A Microbial Exopolysaccharide Produced by Sphingomonas Species

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A Microbial Exopolysaccharide Produced by Sphingomonas Species for Enhanced Heavy Oil Recovery at High Temperature and High Salinity Yajun Li,† Long Xu,*,†,‡ Houjian Gong,† Boxin Ding,‡ Mingzhe Dong,†,‡ and Yanchao Li§ †

School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, PR China Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada § Downhole Service Company, Chuanqing Drilling Co. Ltd, CNPC, Chengdu 610052, PR China ‡

ABSTRACT: Microbial exopolysaccharides secreted by microorganisms during metabolic processes have been widely used in biotechnology because of their environmentally friendly and renewable nature. This study evaluates the potential of a novel microbial exopolysaccharide, diutan gum, which is produced by Sphingomonas species, for enhanced heavy oil recovery at high temperature and high salinity. In addition, two conventional polymers [xanthan gum and partially hydrolyzed polyacrylamide (HPAM)] used in oil exploitation are compared under the same conditions. It is found that the steady apparent viscosity and dynamic modulus of aqueous diutan gum solutions are not sensitive to the temperature and virtually independent of the salinity, while those of xanthan gum and HPAM significantly decrease at high temperature and high salinity. The retention values of the apparent viscosity and the dynamic modulus of diutan gum at 90 °C and 244 121 mg·L−1 salinity are greater than 90%. The gellike structure of diutan gum is dependent on the shear rate rather than the shear time and the aging time. The thermal stability and salt tolerance of diutan gum are mainly attributed to the stability of the gel-like molecular structure, which is greatly related to the double helix. Flow tests in sandpacks demonstrate the excellent mobility control capacity of diutan gum in porous media, and the permeability reduction of porous media is attributed to the adsorption and interception of diutan gum at high temperature and high salinity. Sandpack flooding experiments confirm that the heavy oil recovery efficiency of diutan gum is raised by 20.9% OOIP and is higher than that of either xanthan gum (9.3%) or HPAM (5.4%) at 90 °C and 244 121 mg·L−1 salinity. It is believed that diutan gum will be a promising oil recovery agent for enhanced oil recovery in high-temperature and high-salinity reservoirs. structure.12 The amide groups of HPAM undergo extensive hydrolysis into carboxylic acid units at high temperature, further causing the interaction with ions.13 High shear force is also detrimental to the performance of HPAM on reducing the solution viscosity because of the degradation of molecular chains.14 Therefore, the lack of conventional polymers for use in high-temperature and high-salinity reservoirs has restricted the application of polymer flooding for EOR. Consequently, in polymer flooding processes, it has become of vital importance to determine how to improve the thermal stability and salt tolerance of displacing agents. To date, many researchers have mainly focused on developing HPAM and improving its performance by incorporating thermally stable monomers or incorporating hydrophobic side chains, such as hydrophobically associating polymers, thermo-viscosifying polymers, copolymers, and ter-polymers.15−17 Although modifying HPAM makes it more thermally stable and salt tolerant, increasing the oil recovery, some degradation products of modified HPAM are harmful to the environment because of the derivative functional groups. Therefore, it is important to develop novel resources to improve the efficiency of EOR without negative effects.

1. INTRODUCTION Polymer flooding is a widely used technique in oilfields for enhanced oil recovery (EOR) at present. Generally, for polymer flooding, the EOR mechanism is considered to be such that a water-soluble polymer with high molecular weight added into the injection water can improve the mobility ratio and enlarge the swept volume by increasing the viscosity of the water phase.1−3 Therefore, the oil recovery efficiency is directly related to the viscosity and viscoelasticity of the polymer fluid, which has a lower mobility ratio (less than one), weakening the viscous fingering.4 It has already been proven that, under suitable conditions, polymer flooding improves oil recovery by up to 20% of the original oil in place (OOIP) over water flooding.5,6 At present, partially hydrolyzed polyacrylamide (HPAM) is one of two widely used oil displacement agents in polymer flooding (along with xanthan gum).7−9 However, because of the shift in oil exploitation toward higher-temperature and higher-salinity reservoirs, this conventional polymer cannot be used to viscosify displaced fluids in such harsh conditions. Counterions in the formation brine screen the electrostatic interactions between adjacent HPAM molecular chains, leading to a significant viscosity reduction and even precipitation upon interaction with divalent ions occurs.10,11 Another factor influencing the molecular configuration of polyelectrolytes is temperature. In most cases, heating destroys the molecular © XXXX American Chemical Society

Received: November 9, 2016 Revised: March 9, 2017 Published: March 14, 2017 A

DOI: 10.1021/acs.energyfuels.6b02923 Energy Fuels XXXX, XXX, XXX−XXX

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polysaccharides for EOR in high-temperature and high-salinity reservoirs.

Recently, biotechnology has been found to have increasing application in petroleum engineering, and polysaccharides have attracted more and more attention.18 It has been found that several bacteria are able to secret polysaccharides. For instance, gellan gum is produced by Pseudomonas elodea;19 dextran is found to be secreted by Leuconostoc mesenteroides;20 welan gum can be produced by Alcaligenes species;21 and lewan is one of the metabolic products of the Halomonas species.22 Microbial polysaccharides are confirmed to be environmentally friendly (biocompatible and biodegradable) and renewable resources. In addition, they also exhibit interesting physicochemical characters, such as emulsifying, stabilizing, thickening, gelation, and so on. It is found that an aqueous welan gum solution shows a high viscosity at low concentration and has a superior stabilizing effect on the foam generated by sodium fatty alcohol polyoxyethylene ether sulfate.23,24 The application of brineinitiated gelation of gellan gum for water shutoff operations in field conditions was also reported.25 Sol-to-gel and gel-to-sol transitions of gellan gum occur in saline water, and the step-bystep plugging of highly permeable channels of sandpacks occurs because of gellan gum invasion. Diutan gum secreted by Sphingomonas sp. ATCC 53159 is a natural microbial exopolysaccharides with high molecular weight. It is an anionic polysaccharides consisting of a repeat unit with β-1,3-D-glucopyranosyl, β-1,4-D-glucuronopyranosyl, β-1,4-D-glucopyranosyl, and α-1,4-L-rhamnopyranosyl and twosugar L-rhamnopyranosyl side-chains attached to the (1 → 4)linked glucopyranosyl residue. Two O-acetyl groups are attached per repeat unit to the 2′ and 6′ positions of the (1 → 3) linked glucopyranosyl.26−28 Acting as another conventional oil recovery agent, xanthan gum secreted by Xanthomonas campestris is an extracellular polysaccharide with high molecular weight. The cellulosic backbone of xanthan gum consists of a number of pentasaccharide repeat units formed by five monosaccharides. The molecular backbone is substituted on alternate β-1,4-D-glucopyranosyl residues with trisaccharide side chains of β-D-rhamnopyranosyl, β-1,4-D-glucuronopyranosyl, and α-1,2-D-mannopyranosyl, with various amounts of acetyl and pyruvate substituents.29,30 It is reported that diutan gum can effectively improve the performance of cement paste in terms of the viscosity. The practical application of diutan gum is mainly in the cement and concrete industries at present.31,32 However, the study of diutan gum for EOR has rarely been reported in previous studies. Therefore, here, diutan gum is proposed as a novel oil recovery agent for its excellent physical and chemical properties, especially under harsh reservoir conditions. In this work, the detailed rheological properties of aqueous diutan gum solutions were investigated to obtain steady-state and dynamic molecule-level information at high temperature and high salinity. Comparison with two commonly used polymers (xanthan gum and HPAM) was made under the same conditions. Afterward, sandpack flow tests were performed under reservoir conditions to study the flow characteristics of diutan gum in porous media, including mobility control and adsorption behavior. Finally, the EOR performance of diutan gum was discussed. This article is one of only a few that attempts to use the new microbial exopolysaccharide diutan gum for enhanced heavy oil recovery. The physicochemical properties of diutan gum relative to its potential for EOR are theoretically studied and correlated, which is beneficial for understanding the mechanism and application of microbial

2. EXPERIMENTAL SECTION 2.1. Materials. Diutan gum was kindly supplied by CP Kelco Company, United States. The average molecular weight (Wm) was approximately 5.2 × 106 g·mol−1, and the intrinsic viscosity was 5450 mL·g−1 (25 °C) in pure water. Xanthan gum (Fufeng 80) was produced by the Inner Mongolia Fufeng Biotechnology Co., Ltd., China. The average Wm was approximately 2.0 × 106 g·mol−1, and the intrinsic viscosity was 7627 mL·g−1 (25 °C) in pure water. Partially hydrolyzed polyacrylamide was supplied by the China Petrochemical Corporation. The average Wm was 2.0 × 107 g·mol−1, the degree of hydrolysis 20.4%, and the intrinsic viscosity 2658 mL·g−1 (25 °C) in pure water. NaCl and CaCl2, reagent grade, were purchased from the Sinopharm Chemical Reagent Co., Ltd., China. Sample solutions were prepared by dispersing dry polymer powder in brine water and stirring at ambient temperature (25 °C). This work evaluated the properties of different systems only at a given polymer concentration (1.75 g·L−1). The molecular structures of diutan gum, xanthan gum, and HPAM are shown in Figure 1.

Figure 1. Molecular structures of diutan gum, xanthan gum and HPAM.

In flooding experiments, the heavy oil with no water and gas was collected from a reservoir in Henan province of China. The oil sample had a viscosity of 274 mPa·s at 90 °C and an acid number of 1.36 (mg KOH/g-oil). Two synthetic brines were prepared according to the formation water in the heavy oil reservoir. The compositions of the brines used are listed in Table 1. 2.2. Rheological Measurements. The rheological measurements were investigated via rotational rheometer (Haake MARS III, Germany) using the coaxial cylinder sensor system. Samples were kept stationary for more than 12 h to remove bubbles. The B


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different floodings was also monitored. The final efficiency was determined as the incremental oil recovery by both of the polymer injection and the extended water-flooding. The injection rate in the tests was controlled at 0.5 mL/min.

Table 1. Composition of the Brine Water composition (mg·L−1) Na

+

3 432 95 373

Mg2+

Ca2+

Cl−

total salinity (mg·L−1)

51 102

206 411

5 814 148 235

9 503 244 121

3. RESULTS AND DISCUSSION 3.1. Rheological Properties at High Temperature and High Salinity. The apparent viscosities of aqueous diutan gum, xanthan gum, and HPAM solutions under ideal conditions (25 °C and pure water) and reservoir conditions (75 °C and 9503 mg·L−1 salinity) are shown in Figure 2. With increasing shear

temperature was controlled by SC150 circulator, and the maximum deviation was ±0.1 °C. In the steady-state shearing, rate control mode was selected. The shear rate was increased from 0.01 to 1000 s−1 in 300 s. Before the frequency oscillation was conducted, the stress sweep was conducted from 0.01 to 20.00 Pa at 0.10 Hz. Then, a stress value in the linear viscoelastic region was chosen to conduct the oscillatory frequency, and the frequency sweep was carried out within the frequency range of 0.01−10.00 Hz in the oscillation mode OSC. 2.3. FE-SEM. The morphology of the samples was studied using a Hitachi S-4800 (Japan) field-emission scanning electron microscopy (FE-SEM) instrument operating at an accelerating voltage of 5.0 kV. To avoid structural damage, the samples were freeze−dried in a liquid nitrogen environment before being coated with gold. 2.4. Determination of Hydrodynamic Radius. The hydrodynamic radius (Rh) of molecular aggregates for different polymers was measured on Malvern Zetasizer Nano ZS90 (U.K.) at different temperatures and salinities with a scattering angle of 90° according to the principle of dynamic light scattering (DLS) and equivalent sphere as described below.33 The hydrodynamic size of the polymer refers to the size of the hydrated molecular layer of the polymer in aqueous solution. When the polymer concentration is increased to a certain extent, molecular chains will intertwine to form aggregates, resulting in the increase of the polymer hydrodynamic size. The hydrated polymer molecules are modeled in terms of equivalent hydrodynamic spheres that would increase the viscosity to the same extent as solid spherical particles. Rh was calculated according to the Stokes−Einstein equation:34 Rh =

kT 6πηD

Figure 2. Apparent viscosity (η) versus the shear rate (γ̇) and the complex viscosity (|η*|) versus the angular frequency (ω) for diutan gum (D), xanthan gum (X), and HPAM (H) under the ideal conditions (I) and the reservoir conditions (R, 75 °C and 9503 mg·L−1 salinity).

rate, the apparent viscosity of polymer solutions decreases gradually, showing the behavior of the pseudoplastic fluids. This shear-thinning property is greatly related to the molecular aggregation or disperse states in the shear flow.35 Polysaccharide molecules generally exist as the aggregates at low shear rates. When the shear rate increases, the aggregates are dissociated gradually by the shear force, and the individual molecules rearrange along the flow direction; the apparent viscosity declines as a result. Under the ideal conditions, the apparent viscosity of diutan gum is higher than that of xanthan gum or HPAM at the shear rate of 7.34 s−1; the data are shown in Table 2. The apparent viscosity of HPAM is greater than that of xanthan gum. This means that the order of the thickening ability for three polymers is diutan gum > HPAM > xanthan gum under the ideal conditions. Under the reservoir conditions, the apparent viscosities of the polymer solutions decrease. However, the magnitude of the apparent viscosity reduction is clearly different for three polymers, which can be seen in the viscosity retention rate [ϕ (%), the ratio of the apparent viscosity under the ideal conditions and the viscosity under the reservoir conditions], as shown in Table 2. Under the reservoir conditions, the ϕ value of diutan gum (96.5%) is much higher than those of xanthan gum (30.3%) and HPAM (7.95%). Moreover, when the temperature increases to 90 °C and the salinity increases to 244 121 mg·L−1, the ϕ value of diutan gum is still higher than 90%, and that of HPAM is only 2.19%. The small viscosity reduction of diutan gum under the reservoir conditions indicates that high temperature and the high salinity have little influence on the molecular structure of diutan gum. The apparent viscosity of the HPAM solution is the lowest under the reservoir conditions, although it is much higher than that of xanthan gum under the ideal conditions. For the

(1)

where k is the Boltzmann constant; T the temperature; η the medium viscosity; and D the diffusion coefficient, calculated by fitting the correlation curve to an exponential function, with D being proportional to the lifetime of the exponential decay. Each sample was measured three times, and the average value of Rh was calculated. 2.5. Flow and Flooding Tests. Flow and flooding tests were performed using sandpack holders (15.0 cm in length and 3.8 cm in diameter) equipped with fluid distributors at both sides. Different absolute permeability can be obtained by adjusting the mesh (80− 120) of the sand and the packing pressure. The flow and flooding setups were put inside an air bath where the temperature was controlled at 75 or 90 °C. The pore volume was calculated by dividing the weight difference between water-saturated and dry sandpack by the water density. The porosity was determined by the ratio of the pore volume to the volume of the sandpack. The absolute permeability was calculated according to Darcy’s Law by conducting single-phase (brine) flow at different flow rates. All the sandpacks in the setups were placed horizontally. Three processes were conducted in flow tests. The sandpacks were first injected with the brine until the pressure drop across the sandpack was stable. Then, the polymer solution injection process was followed, and finally the extended brine injection process was implemented. Digital pressure gauges were used to monitor the pressure drop across the sandpack to determine the flow resistance. Before each flooding test, the initial oil saturation was set by injected brine water and heavy oil. An initial water-flooding was first conducted with a heavy oil recovery of 40% OOIP. Then, for the tertiary flooding, the polymer solution was injected until the oil cut was less than 2.0%, which was followed by an extended water-flooding until the oil production was negligible. The pressure drop during C


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Table 2. Retention Rate (ϕ) of the Apparent Viscosity (η) and the Dynamic Moduli (G′ and G″) for Diutan Gum, Xanthan Gum, and HPAM Solutions at Different Temperatures and Salinitiesa steady shear temperature (°C)

salinity (mg·L−1)

ϕη (%)

ϕG′ (%)

ϕG″(%)

none diutan gum

25 25 25 75 75 90 90 25 25 75 75 90 90 25 25 75 75 90 90

0 9 503 244 121 0 9 503 0 244 121 9 503 244 121 0 9 503 0 244 121 9 503 244 121 0 9 503 0 244 121

100 97.2 94.6 99.3 96.5 99.1 90.1 80.6 67.9 55.0 29.1 26.7 15.3 26.4 18.1 99.1 2.95 66.7 2.19

100 96.2 93.1 96.7 92.2 93.5 91.7 76.7 54.4 34.9 29.5 24.4 19.3 0.10 0.02 93.4 0.03 1.82 0.01

100 99.4 99.1 98.7 98.3 98.4 97.3 92.1 63.2 52.6 43.9 29.2 25.7 2.80 0.61 99.3 1.60 12.1 0.59

xanthan gum

HPAM

a

dynamic oscillation

system

γ̇ = 7.34 s−1 and f = 0.1 Hz.

comparison between xanthan gum and HPAM, similar results have been previously reported.36 Figure 2 also shows the complex viscosity (|η*|) of aqueous polymer solutions as a function of the angular frequency (ω) under different conditions. The curve shows that the |η*| of diutan gum has a smaller reduction from the ideal conditions to the reservoir conditions within the angular frequency range studied, compared with those of xanthan gum and HPAM. Under the reservoir conditions, the |η*| of diutan gum is much higher than that of xanthan gum and HPAM at the same angular frequency. It can be seen that the curve of |η*| versus ω is almost identical to that of η versus γ̇. This is well-known as the Cox−Merz rule.37 The Cox−Merz rule is applicable for flexible molecules, and intermolecular interactions are detectable by comparison of η and |η*|.38 Because the derivations from the Cox−Merz rules are due to the energetic interactions, when |η*| is higher than η, the strong interaction occurs between the molecular chains. Under the reservoir conditions, for diutan gum, the values of |η*| are still higher than those of η, and an adverse phenomenon occurs for xanthan gum and HPAM. It indicates that the molecular structures of xanthan gum and HPAM have a significant change at high temperature and high salinity; meanwhile, diutan gum can maintain the molecular structure well. Under the reservoir conditions, aqueous diutan gum, xanthan gum, and HPAM solutions are sheared at the rate of 7.34 s−1 for 120 min, as shown in Figure 3. With the increase of shearing time, the viscosities of diutan gum and xanthan gum are essentially unchanged. However, for HPAM, the viscosity gradually decreases with increasing shearing time, showing a behavior of shear degradation. The viscosity stability of the polysaccharides (diutan gum and xanthan gum) is better than that of HPAM. At the concentration of 1.75 g·L−1, which exceeds the critical aggregation concentrations of diutan gum and xanthan gum, inter- and intramolecular network structures are formed within solutions.39 When the applied stress is constant, the molecular aggregates of diutan gum and xanthan

Figure 3. Apparent viscosity (η) of aqueous diutan gum (D), xanthan gum (X), and HPAM (H) solutions as a function of the shear time at the shear rate of 7.34 s−1 under the reservoir conditions (R, 75 °C and 9503 mg·L−1 salinity).

gum do not vary as the shear time increases, which means that their molecular structures are dependent on the shear rate rather than the shear time. For HPAM, the molecular aggregate is gradually destroyed when the shear time is increased, indicating that both the shear rate and the shear time have important effects on the molecular structure of HPAM. Therefore, because the molecular structure is independent of the shear time, the diutan gum solution can maintain viscosity when passing through pumps, pipelines, perforations, and porous media. The relationship between the complex modulus (G*) of polymer solutions and the stress is shown in Figure 4. The high plateau (Gp*) where the complex modulus is independent of the stress is considered to be the linear viscoelastic region. Diutan gum, xanthan gum, and HPAM solutions, under the ideal conditions, all show the linear viscoelastic region. The value of G* remains nearly constant with an increase of stress until the critical stress value (ιc, the stress at the inflection point) is reached. The sudden drop of G* means that substantial change occurs for the molecular structure. This change is always related to the static yield stress of the macromolecule solution in which the molecular aggregates D


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Figure 4. Complex modulus (G*) of aqueous diutan gum (D), xanthan gum (X), and HPAM (H) solutions as a function of the stress under the ideal conditions (I) and the reservoir conditions (R, 75 °C and 9503 mg·L−1 salinity).

form.40 Here the value of ιc reflects the shear resistance of the molecular aggregate to applied stress; when ι > ιc, the initial molecular aggregate is destroyed. For diutan gum, the values of Gp* and ιc, under the reservoir conditions, are close to those under the ideal conditions. However, for xanthan gum and HPAM, the values of Gp* and ιc significantly decrease when changing to the reservoir conditions. Under both the ideal and the reservoir conditions, the Gp* and ιc values of diutan gum are higher than those of xanthan gum and HPAM, indicating that the molecular structure of diutan gum is more stable and can withstand larger applied stress than those of xanthan gum and HPAM at high temperature and high salinity. In the dynamic oscillation experiment, frequency sweep is always used to characterize the morphological characteristics and motion features of different molecules, coils, and chain segments. Figure 5 shows the effect of the oscillation frequency on the storage modulus (G′) and the loss modulus (G″) of aqueous polymer solutions. G′ and G″ represent the elastic component and the viscous component of the solution, respectively. If G′ is higher than G″, the elasticity of the solution is higher than the viscosity. The elasticity of the solution is the dominating factor, meaning that a gel-like structure is built within the solution. Under the ideal conditions, both G′ and G″ for diutan gum are higher than those for xanthan gum and HPAM at the same oscillation frequency, indicating that the diutan gum solution has a stronger viscoelasticity than other two. Moreover, G′ is higher than G″ for diutan gum and HPAM in the entire range of the oscillation frequency, and for xanthan gum, G′ is higher than G″ in most of the frequency range. The elastic component is the dominant factor in the viscoelasticity, meaning that gel-like structures are formed for three polymers in the aqueous solution, as shown by SEM images in Figure 6. The network

Figure 6. SEM images of diutan gum (D), xanthan gum (X), and HPAM (H) under the ideal conditions. The concentration of the aqueous polymer solution is 1.75 g·L−1.

structure of diutan gum is more compact and complex than those of xanthan gum and HPAM, which is due to the strong inter- and intramolecular interaction of diutan gum. The aqueous diutan gum solution shows a superior viscoelasticity as a result. Different molecular structures of polymers determine distinct responses to the environment conditions. Under the reservoir conditions, G′ for diutan gum is still higher than G″ in most of the range of the oscillation frequency. However, xanthan gum and HPAM solutions show the typical properties of the disordered structure in that G″ is higher than G′ and the dynamic modulus shows strong dependence on the frequency. The viscosity component is dominant in viscoelasticity, indicating that the gel-like structure is destroyed for xanthan gum and HPAM. Both G′ and G″ of diutan gum are several orders of magnitude higher than those of xanthan gum or HPAM, indicating that the viscoelasticity of diutan gum is more prominent.

Figure 5. Storage modulus (G′, solid) and loss modulus (G″, hollow) of aqueous diutan gum (D), xanthan gum (X), and HPAM (H) solutions as a function of the oscillation frequency under the ideal conditions (I) and the reservoir conditions (R, 75 °C and 9503 mg·L−1 salinity). E


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Table 3. At 75°C and 9503 mg·L−1 Salinity, the Retention Rate (ϕ) of the Apparent Viscosity (η) and the Dynamic Moduli (G′ and G″) for Diutan Gum, Xanthan Gum, and HPAM Solutions at Different Aging Timesa

High temperature and high salinity always decrease the viscoelasticity of polyelectrolyte solutions. However, they have smaller influences on the viscoelasticity of the diutan gum solution. This can be clearly seen from the ϕ values of the dynamic modulus at diffjerent temperatures and salinities, as shown in Table 2. At high temperature (90 °C) and high salinity (244 121 mg·L−1), the dynamic modulus retention rates (ϕG′ and ϕG″) of diutan gum are still above 90%. However, ϕG′ and ϕG″ of xanthan gum are only 19.3% and 25.7%, respectively, and those of HPAM are even lower (0.02% and 0.59%, respectively). The results indicate that diutan gum solutions have better temperature tolerance and salinity tolerance. Thus, to achieve similar apparent viscosity or viscoelasticity, a lower dosage of diutan gum could be needed compared with xanthan gum and HPAM. Moreover, for xanthan gum, the temperature has a greater influence on reducing the viscosity and the dynamic modulus; conversely, the salinity is the dominant factor for HPAM. For the polymers used in EOR, one of the most important criteria is the viscosity (and viscoelasticity) retention after long-term aging under the reservoir conditions. Therefore, the effects of the aging time on the apparent viscosity of polymer solutions are investigated, as shown in Figure 7. The apparent viscosity of diutan gum

steady shear

dynamic oscillation

system

time (day)

ϕη (%)

ϕG′ (%)

ϕG″(%)

diutan gum

0.25 3 5 10 15 20 25 0.25 3 5 10 15 20 25 0.25 3 5 10 15 20 25

96.3 95.5 94.9 94.5 94.4 94.1 94.0 25.2 22.0 18.2 17.6 15.7 15.7 15.0 2.4 2.3 2.3 2.3 2.2 2.2 2.1

94.7 94.9 94.5 93.9 93.5 93.3 93.2 11.1 9.3 8.9 8.5 8.4 8.4 8.2 1.5 1.4 1.4 1.3 1.2 1.2 1.1

96.4 95.9 95.7 95.5 95.3 95.1 95.1 21.3 17.2 15.7 14.5 13.9 13.8 12.9 9.8 8.4 8.2 8.2 8.0 8.0 7.9

xanthan gum

HPAM

γ̇ = 7.34 s−1 and f = 0.1 Hz. The baseline is under the ideal conditions.

a

stability in an environment of high temperature and high salinity. In xanthan gum molecules, substituents including acetyl, carboxyl, and pyruvate groups are mainly located on the side chains. Because of the steric effect, irregular double helices are generally formed as a result. It is also reported that, for xanthan gum, a transient network is more likely formed by the chains which are cross-linked by finite lifetime bonds.42 The temperature has a determined effect on the lifetime of the bonds.43 Consequently, the transient network structures of xanthan gum are destroyed easily at high temperature because of the entanglement of chains with a finite lifetime. Moreover, the side chains distribute in the outside of the double helices. Most of the water molecules adhered to the side chains of the double helices break away from the molecular chains because of the weak interaction at high temperature. For HPAM, there are no long side chains stretches around the main chains in the molecule. A regular molecular structure is mainly formed by the long molecular chains because of van der Waals forces. The degree of molecular entanglement and the strength of the network structure are far less than those of diutan gum. Furthermore, the counterions also compact the hydrated layer around HPAM molecules, destroying the original structure. Consequently, the molecular aggregates of HPAM can be more easily changed at high salinity. 3.2. Mobility Control in Porous Media. The applications of polymers in tertiary oil recovery operations are mainly attributed to their ability to control mobility through permeability reduction.44 However, high temperature and high salinity always render conventional polymers ineffective in permeability reduction. Therefore, it is important to identify the mobility of diutan gum in high-temperature and high-

Figure 7. Apparent viscosity (η) of aqueous diutan gum (D), xanthan gum (X), and HPAM (H) solutions as a function of the aging time at the shear rate of 7.34 s−1 under the ideal conditions (I) and the reservoir conditions (R, 75 °C and 9503 mg·L−1 salinity).

solutions is essentially constant after being placed for 25 days under the ideal conditions and changes little under the reservoir conditions. The apparent viscosity of xanthan gum solutions has an obvious decline as time progresses. The G′ and the G″ retention of xanthan gum after 3 days are only 9.3% and 17.2%, respectively, under the reservoir conditions, as shown in Table 3, further indicating the network structure of xanthan gum is just a temporary one and easy to break up. For HPAM, though it shows a good stability of the apparent viscosity in the aging process, the very low retention of the apparent viscosity and the viscoelasticity under the reservoir conditions make it unsuitable for the application. For diutan gum, the intermolecular associations occur between the side and backbone chains of different molecules via the van der Waals force and hydrogen bond.21,41 The molecules form regular double helices, and side chains distribute in the core of them. In diutan gum molecules, there are so many oxygen, hydroxyl, and carboxylate centers which have the interaction with the adsorbed water due to hydrogen bonds.39 Therefore, a great many water molecules adhere to the side chains in the double helices. The water molecules are prevented from fleeing from the double helices because of the internal forces. The water retention of the double helices changes little. This structure can maintain F


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where kbrine is the brine permeability of porous media (μm2) and φ is the porosity (%). Both the Fr and the Frr of diutan gum are higher than those of xanthan gum and HPAM, as shown in Table 4, and those of

salinity environments and understand its mobility control mechanism. Polymer adsorption can reduce the area open to fluid flow in petroleum reservoirs, reduce water permeability, and improve the mobility ratio.45 Polymer molecules are adhered to grain surfaces because of physical forces such as van der Waals forces. Therefore, the polymer adsorption on the solid surface can be measured dynamically by the method of injecting the polymer solution sample into a sandpack. Generally, there are two important factors when studying the permeability reduction and hence the efficiency of mobility control. One factor is the resistance factor (Fr), which is defined as the ratio of the mobility of the polymer solution to that of the brine under the same conditions. Another factor is the residual resistance factor (Frr), which is defined as the ratio of the mobility of the brine after polymer flooding to the mobility of the brine before polymer flooding under equivalent conditions. Frr describes the resistance of the reservoir to the flow of brine injected after the polymer solution. It is useful to control the water fingering by providing a quantitative indication of the permeability reduction. Fr and Frr can be calculated in the following forms: Fr =

Frr =

ΔP(polymer) ΔP(brine before polymer)

ΔP(brine after polymer) ΔP(brine before polymer)

Table 4. Summary of Flow Tests of Diutan Gum, Xanthan Gum, and HPAM Solutions under the Reservoir Conditions (R, 75°C and 9503 mg·L−1 salinity) system

porosity (%)

k (μm2)

Fr

Frr

r (μm)

e (μm)

diutan gum xanthan gum HPAM

45.3 40.6 39.4

1.81 1.91 1.68

42.9 21.2 6.50

12.1 8.30 1.62

5.65 6.13 5.84

2.61 2.52 0.65

HPAM are the lowest. The results indicate that the capacity of diutan gum to reduce permeability is the strongest. The permeability reduction is due to a decrease in free volume for flow and is mainly caused by adsorption or interception of polymers. The main component of the sand used in the experiments is silicon dioxide (SiO2), and there are plenty of hydroxyls forming on the surface of SiO2 in the aqueous solution. The molecular structures of diutan gum and xanthan gum contain many hydroxyl and carboxylate substituents in the backbones or side chains, which can establish hydrogen bonds with SiO2.49,50 Therefore, the adsorption capacities of diutan gum and xanthan gum are stronger than that of HPAM, which has only a weak interaction with the sandstone, which can be confirmed by the e values in Table 4. Moreover, benefiting from the longer molecular length of diutan gum, the adsorbed layer thickness is slightly larger than that of xanthan gum. The macromolecular polymers are always intercepted by pore constrictions when the size of molecular aggregates is larger than that of the pore constrictions. This has a considerable effect on the permeability reduction owing to the blockage of pore throats. Because they are influenced by high temperature and high salinity, the molecular aggregates of xanthan gum and HPAM are broken up into smaller segments, causing a reduction of the molecular dimensions, which can be confirmed by the results of dynamic light scattering shown in Figure 9.

(2)

(3)

where ΔP is the pressure drop across the sandpack in the flow of the liquid. The relationship between ΔP and pore volumes injected (PV), at high temperature and high salinity, is shown in Figure 8. ΔP gradually increases as more polymer solution is

Figure 8. Pressure drop (ΔP) as a function of number of PV in brine, diutan gum, xanthan gum, and HPAM flow tests under the reservoir conditions (75 °C and 9503 mg·L−1 salinity).

injected, indicating that the permeability is reduced by polymer injection. The Frr value reflects the reduction in permeability of porous media induced by polymer adsorption, which can be described by the average thickness of the adsorbed layer on the sand grains as given in the following equation:46,47 e = r ·(1 − Frr −1/4)

Figure 9. Hydrodynamic radius (Rh) of diutan gum, xanthan gum, and HPAM at different temperatures and salinities. The concentration of aqueous polymer solution is 1.75 g·L−1.

(4)

Most of the molecular segments of xanthan gum and HPAM cannot be intercepted by the pore throat. However, the molecular aggregates of diutan gum are minimally changed at high temperature and high salinity. Molecular aggregates of diutan gum are still intercepted by the pore throat under the reservoir conditions, which causes permeability reduction. Therefore, permeability reduction by diutan gum is mainly caused by two mechanisms: adsorption and interception.

where e is the average hydrodynamic adsorbed layer thickness (μm), Frr the residual resistance factor at the steady stage, and r the average pore radius (μm) for brine flow which can be calculated by the following equation:48 ⎛ 8·k brine ⎞1/2 r=⎜ ⎟ ⎝ φ ⎠

(5) G


Article

 25.1 14.4 9.5  20.9 9.3 5.4 43.8 39.0 39.1 39.2 44.6 39.6 39.4 39.7 80.7 84.8 82.7 83.6 81.6 84.4 84.2 82.5 1.57 1.62 1.84 1.47 1.63 1.78 1.58 1.52 44.7 44.1 44.1 44.7 44.6 43.9 44.1 44.5 90 °C, 244 121 mg·L−1

brine diutan gum xanthan gum HPAM brine diutan gum xanthan gum HPAM 75 °C, 9503 mg·L

final recovery (%OOIP) tertiary recovery (%OOIP) water-flooding recovery (%OOIP) system

porosity (%)

permeability (μm2)

initial oil saturation (%) H

temperature and salinity

Table 5. Summary of Flooding Tests under the Reservoir Conditions

4. CONCLUSION The rheological properties, mobility control, and heavy oil recovery efficiency of aqueous diutan gum solution have been characterized at high temperature and high salinity and

−1

3.3. Heavy Oil Recovery Efficiency. Mobilization of residual oil in a reservoir is significantly influenced by the viscoelasticity of the displacing phase because of the mobility ratio reduction. Heavy oil recovery efficiencies of diutan gum, xanthan gum, and HPAM were investigated using sandpack to simulate the reservoir at high temperature and high salinity. Results of the oil recovery are summarized in Table 5. At 90 °C and 244 121 mg·L−1 salinity, 60.5%, 48.7%, and 45.1% of OOIP are final recovered by diutan gum, xanthan gum, and HPAM, respectively, compared to 44.6% of OOIP by water-flooding, as shown in Figure 10. In addition, at 75 °C and 9503 mg·L−1 salinity, 64.1%, 53.5%, and 48.7% of OOIP are final recovered by diutan gum, xanthan gum, and HPAM, respectively. The optimal injection dosage for diutan gum is 1.8 PV, at which the most residual oil is recovered. In addition, the recovery efficiencies of xanthan gum and HPAM, at high temperature and high salinity, are much lower than those in the conventional reservoirs.7,47 Especially for HPAM at 90 °C and 244 121 mg·L−1 salinity, the final recovery is just slightly higher than that of brine water. This result indicates that high temperature and high salinity of the reservoir clearly reduce the effectiveness of xanthan gum and HPAM in EOR. In addition, the amount of residual oil recovered by diutan gum is greater than that recovered by the other two conventional polymers. Diutan gum recovers approximately 11.8% more OOIP than xanthan gum and 15.4% more OOIP than HPAM at 90 °C and 244 121 mg·L−1 salinity, demonstrating that the heavy oil recovery efficiency of diutan gum is much higher than those of xanthan gum and HPAM. The preeminent EOR performance of diutan gum is attributed to its mobility reduction capacity, which results from the associated and enlarged networks. The disadvantage of water-flooding on heavy oil recovery is the serious viscous fingering due to the viscosity difference between the heavy oil and brine. Therefore, the key factor to increase heavy oil recovery is weakening the viscous fingering.51 Figure 11 shows the relationship between water cut and PV. When injecting 2.58 PV brine, the water cut in the producing fluid reached 98%, and heavy oil recovery was nearly negligible with further brine injection. However, injection of polymer solutions significantly reduces the water cut. The minimum water cut is as low as 31.4% for diutan gum, 71.5% for xanthan gum, and 85.3% for HPAM. The results indicate that diutan gum is more efficient than xanthan gum and HPAM in reducing the water cut of the produced fluids. Figure 12 shows the relationship between the pressure drop and PV injection of the water-flooding and the polymer tests. The pressure drop during water-flooding first rises from 0 to 0.37 MPa and then decreases with further brine injection. However, when the polymer flooding is injected at 2.0 PV, the pressure drop increases again. Meanwhile, the heavy oil recovery has a significant increase. The pressure drop maximum of diutan gum is higher than those of xanthan gum and HPAM, which is attributed to the adsorption and interception of diutan gum in the pore constrictions, blocking the water channels. These results, including the rheological properties, flow behavior and EOR performance, confirm that diutan gum is a promising flooding agent for enhancing heavy oil recovery at high temperature and high salinity.

43.8 64.1 53.5 48.7 44.6 60.5 48.7 45.1

Energy & Fuels


Article

Energy & Fuels

the permeability reduction of the porous media is mainly caused by adsorption and interception. Diutan gum overcomes the disadvantages of xanthan gum and HPAM at high temperature and high salinity and recovers more residual heavy oil than xanthan gum or HPAM. The results provide a basic understanding of the application of diutan gum in high temperature and high salinity reservoirs and are also useful for identifying the mechanism of diutan gum for enhanced heavy oil recovery.

■

Figure 10. Cumulative oil recovery as a function of number of PV in brine, diutan gum, xanthan gum, and HPAM flooding under the reservoir conditions (90 °C and 244 121 mg·L−1 salinity).

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-532-86983082. Fax: +86-532-86983082. E-mail: [email protected]. ORCID

Long Xu: 0000-0003-1859-6538 Mingzhe Dong: 0000-0002-2926-5139 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Natural Science Foundation of China (51274225), the 973 Program (2014CB239103), and the National Science and Technology Major Project (2016ZX05023001-003-001).

Figure 11. Water cut as a function of number of PV in brine, diutan gum, xanthan gum, and HPAM flooding under the reservoir conditions (90 °C and 244 121 mg·L−1 salinity).

■

REFERENCES

(1) Zhang, H.; Dong, M.; Zhao, S. Energy Fuels 2010, 24, 1829− 1836. (2) Taylor, K. C.; Nasr-El-Din, H. A. J. Pet. Sci. Eng. 1998, 19, 265− 280. (3) Tam, K. C.; Guo, L.; Jenkins, R. D.; Bassett, D. R. Polymer 1999, 40, 6369−6379. (4) Homsy, G. M. Annu. Rev. Fluid Mech. 1987, 19, 271−311. (5) Gong, H.; Xu, L.; Xu, G.; Dong, M.; Li, Y. Ind. Eng. Chem. Res. 2014, 53, 4544−4553. (6) Wang, W.; Liu, Y.; Gu, Y. Colloid Polym. Sci. 2003, 281, 1046− 1054. (7) Xu, L.; Xu, G.; Yu, L.; Gong, H.; Dong, M.; Li, Y. Polym. Adv. Technol. 2014, 25, 1122−1129. (8) Zhang, Z.; Li, J.; Zhou, J. Transp. Porous Media 2011, 86, 199− 214. (9) Jung, J. C.; Zhang, K.; Chon, B. H.; Choi, H. J. J. Appl. Polym. Sci. 2013, 127, 4833−4839. (10) Wyatt, N. B.; Gunther, C. M.; Liberatore, M. W. Polymer 2011, 52, 2437−2444. (11) Cohen, J.; Priel, Z. Macromolecules 1989, 22, 2356−2358. (12) Matsukawa, S.; Watanabe, T. Food Hydrocolloids 2007, 21, 1355−1361. (13) Leung, W.; Axelson, D.; Van Dyke, J. J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 1825−1846. (14) Wever, D.; Picchioni, F.; Broekhuis, A. Prog. Polym. Sci. 2011, 36, 1558−1628. (15) Zhu, D.; Wei, L.; Wang, B.; Feng, Y. Energies 2014, 7, 3858− 3871. (16) Kamal, M. S.; Sultan, A. S.; Al-Mubaiyedh, U. A.; Hussein, I. A.; Feng, Y. Can. J. Chem. Eng. 2015, 93, 1194−1200. (17) Quadri, S. M. R.; Shoaib, M.; AlSumaiti, A. M.; Alhassan, S. M. International Petroleum Technology Conference, Doha, Qatar, December 6−9, 2015; IPTC 18436. (18) Sun, S.; Zhang, Z.; Luo, Y.; Zhong, W.; Xiao, M.; Yi, W.; Yu, L.; Fu, P. Bioresour. Technol. 2011, 102, 6153−6158. (19) Nakajima, K.; Ikehara, T.; Nishi, T. Carbohydr. Polym. 1996, 30, 77−81.

Figure 12. Pressure drop (ΔP) as a function of number of PV in brine, diutan gum, xanthan gum, and HPAM flooding under the reservoir conditions (90 °C and 244 121 mg·L−1 salinity).

compared with two conventional polymers: xanthan gum and HPAM. Under the ideal conditions, the order of the thickening ability for three polymers in aqueous solution is diutan gum > HPAM > xanthan gum; under the reservoir conditions, the order changes to diutan gum > xanthan gum > HPAM. The apparent viscosity and viscoelasticity of the xanthan gum and HPAM solutions decrease much more than those of the diutan gum solutions in the environment of high temperature and high salinity. The viscosity of the diutan gum solution is dependent on the shear rate rather than on the shear time and the aging time. The gel-like structure formed by diutan gum is much denser and more complex and changes minimally at high temperature and high salinity, while temperature is the main factor reducing the viscosity of xanthan gum; salinity is the main factor for HPAM. The thermal stability and salt tolerance of diutan gum are mainly attributed to the stability of the gellike molecular structure formed by the double helix. Flow and flooding tests demonstrate the superior capacity of diutan gum in mobility control during propagation in porous media, and I


Article

Energy & Fuels (20) Kim, D.-S.; Fogler, H. Appl. Microbiol. Biotechnol. 1999, 52, 839−844. (21) Member, M. W.; Morris, E. R. Carbohydr. Polym. 1995, 27, 23− 36. (22) Poli, A.; Kazak, H.; Gürleyendağ, B.; Tommonaro, G.; Pieretti, G.; Ö ner, E. T.; Nicolaus, B. Carbohydr. Polym. 2009, 78, 651−657. (23) Xu, L.; Xu, G.; Liu, T.; Chen, Y.; Gong, H. Carbohydr. Polym. 2013, 92, 516−522. (24) Xu, L.; Xu, G.; Gong, H.; Dong, M.; Li, Y.; Zhou, Y. Colloids Surf., A 2014, 456, 176−183. (25) Kudaibergenov, S.; Nuraje, N.; Adilov, Z.; Abilkhairov, D.; Ibragimov, R.; Gusenov, I.; Sagindykov, A. J. Appl. Polym. Sci. 2015, 132, 41256. (26) Banerjee, P.; Mukherjee, I.; Bhattacharya, S.; Datta, S.; Moulik, S. P.; Sarkar, D. Langmuir 2009, 25, 11647−11656. (27) Mukherjee, I.; Sarkar, D.; Moulik, S. P. Langmuir 2010, 26, 17906−17912. (28) Sonebi, M.; McKendry, D. Open Construct. Build. Technol. J. 2008, 2, 15−23. (29) Rodd, A. B.; Dunstan, D. E.; Boger, D. V. Carbohydr. Polym. 2000, 42, 159−174. (30) De Jong, S.; Van de Velde, F. Food Hydrocolloids 2007, 21, 1172−1187. (31) Roussel, N.; Lemaître, A.; Flatt, R. J.; Coussot, P. Cem. Concr. Res. 2010, 40, 77−84. (32) Zhang, J.; Weissinger, E. A.; Peethamparan, S.; Scherer, G. W. Cem. Concr. Res. 2010, 40, 1023−1033. (33) Rochas, C.; Geissler, E. Macromolecules 2014, 47, 8012−8017. (34) Edward, J. T. J. Chem. Educ. 1970, 47, 261−270. (35) Chagas, B. S.; Machado, D. L. P.; Haag, R. B.; De Souza, C. R.; Lucas, E. F. J. Appl. Polym. Sci. 2004, 91, 3686−3692. (36) Jang, H. Y.; Zhang, K.; Chon, B. H.; Choi, H. J. J. Ind. Eng. Chem. 2015, 21, 741−745. (37) Cox, W.; Merz, E. J. Polym. Sci. 1958, 28, 619−622. (38) Park, S.; Lim, S.; Shin, T.; Choi, H.; Jhon, M. Polymer 2001, 42, 5737−5742. (39) Xu, L.; Gong, H.; Dong, M.; Li, Y. Carbohydr. Polym. 2015, 132, 620−629. (40) Fang, F. F.; Liu, Y. D.; Choi, H. J.; Seo, Y. ACS Appl. Mater. Interfaces 2011, 3, 3487−3495. (41) Morris, E.; Gothard, M.; Hember, M.; Manning, C.; Robinson, G. Carbohydr. Polym. 1996, 30, 165−175. (42) Tako, M.; Teruya, T.; Tamaki, Y.; Konishi, T. Colloid Polym. Sci. 2009, 287, 1445−1454. (43) Modig, K.; Pfrommer, B. G.; Halle, B. Phys. Rev. Lett. 2003, 90, 075502. (44) Gogarty, W. SPEJ, Soc. Pet. Eng. J. 1967, 7, 161−173. (45) Leslie, T.; Xiao, H.; Dong, M. J. Pet. Sci. Eng. 2005, 46, 225− 232. (46) Wever, D. A.; Picchioni, F.; Broekhuis, A. A. Ind. Eng. Chem. Res. 2013, 52, 16352−16363. (47) Wei, B. Carbohydr. Polym. 2015, 134, 398−405. (48) Zaitoun, A.; Kohler, N. SPE Annual Technical Conference and Exhibition, Houston, TX, October 2−5, 1988; SPE 18085. (49) Dário, A. F.; Hortêncio, L. M.; Sierakowski, M. R.; Neto, J. C. Q.; Petri, D. F. Carbohydr. Polym. 2011, 84, 669−676. (50) Petri, D. F.; de Queiroz Neto, J. C. J. Pet. Sci. Eng. 2010, 70, 89− 98. (51) Fletcher, P.; Cobos, S.; Jaska, C.; Forsyth, J.; Crabtree, M.; Gaillard, N.; Favéro, C. SPE Improved Oil Recovery Symposium, Tulsa, OK, April, 14−18, 2012; SPE 154045.

J


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