328DOI DOI 10.1007/s12182-011-0149-6

Wang Chengwen1 , Wang Ruihe1, Cheng Rongchao2 and Chen Erding31 School of Petroleum Engineering, China University of Petroleum, Qingdao, Shandong 266555, China2 Department of Drilling Strategy and Planning, CNPC Drilling Research Institute, Beijing 100195, China3 Drilling Engineering and Technology Corp., Shengli Petroleum Administration Bureau, Dongying, Shandong 257064, China

China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2011

Abstract: To address present concerns about thickening time and high early-strength in deepwater cementing at low temperatures when using conventional accelerators, a new type of set-accelerating admixture comprising of lithium chloride, aluminium hydroxide and alkaline metal chlorides, named as LS-A, was studied in this paper. Mechanism analysis and performance tests show that the accelerator LS-A accelerated the hydration of tri- and dicalcium silicates (C3S and C2S) at low-temperatures by speeding up the breakdown of the protective hydration fi lm and shortening the hydration induction period. Therefore, LS-A could shorten the low-temperature thickening time and the transition time of critical gel strength from 48 to 240 Pa of the Class-G cement slurry, and improve the early compressive strength of set cement at low-temperatures. It exhibited better performance than calcium chloride and had no effect on the type of hydration products, which remain the same as those of neat Class-G cement, i.e. the calcium silicate gel, Ca(OH)2 crystals and a small amount of ettringite AFt crystals. LS-A provides an effective way to guarantee the safety of cementing operations, and to solve the problems of low temperature and shallow water/gas fl owing faced in deepwater cementing.

Key words: Deepwater cementing, accelerator, lithium chloride, Class-G cement, mechanism

Mechanism and performance of a lithium chloride accelerator

*Corresponding author. email: wangupc@126.comReceived October 2, 2010

Pet.Sci.(2011)8:328-334

1 IntroductionApproximately 57 billion barrels of oil equivalent (BBOE)

hydrocarbons has been discovered in deepwater, and the yet-to-be-discovered resources are estimated to be 85-100 BBOE (Pettingill and Weimei, 2002). Many oil companies are showing an increasing interest in exploration and production of the abundant hydrocarbon resources in deepwater. Successful deepwater cementing plays a critical role in assuring the efficient, cost-effective, and safe development of deepwater hydrocarbon resources. In deepwater wells, the seabed temperature is usually lower than 4 C, and the circulating temperature in surface cementing typically ranges from 10 to 15 C (Rae and Lullo, 2004; Ravi et al, 1999). The temperature is verified to be a key factor contributing to the hydration rate of cement slurry. Low temperature will dramatically reduce the cement hydration rate, which can cause much longer thickening times, slow development of compressive strength of the set cement and insuffi cient shear stress of the annular cement sheath to support the casing within a short period. This will inevitably prolong the waiting-on-cement (WOC) time and increase well construction cost

(Wang et al, 2008; 2009). Therefore, favorable early strength is required for

deepwater cement slurry at low temperatures to achieve a preferable WOC time of less than 24 hours. An accelerator is an oil well cement additive used to shorten the thickening time and enhance the early compressive strength. Concerning the potential issues in deepwater low-temperature cementing, calcium chloride is commonly used to improve the early strength of the set cement, in reference to relevant cementing experience in onshore oilfi elds (Griffi th, 1996). It is of great importance to develop a new accelerator used for deepwater cementing at low temperatures. Many proprietary accelerators have been reported since 2002, such as polyhydroxyamine compounds (Reddy and Fitzgerald, 2002), a set-accelerating admixture comprising an alkaline and alkaline earth metal nitrite (Maberry et al, 2005), and (CaO)m(SiO2)nxH2O (Ravi et al, 2005). These accelerators have, to an extent, addressed some deepwater cementing challenges, but their extended application on a large scale is restricted due to their limited performance.

Alkali metal chlorides are a conventional type of set-accelerating materials, especially sodium and potash chlorides. Lithium chloride is also an alkali metal chloride. In aqueous solution, lithium cations with a small radius are easily hydrated due to their strong polarization and the

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hydrated ions have a large hydration radius. This will affect the cement hydration and thus the compressive strength of set cement. Since 1951, almost all related studies and reports have been focused on the suppression of alkali-silica reaction by lithium chloride (Yu et al, 2009). Brothers and Palmer (2004) investigated the accelerating effect of lithium chloride on the setting of cement slurry at low-temperatures. To date, the acceleration mechanism of lithium chloride and its influence on the performance of the cement slurry, however, have not yet been reported in the literature. In this paper, the authors report the infl uence of lithium chloride on hydration of the Class-G cement at low temperatures. Based on the acceleration mechanism of lithium chloride, a set-accelerating admixture comprising lithium chloride, aluminium hydroxide and alkaline metal chlorides, named as LS-A, was prepared, and its infl uence on the performance of Class-G cement at low-temperatures was discussed.

2 Experimental

2.1 MaterialsA set-accelerating admixture comprising of lithium

chloride, aluminium hydroxide and alkaline metal chlorides, named as LS-A, was prepared in the laboratory and its element composition is as follows: Li 14.17 %, Cl 47.73 %, O 27.34 %, Al 4.70 % and Na 4.01 %. Class G HSR cement was provided by Shengli Huanghe Cementing Corp. and its chemical and mineral composition is shown in Table 1. Calcium chloride, absolute ethyl alcohol, and acetone (analytic pure) were provided by Shanghai Branch of China Pharmaceutical Group.

Table1 Chemical and mineral composition of Class-G cement used

Chemical composition, wt% Mineralogical composition, wt%

SiO2 Al2O3 Fe2O3 CaO MgO SO3 C3S C2S C4AF C3A

24.76 2.89 2.63 65.08 0.83 1.25 53.70 30.46 8.0 2.8

2.2 MethodsAbsolute ethyl alcohol was used to terminate hydration

of selected representative samples of set cement cured at 4 C. After being milled into 75 m powders, the samples were dried under vacuum at 40 C to a constant weight. An Xpert PRO MPD X-ray diffractometer (Panalytical Co. Ltd., Netherlands ) was used for phase identifi cation of hydration products. Test parameters were: Cu K radiation, 40 kV and 40 mA, scanning range 2=5-70. The dried cement powders were dispersed on a copper stub using a conductive adhesive and gold-coated in vacuum, and hydration products of Class-G cement produced at low-temperatures were observed with a field emission scanning electron microscope JSM7600F produced by JEOL Corp., Japan at an acceleration voltage of 3.0 kV.

Cement slurries were prepared following API 10B-3-2004 standards, with a water-to-cement ratio of 0.44. All thickening times of cement slurries at low temperatures were performed on an OWC-2000A pressurized consistometer equipped with a circulation system to control temperature (produced by

Shenyang Petroleum Instrument Research Institute). Static gel strength development was measured with a static gel strength analyzer (Model 5265U with UCA functionality, American Chandler Corp.) for all cement slurries, and all data acquisition was performed with a Chandler 5270 DACS system. The compressive strength of set cement was tested after the prepared cement slurry was cured in a copper mold (50mm50mm50mm) in a self-made multifunctional curing pot SL-B at 4, 10 and 20 C, respectively.

3 Acceleration mechanism of LS-A

3.1 Effect of LS-A on low-temperature hydration of oil well cement

Five successive stages of silicate cement hydration can be defi ned (Nelson, 1990) (I) preinduction period, (II) induction period, (III) acceleration period, (IV) deceleration period, and (V) diffusion period. Once the cement particles contact water, cement hydration will take place, which means the beginning of the preinduction period. After several minutes, calcium silicate hydrate (CSH) gel is formed and precipitates on the surface of cement grains, and the gel gradually covers each grain, acting as a protective fi lm. The generation of the protective film indicates the end of the preinduction period and the beginning of the induction period (Kjellsen and Justnes, 2004; Kjellsen and Lagerblad, 2007). Fig. 1 shows SEM images of cement slurry hydrated at 4 C for different times. The cement slurry was made of tap water and Class-G cement at a water/solid ratio of 0.44, with an addition of 3% LS-A. In Fig. 1(a), a CSH film was observed surrounding the cement particles after seven hours of hydration. As the hydration went on, the concentration of calcium ions outside the fi lm and the concentration of silicate ions inside the fi lm continuously increased, which resulted in the protective fi lm gradually breaking due to high osmotic pressure (Fig. 1(b) and Fig. 1(c)). The breakdown of the protective film marks the beginning of the hydration acceleration period (Nelson, 1990). During this period, exposed tricalcium silicate and dicalcium silicate particles would hydrate. A great amount of CSH gel, produced in a Silicate Garden mode (Double and Hellawell, 1976), cross-linked to form a net structure (Fig. 1(d)), thus causing the cement slurry to harden. The SEM microstructure analysis indicates the LS-A may accelerate the breakdown of the hydration film, resulting in its rapid disappearance within 2 hours. The protective film of the conventional Portland cement, however, can exist as long as 3-4 hours at ambient temperatures (Lu et al, 2004). This shows that LS-A accelerated the breakdown of the protective fi lm and contributed to shortening the induction period.

To further investigate the effect of LS-A, the hydration processes of the class-G cement with different accelerators were tested at 20 C. Fig. 2 indicates that the addition of LS-A did not change the hydration process of the Class-G cement, but significantly speeded the cement hydration and shortened the induction period. This means that the addition of LS-A in cement slurry reduced or practically eliminated the induction period, accelerating signifi cantly the hydration reaction in the acceleration period. Therefore, LS-A would accelerate Class-G cement hydration at low temperatures.

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Fig. 3 XRD patterns of cement samples after 12 hours of hydration at 4 C

10 20 30 40 50S1

2

S2

S1: Dry Class-G cementS2: Neat Class-G cement slurry, w/c=0.44S3: Class-G cement slurry with 3% LS-A, w/c=0.44

S3A: Ca(OH)2B: AFtC: C3SD: C2S

D

DD

D

C

CC

C

C

BBB A

AA

AA

, degrees Fig. 4 XRD patterns of cement samples after 48 hours of hydration at 4 C

10 20 30 40 50

D

D

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C

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A: Ca(OH)2B: AFtC: C3SD: C2S

2, degrees

S3

S2

Fig. 5 SEM images of hydration products of set cement with 3% LS-A

(a)

(c)

CSH CSH

Ca(OH)2

Ca(OH)2AFt

CSH

(d)

(b)

SFI 3.0kv X10.000 1m WD5.9mm SEI 3.0kv X50,000 100nm WD5.9mm

SEI 3.0kv x50,000 100nm WD6.0mm SEI 3.0kv X50,000 100nm WD5.9mm

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of bar-shaped ettringite AFt crystals, which are the same as those produced from the neat Class-G cement slurry.

4 Effect of LS-A on the properties of the cement slurry 4.1 Thickening time of the cement slurry

In deepwater surface-casing cementing operations, the circulating temperature gradually declines due to the temperature gradient and the convective heat transfer between the cold sea water and the riser. The circulating temperature of the cement slurry in deepwater wells is within the range of 10 to 20 C. The thickening time of the cement slurry with 3% LS-A was measured at 10 C/7 MPa, and 15 C/10 MPa, respectively, as shown in Fig. 6. Results show the accelerator LS-A can reduce the thickening time to 4-5 hours at low

Fig. 6 Effect of LS-A on thickening time of the Class-G cement slurry

0 30 60 90 120 150 180 210 240 270 3000

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(100 Bc: 307 min)

(30 Bc: 257 min)

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(a) 10 C/7 MPa

0 30 60 90 120 150 180 210 240 2700

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(100 Bc: 248 min)

(30 Bc: 195 min)

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: Temperature: Pressure: ConsistencyThickening time: 248 min

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(b) 15 C/10 MPa

temperatures, which is helpful to ensure safe cementing operations and a short waiting-on-cement (WOC) time. The measured thickening time of the neat cement slurry at 15 C/10 MPa, however, was approximately 15 hours. Compared with the thickening time of the cement slurry containing 3% CaCl2, which was cured at 15 C/10 MPa (Fig .7), the addition of LS-A had no unfavorab le effect on the initial consistency of the cement slurry, but the addition of CaCl2 led to a sharp increase in the consistency above 50 Bc at 12 min and a rapid decrease to 25 Bc at 21 min, this rapid hydration is known as fl ash set. Moreover, for LS-A-containing slurry system, the transition time that the consistency increased from 30 to 100 Bc was 53 min, but for the CaCl2-containing slurry system it was about 98 min. This indicates that LS-A was helpful to shorten the transition time from 30 to 100 Bc of the cement slurry.

4.2 Static gel strength of the cement slurryIn deepwater surface-cementing cementing operations,

the casing setting depth is always near to 1,000 m below the mud line, where the bottom hole static temperature is about 30 C (OLeary et al, 2004; Tahmourpour and Quinton, 2009). To reveal the effect of LS-A on the cement slurry to prevent upward fluid flow, such as gas migration and fluid flows, through and along the cement slurry, the static gel strength was measured at 30 C/14 MPa, respectively, for the neat cement slurry, cement slurry with 3% CaCl2, and the cement slurry with 3% LS-A. Fig. 8 indicates that the time required for the cement slurry to change from having a static gel strength of 0 Pa to having a static gel strength of 576 Pa was 251 min, and the transition time during which the static gel strength increased from 48 to 240 Pa was 82 min. The addition of 3% CaCl2 in the cement slurry shortened the time required for the slurry to go from a static gel strength of 0 to 576 Pa, to 190 min, but because of the fl ash set caused by CaCl2, the static gel strength of the cement slurry rapidly reached 48 Pa at 14 min. This is consistent with what is shown in Fig. 7, in which the cement slurry with CaCl2 had a

high consistency of about 25 Bc after 12 min. The static gel strength of CaCl2-containing cement system (in Fig. 8) was as high as about 48 Pa after 14 min, and the transition time

Fig. 7 Effect of CaCl2 on thickening time of the Class-G cement slurry at 15 C and 10 MPa

0 30 60 90 120 150 180 210 240 270 3000

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(100 Bc: 287 min)

: Temperature: Pressure : ConsistencyThickening time: 287min

(30 Bc: 189 min)

Time, min

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of critical gel strength from 48 to 240 Pa was 116 min, 1.41 times longer than that of the neat cement slurry. However, the addition of 3% LS-A shortened the time of gel strength from 0 to 576 Pa and transition time of critical gel strength from 48 to 240 Pa to be 122 min and 31 min, respectively.

Experimental results indicates that the LS-A signifi cantly promotes the development of static gel strength of the Class-G cement and shortened its transition time required to go from a gel strength of 48 Pa to 240 Pa. With respect to the mechanism to control gas or fluid migration during cementing, a short transition time required for the cement slurry to go from a critical gel strength of 48 Pa to 240 Pa is of helpful to prevent fl uid or gas intrusion from the reservoir and prevent upward fl uid fl ow along the cement slurry (Rogers et al, 2004). The above-mentioned analysis demonstrates that the LS-A enhanced the cement slurry to control gas or fl uid intrusion into the cement column.

cement at low temperatures. Low temperature leads to a greatly increase in the 12- and 24-hours compressive strength of the LS-A-containing cement system, which indicates that LS-A can signifi cantly accelerate the early compressive strength development of cement slurries at low temperatures.

0 20 40 60 80 100 120 140 160 180 200 220 240 2600

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Class-G cement slurries with different acceleratorsw/c=0.44

95 min

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el s

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th, P

a

Time, min

98 min

Fig. 8 Development of static gel strength as a function of time for Class-G cement slurries

4.3 Compressive strength of the set cementFig. 9 presents the compressive strength development

of the neat cement slurry, and slurry with 3% LS-A. Experimental results indicate that the neat cement slurry exhibited no compressive strength development when cured at low temperatures (4 and 10 C) for 12 hours, which is consistent with the XRD analysis results. This means that Class-G cement particles will not hydrate at 4 C for 12 hours. The cement slurry still had a very low compressive strength (0.83 MPa) after being cured for 24 hours. However, the set cement containing 3% LS-A developed a larger early compressive strength at 12 hours (0.7 MPa) compared with the neat cement slurry and was still able to further significantly increase its compressive strength in the later stage. The 24-hour compressive strength values of the LS-A-containing cement system cured at 4, 10, and 20 C were respectively 11.36, 10.69, and 3.33 times larger than that of the neat cement slurry. The 48-hour strength was respectively 2.06, 2.50 and 1.84 times that of the neat Class-G cement slurry. The LS-A is proved to be capable of significantly enhancing the early strength development of the Class-G

0

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48 hours24 hours12 hoursCuring time

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Class-G set cement with different acceleratorsw/c=0.44

Neat, at 4 C 3% LS-A, at 4 C

Neat, at 10 C 3% LS-A, at 10 C

Neat, at 20 C3% LS-A, at 20 C

Fig. 9 Development of the compressive strength of set cement

5 Conclusions1) The set-accelerating admixture comprising of lithium

chloride, aluminium hydroxide, and alkaline metal chlorides, named as LS-A, could speed up the hydration of C3S and C2S at low-temperatures, and thereby improve the low-temperature properties of the Class-G cement by accelerating the breakdown of the protective film, and shortening the hydration induction period.

2) LS-A had no effect on the types of hydration products of the Class-G cement. The products were still calcium silicate gel CSH, Ca(OH)2 crystals, and a small number of AFt crystals.

3) LS-A significantly shortened the thickening time and enhanced the early strength development of the Class-G cement slurry at low temperatures. It exhibited excellent low-temperature acceleration and early strength enhancement, and could effective reduce the waiting-on-cement time to within 12 hours.

4) LS-A shortened the transition time of critical gel strength from 48 to 240 Pa, so it is helpful to minimize the chance of shallow water/gas migration. LS-A had superior performance over existing accelerators, so it is a practical and reliable new accelerator for deepwater cementing.

AcknowledgementsThe fi nancial support is provided by the Ph.D. Programs

Foundation of Ministry of Education of China (Grant No. 20100133120004), National Major Science and Technology Project of China (Grant No. 2009ZX05060) and National High Technology Research and Development Program of China (863 program, Grant No. 2006AA09Z340). The authors would also like to thank the anonymous reviewers for their valuable suggestion and comments.

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(Edited by Sun Yanhua)

Pet.Sci.(2011)8:328-334

Abstract:Key words:1 Introduction2 Experimental2.1 Materials2.2 Methods

3 Acceleration mechanism of LS-A3.1 Effect of LS-A on low-temperature hydration of oil well cement3.2 Effects of LS-A on low temperature hydration products3.3 Effects of LS-A on the microstructure of hydration products

4 Effect of LS-A on the properties of the cement slurry4.1 Thickening time of the cement slurry4.2 Static gel strength of the cement slurry4.3 Compressive strength of the set cement

5 ConclusionsAcknowledgementsReferences