A novel high temperature retarder applied to a long cementing interval

Abstract

A polymer comprised of sodium styrene sulfonate (SSS), itaconic acid (IA) and hydroxyethyl methacrylate (HEMA) was synthesized by aqueous free radical copolymerization and tested as high temperature performing retarder in oil well cement, which was characterized by gel permeation chromatography (GPC), infrared spectroscopy (IR), and thermal gravimetric analysis (TG). The optimum reaction conditions of polymerization were obtained: a mass ratio of SSS/IA/HEMA (PSIH) 9 : 3 : 1, initiator 2%, a reaction temperature of 60 °C, a reaction time of 5 h; characterization evidenced the PSIH had a certain molecular weight and excellent thermal resistance. Performance evaluation showed that the PSIH had good retarding properties at 150 °C, and demonstrated rapid development of compressive strength at 30 °C, providing a strong technical support for complex deep well cementing. Moreover, the retarding mechanism of PSIH was found, through XRD and SEM, to rely on adsorption onto the surface of the cement and chelation of calcium.

---

Introduction

The oil well cementing operation is non-recurring example of engineering and one of most important links in oil well construction. Retarders are one of three main additives for cement slurry, and are useful to delay the setting time of the slurry, thus providing sufficient pumpability over long distances.¹ For the long sealing interval of the well (Fig. 1) when cementing liners, cement slurry is injected into the bottom with a BHCT (the bottom hole circulation temperature) of about 150 °C, and returned to a specific location (temperature of about 90 °C) or even back to the ground (temperature of about 30 °C); the cement must develop compressive strength at the top of the liner before drilling is resumed.² However, the large temperature difference between the upper and lower intervals of the cement column causes huge challenges, which leads to problems such as dramatic shortening of the thickening time of the cement slurry, cement strength decline at the bottom of the oil well and slow development or super retarding occurring at the top of the cement slurry column with long cementing intervals, making it impossible to meet the cementing requirements.^3–5 Hence, a well designed retarder needs to increase the thickening time without having a significant effect on the compressive strength.⁶

Fig. 1 Schematic of tail pipe cementing.

A high temperature retarder adapted to 180 °C was developed by Guo et al.⁷ using AMPS and itaconic acid, and the best performing retarders were synthesized by Yang et al.,⁸ Constantin¹ and Zhao et al.⁹ using AMPS as the main monomer. Though most of synthetic retarders can prolong the thickening time of cement slurry to over 4 h, the compressive strength of the cured cement is reduced significantly, which not only delays the progress of drilling operations, but also increases the security risks of subsequent drilling operations.^10,11 Besides, Li et al.¹² found that the thickening curve of retarder the AMPS/IA/AM shows a significant bulge phenomenon at 190 °C in consistency tests, as well as a sharp decrease of slurry consistency and poor sedimentation stability when the temperature reaches a certain value. So far, there are very few retarders that can achieve an adjusted thickening time at high temperature, and do not affect the development of cement strength at low temperature for deep and ultra-deep wells.¹³

In order to meet the needs of cementing construction safety and overcome inferior AMPS-based retarders, to synthesize a terpolymer which can meet the performance demands of both thickening time and compressive strength is critical; some novel monomers with specific functions were introduced at the same time. In this study, a new high temperature retarder PSIH containing styrene sulfonate (SSS), itaconic acid (IA) and hydroxyethyl methacrylate (HEMA) was synthesized which is adapted to a large temperature range, 30 °C–150 °C. Furthermore, the structure and thermal stability of the copolymer was characterized, and the high temperature retardation and effects of those properties on the compressive strength of cement were evaluated.

Experimental

Materials

Sodium styrene sulfonate (SSS) were obtained from Loctite Chemicals Co (Beijing, China). Itaconic acid (IA), hydroxyethyl methacrylate (HEMA), sodium hydroxide and potassium per sulfate were obtained from the Kelong Chemical Reagent Co (Sichuan, China). The API class G cement was of high sulfate-resistant grade from Jiahua Enterprise Co (Sichuan, China). Fluid loss control additive G33S and an anti-foaming agent were obtained from Weihui Chemical Co (Henan, china).

Synthesis of PSIH

Copolymers of SSS/IA/HEMA (PSIH) were prepared by an aqueous solution polymerization technique in the laboratory. First, the SSS (18 ± 0.02 g), IA (3 ± 0.02 g) and HEMA (1 ± 0.02 g) were put into the beaker, respectively, with an appropriate amount of deionized water and dispersed with ultrasonic vibration; the pH value was adjusted to the desired value (pH = 7) with sodium hydroxide solution. Then, the solution of SSS and IA was added into a 250 mL three-necked flask and heated to 60 °C. The initiator, potassium persulfate at about 2%, and HEMA were dropped into the flask slowly through a separating funnel in the presence of nitrogen and reacted for 5 h with stirring; the product solution with a certain viscosity was obtained after it naturally cooled to room temperature. The terpolymer was extracted by adding ethanol to the liquid system after copolymerization, drying, and pulverization, then dissolved in distilled water. After extraction, drying, grinding was repeated three times, a powder was obtained. The polymerization equation is shown in Fig. 2.

Fig. 2 Copolymerization equation of PSIH.

Characterization

The pellet samples were prepared by pressing a mixture of PSIH and KBr and then measured with Nicolet 6700FTIR (Bio-Rad, USA) spectrophotometer in a range between 4000 and 400 cm⁻¹. Additionally, a ¹H nuclear magnetic resonance spectrum of PSIH was recorded on a JOEL JMX-GX-270 spectrometer (JOEL GmbH, Germany), that was referenced internally to deuterium oxide which was used as solvent. The molecular weight distribution of PSIH was measured by Alliance e2695 gel permeation chromatography (Waters, USA). The polymer was dissolved into distilled water forming a solution with a concentration of 2 mg mL⁻¹ and measurement was performed at room temperature (23 °C) for 90 min. The heat-resistance of PSIH was measured by a TGA/SDTA85 thermogravimetric analyzer (Mettler Toledo, Switzerland) under N₂ protection, at a heating rate of 10 °C min⁻¹ from 25 °C temperature to 600 °C. Furthermore, the surface and phase composition of set cement samples were analyzed via DX-2000 X-ray diffraction and JSM-7500F scanning electron microscope (20 kV, JEOL, Japan).

Performance evaluation

Cement slurries were prepared in accordance with the procedures described in “Recommended Practice for Testing Well Cements”, API Recommended Practice 10B, issued by the API.¹⁴ The retarder was evaluated according to the oil and gas industry standard SY/T5504.1-2005 “Evaluation Method for Well Cement Additives – Part 1 – Retarder”.¹⁵

Performance evaluation mainly includes the thickening-time test and strength test, which were used to explain the retarding property of PSIH. The thickening time elapsed from the initial application of pressure and temperature to the time at which the slurry reaches a consistency deemed sufficient to make it unpumpable (e.g. 70 Bc or 100 Bc), and the test using the HPHT (high pressure-high temperature) consistometer with different dosage of polymer retarder PSIH under different test conditions (90 °C, 120 °C, 150 °C). The compressive strength of a set cement sample was measured by the force required to cause it to fail in compression, expressed as force per unit area, and the experiment used a YA-300 pressure testing machine under different temperatures (30 °C, 90 °C and 120 °C) in order to investigate the impact of the high temperature retarder on the compressive strength of cement at high, medium and low temperature situations. The basic formula includes: Grade G Jiahua oil-well cement, 35% silica powder, 2% fluid loss additive G33S, and 53% freshwater.

Results and discussion

Molecular weight and chemical structure

At first, the GPC spectrum of PSIH (Fig. 3) shows a uniform copolymer with a relatively smooth curve, which indicates that there are less low-molecular-weight species. Molar masses of 39 027 g mol⁻¹ (M_w) and 16478 g mol⁻¹ (M_n), respectively, were found from the molecular parameters exhibited in Fig. 3. Also, the PSIH possesses a relatively low molecular mass and a broad polydispersity index (PDI) of 2.37, typical for radical polymerization.

Fig. 3 Gel permeation chromatography spectrum of PSIH.

Then, to illustrate the molecular structure of the polymer, the FTIR of SSS, IA, HEMA and the prepared terpolymer PSIH are shown in Fig. 4; a ¹H NMR spectrum of purified copolymer was recorded in D₂O and the desired chemical structure corresponding to the proton NMR peak is shown in Fig. 5.

Fig. 4 FTIR spectra of SSS, IA, HEMA and PSIH.

Fig. 5 ¹H NMR spectrum of PSIH, measured in D₂O.

As seen from Fig. 4, in the spectrum of SSS, the characteristic band at 3207.83 cm⁻¹ is a stretching vibration absorption signal corresponding to the C–H in a benzene ring, whereas 1192.17 cm⁻¹ is the characteristic absorption band of S=O and 885.25 cm⁻¹ is the characteristic absorption band of a para-substituted benzene structure. In the spectrum of IA, the characteristic bands at 3116.12 cm⁻¹ is the stretching vibration absorption signal corresponding to O–H, and 1701.66 cm⁻¹ is the characteristic absorption band of a C=O in a carboxyl group. In the spectrum of HEMA, the characteristic band at 3411.06 cm⁻¹ is the stretching vibration absorption signal corresponding to the O–H in an alcoholic hydroxyl. In the spectra of PSIH, the absorption bands at 3406.91 cm⁻¹, 2955.62 cm⁻¹, 1714.10 cm⁻¹ and 1165.09 cm⁻¹ are respectively produced by the O–H, methyl and methylene groups, C=O and C–O–C in IA and HEMA. 3223.97 cm⁻¹, 1063.83 cm⁻¹ and 884.73 cm⁻¹, respectively, are assigned to the stretching vibrations absorption signal corresponding to C–H, C–O–C and para-substituted benzene structure introduced by SSS. From the above analysis, we can infer that the synthesized polymer contained all of the features of the functional groups of SSS, IA, HEMA. Besides, the characteristic absorption band of –CH=CH₂ at 810.60 cm⁻¹ in the SSS and HEMA spectrum and at 818.14 cm⁻¹ in the IA spectrum did not appear in the spectra of PSIH, indicating that all monomers are involved in the polymerization.

Futhermore, it can be seen from the ¹H NMR spectrum (Fig. 5) that the strongest peak at 4.66 ppm is the vibration peak of the solvent D₂O. The broad peak between 1.00 and 1.95 ppm can be assigned to methylene protons present in the backbone of the polymer and to methyl protons of the HEMA monomer. The CH protons of SSS located along the backbone as well as the methylene protons of IA and HEMA appear as several small overlapping peaks between 2.92 and 3.53 ppm. Moreover, there is a vibration peak of hydrogen in the benzene ring introduced by SSS at 7.45 ppm. Combined with the FTIR spectra in Fig. 4, it can be concluded that the terpolymer AMPS/AM/SSS is successfully synthesized.

Heat-resistance of PSIH

As shown in Fig. 6, the thermal stability of PSIH was studied. From the TG and DTG curve of PSIH, the weight loss of the copolymer sample was divided into three stages. The weight loss of the first stage from 30 °C to 130 °C was about 13.0% caused by volatilization of adsorbed water, crystal water and small molecular impurities in the sample. The slight weight loss of the second stage from 130 °C to 375 °C was about 6.14% mainly because of the thermal decomposition of functional groups in the small molecule polymer. The weight loss became appreciable from 375 °C, attributable to the thermal decomposition of the carbon chain and functional groups of the macromolecules. Correspondingly, the sharp peaks at 75 °C, 245 °C, 295 °C, 435 °C in the DTG curve show the maximum decomposition rate of water, carboxyl, sulfonate groups and carbon chain, respectively, which indicates the PSIH may have excellent heat resistance properties, with a degradation temperature as high as 375 °C.

Fig. 6 TG and DTG curve of PSIH.

Cement slurry thickening time

Fig. 7 shows the thickening time of cement slurry with different dosages of the polymer retarder PSIH under 120 °C and 73 MPa; it can be seen from the curve that PSIH could prolong the thickening time of cement slurry as the dosage of retarder is increased. Specifically, the thickening time of cement slurry with 2.0% PSIH could reach 455 min (Fig. 8); it was found that during increased temperature and pressure the thickening curve was smooth and stable, and the cement slurry maintained a consistency of around 20 Bc and had a short transition time, which is beneficial for cementing. Fig. 9 shows the thickening time of cement slurry with 1.0% PSIH under different temperatures, it reveals that the thickening time decreases as the temperature increases from 90 °C to 150 °C and presents a slight trend until the thickening time of the cement slurry reaches 245 min, which also meets the requirement of site cementing. From Fig. 7 to Fig. 9, it is apparent that PSIH has excellent property of retardation on cement hydration under high temperature and pressure.

Fig. 7 Thickening time with different dosages of PSIH at 120 °C.

Fig. 8 Thickening curves with 2.0% PSIH.

Fig. 9 Thickening time with 1.0% PSIH at different temperatures.

Cement compressive strength at different temperature

Corresponding to the results of the thickening experiment, it is vital for the oil-well cement slurry to be applied in a long cementing interval to develop a satisfactory compressive strength within 24 h and 48 h at 30 °C, 90 °C and 120 °C. The results of cement compressive strength with different dosages of PSIH are shown in Fig. 10. It can be seen that at 30 °C and 90 °C (Fig. 10a and b), the development of cement compressive strength is rapid; within a certain range of retarder dosage the early compressive strength of cement decreases to different degrees, especially at 30 °C, where the strength of cement can exceed the lowest-value requirements (3.5 MPa) for hanging sleeves within 48 h. At 120 °C (Fig. 10c), corresponding to the extended thickening time over 4 h, the cement compressive strength is not attenuated significantly, and can be maintained at more than 20 MPa. Combining all of above, it is shown that the polymer retarder PSIH can meet the requirements of cementing and drilling operations, and can effectively avoid the phenomenon of super-retardation at low temperature when applied to a long cementing interval.

Fig. 10 Compressive strength of cement with PSIH, ((a) at 30 °C; (b) at 90 °C; (c) at 120 °C).

Retardation mechanism of PSIH

The XRD patterns of set cement cured at 90 °C for 24 h without and with different dosages of PSIH are shown in Fig. 11. Combined with the inorganic crystal structure database and XRD patterns, the interlocked grid of set cement was formed from Ca(OH)₂ crystals, SiO₂, C–S–H, AFt crystals and AFm crystals, which showed that the major phase compositions of set cement with 0.5% and 1.0% PSIH were the same as those of set cement without PSIH. However, it is observed that the diffraction peaks of Ca(OH)₂ crystals were weakened when the dosage of PSIH was increased, but silica, AFt and AFM crystals were slightly increased at the same time. Evidenced by the change of component, it can be inferred that the retarder is added to inhibit the growth of Ca(OH)₂ crystal, which leads to a delay in the degree of cement hydration, but promotes the formation of AFt and AFm crystals that are conducive to the development of cement strength.

Fig. 11 XRD patterns of set cement with different dosages of PSIH.

Fig. 12 shows SEM images of a set cement surface which was cured at 90 °C for 24 h without and with PSIH, respectively. In Fig. 12a, a number of spherical, sheet and acicular formations appear, corresponding to hydration products such as AFm crystals, Ca(OH)₂ crystals and AFt crystals which grow on the clinker grains of set cement without PSIH.¹⁶ Analyzed on the micrograph of different areas of set cement with 1.0% PSIH, it can be found that an insoluble and impermeable film was formed and coated on the surfaces of the hydrating cement particles, acting as a barrier for continued contact with water (Fig. 12b). Also, the hexagonal sheet and the spherical formations are significantly decreased, but the acicular formations increase (Fig. 12c), which is in accord with the samples of XRD analysis and it can be deduced that added retarder can hinder the growth of cement hydrates especially Ca(OH)₂ crystals.

Fig. 12 SEM micrographs of a set cement surface at curing conditions of 90 °C, 24 h ((a) without PSIH; (b), (c) different areas with 1.0% PSIH).

Comparison of the microstructures presented in Fig. 11 and 12 with theories in the literature, “adsorption deposition” and “calcium complexation” are proposed to explain how copolymer PSIH slows the hydration rate of cement slurry. Firstly, the retarder adsorbs on the surface of the particles or products of cement hydration, and a semipermeable membrane acting as protective layer is formed gradually, which decelerates the water contact with hydrating cement particles until a certain pressure is produced by osmosis and the diffusion of free water. Besides, the retarder has a strong calcium binding capacity, leading to a decrease in calcium existing in the cement pore solution and free water. Eventually, the amount of Ca(OH)₂ crystals is significantly reduced, and the formation of hydration products is inhibited as less calcium is available for their growth, resulting in the hydration process of cement slurry being delayed.

Conclusions

(1) A novel high temperature retarder PSIH was synthesized by aqueous solution radical polymerization and the optimum reaction conditions as follows: mass ratio of SSS/IA/HEMA (PSIH) 9 : 3 : 1; initiator 2%; reaction temperature 60 °C; reaction time 5 h.

(2) Characterization by gel permeation chromatography (GPC), infrared spectroscopy (IR), nuclear magnetic resonance hydrogen spectrum (¹H NMR) and thermal gravimetric analysis (TG) demonstrated that PSIH is the target copolymer, which has a suitable molecular weight and excellent thermal resistance below 375 °C.

(3) Based on the performance evaluation of PSIH, the two methods of strength development and thickening time indicated that PSIH can significantly prolong the cement slurry thickening time at high temperature (150 °C) and with a good shape of the tthickening curve. It also shows the rapid development of compressive strength at low temperature (30 °C), which is beneficial to long cementing intervals.

(4) Through XRD analysis and SEM analysis of the set cements, the retardation mechanism of PSIH mainly relies on “adsorption deposition” and “calcium complexation”, which effectively reduces the contact of water with the cement particles, and retards the hydration rate by blocking the raw materials (Ca²⁺) from generating hydration products.

Acknowledgements

The authors would like to acknowledge co-financial support by the National Science and Technology Major Project (No. 2016ZX05020004-008 and 2016ZX05052).

Notes and references

1. T. Constantin and P. Johann, J. Appl. Polym. Sci., 2012, 124, 4772.
2. A. B. Oriji and S. B. Zakka, Innovat. Syst. Des. Eng., 2013, 3, 32.
3. C. Ma, Y. Bu and B. Chen, Constr. Build. Mater., 2014, 60, 25.
4. A. S. Al-yami, M. K. Al-arfaj and H. A. Nasr-ei-din, SPE/IADC Middle Drilling Technology Conference & Exhibition, SPE/IADC 107538, 2007.
5. C. V. Umeokafor, and O. F. Joel, SPE Society of Petroleum Engineers, SPE 136973, 2010.
6. E. B. Nelson and J. M. Casabonne, SPE Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, SPE 24556, 1992.
7. S. L. Guo, Y. H. Bu and Z. H. Shen, Res. J. Appl. Sci., Eng. Technol., 2013, 19, 4751.
8. D. J. Chen, Z. Y. Yu and Y. Zhang, Drill. Fluid Completion Fluid, 2012, 29, 63.
9. H. Zhao, Y. Tian and Q. S. Wang, J. Yangtze Univ., Nat. Sci. Ed., 2012, 9, 73.
10. S. Li and R. Tu, Guide to Business (Enterprise Herald), 2012, 5, 276.
11. B. H. Zhao, J. L. Zou and A. P. Liu, Petroleum Drilling Technology, 2012, 40, 55.
12. M. Li, J. Z. Jin and Y. J. Yu, Spec. Petrochem., 2013, 30, 1.
13. J. T. Guo, X. J. Xia and S. Q. Liu, Pet. Explor. Dev., 2013, 40, 656.
14. American Petroleum Institute, API Recommended Practice 10B-2, American Petroleum Institute, Washington, 1st edn, 2005.
15. SY/T 5504. 1—2005 Oil Well Cement Additives Evaluation Method (Part 1): Retarder. Beijing: Petroleum Industry Press, 2005.
16. H. X. Zhang, J. Zhuang and S. Huang, RSC Adv., 2015, 5, 55428.

---