SPE 157106094

Nanotechnology-Assisted EOR Techniques: New Solutions to Old Challenges Shahabbodin Ayatollahi, SPE, EOR Research Center, School of Chemical and Petroleum Engineering, Shiraz University (now with Sharif University of Technology), Mohammad M. Zerafat, SPE, EOR Research Center, School of Chemical and Petroleum Engineering, Shiraz University

Copyright 2012, Society of Petroleum Engineers This paper was prepared for presentation at the SPE International Oilfield Nanotechnology Conference and Exhibition held in Noordwijk, The Netherlands, 12–14 June 2012. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Enhanced Oil Recovery techniques are gaining more attention worldwide as the proved oil reserves are declining and the oil price is hiking. Although many giant oil reservoirs in the world were already screened for EOR processes, the main challenges such as low sweep efficiency, costly techniques, possible formation damages, transportation of huge amounts of EOR agents to the fields especially for offshore cases, analyzing micro-scale multi-phase flow in the rock to the large scale tests and the lack of analyzing tools in traditional experimental works, hinder the proposed EOR processes. Our past experiences on using nanotechnology to the upstream cases, especially EOR processes, revealed solutions to some of the challenges associated with old EOR techniques. This method that utilizes particles in the order of ١to١٠٠nm brings specific thermal, optical, electrical, rheological and interfacial properties which are directly useful to release the trapped oil from the pore spaces in the order of ٥ to ٥٠ microns of tight oil formations. Laboratory tests using nanoparticles as the EOR agent, developing nano computational models to explore the surface properties and utilizing nano-scale analyzing tools such as atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) mostly for nanoparticles distribution in the pore spaces and on the surfaces for wettability alteration studies are the main parts of this investigation. This paper summarizes new findings from several different theoretical, analytical and experimental works which shows the effectiveness of traditional methods when assisted by this new technology. Ultimately, based on the past experiences, a roadmap will be proposed to avoid the ongoing trial and error practice in this area.

Introduction Nanotechnology is the science of materials in a range very close to molecular dimensions (١٠٠-١ nm) which has changed our viewpoint in many scientific aspects and has shown novel pathways for old problems remained unsolved through previous technologies. As a result of new properties and the introduction of special phenomena that occur in this size range, materials find considerable potentials to confront the challenges which seemed far from reach through macro-scale technology. These properties at the nano-scale can be mentioned as follows: (a) Optical (transparency (e.g. Copper) and Color Change (e.g. Gold)), (b) Chemical (catalysis (e.g. Platinum)), (c) Electrical/electronic (Conductivity (e.g. Silicone), (d) Thermal (Faster Cooling, Enhanced thermal properties (heat transfer, insulation)), and (e) Mechanical (Ultra-high strength). Those enhanced properties brought about in the Nano size range which can be of interest in production engineering can be:

١) Large surface to volume ratio: enhanced activity and contact area

٢) Confinements of electrons or positive charge changing the material structure: changing dielectric constant, conductivity, optical properties, chemical, electronic, etc

٣) Chemically modified surfaces (wettability alteration at nano-scale)

Nanoparticles as a bridge in between bulk materials and atomic or molecular structures are of great scientific interest. Unlike the bulk materials having constant physical properties, the properties of materials change as their size approaches the nano-scale and as the percentage of atoms at the surface of a material becomes significant. For example, Copper nanoparticles smaller than ٥٠ nm are considered super-hard materials that do not exhibit the same malleability and ductility as bulk copper. Nanoparticles have a very high surface to volume ratio. This provides a tremendous driving force for diffusion, especially at elevated temperatures. Upon addition of the nanoparticles, the properties of the conventional fluid such as density, viscosity, thermal conductivity and specific heat change critically (Zhang et al., ٢٠٠١). These enhanced properties are very important for petroleum engineers dealing with micro scale pore structure of the fluids (oil, gas and water) saturated rocks to optimize oil production in the early stages or improve the oil recovery efficiency as the reservoir is being matured. As the global energy need increases, the giant oil and gas reservoirs fueled the world industry for the past ١٠٠ years are depleting. Global oil demand is expected to advance ١ percent a year to ١٠٥ million barrels a day by ٢٠٣٠ from ٨٥ million barrels a day today (world energy outlook, ٢٠١٠). The ever-increasing demand for petroleum can just be satisfied through two routes: (١) Exploration of new hydrocarbon reservoirs or, (٢) Enhancing the oil recovery of available reservoirs. As the rate of new oilfield discoveries is declining, and most of the producing oil fields are in the late stages of production, thus the importance of improving oil production efficiency by EOR techniques is highly understood. Because of low sweep efficiencies in many of the world’s reservoirs almost two thirds of the oil in place cannot be recovered by conventional production methods. This fact leaves us in the middle of a challenging trend which entails being equipped with newly found technologies like nanotechnology to find out solutions for the serious problems ahead. Electrical double layer and DLVO Theory Any EOR process used in the past for more oil recovery has been practically gone through a sort of trial and error procedure resulted in economical and environmental problems. Using nanotechnology in the oil fields especially for EOR purposes could be enhanced if the theoretical background of this emerging technology is understood and utilized. For example there seems to be an essential need for a comprehensive theory to describe the interaction between nanoparticles, fluids and rock surfaces in order to understand the expected behaviors in each application. Considering the reservoir rock as a charged surface and in the absence of gravitational forces influencing these tiny particles, charge interactions become more pronounced. Due to the fixed surface charge at the solid interface, an oppositely charged region of counter-ions develops in the liquid to maintain the electro-neutrality of the solid-liquid interface. This screening region is denoted as the electrical Double Layer (EDL) because ideally it consists of opposite charges, some of which are bound while others are mobile. This potential distribution results in a redistribution of ions and introduced charged nanoparticles and can thus be considered as the governing theory in order to predict and design the desired interactions. The governing equation for the distribution of potential at charged interfaces is Poisson-Boltzamn (Schoch et al., ٢٠٠٨):

2 02 sinh B

zen zek Tψψ

ε

∇ =

)١(

This equation can be used for the prediction of NPs behavior in conjunction with transport equations. DLVO (Derjaguin- Landau-Verwey-Overbeek) (Derjaguin & Landau, ١٩٤١; Verwey & Overbeek, ١٩٤٨) shows when two particles come near to each other, the stability of particles in solution is affected by total energy of interactions. This total energy of interactions is constituted of attractive and repulsive terms such as electric double layer repulsion, London van der Waals attraction, born repulsion, acid-base interaction and hydrodynamic forces (Khilar, et al., ١٩٩٨). DLVO theory can also be used to simulate the interactions of nanoparticles with each other (Aggregation), to other particles present in the medium (fines, nano-asphlatene, ions, etc.) and also with rock surface (Adsorption) for wettability changes. Efficient transportation of metal particles through the reservoir is a challenging task. Also, the concentration of metal species is of great importance (Ju & Fan, ٢٠٠٩). The optimum concentration depends on the oil composition. Therefore, the main problem after transporting the metal particles into the reservoir is their homogeneous distribution to provide the maximum efficiency. The injection and propagation of particles in the reservoir is governed by many different forces: van der Waals forces, gravity forces, electrostatic forces, Brownian diffusion, inertia forces, hydrodynamic forces and surface tension (Hamedi Shokrlu & Babadagli, ٢٠١١).

Size effects of Nano particles The diameters of pores in normal oil reservoirs are generally in the order of micrometer (Fig.١), so nanoparticles not only enter oil reservoirs, but also show a penetration effect excited by thermal and dynamic energy and result in desired changes in the desired reservoir locations. As a result, the diffusivity of the materials into the rock space is improved, the fluids interfaces in the micro-scale channels are altered, the cohesive force of crude oil to rocks is reduced, the rock wettability is changed, channels for oil flow are smoothed, some damages because of unwanted particles and fluids movement are treated and eventually the oil recovery factor is increased. Nanoparticles can drastically increase oil recovery by improving both the injected fluid properties (viscosity and density enhancement, density, surface tension reduction, emulsification improvement and also thermal conductivity and specific heat improvements) and also fluid-rock interaction properties (wettability alteration and heat transfer coefficient). Due to the high surface forces of nanoparticles such as van der Waals and electrostatic forces, unique nanoparticles can effectively control formation fines migration (Ahmadi et al., ٢٠١١). Fine migration is a noticeable problem in petroleum production engineering. Plugging of throats in porous media occurs due to detachment of fine particles from sand surfaces. Hence the study of interactions between fines and pore surfaces and investigating the governing forces are important issues to describe the mechanism of fines release process.

Fig. 1. The SEM Image of reservoir rock Nano-sensors There are high hopes that by injecting novel sensors into the petroleum reservoirs, it will be possible to more accurately map them in ٣-D, increase the amount of recovery, and minimize the environmental impacts. Nanotechnology can help in this regard through new metering techniques with tiny sensors to provide more transparent information from the black box reservoirs (Fig. ٢). In the field of EOR, the idea of nano-mineralogy is being discussed of creating nano-robots to examine individual pores and channels, trace the trapped oil and EOR agents, monitor the flow of hydrocarbons in subsurface reservoirs, exchange information to the surface, and determine zones to focus or avoid during any EOR processes.

Fig. 2. Nano sensor implanted in the rock

Besides having the right size compatible with reservoir pore size have proper dispersibility, which may be induced by functionalizationreservoir harsh environment and at the same time produce highly effective nano-EOR agents can assist in locating the bypassed oilformation for pressure maintenanceBabadagli, ٢٠١١). Tailoring/Production NP(s) especially for Petroleum EngineeringIt is essential to use materials whose performances production of hydrocarbons in oil industrymultifunctional performance using the enormous opportunities their size, have different properties that in micro and macroscopic level are imperceptible oradvantages in using nanomaterials are their propertieshence, the amounts required for their application in oil wells will be significantly environmental impact will be minimizedscale, numerous problems need to be solvednanomaterials. Oil industry is usually rigid to any changes however if this is going to show a good trend then the huge amount of materials (Thousands of tons are needed in every the Nano Industry. This was already a trend in surfactant and

Fig. 3. Special chemicals for EOR processes

The nanoparticles used in the design of such fluids are preferably inorganic with properties of no dissolution or aggregation in the liquid environmentenvironmentally friendly. Recent experiments have shown some promising nanofluids with amazing properties such as fluids with advanced drag reduction٣), and anticorrosive coatings.

Nanotechnology Analyzing ApparatusThe laboratory studies of nanotechnology analyzers, surface scanning tools, surface properties measurements such as chemicals and electrical scanning tomography in nano scaleare now available to the petroleum industry and Microscopy (AFM), X-ray photoelectron spectroscopyspectroscopy, Zeta Potential Measurements

patible with reservoir pore size distribution and geometrymay be induced by functionalization. Also, the sensors have to withstand the the same time produce sensitive data from reservoir p

can assist in locating the bypassed oil, injection of colloidal water produced from e, and application of nano-sized silica particles for oil mobilization

pecially for Petroleum Engineeringose performances are adapted to complex situations during the exploitation and

industry. Nanotechnology helps in the development of new technologies with the enormous opportunities that nanostructured materials provide us

s that in micro and macroscopic level are imperceptible orare their properties, which could be empowered or maximized due to their sizeir application in oil wells will be significantly low, and consequently the mized. However, it is quite a fact that before nanomaterials can be applied in full to be solved, such as the production of low-cost and easily industrialized lly rigid to any changes however if this is going to show a good trend then the nds of tons are needed in every stage in a reservoir) cy a trend in surfactant and polymer industry being used in EOR

emicals for EOR processes (Surfactants for wettability alteration

gn of such fluids are preferably inorganic with properties of no dissolution or ment. They can be designed to be compatible with reservoir fluids and are xperiments have shown some promising nanofluids with amazing properties such tion, binders for sand consolidation, gels, products for wettability alteration

pparatushnology invented many techniques for nano structure analyzers such as size urface properties measurements such as chemicals and electrical to digitize rock pore structure in recent years (Fig. ٤).m industry and have been used in the research activities are Atomic Force electron spectroscopy (XPS), Scanning Electron Microscopy rements, Size Distribution Analyzer and micro/nano CT scanners

metry, these sensors should sors have to withstand the

parameters. Nano-sensors or loidal water produced from il mobilization (Shokrlu &

during the exploitation and t of new technologies with erials provide us, which by ble or absent. The greatest

maximized due to their size,d consequently the cost and erials can be applied in full t and easily industrialized show a good trend then the could revolutionary change d in EOR.

lteration)

perties of no dissolution or h reservoir fluids and are ith amazing properties such r wettability alteration (Fig.

ure analyzers such as size d electrical charges and ٣D. Some of these apparatuses ctivities are Atomic Force

Microscopy (SEM), Raman CT scanners.

Fig. 4. Scanning Microscopy Technique (i.e. AFM) to browse the surface for any nanometer changes

Enhanced Oil Recovery Reservoir rock as the hydrocarbon containing formation contains hydrocarbon fluids; oil, gas and brine; based on the type of the reservoir; dry gas, gas condensate, light oil, medium and heavy oils, and the production stage; initial condition or after water flooding, these fluids practice different distributions inside the pore structure. The composition of fluids are also different from one reservoir to the other for both aqueous phase (brine made of different minerals in the form of ions) and oily phase (hydrocarbon type fluids composed of very light molecules up to solid phase asphaltene type, which could affect solid- fluids boundary known as wettability and fluids-fluids front defined as interfacial tension. The rock pore structure depends on the formation type, from very homogenous capillary type openings in the sandstone rocks to very complicated pore-throat networks in carbonates and vuggy rocks. The size distribution of the fluids saturated holes in the rock is very wide from several microns up to millimeter in vuggy ones. Besides, in some cases the main formation rock includes tiny channels known as fractures separating rock matrix blocks apart from each other. The two main parameters related to the rock properties known as porosity showing the capacitance of the system to hold fluids and permeability demonstrating the conductivity of the rock to the fluids is not representing the very complicated rock structure to be used for EOR processes. Oil and gas as the desired fluids are produced from the formation under different mechanisms mostly based on volume replacement. For example in many oil reservoirs, gas and water would replace oil phase to be produced while for the gas reservoirs; gas expansion itself and water encroachment from the aquifer are displacing the produced gas. Any of the mechanisms mentioned here encounter multiphase flow in the reservoir pore structure which is still a very complex transport phenomenon under investigation. Relative permeability of oil, water and gas governing this process are known as the most time consuming, costly and uncertain parameters measured in the labs. Moreover, phase changes (gas-oil-solid) in the rock openings because of pressure differences in the reservoir during production or injection (pressure maintenance) would make this process more complex.

Oil trapping (entrapment) Two major types of different oil trapping mechanisms are known in oil reservoirs (Fig. ٥); macroscopic trapping contains oil saturated matrix and layers bypassed mostly due to the layer heterogeneities of the formation. This very high un-swept volumetric efficiency could be seen in fractures and stratified type reservoirs. The second type of oil remained in the pore scale of the rock mostly known as microscopic trapping is because of pore-throat network heterogeneity, wettability and interfacial tensions, known as microscopic displacement efficiency. One can tabulate the most important explanations behind oil trapping in oil reservoir as several types of heterogeneities:

• Macroscopic rock heterogeneities, stratified and layered formations • Microscopic heterogeneities in the body of the formation rock; pore-throat networks • Surface energy heterogeneities at the boundary of the fluids-rock, known as wettability • The tension heterogeneities at the fluid-fluid interfaces (IFT) • heterogeneities in fluids properties such as differences in viscosity and density (mobility ratio)

Fig.5.

Wettability Wettability of oil reservoir rocks describes the tendency of a fluid to adhere to the solid surface competing with another immiscible fluid. Wetting and spreading are the key parameters for oil production both during primary or enhanced oil recovery. At large scalesin the pore space of reservoir rockswhich govern the oil recovery efficiencyafter primary and secondary oil recovery is shown

Fig.6. General Wettability effects on residual oil saturation(Jadhunandan

The interfacial phenomena of spreading and adhesion of fluid on the rock surfacesimplications because of their impact on multireservoirs. However, the prediction of wetting properties and its alteration during the production or any chemical treatment processes is difficult because of the complex chemical composition of the crude oil and the formation brine as well as the interaction with the minerals very close to the rock surfacethe wettability of mineral surfaces are availablewetting and the mechanisms of wettability changes٧) the interactions taking place between crude oilet al., ٢٠١٠, ٢٠١٢).

Fig. 7. Effect of brine pH on disjoining surfaces charges and pressure isotherms

Imaging of residual oil within a carbonate and a clasti

describes the tendency of a fluid to adhere to the solid surface competing with and spreading are the key parameters for oil production both during primary or ales, wettability plays an important role in fluids saturation and their distribution s. In consequence, it affects capillary pressure and relative permeability curvesiency. The general wettability effects on the residual oil saturation

overy is shown in Fig. ٦:

on residual oil saturation, oil remained after primary and secondary oil recovery hunandan, & Morrow, 1991; Owolabi & Watson, 1993)

ding and adhesion of fluid on the rock surfaces, known as wettabilityct on multi-phase flow in the rock hence the recove

n of wetting properties and its alteration during the production or any chemical cause of the complex chemical composition of the crude oil and the formation h the minerals very close to the rock surface. Many practical techniques to assess are available, but these measurements cannot capture the microtability changes. To understand these mechanisms, one needs to investigate

ween crude oil, brine and rock surfaces close to the solid

isjoining surfaces charges and pressure isotherms (Rahbar et al

lid surface competing with ion both during primary or ration and their distribution elative permeability curves,oil saturation, oil remained

d secondary oil recovery

n as wettability, has serious ery efficiency of petroleum production or any chemical rude oil and the formation

ractical techniques to assess e the micro-mechanisms of ne needs to investigate (Fig. lid-fluids boundary (Rahbar

bar et al., 2010, 2012)

Microbial EOR techniques are also considered to utilize wettability alteration induces by microbial side-productions for more oil recovery. Among these, the effects of growth and metabolite production of a variant of Bacillus thermodenitrificans on the surface properties and wettability of mica and sandstone cores are investigated (Fig. ٨). AFM analyses of the mica surfaces revealed noticeable surface changes for all the treated samples compared with untreated mica (Zargari et al., ٢٠١٠, Karimi et al. ٢٠١٢).

Fig. 8. Bacterial adhesion is evident for the sample treated by the MEOR agent (AFM images) (Zargari et al., 2010)

Wettability alteration due to surfactant injection has also been investigated in some papers (Fig. ٩). AFM topographies and phase images were obtained and used to investigate the effects of aging in crude oil and surfactant treatment of the mica surface (Seiedi et al., ٢٠١١).

Fig. 9. AFM images of (a) aged mica, (b) aged mica surface treated with C16TAB, and (c) aged mica surface treated with Triton X-100 (Seiedi et al., 2011)

Interfacial Tension (IFT) Interfacial Tension (IFT) as the energy difference at the molecular level in the vicinity of two immiscible fluids. IFT plays significant role in fluids distribution and movement in the rock, wettability of the solid surface and most significantly on oil recovery efficiency from the reservoir. It is also the fundamental quantity that determines the

pressure change across a surface due to curvature which is the basis for the stability analysis of both thin films and liquid jets. Thus, profound understanding especially for petroleum engineers.reservoirs is the most important parameters affecting residual saturdrainage processes in porous mediatechnique used in the oil industry to relate residual saturation to capillary number mostly affected by IF

EOR Processes The retarded oil in the reservoir because of all of these heterogeneities and associated problems account for ٩٥ percent of original oil in place. Tas well as gas injection. Any of these processes tends to recover more trapped oil from the reservoir through different mechanism such as IFT reductionas well as incorporating new drive mechanism into the reservoir such as gravity drainageprocesses, mechanisms involved and the challenges associated with each in a microscopic scale

Table 1. Main EOR processes, mechanisms involved and the challenges associated in microscopic scale

Method

Thermal; steam injection, steam stimulation, in-situ combustion, SAGD

Chemical, Alkaline-Surfactant-Polymer flooding

ue to curvature which is the basis for the stability analysis of both thin films and standing of this property of fluids is essential in many engineering practices

IFT value as it affects the capillary forces compared to viscous force in the oil parameters affecting residual saturation of the phases during imbibition and ia. The capillary disaturation curve (CDC) as shown relate residual saturation to capillary number mostly affected by IF

Fig.10. Capillary disaturation curve (CDC)

cause of all of these heterogeneities and associated problems account for The well known processes are: Chemical, Thermal and Microbial EOR processes these processes tends to recover more trapped oil from the reservoir through eduction, wettability alteration, selective plugging, viscosity andmechanism into the reservoir such as gravity drainaged the challenges associated with each in a microscopic scale

chanisms involved and the challenges associated in microscopic scale

Mechanism Challenges

• Viscosity reduction • IFT reduction • Steam distillation • Oil expansion

• High energy cost • Low thermal conductivity of rock and fluids• Heat leakage to the undesired layers• Low effective thermal degradation• Well bore damage due to initial high temperature• Heat loss from heat generator to the reservoir

• IFT reduction • Wettability alteration • Mobility control

• High cost because of excess amount needed• Low effectiveness on IFT and viscosity changes because

of the harsh reservoir condition• Damage due to incompatibility• Unfavorable mobility ratio because of • Slow diffusion rate in pore structure• Unknown mechanisms foe wettability alteration

lysis of both thin films and many engineering practices d to viscous force in the oil ases during imbibition and wn in Fig. ١٠ is a general affected by IFT.

oblems account for ٦٠ up to d Microbial EOR processes from the reservoir through scosity and density changes e. Table ١ shows main EOR c scale.

roscopic scale

enges

vity of rock and fluidsdesired layersdegradationto initial high temperature

nerator to the reservoir

xcess amount neededFT and viscosity changes because ondition

patibilityatio because of solution viscosity

pore structurefoe wettability alteration

Gas Injection; HC gas injection, CO٢ Injection, Air Injection

• Pressure maintenance • Viscosity reduction • Oil expansion • Miscibility

• Gas low viscosity leads to fingering and early breakthrough

• High MMP needed to have miscible flooding for higher recovery efficiency

• In the case of CO٢, corrosive resistance materials are needed

• Side effects such as asphaltene deposition occurs

MEOR; Nutrition injection, Bio-products (biosurfactant, biopolymer flooding)

• Selective plugging • Oil biodegradation • IFT reduction • Wettability alteration

• Very slow mechanism • Undesired plugging • Undesired bio reactions (SRB) • Nutrition delivery to the microorganisms • Unknown mechanisms of surface changes (wettability

alteration) • Slow biodegradation process • Chemically assisted MEOR

These mechanisms involved in these processes are categorized generally into mass, heat and momentum transfer; thermodynamics and surface energy changes are to be done at molecular scale inside micro-scale pore structure. Therefore, Nanotechnology is seen as the solution to most of those challenges as summarized below:

• Nano-scale movement in micro size porous media: The most important challenges to traditional EOR processes are the transfer of injected components into porous media as the pore throats could be plugged causing permeability reduction and increasing the cost of injection. Since the nano-scale components are usually in the order of ٥٠٠-١٠٠ nm, can easily flow into the rock pore structure which is usually greater than ١٠ micron. Dust injection is the well known form of nanoparticles injected into the oil saturated formation to perform a variety of changes from fluid properties to interfacial changes both for fluid-fluid and fluids-solid interfaces.

• Very high specific surface area of nanoparticles: This is the most notable property for enhanced mass transfer between the phases and changing fluid properties more easily and at lower costs, homogenously distributed in porous media, adsorbed on the rock surface to change the wettability and changing the thermal conductivity of the fluids and the rock.

• Tailored molecules: Based on the mechanisms explored in the last section for EOR, the idea of tailoring specific types of chemical molecules to play a specific role in porous media is an exciting idea that was made available through nanotechnology. Very efficient scientifically tailored chemical components in the nano-size range can critically change the fluids IFT and viscosity, change the surface energy and even upgrade oil properties through catalytic reactions using nano-catalysts in micro-reactor type porous media.

• Smart delivery of the EOR agent: using chemicals in mega-scale reservoirs has always been the concern as the injection cost increases. In many cases, excess amounts of chemical solutions are used to locate the isolated oil patches remained in the reservoir for EOR. Moreover, selective plugging of the reservoir layers to prevent undesired fluids to be produced would enhance both recovery efficiency and the performance of production facilities. This promising technology being already used in drug delivery would be very cost effective and more efficient.

• Nanofiltration in EOR processes: Besides the use of filtration technology in surface facilities for efficient phase separation, it would be more effective if done underground in the vicinity of well bore for in-situ separation. Petroleum engineers were struggling for the past decades to shut off the undesired phase flow into the well bore, improving the surface facility conditions and keeping gas and water in the oil reservoir exploiting them as natural energy sources.

• Nano-sensors: Very complicated types of fluids and rock structure persuaded the scientists to provide reservoir engineers with sensors installed in the wells monitoring flow regimes, fluid composition and reservoir properties. Besides, there are several types of tools to be used in the reservoir for scanning, logging and testing the rock, fluid as well as reservoir condition to find the optimized production scenario for higher production rate and better recovery efficiency. All the well bore sensors and monitoring tools are large enough that need preparing extra space for them in the well opening, therefore they can hardly monitor far beyond the well location which is very tiny compared to the reservoir volume. Using nano-scale sensors will give the engineers the chance to:

o Trace the trapped oil, its saturation, location with respect to the pore spaces and associated forces need to recover it for EOR.

o Monitor the rock openings in micro-scale to have digital maps of the pore-throat network for modeling the injecting/producing fluids.

o Assessing in depth heterogeneities especially for vuggy and fractured reservoirs.

o Checking the boundaries, fluids contacts, connectivity with neighboring reservoirs to make an accurate ٣D/٤D seismic view of the formation which enhance the understanding for any EOR activities.

o Checking the success of remediation technique if any. The remedies have been used during many EOR processes such as selective plugging and water shutoff. Nanotechnology through nano scale sensors could be utilized to check if it has been successfully employed or not.

• Nano-Analysis: The analytical equipments available in the field of nanotechnology to analyze the surface changes such as Atomic Force Microscopy (AFM) and X-Ray Photo Spectroscopy (XRP), Raman Spectroscopy, Zeta Potential meter; scanning tools such as Scanning Electron Microscopy (SEM), Micro and Nano CT scanners are very useful tools in assessing the structure of porous media, wettability alteration and many other investigations in this field that needs very accurate measurement in nano-scale changes inside the porous media and at the fluid-fluid/fluids-solid interfaces. Professional expertises are needed first to use these equipments then to mimic the reservoir condition in the lab to be analyzed through these techniques.

Fine Migration Formation damage due to fine migration is one of destructive side-effects of some EOR techniques which may also find solutions from nanotechnological viewpoints. Fines loosely attached to the pore surface are in the equilibrium with the pore fluids. These particles start to flow when the equilibrium state is disturbed by the EOR agent which, may end up in permeability reduction in porous media (Fig. ١١). Different solutions have been suggested to prevent detachment of fines from surface such as ionic clay stabilizer, polymers and resins. Nanofluids containing metal oxide nanoparticles show specific properties (Habibi et al., ٢٠١١). Nanofluids that contain nanoparticles (MgO, Sio٢, and Al٢O٣) show specific properties such as high tendency for adsorption and good candidate for the injection into the near wellbore regions because of their very small sizes (Habibi et al., ٢٠١١). Hence the study of interactions between fines and pore surfaces and investigating the governing forces are important issues to describe the mechanism of this process. The main types of these forces are electric double layer repulsion and London-van der Waals attraction. It is possible to change these forces by the use of nanoparticles as surface coatings. Nanoparticles increase the effect of attraction forces in comparison with repulsion forces. Based on extensive experimental works, the magnitude of the electric double layer repulsion in comparison with the London-van der Waals attraction between fines and media grain particles was considerably diminished when MgO nanoparticle was used to coat the porous media resulting in fine fixation (Ahmadi et al., ٢٠١١).

Fig. 11. SEM images of glass bead coated with MgO nanoparticle with adsorbed fine (Habibi et al., 2011) Nanostructure of Asphaltenic Components Some researchers have claimed that oil is essentially a nano-fluid, with asphaltenes being the dominant part, flowing in a micro and often nanoscopic environment (i.e., the porous media). According to that, it seems justifiable to consider most of the oil and gas production technologies, reservoir treatments and stimulation as nanotechnologies

(Evdokimov et al., ٢٠٠٦; Mullins et aleffect of some EOR processes, mostly miscible gas iprecipitation affects EOR through the wettability alterationprecipitation is very sensitive to the reservoir conditions and fluid propratio, and injected fluid molecular weight conjunction with multi-fractal descriptions of asphaltene surface depositioninvestigate the effect of asphaltene surface structure on wettability alteration

Fig. 1

It was found that the effect of asphaltene structure is more pressure. Asphaltene particles of high complexity and with larger polyhigher pressure than those with smaller polyasphaltene structure can be given through wettability alterationpoly-aromatic rings turn the surface less oil wet at higher pressureand surface topography alteration of different asphaltene sources roots in their different structure al., ٢٠١١). Some EOR techniques may also pronounce asphaltene precipitation due to special reservoir conditions they bring about inside the reservoir. Among thesewhich leads to serious production problems such as wettability alterationblocking the transportation pipelinesresolution microscope and image processing software under these conditions reveals that molecular structure could have a noticeable influence on asphaltene deposition dflocculation behaviors (Zanganeh et al

٥% mole CO٢

Fig. 13. Effect of different amount

et al., ٢٠١١). Several studies have shown that asphaltene precipitation as a side stly miscible gas injection, hinders the oil production from the wellshe wettability alteration, viscosity changes or permeability reductione reservoir conditions and fluid properties, such as pressurer weight (Sayyad Amin et al., ٢٠١٠). Visual studies with AFM and SEM in criptions of asphaltene surface deposition (Fig. ١٢)surface structure on wettability alteration (Sayyad Amin et al

2. AFM descriptions of asphaltene surface deposition

sphaltene structure is more pronounced for asphaltene precipitation at higher gh complexity and with larger poly-aromatic rings tend to be detached easier at

maller poly-aromatic rings. Another evidence to emphasizethrough wettability alteration. It was found that asphaltene particles with larger e less oil wet at higher pressure. It seems that the difference in wetting conditioof different asphaltene sources roots in their different structure

ronounce asphaltene precipitation due to special reservoir conditions they bring g these, CO٢ miscible flooding can significantly cause asphaltene deposition

problems such as wettability alteration, plugging of the reservoir formationes, etc (Fig. ١٣). Monitoring the evolution of asphaltene deposition via a highprocessing software under these conditions reveals that

noticeable influence on asphaltene deposition due to different aggregation and t al., ٢٠١٢).

١٠% mole CO٢ ١٥%

rent amounts of CO2 on Asphaltene deposition (Zanganeh et al

tene precipitation as a side from the wells. Asphaltene

bility reduction. Asphaltene ssure, temperature, dilution

es with AFM and SEM in ) have been performed to in et al., ٢٠٠٩, ٢٠١٠).

ene precipitation at higher end to be detached easier at phasize the significance of altene particles with larger

ference in wetting conditionstructure (Sayyad Amin et

rvoir conditions they bring ause asphaltene deposition,of the reservoir formation,tene deposition via a high-that nano size asphaltene

o different aggregation and

٥% mole CO٢

h et al., 2012)

A few studies are also focused on the existence of various asphaltene nanostructures petroleum and basics of quantum mechanics and statistical thermodynamics are used to predict the potential energy and the intermolecular forces among them in order to characterize their structure and dynamics and to establish the relationship between these properties and petroleum fluid behavior (Sabbaghi et al., ٢٠٠٨). Oil-Field Applications of Nanofiltration Nanofiltration separation processes can be helpful in EOR from two points of view: First, the purification and desalination of the injection water used for secondary or tertiary oil recovery. The injection water is mainly the produced water from the wells both for economical reasons and also as a way for water disposal which must meet certain requirements. With reservoirs of very low permeability, conventional approaches to water flooding cannot provide sufficient recovery factors, due to strong capillary forces in narrow pore channels. Thus, the impurity level of injected water is one of the most important concerns for significant oil recovery in low permeable reservoirs. Furthermore, the injection water should have similar ionic concentration to that of the oil reservoir water. In addition, the injection water containing oxygen and bacteria can be a major source of reservoir souring, reducing the value of produced hydrocarbons and requires expensive production equipment to stand the corrosive nature of produced fluids. Typical contaminants that need to be removed from injection water are hydrocarbons, solids and scaling compounds such as sulphate. In offshore environments, seawater is often the preferred source of injection water, which typically contains ٢,٧١٢ mg/l of sulphate ions (Dickson & Goyet, ١٩٩٤) and formation water contains variable amounts of barium which may react with the injected seawater sulphate ions resulting in the barium sulphate scale. This typical incompatibility causes unusual severe sulfate scale problems in the forms of calcium, strontium or barium (Bader, ٢٠٠٧). A solution can be the removal of sulphate ions from seawater before injection; this also helps prevent well souring by controlling sulphate reducing bacteria, which can be accomplished by the filtration of particulates down to the nanometer range. Nanofiltration membranes with pore diameters less than ٢ nm can be of great help in this regard. NF membranes are capable of highly rejecting divalent anions (e.g., sulfate) while retaining a large portion of monovalent anions (e.g., chloride) from seawater. NF is thus a potential technology to provide nearly sulfate-free seawater for oil fields water injection operations (Bader, ٢٠٠٦). The second section of interest can be in the field of produced water treatment. The contaminants need to be removed to enable the produced water reuse or disposal for which several options are available: surface disposal into the sea or evaporation ponds limited by local environmental regulations, injection into disposal wells, reinjection for pressure maintenance, or reuse for irrigation or as industrial process water. Increasingly stringent environmental regulations require extensive treatment of produced water from oil and gas productions before discharge. Produced water is conventionally treated through different physical, chemical, and biological methods. However, current technologies cannot remove small suspended oil particles and dissolved elements. Besides, many chemical treatments, whose initial and/or running cost are high and produce hazardous sludge. As high salt concentration and variations of influent characteristics have direct influence on the turbidity of the effluent, it is appropriate to incorporate a physical treatment, e.g., membrane to refine the final effluent (Fakhrul-Razi et al., ٢٠٠٩). Depending on the quality of the produced water and the water quality requirements for the beneficial uses being considered, NF may be a viable process for produced water treatment (Mondal & Wickramasinghe, ٢٠٠٨). NF permeate is ideal in this case because it is close to the same mineral background composition as the produced water but with all hardness removed, which greatly reduces the risk for precipitation and plugging of the formation (Ebrahimi et al., ٢٠١٠). The main obstacle against the deployment of desalination technologies for produced water purification has always been the complicated chemical composition and associated high operating cost. Pretreatment and membrane replacement are the major factors that increases the operation cost and limits the economic efficiency of membrane technology for produced water desalination (Muraleedaaran et al., ٢٠٠٩). Nanofiltration technology can also help the produced oil to be separated from EOR agents at the surface. This process is economically important to:

١. Increase the oil/EOR agent separation efficiency as in most cases the produced liquid on the surface is in micro/nano emulsion phase.

٢. In many cases the used materials in the fields are needed to be separated because of the possibility of their reinjection and reducing the risk of environmental problems by disposal.

Conclusions The followings are the main points leaned from our recent activities in the field of EOR and nanotechnology:

(١) Nanotechnology could be a breakthrough to some of the bottlenecks we have already experienced in the field of EOR and flow in porous media. The size of nanoparticles and their very different characteristics, especially surface prosperities, are the main keys to this improvement.

(٢) To ring the bell for the reservoir engineers, it is recommended to start with using this emerging technique in the upstream. Using nanotechnology in the surface facilities for more efficient purification, separation and anticorrosive materials would promote the more rigid-minded engineers to feel safe for application through the formation for EOR processes.

(٣) More joint works between chemists, physicians, chemical and petroleum engineers would shed more light into this route.

(٤) Theoretical investigation and simulation studies are required prior to any nano-technique to be used in the field for reducing the risk and the selection of the best method.

(٥) For a short term plan, IFT reduction, wettability alteration and fine fixation are more applicable followed by nano-sensors, nano-catalysts and nanofiltration. The Invention of nano-robots, nano-pumps and smart particles to search for the trapped oil in the reservoir is also imaginable for a foreseeable future.

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