Journal of Scientili e & Industrial Research

Vol. 6 1. May 2002. pp 370-375

New Insights into CaO-swelling Cements DlIllI Appah

Department of Petro leum Engi neerin g. Uni versi ty of Port Harcourt , Nigeri a

Received: 03 Jul y 200 I ; accepted: 18 Febru ary 2002

Inlluence of CaO in improving slurry swe lling is studi ed, using hi gh pressure-high temperalUre (I-IP-HT) expansion ccll s, and compared with previous results. For varying temperatures and pressures the beginning and end of ex pansion and the du rati or. of expansion arc observed. The expansion values arc determined. The ex tent to whi ch cement shrinks i ~ found to be a balance between the chemical react ion of water and clinker mineral , and the physica l reacti on of gel dipo le water auached to the electrically charged cement sur faces. An optimum water cement factor (WCF) or 0.45 ensures pUlllpabil it y during oil well cement ing, without causing increased hardened-paste porosil y and permeabi lity. Speciall y stab le matrix is needed to usc the swelling agent , and the smaller the CaO reacti vity, the earli er the "Time-window" att ains high temperature. Depending on the retarders used to suppress shrinkage, cement expands at an average value of 6 per cent 1'01' 19, I (), IS and 6 min, at 60, 80,90 and IDS "c, respectivel y. Ca lcium ox ide quantity between 12 and 14 per cent by weight of cement is recommended for good swelling. A second method of graduall y mi xi ng cement slurry components in stages represents an improvement of the CaO swelli ng abil i ty over the old lump method which is quite useful.

Introduction

Appropriate cement slurry formulation ensures defect- free cementing. Hardened cement, with hi gh porosity and hi gher permeability to gas and e lectrol ytes is undes irabl e. Make-up water used to prepare cement slurry has an immense effect on the vo lume of' cement. Concrete or mortar from cement wi th aggregate fill is used in the constructi on industry, whil e slurries for dr illing need hi gh water content and almost no aggregate fi ll. High pressure (up to 120 MPa) and temperatures (above 70 "c), usuall y encountered in deep (above 3000 m) oil and gas well s. accelerate the hyd rati on process.

[n spite of the wide attention th at cement I . I ' k 1-, d f II' t -1 .1) vo umetrIC s lrIn ' age . an use 0 swe IIlg agen s

have received in the las t decade, many questions rcmain unanswered. Ea rl ie r simulations applied to the fi e ld are not comparable to expectations, with

. I 6 8 d "' . I 10 expansion va ues at - an ., per cent , respec ti ve y . The present efforts to bridge the gap between laboratory and fie ld data involes determination of cxpansion values and cement paste properti es, such as compress ive and shear strength s, and system permeability. Whil e a water-cement fac tor (WCF), greater than 0.4, ensures that the formul ated slurries

are pumpable, high WCF results in hi gh poros ity and permeability va lues. These lead to gas migration and fl ow of e lectrolytes, from the porous medium and underground water, into the weil l I. Corrosive attack by sour gases, such as H1S and CO::, decreases cement strength va lue and eventua ll y destroys the cement mantle . Soluble calcium compounds are formed . Swell ing agents are used to counter vo lu me contracti on and more important ly to preclude the formation of mi croannu li. Elevated temperatures, between 70 and 120 "c, encountered ill deep and geothermal well s could cause the hydrated cement clinker minerals to recrystali se. The cement strength as a result further dim ini shes.

The three chemi cals used to swell cement are sulphoaluminate, free ca lcium ox ide and peri clase i.e., MgO. MgO hyd rates ve ry slow ly, expands at 11 7 per cent , and was general I y considered harmfu I to cemcnt 12

, until its li se in bridge const ructi on was reported by Zheng el al. " MgO-Swelling cement is particu larl y useful in deep gas well s with temperatures up to 175 °CI-l. The outcome of thi s study sheds more I ight to the understanding of CaO­swelling cement mixing co mposi ti on and mec hani sm. The results strengthen the use of e aO-swelling cement in both shallow and deep well c mp leti on.

APPA H: CaO-SWELLlNG CEMENTS 37 1

Experimental Procedures

Reacti vi ti es o f CaO, cement s lurri es expans ion and paste tes ts were ca rried out under laboratory conditions on diffe rent eaO products to screen and rank them for f ie ld app l icat ions.

Materials and Methods

Comme rcial blenders, hi gh prec Is ion weighing ba lances, and atmospheri c constant temperature bath , as we ll as hi gh pressure-hi gh temperature (HP-HT) expans ion ce ll s were used for the ex pansion tests. Autoclaves for curing cement slurry and Permeameter were used for late r determinati on of the pas te propert ies . The reacti vities of different CaO sampl es were carried out in the ca lorimeter-type tester.

Different commercial pure CaO at 60-95 °C, Port land c lass G cement , lignosulphor.ate re tarders supp lied by th ree oilfie ld service companies and defoamers were used in th is work. P re-determined amounts (689.65 -720 g) of three types o f CaO, as percentages of the base cement , were separate ly mixed into c lass-G cement, to obta in the homogeneous test composi ti ons of CaO-type swelling cements.

The cement pastes were made up with 0.45 water cement factor (WCF) for ex pansion tes t and for measuri ng permeabi lity as well as mec hanica l strengths. The density, measured by means of a hygrometer and pycnometer (confirmed by Ha lliburton method) of the cement slurry was 1.89 g/cmJ. Diffe rent wate r-cement ratios were tes ted to determine the influence of dens ity on the cement slurry development and paste. The prepared slurry was vacuumated to expel all a ir bubbles before the start of the experiment. A part of the s lurry was poured into six small 22 mm di am, cy lindrical metal cell s hav ing dimens ions 30 x 22 mm, fo r permeabilit y tests and into rectangular (5 x 5 x 5 mm) moulds for mechani cal strengths dete rminati on. The samples in the ce ll s and moulds were cured in moisture (water medium) in an autoclave for 7 d at pressures 11 0, 115, 120, 130 MPa and at 24, 27 , 30, 42 °C ,. Respective ly, The retrieved hardened paste sample was stored in alcoho l and oven-dried to free it from pore-water. The sample , under atmospher ic conditions, was then tested for compress i ve and shear bond strength , and permeability. A second porti on of the slurry was poured into three atmospheri c expansion cell s (Figure I ) and kept at 90 °C bath under atmospheric pressure, to obtain quick results.

T hree ce ll s were used for more re li ab le result s. The ba th surface conta ined small balls to prevent evaporati on. Readings were taken after every ha lf­min . until the ex pansi on va lue became constant. Freewater test was conducted for each run , re. ulting in no free water (0 cm\ A third part was poured into an HP-HT ce ll , for ex te nsi ve ex pe rime nts and more prec ise expans ion percentages and consistencies based on the earlie r resu lts.

Swelli ng behaviour o r CaO, pressure, temperature. and thi ckening time were measured continuous ly under s imul ated boreho le conditi ons on a multi channel strip of pape r and values read-orr. T he principle of operation of the measuring (HP-HT) potenti ometer device (Figure 2) has been described in deatil by Ghofrani and Plack'-l. The readings were reco rded between 60 to 120 "c at 10 "c inte rvals to study both the effec ts of temperature on the swell ing add iti ve and retarders. D ifferent proporti ons (10. 12.5, 14, 16 and 20 per cent respec tive ly. by weigh t of cement BWOC) of CaO and retarders were also

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used with a view to obta ining optimum mixtures for a wide range of field applications. The initi a l and final he ights of the cement were recorded from the ca librated paper. The start ing point and end of expansion were parti cularl y noted to determ ine the duration of expans ion used to ca lculate the percentage ex pansion.

Re tarders, suppli ed by service companies, were added to the cement-CaO mixtures in order to slow the hydration process of cement i.e., to suppress the vo lu metric contract ion of the ha rdening cement. They were the refore used fo r longer thi ckening time o f the cemen t s luury. Coarse-gra ined magnes ium ox ides were calc inated and fine ly divided by grind in g to ac hieve sma ll e r spec ifi c surface areas and spec ified gra in sizes. The clacinati on provided M gO of different reactivities. MgO was tested onl y as a poss ib le a lte rnati ve to CaO. M ost sampl es conta ined aboutof 95 per cent of CaO and traces of magnes ium, sili con, iron, and a luminium oxides in diffe rent

proportions. Calc ium ox ides of differen t reac ti vit ies were used to de te rmine the appropriate CaO-cement mi xtures at 60 to I 10 °C. The results can, therefore, be used for severa l borehol e conditi ons . Two swelling agent- and water-makeup methods, th ar conformed to industry pract ices, were employed. Cement, swe lling agent (CaO or MgO) and re tarders were weighed separate ly and mixed in the commercial bl ender for 15 s at low speed and then 30 s at hi gh speed, for labo ratory purposes:

( i) A ll the quantities at the sa me time, ( ii ) Small, ten equal porti ons o f the ce ment , re tarder and swelling agent. The separate portions we re added to the wate r after I min , 1-4 cn,,' of defoamer, 0.3-0.5 per cent by weight of cement (BWOC) of retarder added and properly stirred with an e lectric paddle for I min more . The need to have diffe re nt methods was to obtain the appropriate mixing energies and to have as much contact or spec ific surface area as poss ible . The experiment conduc ted for 8 to 12 h to de te rmine thic kening time. Reactivity tests o f produc ts , at 60 °C, were conducted in a ca lor imete r. The calorimeter con ta ined a rotator, which was used to hamogenise the temperature .

Results and Discussion

Expallsioll

Tabl e I shows the swelling abi liti es of ca lc ium ox ide for diffe rent ex perimental cond iti ons, expressed as ex pans ion va lues. It is e vident that the swelling abi lity of CaO-cement inc reases, up to a certa in po int , with inc reas ing CaO content and elevated ambient temperatrues. From Table I the ave rage expansion is fo und to be 6.4 pe r cent.

The effect of low te mperature on the swe ll i ng ab i liti es of diffe rent types of CaO (CaO- I, CaO-2 and CaO-3)-swelling cements is illustrated in T able 2. The pressure was kept constant at 21 MPa for the samples that had react ivities of 2 , 4 and 6 min , respectively. As temperature inc reases the expans ion a lso inc reases ( 10.6 to 17.6 pe r cent from 25 to 60 "C, except CaO- I). Six months o ld oxides wv re tes ted at 30 "c, to compare results w ith fres h samples.

The limit o f CaO content in cement is not fixed, but depends on severa l factors , such a~ its fineness and the surface a rea in contac t w ith the cement, the kinds and amounts of minera l and chem ica l admixtures . Chemica l compos iti ons are, the refore, usually spec ified when calcium oxide is suppli ed.

APPAH : CaO-SWELLING CEMENTS 373

Tab le I - Ex pansion of CaO-swelling cement at different temperatu res and pressures

Temperature, "c 60 65 80 95.0 11 0

Pressure, MPa 40 45 55 60.5 72.5

End of expansion, min 130 105 82 70.0 68 .0

Beginning of expansion, min 65 55 52 5 1.0 50.0

Duration of expansion, min 65 50 30 19.0 18.0

Ex pansion value, E, per cent 6.8 6.6 6.3 6.2 6.0

Slurry co mposi ti on: Cemem = 698.8 g; swelling agent (CaO) = 87.35 g ( 12.5 per cent BWOC) ; water = 353.79 g (0.45 per cent W/C rat io) ; and retarder = 0.5 per cent CBWOC)

Table 2 - Compari son of the swellin g abili ti es of diffe rent pure CaO samples

SI No. Temperature , HC Expansion val ues of calcium ox ide cements per cenl

CaO- 1 CaO-2 CaO-3

25 10.63 10.65 9.85

2 30 8.25 8.55 10.00

3 35 10.07 9.25 9.88

4 40 9.85 14 .00 16.77

5 60 10.68 14.37 17.60

A 12 to 14 per cent by weight of cement (BWOC) and water cement ratio or factor (WCF) of 0.45 per cent have been found to be parti cul arly useful. Retarder concentrations were varied in the range of 0 .5 to 1.2 per cent for the different swelling agents, eac h concentrati on for different te mperatures. The ex pansion values were 5 . 1, 2 .3 and 3.6 per cent , respecti vely, for 12.5 per cent BWOC of a particul ar swelling agent at 30 °C and 12.6 MPa, from atmospheric, gradua l and bulk mi xing methods, respecti ve ly. Thi s implies that it is usuall y better to add the substances graduall y over the entire duration of the ex periment . It was observed that both the expansion of CaO and shear bond strength decreased as re tarder content increased .

Calc iu m ox ide reacti vities were found to be between I and 7 min , at 60 °C, depending on the type or chemi cal compos iti on of the CaO. The commonl y supplied CaO (95 per cent by mass of substance) had an average reacti vity of 4 min . Dead-bond products ex hibit no reacti vity. The small er the reacti vity of a product , the earlier the Time-w indow is reac hed. The time-window is the minimum time required to obta in stable matri x, hence it is important to note the

beginning and the end of thi s window, on the strip chart , needed for expansion . A large amount of swell ing agent destroys the cement. Di fferent CaO products ava ilable are just reprocessed or renamed CaO samples, as fac tori es keep economi cs in view rather than indi vidual products.

Different durati ons of expans ion were measured for different temperatures, depending on swelling agents reacti vity and gra in sizes. At hi gh temperatures (above 65 "c for CaO and above 105 °C for M gO), the width of the Time-window was small and sol vati on was acce lerated. Within the window, a higher part of the product was converted for the reaction and a higher va lue of ex pansion was observed. The implication be ing that up to 60 °C, there was no need to use retarders, but the onl y major problem is contro l of the thic kening time of cement. The temperature effect on both the stability of matri x cement paste and so lution of the swe lling agent are independent of each other. For certain 14 per cent (BWOC) pure CaO samples tes ted at 2 1 °C and 17 MPa in HP-HT cell s, expans ion va lues of 0.3 to 0.5 per cent have been recorded. For a 20 and 45 per cent of MgO and water contents, res pec ti ve ly, the experiment was carri ed out for about 72 h with expansion values in the range 3-6 per cent.

Permeability

The slurry was prepared in sweet wa ter and the problem was for obtaining the approp riate mlcropores. Permeabi I iti es of hardened cores , prepared from saturated sa line so luti on, in stead of sweet water were, therefore, measured. Contrac ti on usually leads to increased sys tem permeabi lity. Permeability values for 0 .45 per cent make-up water are shown in Table 3.

Mechanical Strength

Table 3 gives the values of co mpress ive and shear strengths for the autoc lave cured samp les. Density is the ma in fac tor in the development of cement strength. High shear strength results in hi gh contact strength with the wall , and hence lower permeability. The va lues of shear strength were in the range of 5.70 to 3.9 1 MPa and permeability vari ed from 0.0164 to 0 .24 12 nm2

. Tri-calcium a luminate was responsible for earl y stage (3 to 6 month s) and tri -calcium s ilicate determines late deve lopment of strength for one year. At temperatures, in excess of 65 DC, the ge l strength was obtained ve ry rap idl y.

374 ] SCI INO RES VOL 6 1 MAY 2002

Tab le 3 - Mechani cal strengths and sys tem permeabiliti es of cement pastes

SI 0 CaO, Sweet Compr- Shear Perme-BWOC, water, essive st rength , ability, per cent WCF, strength , 1 , MPa k, nm2

per cen t cr, MPa (~O)

12.5 0.45 27.46 5.70 0.0548

2 12.5 0.45 14.32 4. 16 0 .2412

3 14.0 0.45 16.87 5.3 1 0.0616

4 14.0 0.55 49 .05 4.12 0 .0370

5 12.0 0.45 18.24 3 .9 1 0. 1507

6 12.5 0.45 32.37 5.00 0 .0438

7 14.0 0.45 22.95 5.62 0 .1 919

8 14.0 0.45 33.94 4.79 0 .0600

9 14.0 0.45 33.95 4.78 0.0328

10 14.0 0.45 32.86 4.63 0.0 164

Autoclave (Borehole static) cond iti ons: Temperature = 42 "c, Pressure = 21 MPa Medium was water and the cement slu rry was cured for 7 d @

temperature gradient = 1. 14 "C/min

The shear strength va lues of the CaO-swelling cement improved . Thi s indicates that there is effec ti ve bonding of thi s cement at the interfaces with the cas ing and formation. The implication is that good sealing can be achieved .

Conclusions

The new findings, based on laboratory investigati ons, lend strong support to the following:

(i) Temperature is more critica l to CaO­swelling cement setting, and cement matrix stability , than pressure . CaO­Swelling cement exhibits a swelling capaci ty of about 6 per cent between 90 and 105 °C.

(ii ) A 12 to 14 per cent of CaO, 0 .3-0.5 per cent of retarder by weight of cement (BWOC), respecti vely, and 0.45 per cent water-cement ratio (WCR) are considered optimum for preparing the cement s lurry.

(iii ) Slurry components should be grad a ll y int roduced in to the sys tem and carefully mixed with the cement, rather than bulk stirring. The results from th is method are c lose r to those being practi sed in industry .

(iv) CaO improves shear strength, which is responsible for matri x stabi I ity and therefore effecti ve bonding . Above 60 °C the minimum required strength is rapidly obta ined .

Acknowledgement

The author is grateful to the A lexander von Humboldt (AvH) Foundati on for research fellowship grant at the Department of Drilling and Production , Institute of Petro leum Engineer ing (ITE), Technologi cal University, Clausthal, Germany. Efforts of Prof. R. Ghofrani and the guest reseachers and staff of the Cement Laboratory of the same Inst itute who have he lped the author in co llecting data are also acknowledged.

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