Experimental Investigation of a Mechanically Stable and Temperature/Salinity Tolerant Biopolymer toward Enhanced Oil Recovery Application in Harsh Condition Reservoirs

Abstract

In search of robust polymers for enhanced oil recovery (EOR) application in reservoirs with harsh conditions, a water-soluble biopolymer was thoroughly investigated in this work to evaluate its applicability in such reservoirs. The experimental data revealed that compared to the commonly used EOR polymer, HPAM, the biopolymer was more efficient in thickening a brine solution as a result of its peculiar conformation. The presence of an electrolyte has almost no effect on the rheology of the biopolymer solution, even at an extremely high salt concentration (20 wt% NaCl). The relation between viscosity and the concentration curve was well fitted to the power-law model. Moreover, the rigid polymer chains rendered the polymer solution superior tolerance to elevated temperatures and salinity, but also led to considerable retention within tight porous media. The adsorption behavior was characterized by the average thickness of the hydrodynamic adsorbed layer on sand grains. The mechanical degradation was assessed by forcing the polymer solutions to flow through an abrupt geometry at ultra-high shear rates. The slight viscosity loss compared to HPAM proved the high mechanical stability of this polymer. These properties made it a promising alternative to HPAM in polymer flooding in the near future for high permeability oil reservoirs with harsh conditions.

1. Introduction

The use of polymers and polymer-based materials in the petrochemical industry is becoming increasingly important and covers many application areas, such as oilfield produced water treatment [1], oil-in-water separation [2], and wax inhibition coatings [3]. In recent years, enhanced oil recovery (EOR) has attracted considerable attention due to the depleting oil reserves worldwide. Common EOR methods include thermal, miscible, and chemical processes, of which the chemical method is the most promising technique in the petroleum industry. Polymer flooding is considered the simplest chemical EOR process, which has been extensively used in oilfields, such as Daqing (China) [4,5] and Pelican Lake (Canada) [6]. The advantage of this technique is that more oil can be produced than all other chemical EOR processes in combination [7].

The possible mechanisms of polymer flooding have been reviewed in the previous article [8]. It is believed that the enhanced oil recovery performance of a polymer is closely related to three factors: (1) viscosity, (2) elasticity, and (3) permeability reduction capacity. The primary purpose of adding a water-soluble polymer is to increase the viscosity of the aqueous phase, through which the unfavorable mobility ratio responsible for the poor volumetric sweep efficiency can be corrected. In addition to viscosity increase, a more favorable mobility ratio can also be reached by decreasing the permeability of porous media. Indeed, this is very achievable in polymer flooding as a result of polymer retention [9,10]. As one of the unique flow characteristics, the elastic property of polymer significantly benefits the promotion of the displacement efficiency through mobilizing the residual oil trapped by capillary force. Numerous efforts have been made to establish this new concept [11,12,13,14].

In reality, a number of polymers have been proposed for EOR use no matter at laboratory or industry, which can be roughly divided into two categories, i.e., synthetic polymers and biopolymers [15,16]. Hydrolyzed polyacrylamide (HPAM) is the first synthetic and the most commonly used polymer to date as a thickening agent in polymer EOR processes due to its availability in large quantities with customized properties (molecular weight, hydrolysis degree, etc.) [5,17]. However, this polymer is very susceptible to reservoir conditions, such as high temperature, salinity, and shear forces, leading to a noticeable loss in thickening ability and subsequent poor oil recovery efficiency [18,19,20]. Xanthan gum is a typical example of the biopolymer, which has increasingly motivated research interests for its application in enhanced oil recovery processes because of the superior tolerance to harsh conditions present in most reservoirs [21,22]. Nevertheless, the high manufacturing cost and possibility of a plugging problem induced by the presence of cell debris restrict its application for large-scale fields [23]. Following xanthan gum, the biopolymer we investigated is expected to be another biopolymer for potential use in the petroleum industry. This nonionic, water-soluble homoglucan is produced by a fungus in a fermentation process from a carbon source, e.g., glucose, which has a β-(1→3) linked backbone with single β-(1→6) linked glucose side chains at approximately every third residue, as shown in Figure 1 [24]. It has been found to exist as a rigid triple helix structure in water; in the triple helix, three schizophyllan chains assemble by intermolecular hydrogen bonds, and the side chain glucoses are located outwards to the helix core [25]. This peculiar structure offers this biopolymer some advanced properties.

The main objective of this work was to evaluate the potential of this biopolymer as a flooding agent for polymer EOR processes, especially for reservoirs with harsh conditions, which has been rarely reported so far. To accomplish this research objective, the physical properties of this biopolymer including thickening ability, salt tolerance, temperature resistance, mechanical stability, and flow characteristics in porous media were systematically investigated and compared against a commercial EOR polymer, partially hydrolyzed polyacrylamide (HPAM). The effect of the experimental conditions on the rheological properties of the biopolymer was further investigated through fitting data to some well-known and simple numerical models.

2. Experimental Procedure

2.1. Materials

Hydrolyzed polyacrylamide (Degree of hydrolysis, 5%; Molecular weight: 8 × 10⁶ g/mol) was supplied by Beijing Hengju Corp. The biopolymer sample (Molecular weight, 2–3 × 10⁶ g/mol) was customized and purchased from Sichuan Weike Biotech Cor., Chengdu, China. The polymer solutions were prepared by dissolving the corresponding polymer in synthetic brine. The crude oil was taken from Daqing oilfield, and the viscosity of crude oil is 18.6 mPa∙s.

2.2. Steady and Dynamical Rheological Measurements

The measurements of steady rheological were carried out using a Bohlin Gemini HR Nano Rheometer equipped with a cone and plate geometry (2°/55 mm) at 25 °C. Steady shear data (viscosity and shear stress) were obtained over a shear rate range of 0.1 s⁻¹–1000 s⁻¹, while dynamical shear data (viscous and elastic moduli) were obtained from a frequency sweep over a range of 0.1 rad/s–100 rad/s at 10% strain with a parallel plate geometry (60 mm and 1 mm gap size). This strain value was in the linear viscoelastic response of the polymers. Frequency sweep measurements were performed at 25 °C.

2.3. Mechanical Stability Test

As shown in Figure 2 (left), a syringe pump (ISCO, Model 100 DX, Lincoln, NE, USA) from a piston accumulator was used to inject synthetic brine at different flow rates through the capillary (Radius, 0.46 mm; Length, 5 cm) at room temperature (25 °C ± 1 °C). After polymer solution injection, a pressure transducer (Kobold Instruments Inc., Pittsburgh, PA, USA) was used to measure the pressure when the brine and polymer solution flow through the capillary at the time of stabilization. Generally, 10 cm³–15 cm³ of the effluent was collected at each flow rate and then sealed. The viscosity of the effluent was measured one day later to eliminate any flow memory of the polymer chains due to the applied stress. Each test was conducted in duplicate, and the mean value was used. Mechanical degradation (D_R) could be calculated using Equation (1)

$$\mathrm{DR}(\%)=\frac{{\mathsf{\mu }}_{\mathrm{o}}-{\mathsf{\mu }}_{\mathrm{e}}}{{\mathsf{\mu }}_{\mathrm{o}}}\times 100$$

(1)

where μ_o is the initial viscosity at 10 s⁻¹; and μ_e is the effluent viscosity at the same shear rate.

Mobility reduction (R_m) is the ratio of pressure drop between polymer solution (ΔP_p) and solvent flow (ΔP_b) at a certain flow rate. Therefore, the apparent viscosity of polymer solution (η_p) during flow through the capillary tube can be calculated by multiplying the decrease in the mobility of the solvent (Equation (2))

$$R_m=\frac{\Delta P_p}{\Delta P_b}=\frac{{\eta }_p}{{\eta }_b}$$

(2)

Shear rate ($\dot{\gamma }$) for a circular tube of radius (r) can be obtained using Equation (3) [26]

$$\dot{\gamma }=\frac{4\times V}{r}=\frac{4\times Q}{\pi \times r^3}$$

(3)

where V is the average velocity of solution, and Q is the volume flow rate which can be controlled by the injection pump.

The shear rate values corresponding to the flow rates evaluated in this work are listed in

Table 1. Shear rate at different flow rate in the capillary.

Flow Rate (mL/min)	Shear Rate (×103 s−1)
2	3.49
4	6.98
6	10.47
8	13.96
10	17.45
12	20.94
14	24.43
16	27.92
18	31.41
20	34.90

.

2.4. Core Flood Test

Core flood tests were performed to evaluate the flow characteristics of polymer systems in porous media. In the current work, sandstone cores (Berea provided by Kocurek Industries Inc., Hard Rock Division, Caldwell, TX, USA) were used. The plug parameters are listed in

Table 2. Plug Parameters of Core Flood Tests.

Core Plug	HPMA	Biopolymer
Permeability	kbrine = 149 md	kbrine = 130 md
Length	5 cm	5 cm
Cross-sectional area	11.81 cm2	11.81 cm2
Pore volume (PV)	16.43 cm3	15.56 cm3
Porosity	27.8%	26.3%
Average pore radius	2.07 µm	1.98 µm
Shear rate (s−1)	58.6	64.6

. In addition to the given parameters, the average pore radius (r) of the core plugs can be obtained using Equation (4) [27]

$$\mathrm{r}={(\frac{8·{\mathrm{k}}_{\mathrm{brine}}}{\mathsf{\phi }})}^{\frac{1}{2}}$$

(4)

where k_brine is the brine permeability (D), and φ is the porosity (fraction).

The core flood experiments were conducted at room temperature (25 °C ± 1 °C). All the fluids were injected at a linear velocity of 0.5 cm³/min. The brine containing 20 wt% NaCl was used throughout these tests. A differential pressure transducer (Kobold Instruments Inc., Pittsburgh, PA, USA) was connected to read the pressure drop (Δp) between the injector and producer. A data acquisition system was used to record readings from the transducer.

The effective shear rate in the porous media imposed on the fluids was established by the following equation (Equation (5)) [28]:

$$\dot{\gamma }=\frac{3n+1}{4n}·\frac{4Q}{A{\left(8k\phi \right)}^{1/2}}$$

(5)

where $\dot{\gamma }$ is the shear rate (s⁻¹), (3n + 1)/4n is a nonNewtonian correction factor for power-law fluids. Q is flow rate (cm³/s). A is the cross-sectional area of the pack (cm²). k is the permeability (cm²), and φ is the porosity. For the fluids used in this article, n values changed between 0.45 and 0.56.

3. Results and Discussion

3.1. Thickening Capacity

The steady shear viscosity of the polymer solutions at the shear rate of 10 s⁻¹ as a function of concentration is plotted in Figure 3. It is apparent that the biopolymer shows a greater thickening capacity than HPAM over the entire concentration range of 0.04⁻¹ wt%, and the viscosity data were well fitted by a power-law model with a quiet high determination coefficient (R₂ = 0.99). In the case of HPAM, the viscosity data were subjected to two regions, which were respectively described by the power-law model. Based on the established relationship between shear viscosity and concentration, two polymer solutions (biopolymer, 0.12 wt%; HPAM, 0.52 wt%) having identical viscosity at 10 s⁻¹ were prepared for the following evaluations.

The rheological properties of the polymer systems were firstly investigated through the variations of viscosity and shear stress under the steady shear as present in Figure 4, in which the shear viscosity data were fitted to the power-law model, and shear stress data were fitted to the Herschel–Bulkley model as given in Equations (6) and (7) [29,30].

Table 3. Fitting parameters for the biopolymer and HPAM.

Polymer	Shear Viscosity			Shear Stress
	k	n	R2	${\mathit{\tau }}_{\mathit{o} }$	m	p	R2
HPAM	0.13	0.70	0.96	0.015	8.51	0.63	0.99
Biopolymer	0.42	0.31	0.99	0.104	4.02	0.31	0.96

lists the parameters obtained from the above two models:

$$\mu =K{\dot{\gamma }}^{n-1}$$

(6)

$$\tau ={\tau }_{o}m{\dot{\gamma }}^p$$

(7)

where µ is the shear viscosity (cp). K is the consistency index. $\dot{\gamma }$ is the shear rate (s⁻¹). n is the flow behavior index. $\tau$ is the shear stress. ${\tau }_{o }$ is the yield stress. m is the consistency coefficient, and p is the flow behavior index.

As shown in Figure 4, both of the biopolymer and HPAM solutions are typical pseudoplastic fluids indicated by the flow behavior indices (n < 1). Compared to HPAM, this biopolymer seems more responsive to the applied shear rate corresponding to a pronounced shear-thinning behavior. The shear-thinning behavior is one of the critical properties for polymer EOR, which can mitigate flow resistance when polymers are rapidly pumped into the reservoir through injection well (high shear rate), and regains the thickening capacity when permeating in porous media (low shear rate).

The rheological observation can generally be attributed to the conformational state of the polymer system. This biopolymer has a linear and tertiary (triple helix) structure, which results in an improved thickening ability relative to HPAM [31]. This structured and gel-like behavior also renders it prominent viscoelasticity in a low frequency region as indicated in Figure 5, which benefits the promotion of the displacement efficiency in polymer flooding processes. However, when this polymer system is subjected to a high shear flow, the formed associations are destroyed, resulting in a remarkable decrease in solution viscosity.

3.2. Salt Tolerance

To examine the effect of salt on the rheological properties of polymer solutions, the shear viscosity was measured as a function of shear rate for different salt content at 25 °C as shown in Figure 6 and Figure 7.

Table 4. Effect of salt content on the shear viscosity of polymer systems.

Polymer	0 wt% NaCl			2 wt% NaCl			4 wt% NaCl			6 wt% NaCl
	k	n	R2	k	n	R2	k	n	R2	k	n	R2
HPAM	1.58	0.38	0.99	0.13	0.70	0.96	0.12	0.72	0.93	0.13	0.71	0.93
Biopolymer	0.41	0.30	0.99	0.42	0.31	0.99	0.44	0.30	0.99	0.46	0.29	0.99

presents the salt effect on each parameter of the power-law model.

As anticipated, the addition of an electrolyte to the HPAM solution considerably reduced the viscosifying power (Figure 6a). This result has been reported elsewhere [32]. It was due to the screening of negatively charged carboxyl groups leading to a reduction in the electrostatic repulsion within polymer chains. Further addition of NaCl from 2 wt% to 6 wt% slightly influences the HPAM solution viscosity because of the relatively low content of charged groups (5% hydrolysis degree). On the contrary, the presence of an electrolyte has almost no effect on the rheology of the biopolymer solution as shown in Figure 6b even at an extremely high salt concentration (Figure 7). The excellent antisalt property can be ascribed to one of the natures of this biopolymer, i.e., noncharged.

3.3. Temperature Resistance

The temperature effect on the rheology of the polymer systems was investigated as shown in Figure 8 and Figure 9. It is clear that HPAM shows a strong dependence on temperature, and the viscosity values experienced a progressive loss as a function of temperature within the whole shear rate range as indicated in

Table 5. Effect of temperature on the shear viscosity of polymer systems.

Polymer	25 °C			40 °C			55 °C			70 °C
	k	n	R2	k	n	R2	k	n	R2	k	n	R2
HPAM	0.13	0.70	0.96	0.12	0.68	0.97	0.10	0.69	0.98	0.09	0.68	0.99
Biopolymer	0.42	0.31	0.99	0.38	0.32	0.99	0.33	0.33	0.99	0.27	0.36	0.99

resulting from the rapid thermal motion of the molecules [33]. Nevertheless, this biopolymer behaves insensitively to temperature as suggested by the constant viscosity with exception of a slight decrease in the Newtonian region (Figure 7). The viscosity profile of HPAM and the biopolymer against temperature was also plotted in Figure 9. The slope of the tread line provides the rate of thickening power loss, (Δµ/ΔT), HPAM = 0.47 mPa∙s/°C > (Δµ/ΔT), and biopolymer = 0.10 mPa∙s/°C.

According to [34], this biopolymer possesses a transition temperature (around 135 °C), above which the conformational status is transformed from triple helices to single chains leading to a severely viscosity loss. Conversely, no significant viscosity change occurred below this transition temperature owing to the rigidity of the polymer chains [19].

3.4. Mechanical Stability

Figure 10 compares the apparent viscosity and mechanical stability of two polymers during flow through a capillary geometry. As Figure 10a shows, HPAM produces greater apparent viscosity than this biopolymer, and the magnitude of the viscosity gradually decreases with the shear rate, while for the biopolymer, this magnitude nearly holds constant independent of the shear rate. In terms of mechanical stability as present in Figure 10b, the degradation degree of HPAM increases steeply with the shear rate and reaches up to 62% at 34,900 s⁻¹. However, for the biopolymer, the percentage of mechanical degradation shows only a slight increase with the applied shear rate. Usually, production operators can accept a viscosity loss of 10–20% [35], and on the basis of this screen criterion, this polymer seems more appropriate than HPAM for EOR use.

In polymer flooding, mechanical degradation is induced by the flow of a polymer solution through pumps, flow lines, chokes, and valves and rock formation at the sand face. These flow restrictions may cause very high shear stress along polymer chains, which would result in the disentanglement of polymer systems and could even be accompanied by a chain scission as depicted in Figure 2 [36]. This might account for the decrease in apparent viscosity as well as the increase in the degradation extent as a function of the shear rate for HPAM. For this biopolymer, the stiffness of the polymer chains is capable of withstanding the yielded shear stress when it passes through such geometries and accordingly experienced much less viscosity loss compared to HPAM as displayed in Figure 11.

3.5. Flow Characteristics in Porous Media

Core flood tests were performed at laboratory scale to simulate real reservoir conditions. Figure 12 shows the developed pressure drop between the injector and producer of the core plug with the volume of injected fluids, which follow the sequence of brine, polymer solution, and resumed brine.

The pressure drop of HPAM and the biopolymer exhibit a very similar trend meaning that the magnitudes were constant during the brine injection followed by a steady increase in polymer flooding until an equilibrium stage was achieved, and after the high amount of resumed brine injection, the pressure drop ultimately leveled off at different extents. In the interest of further studying the flow behaviors of the polymers in cores, two factors were employed as shown in Equations (8) and (9) [37,38].

Here, the resistance factor (RF) is a measure of the mobility control capacity or effective viscosity (=RF multiplies the viscosity of brine) of the polymer solution during traveling in porous media, while the residual resistance factor (RRF) is an indication of polymer retention or the reduction of formation permeability:

$$RF=\frac{\mathsf{\Delta }P\left(\mathrm{Polymer}\mathrm{injection}\right)}{\mathsf{\Delta }P\left(\mathrm{Brine}\mathrm{before}\mathrm{polymer}\mathrm{injection}\right)}$$

(8)

$$RRF=\frac{\mathsf{\Delta }P\left(\mathrm{Brine}\mathrm{after}\mathrm{polymer}\mathrm{injection}\right)}{\mathsf{\Delta }P\left(\mathrm{Brine}\mathrm{before}\mathrm{polymer}\mathrm{injection}\right)}$$

(9)

As

Table 6. Summary of core flood test.

Polymer	RF e	Effective Viscosity (mPa∙s)	RRF e	r (μm)	e (μm)
HPAM	56.3	56.3	38.2	2.07	1.24
Biopolymer	46.3	46.3	41.0	1.98	1.20

^e represents the equilibrium stage of pressure drop.

displays, HPAM produced a RF of 56.3, which is synonymous with an effective viscosity of 56.3 mPa∙s during propagating in the porous media, and the biopolymer produced a lower magnitude of effective viscosity (46.3 mPa∙s). This result is probably due to the high effective shear rate (≈60 s⁻¹) exerting on the polymers when they flow through the core plug, which was estimated using the expression proposed by Christopher and Middleman [39]. Moreover, surprisingly this nonionic biopolymer gives a greater magnitude of RRF than the negatively charged HPAM demonstrating its higher retention within the porous media. It is known that polymer retention is mainly induced by adsorption and mechanical entrapment [40,41], in which the mechanical entrapment often occurs when larger polymer molecules become lodged in a narrow channel. This might be the primary reason why stiff biopolymers are significantly detained in such a tight formation. Therefore, it is generally recommended for highly permeable reservoirs, particularly those experiencing hard conditions, such as elevated temperatures and salinity.

Moreover, to characterize the adsorption behavior of the biopolymer in porous media, the parameter of average thickness of the hydrodynamic polymer layer (e) was introduced, which can be determined by the following expression (Equation (10)) [42,43]:

$$e=r·\left(1-RR{F}^{-\frac{1}{4}}\right)$$

(10)

where e is the average hydrodynamic polymer layer thickness (µm). r is the average pore radius (µm) for the brine flow which can be calculated using Equation (4 [27], and RRF is the residual resistant factor at the steady stage.

Table 6 lists the thickness of the adsorbed polymer layer on the surface of porous media. The hydrodynamic layer thickness of HPAM is 1.24 μm, which is slightly higher than that of the biopolymer. This result is consistent with the above conclusion, i.e., the mechanical entrapment is dominant in the biopolymer retention relative to HPAM.

4. Conclusions

This work systematically examined the physical properties of a biopolymer for potential use in polymer EOR processes. This biopolymer can function as a more efficient thickener than the partially hydrolyzed polyacrylamide (HPAM). In addition, it also exhibjits advanced tolerance to elevated salinity and temperature in comparison to HPAM. The presence of an electrolyte has almost no effect on the rheology of the biopolymer solution even at an extremely high salt concentration (20 wt% NaCl). The mechanical stability test demonstrates that the proposed biopolymer can withstand an extremely high mechanical shear and thus shows a relatively small decrease in thickening capacity after pass through the capillary geometry. However, it seems that this biopolymer is not very appropriate for low-permeability formations due to the significant entrapment. In other words, this biopolymer is a promising flooding agent for highly permeable oil reservoirs, especially for those that have discarded polymer flooding techniques due to their harsh conditions.