, Kyungmin Ham, Louisiana State University

Copyright 2010, Society of Petroleum Engineers This paper was prepared for presentation at the SPE International Conference on CO2 Capture, Storage, and Utilization held in New Orleans, Louisiana, USA, 1012 November 2010. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract The efficiency of Carbon Capture and Storage projects is directly related to the long term sealing ability of cemented sections

in wellbores penetrating CO2 storage reservoirs. The microfractures inside the wellbore cement and/or microannulus are

possible pathways for CO2 leakage to the surface and/or fresh water aquifers and jeopardize safe and long-term containment

of CO2 in the subsurface.

This paper presents an experimental study which investigates the changes inside the cement internal structure when exposed

to acidic brine through an artificial fracture. A 30-day long flow through experiment was conducted using a 1 in by 12 in.

cement core and CO2 saturated brine, as a permeant, at a flow rate of 2 ml/min in a core flooding apparatus with10 psi and

600 psi of injection and net overburden pressures (Low Pressure-LP experiment). The same experiment was repeated with

1800 psi and 600 psi of injection and net overburden pressures for 10 days in order to account for the effects of pressure on

the degradation process of cement (High Pressure-HP experiment). High-resolution X-ray computed tomography was used to

image several subvolumes extracted from the flow-through cores. The images were processed and thresholded, followed by

calculation of porosity. Total porosity was observed to decrease from 26% to 22% after 30 days of exposure of LP

experiment. The HP experiment did not cause any significant change in total porosity possibly due to the short duration of the

experiment.

Introduction At the current state, Carbon Capture and Storage (CCS) technology offers a feasible solution to mitigate the problem of

increasing atmospheric CO2. Risk assessment of a CO2 storage project requires an evaluation of the integrity of the wellbore

network to be analyzed against the possible leakage scenarios over extended time scales (Bachu, 2009). An understanding

about the behavior of microannulus or cement fractures under sequestration conditions is essential in order to analyze the

integrity of existing wellbores and their extended service life under conditions for which they may not have been designed.

Wellbore cement (pore fluid pH~13.5) will be in contact with acidic brine (pH ~3-5) in the post injection period, where the

incompatibility arises from contact of two systems with widely different pH values. Hence, the cement behavior under

dynamic CCS conditions needs to be investigated before implementing large scale projects. This was the main motivation for

this experimental study.

Wellbore Cement Oil and gas (O&G) wells are cased and cemented in order to provide zonal isolation, structural support for the wellbore and

protection of casing against corrosive fluids such as CO2 and H2S rich brines. Maintenance of zonal isolation is the most

important function of wellbore cement in CCS projects because it prevents both horizontal and vertical hydraulic

conductivity within and into the wellbore. Nelson (2006) reports that some 11,000 casing strings in over 22,000 oil and gas

wells in the Gulf of Mexico have been reported with sustained casing pressure which is an indication of inadequate zonal

isolation. Most oil and gas wells are designed for 30-50 years of service life; however, CCS projects will require wellbore

cements to maintain zonal isolation function for hundreds of years. The cause of wellbore leaking is frequently related to

cement failure, which can be due to inadequate cement design or ineffective placement of cement (procedures, equipment,

and technical inadequacy).

2 SPE 139713

Wellbore Cement Failure Wellbore cement failures can occur during and after cementing operations. During cementing operations, insufficient mud

removal and improper cement placement may finally lead to weak bonding at cement/casing and/or cement/formation

interfaces and eventually gas channeling within the cement column. After the cement slurry has been set, it will be subject to

varying loads due to repeated pressure and temperature cycles caused by field operations (Ravi, 2004). Since the thermal

expansion and elasticity coefficients are different for casing and cement, the casing and cement expand and contract at

different rates which increases the possibility of forming a microannulus between the casing and the cement and may also

cause fracture initiation and propagation within the cement. In this study, the main focus is the interaction between CO2 rich

brine and a single fracture within the cement sheath.

Cement Chemistry The main constituents of Portland cement are calcium and silica. It has four major crystalline products (Nelson, 1999);

Tricalcium Silicate (Ca3SiO5), also known as Dicalcium Silicate (Ca2SiO4), Tricalcium Aluminate (Ca3Al2O6), Tetracalcium

Aluminoferrite (Ca4Al2Fe2O10). The silicate phases comprise more than 80% of the Portland cement. After complete hydration of

cement (hydration starts when cement comes in contact with water), two main products are formed; Calcium Silicate Hydrate

(C-S-H) which constitutes 70% of the hydrated cement and is the main binding material, and Portlandite (Ca(OH)2) which

occupies 15- 20 % of the volume after hydration. There are also other minor minerals formed as a result of hydration such as

Ettringite ((CaO)6(Al2O3)(SO3)3.32 H2O). During preparation of the cement slurry, air bubbles can be trapped and can lower

cement strength.

Chemical Reactions between Cement and CO2 Saturated Brine Portland cement consists of Tricalcium Silicate (C3S), Dicalcium Silicate (C2S), Tetracalcium Aluminoferrite (C4AF)

Tricalcium aluminate (C3A). Hydrated Portland cement consists mainly of calcium silicate hydrate (C-S-H), as the main

binding material, Portlandite (CH), unhydrated cement minerals and, in the case of a well cement slurry, a high proportion of

pore water. When hydrated cement is exposed to CO2 saturated brine, a series of chemical reactions take place; i.e., (adapted

from Lea`s Cement Chemistry Book, 2004)

(1)

(2)

(3) (4)

When all of the Portlandite/calcite are depleted, the next Ca2+

source, C-S-H will start leaching Ca2+

and consequently be

converted into an amorphous structure, resulting in soft cement that has no mechanical integrity (Brandl, 2010).

Methodology Brine (2% salt solution) was prepared using distilled water and NaCl and KCl salts. A CO2 saturated brine solution was

prepared by bubbling CO2 through the brine solution. The pH of brine solution ranged from 4.9-5.2, to simulate potential post

injection composition of the brine, which could be buffered by dissolved minerals. The experimental set-up consisted of a

Hassler cell, syringe pump, hydraulic pump, data acquisition system, filters and pressure gauges which were installed in

several locations in the flow stream (Figure 1). The Hassler cell was mounted vertically on a stand position to mimic the

upward flow of CO2 rich brine through the fractured vertical wellbore cement column.

Cement cores (1 in 12 in) were prepared from class-H cement with water to cement ratio 0.38 according to API RP-10B

specifications. The cement slurry was then degassed using a vacuum pump and poured into molds (Figure 2a). The cement

halves were allowed to set for 24 hours. The cement core halves were cured by soaking for 6 months for LP experiment and

120 days for HP experiment in a tap water bath. The cement halves were glued using epoxy along the edges to obtain a 1 in

12 in cement core (Figure 2b).

A low pressure set of experiments were conducted at atmospheric conditions and pressure transducers (0-50 psi) were

selected to ensure the detection of small pressure changes. Pressure transducers capable of measuring 0-5000 psi were used in

a similar set of experiments conducted at higher pressures (1800 psi). The pressures along the core including the injection and

confining stresses were recorded using software written at the Louisiana State University (LSU) Petroleum Engineering

Research and Technical Training Laboratory.

A dual syringe pump system with a 507 ml maximum capacity was utilized and set to single pump auto-refill mode to

provide continuous flow during the experimental period, approximately 24 hours. A nitrogen-charged back pressure regulator

was utilized to achieve higher injection pressures.

SPE 139713 3

Figure 1. Schematics of experimental set-up

Figure 2. Cement preparation mold (a) and cement core (1 in. x 12 in.) (b)

Figure 3. Cement core halves assembled to mimic single fracture (a) locations of samples prepared for different analytical techniques (b,c)

After CT scan data acquisition each core was dissected as shown in Figure 3, in order to have systematic information

gathered from each section of the core. This was critical since all characterization techniques were of destructive nature and

samples could not be reused, meaning that data sets were from different locations within the core. In order to understand

these processes, various material characterization techniques that complement each other were used. Before starting the

experiments, the cores were scanned using X-Ray Computed Tomography (CT) as a control. Effluent brine (brine after

flowing through the fracture) was analyzed using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES).

After completing the experimental runs, reacted cores were analyzed using X-Ray Diffraction (XRD), Environmental

Scanning Electron Microscopy (ESEM), Mercury Intrusion Porosimetry (MIP), CT and High Resolution CT (micro-CT).

Micro-CT (High

Resolution CT)

MIP analysis

ESEM

Micro-CT (Low

Resolution CT)

Fracture

Surface

3 mm 5mm cement core

Section 1-2

Section7- 8

Inlet (Section 7-8)

4 SPE 139713

Results and Discussion

X-Ray Computed Tomography (CT) Non-destructive CT-scans (Figure 4) were acquired prior to the flow experiments (Figure 4a and 4b) and following the flow

experiments (Figure 4c and 4d). The characterization was performed on as-received samples immediately after cement/fluid

contact, thereby minimizing sample alteration. Cement samples were scanned using a Picker PQ5000 CT scanner. The spatial resolution was 0.25 mm and the slice thicknesses were 2 mm. The cement core was scanned at 8 different locations along the

core from bottom to top (top refers to outlet and bottom refers to inlet) at 140KV.

a

b

c d

Figure 4. CT scans of the unreacted (a,b) and reacted cement (c,d) showing the dissolution along the fracture walls (shown in darker color due to mass reduction)

Visual image analysis of CT scans revealed that there were density variations within reacted cement matrix along 12inch

core. Most notably the density difference was observed between inlet and outlet section, see Figure 4c, 4d. In addition section

labeled 4 on the CT scan (Fig 4c) suggested a presence of higher density material, most likely due to the deposition of

secondary calcite, also supported by microstructural analysis reported in section on ESEM. Based on qualitative image

analysis, sections of the cement core were then selected for further characterization. Thin slices near the outlet (1-2), the inlet

(7-8) and two sections in the main core body (3-4 and 5-6) were cut using a diamond saw for microstructural investigations.

Outlet

Inlet

Outlet

Inlet

SPE 139713 5

High Resolution Imaging High-resolution X-ray tomography images were collected at the Louisiana State University Center for Advanced

Microstructures & Devices (CAMD) Tomography beamline. Cement cores (3 mm diameter by 5 mm length), were drilled

from reacted cores from LP experiment (Figure 5), and imaged at 2.5 m spatial resolution at 34 keV monochromatic X-ray

energy. Reconstruction of the 1522 projection images (0.25 angle increment ) or 766 projection images (0.5 angle

increment) between 0 to 180 was performed using a MatLab program based on filtered back projection and correlated

sampling to reduce most systematic errors.

Figure 5. A small section (2.24 2.25 2.01 mm3) of the

1.5 1.5 mm2 by 5 mm imaged cement core.

Porosity analysis was conducted on this small section.

Porosity Quantification Given the 2.5 micron voxel size of the micro CT

images obtained at CAMD, it is only reasonable to

expect us to be able to identify pore sizes that are 5

microns or larger. Two subvolumes were extracted

from the image volumeone in region A and one in region B. The grayscale images were converted into

binary images (i.e., solid and void) using a simple

thresholding algorithm in ImageJ. Once the two

phases were identified, the porosity was calculated

by simply calculating the volume fraction of each

phase. The porosity values were found to be 0.027

and 0.046 in regions A and B, respectively. For the

particular subvolumes analyzed, that the volume

fraction of void space that is on the order of 5

microns or larger seemed to decrease in region A

(the region in contact with CO2).

A Slice #1 LP_1-2 b Slice# 77 LP_1-2

c Slice #217 LP_1-2 d Slice #297 LP_1-2

d Slice #352 LP_1-2 e Slice #501 LP_1-2

Fracture Surface

B

A

Figure 6. Axial Slices in z-direction showing the internal structure alterations along the 5 mm core, drilled perpendicular to the fracture surface

Inner portion of the core

A B

In previously reported data (Radonjic, 2010) the

similar trend was observed on cement exposed to

acidic brine for 8 weeks, at atmospheric conditions.

Most likely due to the extended time of that

experiment 3 distinct regions were identified in

images scanned at 5.06 micron resolution. The

same procedure as described above was applied

and estimated porosities were 0, 0.005, and 0.02. In

this case, pore sizes that are 10 microns or larger

were identifiable.

B A

B B

B B

A A B

6 SPE 139713

Mercury Intrusion Porosimetry (MIP) The MIP method is one of the standard procedures to establish the pore size distribution of a porous media, in this case

hydrated cement. The MIP can capture connected pores but not disconnected pores and assumes that pore size distribution is

not affected by the drying process, (Abell, 1999).

The number of pores was calculated assuming that all pores are spherical. Given an incremental volume of injected mercury,

the number of pores is the ratio of the incremental volume to the calculated volume of each pore. The designation

Control_LP stands for the unreacted cement core whereas, LP_7-8 stands for the inlet section of the reacted sample at low

pressure.

The high pressure (HP) experiment increased porosity slightly, control sample of 21.26 % to reacted sample with 21.64 %

porosity (Table 1), however this increase may in fact be within the error limits of the measurement tools. On the other hand,

low pressure (LP-five reported sections and the control sample) experiment resulted in reduction of total porosity. There were

four distinct pore size ranges: 30-70 m (large pores), 10-30 m (medium pores), 0.5-10 m (small pores) and 0-0.5 m

(nano pores).

Table 2 tabulates results obtained from the comparison of unreacted and reacted samples for low pressure and high pressure

experiments. The increment or reduction in percentage pore volumes was observed even in the sub-ranges of defined pore

sizes, as shown in Table 2-section on nano-pores. The reduction in the total pore volume of large pores in LP and HP

experiment may be caused by two potential mechanisms: calcite (Ca(CO3)) precipitation inside the pores, causing the actual

pore volume reduction or due to the plugging of pore throats connecting the large pores.

Table 1. Total porosity values derived from MIP

Pore Size Range

(m) Low Pressure Experiment_ Section 7-8

(Inlet for Flow-through fluid)

Percentage of a given pore range over

the total pore volume

Low Pressure Experiment_1-2

(Outlet for Flow-through fluid)

Percentage of a given pore

range over the total pore

volume

High Pressure

Experiment

Percentage of a given

pore range over the

total pore volume

Large Pores

30-70 m

Unreacted>Reacted Reacted >Unreacted Unreacted>Reacted

Medium pores

10-30 m

Reacted>Unreacted Reacted>Unreacted I. Unreacted=Reacted

around 30 m

II. Reacted>Unreacted

around 20 m

III.Unreacted>Reacted

around 10 m

Small pores

0.5-10 m

I. Between 5-10 m, Reacted>Unreacted

II. Reacted>Unreacted

Reacted>Unreacted Unreacted>Reacted

Nano pores

0-0.5 m

I. Between 0.1-0.5 m,

Unreacted>Reacted

II. Between 0.005-0.1 microns,

Unreacted>Reacted

I. Between 0.1-0.5 m,

Unreacted>Reacted

I. Between 0.1-0.5

m,Reacted>Unreacted

II.Unreacted>Reacted,

0.1-0.05 m

Table 2. Comparison of percentage pore volumes of different pore size ranges: Large pores 30-70 m, Medium pores 10-30 m, Small pores 0.5-10 m and Nano pores 0-0.5 m. U refers to unreacted cement sample and R refers to reacted cement sample. I, II and III refer to different sub-ranges within each pore size range.

Sample Identification Total Porosity

Control_LP 26.33%

LP_1-2 20.43%

LP _3-4 22.94%

LP_4-5 22.01%

LP_5-6 20.79%

LP_7-8 20.50%

Control_HP 21.26%

Reacted_HP 21.64%

SPE 139713 7

Figure 7 shows the plot of applied injection pressure versus the cumulative intrusion of mercury. The most dramatic changes

in pore size are observed between 100-2,000 psi for HP experiment, with a substantial porosity increase with acidic brine

exposure. For the LP experiment most evident difference in pore size distribution is observed between 3,000-60,000 psi,

where porosity decreases with exposure to acidic brine. The possible explanation for these contradicting trends in pore

volume of cement matrix affected by CO2 rich brine for HP and LP experiments is simultaneous presence of two reaction

mechanisms: leaching of Ca, causing increase in porosity and secondary carbonation causing reduction in porosity. This data

is in agreement with data published by Van Gerven et al. (2007).

Figure 7.Cumulative Intrusion versus Mercury Injection Pressure

Figure 8a and 8b outlines pores size distribution differences between control samples cured for LP and HP experiments. It is

clear that for pore size range 0-0.075 m HP control sample has higher porosity than LP control sample. However, the next

pore size range 0.075-0.5 m LP cement appears to have higher porosity.

In the next pore range 0.5- 50 m LP continues to have higher porosity than HP. The final measured pore range 50-70 m

both samples have similar porosity.

Figure 8c and 8d shows MIP data for HP experiment, pore volume distribution change due to acidic brine exposure. The first

pore range 0-0.045 m there is no apparent change on porosity. From 0.045-0.07 m pore volume distribution shifts to the

left, effectively pointing to reduction in pore volume. The next range 0.008-0.12 m the reduction is minimal. Pore range

0.12-0.5 m increase in pore volume in reacted sample is more evident. From 0.5 to 3.5 m minimal number of pores was

identified in both samples. Reacted cement has undergone further pore volume reduction in the range of 3.5-20 m. In the

final measured range 20-80 m (apart from few data points) reduction in pore volume is still observed.

Figure 8e and 8f represent MIP data for LP experiment for the inlet section of the core. We primarily observe reduction in

pore volume apart from the pore size range from 0.5-30 m where substantial increase of pore volume is measured.

For LP experiment of the outlet section reduction of the pore volume is only observed in ranges from 0-0.05 m and 0.1-0.5

m. The large pore sizes are more present in reacted samples, therefore result in increased porosity.

8 SPE 139713

Figure8. Pore Size versus its volume percentage in total pore volume for each experiment

a. Pore size range 0-0.5 m for unreacted samples b. Pore size range 5-70 m for unreacted sample

c. Pore size range 0-0.5 m for HP experiment

d. Pore size range 5-70 m for HP experiment

e. Pore size range 0-0.5 m for LP_7-8 f. Pore size range 5-70 m for LP_7-8

g. Pore size range 0-0.5 m for LP_1-2 h.Pore size range 5-70 m for LP_1-2

SPE 139713 9

Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) Effluent brine samples were analyzed using ICP-OES in order to verify Ca

2+ was leaching from the cement during acidic

brine exposure (Figure 9). This method is also called as ICP-AES (Atomic Emission Spectroscopy). In these measurements,

excited electrons emit energy at a given wavelength as they return to ground state after excitation by high temperature Argon

Plasma. The fundamental characteristic of this process is that each element emits energy at specific wavelengths peculiar to

its atomic character. The intensity of the energy emitted at the chosen wavelength is proportional to the amount

(concentration) of that element in the sample being analyzed. Thus, by determining which wavelengths are emitted by a

sample and by determining their intensities, elements can be found qualitatively and quantitatively from the given sample

relative to a reference standard. Figure 9 shows that Ca2+

leached from cement quickly at the beginning of the experiment.

Later, leaching is reduced probably due to the formation of calcite, which provides an impermeable layer to prevent further

ingress of acidic brine into cement matrix.

Figure 9. ICP-OES analyses for the effluent brine showing Ca2+

leaching from the cement.

Environmental Scanning Electron Microscopy (ESEM) Portions of unreacted and reacted cement core samples were imaged using ESEM for microstructural characterization. ESEM

was deployed to further investigate the nature of altered zones within the cement at a much finer scale and under low vacuum

conditions in order to prevent cement dehydration during the analysis. Energy Dispersive Spectroscopy (EDS) was also

utilized to provide chemical composition of various features within the cement structure.

Figure 10a, c and e represents a sequence of ESEM images from low pressure experiment at the inlet region of the core.

Figure 10a taken from the cross section of the core shows sustainably enlarged fracture aperture and secondary

microfractures across the entire sample. EDS analyses Figure 10b) from this region shows Ca/Si ~2:1. In Figure 10c, image

was taken from the fracture wall exposed to the acidic brine flow and supported by EDS analysis (Figure 10d). This region

has an increased Ca/Si and Fe rich phases present. The final image at much higher magnification shown in Figure 10e reveals

columnar crystal growth on the fracture wall coupled with EDS which suggests Ca rich minerals. Similar observation of Fe

rich deposits in reacted cement was reported by Duguid et al. (2005).

Similar trends were observed for LP outlet region of the cement core shown in Figure 11a, c and e. However, major

difference between inlet and outlet regions is the chemical composition of deposits on the fracture wall (Figure 11e,f). In the

outlet region fracture wall is covered by calcite like minerals. Furthermore, the outlet region appears to have a set of

perpendicular fractures to the main fracture wall that are clearly undergone dissolution and secondary precipitation, as shown

in the central portion of the micrograph in Figure 11c.

10 SPE 139713

Figure10. ESEM micrographs of Low-pressure reacted cement core at the first contact with acidic brine (LP 7-8- INLET)

a Low mag. BSD ESEM image of reacted fracture top view b EDS analysis of the cement not in direct contact with brine

c Micrograph of the fracture wall after splitting the core d EDS of the fracture wall surface reveals lower Ca/Si from above

e High mag ESEM micrograph from fracture wall flow area f EDS from fibrous - Ca rich deposits within fracture wall

SPE 139713 11

a Low mag BSD ESEM image of reacted fracture top view b EDS analysis of the cement not in direct contact w/t brine

c Micrograph of the fractured wall surface secondary fracs. d EDS data confirms Ca-rich deposits within secondary fractures

E High mag ESEM micrograph from fracture wall flow

area

f EDS from highly porous area within fracture wall / Ca-rich plates

Figure11. ESEM micrographs of Low-pressure reacted cement core at the final contact with brine (LP 1-2-OUTLET)

12 SPE 139713

Conclusions

Impact of CO2 saturated brine on well cements under dynamic (flow through) conditions, appeared to have double-sided

effect on porosity of cement matrix. MIP data revealed porosity reduction for small pore size ranges in LP experiment

whereas porosity increase was detected in HP experiment for medium pore size ranges. Micro-CT scans showed porosity

reduction for medium to large pores sizes.

ESEM/EDS revealed different reaction products at the fracture wall surfaces for HP and LP experiments, whose difference is

in the presence/absence of Fe. Furthermore, outlet section from LP experiment showed secondary fracture network developed

perpendicular to main fracture, which aided dissolution of cement and provides space for secondary calcite precipitation.

Other major observation was the impact of time on overall pore size distribution change. LP experiment conducted for 30

days show more evident porosity reduction for small pore size ranges, while 10 days long HP experiment appears to have

increased porosity for large pore size ranges. In terms of long term behavior of wellbore cements under CCS conditions, it is

possible that cements will undergo both dissolution and precipitation of new minerals. Further research is ongoing to quantify

the change in permeability of cement with acidic brine exposure.

References

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Bachu S., Bennion B.D., Experimental Assessment of Brine and/or CO2 Leakage through Well Cements at Reservoir Conditions, International Journal of Greenhouse Gas Control, 3 (4), 494-501, 2009.

Brandl A., Cutler J., Seholm A., Sansil M., Braun G., BJ Services Co., Cementing Solutions for Corrosive Well Environments, SPE 13228 presented at the CPS/SPE International Oil &Gas Conference and Exhibition, 8-10 June, Beijing, China, 2010.

Duguid A., Radonjic M., Scherer G., Degradation of Well Cements Exposed to Carbonated Brine, 4th Annual Conference on Carbon Capture and Sequestration DOE/NETL, May 2-5, 2005.

Hewlett P., Lea F., Lea`s Cement Chemistry of Cement and Concrete, Fourth Edition, Elsevier, 2004.

Nelson B. E., Guillot D., Well Cementing. Second Edition, Schlumberger, 2006.

Radonjic M., Yalcinkaya T., Willson C.,S., Conductivity of Fractures in Oilwell Cement upon Contact with Sequestered CO2, Proceedings (CD) of the 3nd International Conference on Porous Media and its Applications in Science and Engineering, June 20-25,

Italy, 2010.

Ravi K., Bosma M., Gastebled O. Improve the Economics of Oil and Gas Wells by Reducing the Risk of Cement Failure, SPE 84498 presented at the IADC/SPE Drilling Conference, Dallas, Texas, USA, 2002.

Van Gerven T., Cornelis G., Vandoren E., Vandecasteele C., Effects of Carbonation and Leaching on Porosity on Cement-bound waste, Waste Management 27 (977-985), 2007.

Acknowledgements We would like to thank Sultan Anbar and Corey Klibert for their help in interpreting the results. We also thank Stefan Bachu

for fruitful discussions. Also, the help from Barry Newton is greatly appreciated in imaging the cement cores at low

resolution. We would like to thank Craft & Hawkins Department of Petroleum Engineering for funding this project. We also

thank Chevron ETC for allowing us to use cement laboratory facilities. Tomography beamline at CAMD acknowledges the

support of the NSF IMR-0216885 (monochromator) and the State of Louisiana through the CAMD operational budget.