Effect of Degree of Branching on the Mechanism of Hyperbranched

Jul 6, 2016 - State Key Lab of Oil and Gas Reservoir Geology and Exploitation, ... As the degree of branching increased, the dominant mechanism of the...
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Effect of Degree of Branching on the Mechanism of Hyperbranched Polymer To Establish the Residual Resistance Factor in HighPermeability Porous Media Nanjun Lai,*,†,‡ Yan Zhang,† Fanhua Zeng,§ Tao Wu,† Ning Zhou,† Qian Xu,† and Zhongbin Ye†,‡ †

School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China State Key Lab of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China § Faculty of Engineering and Applied Science, University of Regina, SK S4S 0A2, Canada ‡

S Supporting Information *

ABSTRACT: To improve a polymer’s oil recovery power, it is imperative to discuss its residual resistance factor (RRF), which is a significant parameter in the field and is associated with the polymer molecular structure. This study investigated the capability of three kinds of hyperbranched polymers (HPDAs) with various degrees of branching to establish the RRF in high permeability porous media by way of a one-dimensional sandpack model under different polymer solution concentrations, permeabilities, and injection rates. In addition, the mechanisms of these polymers to establish the RRF were surveyed through altering the wettability of the rock surface. Furthermore, the diameter distribution and microstructure of the injected and produced polymer solution were determined by utilizing dynamic light scattering and scanning electron microscopy, respectively. The experimental results showed that the RRFs of three kinds of hyperbranched polymers were different in variation trend with changes in external conditions. As the degree of branching increased, the dominant mechanism of the polymer to establish retention and the RRF gradually shifted from surface adsorption to mechanical trapping. And the larger the proportion of the mechanical trapping effect was, the stronger the ability to build the RRF became. This was mainly because that the higher the degree of branching of the polymer is, the higher the branched chain number is, the larger the hydrodynamic radius of polymer solution becomes, the stronger the structure formed between end branches becomes, and the lesser the damage caused to the polymer by high permeability medium is. In polymer flooding studies, the apparent viscosity of the displacing phase has been promoted overtly via increasing the molecular weight9,14,15 and modifying the molecular structures of conventional polymers.16−18 With the increase of molecular weight, the thickening performance of the polymer solution has been improved significantly, but this method cannot solve the problem that the viscosity of the polymer solution would become poor in high-temperature and high-salinity reservoirs. Moreover, through adding functional groups into the linear molecular chain of polyacrylamide,17,19 the relevant performance of polymer solution, such as temperature resistance and salt resistance, can be improved to a certain extent. However, the unchanged linear molecular structure still causes a problem that the viscosity of the polymer solution is poorly maintained under the shearing action of the reservoir,20,21 and finally the polymer solution cannot establish a certain flow resistance by relying on its viscosity in high-permeability reservoir.22 Therefore, it is difficult to relieve the mobility ratio between the displacing and displaced phase through increasing the displacing phases viscosity. Fortunately, there is another method to assuage the fluid ability of the displacing phase that is to lower the reservoir permeability by retention of the polymer solution. In actual

1. INTRODUCTION It is well-known that there is an unfavorable mobility ratio between the displacing phase and the displaced phase during water flooding development, which causes a fast increase for the moisture content of produced fluid in high permeability porous medium.1 Therefore, the chemical flooding is an indispensable stage after water flooding, which means adding some chemical agents to injected water to alleviate the mobility ratio.2 Among these, surfactant flooding can indirectly increase the mobility of the displaced phase through a series of mechanisms such as interfacial tension reduction, wetting conversion, and emulsification.3,4 But it mainly work only when the viscosity of oil is less than 40 mPa·s, and the surfactant would easily be adsorbed on the rock surface during flooding.5−7 Given the difficulty of enhancing the mobility of the displaced phase, researchers give priority to the reduction of the displacing phase mobility. At present, polymer flooding is a relatively mature technology to control the displacing phase mobility after water flooding, and it has been industrially applied in most oilfields.8−12 λ = K/μ

(1)

Through analyzing the formula of mobility (eq 1),13 it could be found that the decrease of the flow ability of the displacing phase, namely lowering the λ value, can be accomplished by two approaches: (1) increasing the viscosity of the displacing phase and (2) dropping the reservoir permeability. © XXXX American Chemical Society

Received: April 8, 2016 Revised: July 6, 2016

A

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Figure 1. Structures of the three hyperbranched polymers (HPDAs).

applications, the residual resistance factor (RRF) is used to evaluate the capability of the polymer solution to lower permeability (eq 2), and it is one of the important parameters and technical indicators for the field construction design of polymer flooding.23,24 The research conducted by Hu et al.25 showed that, under the polymer injection conditions of the Daqing Oilfield, the recovery efficiency was improved by about 3% through polymer flooding when the RRF of the polymer solution rose from 2 to 3, which indicated a remarkable improvement for the oilfield. RRF = K wb/K wa

Furthermore, these polymers with different degrees of branching exhibited discrepant solution properties due to their different molecular structures, which determined the different abilities and mechanisms for them to establish RRF in porous media. Therefore, the aim of this study is to use a one-dimensional sandpack model to conduct research on the effect of external conditions on the capability of hyperbranched polymers with various degrees of branching to establish the RRF in highpermeability reservoir. Besides, the effect of the degree of branching on the mechanism of polymer to build the RRF was studied by altering the rock wettability. Finally, the dynamic retention, molecular structures, and molecular dimension of polymer solutions before and after flowing through the reservoir were also performed to explain the underlying reasons for the formation of those mechanisms.

(2)

During the seepage in the porous medium, the polymer solution would be partly detained in the reservoir due to a series of interactions with the porous medium,26,27 which would induce the establishment of a certain RRF and the reduction of reservoir permeability. The stronger the interaction between rock and the polymer solution became, the larger the retention of the polymer solution in porous media got, and the RRF value was higher.28 In general, the retention mechanism of the polymer solution in porous media can be divided into three aspects: (a) the mechanical trapping effect, (b) the surface adsorption effect, and (c) hydrodynamic retention.25 Many factors can influence the capability and mechanism of a polymer solution to establish the RRF in porous medium, including the relative molecular mass of the polymer, the concentration of the polymer solution, the permeability of the rock core, and the injection rate of the polymer solution.29,30 Considering the uncertainty of the above factors, it is increasingly urgent to improve the ability of polymer solution to establish RRF from the structural modification. Plenty of research showed that the branching structure could heighten the shear performance of a polymer solution and increase its adsorption site as well as the trapped probability by pore throat.31,32 In view of this, our research team proposed using a dendritic branched structure to improve the RRF of polymer solution in the early stage. A series of water-soluble hyperbranched polymers based on a dendritic structure were synthesized successfully and had excellent RRF values.33

2. MATERIALS AND METHODS 2.1. Hyperbranched Polymers with Different Degrees of Branching. Three kinds of water-soluble hyperbranched polymers (hereafter referred to as HPDAs) with various degrees of branching used in this study consisted of a dendrimer as the inner core and linear polyacrylamide chains as the outer flexible layer. Their structures are shown in Figure 1. The detailed synthesis, according to a method reported elsewhere by our team,33 is provided in the Supporting Information. The IR spectrum and the 1H NMR spectra of typical HPDAs are also shown in Figure S2. The weight-average molar mass Mw of HPDA-1, HPDA-2, and HPDA-3 are 5.02 × 106, 9.67× 106, and 1.89× 107 g/mol, respectively (Figure S3). 2.2. Measurement of Solution Properties for HPDAs. The apparent viscosities of HPDA solutions with different concentrations were tested by using the Brookfield DV-III Programmable Rheometer (Brookfield Co., America) at 60 °C and 7.34 s−1. Then we measured the apparent viscosities of samples at the same condition, which were being sheared by a blast-hole model33 at a 40.4 mL/min flow rate. To investigate the antishearing performance of a polymer solution, the viscosity retention rate was used as the target, which was B

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Energy & Fuels calculated by dividing the current viscosity by the initial viscosity. Throughout the experiment, the simulated injection water was used and its ion composition is shown in Table 1. Table 1. Ion Content of Simulated Salt Water ion

Na+

Ca2+

Mg2+

HCO3−

Cl−

SO42−

content (mg/L)

3130.2

99.9

97.7

200.0

5099.9

100.0

2.3. Wettability Changing of Quartz Sand. Quartz sand with a particle size of 60−80 mesh was washed using 18% hydrochloric acid and then washed using a large amount of pure water until the pH of the precipitated aqueous solution was 7. Then the sand was dried to form water-wet quartz sand. Afterward, a mass of methyl silicone oil was mixed with the water-wet quartz sand thoroughly. The mixed system was placed at 60 °C to age for a week, and then it was placed at 120−140 °C in order to the solidification of the silicone oil. After that, the groups at the quartz sand surface altered from a hydrophilic Si−O structure into a hydrophobic methyl group, causing an oil-wetting surface. 2.4. Sandpack Tube Displacement Experiment. In this segment, a one-dimensional sandpack model with a length of 25 cm and an inner diameter of 2.5 cm was provided with a pressure test point in the middle. The water- and oil-wet quartz sands were employed to simulate the porous medium. And the sandpack tube displacement experiments used to study the RRF of the hyperbranched polymers were carried out at 60 °C approximately as described in ref 34. The flow rate was 1 mL/ min when the polymer and subsequent water flooding occurred, respectively. Throughout the experiment, water flooding was performed to achieve a stable pressure, followed by the injected polymer solution until the concentration of produced fluid determined by the starch-cadmium iodide method35 was the same as or similar to that of polymer dope, then a follow-up water flooding was conducted until the polymer concentration of output fluid was close to zero. The standard curve for concentration detection is displayed in Figure S4. The RRF was calculated according to eq 2 by simplifying the Darcy formula,36,37 and the dynamic retention of the polymer solution was obtained via eq 4.38 The hydrodynamic radius (Rh) of both the injected fluid and produced fluid of the HPDA solution were tested by using a BI-200SM wide-angle scattering meter (Brookhaven) with a lamp-house wavelength of 532 nm and a scattering angle of 90°. The polymer solutions were filtered with a 0.8 μm pinhole filter membrane before use. In addition, the environment scanning electron microscope (FEI Quanta450) was employed to test the solution morphology. RRF =

ΔPwa ΔPwb

Figure 2. Apparent viscosity as a function of polymer concentration for HPDA solutions.

Figure 3. Effect of shear action on the apparent viscosity of HPDA solutions.

apparent viscosities displayed a rising trend with increasing concentration, which manifested an admirable thickening ability for the three kinds of hyperbranched polymers. Besides, the effect of the degree of branching on the apparent viscosity was discovered distinctly from Figure 2. It was always stable of viscosity increment for HPDA-1. But there was an evident breakpoint at 1750 mg/L for HPDA-2 and at 1250 mg/L for HPDA-3. After that, a sharp increase was observed for the apparent viscosity with the change in polymer concentration. As is shown in Figure 3, a same change trend of the apparent viscosity to concentration could be observed for sheared polymer solutions, and the viscosity value appeared to reduce in varying degrees. Whereas the hyperbranched structure endowed the HPDA solutions with high viscosity retention that signified an excellent shear resistance. Moreover, the HPDA-3 solution presented the greatest antishearing performance due to its plentiful branched chains and strong interaction between chains, which is conducive to establishing the remarkable RRF. 3.2. Capability of HPDAs To Establish RRF. With the changes of degree of branching, polymer concentration, permeability of the porous medium, and injection rate of polymer solution, the special behaviors of the hyperbranched polymer solutions to establish the RRF in porous medium were investigated. In this section, the displacement model was equipped with water-wet quartz sand. Since the filling of the model with quartz sand had a small error, a corresponding error

(3) n

R=

C0V0 − ∑i = 1 C iVi W

(4)

3. RESULTS AND DISCUSSION 3.1. Thickening Ability and Antishearing Performance of HPDA Solutions. The apparent viscosities of the HPDA solutions related to polymer concentration before and after being sheared by the blast-hole model were curved, as is shown in Figure 2 and Figure 3. For unsheared polymer solutions, the C

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Energy & Fuels was caused for permeability. As a result, the measurement error of the RRF was plus or minus 3 percentage points. 3.2.1. Effect of Polymer Concentration. When the flow rate of polymer-flooding was 1.0 mL/min and the permeability of the porous medium was about 2000 mD, the RRF of the polymer solution was basically built in direct proportion to its concentration (Figure 4). This could be attributed to the fact

Figure 5. Effect of permeability on the RRF of HPDA solutions.

under the conditions of polymer concentration for 1500 mg/L and the flow rate of polymer-flooding for 1.0 mL/min. It was found that the increase of permeability would gradually weaken the RRF value of three polymers, but the respective decline extent was different. The smaller the degree of branching was, the greater the decreased degree of the RRF became, and vice versa. When the permeability of the porous medium changed from 300 mD to 500 mD, the RRF constructed by HPDA-1 was reduced by 30.23% (from 21.6 to 15.07), which was the biggest among three polymers. As for HPDA-3, the decline trend gradually slowed down after the permeability of porous medium increased to 1500 mD. In light of the research and theory of Huang,40 the permeability of porous medium was correlated negatively with its surface area. Consequently, the porous medium with high permeability could provide less adsorption sites for the polymer molecule.41 Meanwhile, the increase of permeability would enlarge the rock holes and pore throats, so the trapped amount of polymer solution by the porous medium would also be reduced.42 For these two reasons, the retention of polymer solution in porous media was reduced, and the ability to construct the RRF was also gradually reduced with the increase of permeability. 3.2.3. Effect of Injection Rate. At this point, the permeability of the porous medium was approximately 2000 mD and the concentration of polymer solution was 1500 mg/L. From Figure 6, it was clear that the capability of the polymer solution to build flow resistance weakened monotonically with the injection rate of the polymer solution increasing. This was quite different from the result of relevant research43 conducted previously, which reported that the RRF of polymer solution in

Figure 4. Effect of polymer concentration on the RRF of HPDA solutions.

that the higher the polymer solution concentration rose, the more the molecular aggregations became, the stronger the mutual entanglement became, and the larger the aggregation size and apparent viscosity became, thereby improving its adsorption and trapped capacity by narrow pores that resulted in the decrease of reservoir permeability to a certain extent. In addition, after polymerization, the branched chains of dendritic skeletons were equipped with a large amount of hydrophilic functional groups, such as the acylamino and sulfonic groups. On this occasion, the hydrogen bonds could be formed between the molecular chains and the silica structures of the porous medium surface.30,39 With the increase of solution concentration, the number of functional groups also increased by different degrees, and the polymer solution could adsorb more on the surface of the medium. These two reasons jointly increased the polymer retention in porous media, and finally enhanced the RRF. Additionally, Figure 4 also showed an obvious difference in the rising trend of the RRF for HPDA solutions. The RRF established by HPDA-1 increased in a linear manner with the increase of polymer concentration. As for HPDA-2, when the concentration increased from 500 mg/L to 1000 mg/L, the RRF value was improved by 21.12% (from 4.07 to 5.16). However, as the polymer concentration being sequentially increased, the RRF was enhanced by a larger rate. Concerning HPDA-3, when the concentration increased from 500 mg/L to 1000 mg/L, the RRF rose by 38.33% (from 5.02 to 8.14), which was higher than that of HPDA-1 and HPDA-2. These phenomena could be explained from the molecular structure. As mentioned before, the polymer with high degree of branching possessed more branched chains and could form the larger framework, finally causing stronger trapped and adsorption capacity. Therefore, a high degree of branching could generate a faster rising trend of the RRF for hyperbranched polymer with the increase of solution concentration. 3.2.2. Effect of Permeability. Figure 5 exhibited the effect of porous medium permeability on the RRFs of HPDA solutions

Figure 6. Effect of injection rate on the RRF of HPDA solutions. D

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Energy & Fuels porous medium would improve first and then drop with increasing injection rate of the polymer solution. The discrepancy is derived from the fact that the injection rates adopted by this research were larger than those of previous researches. Thus, the shearing action applied on the polymer solution by pore throats was much stronger than the stretching effect44 and the molecular structure of the solution was largely destroyed, so the RRF of the polymer solution was gradually weakened with the increase of the injection rate. It also could be observed from Figure 6 that the three kinds of polymers were quite different in the weakening trend of the RRF value based on injection rate. With the increase of the degree of branching, the decline of the RRF was gradually slow. The injection rate changed from 0.25 to 1.5 mL/min, the descent rate of RRF for HPDAs were in order 90.2, 59.1, and 26.3%. This consequence may be supported by the antishearing performance. After increasing the injection rate, the shearing action of the porous medium applied to the polymer solution was extremely horrible, but only a small part of the branch structures could be damaged for polymer with a high degree of branching. And its overall structure could be unaffected obviously due to a great shearing resistance,31,33,34 which caused the molecular size of the polymer solution to not decrease significantly, and naturally the dynamic retention in porous medium did not decrease excessively. 3.3. Mechanisms of HPDAs To Establish the RRF. The HPDA solutions with various degrees of branching presented different abilities to establish a RRF in porous media, which is necessarily related to the establishment mechanism. Given that the contribution of hydrodynamic retention in the total retention volume can be ignored under the constant injection rate,45 this study made a quantitative inspection on the surface adsorption effect and mechanical trapping effect under 2000 mD permeability. The concentration of the polymer solution was 1500 mg/L, and the polymer injected rate was 1 mL/min. When the wettability of quartz sand ranged from water-wet to oil-wet, the surface adsorption between the quartz sand and the polymer solution would be eliminated absolutely, and consequently only the mechanical trapping of the polymer was retained.46 Then, the difference of the retention value of the polymer solution under these two kinds of media were employed to determine the contributions of surface adsorption in total retention volume, respectively. A concentration profile curve based on the output fluid concentration measured at different sampling points was drawn (see Figure 7). It was observed that the concentration profile curves of the HPDA solutions had identical variation trends both in water-wet and oil-wet porous media. The higher the degree of branching of the polymer was, the lower the relative concentration of the output fluid got. Meanwhile, the injection volumes (PVs) of HPDA-1, HPDA-2, and HPDA-3 in oil-wet medium showed a slight fall compared with those in water-wet medium when the dynamic retention reached a balance. In addition, the offset of HPDA-1 in PV was the largest, which suggested that the change in the surface wettability of the porous medium had the greatest impact on HPDA-1. In view of this, the retained polymer solutions in oil-wet and water-wet porous media calculated by the material balance method35 was shown in Figure 8. The retention volume of the three kinds of polymers appeared to decrease in different degrees after the surface adsorption effect was eliminated. To be specific, the dynamic retention of HPDA-1 dropped from 115.05 μg/g sand (in a water-wet medium) to 26.92 μg/g sand with a loss of

Figure 7. Concentration profile curve of the HPDAs solutions in water- and oil-wet porous media.

Figure 8. Dynamic retention of the HPDAs solutions in water- and oil-wet porous media.

76.71%. This indicated that the contribution of the surface adsorption of HPDA-1 in total retention is as high as 76.71% in water-wet medium. As for HPDA-2, the proportion of surface adsorption in total dynamic retention was only 41.75%. Further, the dynamic retention loss of HPDA-3 in oil-wet medium was as low as 17.40%, suggesting that surface adsorption made only a small contribution to total retention with the degree of branching of polymer increasing. Any change in dynamic retention would affect the RRF value immediately. From Figure 9, it could be noted that the RRF built by HPDA-1 in oil-wet porous medium was as low as 1.1, with a sharp decrease of 59.41% compared to that built in water-wet porous medium. Therefore, it could be obtained that the surface adsorption effect is the major mechanism to create the RRF for HPDA-1. In contrast, HPDA-2 and HPDA-3 were weakly dependent on the elimination of the surface adsorption effect, and their RRF values were 4.81 and 11.63, respectively (decreasing by 34.38% and 20.67% compared with the values in water-wet porous medium). This phenomenon indicated that the mechanical trapping, instead of the surface adsorption effect, gradually turned into the major mechanism to establish flow resistance as the degree of branching of polymer increased. This is the main reason why HPDA-3 greatly influenced by mechanical trapping can have a high retention in high permeability porous medium. The same observation could be obtained from the pressure drop curve (Figure 10). In waterE

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Figure 9. RRF of HPDA solutions in water- and oil-wet porous media. Figure 11. Diameter distributions for the HPDA solutions.

broad peak and Rm moved leftwards dramatically. Moreover, the small peak at 1−3.16 nm extended and became a broadtailed peak due to the destruction of most molecular aggregations by the shearing action of the porous medium. Productively, the Rh of the HPDA-1 solution dropped from 162.6 to 52 nm, with a massive loss of 67.97% (Figure 12).

Figure 10. Displacement pressure curve of the one-dimensional sandpack model.

wet porous medium, the balance pressure drop (ΔP) during polymer solution injection and the subsequent water flooding increased with the increase of the degree of branching of polymer, implying that the power to establish the RRF gradually improved. Simultaneously, the ΔP of the polymer solution with high degree of branching grew slowly, and the behavior of constructing resistance point by point was more obvious. After the surface adsorption effect of quartz sand was eliminated, the ΔP of polymer flooding and the subsequent water flooding in the oil-wet medium was much lower than that in the water-wet medium as a result of the dominant mechanical trapping. The accordant phenomenon was that the ΔP loss for HPDAs was increasingly small with the degree of branching increasing. The mechanisms of HPDAs to establish the RRF are associated with their molecular structure and dimension.47 Also, the performances of produced fluid can affect the flow behavior of the polymer solution in porous medium to a certain extent.48 Thus, the diameter distribution and morphology both of the injected and produced HPDA solutions were studied further, as discussed in Sections 3.4 and 3.5. 3.4. Diameter Distribution of HPDA Solutions. The molecular dimension of the 1500 mg/L injected HPDA-1 solution scaled a broad peak in the range of 3.16−3162.28 nm and a small peak in the range of 1−3.16 nm and 3162.28− 10000 nm. The particle size in the main peak (Rm) was 93.33 nm (Figure 11). As for the produced polymer solution, the

Figure 12. Mean diameter of injected and produced HPDA solutions.

Small-size molecular aggregations were tough to be captured by the porous medium under shear action, which confirmed that it was difficult for the HPDA-1 solution to establish dynamic retention by mechanical trapping. Compared with HPDA-1, the broad peak of the 1500 mg/L injected HPDA-2 solution moved rightwards and the Rm increased to 316.2 nm. After passing through the porous medium, the broad peak also moved leftwards as it was in the HPDA-1 solution. However, due to the high molecular structure strength and great shear resistance, the movement degree of diameter distribution was not significant in the radial direction. The Rm shifted from 316.2 to 177.8 nm only. Consequently, the Rh of the HPDA-2 solution decreased from 321.3 to 195.4 nm with a decrease rate of 39.19% (Figure 12), which was much lower than that of HPDA-1. Thus, the HPDA2 solution can establish dynamic retention in porous medium through mechanical trapping. Expectedly, the Rh increased continually with an increase of the degree of branching, and the highest value of 469 nm was assigned to the 1500 mg/L HPDA-3 solution. After seepage in F

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Figure 13. SEM morphologies of (a) injected HPDA-1 solution, (b) injected HPDA-2 solution, (c) injected HPDA-3 solution, (d) produced HPDA-1 solution, (e) produced HPDA-2 solution, and (f) produced HPDA-3 solution in brine.

molecular structures presented the features of compacting, tightness, and multilayer (Figure 13b). From Figure 13e, some similar phenomena with produced HPDA-1 could be observed that most frameworks of the produced HPDA-2 solution became thinner due to the stretching action49 of the porous medium, and the compact network structures slightly became loose. But the overall molecular structure of the solution was unaffected and the relatively complete spatial network structures could be found in the photograph. Therefore, it could establish effective retention and the RRF based on building firm spatial structures in the porous medium. Figure 13c showed that the injected HPDA-3 solution had compact multilayer spatial network structures, which were equipped with oval, evenly short and thick molecular frameworks. These frameworks heightened the overall strength of the structures and improved the shearing resistance of the branching chains. Because of this, the frameworks of the molecular aggregates were still short and thick after migrating in porous medium, and the network structures were mostly circular or oval (Figure 13f). This indicated that the shearing and stretching effects of the porous medium do not have a significant influence on the spatial network structures of the molecular aggregates for HPDA-3. Thus, the dense nodes of each network structure could be adsorbed on the rock surface and molecules would be captured by the porous medium. In consequence, the polymer solution built spatial network structures of certain strength in the porous medium, which could improve the retention.

porous medium, some branching chains of the polymer molecules were shorn, resulting in two feeble peaks at 1− 3.16 nm and 3.16−17.78 nm. But its Rm stayed the same, which was attributed to the excellent shearing resistance of the molecular structure. In this way, the Rh of the HPDA-3 solution was reduced to 370.2 nm, with a small drop rate of 21.06% (Figure 12). This result showed that the porous medium had little impact on the size of the aggregate molecules in the HPDA-3 solution. Hence, these aggregates could be captured easier by the narrow pore throats due to their great hydrodynamic radius. 3.5. Morphology of HPDA Solutions. As seen in Figure 13, the three kinds of polymer solutions of 1500 mg/L had different sizes of spatial network structures. This could be supported by the fact that the interaction among branching structures produces molecular aggregations and forms space net structures.31,33,34 However, the polymers with various degrees of branching varied in interaction intensity among branching chains, which resulted in differences between their network structures in size and shape. In terms of HPDA-1, the injected polymer solutions had a relatively complete but loose structure due to poor power among branching chains (Figure 13a). Besides, the frameworks used to connect the aggregations were small and thin wire-drawing, with the result that the spatial network structures were feeble. After passing through a 2000 mD high permeability medium, it could be seen that the network structure of the HPDA-1 solution was damaged mostly (see Figure 13d). Furthermore, the frameworks of the spatial structures became thinner and even broke since the molecular aggregates were stretched by the narrow pore throats. At this point, the HPDA-1 could only rely upon the small faulted molecular chains to adsorb on the rock surface by physical adsorption, but this adsorption type was startlingly weak. So its abilities to establish RRF and reduce reservoir permeability were low. As the degree of branching increased, the strength of the structures was greatly improved. For injected HPDA-2 solution, the network frameworks were shorter and thicker, and the clearance among the structures became smaller. Therefore, the

4. CONCLUSIONS In this study, the mechanisms of three kinds of hyperbranched polymers (noted HPDAs) with various degrees of branching to establish the RRF in high-permeability porous medium were studied from the perspective of molecular structure, solution properties, and dynamic retention. The main results are listed as follows: (1) Due to the relatively low thickening ability, antishearing performance, small diameter, and weak structural strength for aggregates, the HPDA-1 solution mainly G

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Energy & Fuels relied upon surface adsorption to construct dynamic retention and the RRF, so it was low. (2) As the degree of branching increased, the solution diameter and shearing resistance increased, and the solution could build strong network structures based on the interaction among tail chains. Hence, the major mechanism for the retention and the RRF of hyperbranched polymer gradually turned into mechanical trapping. (3) The HPDA-3 solution had the greatest contribution of mechanical trapping. Thus, its capability to generate retention and establish the RRF was the highest in highpermeability porous medium. The research findings suggested that, on the premise of meeting the injectivity, it was required to increase the hydrodynamic diameter and molecular structure strength of the polymer solution and promote the probability that polymer molecule aggregates were captured by porous medium, so as to improve the retention ability and the RRF of the polymer solution with the purpose of outstanding oil recovery.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00826. Contains the experimental details, information on the typical FTIR spectrum and 1HNMR spectrum, the determination of the molecular weight of the HPDAs, and the standard curve for the polymer concentration dectection (PDF)

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ΔPwb = Pressure drop in water injection, (MPa) ΔPwa = Pressure drop in subsequent water injection, (MPa) R = Retention of polymer solution in porous medium, (μg/g sand) C0 = Concentration of injected polymer solution, (mg/L) Ci = Concentration of i produced sample, (mg/L) V0 = Injection volume of polymer solution, (mL) Vi = Volume of i sample in produced polymer solution, (mL) n = Number of produced samples collected in model exit W = Dry weight of porous medium, (g)

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-13094484238; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (Grant No. 51404208), the China Scholarship Council, the second Youth Backbone Teachers Project of Southwest Petroleum University, the Open Fund (PLN1433) of the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), the National Undergraduate Training Programs for Innovation and Entrepreneurship (Grant No. 201510615008), the Scientific research project of Southwest Petroleum University (Grant No. 2014QHZ014), and the Undergraduate Extracurricular Open Experiment of Southwest Petroleum University (KSZ15070).

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NOMENCLATURE λ = Mobility K = Reservoir permeability, (mD) μ = Apparent viscosity, (mPa·s) RRF = Residual resistance factor Kwb = Water phase permeability before polymer injection, (mD) Kwa = Water phase permeability after polymer injection, (mD) H

DOI: 10.1021/acs.energyfuels.6b00826 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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DOI: 10.1021/acs.energyfuels.6b00826 Energy Fuels XXXX, XXX, XXX−XXX