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Zwitterionic copolymer for controlling fluid loss in oilwell cementing: preparation, characterization, and working mechanism.

INTRODUCTION

Currently, oil/gas exploration and development are faced with severe challenges, such as the inferior quality of resources, complication of oil/gas development, the engravecer of safety and environmental protection, etc. [1,2] The tendency of the globe oil/gas exploration and development is gradually from the conventional reservoirs to unconventionality, from shallow to deep and ultra-deep, and from land to ocean [3]. As the rapid development of oil/gas exploration technology, the while-drilling formation is increasingly complicated, and the number of deep and ultradeep-, complex-wells is found to increase gradually [4]. Thus, the cement additives are becoming more and more important in the process of oil/gas well cementing, especially in the aspect of high- temperature resistance and good comprehensive performance. It is clearly acknowledged that the inferior fluid loss control cement slurry may be responsible for primary cementing failure due to the excessive density increase or annulus bridging, and the formation invasion by the filtrate of cement slurry may be contaminative to the reservoir. In multitudinous cementing additives, the fluid loss additive is an essential controlling agent which is widely applied to prevent the cement slurry from dehydrating to keep its pumping ability and other predetermined properties. Deep and ultra-deep wells are characterized by high bottom hole circulation temperature (BHCT) which may reach 260[degrees]C, high pressure and even high salinity [5]. Therefore, the principal function of fluid loss additives is to control the loss of liquid from cement slurry to the porous formation.

In the harsh conditions of high temperature, high pressure and high salinity, the traditional fluid loss additives, including polyvinyl alcohol, polyacrylamide and cellulose, are easily degraded, leading to the performance invalidation. Polyvinyl alcohol fluid loss additive has good performance at low temperature (below 100[degrees]C), but it is scarcely any salt-resistance [6]. The ployacrylamide (PAM) fluid loss additive is applied widely, but whose amide groups will gradually hydrolyze with the increasing of BHCT which lead to super retarding for cement slurry system [7, 8]. Cellulose filtrate control agents, hydroxyethyl cellulose (HEC) and carboxymethylhydroxyethyl cellulose (CMHEC), etc., are applied in the medium temperature range (50-150[degrees]C), however, they may cause the slurry thickening at high temperature and super-retarding at low temperature, because their performance is subject to the formation of large colloidal polymer and the adsorption on the surface of cement grains [9]. Meanwhile, other drawbacks of the cellulose additives are poor shearing ability, inferior thermal-stability, weak salt-resistance and susceptibility of multi-valence metal ionic, etc. Hence, the development of a high temperature and salt-resistant fluid loss additive is an inevitable trend in the deep and ultra-deep well cementing operations.

In this study, a novel zwitterionic polymeric fluid loss additive has been synthesized with 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS), dimethyldiallyl ammonium chloride (DADMAC), N,N-dimcthylacrylamide (DMAM) and acrylic acid (AA), and characterized with respect to its effectiveness. The results indicate that the zwitterionic polymer has a great potential to be used for the oil/gas wells cementing in the conditions of high-temperature and high salinity, and its performance evaluations are as follow.

EXPERIMENTAL

Design of Molecular Structure

The molecular structure of quadripolymer is polytropical, varying not only in the sort of monomer and the position of the functional groups, but also in the molecular weight and its distribution. The properties of fluid loss additive are based on different functional groups of copolymer to be designed in the main chains. Hence, the staggered space network structure of macromolecular is very important.

The zwitterionic polymer AMPS/DADMAC/DMAM/AA was synthesized by aqueous solution radical polymerization technique. Sulfonic acid groups in the lateral chain of AMPS possess high charge density and strong hydration tendency, and a negative charge is shared by combination of two it bonds and three high voltage negative oxygen atoms makes the copolymer having good salt-resistance [10]. AMPS shows outstanding adsorbability and complexation with cement grains due to its carbonyl groups. The major lateral chain in AMPS enhances the rigidity of polymer molecular chains, sequentially reinforces the thermal-stability of zwitterionic copolymer [11]. The introduction of DMAM makes the polymer more stable against hydrolysis under the strongly alkaline pH conditions of aqueous cement dispersions due to the steric hindrance of double methyl groups, especially in high temperature [12, 13]. DADMAC is geared to the cationic type quaternary ammonium salt, containing diolefinic bonds which can form a steady five carbon ring to improve the high temperature resistance and salt tolerance of copolymer [14]. To some extent, AA endows the copolymer high dispensability to improve the rheological behavior of cement slurry. Meanwhile, both the reduction in fluid loss and the improvement in compactness of filter cake rely on the adsorption of carboxyl groups on the surface of cement grains and hydration products [11, 15]. Because AMPS-based polymers have the feature of excessively increasing the consistency of the slurry as well as other aqueous soluble polymers, numerous dispersant should be introduced to improve the rheological behavior of slurry and to ensure the safety of cementing operation. However, the dispersant can impair the fluid loss control ability due to the competitive adsorption [16]. Excitedly, these problems can be solved by introducing DADMAC and adjusting the amount of AA to enhance the polymer absorbability on cement grains.

Chemical Materials

AMPS and DMAM were received from Ruibolong Oil Tech& Development (Beijing/China). DADMAC (AR) and AA (AR, distilled prior to polymerization) were obtained from Luyue Chemical (Shandong/China) and Eastern Petrochemical (Beijing/China), respectively. Ammonium persulfate (APS, oxidizer, AR) and N,N,N',N'-tetramethyl ethylenediamine (TMEDA, reducing agent, AR) were purchased from Sincere Chemicals (Shijiazhuang/China), which were prepared to solution concentration of 20-50% and then added separately to the reaction system. Dodecylmercaptan and sodium hydroxide (NaOH) were provided from Jiangtian Chemical Tech (Tianjin/China). Unless otherwise specified, all chemicals were used as received.

The mineral compositions and chemical compositions of Class G oil well cement used in our laboratory (HSR, Jiahua Special Cement, Sichuan, China) are shown in Table 1. Its average size, density, and Blaine specific surface area are 14 [micro]m, 3.15 g [cm.sup.-3], and 428.6 [m.sup.2] [g.sup.-1], respectively. Tartaric acid is used as a high temperature retarder to prevent the cement slurry from setting prematurely, which was obtained from Jiangtian Chemical Tech. Silica fume (20 [micro]m) is received from Yuyang Construction Group (Tianjin, China) and applied to prevent the strength retrogression of set cement under high temperature.

Synthesis of FDA-II

The quadripolymer was synthesized by the free-radical aqueous solution polymerization. The appropriate amounts of AMPS and AA were placed in a 250-mL four-necked round flash equipped with a mechanical stirrer, reflux condenser tube, thermometer, and distilled water. The pH value was adjusted to 5-7 by sodium hydroxide solution (3 mol [L.sup.-1]). Next, DMAM and DADMAC were mixed into the solution with 210 rpm constant stirring speed. A certain amount of chain transfer, the dodecylmercaptan was also added into the reaction solution which was used to control the molecular weight of copolymer. When the system was gradually heated to the predetermined temperature, the initiator solutions, APS and

TMEDA with the weight ratio of 2:1, were dropped into the solution respectively. The copolymerization proceeded being thermally insulated for 2-4 h with constantly stirring until the reaction ended. After the products were cooled to room temperature, the resulting viscous and yellowish aqueous polymer solution was freeze-dried directly, yielding a white powder to use for performance evaluation, coded as FDA-II. The synthesis route was shown in Fig. 1.

Characteristics of Zwitterionic Copolymer FDA-II

The zwitterionic copolymer was extracted by absolute ethyl alcohol, repeating for three times. Then, the precipitate was dissolved in deionized water again for dialysis treatment. Dialysis of copolymer was carried out in deionized water at room temperature for 3 days using dialysis membranes with a molecular weight cut off value (MWCO) of 14,000. Employing an vacuum freeze dryer apparatus (BenchTop Pro, VirTis, US; condensation temperature of -55[degrees]C, vacuum pressure of <20 Pa), the copolymer was freeze-dried for 24 h at a final pressure of 10 Pa and a white powder was obtained. Gel permeation chromatography (GPC, Agilent 1100, USA) was employed to determine the molecular weight of FDA-II, including the number average molecular weight ([M.sub.n]), weight average molecular weight ([M.sub.w]), z-average molecular weight ([M.sub.z]), polydispersity index (PDI), and so on. The structures of FDA-II were characterized by Bio-Rad FTS 3000 Fourier infrared spectrometer (Bruker DaltonicInc, America, KBr pellet, and spectral range from 400 to 4000 [cm.sup.-1]). [sup.1]H-NMR spectrum of FDA-II was recorded on Bruker Avance III 400 NMR (BrukerBioSpin, Switzerland) spectrometer by dissolving the samples in deuterium oxide ([D.sub.2]0). Model Netzsch DSC204F1 differential scanning calorimeter (NETZSCH-Geratebau GmbH, Selb, Germany) and Model TGA-Q500 thermal gravimetric analyzer (Shimadzu, Japan) were used to investigate the thermal stability of the copolymer in a nitrogen atmosphere at a heating rate of 10[degrees]C [min.sup.-1] with temperature range from 20 to 600[degrees]C. The apparent viscosity of copolymer aqueous solutions was tested by NDJ-1 Rotational Viscometer (Shanghai, China) at 30[degrees]C at 9500 mg [L.sup.-1] for the salt tolerance investigation. The surface morphology of FDA-II was investigated by a scanning electron microscope (SEM; XL-30, Philips co., LTD, Netherland).

Preparation of Cement Slurry

The cement slurry was prepared in accordance to the procedures described in "Recommended Practice for Testing Well Cements," API Recommended Practice 10 B issued by the American Petroleum Institute (API) standard [17]. The dosages of solid materials and liquid were accurately weighted according to the liquid/solid ratio (l/s) in order to meet the density of cement slurry. Next, the cement was dry-blended with silica powder at a weight ratio of 100:35 when the slurry was used for the high temperature (above 120[degrees]C). And, there was no silica powder added to the cement slurry when the temperature was below 120[degrees]C. Afterward, the liquid additives were dissolved in water in a consecutive way and stirred well. The mixture was added to the liquid placed in the cup of Waring blender within 15 s at 4000 rpm and mixed for 35 s at 12,000 rpm by using a model OWC-2000D Waring blender (Shenyang Tiger co., LTD, China).

Performance Tests of Conventional Cement Slurry

The cement slurry containing additives or/and admixture was transferred into the slurry cup of OWC-9350 Constant Pressure Thickening Instrument, heated to the target temperature and kept for 20 min. Next, the cement slurries were added to the TG-71 high-temperature and high-pressure (HTHP) water loss meter. The testing conditions were 30 min and 6.9 MPa. If the testing temperature was above 90[degrees]C, a pressure of 3.5 MPa [N.sub.2] was applied and the cylinder was heated up to the predetermined temperature within 1 h with constant stirring (150 rpm) after decanting the cement slurries into HTHP turnover water loss meter (Chandler Engineering, USA). When the temperature was reached, stop stirring and turn the instrument over. The filtrate of cement slurry was carried out with a pressure of 6.9 MPa [N.sub.2] in 30 min which was doubled as required by API RP 10 B and designated as API fluid loss of the corresponding cement slurry [17].

The compressive strength of set cement in different temperatures was obtained by employing a Chandler 5265U static gel strength analyzer through ultrasonic method, which could also reliably simulate the static gel strength development of cement slurry by adopting non-destructive measurement technology. Its run temperature can be high up to 205[degrees]C, and the maximum operating pressure (MOP) up to 137 MPa. Hence, the influence of cement additives on the compressive strength development of set cement could be observed conceivably via this method. To better understand the effect of different polymer amounts on the compressive strength of set cement, the curing temperatures of 30 and 60[degrees]C were chosen in this study.

Rheological Properties of Cement Slurries

The rheological parameters of cement slurries containing different amount of zwitterionic copolymer, such as shear stress, flow behavior index (n) and consistency index (k) were determined through measurements of the viscosities at different shear rates (or rotation rates) using a ZNN-D6 rotational viscometer (Qingdao, China) at different temperatures. [[theta].sub.300] represents the numerical value shown on the indicator dial of the six-speed rotational viscometer when it is rotated with 300 rpm (shear rate of 511 [s.sup.-1]). Similarly, [[theta].sub.100] represents the reading of the viscosity at 100 rpm (shear rate of 170 [s.sup.-1]). The computation formulas were as follows:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

SEM Analysis

The surface morphologies of zwitterionic copolymer and cement filtrate cake were determined by Model XL-30 SEM instrument. After slicing horizontally the filter cake, the original morphologies of filtration section could be observed through SEM equipped with GSE detector and 3.0 kV accelerating voltage.

Permeability of Cement Filter Cake

The permeability of set cement was measured by OFITE Model 360 Air Permeameter (OFI, TEXAS, USA), which could be calculated by the formula shown as follow [18, 19]:

K = [V.sup.2.sub.t] x [eta] x R/2[A.sub.2] x [DELTA]P x t

Where [V.sub.t] is the filtrate volume collected by the filtration experiment within assay time; [eta] is the viscosity of filtrate; R is the volume ratio of filtrate cake and filtrate collected; A is filtration area, usually 22.6 [cm.sup.2]; [DELTA]P is the differential pressure, usually 6.9 MPa; t is assay time for 30 min. The permeability measurement of cement filtrate cake is in favor of understanding the working mechanism of fluid loss additives.

TOC Analysis

Adsorbed amounts of polymer are determined from the filtrate collected from the filtration experiment of cement slurry. Generally, depletion method is applied to investigate the polymer adsorption on the mineral surface, which is the decrease in the polymer concentration before and after the contact with cement solely. It has been confirmed through a solubility test [20]. For this purpose, a high total organic carbon apparatus (TOC-VCPN, Shimadzu, Japan) equipped with a C[O.sub.2] and N[O.sub.x] detector is used to quantify polymer concentrations in the filtrate which collected from the filtration experiment of cement slurry. Before the TOC analysis, the alkaline cement filtrate containing the unabsorbed polymer was adjusted to pH 7.0 by adding 0.01M [H.sub.2]P[O.sub.4]. There is small amount of polymer in the filtrate which can be figured out by the method of TOC analysis. From the difference in polymer concentrations contained in the initial solution and the filtrate of cement slurries, the adsorbed amount of polymer on cement grains can be calculated.

RESULT AND DISSCUSSION

Optimal Synthesis Conditions of the Zwitterionic Polymer

The zwitterionic polymer was synthesized by free radical polymerization in aqueous solution. In this reaction, the redox initiator APS/TMEDA (weight ratio of 2:1) was applied to trigger the copolymerization and the dodecylmercaptan was selected as polymer molecular weight controller with only 0.5% by weight of the total monomers. To obtain the optimal synthesis conditions of copolymerization, the reaction was investigated by examining the influence of synthesis conditions with reaction temperature, pH value, initiator concentration, weight fraction of AA, weight ratio of monomers, monomer concentration and reaction time on the apparent viscosity of polymer solutions and API fluid loss ([FL.sub.API]) of cement slurry by single-factor variable analysis. The apparent viscosity of zwitterionic polymer aqueous solutions produced with different synthesis conditions was tested by employing a NDJ-1 rotational viscometer at 30[degrees]C at 150 g [L.sup.-1]. The [FL.sub.API] of cement slurry was measured by TG-71 HTHP Water Loss Meter at the temperature of 90[degrees]C and pressure of 6.9 MPa. Hereinto, the amount of the resulting copolymer solution is 3.0% by weight of cement (bwoc). A series of curves among apparent viscosity, [FL.sub.API] and influence factors are obtained, shown in Fig. 2. The corresponding optimal synthesis conditions can be concluded from the curves: the reaction temperature is 60[degrees]C; the pH value is 6; the monomer concentration of AMPS/DADMAC/DMAM/AA is 18 wt%; the initiator concentration is 0.7% by weight of total monomers; the weight ratio of AMPS, DADMAC, DMAM, and AA is 6.3:1.8:1.5:0.4; and the reaction time is 140 min. The optimum copolymer solutions are freeze-dried directly to evaluate its performance. Also, Fig. 2 manifests that the [FL.sub.API] of cement slurry decreases gradually with the apparent viscosity increasing and a moderate viscosity results in a minimum [FL.sub.API]. Hence, the augment of liquid viscosity is one of working mechanisms of zwitterionic copolymer used for fluid loss control.

Characterization of the Zwitterionic Polymer

The molecular properties of the synthesized polymer were investigated by gel permeation chromatography. The various molecular weight of FDA-II and its distribution are listed in Table 2. It can be concluded that the synthesized polymer possesses a relatively high molecular weight ([M.sub.n] = 298,426 g [mol.sup.-1], [M.sub.w] = 645,580 g [mol.sup.-1]) and a broad polydispersity index (PDI) of 2.1934, typical for radical polymerization [21]. The high molecular weight is an important characteristic of the synthetic polymer used as fluid loss additive for cementing, which can keep the liquid phase viscosity effectively and prevent the filtrate from invading into the formation or reservoir stratum through aggrandizing its fluid resistance.

Next, the chemical compositions of the polymer were also characterized by FTIR and 'H NMR to prove the successful incorporation of all comonomers into the polymer. The FTIR spectrum of the purified polymer is shown in Fig. 3. The adsorption band at 3455.8 [cm.sup.-1] originates from the N--H stretching vibration in AMPS. The C--H stretching vibrations of methyl and methylene groups appear around 2937.1 [cm.sup.-1] The stretching vibrations of carbonyl moiety of DMAM and deformation vibrations of the secondary amine in AMPS occur between 1646.9 and 1552.4 [cm.sup.-1] [22]. The peaks at 1454.1 and 1402.0 [cm.sup.-1] are the symmetric bending vibration of --C[H.sub.3] in DMAM. At 1720.2 [cm.sup.-1], a typical band for carbonyl moiety in carboxyl group (--C=0 stretching vibration) was observed which is specific for AA. The symmetric and asymmetric stretching vibration absorption peaks of --S[O.sup.-.sub.3] in AMPS occur at 1043.3 and 1216.8 [cm.sup.-1], respectively. The absorption peak around 3014.4 [cm.sup.-1] can be attributed to the stretching vibration of methylene group in the carbon-nitrogen quinary heterocyclic structures of DADMAC and its deformation vibration is around 960.4 [cm.sup.-1] [23].

As further proof, the [sup.1]H NMR spectrum of FDA-II was recorded in [D.sub.2]O (shown in Fig. 4). It can be observed that the chemical shift in peak of 4.699 ppm is the deuterium in [D.sub.2]O [24]. The peaks located at chemical shift of [delta] = 3.807 and 3.827 can be assigned to the protons of methylene group in carbon-nitrogen quinary heterocyclic structures. The peaks at 2.484-2.643 ppm are corresponded to the protons of [H.sub.c] and [H.sub.f] in DADMAC and DMAM respectively, and the methyl protons (--C[H.sub.3]) located at the chemical shift of [delta] = 2.866 and 2.941 [25]. Hence, all peaks of five-membered heterocyclic protons indicate that DADMAC was copolymerized with other monomers successfully. The peak at 1.421 ppm can be attributable to the methyl protons in AMPS and the polymeric chain ([H.sub.b], --C[H.sub.2]--) protons [20]. The signals at broad chemical shifts in the region of 1.794-2.196 ppm are associated with the polymeric chain ([H.sub.a], --CH--) protons. Additionally, the chemical shifts that appear at 3.047 ppm can be attributed to the methane protons ([H.sub.e], --C[H.sub.2]--) in AMPS. All the peaks were assigned to the formula. From the above analysis, the results indicate that all co-monomers were involved in the copolymerization of FDA-II.

Thermal Analysis of the Zwitterionic Polymer

The polymer property depends on its structure [26, 27]. Generally, with the temperature increasing continuously, the thermal motion of polymer molecular chains intensifies, and the significant heat change and mass loss will occur if the polymer chains are broken. The valid performance of polymer will be weakened gradually or even be lost completely. Hence, an excellent copolymer used as oil-well cement fluid loss additive should have good high-temperature tolerance and structural stability.

The thermal analysis curves for the prepared polymer are shown in Fig. 5. From the DTG-DSC curves, it can be seen that the main decomposition occurred around 311.9[degrees]C. The TG curve could be divided into four weight loss stages. The first stage of weight loss was from room temperature to 142.1[degrees]C, showing a weight loss of 4.557% responsible for the elimination of water molecules coordinated with polymeric chain via hydrogen bonds [20]. In the second stage, the loss increased to 28.541% at 306.9-323.6[degrees]C due to the side-chain cleavage and degradation of polymer backbone. The third contribution came from the various oxidation forms of the polymer chains and the weight loss of 11.263% occurred from 379.6 to 421.4[degrees]C. When the processing temperature was above 500[degrees]C, the polymer could be completely oxidized and its weight remains about the same. So, all data illustrated that the zwitterionic polymer has the stable structure and good thermal-resistance.

Temperature-Resistance

The temperature resistance of cement slurry embracing the zwitterionic polymer was investigated. Figure 6 showed the relationship between the [FL.sub.API] of cement slurry and test temperature. In the temperature range of 30-200[degrees]C, an obvious phenomenon was observed that the [FL.sub.API] of slurry increased gradually with increased temperature in the condition of identical polymer volume (0.6, 0.8, and 1.0 bwoc%, respectively) and decreased by increasing the polymer amounts. The reasons for these phenomena are that [28, 29]: (a) with temperature increasing, the thermal motion of polymer molecular chains intensifies, resulting in the molecular chain not being fully entangled which leads to the reduction of apparent viscosity of polymer solution, while the high viscosity of polymer plays an important role in filtration reduction; (b) due to the hydrogen bond and van der Waals force, the polymer associative structure is easily affected by the elevated temperature; (c) to some extent, the molecular chains are attacked by strong electrolyte and cations in cement slurry phase, leading to the molecular chains entanglement, viscosity decrease and functional groups embedment into the curly polymer structure. Hence, the [FL.sub.API] of cement slurry increased gradually with the increasing test temperature. According to the requirements regarding the liner cementing for deep wells issued by the American Petroleum Institute, the [FL.sub.API] of cement slurry should be <100 mL to improve the cementing quality [17]. For the FDA-II slurry, the [FL.sub.API] could be controlled within 100 mL at 30-200[degrees]C when the polymer volume was no <0.8 bwoc%, which satisfied the cementing requirements in deep and ultra-deep wells. In addition, for 1.0 bwoc% of FDA-II system, the [FL.sub.API] was very stable, exhibiting excellent fluid loss control performance in the temperature range. Hence, the cement slurry incorporating the zwitterionic polymer exhibited a good high temperature resistance.

Salt-Tolerance

As is well known, Portland cement is an inhomogeneous cementitious material which has large amounts of cationic salts in cement slurry phase, especially [Ca.sup.2+] and [Na.sup.+]. Simultaneously, the cationic in formation water will penetrate into the annulus cement slurry in the period of waiting on cement (WOC), especially for the cementation in high salinity wells in oil field. The traditional copolymer used as fluid loss agent usually shows a curly state in ionic liquids, which will affect the fluid loss controlling performance of polymer and cementing quality [30, 31]. Therefore, it is necessary to evaluate the salt-tolerance of the zwitterionic copolymer. In the conditions where the concentration of polymer was 9500 mg [L.sup.-1] and the total salinity of calcium chloride (Ca[Cl.sub.2]) and sodium chloride (NaCl) was 120 and 240 g [L.sup.-1], respectively, the apparent viscosity of zwitterionic polymer aqueous solutions at 30[degrees]C and the [FL.sub.API] of cement slurry curing at 90[degrees]C were investigated with different salt concentrations [32]. It can be seen from Fig. 7 that the apparent viscosity of polymer solutions decreased exponentially with increasing salinity. It was attributed to the addition of Ca[Cl.sub.2] and NaCl electrolytes and the increased amount of counter ions which could penetrate into the polymer conformations and cover the effective charges, resulting in a small hydrodynamic volume and reduced viscosity. Moreover, the inorganic salts broke the electrovalent bonds of zwitterionic copolymer and damaged its intermolecular association. The polymer chains contracted acutely when the electrolyte was added, inducing the apparent viscosity of polymer solutions decreasing dramatically. However, when the salt concentrations of Ca[Cl.sub.2] and NaCl were >30 and 40 g [L.sup.-1] respectively, the variable apparent viscosity became relatively stable with the augment of salinity which was due to the increase of steric hindrance of molecular conformation changes and chain shrinkage [33]. Although the [FL.sub.API] of cement slurry gradually increased with the raise of salinity, it was still <100 mL in the range of total salinity. Thus, the zwitterionic polymer exhibited an excellent salt-tolerance and could be applied in brine cement slurry system.

Fluid Loss Performance

The effects of zwitterionic polymer concentrations on the filtrate volumes of cement slurry with fresh water, salt-water (8%, 15% and saturated one) under different temperatures are shown in Fig. 8. Hereinto, the cement slurries contain a fixed dosage of 1.5 bwoc% tartaric acid retarder which is necessary to prevent their premature setting under high temperature. The results illustrated that the [FL.sub.API] gradually decreased with the increasing polymer volumes. It can be also concluded that filtrate volumes increased with temperature rising, and the similar variations in [FL.sub.API] as the increasing salinity were presented. The corresponding explanation of these variations has been illustrated in the aforementioned parts. Considering the extremely harsh conditions of 200[degrees]C, the zwitterionic polymer reached maximum effectiveness at a dosage of 0.8 bwoc% where the [FL.sub.API] of 66 mL was achieved. The [FL.sub.API] could be <100 mL for the saturated salt cement slurry at 90[degrees]C, and the unsaturated one at 150[degrees]C when the synthesized polymer amount was over 0.80 bwoc%. Meanwhile, the effective addition of zwitterionic polymer decreased when the temperature or salinity was less than harsh conditions. Additionally, if the polymer volume was more than 1.0 bwoc%, the filtrate of cement slurries remained largely untouched for high temperature and salt-cement slurry. Hence, it is demonstrated that the synthesized zwitterionic polymer exhibited excellent fluid loss control performance for the high temperature and high salinity wells cementing.

Rheological Properties

Fig. 9 shows the effects of zwitterionic polymer on cement slurry rheological properties under different temperatures. It can be seen that the cement slurries containing FDA-H are plastic fluids belonging to non-Newtonian fluids. The curves of shear rate and stress illuminated that a certain force was required to let the slurry flow, which could not be uniformly changed. With the polymer volume augmenting, the shear stress required to let the slurry flow increased gradually at relatively low temperature (30[degrees]C). However, it reduced to the minimum value and then increased when the temperature was above 60[degrees]C. For example, at 90[degrees]C, the decrease of shear stress was observed at an addition of 0.2 or 0.4 bwoc% of polymer. The shear stress increased obviously which was measured at high shear rate, though it decreased at low shear rate. Similarly, the consistency index of cement slurry first decreased and then increased slightly with the addition of FDA-II, which was diminished at the high dosage of polymer with rising temperature. The results are shown in Fig. 9d.

The difference between two variation trends is mainly due to the increasing viscosity of polymer solution and its shear-resistance. At a low temperature, the intramolecular and intermolecular associations of copolymer are formed through hydrogen bond, van der Waals force and electrovalent bond, which interact with the active sites on cement grains to exhibit a three-dimensional network structure, leading to the particle agglomeration. Thus, the shear stress of cement slurry increases with polymer volume increasing. As the temperature rises, the thermal motion of polymer molecular chains intensifies and its associative structure is partially destroyed to make some sulfonate and carboxyl groups naked. Moreover, the excess negatively charged AA interacts with positively charged sites existing on the surface of cement grains, which lead to an electrostatic repulsion. The intensity of intramolecular and intermolecular association increases with the addition of zwitterionic polymer. So, the shear stress and consistency index increased slightly when its volume is above 0.3 bwoc%. Such property presents a huge advantage over common synthetic fluid loss agent which exhibits a strong viscosifying effect [34]. However, the combination of viscosifying effect and dispersing property makes the zwitterionic polymer superior over the others used as high temperature fluid loss additive, which is beneficial to the high temperature stability and rheology of cement slurry.

Ultrasonic Compressive Strength

The ultrasonic compressive strength trends of conventional cement slurries measured at 30 and 60[degrees]C are exhibited in Table 3. Time to reach 0.35 and 3.45 MPa are considerably important from an operational perspective, because 0.35 MPa is related to the initial setting and 3.45 MPa is considered as the essential condition required to support the casing string and ensures the further drilling [35]. From the operational viewpoint, the compressive strength values at 12, 24 and 48 h are also given.

According to the data in Table 3, it is evident that the synthetic polymer has a few consequences over the early strength development of set cement, whether at 30 or 60[degrees]C, given an increase in times to reach 0.35 and 3.45 MPa. For the synthesized polymer slurry (0.36 bowc%), the time to reach 0.35 MPa increased by 1.17 h compared to the reference one at 30[degrees]C while the time to obtain 3.45 MPa was 1.2 h more. Fortunately, the less impact appeared in the higher temperature circumstance. At 60[degrees]C, the initial time (0.35 MPa) of polymer modified cement was only 0.17 and 0.26 h more than that of neat slurry, for 0.36 and 0.64 bwoc% polymer system correspondingly. Additionally, the time to reach 3.45 MPa increased by 0.31 and 0.43 h relative to the reference sample. The reasons for these phenomena are that the sulfonate and carboxylic groups in polymer chains absorb or anchor strongly on the surface of cement grains to form a dense net structure in the hydrates to reduce the filtrate, but which can also retard the cement hydration in some extent and slow the strength development of hardened cement down. Furthermore, it can be also concluded from the resulting data that the 12-h compressive strength of polymer modified cement was slightly lower than that of the reference one at 30[degrees]C while higher at 60[degrees]C. At later ages, the 24 and 48-h ultrasonic compressive strength values of polymer cement (0.36 bwoc%) were only 9.6 and 16.45% higher than those of the reference cements while 14.4 and 22.9% higher for cements (0.64 bwoc%), respectively. Consequently, the synthetic polymer has less influence on the setting and early compressive strength of set cement and enhances the post-stage strength development.

SEM Analysis for Polymer and Filter Cake

The zwitterionic polymer solution was prepared with the concentration of 4000 mg [L.sup.-1] by dissolving in distilled water. And then, the solution was freeze-dried for 24 h at the conditions of 10 Pa and -55[degrees]C. Figure 10a-c illustrates SEM images for polymer recorded at X60, X300 and X500, respectively. It can be seen that the three-dimensional (3D) network structure indwelled in the polymer matrix, containing high porous structures. The main reasons for the formation of 3D structure are the static electricity, hydrogen bonding, van der Waals force and ion association between the intermolecular and intra-molecular to develop a rigid and reversible associative structure. The structure improves considerably the high temperature dilute performance and shear-thinning of polymer to increase the thermal stability of cement slurry. SEM images show the morphologies of filter cake with different polymer recruitments, which are displayed in Fig. 10d-f. It is obvious that the compactness of filter cake gradually increased with the addition of zwitterionic polymer. Moreover, it can be obseved from Fig. 11 that the permeability of set cement and the [FL.sub.API] of cement slurry decreased with the augment of polymer volume. The results reveal that the filtrate control performance of the zwitterionic polymer is closely linked with the filter cake quality. Because of the adsorption and hydration groups absorbed or clinched on cement grain surface, a dense mesh texture of "cement grain-polymer-hydrone adsorption layer" is developed to bind more free water and fill the voids among cement particles, improving its structural compactness and creating a low permeable cement [36, 37].To summarize, the permeability and compactness of filter cake can be improved tremendously after incorporating the zwitterionic copolymer in cement slurry.

Adsorption Behavior of Zwitterionic Polymer

To further investigate the working mechanism of the zwitterionic polymer, the relationships between its adsorption and [FL.sub.API] of cement slurry were carried out, wherein the adsorbed amount of FDA-II was evaluated by TOC analyzer. The results are shown in Fig. 11. As adding the zwitterionic polymer, the adsorbed amount of FDA-II on cement grains increased gradually and the [FL.sub.API] of cement slurry deceased obviously. When the polymer amount was 0.4 bwoc%, it provided an outstanding fluid loss control property (96 mL of [FL.sub.API]) and adsorbed in high amount (3.2 mg [g.sup.-1] cement). Similarly, the [FL.sub.API] decreased substantially (48 mL of [FL.sub.API]) and the polymer adsorbed amount was 7.1 mg [g.sup.-1] cement when the dosage increased to 0.8 bwoc%. Further adding the polymer, the permeability of set cement, [FL.sub.API] of cement slurry and adsorbed amount of FDA-II on cement grains remained about the same. Hence, it became clear that the filtrate control property of the zwitterionic polymer exhibited direct relationships with its adsorption and permeability of filter cake, which was in agreement with the results of Salami [11]. It can be concluded that the working mechanisms of zwitterionic polymer mainly rely on its adsorption on the surface of hydrated cement and the improvement in filter cake quality to control fluid loss.

CONCLUSIONS

In this study, a novel quadripolymer was successfully synthesized via free radical aqueous solution polymerization. The optimum conditions were determined by the single factor variable method according to the apparent viscosity and API fluid loss. Various techniques, including GPC, FT-IR, [sup.1]H NMR, DTG, DSC, and SEM were used to investigate the molecular weight ([M.sub.n] = 298,426 g [mol.sup.-1]), compositions and microscope structure of synthetic polymer. Its temperature- and salt-resistance of cement slurry were also investigated, confirming it could be applied in the harsh conditions of 200[degrees]C and saturated brine. Besides, it had excellent rheological property at different temperature, making for the sedimentary stability and pumping property of field cement slurry. The synthetic polymer had less influence on hardening cement and accelerated the post compressive strength development of set cement, proved by the ultrasonic method. Moreover, the working mechanism of synthesized polymer was studied through the adsorbed amount of polymer on cement grains, SEM observation and permeability of filter cake, mainly relied on its chemisorptions on the surface of hydrating cement and improving the filter cake quality. In conclusion, the novel zwitterionic quadripolymer is suitable for improving comprehensive performance of cement slurry and has great potential in high-temperature and high-salinity oilfields.

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Xiujian Xia, (1) Yakai Feng, (1) Jintang Guo, (1) Shuoqiong Liu, (2) Jianzhou Jin, (2) Yongjin Yu (2)

(1) School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People's Republic of China

(2) Drilling Research Institute of CNPC, Beijing 102206, People's Republic of China

Correspondence to: J. T. Guo; e-mail: jtguo@tju.edu.cn

Contract grant sponsor: National Science and Technology Major Project of China; contract grant number: 2011ZX05021-004.

DOI 10.1002/pen.24387

TABLE 1. Mineral and chemical compositions of class G oil
well cement.

                            Mineral compositions (wt%)

[C.sub.3]S    [C.sub.2]S    [C.sub.3]S    [C.sub.4]AF    free-CaO

53.29            21.17         6.34          8.97          0.58

                            Chemical compositions (wt%)

                            [Al.sub.2]    [Fe.sub.2]
CaO           Si[O.sub.2]    [O.sub.3]     [O.sub.3]

63.57            21.89         3.12          3.47

                            Chemical compositions (wt%)

MgO           S[O.sub.3]    [K.sub.2]0       L.O.I

1.59             2.86          0.59          1.92

Note: L.O.I is loss on ignition; [C.sub.3]S: tricalcium
silicate (3CaOSi[O.sub.2]); [C.sub.2]S: dicalcium silicate
(2CaO Si[O.sub.2]); [C.sub.3]A: tricalcium aluminate
(3CaO[Al.sub.2][O.sub.3]); [C.sub.4]AF: tetra calcium aluminate
ferrite (4CaO[Al.sub.2][O.sub.3] [Fe.sub.2][O.sub.3]).

TABLE 2. Molecular properties of FDA-II.

[M.sub.n]             [M.sub.w]          [M.sub.z]
(g [mol.sup.-1])   (g [mol.sup.-1])   (g [mol.sup.-1])    PDI

298426                  645580             875360        2.1934

TABLE 3. Compressive strength development of the different
conventional cement slurries tested at 30 and 60[degrees]C.

                 [t.sub.0.35 MPa]   [t.sub.3.45 MPa]   [P.sub.12 h]
Items    T (C)         (h)                (h)             (MPa)

Neat      30           4.50               7.25             6.32
          60           2.56               3.60            11.21
0.36%     30           5.67               8.45             5.84
          60           2.63               3.86            12.12
0.72%     30           5.83               8.16             5.98
          60           2.60               3.98            12.86

                 [P.sub.24 h]   [P.sub.48 h]
Items    T (C)      (MPa)          (MPa)

Neat      30        11.38          16.82
          60        16.94          22.39
0.36%     30        12.86          18.94
          60        18.57          26.05
0.72%     30        13.44          19.45
          60        19.38          27.51

Note: [t.sub.0.35MPa] and [t.sub.3.5MPa] are the time to reach
0.35 and 3.5 MPa of compressive strength of set cement,
respectively; [P.sub.24h] and [P.sub.48h] are the 24 and 48 h
compressive strength of set cement.


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Author:Xia, Xiujian; Feng, Yakai; Guo, Jintang; Liu, Shuoqiong; Jin, Jianzhou; Yu, Yongjin
Publication:Polymer Engineering and Science
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Date:Jan 1, 2017
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