Shanghai Key Laboratory of Mutiphase Materials Chemical Engineering,Lab of Chemical Engineering Rheology,Research Center of Chemical Engineering,East China University of Science and Technology,Shanghai 200237,China

2 Langfang Filial of Research Institute of Petroleum Exploration and Development,PetroChina,Langfang 065007,China

Keywords:Hydroxypropyl xanthan gum Rheological properties Drag reduction Cryo-FESEM

A B S T R A C T Hydroxypropyl xanthan gum(HXG)was prepared from xanthan gum(XG)and propylene oxide under alkaline condition.Rheological and drag reduction properties of different concentrations of aqueous HXG and XG solution were studied.The micro-structure network of HXG and XG solutions was investigated by Cryo-FESEM.The results showed that HXG and XG solutions could exhibit shear thinning property.The apparent viscosity of 6 g·L?1 HXG solution was 1.25 times more than that of 6 g·L?1 XG solution.The storage modulus G′and the loss modulus G″of HXG solutions were greater than those of XG solutions,and thixotropic and viscoelastic properties were more significant in HXG solutions.The HXG and XG solutions reduced the pressure drop of straight pipe,and the maximum drag reduction of 1 g·L?1 HXG and XG in smooth tube reached 72.8%and 68.1%,respectively.Drag reduction rate was increased as the concentration increased.The HXG solution may become a new polymeric drag reducer.

1.Introduction

Xanthan gum(XG)is a kind of natural polysaccharides with high molecular weight[1,2],which comes from aerobic fermentation process of Xanthomonas campestris[3–5].Since Kelco applied XG into industrial production at early 1960s,it has been widely used in food,medicine,textile,cosmetics,fire and oil fields[6].Due to the characteristics of high viscosity,high resistance to acid,alkali and salt[7],xanthan gum possesses good prospects in the field of petroleum recovering.In order to improve its dissolution rate,XG was modified with formaldehyde by Su et al.[8].XG derivatives were also obtained by reaction with an unsaturated organic acid(acrylic acid)or with acidic reactive derivatives(acryloyl chloride,maleic anhydride)[9–11].But the modification processes of XG were complicated,and the reaction conditions need stringent control.To the authors'best knowledge,the hydroxypropyl modification of the xanthan gum has not been reported yet.

Liquids are mostly transported through pipes[12–14].Addition of a small amount of polymer to the Newtonian fluid could significantly reduce the friction coefficient in pipe flow.This phenomenon was known as the drag reduction effect[15–17],which was discovered in 1949.Since then,drag reduction has been documented in a variety of systems with different results[18–23].Lumly[24]believed that the key to drag reduction was the increase in apparent viscosity of the molecular extension of polymers in the flow outside the viscous sublayer effect.Virk and Wagger[25,26]also proposed that polymer molecules elongated(in a rod or an extended thread)may lead to effective drag reduction.

In this work,hydroxypropyl xanthan gum(HXG)was prepared by the reaction of hydroxyl of XG with propylene oxide under alkaline condition.Rheological and drag reduction properties of aqueous HXG and XG solutions were studied.HXG solutions exhibited higher apparent viscosity,more obvious thixotropy,much stronger viscoelasticity and better drag reduction than XG solutions did at the same concentration.HXG can be a new drag reducer polysaccharide for use in shale gas and petroleum fracturing fluid fields.

2.Materials and Methods

2.1.Material

Xanthan gum was food grade and purchased from Runchuang Food Company(Shanghai).1,2-propylene oxide was the etherified reagent and purchased from Shanghai Lingfeng Chemical Reagent Ltd.The reaction solvent was ethanol,chemical pure,from Sinopharm Chemical Reagent Co.The pH adjusting agents were NaOH and acetic acid,all analytical pure(AR).

2.2.Experimental methods

2.2.1.Preparation of hydroxypropyl xanthan gum(HXG)

To remove moisture,the XG powder was previously dried at 45°C for 8 h.The syntheses were performed in ethanol–water solution(70wt%),in the presence of etherification agent,1,2-propylene oxide.XG(20 g)was dispersed in 28 g ethanol(AR)and 12 g 0.5 mol·L?1NaOH solution in a three-neck flask under constant stirring to alkalize XG for 1 h at 25°C.Then the system was adjusted to pH 9–10 by diluting acetic acid.Heated up to 75°C with reflux under stirring,8 ml of 1,2-propylene oxide was added to the system by drop wise,and the reaction lasted for 4 h.Finally,the obtained suspension was filtered,rinsed 4 times with ethanol/water mixtures(50wt%,80wt%,90wt%ethanol solution)(40 ml)and finally with ethanol(AR,50 ml).The final HXG product was light yellow powder with particle size in 0.149-0.178 mm(80–100 mesh)after drying at 50°C for 12 h and grounding.The yield was 92%.

2.2.2.Measurement of hydroxypropyl molar substitution degree

The content of hydroxypropyl in HXG was measured by ultraviolet visible light spectrophotometer(UV762,Shanghai Lengguang Technology Co.,China)at 595 nm,referring to the standard Q/SH 0050-2007[27].The average hydroxypropyl weight fraction of HXG was 4.38%over three measurements.

2.2.3.Preparation of HXG and XG solutions

Aqueous HXG and XG solutions with different concentrations ranging from 1 to 10 g·L?1were prepared by dissolving the dry biopolymer(XG and HXG)in deionized water with agitation.Additionally,the aqueous solutions were fully swelled for 6 h before rheological measurements.

For drag reduction tests as noted below,40 L solutions of different concentrations ranging from 0.4 to 1.0 g·L?1were prepared in tapwater.

2.2.4.Rheological measurements

The rheological measurements were performed in a rotational rheometer(Physica MCR 101,Anton Parr,Austria),using a cone and plate geometry(CP25-1-SN10665,cone angle 1.001°,diameter 24.955 mm,plate distance 0.05 mm)and a Peltier-based temperature control.The steady shear viscosity was measured at a constant shear rate of 170 s?1.In the shear-thinning measurements,the viscosity was recorded in the range of shear rate from 0.01 to 1000 s?1.In oscillatory measurements,a strain sweep was performed from 0.01%to 100%at a given frequency 1.0 rad·s?1to fix the upper limit of the linear viscoelastic zone at a strain value of 30.0%and the storage modulus G′and the loss modulus G″were determined through small amplitude oscillatory shear at frequencies ranging from 1 to 100 rad·s?1.In thixotropy measurements,the shear rate increased from 0.01 to 170 s?1in 40 s,and then decreased from 100 to 0.01 s?1in the same time.All measurements were performed at 30.0°C±0.1°C.

The shear resistance and heat resistance of solutions were measured on a high temperature rheometer(RS6000,Haake,Germany),using a parallel plate(PZ38,plate diameter 38 mm).The temperature was raised from 30°C to set temperature(80°C or 100°C)within 30 min,then the viscosity of solution was recorded with time at constant temperature and constant shear rate 170 s?1for 90 min.

2.2.5.Drag reduction measurement

The experimental facility of drag reduction was a circulation pipeline testing system,composed of a liquid storage tank,a centrifugal pump,a turbine flow meter,a pressure sensor and a stainless steel pipe.The pipe flow chart was given in Fig.1.The test section was a smooth tube(length 2.0 m;inner diameter 20.5 mm).Tap water was used in equipment calibration.The tube length is about 3.3 m before the entrance of test section,which is long enough for full development of tubular turbulent flow.In addition,this is supported by the fact that the curve of water friction coefficient with Reynolds number is coincided with the Prandtl–Karman curve in Section 3.3.

Fig.1.The experimental flow chart.1—liquid storage tank;2—centrifugal pump;3—thermometer;4—flowmeter;5—smooth pipe;6—pressure sensor.

According to the rheological measurements,the drag reducing solutions showed the shear thinning behavior which could match with the power law.The generalized Reynolds number ReG[28]and the Fanning friction factor f of the power fluid are given by Eqs.(1)and(2),respectively:

where qvis the volume flow rate(m3·s?1);v is the flow velocity(m·s?1);ΔP is the pressure drop in straight pipe(kPa);l is the length of testing section (m); ρ is the density of the fluid(kg·m?3);d is the inner diameter of the testing pipe(m).

The two asymptotic regimes[25,29]describe the turbulent drag reducing performance.One is the Prandtl–Karman regime for Newtonian turbulent flow.The curve of water friction coefficient f with ReGin smooth tube was coincided with the Prandtl–Karman regime under the same conditions in this experiment:

The other one is the Virk regime for maximum possible drag reduction of polymer solution,which is given by

The drag reduction rate R%could be computed as

where f1is the friction factor of water and f2that of HXG or XG solution.

2.2.6.Cryo-FESEM

Sample solutions for cryogenic field emission scanning electron microscopy(Cryo-FESEM)were loaded into copper gauze and plunged frozen in liquid nitrogen at its boiling point.Then the samples were transferred into the cryo chamber of the microscope,which was held at a temperature of ?130°C.The samples were then sublimed at?130°C for approximately 30 min.Once coated the samples were viewed at ?130°C.Images of the treated samples were obtained with Model S-4800 field emission scanning electron microscope(Hitachi High-Technologies Co.,Japan)and operated at 15.0 kV.

3.Results and Discussion

3.1.Rheological properties of HXG and XG solutions

3.1.1.Apparent viscosity of different concentrations

Fig.2 shows the apparent viscosity(η)of the aqueous HXG and XG solutions at different concentrations.The viscosity increases along with the increasing concentration,but the increase in viscosity of modified xanthan gum solution is significantly greater than XG solution.For the concentration of 6 g·L?1,the viscosity value of HXG and XG solution are 167.1 mPa·s and 74.3 mPa·s,respectively,increased by 125%.This may be because propylene oxide in the modification process could mainly react with hydroxyl groups on the XG molecules.In addition,the hydroxypropyl modified xanthan gum could also form intramolecular hydrogen bonds,and the intertwining segments among the chains do increase irregularity and internal flow resistance,therefore the apparent viscosity increases.While because of hydroxyl grafted to xanthan gum,the water solubility and dissolution rate are also improved significantly.

Fig.2.Apparent viscosity η of aqueous HXG and XG solutions as a function of concentration(30°C,170 s?1).(△HXG;□XG).

3.1.2.Shear thinning behaviors

Fig.3.Apparent viscosity η as a function of shear rate.(a)◇0.8 g·L?1 HXG;◆0.8 g·L?1 XG;△0.6 g·L?1 HXG;▲0.6 g·L?1 XG;○0.4 g·L?1 HXG;*0.4 g·L?1 XG;(b)◇6 g·L?1 HXG;◆6 g·L?1 XG;△3 g·L?1 HXG;▲3 g·L?1 XG;○1 g·L?1 HXG;*1 g·L?1 XG.

Typical flow curves in different concentrations,as a function of shear rate,are shown in Fig.3.In general,the solutions behave as shear thinning fluids which mean that their viscosity decreases as the rate of deformation increases.For this case,the shear thinning behavior could be fitted quite well with the Ostwald–de Waele model(the power law)[30]given by

Parameters n and K were obtained by the slope of straight line and the intercept.The linear correlation coefficients used in Eq.(6)are greater than 0.99.The values of n and K are also given in Table 1.

Table 1 Influence on values of the parameters n and K with different concentrations

For a power law fluid,the value of consistency index K indicates the viscosifying capacity;while the fluid behavior index n can also imply the property of shear thinning.From Table 1,the value of n is between 0 and 1 under the experimental conditions,which shows that HXG and XG solutions exhibit shear thinning behavior.At the same concentration the value of K of HXG is higher than that of XG,while the value of n is lower than that of XG.When the concentration increases,K increases and n decreases.This illustrates that the viscosifying capacity of HXG is stronger than XG,and at higher concentration,shear thinning and viscosifying capacity are stronger.

3.1.3.Thixotropy

Thixotropy is the property that a time-dependent decrease in the viscosity of a liquid subjected to shearing,stirring,or otherwise stressing,followed by a gradual recovery when the action is stopped,corresponding to the breaking down and subsequent partial buildingup of some form of structure.Thixotropy reveals the relation between viscosity of the material and time,and reflects changing process of the structure along with time after stress.

Fig.4.Thixotropy of HXG and XG solutions.(a)3 g·L?1:■XG up-line;□XG down-line;▲HXG up-line;△HXG down-line;(b)6 g·L?1:■XG up-line;□XG down-line;▲HXG up-line;△HXG down-line.

The thixotropic properties of different concentration aqueous HXG and XG solutions were measured at 30°C.The area of hysteresis loop could characterize the thixotropic property of aqueous HXG and XG solutions.Fig.4 shows that the hysteresis loop is developed;but compared to HXG system,hysteresis loops in XG systems are very small,namely little thixotropy in the same concentration.Therefore,it is suggested that the network structure of HXG solutions is stronger than that of XG solutions.

3.1.4.Viscoelasticity

Under the oscillatory mode and 30°C,the frequency sweep is applied for aqueous solutions of different HXG and XG concentrations.The results in Fig.5 show that the elastic modulus G′is predominant over the entire angular frequency.It means that the aqueous XG and HXG solutions show weak gel behaviors.Figs.4 and 5 indicate that elastic modulus G′and viscous modulus G″of HXG solutions are greater than those of XG solution with the same concentration.It is proved that the modified xanthan gum HXG solution has better rheological properties than XG solution.

3.2.Heat resistance performance

The changes of viscosity with temperature and the resistance to shear and temperature measured by HAKKE RS-6000 rheometer are shown in Figs.6 and 7.Under different temperatures the viscosity of 6 g·L?1HXG solution are always higher than 6 g·L?1XG solution,and even at 100°C the value is still nearly 2 times of XG solution.The temperature was raised from 30°C to 80°C within 30 min,then at the constant temperature and constant shear rate 170 s?1for 90 min.The 3 g·L?1HXG could keep the viscosity more than 50 mPa·s,while the viscosity of 3 g·L?1XG solution was less than 50 mPa·s at room temperature.Therefore,the modified xanthan gum HXG would be suitable for low concentrations of non-crosslinking xanthan gum fracturing fluid applied in oil recovery enhancement.

3.3.Drag reduction

We tested the drag reducing properties of both the HXG and XG solutions in a 20.5 mm smooth tube.The time of practical fracturing process for tight sandstone is much shorter than the testing time of drag reduction experiments(about 15 min).Moreover,before the test of drag reduction,drag reducing fluid was cycled for 10 min at the maximum flow of 2.8 L·s?1and the pressure drop was observed quite constant.It could be considered that there was no significant degradation for drag reducing fluid during test by flow friction and pumping.

As shown in Fig.8,the friction coefficient f of the HXG and XG systems are smaller than water but above the Virk regime and f decreases with generalized Reynolds number ReGincreasing in the experimental range.It illustrates that HXG and XG solutions possesses drag reducing properties,and the drag reducing behavior become remarkable with ReGincreasing.

Fig.5.G′and G″vary with the frequency ω for different aqueous HXG and XG concentration solutions.(a)3 g·L?1:◆HXG-G′;▲XG-G′;◇HXG-G″;△XG-G″;(b)6 g·L?1:◆HXG-G′;▲XG-G′;◇HXG-G″;△XG-G″.

Fig.6.Viscosity varies with temperature of 6 g·L?1 HXG and XG solutions at 170 s?1.(■6 g·L?1 HXG;◇6 g·L?1 XG).

Fig.7.Heat resistance and shear resistance curve of 3 g·L?1 HXG solution at 170 s?1.

Fig.8.Friction coefficient f as a function of generalized Reynolds number for aqueous solutions of different HXG and XG concentrations.(a)◆water;◇0.4 g·L?1 XG;▲0.4 g·L?1 HXG;*0.8 g·L?1 XG;○0.8 g·L?1 HXG;(b)◆water;◇0.5 g·L?1 XG;▲0.6 g·L?1 HXG;*1.0 g·L?1 XG;○1.0 g·L?1 HXG.

Fig.9 shows the drag reduction properties of the HXG and XG solutions in which R(%)was plotted against generalized Reynolds number ReGat different concentrations.For each solution,the R(%)first increases with ReGand the R(%)remains largely steady with further increase in ReGat higher flow rates.The maximum drag reduction rate for 1 g·L?1,0.8 g·L?1,0.6 g·L?1and 0.4 g·L?1XG solutions are 68.1%,59.6%,53.9%and 43.5%,respectively.While for the same concentration of HXG solutions,the maximum drag reduction rate are 72.8%,66.2%,59.1%and 48.7%,respectively.Under the same concentration,the drag reducing behavior of HXG solution is superior to the XG solution.Because the viscoelastic network structure in HXG and XG solutions could inhibit the formation of vortex in turbulent flow[28],to reduce the loss of mechanical energy in fluid flow.Therefore,the loss of mechanical energy is reduced.As the concentration increases,the network structure and viscoelasticity enhance,which could resist strong shear,and the drag reduction effect becomes better.

3.4.Cryo-FESEM

To observe the difference structure in HXG and XG solutions,FESEM images were taken and shown in Fig.10.It shows honeycomb network in 6 g·L?1aqueous HXG and XG solutions,and the former structure is much tighter than the latter.The microstructure of HXG and XG solutions may explain why XG and HXG solutions could have a good drag reducing property.

4.Conclusions

Through the hydroxypropyl modification of XG under alkaline condition,the apparent viscosity and viscoelasticity of HXG solution were increased,and the network was further strengthened.The HXG and XG solutions showed significant shear thinning property,which could be described by the Ostwald–Dewaele equation.Cryo-FESEM images also proved that the HXG solutions had stronger honeycomb structure than the XG solution.

For 1 g·L?1of XG and HXG solutions,the maximum drag reduction rate in the smooth tube reached 72.8%and 68.1%,respectively.The HXG solutions could significantly reduce the straight pipe pressure and increase flow.So HXG will become a new drag reduction polymer.The modified xanthan gum could be applied to the low concentration of non-crosslinked xanthan gum fracturing fluid.

Fig.9.Drag reduction rate as a function of generalized Reynolds number for aqueous solutions of different HXG and XG concentrations.(a)◆0.8 g·L?1 HXG;◇0.8 g·L?1 XG;●0.4 g·L?1 HXG;○0.4 g·L?1 XG;(b)◆1.0 g·L?1 HXG;◇1.0 g·L?1 XG;●0.6 g·L?1 HXG;○0.6 g·L?1 XG.

Fig.10.Cryo-FESEM images of 6 g·L?1(a—XG;b—HXG solutions).

Nomenclature

d inner diameter of the testing pipe,m

f1friction factor of water

f2friction factor of HXG or XG solution

G′ storage modulus,Pa

G″ loss modulus,Pa

K consistency index,Pa·sn

l length of testing pipe,m

n fluid behavior index

ΔP pressure drop in straight pipe,kPa

qvvolume flow rate,m3·s?1

R drag reduction rate,%

ReGgeneralized Reynolds number(=(4n/(3n+1))n(ρv2-ndn)/(8n-1K))

v flow rate,m·s?1

η viscosity of the fluid,Pa·s

ρ density of the fluid,kg·m?3

τ shear stress,Pa