Ghada Bassioni*a,
Mohammed Mohammed Alib,
Ali Almansoorib,
Gabriele Raudaschl-Sieberc and
Fritz E. Kühn
c
aChemistry Department, Faculty of Engineering, Ain Shams University, P. O. Box 11517, Cairo, Egypt. E-mail: ghada_bassioni@eng.asu.edu.eg; Fax: +20 22630470; Tel: +20 1001832728
bChemical Engineering Program, The Petroleum Institute, P. O. Box 2533, Abu Dhabi, UAE
cMolecular Catalysis, Department of Chemistry, Catalysis Research Center, Technische Universität München (TUM), Lichtenbergstr. 4, D-85747 Garching bei München, Germany
First published on 17th January 2017
The chemistry of oil-well cement is very complex. As a result of incompatibility of some simultaneously added chemical combinations to obtain better cement workability, undesirable interactions reflect on the properties of the cement slurry. Experimental investigation of these properties is lengthy and depends on accuracy. In this work, the compatibility between lignosulfonate as a retarder R and NSF polycondensates as dispersants D is studied. Furthermore, optimum dosages are proposed based on the zeta potential technique at ambient conditions. The results show that the optimum dosage for (R2 + D1) is 0.2% by weight of cement. Moreover, the obtained results are supported by adsorption isotherms. The competitive adsorption is attributed to the differences in anionic charge densities between the additives and is postulated to take place on C3A hydrates. The effect of the proposed additive dosages is studied on cement hydration. The obtained dosages show a good setting time (21 h) at a water-to-cement ratio of 0.4 and under ambient conditions. Furthermore, due to complex interactions with cement hydrates, mathematical models are proposed that are able to validate experimental results of surface properties and hydration processes.
As cement is being pumped into the well, the slurry is exposed to aggressive conditions such as high temperature, high pressure and salinity making the process quite challenging and rather complex. Therefore, certain additives are combined and employed in cement formulation in order to enhance its properties and achieve the optimum performance. Retarders and dispersants combination are commonly utilized due to their ability to control the cement early hydration and flow-ability under extreme well conditions. Retarders are defined as the chemical additives that delay cement setting times and prevent premature hardening. Typically, the perfect retarder for oil-well cement is the one which extends the cement setting for adequate time and then suddenly allow the cement hydration to proceed at a rapid rate.2 There are different kinds of oil-well cement retarders such as calcium and sodium lignosulfonates.3 As summarized recently the proposed retardation mechanisms fall into four categories: (1) calcium complexation by chelating the free calcium ions preventing C–S–H formation and/or portlandite precipitation, (2) formation of a semipermeable layer, later broken down by osmotic pressure gradients, (3) direct surface adsorption on cement anhydrous particles, (4) nucleation and growth poisoning of cement hydrates.4 A fifth mechanism (“dissolution–precipitation”) has been proposed and considered a special case of surface adsorption.5 Dispersants are used to improve the rheological behavior of cement slurries. In deep oil well – because of the increased temperature – the viscosity of cement slurry is low leading to undesirable flow characteristics of the cement slurry.6 This behavior affects the cementing job negatively and thus zonal isolation does not get complete since the low viscosity jeopardizes the success of cement placement. Dispersants have the ability to overcome the van der Waals attractive inter-particle forces by imparting a stronger repulsive force at the surface–liquid interface, freeing the entrapped water.4 This process is described to occur as follows: (1) the mixing actions break down the cement agglomerates and distribute the dispersant agents, fully miscible in the high ionic strength pore water, through the cement slurry; (2) an attractive force between the dispersant additives, which usually bears a negative charge at the relatively high pH of the pore water, and the positively charged cement surfaces, causes fairly rapid adsorption of the dispersants onto the cement particles; and (3) the adsorbed compounds, forming a layer of a certain thickness and conformation, produce some combination of electrostatic and steric repulsive forces (steric hindrance) that prevent re-agglomeration of cement particles and liberate water that has been trapped within the flocculated structure. The types of dispersant include polymelamine sulfonate (PMS), polynaphthalene sulfonate (PNS or NSFC), polystyrene sulfonate and other types of hydroxycarboxylic acids such as citric acid.6
The use of different additive formulations in cement systems make the cement chemistry very complex and may cause undesired interaction. As a result, well cementing operation is compromised due to the loss in cement/additives system performance. This loss can be referred to the low flow-ability of cement slurry, over retardation, low early strength development and high cement fluid loss. Incompatibilities in the cement system are mainly attributed to the competitive adsorption of the additives on limited adsorption sites on cement and hydration products surfaces and it is depended mainly on the chemical nature of additives.
The main goal of this work is to find a way to simplify data access, usually resulting from complex experimental measurements. First, a systematic study is conducted that produces highly economic cement recipes with maximum efficient workability and experimental results are validated with simple mathematical models. This objective can be achieved by linking the chemical and mechanical properties. This work is focusing on two important additives, retarders and dispersants. The study starts with analyzing chemical characteristics of two commercially available additives: sodium lignosulfonate (retarder R) and sodium naphthalene sulfonic acid–formaldehyde condensate (dispersant D). Moreover, the study proposes the optimum dosages for those additives based on zeta potentials, adsorption isotherms and studying the interaction between additive systems as well as the compatibility between different additive dosages on the adsorptions sites. Furthermore, the effect of the proposed dosages on the cement hydration is shown. Also, mathematical models validating the zeta potential measurements and hydration processes are suggested.
1H NMR spectroscopy on the retarders is carried out using a 400 MHz spectrometer (Bruker AMX 400, BRUKER, Germany) at a resonance frequency of 400.13 MHz for 1H with D2O as a solvent.
29Si MAS-NMR and 27Al MAS-NMR measurements are conducted for hydrated cement with 0.5% by weight of cement (bwoc) of both retarders (R1 and R2) and compared to the blank one by a 300 MHz spectrometer (Bruker Avance 300, BRUKER, Germany) using Bruker TopSpin 2.1 software. 29Si MAS-NMR measurements (resonance frequency of 59.62 MHz) are performed in 7 mm ZrO2 rotors at a spinning speed of 5 kHz with 1050–1200 scans. 27Al MAS-NMR measurements (resonance frequency of 78.21 MHz) are performed in 4 mm ZrO2 rotors at a spinning speed of 15 kHz with 3000 scans.
Elemental analysis (C, H, N, S) of the utilized retarders and dispersant is determined with a CHNS-O elemental analyzer (EA 3000, Euro Vector SPA, Italy).9 In order to determine the amounts of calcium, sodium and potassium in the cement's additives atomic absorption spectroscopy technique is used. Standards solutions of Ca, Na and K are prepared at different concentrations (1 ppm to 6 ppm). The samples are dissolved in 1% of sulfuric acid (H2SO4) and 1% (HNO3) and heated until the color of the solution become light. After that the sample is diluted in distilled water. Solutions of the acids and the distilled water are prepared as background. The AAS measurements are conducted using (AAS, Varian AA280 FS, Australia) coupled with SpectrAA software.
Ion chromatography technique is utilized to evaluate the sulfate ions concentration (degree of sulfonation) presented in the studied additives. Solutions of 200 mg L−1 of the chemical additives (R1, R2 and D1) are prepared and measured using (DIONEX, ICS3000, USA). External anions standards with sulfate (SO42−) concentration 0, 5, 10, 15 ppm are obtained from (DIONEX, USA) and 4 points calibration curve are generated in order to measure the sulfate anions' concentration in the samples.
The molecular weights for the used additives (R1, R2 & D1) are determined by using (Agilent HPLC 1200, Agilent Technologies, USA) using PL aquagel-OH MIXED-H 8 μm, inner diameter (I.D) 7.5 × 300 mm column obtained from (Agilent Technologies, USA) and it's equipped with a refractive index (RI) detector. The additives are dissolved in 0.2 mol L−1 NaNO3 aqueous solutions to a concentration of 0.2 wt% and the samples are filtered through 0.2 μm syringe filter (Supor membrane, Pall life sciences, USA). A 0.2 mol L−1 of NaNO3 aqueous solution is used as an eluent (adjusted to pH = 8 by adding 10 drops of 0.01 mol L−1 NaOH) at a flow rate of 1 mL min−1 and the sample is injected with volume of 50 μL. The column is calibrated using polyethylene oxide/polyethylene glycol (PEO/PEG) standards and the separation range is 100 to 1000k Da (dalton). The molecular weight calculations are carried out using GPC software and a 6th order polynomial fit is used.
The zeta potential approach is used in order to determine the surface charge on the cement particles and the electrostatic stabilization of the cement/additive system. The electro-acoustic instrument (DT 1200, Dispersion Technology, USA) is used. The instrument is opted due to its ability of measuring zeta potential for high volume fraction solid/liquid system such as cement slurry.11 Prior to zeta potential titration measurements for cement slurry, 6.4 mL of distilled water is poured into the centrifuge tube using a micropipette. Then, 16 gram of oil well cement class G is added to the distilled water. The sample tubes are mixed for 1 minute homogeneously by a test tube shaker (Vortex-Genie 2, Scientific Industries, USA) at 2550 rpm. The sample tubes are centrifuged by (Eppendorf Centrifuge 5804 R, Eppendorf, Germany) for 20 min at 4000 rpm and 25 °C. Then the extracted pore solution is transferred to zeta probe container in order to determine the ionic vibration current (IVI). The ionic vibration current is sat to be subtracted from the measured zeta potential values during additives titration experiment.11 Then, additives solutions of 2% bwoc are prepared as follows: (1) R2, (2) D1, (3) D2, (4) R2 + D1 (1:
1). The preparation of cement slurry is performed at room temperature 25 °C and at constant water to cement ratio (w/c = 0.4) according to ASTM C-305.12 A 550 g of oil-well cement class G and 220 g of distilled water are weighted using an electronic balance. The cement slurries are prepared and mixed using cement blender (ToniMIX, Toni Technik Baustoffprüfsysteme GmbH, Germany). The water is subsequently poured into the mixing bowl. The cement is added gently to the water and is allowed to settle for almost 30 s. The cement is mixed at low speed (145 ± 5 rpm) for 30 s. The mixer is stopped for 15 s. During this time the cement on the side of the bowl is scraped down into the batch. The cement is mixed again at medium speed (285 ± 10 rpm) for 1 min. Then cement slurry is poured immediately into the special glass cell as with motorized stirrer speed of 475 rpm.12 The additives 2% bwoc (50 mg mL−1) solution is then added with an increment of 1 mL with a rate of 1 mL min−1 until a plateau is reached.
![]() | (1) |
The coefficients {wi}li=1 and b are calculated by minimizing the regularized risk function in eqn (2).14
![]() | (2) |
![]() | (3) |
Data are normalized into the interval [0, 1] by using eqn (4).
![]() | (4) |
The comparison parameters are the correlation coefficient (R2), Mean Absolute Error (MAE), Root Mean Squared Error (RMSE). These parameters can be written as show in eqn (5)–(7).
![]() | (5) |
![]() | (6) |
![]() | (7) |
Although FT-IR and 1H NMR results indicate a similar structure for R1 and R2, differences in the mechanical behavior of the retarders necessitated the conduction of additional tests.7 Therefore, further chemical analyses have been conducted to evaluate the differences between both retarders and the results are shown in Table 1.
Additive | Mw (Da) | Mn (Da) | % C | % H | % N | % S | % Na | % Ca | SO42− | ACD (μeq. g−1) |
---|---|---|---|---|---|---|---|---|---|---|
R1 | 2164.5 | 801.14 | 39.16 | 4.94 | 0.17 | 5.05 | 0.8 | 8.4 | 0.90 | 1521 |
R2 | 1237.3 | 534.34 | 43.86 | 4.6 | 0.14 | 3.28 | 7.6 | 0.1 | 2.48 | 2353 |
D1 | 1063.6 | 390.41 | 42.14 | 3.86 | <0.1 | 12.16 | 10.5 | 0.5 | 7.63 | 3050 |
Molecular weight distribution of different cement additives is considered as one of the factors that affect the adsorption behavior. As can be seen, R1 has a higher average molecular weight (Mw = 2164.50 g mol−1) compared to R2 (Mw = 1237.30 g mol−1). Moreover, the anionic charge density (ACD) is considered a key factor that controls the interaction with cement components. According to Table 1, the sulfate content is higher for R2 but the sulfur amount in R1 is higher than in R2. This variation between the two analytical methods indicates the presence of other sulfur components in R1, which is attributed to the existence of some impurities. This has a significant impact on compatibility with other additives like dispersants. The results show that sodium is the counter-ion in retarder R2 while calcium is the counter ion of R1. Interestingly, the findings show remarkable differences between R1 and R2.
The findings on the effect of molecular weight on the adsorption performance show a high consistency with previously presented studies.20 The fraction with higher molecular weight is found to have more sulfonic groups in each molecule and it is more likely to twist.14 Based on previous studies it is expected that the adsorption amounts (retardation effectiveness) of R1 would be higher than R2. However, the thermal analysis results show a different behavior.18 Moreover, the slump loss value of cement/additives paste with higher molecular weight additive is more effective among the lowest molecular weight additives. This behavior is attributed to the movement variation in pore solution between different molecular sizes of the additive towards the cement active sites; the bigger molecules take a longer time to reach there and hence, the smaller ones are consumed first. As a result, no free additive in solution is left for the cement to maintain its high fluidity (no slump loss).
Although R1 possesses a higher average molecular weight (Mw) than R2, the results show that ACD of R2 is higher than R1 by approximately 35%. These differences in ACD between the retarders are assigned to the variation in the degree of sulfonation. Lignosulfonate contains different polar groups such as sulfonic, phenylic, hydroxyl and alcoholic hydroxyl groups so that these values for the charge densities of both retarders represent the sum of the charges of all those groups.21 In other words, the actual charges of the sulfonate groups for both retarders are lower than the values represented in Table 1. Quyang et al. modified lignosulfonate by increasing the degree of sulfonation (sulfonic groups) and compared its performance on zeta potential to the non-modified ones.21b It is reported that the modified component possesses a higher charge density compared to the original ones, and thus a higher zeta potential is achieved.
Presumably, the differences between R1 and R2 or lignosulfonates in general are related to the different types of wood from which they are isolated, the pulping process, method of sulfonation and other variables.22 D1 and D2 dispersants share similarities in their chemical composition. Some minor differences exist in the sulfate contents and anionic charge densities, while microstructure analysis reveals a major difference between them.
Particular focus will be further made on the mixture R2/D1 due to the above described indications regarding performance, especially that dispersant (D1) has a higher anionic charge density (ACD) compared to the retarder (R2) by almost 22%. Heat flow calorimetric results obtained are in favor of that combination, too.7
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Fig. 2 29Si MAS NMR spectra of hydrated blank cement, hydrated cement + 0.5% bwoc R1 and hydrated cement + 0.5% bwoc R2. |
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Fig. 3 27Al MAS NMR spectra of hydrated blank cement, hydrated cement + 0.5% bwoc R1 and hydrated cement + 0.5% bwoc R2. |
The 27Al MAS NMR analysis of cement/additives is executed and compared to the blank sample as illustrated in Fig. 3. For the blank sample the expected two main signals are found, one at 13.0 ppm (assigned to ettringite) and one at 9.2 ppm (attributed to monosulfate). It is reported that anhydrous cement exhibits tetrahedral and octahedral coordination at chemical shifts of 80–90 ppm and 0–20 ppm, respectively.23 As can be seen, the peak maximum of tetrahedral coordinated aluminum is shifted to 58.0 ppm due to the hydration process because Si is substituted by Al in the C–S–H phase as reported in the literature.27 For the additive system it can be seen that the ettringite formation is affected due to the addition of retarders and the peak is found at 12.6 ppm while, for the R2 system, the ettringite peak intensity does not change. In addition, the monosulfate peak is found at 8.8 ppm for both retarders. Therefore, it can be concluded that R1 is more effective on ettringite hydration than R2.
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Fig. 4 Zeta potential of oil well cement class G treated with 0–33 mL of 50 mg mL−1 of R2, D1 and R2 + D1 (1![]() ![]() |
The combination of both retarders and dispersants are studied in order to evaluate the compatibility between both of them. From Fig. 4 it can be seen that the zeta potential values for the mixture of (R2 + D1) are increased (towards positive value), relatively, when compared to D1. This behavior is referred to a competitive adsorption between the highest anionic charge density (D1) and the lowest (R2) onto cement active sites. The zeta potential of the mixture reaches a plateau approximately at 0.2% bwoc. Therefore, depending on the obtained zeta potential results, the optimum dosage for the studied cement/additive systems appears to be 0.2% bwoc.
The zeta potential measurement of the cement/additives systems is recognized as a method to study the additives' adsorption on the hydrated cement surface (Fig. 5). In general, it can be assumed that the lower the zeta potential, the higher the adsorption of additives on the cement particles. This statement may hold only when the dispersion mechanism of the additive on cement suspension is induced by their electrostatic repulsion. It is stated that if the zeta potential only changes slightly during continuous addition of superplasticizer, the cement particles will be dispersed by the steric hindrance mechanism.30,31
Although, both R1 and R2 have the same structure, their retardation potential is different. This can be attributed to a number of reasons. It is proposed that the lignosulfonate retardation mechanism is appearing by a combination of calcium complexation and precipitations onto anhydrate clinker phases. The origin of calcium ions is due to the rapid dissolution of anhydrous cement clinker phases especially, C3S once it gets contacted with water. Furthermore, the experimental work proofed that lignosulfonate concentration decreases with increasing calcium ion concentration in the solution. Furthermore, the microscopy analysis showed a layer of polymer (LS–Ca complexes) precipitate on anhydrous cement clinker phases providing a barrier preventing the water to penetrate into the cement hydration shell to produce more hydration products. Assuming this hypothesis is valid although it is stated that the complexation mechanism is unlikely because chelators can be moderate retarders while strong retarders can be moderate chelators the counter-ion effect should not be discounted.4 From the elemental analysis results it can be seen that calcium is present in R1 by about 8.4 wt% while, IC result shows the calcium ions in water present by about 17.8 mg L−1 in 200 mg L−1 sample. That means that by adding R1 to the cement slurry, the calcium concentration will increase and thus, the lignosulfonate concentration in the suspension will decrease because it is consumed by additional calcium ions if our assumption still holds. Consequently, the amount of lignosulfonate available for hydration retardation is not enough and provides better effectiveness. R1 contains higher amounts of inorganic carbon in form of CO32− ions. As a result those ions compete with lignosulfonate particles for the positive active sites on hydration products mainly on early C3A hydrates and therefore reduces the effectiveness of R1. The anionic charge density differs from R2 and is almost by 35% less. That means that at the same dosage the amount available is not effective enough to retard the cement hydration for a certain time. Moreover, the R1 amount is adsorbed or even chemisorbed. The effective functional group (sulfonate and hydroxyl) amounts are not sufficient to retard the cement hydration in comparison to the performance of R2 (Table 2).
Blank | 0.1% bwoc | 0.2% bwoc | 0.3% bwoc | 0.4% bwoc | 0.5% bwoc | 0.6% bwoc | 0.7% bwoc | |
---|---|---|---|---|---|---|---|---|
Time elapsed to reach peak maximum for R1 | 10.7 h | 16.9 h | 31 h | 1.76 days | 2.3 days | 4.46 days | 6.87 days | 15.8 days |
Time ratio between two concentrations for R1 | 0.1/0.0% bwoc | 0.2/0.1% bwoc | 0.3/0.2% bwoc | 0.4/0.3% bwoc | 0.5/0.4% bwoc | 0.6/0.5% bwoc | 0.7/0.6% bwoc | |
1.58 | 1.84 | 1.36 | 1.33 | 1.91 | 1.54 | 2.30 | ||
Time elapsed to reach peak maximum for R2 | 10.7 h | 16.1 h | 37 h | 4.23 days | 8 days | 15 days | 18.4 days | 24.76 days |
Time ratio between two concentrations for R2 | 0.1/0.0% bwoc | 0.2/0.1% bwoc | 0.3/0.2% bwoc | 0.4/0.3% bwoc | 0.5/0.4% bwoc | 0.6/0.5% bwoc | 0.7/0.6% bwoc | |
1.51 | 2.30 | 2.74 | 1.88 | 1.88 | 1.22 | 1.35 | ||
R2/R1 ratio at the same concentration | 0.95 | 1.19 | 2.41 | 3.43 | 3.37 | 2.67 | 1.57 |
Results using the support vector regression method for different dosages of R1 are listed in Table 3 and compared to the model tree method. It can be seen from the results, that the model indicates the surface behavior of the slurry in the presence of different additives and it would be able to predict any possible combinations of these additives on the cement slurry. Fig. 6 shows that there is some deviation to the actual experimental result, but the model can be surely used as a trend for dosage optimization.
Polymer | Performance measures | |||||
---|---|---|---|---|---|---|
R2 | MAE | RMSE | ||||
MT | SVR | MT | SVR | MT | SVR | |
R2 | 0.8177 | 0.6325 | 0.1128 | 0.1096 | 0.1558 | 0.1586 |
D1 | 0.9084 | 0.8548 | 0.0892 | 0.1092 | 0.1163 | 0.1484 |
D2 | 0.8595 | 0.8001 | 0.1105 | 0.0952 | 0.1447 | 0.1543 |
Comparison of the predicted to the experimental heat evolution values during the cement hydration process with and without polymer R1 is shown in Fig. 7. The data are obtained by the Support Vector Regression method (SVR) and the Model Tree (MT) method, the M5 model rules (see Table 4).
Polymer | Performance measures | |||||
---|---|---|---|---|---|---|
R2 | MAE | RMSE | ||||
MT | SVR | MT | SVR | MT | SVR | |
Blank | 0.9177 | 0.6349 | 0.0354 | 0.2509 | 0.3809 | 0.7413 |
0.1 R1 | 0.8914 | 0.8015 | 0.0441 | 0.1652 | 0.4118 | 0.5431 |
0.2 R1 | 0.9491 | 0.9012 | 0.0305 | 0.2461 | 0.296 | 0.4036 |
0.3 R1 | 0.9662 | 0.7192 | 0.0268 | 0.3108 | 0.2111 | 0.5521 |
As can be seen in Fig. 7, the model tree method applied on the investigated cement mixtures revealed that calculated values are in good agreement with experimental data and are in better accord than the equivalent calculations using support vector regression. Despite a slight deviation from the actual heat evolution compared to the experimental data the results of the mathematical models can be used to identify the setting time of the cement pastes. The good correlation obtained with these mathematical models in comparison to experimental results encourages the complete reliance on them rather than using expensive and lengthy experiments to assess the performance of additives on oil well cements in the oilfield.
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