Preparation and property of 2-acrylamide-2-methylpropanesulfonic acid/acrylamide/sodium styrene sulfonate as fluid loss agent for oil well cement.
In the cementing process of oil and gas well, various additives were usually added into the cement slurry to regulate the performance of cement slurries and to meet the requirements of cementing. Among all properties of the cement slurries, the fluid loss (amount of water loss) is one of the most critical concerns for cement slurry formulation, especially at high temperature and in high pressure. The main function of fluid loss additives is to prevent the cement slurries from dehydrating to keep its pumping ability and other designed properties. When the fluid loss of the cement slurries reaches a certain value, some severe problems could be arisen, such as shortening of thickening time, deterioration of rheological behavior, and appearance of annular channeling (1-3).
As the development of oil exploration all over the world moving to the deep formation, the temperature and pressure of the formation have been rising (4). In the conditions of high temperature and high pressure, the traditional fluid loss additives of oil well cement, such as cellulose and polyvinyl alcohol, are easy degraded (5). So, the new synthetic polymers were studied to replace the traditional fluid loss additives. So far, the homopolymers and copolymers of 2-acrylamide-2-methylpropanesulfonic acid (AMPS), acrylamide (AM), N-methylacrylamide salts, N-isopropylacrylamide, N-butylacrylamide, N-diethylamide, N-hydroxymethylamide, N-hydroxymethylacrylamide, and N-alkylacrylamides (6-11) have been used as fluid loss additives. However, these acrylamide copolymers are less thermally stable and easy to hydrolyze at severe operating conditions especially in high temperature and caustic environment.
Therefore, in this paper, a new polymeric fluid loss additive has been synthesized with AMPS, AM, and sodium styrene sulfonate (SSS). The rigidity of its molecular chain has been enhanced by introducing the benzene ring structure, so the terpolymer has higher thermal stability than AMPS/AM copolymer.
There were two main mechanisms of fluid loss control reported in the literature (12): First, the diameter of cement pore was reduced because of the adsorption of the long-chain polymers on the surface of cement particles. The cement pores were blocked as the hydration and expansion of polymers. Second, the compactness of filter cakes was improved. In this paper, the mechanism of polymer fluid loss control has been discussed with [zeta] -potential measurement and scanning electron microscopy (SEM).
Synthesis of AMPS/ AM/ SSS
The AMPS was refined by recryslallization. In a 250-mL round-bottom three-necked flask, 15 g of monomers (the molar ratio of AMPS, AM, and SSS was adjusted according to the orthogonal test) was dissolved into 85 mL distilled water, the pH was adjusted to 6-7 with NaOH. The nitrogen gas was pumped into the reaction system at room temperature with slow stirring for 30 min. Then, the aqueous solution of azobisisobutryamide chloride was dropped the reaction mixture, keeping at a setting temperature (35, 40, or 45 [degrees] C according to the orthogonal test) for 6 hr with constantly stirring. During the polymerization, the reaction system became more and more viscous. After the reaction, the product was extracted with acetone for three times. The reaction equation is shown in Fig. 1. The mechanism of the terpolymerization was a free radical chain polymerization.
Fluid Loss Test
The preparation and fluid loss test of cement slurries were all in accordance with American Petroleum Institute (API) standard, and the concrete steps are as follows: Oil well cement of API Class G, deionized water, and AMPS/ AM/SSS were mixed in waring blender, weight ratio of water to dry-cement 0.44, and terpolymer 1.5 wt% by weight of cement (BWOC) and dispersant 0.3 wt% BWOC. Then, the cement slurries were transferred into OWC-9350 Constant Pressure Thickening Instrument (Institute of Applied Technology, Shenyang Aerospace University, China), heated to 150 [degrees] C and held for 20 min. Finally, the cement slurries were added into GGS-71 High-Temperature and High-Pressure Water Loss Meter (Tongchun analytical instrument factory of Qingdao, China), and the fluid loss of the slurries were determined under the conditions of 150 [degrees] C, 6.9 MPa, in 30 min.
Nicolet 7600 Fourier Infrared Spectrometer (KBr pellet, from 400 to 4000 [cm.sup.(-1)] ) and Bruker Avance 600 NMR Spectrometer (solvent [D.sub.2]O) was used to investigate the structure of the polymer. Mettler Toledo thermogravimetric and simultaneous differential thermal combined analyzer of type TGA/SDTA851e was used to checkout the thermal stability, from 30 to 550 [degrees] C, 10 [degrees] C [min.sup.(-1)]. NDJ-1 Rotational Viscometer (Shanghai Geology Instrument Institute, China) was used to test the apparent viscosity of solvent [H.sub.2]O at 30 [degrees] C, 10 rpm. The [zeta] -potential of cement slurries after thickening for 20 min in atmospheric pressure was tested with a zeta potential analyzer (ZetaProbe, Colloidal Dynamics). The fresh filter cakes after fluid loss tests were cut into two equal halves. The cross section of the filter cake was observed with a JSM-5900LV scanning electron microscope (20 kV, JEOL, Japan).
RESULTS AND DISCUSSION
Conditions of Terpolymerization
The orthogonal experiments ([L3.sup.3] ) were arranged to investigate the main effects (see Table 1) of the reaction temperature, the amount of initiator, and the molar ratio of monomers on the fluid loss. The less fluid loss showed the better performance of fluid loss agents. From the results of orthogonal experiments shown in Table 2, the optimal polymerization conditions were obtained as follows: reaction temperature, 40 [degrees] C; amount of initiator, 0.1 wt% (by weight of monomers); molar ratio of monomers AMPS/AM/SSS, 20/5/1. On optimal polymerization conditions, a new sample was synthesized and found the fluid loss was 40 mL in the same test condition.
TABLE 1. Factors of the orthogonal experiments ([L3.sup.3]). Levels 1 2 3 Reaction temperature (C) 35 40 45 Amount of initiator (%) 0.1 0.2 0.3 Molar ratio of monomers AMPS/AM/SSS 16/9/1 18/7/1 20/5/1 TABLE 2. Results of the orthogonal experiments ([L3.sup.3]). No. Reaction Amount of Molar ratio of Fluid temperature initiator monomers loss ([degrees] C) (%) AMPS/AM/SSS (mL) 1 35 0.1 16/9/1 76 2 35 0.2 18/7/1 78 3 35 0.3 20/5/1 84 4 40 0.1 18/7/1 48 5 40 0.2 20/5/1 82 6 40 0.3 16/9/1 86 7 45 0.1 20/5/1 44 8 45 0.2 16/9/1 86 9 45 0.3 18/7/1 94 [k.sub.1] 79 56 82 [k.sub.2] 72 82 73 [k.sub.3] 74 88 70 R 7 32 12 Note; [k.sub.1], average fluid loss of level one; [k.sub.2], average fluid loss of level two; [k.sub.3], average fluid loss of level three; R, range.
By comparing the R of three factors, it was found that the impact increased in the following sequence: reaction temperature < molar ratio of monomers AMPS/AM/SSS < amount of initiator. It is obvious that the main influence factor is the amount of initiator.
The Dosage Impact of Initiator
According to the orthogonal experiments, the amount of initiator is the most important influence factors of the terpolymerizaton, so we carried out single factor experiments. The dosage impact of initiator is shown in Fig. 2.
It can be seen from Fig. 2 that the fluid loss of cement slurry decreases first and increases later, and the minimum is 44 mL with the initiator of 0.1 wt%. This may be due to the terpolymers that have the maximum molecular weight at that point. When the dosage of initiator was too small, less free radical was generated, and free radical was stabilized and loss their activity by attracting electron effect of sulfonate group in AMPS, resulting in the low molecular weight and poor performance of fluid. When the dosage of initiator was too large, excessive free radicals were produced, the reaction was too fast, more heat was produced, the reaction temperature was increased, the self-accelerating effect was generated, and the chain termination rate was accelerated too, so the molecular weight is not high enough, resulting in poor performance of fluid loss control . Only the appropriate dosage of initiator could produce the appropriate reaction speed and obtained terpolymers with high molecular weight and good performance of fluid loss. The experimental results show that this appropriate initiator dosage was 0.1 wt%.
Fourier Transform Infrared Spectrometer Analysis
The Fourier transform infrared spectrometer (FTIR) spectra of AMPS, AM, SSS, AMPS/AM, and AMPS/AM/ SSS are shown in Fig. 3. In the spectrum of AMPS, the characteristic bands at 3237.1 [cm.sup.(-1)] is stretching vibrations absorption signal corresponding to N--H in--CO--NH--, whereas 2947.2 [cm.sup.(-1)] is the characteristic absorption band of --[CH.sub.3] and 1668.7 [cm.sup.(-1)] is the characteristic absorption band of C=O. In the spectrum of AM, the characteristic bands at 3354.2 and 3181.9 [cm.sup.(-1)] are stretching vibrations absorption signals corresponding to N--H in--CO--[NH.sub.2], and 1668.7 [cm.sup.(-1)] is the characteristic absorption band of C=O. In the spectrum of SSS, the characteristic band at 3085.7 [cm.sup.(-1)] is stretching vibration absorption signal corresponding to C--H in benzene ring. In the spectra of AMPS/AM and AMPS/AM/SSS, the large absorptions band from 3100 to 3600 [cm.sup.(-1)] were coproduced by the N--H in AMPS and AM. The characteristic absorption bands of--CH--[CH.sub.2] at 980.3 [cm.sup.(-1)] in AMPS spectrum, at 989.4 and 961.7 [cm.sup.(-1)] in AM spectrum, and at 989.0 [cm.sup.(-1)] in SSS spectrum did not appear in both spectra of AMPS/AM and AMPS/AM/SSS, indicating that all monomers are involved in the polymerization. In the spectra of AMPS/AM/SSS, 2993.2 [cm.sup.(-1)] is the characteristic absorption band of --[CH.sub.3] introduced by AMPS; 1673.9 [cm.sup.(-1)] is the characteristic absorption band of C=0 introduced by AMPS and AM. From the above analysis, we can infer that the synthesized polymer at least contained AMPS and AM.
Nuclear Magnetic Resonance Hydrogen Spectrum Analysis
The nuclear magnetic resonance hydrogen spectrum ([.sup.1]H NMR) spectra of AMPS/AM and AMPS/AM/SSS are shown in Fig. 4. In the [.sup.1]H NMR spectrum of AMPS/AM, 1.400 ppm is the vibration peaks of --[CH.sub.3], 2.134 ppm is the vibration peaks of --[CH.sub.2], 3.179 ppm is vibration peaks of CH, and 4.777 ppm is the vibration peak of solvent [D.sub.2]O. The [.sup.1]H NMR spectrum of AMPS/AM/SSS has almost the same vibration peaks, but the only difference is that there is vibration peak of hydrogen in benzene ring introduced by SSS at 7.571 ppm. Combined with the FTIP spectra in Fig. 3, it can be concluded that the terpolymer AMPS/AM/SSS is successfully synthesized.
Thermogravimetric and Simultaneous Differential Thermal Analysis
With initiator azobisisobutryamide chloride of 0.1 wt% (by weight of monomers) and monomers mass ratio AMPS to AM of 20/5, a copolymer AMPS/AM was synthesized at 40 [degrees] C. As shown in Fig. 5, the thermostabilities of AMPS/AM/SSS and AMPS/AM are studied with thermogravimetric analysis and simultaneous differential thermal analysis (TGA-SDTA).
In the TGA curve of AMPS/AM/SSS. the weight loss from 25 to 320 [degrees] C might be caused by volatilization of free water and small molecular impurities in the sample. The main weight loss at 350 [degrees] C, indicating that the terpolymer started to decompose at 350 [degrees] C, corresponds to the clear heat absorption peak at 350 [degrees] C in SDTA curve of AMPS/AM/SSS.
The same variation also emerged in TGA-SDTA curve of AMPS/AM, but they have remarkable differences. The copolymer AMPS/AM started to decompose at about 290 [degrees] C, but the terpolymer AMPS/AM/SSS started to decompose at 350 [degrees] C. This meant that the thermal stability of terpolymer AMPS/AM/SSS was much better than that of AMPS/AM. The reason is that the rigidity of molecular chain of AMPS/AM/SSS was enhanced by benzene ring introduced by SSS. The steric hindrance of internal rotation was increased, and the heat movement of molecular chain was difficult, thus enhancing the polymer heat resistance.
To discuss the dosage effect of SSS on thermal stability of the terpolymer AMPS/AM/SSS in detail, the polymer was synthesized with difference molar ratios of monomers, 20/5/0.5 and 20/5/2, at the same reaction conditions, azobisisobutryamide chloride 0.1 wt%, 40 [degrees] C, and compared with AMPS/AM/SSS with monomers molar ratio of 20/5/1. The TGA curves of AMPS/AM/SSS with monomers molar ratio of 20/5/0.5, 20/5/1, and 20/5/2 are shown in Fig. 6.
From Fig. 6, the terpolymer with monomers molar ratio of 20/5/0.5 started to decompose at 260 [degrees] C, the terpolymer with monomers molar ratio of 20/5/2 started to decompose at 410 [degrees] C so the thermal stability of AMPS/AM/SSS was enhanced with the dosage increasing of SSS. However, does the performance of fluid loss control of the terpolymer have the same enhancing trend? That is the study subject of our future.
Performance of Fluid Loss Control
In Fig. 7, the fluid loss of terpolymer AMPS/AM/SSS and copolymer AMPS/AM were compared from 60 to 160 [degrees] C. It can be found that the major advantage of terpolymer AMPS/AM/SSS over copolymer AMPS/AM was the ability to maintain fluid loss control over a broad range of temperatures. In this study, when the temperature surpassed 120 [degrees] C, the fluid loss control of copolymer AMPS/ AM was deteriorative, because it exceeded 100 mL and could not meet the requirements of engineering any longer (SY/T 5504.2-2005). But the terpolymer AMPS/AM/ SSS was very stable and exhibited excellent fluid loss control performance with temperature range from 60 to 160 [degrees] C, indicating again that the terpolymer AMPS/AM/ SSS had good thermal stability.
The Apparent Viscosity
The aqueous solution of AMPS/AM/SSS was prepared with mass concentration of 0.5%, and heated in the roller oven at 30, 60, 90, 120, and 150 [degrees] C for 12 hr. Then, the apparent viscosity of the terpolymer was measured with rotational viscometer at 30 [degrees] C, 10 rpm. The apparent viscosity and viscosity retention are shown in Fig. 8.
It can be seen in Fig. 8, the apparent viscosity of AMPS/AM/SSS was on a downward trend, but even after treating at 150 [degrees] C, the apparent viscosity of the terpolymer was still above 43 mPa S, and the viscosity retention retained around 75%. This is because the performance of heat resistance has been improved due to the introduction of the rigid benzene ring structure by SSS. Increasing of the liquid viscosity is one method of controlling the fluid loss in cement slurries , so the good performance of viscosity retention is another proof of the excellent fluid loss control performance of AMPS/AM/SSS under high temperature.
[zeta] -Potential Measurement
After the cement ash was mixed with water, the flocculated structure of cement slurry would be produced because of van der Waals force among cement particles. The schematic diagram of the flocculated structure is shown in Fig. 9. This flocculated structure was an anomalous netted body, which was constituted with different sized cement particles and free water in pores. The free water was very easy to spread out from the pores under high pressure, so a large amount of fluid loss was produced, resulting in serious problem in cementing engineering of oil well.
Cement slurries were prepared in accordance with API standards, adding certain amount of dispersant and fluid loss, [zeta] -potential of cement slurry was measured after thickening in OWC-9350 Constant Pressure Thickening Instrument under atmospheric pressure for 20 min. The fluid loss and [zeta] -potential in different cement slurries are compared in Table 3.
TABLE 3. Comrast table of fluid loss and [zeta]-potential in different cement slurries. Experiment Dispersant (wt%, AMPS/AM/SSS (wl%, [zeta] Fluid code BWOC) BWOC) (mV) loss (mL) 1 0 0 -3.5 1530 2 0.3 0 -21.8 1020 3 0.3 1.0 -29.5 62 Note: BWOC here is short for "by weight of cement."
In Experiment 2, the dispersant (SXY-2, condensation polymer of acetone and formaldehyde, see Fig. 10, Chuan Feng Chemical Engineering, Sichuan, China) was added into the cement slurries, the absolute value of [zeta] -potential on the surface of cement particles was increased, and the performance of fluid loss control was decreased. The reason is that the macromolecules of dispersant wrapped around the cement particles and increased the charge density (15).
And then, the fluid loss additive AMPS/AM/SSS is added in Experiment 3, the absolute value of [zeta]-potential on the surface of cement particles was further improved, because the terpolymer with sulfonic acid group could wrap on the surface of cement particles and enhanced the surface charge. So, the electrostatic repulsion of cement particles was further increased and the flocculation structure could not generate any longer (16), (17). With the increase of [zeta] -potentkl, the fluid loss of cement slurries was decreased and finally got to 62 mL, as shown in Table 3. So, we believed that one of the mechanisms of fluid loss control was preventing the generation of flocculated structure in cement slurries.
The SEM images of filler cake of the three cement slurries in Table 3, after fluid loss test at 150 [degrees] C, 6.9 Mpa, and 30 min, in different magnifications are shown in Fig. 11. From Fig. 1 la and b, SEM images of filter cake from cement slurry without any additives at different magnifications, we could see the uneven distribution of cement particles and big pores among the cement particles. These pores became channels of free water. The experimental result showed that the fluid loss of this cement slurry was 1530 mL. In Fig. 11c and d, the pores still existed, free water in the cement slurry could flow out through these pores, and its fluid loss was 1020 mL. From Fig. 1 le and f, we could see lots of small particles blocked the pores, so the porosity of the filler cake was the lowest, and the fluid loss was reduced by 62 mL.
Two reasons account for this phenomenon: First, the terpolymer AMPS/AM/SSS adsorbed and wrapped on the surface particles, and became bridges among these particles. These particles could squeeze together, and left less pores for free water. Second, the terpolymer AMPS/AM/ SSS on the surface of cement particles could produce a hydrated layer, which made the cement particles easily squeeze into the pores among the cement particles, even fully plugging the pores among the cement particles. Thereby, the channels of fluid loss were blocked. Both of these two reasons lead to the decrease of porosity. So. we found that another mechanism of fluid loss control in cement slurries was blocking the channel of fluid loss by reducing the porosity of the filter cake.
The terpolymer AMPS/AM/SSS was perfectly synthesized approved by FTIR and [.sup.1]H NMR with the initiator of azobisisobtltryamide chloride. The optimal polymerization conditions were obtained by orthogonal tests as follows: reaction temperature 40 [degrees] C, amount of initiator 0.1 wt%, and molar ratio of monomers AMPS/AM/SSS, 20/5/1. The TGA-SDTA showed that the terpolymer AMPS/ AM/SSS started to decompose at 350 [degrees] C. The fluid loss test showed that the terpolymer AMPS/AM/SSS exhibited excellent fluid loss control performance from 60 to 160 [degrees] C. After heating at 150 [degrees] C for 12 hr, the apparent viscosity of the terpolymer was still above 43 mPa S, and the viscosity retention retained around 75%. All these tests showed that the terpolymer AMPS/AM/SSS has better thermostability than AMPS/AM, however, we found that the thermal stability of AMPS/AM/SSS was enhanced with the increasing dosage of SSS, the effect of the dosage to the properties of AMPS/AM/SSS can be our next study topic. The mechanisms of fluid loss control of the terpolymer AMPS/AM/SSS, investigated by means of [zeta]-potential measurement and SEM, were that the terpolymer can prevent the generation of flocculated structure in cement slurries and reduce the porosity of filter cakes.
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Correspondence lo: Jia Zhuang; e-mail: Zhuangjia@swpu.edu.cn
Published online in Wiley Online Library (wileyonlinelibrary.com).
[C] 2011 Society of Plastics Engineers
Huan-Ming Li, Jia Zhuang, Han-Bin Liu, Li Feng, Wenbo Dong
School of Material Science and Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, People's Republic of China
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|Author:||Li, Huan-Ming; Zhuang, Jia; Liu, Han-Bin; Feng, Li; Dong, Wenbo|
|Publication:||Polymer Engineering and Science|
|Date:||Feb 1, 2012|
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