UTILIZATION OF SUPPLEMENTARY CEMENTITIOUS MATERIALS IN GEOTHERMAL WELL CEMENTING

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1 PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 11-13, 2013 SGP-TR-198 UTILIZATION OF SUPPLEMENTARY CEMENTITIOUS MATERIALS IN GEOTHERMAL WELL CEMENTING Baris Alp 1, Serhat Akin 2 1 Turkish Petroleum Corporation, Research Center, 2180 Street No: , Cankaya, Ankara, Turkey 2 Middle East Technical University, Petroleum & Natural Gas Eng Dept, Dumlupinar Blvd No: Cankaya, Ankara, Turkey baralp@tpao.gov.tr, serhat@metu.edu.tr ABSTRACT In high temperature geothermal wells commonly conventional cement slurries based on silica blended mixes, are prepared to catch up with the required mechanical properties (compressive strength, thickening time, fluid loss, etc.) for the fresh and hardened cement slurry. Typically, 35 to 40 percent silica is added to blends to decrease the Ca/Si ratio of cement slurries to 1 in order to prevent retrogression in the physical and chemical properties above temperatures of 230 ºF (110 ºC). Ground granulated blast furnace slag (GGBFS) has a Ca/Si ratio lower than 1 and thus silica does not need to be added. The hydration of GGBFS blended cement slurries are improved at elevated temperatures that has vital importance when drilling wells in high temperature conditions. This study presents an experimental study to investigate the applicability of GGBFS in cementing of geothermal wells. The materials used in the analysis are API Class G cement, silica flour, GGBFS and sodium silicate (water glass). In addition to these materials, some chemical additives are used to provide fluid loss control (as fluid loss control agent), to arrange setting time (as retarder) and flow properties (as dispersant). Compressive strength by ultrasonic cement analyzer, HPHT (high pressure high temperature) static fluid loss, and thickening time analyses are conducted. The temperature of the analyses and/or the curing temperature of cement slurries conducted are 194 F (90 ºC), 248 F (120 ºC) and 374 F (190 ºC) which correspond to typical low to high temperature geothermal wells. It has been found that GGBFS improves compressive strength of the set cement at high temperatures. Presence of GGBFS in the slurry decreases fluid loss amount and increases setting time when compared to conventional silica blended cement slurries. GGBFS is a byproduct of iron industry and its cost is generally quite lower than Class G cement. Utilization of GGBFS in geothermal wells is not only economical but also environmentally appropriate. INTRODUCTION Hydrothermal Hydration of Portland Cement Restricting movement of fluids between formations, keeping casing in place and preventing corrosion e.g. from saline and sulfated underground water are important tasks accomplished by cementing. It is typical to use API Class G cement with additives to control properties of fresh or hardened cement paste (also called as cement slurry in oil well cementing) such as compressive strength, fluid loss control, consistency and thickening time. Silica flour is added to prevent strength retrogression. Bottom hole temperature in a geothermal well can be as high as 700 F (370 ºC), and the formation brines are often extremely saline and corrosive. As a result, geothermal well cement should withstand higher temperatures and tackle more aggressive environments than those encountered in oil and gas wells. Hydrothermal Hydration of Portland Cement In the hydration of Portland cement (PC) at elevated temperatures, hydration rate of C 3 S increases at early ages, on the other hand, hydration rate of C 2 S increases both at early and later ages (i.e. months later) especially at high temperatures (Odler, 2004). The overall hydration rate of Portland cement increases at elevated temperatures. The hydration product of Portland cement, C-S-H gel, is thermodynamically stable up to 110 C; at higher temperatures C-S-H gel metamorphose to more stable structures. Hydration at high temperatures leads to the formation of highly crystalline silicate hydrates with more Ca/Si ratio. It takes free lime (CH), which is already available due to C 3 S and C 2 S hydration, and converts to the phases called mainly alpha

2 dicalcium silicate hydrate (α-c 2 SH) and / or Jaffeite (C 3 SH 1.5 ) (Andrew et al, 2008). α-c 2 SH is highly crystalline and much denser than C-S-H gel. Conversion of C-S-H to α-c 2 SH occurs with an associated volume reduction and therefore, is deleterious to the hardened cement. As a result, compressive strength and permeability of the hardened cement is adversely affected by the formation of α-c 2 SH (Taylor, 1997; Nelson, 1990). Hydrothermal Hydration of Portland Cement in the Presence of Silica The strength retrogression of cement at high temperatures can be prevented by reducing Ca/Si ratio in the cement slurry. It can be reduced to 1.0 with addition of 35 to 40 percent silica by weight of cement (Nelson, 1990). In the presence of finely ground silica, pozzolanic reaction takes place and C- S-H gel tend to form 1.1 nm tobermorite, (C 5 S 6 H 5 ) (Odler, 2004). At temperatures above 150 C, tobermorite converts to mainly xonotlite (C 6 S 6 H) and gyrolite (C 6 S 3 H 2 ). At 250 C truscotite begins to appear and both xonotlite and truscotite are stable up to 400 C (Nelson, 1990). Among pozzolans α-quartz is the most effective pozzolanic material due to its high silica content and is frequently used in thermal wells to prevent strength retrogression (Nelson, 1990). Supplemantary Cementitious Materials The use of supplementary cementitious materials (SCM) dates back to the ancient Greeks who incorporated volcanic ash with hydraulic lime to create a cementitious mortar. Most concrete mixture contains supplementary cementitious material that forms part of the cementitious component. These materials are majority byproducts from other processes or natural materials. The major benefit of SCM is its ability to replace certain amount of Portland cement and still be able to display cementitious property, thus reducing the cost of using Portland cement. Ground granulated blast furnace slag (GGBFS) is such a material that can be used as a substitute to Portland cement. Replacement of GGBFS to Portland cement not only contributes to waste management but also improves the properties of fresh and hardened cement slurry. Pozzolanic Reaction During cement hydration, CH is liberated as a result of hydration of calcium silicates. CH does not contribute to the strength of hardened cement slurries but decrease chemical resistance of the cement slurries. In the presence of a pozzolan, silica reacts with free CH to form more stable cementitious compounds (called secondary C-S-H). Figure 1 shows the effect of slag content on the CH content of the hydrated cements by time. CH content can go down to zero percent with increasing content of GGBFS in the cement paste due to the pozzolanic reaction. Figure 1: Effects of curing age and proportion of slag on the calcium hydroxide content of the Portland-slag cement paste (Lea, 1971) Ground Granulated Blast Furnace Slag GGBFS has hydraulic setting property and can be utilized as a substitute to PC to produce slag blended PC. However, its hydration rate is much slower than PC at ambient temperatures According to ASTM C595, slag content in the slag blended cement can be up to 70 percent (by mass), whereas, EN makes limitation of GGBFS in slag-cement blend up to 95 percent by mass (CEM III/C). GGBFS is the maximum amount of mineral additive that is allowed to be used in the cement blends according to EN The formation of the secondary C-S-H gel in the cement reduces porosity because of pozzolanic reaction between cement and GGBFS. Also increased hydration rate of GGBFS at elevated temperatures decreases porosity of hardened cement with the contribution of pozzolanic reaction. The porosity reduction can be less than five folds when compared to hardened cement slurries prepared with neat cements (Figure 2). All GGBFS blended cements show lower porosity than neat Class G cement paste at all ages. 60 percent of GGBFS substitution in the cement paste distinctively decreases porosity. It is also stated that the pores in the hydrated GGBFS blended cements are finer than that of the hydrated neat cements. (Uchikawa, 1986).

3 Compressive strength as estimated by UCA (MPa) Figure 2: Porosity of hardened cement pastes at 80 ºC with w/c ratio of 0.44, G is prepared with neat Class G cement and S20, S40, S60 and S80 are PC blended cement pastes with ratios by mass (80:20), (60:40), (40:60) and (20:80) respectively, (Alp, 2012) The presence of GBBFS in the blend not only decreases porosity but also increases the compressive strength of hardened cement pastes at high temperatures. Figure 3 shows the effect of GGBFS on the compressive strength of hardened cement pastes. GGBFS blended cement pastes show higher compressive strength than neat cement pastes. Mueller (1995) observed similar results at 24-hours with GGBFS-PC ratio of 40:60. The compressive strength of GGBFS blended cement pastes were higher than reference cement paste at temperatures of 77 C and 93 C at 24-hours. The differences were even higher at 72 hours G S20 S40 S60 S Time, day Figure 3: Compressive strength of hardened cement pastes by UCA at 80 ºC with w/c ratio of 0.44, G is prepared with neat Class G cement and S20, S40, S60 and S80 are PC blended cement pastes with ratios by mass (80:20), (60:40), (40:60) and (20:80) respectively, (Alp, 2012) The hydraulic property of GGBFS can be improved by activators. Alkali hydroxides and alkali salts are generally activators, but the most popular ones are sodium hydroxide, sodium silicate, sodium sulfate, calcium sulfate and calcium hydroxide. Even Portland cement can be used as a GGBFS activator. Alkalis increase the ph of the aqueous solution which contributes to the dissolution of slag particles. These activators break of the bonds in the threedimensional network of the glass phase of GGBFS and release the ions of calcium, silica, aluminum and magnesium. Conventional silica blended cements can withstand up to 400 ºC (Taylor, 1997; Nelson, 1990), however, alkali activated slag can be used up to ºC, (Odler, 2000). The chemical corrosion resistance of alkali activated slag is very high. It is completely resistant to sodium sulfate and has high resistance to magnesium chloride and nitrate attack (Talling and Brandsetr, 1993) EXPERIMENTAL STUDY The materials used in this study are API Class G cement, GGBFS, silica flour and liquid sodium silicate (water glass). API Class G cement and GGBFS are obtained from Bolu Cement plant. Chemical analysis of these materials and mineralogical composition of Class G cement are presented in Table 1 and Table 2 respectively;. Table 1: Chemical composition (%) of materials Components Class G cement Materials GGBFS Sodium silicate CaO SiO Al 2 O Fe 2 O MgO SO Na 2 O K 2 O Cl TiO Mn 2 O LOI Free CaO

4 Table 2: Mineralogical composition (%) of Class G cement clinker C 3 S C 3 A 2C3A+ C4AF Percentages The specific surface area (Blaine s fineness) of GGBFS is 5092 cm 2 /g which is quite higher than that of API Class G cement (3220 cm 2 /g). It is stated that, an increase in the fineness of slag two to three times that of normal Portland cement contributes in a variety of engineering properties such as segregation, time of setting, heat evolution, better strength development and excellent durability (Swamy, 1998). The specific gravity of GGBFS used in the study is It is highly vitreous and glassy in structure that also improves the slag reactivity. The SiO 2 /Na 2 O molar ratio of sodium silicate used in the study is 3.2. It is stated that SiO 2 /Na 2 O is one of the most important factor that influences hydration of GGBFS and strength development of slurries at hydrothermal conditions (Sugama, 2006). Six cement slurry compositions are prepared. First composition is the conventional silica flour blended Class G cement (G-SI). The second and third one is the blends of GGBFS and Class G cement in different proportions (G-S1 and G-S2). The forth composition is the ternary mix of GGBFS, Class G cement and silica flour (G-S-SI). The fifth composition is prepared with neat GGBFS (S) and the last one is the alkali activated GGBFS (AA-S). Silica is added to slurry BWOC (by weight of cementitious materials; total of Class G cement and GBBFS). Ratio of water to solid constituents of the cement compositions are taken as 0.44 and the compositions are illustrated in Table 3. Table 3: Composition of cement slurries, (Alp, 2012) Constituents Class G, % GGBFS, % Silica Flour, % BWOC Na 2 SiO 3, ml/100 gr Water, % * * Less water is added due to presence of water in sodium silicate The compressive strength of the cement slurries are investigated by Ultrasonic Cement Analyzer (UCA). UCA measures the transit time (second/meter) of ultrasonic waves through the cement slurry. It is a non-destructive test method and simulates the wellbore conditions of temperature and pressure. The freshly mixed cement slurries are put into slurry cup and investigated for 24 hours at a constant pressure of 3000 psi (20.7 MPa). The temperature gradually increases up to 190 ºC (374 ºF) at 240 minutes and this temperature continues to the end of 24 hours. In the HPHT static fluid loss analysis, cement slurries are first conditioned at 100 ºC (212 ºF) in the atmospheric consistometer for 20 minutes. Then, recently conditioned cement slurry is put into HPHT filter cup and a differential pressure of 500 psi is applied at a static temperature of 150 ºC (302 ºF). The aqueous phase of cement slurry is forced to filter out for 30 minutes and the amount of filtrate is noted. The amount of fluid loss is proportional to the square root of time. If blowing out occurs within 30 minutes then the API fluid loss is calculated according to Equation 1. (API Spec 10B) Calculated API Fluid Loss = 2 Q t (1) Pressurized consistometer is used to measure the consistency and thickening time (pumpability time) of the cement slurries under the pressure of 5000 psi and at a temperature of 248 ºF (120 ºC). Chemical additives are needed to be used in the slurries to control fluid loss, consistency and setting time. Therefore, cement slurries are mixed with fluid loss additive (Halad-23), dispersant (CFR-3) and retarder (HR-12). On the other hand, no chemical additives are used in the compressive strength analysis. The amounts of chemical additives (Table 4) are calculated by weight of total solid constituents in the slurry (BWOS). Table 4: Chemical additives of cement slurries Chemical Additives, % (BWOS) Components Halad-23 CFR-3 HR-12 %, (BWOS) RESULTS AND DISCUSSION Compressive Strength Compressive strength of the set cements is important as it commonly represents the overall quality of cements. Higher compressive strength generally means lower porosity and increased durability. The UCA actually measures the compressibility of samples, but a previously developed correlation with

5 Compressive strength, x10 3 psi compressive strength (Nelson, 1990) is used. Figure 4 shows the time dependant compressive strength of hardened cements measured by UCA at 374 ºF G-SI GS-1 GS-2 G-S-SI S AA-S Time, day Figure 4: Compressive strength of the hardened cements measured by UCA The compressive strength data contained in Table 5 shows time to reach (TTR) compressive strength of hardened cements to 50 and 500 psi (0.34 and 3.44 MPa), maximum achieved compressive strength within 24 hours and final compressive strength at 1 day. Despite the high compressive strength of neat GGBFS after 1-day, it has the highest period to reach compressive strength of 50 psi and 500 psi. However, sodium silicate activation clearly decreases these periods. GGBFS blended cements; G-S1 and G-S2 have the lowest time to reach 50 psi and 500 psi. Table 5: Parameters of compressive strength analysis of hardened cement slurries TTR 50 psi, TTR 500 psi, Max. comp. strength, psi Final comp. strength, psi 01:56 01:15 01:31 01:16 04:54 03:14 02:54 02:00 02:16 02:04 07:49 03: According to Figure 4, conventional silica blended Class G cement slurry (G-SI) shows moderate compressive strength. The strength retrogression is prevented as mentioned in the literature. GGBFS blended cement slurry (G-S1) showed lowest compressive strength. The strength increases up to a threshold point. Then retrogression occurs within the cement because of exposure to high temperatures. G- S2 with 75 percent of GGBFS in the slurry shows comparable results with G-SI. Ternary mix prepared with Class G cement, GGBFS and silica flour (G-S- SI) in the 2 nd place with a compressive strength of nearly 2500 psi in the middle period. However, after 1-day its compressive strength is lower than that of G-S2 and G-SI. Cement slurries prepared with neat GGBFS has lowest compressive strength at early periods however; it is in 2 nd place with a compressive strength of more than 3000 psi after 1-day. Alkali activated GGBFS has a compressive strength of nearly 4000 psi and shows the highest compressive strength among slurries. Alkali activation clearly increases both initial and final compressive strength (after 24 hours) of hardened GGBFS. No strength retrogression is observed both in S and AA-S, and also negligible strength retrogression is observed in GS-2. The compressive strength of S and AA-S tends to increase gradually after 1-day while the other slurries go more asymptotic to x axis. Thickening Time The results of the laboratory thickening time tests provide an indication of the length of time that cement slurry remain pumpable. Consistency of cement slurry is expressed in Bearden units (B c ). Consistency of 40 Bc indicates the maximum pumpability while 70 Bc indicates the starting of cement setting. Table 6 shows times to reach (TTR) 40 Bc and 70 Bc of cement slurries at 248 ºF and under pressure of 5000 psi. Table 6: Thickening time of cement slurries TTR 40 Bc, 02:05 03: * 03:40 2:22* NA** TTR 70 Bc, 02:09 03: * 03:43 3:10* NA** * Without retarder ** Workable cement slurry cannot be achieved. Lower amounts of Class G cement in the cement slurry decreases setting time as seen in the Table 5. Because, decreasing cement amount in the slurry also decreases the amount of rapid hydrating C 3 S and C 3 A in the blend. The hydration rate of GGBFS is much slower than cement because it requires alkaline rich

6 environment. This alkaline environment can be provided by releasing lime in the hydration of cement. Similar to GGBFS, silica flour also needs lime to form calcium silicate hydrates within set cement. Therefore, increase of total amounts of GGBFS and silica flour decreases setting time of cement slurry. In the neat GGBFS slurry, setting cannot be achieved within 8 hours but without retarder its setting time is 3 hours and 10 minutes. On the other hand, it is not possible to mix workable alkali activated GGBFS slurry with specified chemical additives that are given in Table 4. Fluid Loss Control A series of tests are conducted to determine fluid loss efficiency of cement slurries and findings are contained in Table 7. Fluid loss performance is better in the GGBFS systems. The increased fineness of GGBFS improves fluid loss control of the cement slurry when compared to systems of Class G cement and silica flour. Even small amounts of GGBFS replacement in the cement blend contribute to fluid loss control as seen in the ternary mix of G-S-SI. However, it is not possible to mix a workable alkali activated GGBFS with specified chemical additives that are given in Table 4. Table 7: HPHT fluid loss of cement slurries API Fluid 96* NA** Loss, cc * Blowing out at 25 min., calculated using to Eq. 1 **Workable cement slurry cannot be achieved. Density of the cement slurries are shown in Table 8. It is possible to make GGBFS blended slurries with lower density than Class G cement systems. In addition, water requirement of GGBFS is higher than neat cement due to its high fineness. Therefore, water to cement ratio of the GGBFS slurries can be increased more than 0.44 and density can even be lower than values in Table 8. Table 8: Density of cement slurries Denstity, gal/ppcuft CONCLUSION Several laboratory tests were conducted to study high temperature application of ground granulated blast furnace slag. The results showed that: It is possible to prepare GGBFS blended cement slurries with higher compressive strength than conventional silica blended cement slurries. Strength retrogression is not observed in the neat GGBFS and sodium silicate activated GGBFS. GGBFS shows superior performance in HPHT static fluid loss than Class G cement and silica flour. GGBFS and silica flour increases setting time of cement decreasing the required amount of retarder used in the cement slurry. Chemical additives that are used in the silica blended cement slurries can also be used in the neat GGBFS slurry and GGBFS blended cement slurries. Sodium silicate activated GGBFS slurry shows the highest compressive strength but it is not possible to mix workable slurry with fluid loss control additives. It is possible to prepare GGBFS blended cement slurries with lower density than conventional silica blended cement slurries. REFERENCES Utilization of GGBFS in geothermal well cementing is both economical and environmental friendly. Alp, B., Utilization of GGBFS blended cement pastes in oil wells, Ms. Thesis, METU, Ankara. Andrew, C. J., Wilkinson, A. P., Luke, K., Funkhouser, G. P., (2008), Class H cement hydration at 180 C and high pressure in the presence of added silica, Cement and Concrete Research, Volume 38, pp API Specification 10B, (1997), Recommended Practice for Testing Well Cements, American Petroleum Institute.

7 ASTM C595, (2008), Standard Specification for Blended Hydraulic Cements, American Society for Testing and Materials. EN 197-1, (2000), Cement-Part 1: Compositions and conformity criteria for common cements, European Standards. Lea, F. M., (1971), The Chemistry of Cement and Concrete, 3rd edition, Chemical Publishing Co., New York. Mueller, D. T., Gino, D., Hibbeler, J., Kelly, P., BJ Services, (1995), Portland Cement Blast Furnace Slag Blends in Oilwell Cementing Applications, SPE Annual Technical Conference and Exhibition, Dallas Nelson, E. B., (1990), Well Cementing, Sclumberger Educational Services, Texas. Odler, I., (2000), Special Inorganic Cements, Modern Concrete Technology Series, London. Odler, I., (2004), Hydration, Setting and Hardening of Portland Cement, in P. Hewlett (ed.), Lea s chemistry of cement and concrete, 4th edition, Arnold, London. Sugama, T., (2006), Advanced Cements for Geothermal Wells, Brookhaven National Laboratory, Upton, New York. Swamy, R. N., (1998), Design for Durability and Strength Through the Use of Fly Ash and Slag in Concrete, CANMET/ACI International Workshop on Supplemantary Cementing Materials, American Concrete Institute, Toronto, pp Talling, B., and Brandstetr, J., (1993), Clinker-free concrete based on alkali-activated slag. Mineral Admixtures in Cement and Concrete (ed. S.N. Ghosh), ABI Books, New Delphi, pp Taylor, H. F. W., (1997), Cement Chemistry, Thomas Telford, London. Uchikawa, H., (1986), Effect of blending components on hydration and structure formation, 8th ICCC, Volume 1, Rio de Janeiro, pp