Observation of Cement / Sandstone Interface after Reaction with Supercritical CO 2 Using SEM-EDS , μ-XRD , and μ-Raman Spectroscopy

We observed cement/sandstone interfaces after reaction with supercritical CO2 using SEM-EDS, μ-XRD, and μ-Raman spectroscopy in order to evaluate chemical reactions of well-cement in CO2 storage site. To model actual well in geological formation, we prepared a well composite sample consisting of steel casing, Portland cement, and Berea sandstone. The batch experiment was performed for 56 days under a condition of 10 MPa and 50◦C. After the batch experiment, a carbonation zone appeared at the cement/sandstone interface, however, the carbonation depth was limited within a few millimeters and the inner part of the cement was not altered. The Ca concentration in the carbonation zone increased 13% in comparison to that in the unaltered cement zone while the Mg, Si, and S concentrations decreased significantly. The predominant crystalline phases in the carbonation zone were calcite, aragonite, and vaterite. In addition, sparse precipitation of CaCO3 was observed in the pore spaces of the sandstone along the cement/sandstone interface. [DOI: 10.1380/ejssnt.2016.198]


INTRODUCTION
Underground CO 2 storage in porous and permeable reservoir rocks is seen as one of the most feasible options for reducing CO 2 emissions from a large-scale point source [1].In a CO 2 geological storage, when a well is drilled for CO 2 injection or observation, a steel casing is inserted into the hole, and usually (Portland) cement is poured along a portion of the annular space between the casing and the formation for the purpose of bonding and supporting the casing, and protecting it from corrosion.After the CO 2 is injected in the subsurface, the well cement is exposed to corrosive CO 2 -saturated fluid.Moreover, the formation temperature and pressure at CO 2 injection depths are enough to maintain the supercritical state having a dissolving power comparable to that of liquids, and having transport properties intermediate between gases and liquids.In order to ensure the long-term integrity of the wells in CO 2 sequestration, it is essential to evaluate chemical reaction of the cement due to its exposure to supercritical CO 2 .
Ordinary Portland cement (hydrated) paste is a highly alkaline material of more than pH12 because the hydrated cement paste contains highly concentrated calcium hydroxide (portlandite; Ca(OH) 2 ).This alkaline environment of the cement provides the capability of protecting the steel casing against acid corrosion.In contrast, it is well known that ordinary Portland cement is thermodynamically unstable in CO 2 -rich environments [2].When Portland cement is exposed to CO 2 , portlandite is carbonated to form calcium carbonate (CaCO 3 ) at the cement surface by the following chemical reaction: This chemical reaction is called cement carbonation.It is well known that significant cement carbonation per- mits a harmful effect on cement durability because CO 2 in solution is a corrosive fluid [3].On the other hand, precipitation of the calcium carbonate at the cement surface provides a protection effect to further CO 2 attack [4][5][6].
We have studied a chemical interaction between well cement and supercritical CO 2 by laboratory experiments [7,8].In this paper, we focused on the chemical reaction at the cement/sandstone interface with supercritical CO 2 based on SEM-EDS, µ-XRD, and µ-Raman spectroscopy.To simulate the actual wellbore system, we prepared well composite samples consisting of steel casing, Portland cement, and Berea sandstone.Using the samples, batch experiments were conducted under a reservoir condition.ameter carbon steel casing (J55) was embedded and fixed with Portland cement corresponding to API (American Petroleum Institute) 10A [9] Class A (water/cement ratio: 0.46) in a hole, 13 mm in diameter and 40 mm in depth, located in the centre of the sandstone sample.The sandstone type that was selected was Berea sandstone, which is quartz-rich sandstone with a homogeneous pore structure.

B. Batch experiment
A batch experiment of the samples with CO 2 was performed at 50 • C and 10 MPa of CO 2 pressure corresponding roughly to the conditions around 1 km below the ground surface.Figure 2 is a schematic diagram of the experimental setup.Two well composite samples set in the pressure vessel (63.5 mm in inner diameter and 200 mm in depth) were filled approximately half-full with 0.5 M NaCl solution (brine).Subsequently, the vessel was vacuumed to remove any remaining air.Then, CO 2 was injected into the vessel by means of a high-pressure syringe pump (Model 500D, Teledyne ISCO Inc., USA), and exposed to the samples for 56 days under the conditions of 50 • C and 10 MPa in which CO 2 is in a supercritical state.During the batch experiment, the upper sample was exposed to a water-saturated supercritical CO 2 (called wet-CO 2 ) condition while the bottom sample was exposed to a CO 2saturated brine condition.After the batch experiment, the CO 2 -exposed samples were longitudinally halved and polished the cross section with 6000 mesh SiC abrasive powder for elemental and crystalline phase analysis by SEM-EDS, µ-XRD, and µ-Raman spectroscopy.

C. Apparatus
Observation and elemental analysis of the CO 2 -exposed samples were performed: using a SEM (VE-9800, Keyence Corporation) with an energy dispersive X-ray spectrometry (EDS; OXFORD INCA energy) system equipped with a Si(Li) semiconductor detector [PENTA FETx3 (Oxford Instruments), sensitive area: 30 mm 2 , energy resolution: < 133 eV@MnKα].The SEM observation and EDS analysis were operated at an acceleration voltage of 20 kV and a beam current of approximately 0.3 nA with a working distance of 29 mm.The quantitative procedure of the XPP matrix correction method was used in the EDS system [10].An X-ray diffractometer (SmartLab, Rigaku Corporation) equipped with a 9 kW rotating anode (Cu) X-ray generator (operated at 45 kV and 200 mA) and a high speed 1D detector (D/teX ultra250 1D detector) was used for crystalline phase analysis in a micro area of the reacted samples.For the nondestructive µ-XRD analysis, a polycapillary X-ray focusing optics (called a CBO-f unit) and a parabolic multilayer mirror optics (called a CBO unit) was incorporated with the XRD spectrometer.These units enable focusing the brilliant X-rays to ϕ0.4 mm at the focal point on the sample surface.Raman spectra were obtained on a laser confocal µ-Raman spectrometer (LabRAM HR Evolution, HORIBA Ltd.) equipped with a Nd:YAG laser emitting 532 nm radiation at the sample and a Peltier-cooled CCD detector (1024 × 256 pixels).The spatial resolution of the laser focal point was 0.5 µm(X) × 0.5 µm(Y) × 1.5 µm(Z); the spectral resolution was better than 0.2 cm −1 .

III. RESULTS AND DISCUSSIONS
A. Observation of the well composite samples after reaction with CO2

B. SEM-EDS analysis at the cement/sandstone interface
Figure 3 shows optical microscope images at cement/sandstone interface of the well composite samples after the reaction under the wet-CO 2 condition (Fig. 3(a)) and the CO 2 -saturated brine condition (Fig. 3(b)).An orange-brown zone was clearly observed on the surface of the cement at the cement/sandstone interface under both wet-CO 2 and CO 2 -saturated brine conditions.The orange-brown colored zone was calcium carbonate (CaCO 3 ) formed by the CO 2 reaction as will be describe in the µ-XRD and µ-Raman analysis.A white zone was observed in the intermediate cement under the CO 2saturated brine condition (Fig. 3(b)).We defined the orange-brown zone as a "carbonation zone".
The depth of the carbonation zone exposed to the wet-CO 2 condition was larger than that exposed to the CO 2 -saturated brine condition.However, the carbonation depth under the wet-CO 2 condition was limited within a few millimeters and the inner part of the cement was not altered.This indicates that the sandstone surrounding the cement serves as a buffer against a direct CO 2 attack.In addition, the steel casing was remained in nearly excellent condition and showed few signs of corrosion.Based on these results the predicted 30-year carbonation depth evaluated by logarithmic approximation was estimated at 4.5 mm for the wet-CO 2 condition and 0.76 mm for the CO 2 -saturated brine condition in the result of our other study [8].
Figure 4 shows the SEM-EDS maps of Ca, Na, Mg, Cl, and S at the cement/sandstone interface of the well composite samples after reaction under the wet-CO 2 condition (a) and the CO 2 -saturated brine condition (b) and the non-reacted sample (c).The X-ray collection time was 150 ms per pixel and the total measurement (live) time was 2.2 h.As shown in Fig. 4, we found that the Ca and Na maps obviously differed from the other elemental maps, Ca concentrated in the carbonation zone in comparison to that in the unaltered cement while Mg, Cl, and S in the carbonation zone were poor.Na highly concentrated at the outer cement surface rather than in the carbonation zone.Figure 5 shows the SEM-EDS line profiles of CaO, Na 2 O, MgO, Cl, and SO 3 at the cement/sandstone interface of the well composite samples (white arrow lines shown in Fig. 4(b) and (c)) after the reaction under the CO 2 -saturated brine condition (black square in Fig. 5), and overlaps the results of the nonreacted sample (white square in Fig. 5).Each plot in Fig. 5 consists of extracted data in the 60 µm times 60 µm area from all the EDS map data in Fig. 4. From the results of the line analysis, it was found that the CaO concentration in the carbonation zone increased by 13% in comparison to that in the unaltered cement zone while the concentrations of MgO, Cl, and SO 3 in the carbonation zone decreased significantly.The concentration of Na was increased drastically at the at the cement/sandstone interface.In addition, the concentrations of Na and Cl in the unaltered zone with CO 2 -exposed sample were higher Focusing on the sandstone side, it was observed that small points rich in Ca distributed in the pore spaces of the sandstone as shown in Fig. 4 and Fig. 6.This suggests that the precipitation of fine CaCO 3 crystal is as result of Ca 2+ diffusion out of the cement coupled with inward diffusion of carbonate ion.

C. µ-XRD and µ-Raman analysis at the cement/sandstone interface
Figure 7 shows the XRD patterns at the carbonation zone of the well composite samples after reaction under the wet-CO 2 condition (a) and the CO 2 -saturated brine condition (b).The crystalline phases in the carbonation zone were predominantly CaCO 3 and traces of Friedel's salt (Ca which is formed by the reaction of chlorides with the C3S (monosulfate: Three polymorphs of CaCO 3 (calcite, aragonite, and vaterite) were identified in the carbonation zone under both conditions.Aragonite and vaterite are metastable forms relative to calcite, in particular, vaterite is very rare in natural environments, but vaterite creation during the cement carbonation process has often been reported in other studies [6,[11][12][13][14].To confirm more detailed polymorphs of CaCO 3 in the carbonation zone, µ-Raman analysis was performed.Figure 8 shows a Raman mapping image of calcite and aragonite in the carbonation zone under the CO 2 -saturated brine condition.Unfortunately, Raman spectra derived from vaterite (e.g., 752 cm −1 , ν 4 in-plane bending mode) were not detected.As shown in Fig. 8, an interesting distribution of calcite and aragonite was observed in the carbonation zone.Calcite was dominant at  the side of the cement/sandstone interface, whereas aragonite was dominant at the side of the inner (unaltered) cement.The polymorphic crystallization of CaCO 3 is an effect of the inorganic ions and pH condition in the reaction field.Magnesium ions in the mother solution favor aragonite formation very strongly while NaCl inhibited aragonite formation [15].This result is consistent with results of the elemental mapping and line analysis by SEM-EDS, as shown in Fig. 4 and Fig. 5.A high pH condition in cement favors the formation of vaterite and calcite.In addition, vaterite is preferentially formed from the C-S-H (calcium silicate hydrate) phase in cement by the following reaction, while calcite and aragonite are created from portlandite [13][14][15][16].

IV. CONCLUSIONS
To investigate chemical reactions of Portland cement with supercritical CO 2 along the cement/sandstone interface of well-cement in CO 2 storage site, we performed a batch experiment using a well composite sample consisting of casing, Portland cement, and Berea sandstone.We initially expected that cement durability was significantly increased by the harmful cement carbonation due to CO 2 attack.However, the cement carbonation of the well composite sample was limited at the cement/sandstone interface and the inner part of the cement and steel casing were not altered.This result was indicated that precipitation of dense CaCO 3 at the cement surface increases its compressive strength and provides an effective barrier to further CO 2 attack.
Crystalline phases in the carbonation zone were calcite, aragonite, vaterite.Vaterite is preferentially formed from the C-S-H phase in cement.In the cement carbonation, it is assumed that carbonation of C-S-H phase is occurred after the carbonation of portlandite [16].Therefore, vaterite would be potentially key material for evaluation of the cement degradation.

Figure 1 FIG. 2 .
Figure1shows the prepared cylindrical well composite sample, 30 mm in diameter and 50 mm in length.To model an actucal well in geological formation, a 4 mm di-

FIG. 3 .
FIG. 3. Optical microscope images at the cement/sandstone interface of well composite sample exposed to the wet-CO2 for 56 days (a) and the CO2 saturated brine for 56 days (b).

FIG. 4 .
FIG. 4. SEM-EDS maps of Ca, Na, Mg, Cl and S in the cement/sandstone interface of well composite sample.(a) After reaction under the wet-CO2 condition for 56 days; (b) after reaction under the CO2 saturated brine condition for 56 days; (c) non-reacted sample.

FIG. 8 .
FIG. 8. (a) Overlay of a Raman mapping on a reflected light microscopy image through the calcite (blue) and aragonite (red) polymorph at the carbonation zone under the CO2 saturated brine condition.(b) Raman spectra at the red area (aragonite) and the blue area (calcite).