Sodium silicate plays an important role in the hydration and self-degradation of geothermal temporary sealing materials, prepared by introducing sodium carboxymethyl cellulose (CMC) into sodium silicate-activated slag/fly ash cement.1,2 Solid products resulted from interactions between CMC and sodium hydroxide from sodium silicate. It was found that dissolution of these products upon contact with water-generated heat plays a pivotal role in the self-degradation performance of CMC-modified alkali-activated cement (CAC).2 However, the influence of modulus and quantity of sodium silicate on the self-degradability of CMC-modified alkali-activated cement had not previously been investigated.
An ideal material should provide sufficient strength (>3.5 MPa) to seal fractured formations and prevent loss of circulation at about 85 °C,3 but should degrade after drilling at 200 °C and upon contact with water. The current study investigated the effect of sodium silicate with different moduli on the compressive strength of CAC and the in situ exothermic heat generated by CAC contact with water to explore sodium silicate modulus and dosage on the self-degradability of CAC.
Experimental
Materials
Slag was obtained from Xinding Minerals Processing Plant (Lingshou, China), fly ash was supplied by Luyuan Power Resource Development Groups Co. Ltd. (Dongying, China), and sodium silicate was from Qingdao Yousuo Chemical Technologies, Inc. (Qingdao, China). The chemical compositions of slag and fly ash measured by inductively coupled plasma/atomic emission spectrometry (ICP/AES) are shown in Table 1.
Table 1 – Chemical compositions of slag and fly ash detected by ICP/AES 
According to Refs. 2, 4, and 5, the current study used slag/fly ash (80/20) + w/b (0.6)+6% sodium silicate + 1.5% CMC as the base formula, except for the modulus and amount of sodium silicate. The sodium silicate (Na2O·nSiO2) with different moduli (n = 1 and 3) were added in increments of 2, 4, 6, 8, and 10%.
The components were blended thoroughly and then added to water. These slurries were placed in air at room temperature for 72 hr. Following this, the set cements were cured at 85 °C for 24 hr. Some specimens were heated further for 24 hr in an oven at 200 °C; some of the 200 °C-heated specimens were cooled to room temperature for 24 hr and immersed in water for 1 hr.
Measurements
Compressive strength was tested according to Chinese National Standard GB 10238-20056 and Chinese Petroleum and Natural Gas Industry Standard SY/T 6544-20107 using a YAW-300B servo universal testing machine with a loading rate of 17.1 kN/min. Three cubic specimens (50.8 × 50.8 × 50.8 mm) cast from each composition were prepared to measure compressive strength.
Generation of in-situ exothermic heat was determined by a temperature data logger. The internal temperature of specimens in contact with water was tested with a high-temperature recorder. Specimens were heated at 200 °C, cooled to room temperature for 24 hr, and immersed in water. The cubic specimen was immersed in 400 mL of water. To test internal temperature, a round hole was made before the cement was set to allow insertion of the probe.
Results and discussion
Compressive strength
Figures 1 and 2 show the compressive strength of the material with different dosages of sodium silicate with moduli 1 and 3. After mixing, the material without sodium silicate was gel-like with poor rheological properties. The 85 °C-cured compressive strength of these samples was only 0.382 MPa, and this decreased to 0.352 MPa after heating at 200 °C. This phenomenon indicates that the hydration reaction of the slag/fly ash cement is difficult to conduct without an alkali activator (the slag and fly ash were not activated).
Figure 1 – Compressive strength of material with different quantity of sodium silicate (modulus = 1) after different periods.
Figure 2 – Compressive strength of material with different quantity of sodium silicate (modulus = 3) after different periods.The 85 °C-cured compressive strength improved with increased sodium silicate (Figure 1)—from 7.12 to 20.87 MPa—when the amount increased from 2 to 10%, indicating that the increase in sodium silicate (modulus = 1) enhances the 85 °C-cured compressive strength.
As seen in Figure 1, the 200 °C-heated compressive strength first increased and then decreased as the sodium silicate (modulus = 1) dosage increased. Peak value obtained by the addition of 6% was 18.06 MPa. The water-immersed compressive strength also increased first and then decreased. After immersion in water for 24 hr, the compressive strength of the material with 2, 4, 6, 8, and 10% sodium silicate (modulus = 1) decreased by 0.18, 2.50, 2.86, 6.02, 7.84, and 7.71 MPa compared to the 200 °C-heated specimens. Loss of strength increased as the increasing dosage when the dosage was under 8%, and reached a peak value of 7.84 MPa at 8%. Strength loss changed as the quantity of sodium silicate increased, indicating that the amount of sodium silicate impacts the material’s self-degradation.
In contrast to the material with modulus = 1 sodium silicate, the 85 °C-cured compressive strength decreased with increasing modulus = 3 sodium silicate (Figure 2).The maximum value of the 85 °C-cured compressive strength was 8.70 MPa, obtained by the addition of 2% sodium silicate. Minimum value was 3.77 MPa, obtained by the addition of 10% sodium silicate. After resolving the circulation losses, the accepted compressive strength to resume the drilling operations was 3.5 MPa; all of the 85 °C-cured samples in this study met this requirement.
As in the addition of the modulus = 3 sodium silicate, the 200 °C-heated compressive strength decreased in comparison to the 85 °C-cured samples, except for the addition of 6%. The water-immersed compressive strength decreased as the dosage increased. Maximum strength loss was 6.24 MPa, obtained after adding 6%. The water-immersed compressive strength of the material with 6% sodium silicate was 1.27 MPa, which decreased by 6.24 MPa in comparison to the 200 °C-heated sample.
The optimal temporary material should degrade upon contact with water after heating at 200 °C, while the compressive strength of the water-immersed samples should be low, and even lost completely. Following immersion in water, the compressive strength of the material with modulus = 1 sodium silicate was larger than 6.0 MPa, which did not meet the requirement. However, this can be achieved by adding a modulus = 3 sodium silicate in an amount greater than 6%.
Exothermic temperature
Figures 3 and 4 show the relationship between elapsed time and exothermic temperature for the material with different quantitites of sodium silicate with moduli = 1 and 3. The 200 °C-heated specimens were cooled at room temperature for 24 hr prior to testing; the initial temperatures of the samples were thus constant.
Figure 3 – Exothermic temperature of the specimens with different quantity of sodium silicate (modulus = 1) changed as the immersed time.
Figure 4 – Exothermic temperature of the specimens with different quantity of sodium silicate (modulus = 3) changed as the immersed time.As shown in Figure 3, the temperature of the sample without sodium silicate exhibits a minimum temperature increase, from 16.3 °C to 19.3 °C. The largest peak value of the temperature of the material with modulus = 1 sodium silicate was obtained by addition of 8%, followed by increments of 10, 6, 2, and 4%, with temperatures of 23.6, 23.3, 23.1, 22.2, and 21.3 °C, respectively. The sequence of the peak value was similar to that of the strength loss shown in Figure 1.
When the modulus of sodium silicate was 3 (see Figure 4), the largest peak in temperature was obtained by addition of 6%, the strength loss of which was 6.24 MPa, followed by 4, 2, 8, and 10%, with temperatures of 23.0, 22.8, 21.8, 21.0, and 20.1 °C, respectively. The strength loss of the samples with 6, 4, 2, 8, and 10% modulus = 3 sodium silicate were 6.24, 3.36, 2.41, 0.60, and 0.13 MPa. The sequence of the strength loss is the same as that of the peak value of the temperature. A comparison of all the samples in this study reveals that the same difference in peak value of temperature and strength loss is not applicable for the material with different sodium silicate moduli.
The peak temperature value of the samples with modulus = 1 sodium silicate were larger than that of the samples with modulus = 3 in the same quantity, except for 4%. Average strength loss of the material with modulus = 1 sodium silicate was 4.52 MPa, which is larger than that of the samples with modulus = 3 sodium silicate at 2.15 MPa. The 200 °C-heated compressive strength of the samples with modulus = 1 sodium silicate were larger than that of the samples with modulus = 3 sodium silicate, although the strength loss of samples with modulus = 1 sodium silicate were larger, and the residual strength was too high to meet the degradation requirement.
Conclusion
Dissolution of the solid products formed during heating at 200 °C produces in situ cement heating, which promotes self-degradation of alkali-activated slag/Class C fly ash blend cement.2 The formation of these products is closely related to sodium silicate. Results of the compressive strength and exothermic temperature verify that the exothermic temperature influences self-degradation. In addition, results of the exothermic temperature indicate that both the modulus and quantity of sodium silicate impact the insitu exothermic heat of CAC, and thus its self-degradation.
References
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- Sugama, T. and Pyatina, T. Effect of sodium carboxymethyl celluloses on water-catalyzed self-degradation of 200 degrees C-heated alkali-activated cement. Cement and Concrete Composites 2015, 55, 281–9.
- Thiercelin, M. Mechanical properties of well cements. In Well Cementing, 2nd ed. Nelson, E. and Guillot, D., Eds.; Schlumberger: Sugar Land, TX, 2006.
- Li, F.Y. Experiments Research on Self-Degradable Slag/Fly Ash Cement for EGS Geothermal Wells. Master’s Thesis, China University of Geosciences, Beijing, China, 2015.
- Tan, H.J.; Zheng, X.H. et al. Optimization and characterization of the self-degradable cement for geothermal wells. Geothermal Resources Council Transactions 2016, 40, 255–61. Geothermal Resources Council 2016 Annual Meeting, Sacramento, CA.
- Chinese National Standard GB 10238-2005. Oil well cement, 2005.
- China Petroleum and Natural Gas Industry Standard SY/T 6544-2010. Performance requirements for oil well cement slurries, 2010.
Huijing Tan, Xiuhua Zheng, and Bairu Xia are with the School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, P.R. China; e-mail: [email protected]. Shiwei Yang is with the School of Civil Engineering, Southwest Jiaotong University, Chengdu, P.R. China. Zhongli Lei is with the Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, P.R. China. This work was supported by the National Natural Science Foundation of China (grant no. 41572361), “Design and Preparation of Green Environmental Friendly of Self-Degradable Foamed Temporary Cementitious Sealing Materials for Geothermal Reservoir,” and the Fundamental Research Funds for the Central Universities (grant no. 2652017068).