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Fuel

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Solid-shelled microspheres loaded with solvent as diluents for extractingblockages by heavy-oil and asphaltene precipitates

Li Haoa,b, I-Cheng Chenb, Jun Kyun Ohb, Cengiz Yeginc, Nirup Nagabandib,Jyothsna Varsha Talarib, Luhong Zhanga, Mustafa Akbulutb,c,d,⁎, Bin Jianga,⁎

a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR ChinabArtie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, United Statesc Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843-3003, United Statesd Texas A&M Energy Institute, Texas A&M University, 3372 TAMU, College Station, TX 77843-3372, United States

A R T I C L E I N F O

Keywords:Solid-shelled microspheresDiluentsConfinement-induced releaseLow permeabilityEnhanced hydrocarbon recovery

A B S T R A C T

A novel type of solid-shelled microspheres was prepared by emulsion templating technique through layer-by-layer self assembly. The regulation controlled by different payloads, surfactants, 3-aminopropyltriethoxysilane(APTES) and tetraethoxysilane (TEOS) ratio produced three solid-shelled microspheres with various mor-phology, sizes, shell thicknesses and brittleness. Such microspheres can be considered as diluent-loaded eggs thatcan be cracked when forced between narrow rock surfaces or against blocking fluid flow particles. The structuralcharacteristics, size distribution, stability, breakable and release behavior of three payload-in-silica microsphereswere investigated by optical microscopy techniques and scanning electron microscopy (SEM) technique, dy-namic light scattering (DLS), zeta potential, and ultraviolet-visible (UV–vis) spectroscopy. The results indicatedthat these three microspheres were relatively uniform in size (1–5 μm) and had a smooth surface with shellthicknesses of 40–120 nm, which also possessed excellent stability. Yet the brittleness of silica shell was ob-viously different for three solid-shelled microspheres, in detail, the silica shell of toluene microspheres was morebrittle and easier to be ruptured than hexane microspheres under external hydrodynamics forces. Sand columntests revealed that the easy-broken toluene-in-silica microspheres prepared from tween-80 template matched thepore throat of reservoir better and achieved higher heavy oil recovery efficiency (94.60%). Such breakablemicrospheres can provide a selective and targeted diluting approach to extract blockages by heavy-oil andasphaltene precipitates.

1. Introduction

As easy-to-recover hydrocarbons are globally running out, devel-opment of approaches for the recovery of hydrocarbons trapped in lowand very-low permeability reservoirs is in great demand [1–6]. Typi-cally, crude oil is trapped in the narrow regions between the grains of asedimentary rock. During recovery, the oil needs to flow through in-terconnected pores with sizes that vary from the sub-micron to sub-millimeter range [1,7].

The current efforts to recover such hydrocarbons have faced severalcritical challenges associated with the low permeable nature of suchreservoirs. For instance, the loss of permeability and poor hydraulicconductivity usually lead to serious heterogeneity and low fluid pro-duction, and thus influence oil production [8]. Considerable efforts

have been made to develop effective treatment techniques for over-coming two challenges: (i) pore blockages by small sandstone rocks and(ii) low mobility of hydrocarbons and displacement fluids in narrowchannels due to the attractive intermolecular interactions between rocksurfaces and fluids. Besides, heavy crude oils typically have high visc-osity values, which poses challenges for their extraction and productiondue to the low mobility associated with high viscosity. The commonmethods for improving the mobility of such oils in reservoir settingseither rely on increasing the permeability of the reservoir or decreasingthe viscosity of heavy oil. The former is typically achieved using etch-ants such as HCl and HF while the latter can be accomplished byheating or dilution of heavy crude oil [9–11]. Among various treatmentmethods for heavy hydrocarbon resources, dilution with aromatic andorganic solvents such as benzene and toluene has traditionally attracted

https://doi.org/10.1016/j.fuel.2018.07.006Received 10 December 2017; Received in revised form 30 June 2018; Accepted 2 July 2018

⁎ Corresponding authors at: Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, United States (M.Akbulut).

E-mail addresses: makbulut@tamu.edu (M. Akbulut), binj@tju.edu.cn (B. Jiang).

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much attention because the free energy of mixing of these solvents withheavy crude oil is thermodynamically favorable for these solvents[12–17]. The mixture of shorter chain hydrocarbons such as ethane,propane, and butane, has also be utilized as diluents to ease heavy oil/natural bitumen flow [2]. The main concern with this approach is thehigh cost of solvents relative to heavy crude oil, the waste of largevolume of solvents to the porous media, and the reduction in oil qualitywith the addition of large amount of solvents. Thus, development ofdiluents with selective and targeted property is a technologically ad-vantageous concept to improve permeability and flow properties, andthen enhance recovery efficiency in heterogeneous reservoirs.

Core-shell microspheres are micron-sized objects, encapsulating anactive ingredient with an organic or inorganic shell which can controlthe release of core by changing its porosity, size and thickness [18–22].Encapsulation is used to protect the core materials from harsh thermal,chemical or mechanical degradation environments [23–25]. SiO2 isstable under harsh conditions, and hence functions as an ideal shellcomposite to protect the inner core. Core-shell microspheres possessmany promising properties, such as high surface area, thermal andchemical stability, high and adaptable dispersibility, active interfaces,and readily modifiable, which render them with the potential as aneasier diluent agent and mobilization for improving permeability andrecovering hydrocarbons trapped under challenging conditions[26–32]. Zhao et al. reported poly(octadecyl methacrylate)/3-metha-cryloxypropyl trimethoxysilane/silica multi-layer core-shell nano-composite with thermostable hydrophobicity and good viscosity breakproperty by surface polymerization, and the resulting material showedmarked ability as nano viscosity depressants for crude oil [33]. The useof core-shell microspheres in oil reservoirs during enhanced recovery isnot common likely due to the lack of appropriate particle size for lowpermeability reservoirs and harsh reservoir conditions with high sali-nity and temperature.

In our previous work [4], we designed a breakable, solid-shelledmicroemulsion for reservoirs, which can be considered as diluent-loaded eggs. The aim is to confinement-induced release their payloadupon encountering solid particles in narrow channels and confiningpores blocking hydrocarbon transport recovery. Compared to the tra-ditional bulk diluents distributing throughout heterogeneous reservoirs,diluents-loaded silica-shell microspheres can be targeted and selectivelyreleased locally when they are forced into the narrow and confinedregions having comparable size with the dimensions of such particles.Therefore, as illustrated in Fig. S1, this approach has potential for re-ducing the loss of diluents/solvents. For bulk/free diluents, the largevolume of solvent not encountering the pore blockages is waste and canharm the quality of oil, whereas for microencapsulated diluents, solventnot reaching the pores is still isolated and can readily be separated andreused. To the best of our knowledge, this is a novel concept about theapplication of core-shell organic-inorganic microspheres with breakableshells in process of improving relative permeability of heavy oil. Al-though the proof-of-concept of toluene-in-silica microspheres has beendemonstrated in our past work, a systematic investigation of the re-lationship among various factors, including core materials, surfactants,shell materials, and their concentrations, is still lacking.

Herein, we reported a systematic study on the coating regulation ofpayload-in-silica microspheres with different sizes and shell thicknessesand brittleness. The roles and correlations of the factors, such as oilphases, surfactants, APTES, TEOS, were analyzed and discussed. Thiskind of breakable core-shell microspheres provides an alternative can-didate for heavy oil recovery in low permeability reservoirs.

2. Material and methods

2.1. Materials

Heavy oil (API of 18.8° and 1.97% S) was obtained from Ecopetrol(Colombia) and used as received. Tetrahydrofuran (THF, ≥99%),

toluene (≥99.5%, ACS grade), tetraethoxysilane (TEOS, ≥98%),polyoxyethylene (n=9) nonylphenylether (Igepal Co-630, averageMn=617) were purchased from Sigma-Aldrich (St. Louis, MO). Hexane(≥98.5%, ACS grade) was supplied by Alfa Aesar (Ward Hill, MA).Tween 80 was purchased from TCI America (Portland, OR). 3-Aminopropyltriethoxysilane (APTES, ≥99%) was obtained from GelestInc. (Morrisville, PA). All chemicals were used directly without furtherpurification. Quartz sand (pure, 99.8%, 40–100 mesh, Acros Organics,Geel, Belgium) was used for preparing sand columns as model porousmedia. Unless stated otherwise, all references to water shall be under-stood to mean Milli-Q water (resistivity of ∼18.2MΩ·cm−1).

2.2. Preparation of surfactant-stabilized emulsions

The oil-in-water emulsions were prepared using toluene or hexaneand Milli-Q water in the presence of one of the two non-ionic surfac-tants (Tween 80 or Igepal Co-630). In a typical run, a stock solution ofthe surfactant was prepared by dissolving 0.2 mL Tween 80 or IgepalCo-630 in 44.8 mL Milli-Q water (for a concentration of 0.4% v/vsurfactant). Then, 5mL (10% v/v) toluene or hexane as the oil phasewas added to the continuous phase and the system was emulsified byvigorous stirring for 2 h. Then, the system was homogenized via ul-trasonication in the absence of vigorous stirring for 30min. Theseconditions led to a turbid and homogenous suspension, indicating theformation of emulsion droplets. We varied the concentration of Tween80 or Igepal Co-630 used to prepare the emulsions in the range from0.2% to 2% v/v. We also changed oil phase concentration from 5% to10% v/v.

2.3. Encapsulation of emulsion droplets with solid shells

Breakable microspheres were prepared by encapsulation of emul-sion droplets in silica-solid shells using APTES and TEOS as comonomervia an emulsion templating technique through layer-by-layer self as-sembly. The quaternization of the amine group which produced hy-droxide ions made APTES a self-catalyst for the hydrolysis and con-densation of silicate without the need of external catalyst [34]. As atypical synthesis, various amounts of APTES, ranging from 0.53mL to2.12mL were added into Milli-Q water, producing final APTES con-centrations in the range of 0.034–0.138M (volume ratio of APTES andTEOS ranging from 1:1 to 4:1). Then, the mixture was vigorously stirredto fully dissolve the reactants. Afterwards, the solution was slowlyadded into the surfactant-stabilized emulsion while stirring. Sequen-tially, 0.53mL TEOS (0.036M) was added into the freshly preparedemulsion system while stirring and reacted for 24 h at room tempera-ture to obtain toluene-in-silica or hexane-in-silica microspheres dis-persion.

2.4. Characterization of solid-shelled microspheres

The particle size distribution and zeta potential of solid-shelledmicrospheres in different conditions were measured by use of dynamiclight scattering (DLS) and laser Doppler electrophoresis (LDE) methods,respectively via Zetasizer Nano ZS 90 (Malvern Instruments, Ltd.,Westborough, MA). The DLS measurements were carried out at ascattering angle of 90° at 25 °C. If the particle size was larger than 3 μm,we utilized optical microscope images to calculate the particle size. Thesize and shape of different solid-shelled microspheres and microscopicvisualization were observed by use of an optical microscope (OlympusBX51, Japan). Surface morphology and shell thickness of different kindsof microspheres were characterized by a scanning electron microscope(SEM, JSM-7500F; JEOL, Tokyo, Japan). In SEM measurements, sam-ples were prepared by placing a droplet of microspheres dispersion ontoa clean silica wafer, and then evaporating water at ambient tempera-ture. The amount of payload in solid-shelled microspheres was quan-tified using the ultraviolet-visible (UV–Vis) spectroscopy (UV-1800,

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Shimadzu, Japan).

2.5. Heavy oil recovery experiments

Heavy oil recovery was evaluated using sand column experiments asdescribed in our previous work [4,35]. Using the gravimetric analysis,the porosity of the sand column was calculated to be 42.05 ± 0.5%,and the corresponding pore volume (PV) was 1.44 ± 0.02 cm3. After-wards, a pre-defined concentration (10% v/v) of solid-shelled micro-spheres dispersion in water was injected into the column using a pro-grammable syringe pump (Nexus-6000, Chemyx Inc, Stafford, TX) at aselected flow rate (3.30mL/min). Pressure gauges were installed atinlet and outlet of the column. Using this setup, the pressure drop wasmeasured to be in the range of 22–40 kPa, corresponding to a pressuregradient in the range of 340–620 kPa/m. Based on these values, wecalculated the permeability to be 1.71–3.11 D using Darcy's Law [36].According to the Carman-Kozeny equation [37], the corresponding porethroat diameter was calculated to be 35.8–47.9 μm. The supernatant ofthe recovered mixture, heavy oil phase, was collected and then dried at120 °C under vacuum for 24 h to evaporate residual toluene. The pre-sence of toluene was detected using olfactory analysis, which can re-solve concentrations as low as low as 10 ppm [38]. The fraction re-covery calculated by taking the mass ratio of the recovered heavy oil tothe initial heavy oil loading in the column. All experiments were carriedat least in triplicate.

3. Results and discussion

3.1. Structural characterization

In this paper, we developed three solid-shelled microspheres usingvarious payloads, surfactants, and APTES/TEOS ratio. The experi-mental conditions used for preparing these solid-shelled microspheresare listed in Table 1. We found that the morphology, size, shell thick-ness and brittleness of solid-shelled microspheres depended strongly onsuch these parameters.

Fig. 1 displayed morphology of hexane-in-silica and toluene-in-silicamicrospheres prepared from different conditions. From optical micro-scope images in Fig. 1(a), (d), (g), it can be seen that all of these threesolid-shelled microspheres were relatively uniform with smooth sur-faces and particle size was estimated to be 4.80 ± 0.21 μm,1.72 ± 0.20 μm, and 0.96 ± 0.12 μm, respectively. While the corre-sponding emulsion droplet sizes without silica layer were obtained tobe 1.10 ± 0.04 μm, 0.83 ± 0.09 μm, and 0.60 ± 0.18 μm from DLSmeasurements, respectively. It also indicated that both Tween 80 andIgepal Co-630 can act as a stabilizer to preserve a smooth surface forsolid-shelled microspheres. Different surfactants produced variousparticle sizes because of interfacial tension differences.

As observed from SEM images in Fig. 1(b, c), (e, f), (h, i), three solid-shelled microspheres possessed breakable silica shell structures underultra-vacuum system in SEM and then hexane or toluene encapsulatedin microspheres was released from the broken shell. However, thebreakable shell surface morphology was obviously different amongsuch three microspheres. Most of toluene-in-silica microspheres

stabilized by Tween 80 (marked as B) can be completely broken withonly a few microspheres forming small pores on shell surface. And thethickness of the silica shells was 95.22 ± 4.50 nm for microspheres B.Comparatively, toluene-in-silica microspheres stabilized by Igepal Co-630 (marked as C) have more easily breakable shells and solid-shelledmicrospheres C with shell thickness of 41.52 ± 4.40 nm can be totallybroken, indicating the high brittleness of the thin silica shell, which wasprobably because of a low condensation degree of TEOS with lessamount of APTES. It was also worth noting that most of hexane-in-silicamicrospheres stabilized by Tween 80 (marked as A) maintained relativegood spherical structures, and hexane solid-shelled microspheres A justwere cracked to form small holes on shell surfaces, which thickness ofsilica shell was estimated to be 116.36 ± 4.80 nm.

3.2. Colloidal stability of solid-shelled microspheres system

Three solid-shelled microspheres formed by silica layer were verystable against aggregation for an indefinite period of time. Such mi-crosphere suspensions that had been kept for 30 days or more showedno significant changes in particle size based on optical microscope andDLS measurements. The surfactants and surface amine groups of APTESprovided stabilizing effect to the microspheres through steric andcharge repulsion and increased in the interfacial elasticity resulting inenhanced colloidal stability [39,40].

In order to clarify the colloidal stability, zeta potential of hexane-in-water and toluene-in-water emulsion stabilized with Tween 80 orIgepal Co-630 without silica shells was measured and the obtainedvalues for emulsions related to microspheres A, B, C were15.00 ± 4.81mV, −28.60 ± 6.29mV, and −34.73 ± 3.19mV,which indicated that all emulsions possess good colloidal stability andcan be applied for solid-shell microsphere formation as emulsion tem-plates. Subsequently, zeta potential of three microsphere suspensionswith silica shell prepared using different parameters was investigated.The loading of APTES and TEOS on the hexane-emulsion or toluene-emulsion droplets surface decreased magnitude of zeta potential to2.70 ± 0.62mV, −7.42 ± 0.86mV, and −8.04 ± 1.14mV for solid-shelled microspheres A, B, C by overcoming the repulsive effects of theelectrical double layers to allow the finely sized oil droplets to formlarger droplets through coalescence, which presumably owing to theprotonation of amino groups in APTES resulting in a condensation re-action with TEOS. This reduction was presumably owe to the hydrolysisof APTES and protonation of amino groups which led to NH4

+ ionsform and adsorb on the surface of hexane-in-water or toluene-in-wateremulsion. Afterwards, hydrolysis of TEOS and condensation reaction ofAPTES and TEOS continued to decrease surface charge density.

3.3. Toluene loading behavior

UV spectroscopy was utilized to prove the presence of toluene insolid-shell microspheres B and C, and to quantify and compare the to-luene amount in them. To this end, the absorption at a wavelength of261 nm, which is a characteristic peak for toluene, was selected in theanalysis. For a given amount of toluene, toluene remained in the so-lution was 16.57% more for the microsphere suspension B compared tosuspension C after 1 day of preparation (Fig. 2). The difference in to-luene concentration as indicated by the reduction in the intensity of thetoluene peak at 261 nm was due to evaporation over 1 day of samplepreparation. It indicated that toluene-in-silica shell microspheres B hadstronger silica shell to restrict the evaporation of volatile toluene pre-serving higher amount of toluene and possessed better encapsulationproperty, which was in agreement with SEM images.

3.4. Inducing release of organic solvents from solid-shelled microspheres

To compare the breakability or forced release behavior of threesolid-shelled microspheres, we tried the forced injection approach into

Table 1Experimental parameters for preparing different formulations of solid-shelledmicrospheres.

Sample Payload Payloadconcentration(%)

Surfactant Surfactantconcentration(%)

APTES/TEOSratio (v/v)

A hexane 10 Tween 80 0.4 2.8B toluene 10 Tween 80 0.4 4.0C toluene 10 Igepal Co-630 0.6 2.0

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a sand column as breaking stimuli, which was relevant and applicableto the reservoir conditions. The changes in the mean particle size wereused as criteria of release. Fig. 3 displayed the particle size distributionafter treating under forced injection for three solid-shelled micro-spheres. As observed in aspect of particle size changes, forcing solid-shelled microsphere suspensions into a porous media was an effectiveapproach to rupture their silica shell resulting in the leakage of tolueneor hexane from the solid-shelled microspheres. Among them, toluene-in-silica microspheres C was most easily broken with fragile silica shellevidenced from no microparticle detecting after forced injection pro-cess. While after the microsphere suspensions A and B passed throughthe column, it displayed two different peaks in particle size distribution,

indicating the releasing of organic solvents but the partially breakage ofsolid-shell microspheres.

The aforementioned comparison among three solid-shelled micro-spheres demonstrated that solvents, surfactants, and APTES/TEOS ratiogave a significant effect on particle size, silica shell thickness, evenbrittleness and fragility of final products. Thus, it is necessary to in-vestigate the roles and correlations of these factors.

3.5. Effect of payload

Fig. 4 presented optical microscope images for hexane-in-wateremulsion and hexane-in-silica shell microspheres with differentamounts of payload. For hexane emulsion prepared from 0.4% Tween80, the average droplet size was 1.02 ± 0.16 μm, then the value wasdramatically increased up to 3.10 ± 0.12 μm after the silica shell grewwith 0.095M APTES and 0.036M TEOS coating on 5% hexane emul-sion. And with the increase of hexane concentration, the average dro-plet size increased to 4.80 ± 0.21 μm for 10% hexane-in-silica micro-sphere. In our previous work using toluene as payload, the averageparticle size was 2.31 ± 0.09 μm and 2.67 ± 0.18 μm for 5% and 10%toluene-in-silica shell microsphere, respectively [4]. The results re-vealed that larger concentration of hexane or toluene supplied morereaction sites for the monomer and thus led to more active condensa-tion and formation of larger particles. It also indicated that oil phasetype and content in emulsion determined the size of solid-shelled mi-crospheres.

3.6. Effect of surfactant

The system with larger concentration of surfactants producedsmaller droplet-sized solid-shelled microspheres, as displayed in Fig. 5.

Fig. 1. Optical microscope and SEM images of three kinds of solid-shelled microspheres (marked as A, B, C) prepared from different process parameters: (a-c)microsphere A, (d–f) microsphere B, (g–i) microsphere C.

Fig. 2. UV spectra of toluene-in-water emulsions B and C with silica shell.

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In detail, it can be observed that a significant reduction in the averageparticle size as the concentration of Tween 80 was increased from 0.2%to 0.4% for solid-shelled microspheres, and then when continuing toincrease the surfactant amount, the particle size decreased slightly. Formicrospheres prepared from Igepal Co-630-stabilized emulsion, theparticle size decreased from 0.2% to 0.6% Igepal Co-630, and then thedecrease trend became slow. In the synthesis procedure, growing solid-shelled microspheres were stabilized against aggregation by surfactantmolecules. The larger amount of surfactants led to less free aqueousspace, resulting in the formation of fewer monomers in each micelle,and thus, a reduction in particle size [41]. Moreover, the particle size ofIgepal Co-630-stabilized solid-shelled microsphere was obviously

smaller than that of Tween 80-stabilized microsphere. This was mainlyrelated to the hydrophilic-lipophilic balance (HLB) value of surfactantwhich influenced the droplets through a reduction in interfacial tensionof the oil/water interface. Igepal Co-630 has a lower HLB which gives alower activation energy for homogeneous nucleation and leads to morenuclei and smaller droplets.

3.7. Effect of APTES/TEOS

The concentration of APTES also plays a role in determining particlesize. As shown in Fig. 6, solid-shelled microsphere used Igepal Co-630as surfactant increased in size from 0.53 ± 0.07 μm to

Fig. 3. The particle size distribution of hexane-in-silica and toluene-in-silica microspheres (A, B, C) after triggered releasing in porous media. The inserts show initialparticle sizes from optical microscope images.

Fig. 4. Optical microscope images for (a) hexane-in-water emulsion and (b, c) hexane-in-water emulsion with silica shell using 5% and 10% hexane as payload,respectively.

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1.72 ± 0.11 μm with increase in self-catalyst APTES concentration. Itcan be explained that higher concentration of APTES increased the pHof the solution, which raised the rates of hydrolysis and condensation.The APTES molecules seemed to be mainly incorporated on the surfaceof the emulsion droplets and the particle size was enlarged. Besides,APTES provided more comonomer to the system and the increase in theamount of monomer resulted in the increase of particle size. However,with an increase in APTES content, the TEOS content should appro-priately decrease to avoid the homogeneous nucleation of monomers[42]. The good match between APTES and TEOS content is essential toavoid the formation of core-free silica particles.

3.8. Heavy oil recovery behavior

In order to evaluate the effectiveness of solid-shelled microsphereswith different parameters for enhanced hydrocarbon recovery in lowpermeability reservoirs, a series of heavy oil flooding tests were con-ducted in a sand column.

The comparison of three solid-shell microspheres (A, B, C) preparedfrom different parameters on heavy oil recovery efficiency were pre-sented in Fig. 7. As observed, at low injection volumes, solid-shelledmicrosphere C was the most effective means to recover heavy oiltrapped in the porous media with recovery efficiency of

Fig. 5. (a–e) Optical microscope images for hexane-in-silica microspheres using different concentrations of Tween 80 as surfactant in emulsion template, (f) Averageparticle size of hexane-in-silica microspheres using different concentrations of Igepal Co-630 as surfactant in emulsion template.

Fig. 6. Particle size distribution of solid-shelled microsphere with differentvolume ratios of APTES/TEOS (condition: 10% toluene, 0.6% Igepal Co-630 asa surfactant).

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54.27 ± 4.06% using 13.9 PV dispersion. At higher injection volumes,solid-shelled microsphere B acquired a higher percent recovery withheavy oil recovery efficiency up to 94.60 ± 3.41% using 34.7 PVdispersion, exceeding the efficiency of microsphere C at the same in-jection volume (73.69 ± 9.13%). Solid-shelled microsphere B had alow adsorption capacity and was easily washed-out and displaced moreheavy oil in porous media. As stated above, solid-shelled microsphere Chad more fragile and thin silica shell and was easier to break and re-lease its payload under external force resulting in higher recovery ef-ficiency initially. However, subsequently, it easily led to the particlesaggregation because of larger surface-to-volume ratio resulting from itssmall particle size, and therefore it possessed high surface energies,which may limit fluid flow in the channels and slow down the recoveryrate. Unfortunately, the heavy oil recovery efficiency of solid-shelledmicrosphere A (hexane-loaded microspheres) was not much satisfactorywhich may because its silica shell thickness was a little thick and not sobrittle to be broken upon encountering solid particles in narrow chan-nels and pores blocking heavy oil transport recovery. The relativemagnitude of particle size and pore throat of reservoir is shown to in-fluence the efficiency of heavy oil recovery using diluent loaded mi-croencapsulates [37]. The current heuristics is that the initial particlesize of microcarrier is smaller than the one tenth of diameter of porethroat to ensure the effective transport of carriers in pore channels [43].The low efficiency of microspheres A in mobilizing heavy oil can partlybe ascribed to the mismatch between microsphere size and pore throatdiameter of the column, which can prevent the transport of micro-spheres into the lower section of the column. Comprehensively, to-luene-loaded microspheres B stabilized with Tween 80 possessed moreexcellent ability to recover heavy oil trapped in porous media.

4. Conclusions

In summary, a series of breakable solid-shelled microspheres withdifferent preparation parameters was developed and investigated asdiluents applied in narrow channels and pores blocking hydrocarbontransport recovery. The roles and correlations for systematically syn-thesizing silica-shelled microspheres with controllable surface

morphologies, particle sizes, and silica shell thickness were introduced,which can be regulated by tuning preparation factors such as payloadtype and concentration, surfactant type and amount, and APTES/TEOSratio. Hexane-in-silica shell microsphere had large particle size andthick silica shell and was not brittle to be broken, while toluene-in-silicashell microsphere stabilized with Igepal Co-630 exhibited totally dif-ferent property with small particle size and very fragile and brittle silicashell. The core–shell structure of microspheres formed by hexane ortoluene wrapped with an inert and breakable silica layer provided easeof protection and releasing payload. The present study also showed theheavy oil recovery efficiency results obtained from sand column testsusing three different solid-shelled microsphere dispersions. Amongthem, toluene-in-silica solid-shelled microsphere using Tween 80 as asurfactant provided excellent heavy oil recovery efficiency up to94.60%. The realization of breakable solid-shelled microspheres will bebeneficial to enhanced hydrocarbon recovery and improving perme-ability of reservoirs.

Declarations of interestThe authors declare no competing financial interest.

Acknowledgement

The authors gratefully appreciate the financial support from theChina Scholarship Council (CSC).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in theonline version, at https://doi.org/10.1016/j.fuel.2018.07.006.

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