Alain Rivereau

Gwenn Le Saoût

Hélène Zanni

Éric Lécolier

Comptes Rendus Chimie

Preliminary communication / CommunicationStudy of oilwell cements by solid-state NMRGwenn Le Saoût a,b,*, Éric Lécolier a, Alain Rivereau a, Hélène Zanni ba Institut français du pétrole, 1–4, av. de Bois-Préau, 92852 Rueil-Malmaison, Franceb Laboratoire de physique et mécanique des milieux hétérogènes, UMR CNRS 7636, ESPCI,10, rue Vauquelin, 75231 Paris cedex 05, FranceReceived 20 March 2003; accepted 15 October 2003Available online 1 April 2004AbstractThe prime objective of the plug-and-abandon operations is to provide zonal isolation for infinite time. Cement-basedmaterials are generally used as plugging materials. Therefore, it is important to understand physical and chemical processescausing cement degradation in downhole environment. In this study, we have characterised two cement formulations at differentageing conditions using NMR and XRD techniques. In particular, we evidence that an increase in pressure and temperature leadsto more polymerised calcium silicate hydrates (C–S–H). In the low permeability cement samples, it was shown that thepozzolanic activity of silica fumes increases with temperature and pressure and leads to the consummation of all the portlanditereleased during cement hydration. To cite this article: G. Le Saoût et al., C. R. Chimie 7 (2004).© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.RésuméL’objectif d’une opération d’abandon de puits est d’établir une zone d’isolation pour une durée infinie. Les bouchons mis enplace pour fermer les puits sont majoritairement des bouchons de ciment. Il est donc important de comprendre les différentsprocessus physiques et chimiques de la dégradation du ciment dans les conditions de puits. Dans cette étude, nous avonscaractérisé la structure de deux formulations dans différentes conditions de vieillissement, en utilisant les techniques de RMN etde DRX. Nous avons pu montrer que l’augmentation de pression et de température conduisait à une polymérisation plusimportante des silicates de calcium hydratés (C–S–H). Dans le cas des ciments de faible perméabilité, nous avons mis enévidence une accélération de l’activité pouzzolanique de la fumée de silice lorsque la température et la pression augmentent.Pour citer cet article : G. Le Saoût et al., C. R. Chimie 7 (2004).© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.Keywords: Oilwell cements; XRD; NMRMots clés : Ciments pétroliers ; DRX ; RMN* Corresponding author.E-mail address: lesaout@pmmh.espci.fr (G. Le Saoût).C. R. Chimie 7 (2004) 383–388© 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.doi:10.1016/j.crci.2003.10.0181. IntroductionWells have been drilled for different purposes: ex-ploration, disposal, oil and gas production... All thesewells will be abandoned and will require plugging. Thegoal of each well abandonment should be to controlany leakage of fluids within the well to avoid the risk ofcontamination of freshwater aquifer and to monitor thestate of well sealing in order to prevent surface or seapollution [1]. Many formulations have been developedfor oilwell cementation; however, only few studieshave been devoted to the physical and chemical pro-cesses causing cement degradation under downholeconditions [2–5].The objective of this work is to study the structureand the properties of cement formulations cured andstored under different conditions of temperature andpressure. The XRD allows us to investigate the highlycrystallized phase, whereas NMR is very adapted tothe study of such heterogeneous and multiple phasematerial [6]. It is also one of the only available tech-niques able to investigate the calcium silicate hydrates(C–S–H), which are the main constituents of the Port-land cement paste. It is well known that C–S–H presenta poorly crystalline structure and variable composition[7].2. Experimental detailsTwo cement formulations, a class G Portland ce-ment (labelled C in the text) and a Low PermeabilityCement (LPC), with two curing conditions were pre-pared for this study as described in Table 1. The unhy-drated cement used was a class-G Portland cementfrom the Dyckerhoff Company (Bogue composition(wt%): 51.2 Ca3SiO5, 27 Ca2SiO4, 2.3 Ca3Al2O6,14.4 Ca4Al2Fe2O10). In the case of LPC samples, sandand silica fume were added to the class-G Portlandcement to increase mechanical and durability proper-ties by optimising the material compacity [8]. Theaddition of silica fume produces secondary hydrates bypozzolanic reaction with the lime resulting from pri-mary hydration (SiO2 + Ca(OH)2 → C–S–H) [9].Silica fume has a very high water demand, because ofits high specific-surface area, so a cement-dispersant,polynaphtalene sulfonate (PNS), was incorporated inthe mixes to maintain adequate consistency at a reason-able water/cement ratio.X-ray diffraction (XRD) data were collected using aPhilips PW 1820 diffractomer employing the Co–Karadiation (k0 = 1.789 Å). The samples were scanned at0.6° per minute between 2 and 82° 2h.The 27Al and 29Si NMR experimental details arereported in Table 2. Single-pulse experiments werecarried out in order to respect the relaxation times ofTable 1Characteristics of cement mixes (weight ratios relative to cement mass) and curing conditionsFormulations LPC CComponents LPC I LPC II C I C IICement class G 1 1Silica fume 0.24 —Sand 0.2 —Cement dispersant 0.018 —Water 0.27 0.44Curing conditions (30 days in tap water) T = 293 K T = 353 K T = 293 K T = 353 Kp = 105 Pa p = 7 × 106 Pa p = 105 Pa p = 7 × 106 PaTable 2Experimental details for NMR measurementsNucleus Sample Spectrometer Bruker ZrO2rotorFrequency(MHz)Pulse width Relaxationdelay (s)Spinning rate(kHz)27Al LPC, C and raw materials ASX 500 2.5 mm 129.80 p/12 (0.5 µs) 1 2529Si LPC 11.7 T 7 mm 99.305 p/2 60 5.5C and raw materials ASX 300 4 mm 59.591 3 77.05 T384 G. Le Saoût et al. / C. R. Chimie 7 (2004) 383–388the species present in the samples except for sand. The27Al and 29Si chemical shifts were respectively refer-enced relative to a 1.0 M AlCl3–6 H2O solution and totetramethylsilane Si(CH3)4 (TMS) at 0 ppm, usingSi[(CH3)3]8Si8O20 (Q8M8) as a secondary reference(the major peak being at 11.6 ppm relatively to TMS).3. Results and discussion3.1. X-ray diffractionPhases identification of hardened cements after a30 days-cure at ambient temperature and atmosphericpressure by XRD analysis (Fig. 1) shows the presenceof unhydrated phases alite Ca3SiO5, belite Ca2SiO4,ferrite phase Ca2(AlxFe1–x)2O5 (where 0 < x < 0.7)along with a-quartz due to siliceous aggregate used inLPC samples and usual hydrated phases such as port-landite Ca(OH)2, and ettringite [Ca3Al(OH)6·12H2O]2·(SO4)3·2 H2O (AFt phase). In the samples curedat high temperature and high pressure, ettringite phasewas absent. In the CII sample, instead ettringite, hydro-grossular CaO3Al2O3(SiO2)3–x (H2O)2x (where x = 0 to3) have been formed. Under conditions of elevatedtemperature, previous studies have shown that hydro-grossular Si-free will form, which is the most thermo-dynamically stable and the least soluble of the calciumaluminate hydrates [10]. In the LPCII spectra, all peakscharacterising the portlandite phase [Ca(OH)2] havecompletely disappeared. The calcium hydroxide re-leased during cement hydration is actually consumedas a result of interaction with active silica fume to formC–S–H phases.3.2. Magic Angle Spinning Nuclear MagneticResonance Spectroscopy3.2.1. 29Si MAS NMRAn example of the spectra decomposition is pre-sented in Fig. 2d, where we have also reported NMRspectra of the raw materials (Fig. 2a–c). It is importantFig. 1. XRD patterns of samples hydrated for 30 days at (a, c):T = 293K, p = 105 Pa and (b, d): T = 353 K, p = 7 × 106 Pa, Co Karadiation.Fig. 2. 29Si MAS NMR spectra of raw materials: (a) sand, (b) silicafume, (c) cement, and (d) LPCII sample hydrated for 30 days atT = 353 K and p = 7 × 106 Pa with its deconvoluted peaks. Theasterisks mark spinning sidebands.385G. Le Saoût et al. / C. R. Chimie 7 (2004) 383–388to note that even though the presence of Fe can causepeak broadening and the Fe-content of oilwell Portlandcement is quite large, reasonably well-defined spectracan be obtained. The 29Si NMR spectrum in Fig. 2adisplays a narrow line of Q4 type at –107 ppm and ischaracteristic of crystalline quartz, whereas the broadpeak at –110 ppm in the spectrum for silica fume(Fig. 2b) is characteristic of amorphous SiO2. Fig. 2cshows the 29Si NMR spectrum of class-G cement. Itcontains a broad Q0 component near –71 ppm, which isthe sum of alite and belite. The observed line broaden-ing arises from the incorporation of metal (Mg2+, Al3+and Fe3+) and other impurity ions into the crystallattice [11]. All raw materials are still present in thehydrated cement (Fig. 2d) with usual non-stoichiometric and non-crystalline calcium silicate hy-drates (C–S–H). The main resonance lines at–79.2 ppm and –85.5 ppm are respectively due to theend-chain tetrahedra Q1 and nonbridging tetrahedra Q2of the C–S–H [12]. The peak for Q2 sites is asymmetricor has a small shoulder at about –82.5 ppm. In calciumsilicate hydrate, this peak has been already assigned tothe middle tetrahedral of the dreierkette C–S–H chainstructure (Q2L) [13, 14], to distorted Q2 sites producedby various kinds of staking disorder [15] or to Q2species originating from precipitation of hydrate fromthe small quantities of bleed water resulting from thelarge centrifugal forces induced by the MAS technique[16]. Furthermore, in our samples, the C–S–H can alsocontain Al and the Q2(1Al) are expected in the sameshift range as the Q2L [17]. Thus, assignment of thispeak is a difficult problem that remains unresolved.The Q3 peak observed only as a shoulder near –90 ppmis ascribed to the cross-linked C–S–H structure [18].The 29Si MAS NMR spectra of C and LPC samplesare presented in Fig. 3. As temperature and pressureincrease, the relative intensity of the Q0 peak de-creases. This is linked to an acceleration of the hydra-tion kinetics with temperature and pressure, the Q0anhydrous species being transformed into calcium sili-cate hydrate (C–S–H). Furthermore, the ratio Q2/Q1increases with temperature and pressure. In addition,there may be some Q3 sites present in the treatedsamples. These results indicate that increasing tem-perature and pressure increases the polymerisation ofthe C–S–H structure. Such structural changes havebeen previously observed on hydration of tricalciumsilicate at high temperatures and high pressure [19].In the case of LPC samples, we can observe animportant decrease of the relative intensity of the silicafume peak with increasing temperature and pressure.This phenomenon can be attributed to the progressionof the pozzolanic activity of silica fume and is in goodaccordance with the disappearance of portlandite inXRD spectra of the LCPII sample. This result is inagreement with studies by Zanni et al. that indicate aweak and low activity of the silica fume at ambienttemperature, which increases with temperature [20].3.2.2. 27Al MAS NMRThe main part of the bulk Al2O3 content in OrdinaryPortland Cement is present in the so-called interstitialmaterial, which contains the aluminate and ferriteFig. 3. 29Si MAS NMR spectra of samples hydrated for 30 days at (a,c): T = 293 K, p = 105Pa and (b, d): T = 353 K, p = 7 × 106 Pa. Theasterisks mark spinning sidebands.386 G. Le Saoût et al. / C. R. Chimie 7 (2004) 383–388phases. However, for oilwell cements that contain alow bulk Al2O3:Fe2O3 ratio, the aluminate phase isusually absent or present in very small quantities andAl is mainly in the ferrite phase [21].The 27Al NMR spectrum of anhydrous cement(Fig. 4a) shows a broad asymmetric band of intensitynear 80 ppm corresponding to tetrahedrally coordi-nated Al. The peaks near 13 ppm and 10 ppm areattributed to Al in octahedral sites. The coordinationenvironment of aluminium in anhydrous cement isvariable; anhydrous cement may contain both octahe-dral and tetrahedral sites or only tetrahedral sites[11,18]. However, octahedral sites observed may beassigned to hydration product during storage. Skibstedet al. [22] provides compelling evidence that the reso-nances in the tetrahedral region arise from Al for Sisubstitution in the alite and belite phases. They alsoshow that Al present in the tetracalcium aluminoferritephase of cement, either in the antiferromagnetic or theparamagnetic form, contributes little or no to the ob-served 27Al MAS NMR spectrum [23]. On reactionwith water, it initially forms ettringite [Ca3Al(OH)6·12H2O]2·(SO4)3·2 H2O (AFt phase) and later thethermodynamically stable monosulfoaluminate[Ca2Al(OH)6]2·(SO4)·12 H2O (AFm phase), which allexclusively contain octahedrally coordinated Al [24]and leads to peaks in 27Al NMR spectra respectively at13 and 10 ppm [17,22,25]. We can notice that AFmphases are not detected by XRD, indicating that theAFm material is poorly crystalline. Taylor has sug-gested that AFm phases could be incorporated withinthe C–S–H, perhaps within or near the silicate layers[24]. The peak attributed to AFm is present in all oursamples but the AFt peak is not or weakly present intreated samples CII and LPCII.We can also notice the presence of a band at 4 ppm,particularly intense in the LPCII sample and a broadband near –20 ppm in the CII sample. The broadest linenear –20 ppm has been attributed to hydrogrossularphases [17]. The important width of this band can beexplained by the highly distortion of site because ofSi-site partial substitution by OH and Al-site by iron.This attribution is in agreement with XRD results thatshow the presence of hydrogrossular phases in the CIIsample. The attribution of the peak at 4 ppm is not clearand has been also assigned to hydrogrossular [25], butin the investigations of aluminium incorporation in theC–S–H, this peak has been assigned to Al3+ substitut-ing Ca2+ in the octahedral sheet of the C–S–H structure[26]. However, as Taylor pointed out, the large differ-ence in ionic radius between Al3+ and Ca2+ makesunlikely that Al3+ would replace Ca2+ randomly. Tay-lor proposed that Al replacement of Ca2+ could onlyoccur with the formation of AFm or other phases inwhich Al is octahedrally coordinated, leaving theC–S–H with very limited substitution [24].In sample LPCII, a band is clearly detected near36ppm and tentatively assigned by Faucon et al. topentacoordinated Al3+ substituting for Ca2+ ions situ-Fig. 4. 27Al MAS NMR spectra of (a) anhydrous cement and sam-ples hydrated for 30 days at (b, d): T = 293 K, p = 105 Pa and (c, e):T = 353 K, p = 7 × 106 Pa.387G. Le Saoût et al. / C. R. Chimie 7 (2004) 383–388ated in the interlayers of the C–S–H structure [26]. TheMAS spectra of the hydrated samples (Fig. 4b–e) alsoyield signal in the Al(IV) range particularly intense inthe LPCII sample, but the peak maximum has changedto about +65 ppm from +82 ppm for the unhydratedcement. According to Skibsted et al. [23], the moreshielded peak may arise from Al incorporated in theC–S–H.4. ConclusionTwo cement formulations with different curing con-ditions were analysed by XRD and NMR measure-ments. The raw materials are still present in all thesamples hydrated one month. The increase of pressureand temperature lead to a more polymerised C–S–H. Inthe cement C, we observed an acceleration of thehydration kinetics with temperature and pressure. Thehydrated phases of the CI sample consisted of poorlycrystalline C–S–H, AFm with AFt and portlandite. Inthe sample cured at 353 K, 7 × 105 Pa, AFt phase isabsent; hydrogrossular had formed instead. In the LPCsamples, it was shown that the pozzolanic activity ofsilica fumes increases with temperature and pressureand lead to the consummation of all the portlanditereleased during cement hydration. This feature is quiteimportant and positive for long-term durability as-pects. Indeed, it is well known that calcium hydroxideis easily soluble as soon as pH is lower than 12.5 atroom temperature. In order to have a better characteri-sation of the minor phases present in the cement, selec-tive dissolution is underway. Furthermore, mechanicaltests and porosity measurements are in progress to tryto establish a correlation between the structure andmacroscopic properties.AcknowledgementsThe authors would like to thank ‘Institut français dupétrole’ for the permission to publish this paper. Theyare grateful to Annie Audibert for valuable discussions,and Bernadette Rebours, Sylvie Massot, Isabelle Clé-mençon for their help in XRD measurements.References[1] I. Barclay, J. Pallenbarg, F. Tettero, J. Pfeiffer, H. Slater,T. Staal, D. Stiles, G. Tilling, C. Whitney, Oilfield Rev. 13(2001) 28.[2] E. Grabowski, J.-E. Gillot, Cem. Concr. Res. 19 (1989) 333.[3] C. Noïk, A. Rivereau, C. 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Study of oilwell cements by solid-state NMR

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Study of oilwell cements by solid-state NMR

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