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Stabilization of Foam Lamella using Novel Surface-grafted Nano-cellulose based Nano-fluids Bing Wei, Hao Li, Qinzhi Li, Yangbing Wen, Lin Sun, Peng Wei, Wan-Fen Pu, and Yibo Li Langmuir, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 14, 2017
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Stabilization of Foam Lamella using Novel Surface-grafted Nano-cellulose based Nano-fluids Bing Wei,a,* Hao Li,a Qinzhi Li,a Yangbing Wen,b Lin Suna, Peng Wei,a Wanfen Pu,a Yibo Lia, c a) State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, China b) Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science & Technology, Tianjin, 300457, China c) Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science and Technology, Rolla, 65401, USA
ABSTRACT: To solve the potential risk of present oilfield chemistries to subterranean environment, our group contributes to the development of "Green" petroleum production processes. This proof of concept research studied the well defined nano-cellulose based nano-fluids, i.e. original (NC), AMPS grafted (NC-KY), and AMPS and hydrophobic chains grafted (NC-KYSS), in stabilizing foam lamella for potential use in Enhanced Oil Recovery (EOR). The data showed that the collaboration of the surface-functional nano-cellulose considerately improved the foam stability particularly in the presence of hydrocarbons due to the thickened foam film coupled with the molecular interactions at interior lamella. Since the grafted AMPS and alkyl chains, NC-KYSS noticeably enhanced foam quality compared against NC and NC-KY. With the increase in gas pressure, the lamella stabilizing effect of NC-KYSS became increasingly significant. The co-flowing behaviors of foam with oleic phase in porous media were examined in a five-spot visualization micro model (15 cm×15 cm×1 cm) and identified using a digital analysis method. The defoaming/destabilizing effect of hydrocarbons was fairly notable in porous media causing the foam to finger through the formed "oil bank". However, a tough displacement front was constructed when the surfactant synergized with NC-KYSS due to the stabilized foam lamella and 12% of incremental oil recovery was produced.
INTRODUCTION
It has been well-known that for most of the oilfields in the world, about 65% of hydrocarbons still remain unproduced even after natural drive and water flooding are exhausted.1, 2 These remaining resources are therefore the primary target for enhanced oil recovery (EOR) processes. After decades of research efforts and pilot tests, numerous EOR methods have been proposed such as gas injection, chemical flooding, and steam injection, etc.3-6 Although gas-based EOR methods
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show great potential in stimulating oil production, their drawbacks such as viscous fingering and gravity segregation are also very significant, which accordingly detract their EOR performance during displacing the oil in place.7 Given this issue, utilizing foam was proposed and proved to be able to offset the aforementioned shortcomings of the conventional gas injection.8, 9 According to Hirasaki, foam is defined as a dispersive system, in which liquid is the continuous phase with gas phase trapped in the thin liquid films named lamellae.10 Due to the presence of bubbles, the viscosity of the displacing phase can be noticeably increased, and subsequently divert gas to sweep low permeability region.11 In addition, the surface activity of foaming agents (surfactant) is capable of reducing the capillary forces and promoting the micro scale displacement efficiency.12 As a result of the synergetic mechanisms, foam flooding plays an increasingly important role in oil production stimulation compared to other methods. The flow behaviors of foam flooding are highly governed by foam stability. However, due to the complex reservoir conditions (pressure, temperature, geological properties, crude, etc.), the stability of foam during travelling in porous media is still a challenge particularly in the presence of hydrocarbons, which are considered as anti foaming agents and detrimental to foam stability.13, 14
Therefore, in the past decades, substantial efforts have been made addressing the foam stability
enhancement from both theoretical and experimental levels.15 The strategy that utilizes chemicals to prevent film drainage has been technically proved in the pioneering works. The chemicals are generally categorized into two groups, i.e. polymer and nano-particle.14, 16-18 Sydansk studied the physical properties of a polymer-enhanced foam (PEF) system and observed that adding polymer to the aqueous phase resulted in the improvement of the foam viscosity and stability.19 Shen et al. investigated the stability of HPAM- and xanthan gum-enhanced foam system during migrating in porous media.15 More recently, an associative polymer was utilized to stabilize foam lamella through the molecular interactions with surfactants.20 Using starch particle to improve the physical properties of foam was proposed by Zhang et al. They observed that the film drainage was considerably mitigated owing to starch participation, and the resultant foam strength and viscosity were accordingly promoted.21 As for nano-particle, silica is the most widely used nano-particle according to reports.22,
23
Farhadi et al. examined the stability and mobility control of a
silica-stabilized CO2 foam system. It was proved that this system produced uniform and small
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foams with high apparent viscosity compared to the surfactant-only system.24 Singh and Mohanty studied the synergy between nano-silica and surfactant, and fairly similar results were obtained as Farhadi et al.25 Hydrophobic silica was also adopted to stabilize a conventional foam system and the resultant foam system was more rough than that of PEG-coated nano-silica stabilized foam.26 In addition to silica, the possibility of aluminum, fly-ash and CaCO3 as foam stabilizers, were also assessed.27-29 Based on the previous works, three possible mechanisms for foam stabilization were elucidated by Horozov, which are monolayer of bridging particles, bilayer of close-packed particles, and network of particle aggregates.30 Nevertheless, in this work, surface-grafted nano-cellulose based nano-fluids, which are sourced from an eco-friendly biomaterial, cellulose, were proposed to stabilize the foam lamella as well as raise the environmental compatibility of oilfield chemistries. Cellulose is considered one of the most abundant natural polymers on the earth.31-34 After cellulose is nano-sized, many distinctive chemical and physical properties are subsequently created as high strength, surface accessibility, and stability.35, 36 Due to the eco-friendly features, nano-cellulose is believed to be a promising injectant for upstream petroleum industry as an alternative to HPAM and other synthetic polymers/gels. In the previous works, we have evaluated the physical properties and potential of this type of nano-material in enhanced oil recovery (EOR).37 Following up this avenue, to further tune its properties towards EOR, surface grafting by AMPS (2-acrylamido-2-methylpropane sulfonic acid monomer) and hydrophobic groups (alkyl chain) was conducted on NC backbone38 and this proof of concept research assessed the performance of these well defined nano-fluids in stabilizing foam lamella. To accomplish this task, the foamability and foam stability in the presence and absence of hydrocarbons were first examined, through which the optimal foaming systems were formulated. Then, the microscopic characteristics of foam with and without nano-fluids presence were compared in order to find out the underlying mechanism. The pressure influence on foam stability was also investigated using a high-pressure transparent windowed cell. Water flooding and subsequent foam flooding as an EOR mode were mimicked using a five-spot visualization micro model, from which high resolution images were captured and analyzed using a programmed MatLab to distinguish the water/oil distribution. The co-flowing behaviors of foam and oil in porous media were also identified. Eventually, the prospect and current challenge of this work were stated based on our accumulated experiences, which might be of significance to the
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relevant researches.
EXPERIMENTAL SECTION
Materials Nonionic surfactant alkyl polyglycoside (APG, 98%) was purchased from Chengdu Kelong Chemicals Inc., China. Crude oil sample was kindly provided by Xinjiang Oilfield Co. with a viscosity of 9.2 mPa·s at 25oC. Three nano-cellulose samples, the control (NC), NC-KY (AMPS grafted, 0.525 mmol/g NC), and NC-KYSS (AMPS, 0.525 mmol AMPS/g NC, hydrophobic chains grafted, 0.094 mmol/g NC), were supplied by Tianjin Woodelf Biotechnology Co., Ltd. (Tianjin, China).38,
39
The basic chemical composition and microstructure of the samples are
shown in Fig. 1. 0.5 wt% of NaCl brine was used throughout this work.
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Figure 1. Structure of NC, NC-KY and NC-KYSS38 Foamability and foam stability The foamability of the formulated chemical systems and also foam stability in the presence and absence of hydrocarbons were initially examined at ambient pressure and temperature using Waring blender method. 100 ml of foaming agents were mechanically agitated at 6000 rpm for 1 min and then transferred to graduated cylinders. Afterwards, the foamability, drainage half-time and foam volume decay were quantified upon storage. In addition, the foam quality under the effect of pressure was also evaluated in a high-pressure transparent chamber as illustrated in Fig. 2 following the procedures as: 30 ml of foaming solution was loaded into the chamber followed by air injection to a pre-specified pressure. Once the system reached the equilibrium, magnetic bar stirring was conducted at 2000 rpm for 1 min to prepare foam. The foamability, drainage half-time and foam height decay were subsequently recorded through the transparent window.
Figure 2. Experimental set-up of the high pressure transparent windowed apparatus Microscopy studies of the foam The microscopic features of the prepared foams were analyzed at ambient pressure and
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temperature using a Nikon 501 microscope (Japan). A small volume of the foam was placed onto a glass slide for observation, and then a series of magnified images were collected. Interfacial surface tension (IFT) measurements The IFTs of oleic phase-brine, oleic phase-air and brine-air systems were measured at 25oC utilizing a SVT 20N Spinning Drop Tensiometer (Dataphysics Instruments GmbH, Germany) with high accuracy (±0.01mN/m) ranging from 0.001-2000mN/m (Fig. 3). Each measurement was conducted in triplicate to ensure repeatability and consistency, and the mean value was obtained.
Figure 3. Schematic diagram of IFT measurement Flowing behaviors in porous media To mimic oil recovery process, a five-spot visualization micro model was designed and assembled as illustrated in Fig. 4. The micro model was composed of a stainless steel tank (15 cm×15 cm× 1cm) and a transparent glass cap. The porous media was packed with homogeneous 80-mesh glass beads. The flow rate of air was controlled by an F-112AI digital mass flow meter (Bronkhorst Ltd., Netherlands). The downstream pressure of the model was maintained at 0.5MPa by a back-pressure regulator. The model was first saturated with brine to measure the pore volume (PV). Then, the crude oil was flooded to displace the brine in place to establish the connate water saturation (Swi) and initial oil saturation (Soi). The brine flooding was later conducted at 1 ml/min until no additional oil was produced. As an EOR mode, foaming solution and air were co-injected into the model at a constant flow rate of 1.0 ml/min (1:1) to simulate foam flooding (1 PV) followed by resumed brine injection. During this test, oil recovery factor, pressure drops and displacing behaviors were recorded along fluid injection. In addition, the visual images that were captured to illustrate the co-flowing behaviors of foam and oleic phase were analyzed using a coded MatLab program.
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Figure 4. Schematic diagram of foam flooding experiments
RESULTS AND DISCUSSION
Foamability and foam stability To optimize the foaming formulations, a series of static experiments with different compositions were conducted as shown in Fig. 5. Initially, the optimal concentration of the foaming agent (APG) was determined, which corresponded to the critical concentration (0.2 wt%) on the curves presented in Fig. 5a. It was because the increasing rate of the foamability and drainage half-time after 0.2 wt% varied slightly with the surfactant concentration further increasing. Moreover, 0.2 wt% of surfactant is thought to be the economic acceptable concentration for scale utilization in oilfields. Thereafter, three nano-cellulose based nano-fluids were mixed with 0.2 wt% of APG, respectively. Their foaming behaviors were examined as shown from Fig. 5b to Fig. 5d. Clearly, as a result of the nano-cellulose, these two factors that are closely related to the foam quality have been enhanced. From the point of the optimizations coupled with the comparison purpose, the concentration of 0.1 wt% nano-fluid was used to formulate the foaming systems. Figure 6 graphs the foam indexes of the optimized foaming systems. The results indicated that after nano-fluids addition the parameter of drainage half-time was significantly improved especially for NC-KY and NC-KYSS. In contrast, the presence of nano-fluids was detrimental to the foamability of the foaming agents, as proved by the reduced foam volume under the same conditions.
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Figure 7. Morphology of the foam systems: nano-fluid (0.1 wt%) and surfactant (0.2 wt%) (Bottom: Zoomed images at 30min) 1000
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Figure 8. Volume decay profiles of four foaming systems The stability of the prepared foam was further examined by plotting the volume decay with storage time. Figure 7 shows the microscopy images of the four foaming systems. The images clearly show that the nano-fluid stabilized foams remain uniform bubble size after 30 min; on the contrary, the surfactant-only foaming system (the leftmost cylinder) becomes coarser due to gas diffusion and/or coalescence as observed in the close-up images. The superior stability of the nano-fluid stabilized foam was also confirmed by the relative volume in the cylinder, and the leftmost foam (surfactant-only) was found to nearly vanish after 5 hours. The foam decay profiles of the four systems in the absence of hydrocarbons are shown in Fig. 8. The profiles can be generally divided into three regions. As a result of the confined film drainage, a noticeably long duration of the first plateau occurs for the nano-fluid stabilized three forming systems, after that
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the foam volume rapidly declines with time until the foam completely ruptures. The hypothesized mechanisms of foam lamella stabilization are presented in Fig. 9. Due to the synergism of the nano-cellulose molecules, the close-packed foam lamella is further strengthened, which consequently renders the generated bubbles less probability to rupture.40 Moreover, the substantial OH- groups on the nano-cellulose surface are also capable of impeding liquid drainage owing to the hydrogen bonding interactions between water, nano-cellulose, and surfactant molecules in the film lamella. The close-packed foam lamella leads the foam volume to shrink due to the increased liquid viscosity and density, whereas the molecular interactions at the foam lamella can stabilize the foam and accordingly extend the drainage half-time. The proposed mechanism well interpreted the results shown in Fig. 6.
Figure 9. Hypothesized mechanism of the stabilized lamella Sensitivity studies of the foam to hydrocarbons In this section, the foam stability in the presence of hydrocarbons was investigated at the bulk scale to assess the oil sensitivity. This parameter is considered one of most critical screening
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criteria to chemical agents since foam is demanded to permeate through oil-bearing zones to displace the oil in place. A series of static experiments were conducted to investigate the foaming capacity of the chemical agents with hydrocarbon presence. Figure 10 shows two parameters of the four foaming systems with 1vol % of hydrocarbons. As for the first three systems, the presence of hydrocarbons imposed a significant detriment on both of foamability and foam stability. In contrast, when NC-KYSS was used, the resultant foam was fairly oil-tolerant, and the comparable physical properties were obtained as shown in Fig. 6, suggesting the insensitivity of this system to hydrocarbons. This result is ascribed to the surface activity of NC-KYSS as result of the surface-grafted AMPS as well as the hydrophobic chains pending on NC backbone as illustrated in Fig. 1. Nevertheless, the addition of NC and NC-KY exerts a severely negative influence on foam quality with hydrocarbon presence, probably caused by the alignment of the surfactant molecules at the surface under the obstruction of NC and NC-KY induced by the created repulsive effect between hydrocarbon and hydrophilic nano-cellulose. Figure 11 plots the foam decay profiles as a function of storage time in the presence of hydrocarbons. The curves conclusively evidence the detrimental effect of hydrocarbons in destabilizing foam bubbles. As indicated in Fig. 11, the nano-fluid stabilized foam possesses a prominent oil-resistance particularly NC-KYSS, which creates the widest plateau compared against other three systems. This hydrocarbon effect can be also understood in Fig. 12, which presents the dynamic behaviors of the foam macro-morphology with time. The images clearly indicate that the defoaming effect of hydrocarbons is extremely pronounced for the surfactant-only system. As reported by Farajzadeh et al., some briefly accepted mechanisms of the destabilizing effect include surfactant depletion due to the competitive adsorption at oil/water or gas/water interface, oil entering and spreading in film, and hydrophobic bridging between two films.41 As a consequence, the negative van der Waals (Πvw) which is one element of the disjoint pressure (Π=ΠEL+Πvw) becomes strong and subsequently leads the foam film to rupture. When the foam film is equipped with the nano-cellulose as the rightmost cylinder (NC-KYSS) shows, the defoaming effect of hydrocarbons is efficiently diminished. This effect is ascribed to the aforementioned thickened foam lamella as well as the hydrogen bonding associations. The synergistic effect likely provides obstacle to oil droplets at the interface, and also impedes the film lamella to drain as shows in Fig. 9.
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Figure 10. Foam volume and drainage half-time of the optimized systems in the presence of hydrocarbons
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Figure 12. Morphology of the foam systems with 1 vol% hydrocarbon presence: 0.2 wt% surfactant and 0.1 wt% nano-fluid (Bottom: zoomed images at 30min) Pressure influence The foam decay with and without hydrocarbons presence in the pressured visualization chamber versus storage time were respectively plotted and then fitted to delineate the influence of pressure in foam longevity. Based on the experimental results (foam stability, oil-resistance, etc.), NC-KYSS was finally selected for the following studies. The foam volume-pressure-time dependent behaviors of the NC-KYSS stabilized foam are presented in Fig. 13 (a-d). According to reports, the foam stability usually varies differently with pressure depending on the natures of gas, surfactant, minerals, and pH values.42, 43 As illustrated in Fig. 13, the foam stability is nearly proportional to the system pressure, i.e., the plateau (stabilizing term) was considerably widened with the increase in pressure (Fig. 13a). In addition, the longevity of the foam from preparation to complete rupture at 15 MPa is also noticeably longer than that at 1 MPa. The profiles in Fig. 13b reveal that when hydrocarbons presence the foam bubbles continuously rupture towards vanishing due to the deforming effect as confirmed in Figs. 11 and 12. Nevertheless, for the NC-KYSS stabilized foam (Figs. 13c and d), the data demonstrate that the synergistic effect of the nano-fluid renders the resultant foam superior stability and oil-resistance, which consequently generates a fairly wide plateau even in the presence of hydrocarbons. Furthermore, the stabilizing functionality of the nano-fluid is more notable with the pressure increasing. The interpretations to this result might be associated with the interfacial behavior, disjoint pressure, and foam/oil interactions, etc. However, the dominant mechanism is thought to be the increase of viscosity and density upon pressure as reported by Holt et al.42 a
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Figure 13. Foam volume decay profiles under pressure a: surfactant-only foam; b: surfactant-only foam with 1vol% of hydrocarbon; c: NC-KYSS stabilized foam and d: NC-KYSS stabilized foam with 1vol% of hydrocarbon Microscopy studies of the foam morphology The comparison of foam microscopic morphology was performed in this section to gain the deep insights regarding the alternation of foam lamella and bubble size after NC-KYSS stabilization. As indicated in Fig. 14, the participation of NC-KYSS made the resultant bubbles smaller in contrast to the surfactant-only foam (Fig. 14a). Furthermore, the foam lamella in Fig. 14 (c, d) was noticeably thicker due to the stabilizing effect, which subsequently enables the prepared foam to retard inter-bubble gas diffusion. In the presence of hydrocarbons as indicated in Fig. 15, the improvement of the foam lamella thickness was even more significant compared to Fig. 14. The microscopic observations were highly in agreement with the results obtained in the macroscopic scale tests (Fig. 12). The coarsening rate of the foam bubbles was accelerated when hydrocarbons were involved and consequently resulted in large bubbles as presented in Fig. 15a. Although the hydrocarbons have exerted a detriment to foam morphology, considerably thick foam lamella could be still constructed due to the collaboration of NC-KYSS (Fig. 14c-d) compared to the surfactant-only foam. The statistical analysis of the bubble size distributions of Figs. 14 and 15 shown in Fig. 16 also demonstrated that the NC-KYSS stabilized foam produced quite small and more uniform bubbles, meaning the thicker foam lamella compared to the surfactant-only foam.
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Figure 14. Microscopy images of the foam without hydrocarbons at the onset of experiments (a and b, surfactant-only foam at 40 and 100×; c and d, NC-KYSS stabilized foam at 40 and 100×)
Figure 15. Microscopy images of the foam with hydrocarbons at the onset of experiments (a and b surfactant-only foam at 40 and 100×; c and d NC-KYSS stabilized foam at 40 and 100×)
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Figure 16. Bubble size distributions of the surfactant-only foam and the NC-KYSS stabilized foam Interfacial behaviors Direct surface interactions between foam and oleic phase are thought to govern the destabilizing effect of hydrocarbons.41 To qualitatively analyze the destroying effect, three parameters are commonly adopted to describe the main mechanisms including the entering coefficient (E), spreading coefficient (S), and bridging coefficient (B), which are defined as follows:
= ܧσ୵/ + σ୵/୭ − σ୭/
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ܵ = σ୵/ − σ୵/୭ − σ୭/
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= ܤσଶ୵/ + σଶ୵/୭ − σଶ୭/
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where ߪ is the interfacial tension and the subscripts w, g and o represent water, gas and oil, respectively. Thermodynamically, when E is negative the oil droplet remains in water phase implying the stability of the foam with oil, whereas it would be favorable for oil droplet invading the gas/water interface if E >0. Similarly, oil spreading does not happen when S is negative. When B≥0 the film is unstable, while negative value of B corresponds to a stable foam file.41 In addition to the aforementioned three parameters, Schramm and Novosad proposed another simple parameter, lamella number (L), to assess the effect of hydrocarbons in foam lamella as follows:44 ౭/ౝ
= ܮ0.15
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౭/
On the basis of L value, foams are categorized into three types, i.e. L
