38 Oilfield Review

Big Things in Small Packages

Many industries are developing methods to harness the potential of nanoscale

objects and produce them en masse. Nanotechnology might also revolutionize

key areas of hydrocarbon recovery. However, successes in other industries have

often been in conditions far from the harsh realities of oilfield environments. Research

is now underway to solve E&P-specific challenges and progress is being made in

several areas.

Andrew R. BarronJames M. TourRice UniversityHouston, Texas, USA

Ahmed A. BusnainaYung Joon JungSivasubramanian SomuNortheastern UniversityBoston, Massachusetts, USA

Mazen Y. KanjSaudi AramcoDhahran, Saudi Arabia

David PotterUniversity of AlbertaEdmonton, Alberta, Canada

Daniel ResascoUniversity of OklahomaNorman, Oklahoma, USA

John UlloConsultantSudbury, Massachusetts

Oilfield Review Autumn 2010: 22, no. 3. Copyright © 2010 Schlumberger.For help in preparation of this article, thanks to Hélène Berthet, Schlumberger Riboud Product Centre, Clamart, France; and Tancredi Botto and Joyce Wong, Schlumberger-Doll Research, Cambridge, Massachusetts.

Imagine a reservoir infiltrated with devices that could report back their location as well as the properties of the surrounding fluids. Or tiny sen-sors that could seek out an oil/water contact and be tracked by detection methods at the surface. These are not merely far-fetched dreams; they are the long-term goals of research groups inves-tigating nanotechnologies for the oil and gas industry. However, nanotechnology is still in its

infancy and even describing what it is can gener-ate discussion and debate.

Nanoparticles can be found in the glazes of ancient pottery, but such inadvertent use of nanostructures is far from the complex science of nanotechnology, which is grounded in research, development and manufacturing. Scientists have theorized about nanostructures for more than half a century, but it wasn’t until

1. For one of the first recognized discussions on nanoscale research: Feynman RP: “There’s Plenty of Room at the Bottom: An Invitation to Enter a New World of Physics,” transcript of talk presented at the 1959 Annual Meeting of the American Physical Society, first published in the journal Engineering & Science (February 1960), http://www.zyvex.com/nanotech/feynman.html (accessed July 26, 2010).

2. For comparison, nanocrystals can be as small as 10 nm; a human red blood cell is approximately 5,000 nm.

For more on oilfield nanotoxicology: Nabhani N and Tofighi A: “The Assessment of Health, Safety and Environmental Risks of Nanoparticles and How to Control Their Impacts,” paper SPE 127261, presented at the SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production, Rio de Janeiro, April 12–14, 2010.

Autumn 2010 39

the 1980s that it became possible to physically construct what was envisioned.1 In addition to the study of materials at the nanoscale, nano-technology includes developing tools to create, observe and manipulate nanostructures experi-mentally and, ultimately, at mass production levels.

Additionally, scientists must carefully study and define the physical properties of newly cre-ated nanomaterials to make safe and effective use of them. Such evaluations are especially needed when nanomaterials are constructed at molecular and atomic scales because their behaviors can change significantly with size. These types of studies, which will help identify nanomaterials useful for specific applications or new research investigations, are common practice for bulk materials such as ores and chemicals.

Furthermore, past scientific discoveries, such as radioactive materials, chlorofluorocarbons and asbestos, presented significant toxicological risks that were not identified until after loss of lives or serious damage to the environment. Similarly, because nanomaterials are small enough to bypass biological membranes or bond to tissue cells, they could present serious toxicological threats, and these risks must also be part of materials evaluations.2 Despite these challenging caveats, many experts anticipate revolutionary benefits from nanotechnology applications.

Some of the most advanced materials in use today are alloys, with widespread application in aircraft, automobiles, ships and buildings. Specialists in materials science can create an alloyed metal that is strong but also light, combin-ing the best properties of each component metal. Nanomaterials, like alloys, can also be fine-tuned to fit the needs of a specific application. There are two main approaches to designing nanomaterials: • Top-down materials design: Scientists perform

nanoscale modifications of existing materials, typically of the surfaces, to enhance their origi-nal properties.

• Bottom-up materials design: Scientists develop new materials from nanoscale subcomponents, which can often provide greater refinement of a material’s performance.

The top-down approach, which uses methods such as nanolithography, nanoablation or nanoetching, creates nanostructures from mac-roscopic objects, or bulk materials. These nano-fabrication methods are typically scaled-down versions of larger fabrication methods. Top-down enhancement of an existing material can be thought of as incremental nanotechnology because the original material is not radically altered. However, the impressive results it can

yield were demonstrated by scientists at the National Institute of Standards and Technology (NIST), in Gaithersburg, Maryland, USA.

NIST scientists improved the luminescent properties of high-brightness light-emitting diodes (HBLEDs) through a nanoetching technique. HBLED technology has many practical uses, such as home lighting, back-lighting for flat-screen

televisions and lasers, because these diodes require much less power and are smaller than more-traditional lighting technology. To improve their light-emitting capabilities, the surfaces of HBLEDs are nanoetched with a circular Bragg grating (CBG) pattern. The CBG reduces inter-nal refraction, which allows more light to be emitted from the HBLEDs (above). The process

> Enhanced HBLEDs. A distributed Bragg reflector (DBR, top) enhances light extraction by significantly decreasing light that is reflected onto the substrate layer of HBLEDs. A circular Bragg grating (CBG) extracts light from outside the unmodified DBR reflection cone. Electron beam lithography creates CBG patterns, which collectively form a CBG mask. Chlorine-assisted ion-beam etching of the mask produces a 150-nm deep CBG (middle). A trench is wet etched around the outside of the device to allow internally refracted light within DBR layers to escape laterally. Results (bottom) show that a 525-nm pitch CBG (red curve) provides the best improvement of light intensity over the standard HBLED (black curve). (Photographs courtesy of the National Institute of Standards and

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improves the light-emitting efficiency of the HBLEDs used in the experiments from 2% to 41%.3

Using the bottom-up materials design pro-cess, scientists grow nanomaterials from molecu-lar or atomic subcomponents or from other nanostructures. There are many methods to grow nanomaterials, each with its own variations:• Molecular self-assembly: Molecules adopt

defined arrangements based on their shape and functional groups.

• Aerosol-based processes: Chemicals sprayed onto macroscopic surfaces react with the sur-face and form droplets, creating nanoparticles.

• Atomic condensation: With intense heat, bulk materials such as metal are atomized in a vacuum; scientists then direct the dispersed matter into a collection chamber containing

gas. Vaporized atoms collide with gas mole-cules, resulting in rapid cooling, with subse-quent condensation that forms nanoparticles.

Self-assembling bottom-up materials design is often referred to as evolutionary nanotechnol-ogy because the materials created can be highly customized and therefore may offer unique abili-ties compared to materials created using other methods. Bottom-up approaches have been used in the medical industry to create contrast agents that enhance medical images and drug-delivery systems designed to carry payloads of treatments to targeted areas or specific cells within the body. There are also nanosensors that detect proper-ties in situ and nanogenerators that can harvest energy, such as heat or movement, and convert it into electricity.

Yet, to even begin to work at the nanoscale, it took two inventions from the 1980s to allow scien-tists to see, and later manipulate, nanostructures. The scanning tunneling microscope (STM), invented in the early 1980s, enabled scientists to observe single atoms within materials. The atomic force microscope (AFM), introduced in 1989, enabled scientists to then manipulate individual atoms (below left). In 1985, the STM was an essen-tial tool in the discovery of fullerenes, molecules composed entirely of carbon with structures in the form of hollow spheres, cylinders or ellipsoids.4 Cylindrical or tube-shaped fullerenes are more commonly known as carbon nanotubes (CNTs); they are utilized in a significant portion of nano-technology projects to this day.

This article first defines nanostructures, then describes recent research efforts in several fields including the electronics, medical and cosmetic industries. A description of the work of the Advanced Energy Consortium (AEC) highlights precompetitive research in nanosciences and potential areas of impact within the E&P work-flow.5 The consortium, managed by the Bureau of Economic Geology at the University of Texas, Austin, USA, includes Baker Hughes, BP, ConocoPhillips, Halliburton, Marathon Oil Corporation, Occidental Petroleum Corporation, Petrobras, Schlumberger, Shell and Total. The AEC’s primary goal is to develop intelligent nano-sensors that can be injected into oil and gas res-ervoirs to improve resource recovery. Other examples from the oil and gas industry provide a glimpse of applications outside the scope of the AEC.

An Introduction to NanotechnologyIn science, the term nano means one billionth; however, it is commonly prefixed to describe any-thing belonging to or inferring a nanotechnology, such as nanoscience, nanoelectronics and nanorobotics. Although exact sizes have yet to be standardized, nanostructures typically range in size between 1 nm and 1 µm (1 × 10-9 and 1 × 10-6 m) (next page). By comparison, microstruc-tures are between 1 and 100 µm in size. In this article, a standard classification is used to define primary types of nanoscale building blocks.6 These building blocks form nanomaterials such as nanoaerosols, nanopowders or nanosheets:• Zero-dimensional (0D) building blocks (length

equals width) include nanoparticles, nanoclus-ters and nanocrystals.

• One-dimensional (1D) building blocks (length is greater than width) include nanotubes, nanofibers and nanowires.

> Atomic force nanotechnology. The first commercially available atomic force microscope (AFM) was introduced in 1989. A cantilever with a sharp tip (top right) physically scans the surface of specimens. Cantilever deflection is measured by detecting movement of a laser dot using a photodiode sensor (left). A piezoelectric (PZT) scanner controls movement of the cantilever to ensure constant force against the specimen. As input to Hooke’s law, the deflection distance and stiffness coefficient of the cantilever are used to determine force. When the tip is in contact with an atom of a material being scanned, a voltage is applied to lift the atom onto the tip. The atom can then be moved to another location and the polarity of the voltage reversed to place the atom at a new location. Scientists at the National Institute of Standards and Technology used this technique to create their logo with an AFM placing cobalt atoms on a copper surface (bottom right). Each atom (the points of each letter in the logo) acts like a pebble in a pond as the disturbed electrons on the copper surface give a ripple-like appearance.

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Of the 0D objects, nanoparticles are either amorphous or semicrystalline structures, and they range in size from 10 nm to 1 µm. During fabrication, the nanoparticles that form nano-materials can be of different sizes, which can vary more than 15%, and still successfully form a nanomaterial without affecting its design specifi-cation. Unlike nanoparticles, nanoclusters are sensitive to their size and can be more reactive when scaled up or down.7 Hence, producing nano-materials from nanoclusters requires much smaller size variations, typically less than 15%, or the nanomaterial will behave differently; for example, nanoclusters in a nanoemulsion might undesirably agglomerate, resulting in a failed batch.8

Nanoclusters, like nanoparticles, have either an amorphous or semicrystalline structure, but are smaller than nanoparticles: between 1 and

10  nm in diameter. At the smaller end of this scale, they are sometimes characterized by their number of atoms: from about 200 to 1,000. Nanocrystals are single-crystal nanostructures between 1 and 30  nm in size.9 Semiconductive nanocrystals are more commonly known as quan-tum dots and have many potential applications in nanosensors and other electrical components; they are also used as easily detectable markers in applications such as medical imaging.

Nanosized building blocks that are 1D have diameters of 1 nm to 1 µm, but their lengths are unconstrained, often exceeding 1 µm. Nanotubes belong to the 1D building block group and have a hollow core, whereas nano wires, nanofibers and nanorods are solid. Nanofibers are amorphous and typically nonconducting. Nanowires are crys-talline and can be conducting, semiconducting or

insulating. They are found in many existing nano-circuits, such as those on microchips.

3. Su MY and Mirin RP: “Enhanced Light Extraction from Circular Bragg Grating Coupled Microcavities,” Applied Physics Letters 89, no. 3 (July 17, 2006): 033105.

4. Kroto HW, Heath JR, O’Brien SC, Curl RF and Smalley RE: “C60: Buckminsterfullerene,” Nature 318, no. 6042 (November 14, 1985): 162–163.

5. For more on the AEC: http://www.beg.utexas.edu/aec/ (accessed September 8, 2010).

6. Fahlman BD: Materials Chemistry. Dordrecht, The Netherlands: Springer (2007): 275–357.

7. Organometallic chemistry uses the term “cluster” to describe small molecular cages of fixed sizes.

8. For more on nanocrystal size dispersion: Wan YM, Van Der Jeugd K, Baron T, De Salvo B and Mur P: “Improved Size Dispersion of Silicon Nanocrystals Grown in a Batch LPCVD Reactor,” in Claverie A, Tsoukalas D, King T-J and Slaughter JM (eds): Materials Research Society Symposium Proceedings 830. Warrendale, Pennsylvania, USA: Materials Research Society (2005): 257–262.

9. A single-crystal material has a crystal lattice that is continuous to the edges of the material, with no grain boundaries.

> Nanostructures up close. A solution of colloidal silica nanocrystals suspended in water (top left) is commonly used as an abrasive for finely polishing silicon wafers. Another form of nanocrystals, vanadium dioxide (top center), has been used for high-performance optical shutters. The transmission speed between transparent semiconductive and reflective conductive phases can occur in one-tenth of a trillionth of a second. Nanowires (top right) have potential for future nanocircuitry. Quantum dots

(bottom left) are semiconductors with applications in solar panels, lasers and markers for imaging. Carbon nanotubes (bottom center) are among the most widely used forms of nanostructures. Nanocubes (bottom right) illustrate the diversity of shapes that can now be fabricated. [Images Courtesy of the National Institute of Standards and Technology and Furmanj (top center) at the English language Wikipedia, http://en.wikipedia.org/wiki/File:Nanostars-it1302.jpg (accessed November 9, 2010.)]

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Industrial ApplicationsOne of the most common applications of nano-technology today is in the electronics industry, in central processing units (CPUs). These compo-nents are used in computers; their job is to exe-cute binary-based instructions as efficiently as possible. They are made up of millions of transis-tors, which are effectively two-state switches. The closer the transistors are to each other and the smaller they are, the faster an electrical sig-nal can be transmitted. This combination of prox-imity and small size increases the number of instructions that can be calculated per second and improves the electrical-power efficiency of the processor.

CPUs are manufactured typically from silicon wafers. A layer of photoresistive material is applied to the wafer using a spin-coating process that evenly distributes the solution on the surface. A nanofabrication process, known as immersion lithography, cures channels on the photoresistive layer to create a circuit pattern (above). Light from a narrow-wavelength source, such as a 193-nm argon fluoride excimer laser, passes through a mask in the form of the circuit design pattern and then through a series of lenses that focus the pattern down to the

nanoscale. The exposure of the photoresistive layer leaves a nanoscale circuit pattern. The light-exposed pattern is removed by a chemical etching process that doesn’t affect the unexposed photoresist. More layers of semiconductor, insu-lator and photoresist are added and etched away to build highly complex three-dimensional cir-cuits. Using immersion lithography, it is now pos-sible to achieve features less than 32 nm wide.10

In the pharmaceutical industry, key motiva-tions for new-product research are to develop drugs that can combat specific ailments with minimal patient side effects and that deliver treatments as quickly as possible. While cancer therapeutic methods have improved significantly over the last 50 years, prolonging many lives, some elements of the treatments, such as antineoplas-tic chemotherapy, have remained conceptually the same since the 1950s.11 This chemical-based medication slows cell-division rates. Combined with surgical removal of cancerous tumors and radiation therapy, chemotherapy can be an effec-tive means to stop the spread of cancer. However, since such treatment methods contain processes that are harmful to healthy cells, researchers are striving to discover medical procedures that are less harmful or completely safe to use on patients.

Nanotechnology drug-delivery systems (DDSs) are an area of research that has the potential to significantly improve many treatments.12 Using this technique, several drug molecules can be covalently attached to the relatively large surface area of carbon nanotubes (CNTs). In addition to carrying the bonded medication, the CNT package is configured with targeting molecules that seek out receptors of specific cells. This process limits negative effects of medication on healthy cells in the body, improving treatment efficacy.

A recent research project, with collaboration between the University of Connecticut Health Center, Farmington, USA, and the National Institutes of Health, Bethesda, Maryland, focused on using single-walled carbon nanotubes (SWNTs) as a transport mechanism for antican-cer agents.13 A complex multistage process is applied to the SWNTs to prepare them to accept anticancer drugs. The tubes are oxidized in acid, producing carboxylate groups on their surfaces. A chemical promoter is then used to create an amide-based reaction, known as amidation, to bond anticancer medication to the SWNTs.

One pharmaceutical being tested is a platinum-based chemotherapy drug that inter-feres with mitosis cell division. The drug is com-bined on the SWNT with epidermal growth factor, which performs the targeting component of the DDS (below). A test group of laboratory mice was injected with cancerous cells and monitored as

> Immersion lithography. A narrow-wavelength light source is placed above a mask containing a circuit pattern (left). The channeled light is then focused through a lens that reduces the size of the circuit pattern to the nanoscale. A liquid medium that has a refractive index greater than that of air, which is 1.0, takes the place of the air gap used in photolithography. The greater refractive index of the liquid layer increases the focal length and the angle of refraction (right). In central processing unit (CPU) manufacturing, this process can be repeated more than 50 times; each cycle involves chemically treating, cleaning, doping and adding more photoresist to build many layers of circuitry.

> Single-wall nanotube (SWNT) drug-delivery system. Epidermal growth factor receptors (EGFRs) are produced in excess in cancerous skin cells. Bonding epidermal growth factor (EGF) proteins to SWNTs enables the nanotubes to favor cancerous cells over healthy cells. Once in situ, each SWNT can carry a selection of cancer drug treatments that are exposed to the cancer cells, such as by chemical bonding. In this example, fluorescing quantum dots (< 10-nm semiconduc-tive nanoparticles) were also attached to help scientists locate and confirm successful attach-ment of the SWNTs to cancer tumors.

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10. For more on immersion lithography: “Technology Backgrounder: Immersion Lithography,” http://www.icknowledge.com/misc_technology/Immersion%20Lithography.pdf (accessed August 30, 2010).

11. Hirsch J: “An Anniversary for Cancer Chemotherapy,” JAMA 296, no. 12 (September 27, 2006): 1518–1520.

12. Prato M, Kostarelos K and Bianco A: “Functionalized Carbon Nanotubes in Drug Design and Discovery,” Accounts of Chemical Research 41, no. 1 (January 2008): 60–68.

13. Bhirde AA, Patel V, Gavard J, Zhang G, Sousa AA, Masedunskas A, Leapman RD, Weigert R, Gutkind JS and Rusling JF: “Targeted Killing of Cancer Cells in Vivo and in Vitro with EGF-Directed Carbon Nanotube-

they developed 500-mg tumors. Half of this group was treated with the new DDS, which stopped further growth of tumors over a 10-day period. The remaining half of the test subjects were a control group, in which tumors grew to more than 2,000 mg.

The cosmetics industry also has a well- established nanotechnology research and devel-opment sector.14 For example, metal-oxide parti-cles such as titanium dioxide [TiO2] and zinc oxide [ZnO] are used in many modern sunscreens because their small size allows them to limit ultraviolet adsorption to safe parameters. The particles are also invisible to the naked eye when applied to the skin, and they do not agglomerate, which makes the sunscreen easy to apply.

Some concerns have been raised about the toxicological risks in applying nanoparticles directly to the skin, and certain results suggest TiO2 can penetrate the epidermis, while other results contradict such findings. Currently the US Food and Drug Administration is conducting one study on the effects of nanoparticle size on skin penetration and a second study on the specific effects of TiO2 and ZnO on excised human skin over a 24-hour period.15

Nanotechnology in E&P EnvironmentsWhile nanotechnologies are increasingly used in other industries, the oil and gas industry is only in the early stages of exploring this new field. The main difficulty in applying the successes of other industries is the hostile operating environment encountered downhole, including high tempera-ture and high pressure, and often a variety of cor-rosive fluids. Therefore, not only must the E&P industry consider how to take advantage of nanotechnology to solve oilfield problems, but research must also include development of nano-structures capable of surviving the rigors of such harsh conditions. Nevertheless, the industry has made some headway in several areas.

Characterizing reservoirs demands advanced modeling and simulation software to predict how the reservoir fluids and rocks will behave during production. Accurate physical data are essential to reduce the uncertainty of these predictions. Several sources, including geologic studies, seis-mic surveys, logs, well tests and production data, provide input to build an accurate picture of the reservoir. But high-resolution logging tools can gather data only close to the wellbore, and seis-mic surveys, although covering large areas, are of comparatively low resolution.

However, if it were possible to know the physi-cal and chemical properties of the reservoir and

its fluids while maintaining data resolutions closer to that of logs, operators could find ways to greatly improve recovery efficiency. To this end, researchers are investigating the use of nanoparticles that can be injected into a reser-voir to help with reservoir characterization. An early goal is to create nanoparticles that have the right properties, such as size, to pass through reservoir structures, and resistance to flocculation to avoid clogging the structures. A later step will be to enable these particles to measure reservoir properties as they are being transported. Two types of applications are being developed to realize this potential: nanosensors and contrast-enhancing nanoparticles.

When injected into reservoirs, nanosensors will physically probe reservoir fluids and rock as they are transported by fluid flow. Data will be retrieved either by performing a direct analysis of nanosensors recovered from production fluids or, more ambitiously, by wireless communication

with sensors in situ. Contrast-enhancing nanoparticles, on the other hand, have no data gathering capabilities. Instead, these nanoparti-cles will be used to enhance traditional data acquisition methods, such as nuclear magnetic resonance (NMR) imaging or controlled source electromagnetic (CSEM) surveys, in much the same way as contrast-enhancing agents improve medical imaging.

Transportation While SensingA pioneering example of oilfield nanotechnology comes from the Middle East. In 2007, Saudi Aramco began a long-term research project to develop nanosensors that could be used in the Arabian carbonate reservoirs. An early phase in the project determined how small nanoparticles needed to be to pass through pore throats, the smallest permeable fluid channels found in reser-voir rocks (above). High-pressure mercury injec-tion tests were performed on approximately

> Pores and throats. Pores are microscale cavities (blue-dyed areas) between grains (white and yellow areas) that contain fluids or gases. Pore throats are the narrow conduits that link pores (top left) and are the smallest permeable structures in reservoir rock. For nanotechnology to succeed in a reservoir, contrast agents or nanosensors must be able to traverse (red lines) a formation without flocculating, bridging pores or damaging reservoir permeability. A more-complex 3D visualization of pores and throats is demonstrated in the porecast image (top right). (Photograph courtesy of Dave L. Cantrell, Saudi Aramco.)

Based Drug Delivery,” ACS Nano 3, no. 2 (January 13, 2009): 307–316.

14. In 2008, the sixth greatest number of US patents for nanotechnologies belonged to a cosmetics company: Chen H, Roco MC, Li X and Lin Y: “Trends in Nanotechnology Patents,” Nature Nanotechnology 3, no. 3 (March 2008): 123–125.

15. Katz LM: “Nanotechnology and Applications in Cosmetics: General Overview,” in Morgan SE, Havelka KO and Lochhead RY (eds): Cosmetic Nanotechnology: Polymers and Colloids in Cosmetics. Washington, DC: American Chemical Society, ACS Symposium Series 961 (2007): 193–200.

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850 core plugs from different areas of the Ghawar field. The resulting analysis indicated pore-throat sizes as small as 0.5  µm in diameter.16 Based on previous transport phenomena studies, scientists concluded a safe size limit for nanoparticles as

one-fifth to one-seventh of the pore-throat limit, or 70 to 100 nm. With these constraints, the scientists investigated several nanoparticle functionaliza-tion methods to address compatibility with the harsh reservoir conditions (above).

The project has recently reached a major mile-stone of injecting and producing nanoparticles from an active reservoir. In the first half of 2010, scientists dispersed 10-nm–sized nanoparticles into 250  bbl [40  m3] of injection water.17 The nanoenriched 100-parts per million (ppm) solu-tion was then pumped into the Arab-D Formation of the Ghawar field. This was followed by injection of brine to drive the mix an estimated distance of 15 to 20 ft [5 to 6 m] away from the wellbore. The injection well was then shut in for three days before the reservoir was produced. Engineers con-ducted extensive sampling of production fluids over two days, determining the presence of nanoparticles using fluorescence spectroscopy.

By comparing the concentrations of nanopar-ticles in the produced water samples and in the water to be injected, Saudi Aramco researchers confirmed a high recovery factor of approximately 86%, which demonstrated the nanoparticles’ ability to remain colloidally stable under high-temperature and high-salinity conditions. The large yield also indicated that the nanoparticles were able to traverse pore throats with no affinity to the carbonate formation. Accelerated labora-tory tests and wellhead pressure measurements during the field test showed no signs of reduction in fluid flow rates or reservoir permeability.

Academic institutions are pursuing similar projects that investigate the transport of nano-sensors through reservoirs. Several potential applications include detecting residual oil and other reservoir properties such as acidity, salin-ity, pressure and concentrations of carbon diox-ide [CO2] and hydrogen sulfide [H2S]. While only in their infancy, several projects have shown progress.

One team has begun exploring transport and retention of nanoparticles through reservoir rock samples.18 Recently this work has been extended to demonstrate a method for estimating residual oil in place (ROIP). Scientists at Rice University in Houston and Nankai University in Tianjin, People’s Republic of China, prepared a coreflood setup to determine how changing injection-fluid composition and rock properties might affect nanoparticle transport (left).

During one set of fluid-property tests, the sci-entists discovered that increasing the ionic strength of the fluid, by adding potassium chlo-ride [KCl], increased the time it took to detect nanoparticles in the effluent. Increasing ionic

> The importance of nanoparticle surface functionalization. In this example, following injection of inadequately functionalized nanoparticles, large quantities of nanoparticles remained on the surfaces of rock grains (left). This event would almost certainly reduce the permeability of the reservoir; scientists therefore looked for ways to alter the nanoparticles’ surface chemistry. After scientists injected well-functionalized nanoparticles, rock grains remained clean (right).

> Laboratory setup. Scientists crushed core samples of dolomite and Berea sandstone then filtered the residue to obtain a distribution of grain sizes from 106 to 250 µm. The grains were washed with toluene and methanol to remove trapped oil and then packed into individual glass columns. Scientists calculated breakthrough and concentration of nanoparticles [scanning electron microscope (SEM) image, inset] by injecting one pore volume and then using a spectrophotometer to detect the ultraviolet fluorescence of nanoparticles in the effluent.

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strength also significantly reduced the nano- particle concentration in the effluent (above). However, in the worst case, when tests detected only 40% of the injected nanoparticles, flushing with deionized water then enabled the recovery of more than 90% of the trapped nanoparticles.

Another study considered how divalent cat-ions, abundant in seawater, would affect trans-port through a packed dolomite column. Divalent cations are molecules, or ions, missing two elec-trons, which makes them highly reactive. Calcium ions [Ca2+] and magnesium ions [Mg2+] are two examples of divalent cations found in large quantities in both seawater and in dolomite, one of the rock samples used in the test. Scientists confirmed the occurrence of salt bridging the nanoparticles and the Ca and Mg ions in the injection fluid and dolomite column (right).

16. Kanj MY, Funk JJ and Al-Yousif Z: “Nanofluid Coreflood Experiments in the ARAB-D,” paper SPE 126161, presented at the SPE Saudi Arabia Section Technical Symposium and Exhibition, Al-Khobar, Saudi Arabia, May 9–11, 2009.

17. For the recent status of the project: Bence H: “Nanobot Trial a Winner,” http://www.aramcoexpats.com/Articles/Pipeline/Saudi-Aramco-News/Dhahran-Media/6346.aspx (accessed August 20, 2010).

18. Yu J, Berlin JM, Lu W, Zhang L, Kan AT, Zhang P, Walsh EE, Work SN, Chen W, Tour JM, Wong MS and Tomson MB: “Transport Study of Nanoparticles for Oilfield Application,” paper SPE 131158, presented at the SPE International Conference on Oilfield Scale, Aberdeen, May 26–27, 2010.

> Effect of potassium chloride [KCl]. Deionized water (blue curve) was used to characterize the flow through a dolomite column. When increasing concentrations of KCl were added (purple, green and red curves) to a deionized nanoparticle solution, the recovery factor was reduced significantly. Total recovery of nanoparticles was determined by flushing with deionized water (black circles). Results suggest that the increase of ionic strength causes nanoparticles to be deposited on dolomite particles, as predicted.

> Divalent cations and modified nanoparticle surfaces. Naturally negatively charged nanoparticles are easily bridged by magnesium [Mg2+] and calcium [Ca2+] ions, commonly found in seawater (top). Scientists performed several experiments to determine the impact of these ions on fluid flow through packed rock columns. When added in increasing concentrations to nanoparticle solutions (middle), in all tests, nanoparticle concentration in the effluent increased at approximately the same rate as that in deionized water, before decreasing rapidly. In the solution containing the highest concentration of divalent cations (purple curve), the quantity of nanoparticles in the effluent was much lower than that at lesser concentrations. Synthetic seawater, which contains monovalent ions, was tested with Mg2+ and Ca2+ (blue curve). Without divalent cations in the seawater (not shown), nanoparticle concentration in the effluent was similar to that of deionized water. The surfaces of the nanoparticles were then treated to adjust their charge, significantly reducing the bridging effect observed with synthetic seawater mixed with divalent cations in the dolomite rock columns (bottom).

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The fluid tests were then repeated using a packed sandstone column. One of the predomi-nant characteristics of sandstone is that it is composed mainly of silica grains, which, unlike dolomite minerals, are negatively charged. In one test, scientists noted that after 15 pore volumes had been injected, nearly 100% of the nanoparti-cles were recovered compared with only 55% in dolomite under the same fluid conditions.

To improve the breakthrough speed and recov-ery factor, the scientists next looked at treating the surfaces of the nanoparticles to minimize charge interactions. When tests on both dolomite and sandstone were repeated, treated nanoparticles reached 100% breakthrough on the third pore vol-ume. As a result of this study, scientists were able to demonstrate, under laboratory-simulated reser-voir conditions, that nanoparticles can be injected into reservoirs with a very high rate of recovery. The team is now looking at using similar nanopar-ticles and hydrocarbon-marker molecules to detect ROIP. Initial results have shown that the nanosensors were able to distinctly differentiate oil-free and oil-saturated columns.

Small-Sized Chip Fits AllAnother area of nanotechnology research within the AEC charter extends existing tool paradigms for fluid-sensor chips: in this case, for sensors that detect highly corrosive chemicals such as H2S gas. Scientists at Northeastern University, Boston, Massachusetts, USA, have developed a chip that incorporates nanosensors to detect small concentrations of H2S in air, nitrogen, water vapor and liquefied petroleum gas. Because each component is tiny, future chip develop-ments could include multiple sensors of various types to detect several downhole chemicals and conditions. For failure redundancy and increased accuracy, one or more of these multisensor chips could then be embodied in a downhole tool to detect reservoir properties.

The existing H2S sensing chip is based on a two-terminal electronic circuit design (above). These two gold terminals are connected across a channel composed of functionalized SWNTs. The SWNTs are chemically treated during the func-tionalization process. Scientists then add a solution containing millions of 4-Amino-TEMPO molecules that covalently bond with the surface of the modified SWNTs.19 These molecules have a

special effect on the SWNTs: In the absence of H2S, current passes freely through the SWNTs, but when H2S gas is present, the TEMPO mole-cules break down and reduce the conductivity of the channel. The conductivity of the device, therefore, is a measure of the H2S level.

When exposed again to atmospheric condi-tions, the TEMPO molecules reform and the sensor can recover completely, thus making the chip reus-able. In laboratory conditions, the scientists have shown that the chip can be used to detect H2S gas in very small concentrations and in several differ-ent environments (next page). To be commercially viable for oilfield applications, the chip will need to be further developed for low-cost nanofabrica-tion and then tested in downhole conditions.

Contrasting NanoparticlesContrast-enhancing agents are widely used in medical imaging. For example, they are injected into a patient to improve the resolution of X-rays or magnetic resonance images.20 Similarly, experts predict that near-borehole measurements, such as nuclear magnetic resonance (NMR), magnetic susceptibility, acoustic and resistivity, can be improved using specially prepared nanoparticles

> H2S sensing chip. Under initial conditions (without H2S), current is able to flow from Terminal 1 through the SWNT channel to Terminal 2 (bottom). However, when H2S is introduced into the system, the 4-Amino TEMPO molecules are chemically reduced, which increases the resistance of the SWNT channel, ultimately blocking the current completely (top left). Hence, conductivity is a measure of the amount of H2S in the system. The photograph (right) shows the terminals (gold wires) leading to the nanosensor area.

Autumn 2010 47

that respond better than the in situ reservoir fluids or reservoir rock. These particles would be trans-ported throughout a reservoir using reservoir-flooding techniques similar to those in today’s oilfield operations. This particular concept has been adapted to illustrate potential uses of nano-technology in hydraulic fracturing detection.

Operators would like to know the outcomes of fracturing jobs before beginning well testing. Following pumping, fractures that were opened by high-pressure fluid entering formations close once pumping is stopped. Proppant is added to the fracture fluid to hold fractures open as

downhole pressures force them to close. However, proppant might not entirely fill a frac-ture, or it can flow back into the wellbore before the proppant has a chance to stabilize. If the final volume of resulting fractures is too small, the fracturing job will be ineffective at increas-ing permeability to commercially viable levels. If this is the case, refracturing wells using impro-vised fracturing fluids and higher pumping pressures may be necessary.

Identifiying these missed fracturing opportu-nities before well testing is very challenging, since high-resolution near-borehole data, such as NMR or resistivity logs, can’t determine the full

extent of deeper fractures. However, to visualize deeper into the formation, operators can listen to fractures using microseismic monitoring. Yet this

19. For more on 2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO): Barriga S: “2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO),” Thieme Ejournals, https://www.thieme-connect.de/ejournals/pdf/synlett/doi/10.1055/ s-2001-12332.pdf (accessed October 11 2010).

20. Ananta JS, Matson ML, Tang AM, Mandal T, Lin S, Wong K, Wong ST and Wilson LJ: “Single-Walled Carbon Nanotube Materials as T2-Weighted MRI Contrast Agents,” Journal of Physical Chemistry C 113, no. 45 (November 2009): 19369–19372.

> H2S sensing chip results. An H2S sensing chip was tested in several laboratory-simulated reservoir fluids. Scientists introduced H2S into a sealed gas chamber containing the chip and measured the drop in electrical current. To allow the chip to recover, scientists opened the chamber, extracted the H2S and filled the chamber with air. In all environments, the chip was able to detect H2S; the greatest sensitivity observed was approximately 10 ppm in air (top left). In one test (blue curve, top right), scientists did not allow the chip to recover after each increment of H2S; when scientists opened the chamber to extract the H2S and fill it with air, the chip recovered completely to its initial state.

48 Oilfield Review

method may lack the level of resolution needed to define fractures precisely enough to accurately calculate volumes.21 Seeking a solution, AEC part-ners Rice University and the University of Alberta, Edmonton, Canada, are investigating nanoscale

contrast agents to improve magnetic susceptibil-ity measurements.

Superparamagnetic nanoparticles have a larger magnetic susceptibility than any natural material within a reservoir, and when their loca-

tion is constrained, they can be used to highlight formation fractures. To image fracture volumes, the researchers plan to bond nanoparticles to proppant (above). This enables detection of the proppant, from which the fracture volume can be calculated. This information can provide opera-tors with reasons to change proppant material or pumping criteria and could also invite new research into improved proppant materials.

Contrast-enhancing nanoparticles must maintain their higher magnetic susceptibility while downhole, but reservoir conditions such as high temperature can lower the responsiveness of certain nanomaterials. The Rice University group fabricated several potential nanoparticles with superparamagnetic properties. Scientists at the University of Alberta tested the nanopar-ticles at reservoir temperatures and found that only some of them maintained acceptable magnetic properties.

Both groups are now focusing on the temper-ature-resilient particles and have begun trans-port studies. Researchers are also investigating a downhole magnetic susceptibility tool that will ultimately detect the contrast agents in reservoir fractures. Similar paramagnetic particles can also be transported through formations and used to improve traditional NMR measurement sensitivity with little or no adaptation of the existing tools.22

> Nanosensor-coated proppant. Specially coated sensing proppant is injected into target formations using traditional methods (left). When the fracture begins to close, the sensing proppant remains to hold the fracture open (center). The proppant stabilizes under the pressure of the closing fracture and stops further shrinkage (right). The proppant can then be detected using magnetic susceptibility measurements.

> Janus nanoparticle fabrication. Scientists use sonification to make an emulsion of water, wax and silica nanoparticles; the silica nanoparticles become partially embedded in the wax droplets (top left). In the next phase (top right), metal particles are added. These particles bond only to the exposed surface of the silica particles. The solution is then dried in a vacuum, which removes the wax and water, leaving bicoated nanoparticles (bottom left). The metal-coated surfaces of the particles enable them to attract SWNTs in the final stage of the process (bottom right).

Autumn 2010 49

Enhanced Oil RecoveryNanotechnology has the potential to improve res-ervoir characterization by enhancing data gath-ering techniques. However, the greatest impact on the industry may be the use of nanotechnology to increase the amount of recoverable hydrocar-bons beyond the capabilities of existing enhanced oil recovery methods. To that end, nanoparticles have been shown to be highly customizable and can be developed with many properties that can be triggered in very specific conditions. For example, nanoparticle suspensions could be injected into depleted formations to locate immoveable oil and cause in situ reactions to free entrapped hydrocarbons.

Scientists at the University of Oklahoma have shown how Janus nanoparticles can be finely tuned to seek out, stabilize and cause reactions at oil/water interfaces.23 Janus nanoparticles, so called because of their two-faced nature, have opposing properties at either end, such as a hydro-phobic side and a hydrophilic side. This feature

gives the nanoparticles a tendency to seek out oil/water interfaces. By bonding metal catalysts such as palladium to these nanoparticles, the scientists have also been able to induce phase migration reactions using temperature as a control. With funding from the AEC, the scientists are now exploring use of particles with similar properties to mobilize oil remaining after waterflooding.24 This resource represents about two-thirds of origi-nal oil in place (OOIP).25

Creating a bipolar nanoparticle is a multistage process that includes coating hydrophilic parti-cles, such as silica, with a hydrophobic compo-nent, such as SWNTs (previous page, bottom). In this case, by controlling the concentration of SWNTs on the silica particles, scientists can change the contact angle from hydrophobic, through amphiphilic, to hydrophilic. Then, by depositing catalysts on a specific side of the nanoparticles, reactions such as oxidation, reduc-tion or condensation can be selectively triggered.

Scientists at the university are now envision-ing nanotechnology-based secondary and tertiary recovery methods with the ultimate goal of extracting as much as 100% of the OOIP. Their latest work investigates modifying the rheologi-cal properties of trapped hydrocarbons as well as improving the pushing performance of injection fluids (left).

Nanotechnology BreakthroughAs searching for and recovering oil becomes ever more complex, new technology is often required that resets what operators consider standard explo-ration and production practices. However, the cost of technology is a contributing factor to what are termed recoverable hydrocarbons. The true cost of nanotechnology to the oil and gas industry is largely unknown, especially as it currently exists almost entirely in research. Cost factors may include nano-structure recyclability and fabrication yields, envi-ronmental cleanup and nanomaterial composition. Widespread use of similar nanotechnologies in other industries can also help drive down costs; for example, there is now a growing market for off-the-shelf nanomaterials.

Perhaps what will help support broader research and development of nanotechnology is the first successful commercial tool for hydrocar-bon recovery. Some expect this to be nanoscale contrast agents since they may be made afford-able, resilient, recoverable and reusable. This nanotechnology would also integrate seamlessly with current operational workflows, meaning operators can choose to add them to improve their sensing package as they would any other measurement tool. Nanotechnology will undoubt-edly become a much more familiar term within E&P in the next decade. —MJM

> Underground catalysis. Catalytic nanoparticles that stabilize at the water/oil interface can be injected with oxidizing (air) or reducing (hydrogen [H2] and carbon monoxide [CO]) gases to cause reactions that modify the viscosity of the injection fluid and the rheological properties of the fluids, such as water/oil and oil/rock interfacial tensions. Air can be directly injected and H2 and CO could be produced in situ by partial oxidation of the natural gas within the reservoir. This process enables secondary and tertiary hydrocarbon recovery.

21. For more on microseismic monitoring: Burch DN, Daniels J, Gillard M, Underhill W, Exler VA, Favoretti L, Le Calvez J, Lecerf B, Potapenko D, Maschio L, Morales JA, Samuelson M and Weimann MI: “Live Hydraulic Fracture Monitoring and Diversion,” Oilfield Review 21, no. 3 (Autumn 2009): 18–31.

22. Yu H, Kotsmar C, Yoon KY, Ingram DR, Johnston KP, Bryant SL and Huh C: “Transport and Retention of Aqueous Dispersions of Paramagnetic Nanoparticles in Reservoir Rocks,” paper SPE 129887, presented at the SPE Improved Oil Recovery Symposium, Tulsa, April 24–28, 2010.

23. Crossley S, Faria J, Shen M and Resasco DE: “Solid Nanoparticles That Catalyze Biofuel Upgrade Reactions at the Water/Oil Interface,” Science 327, no. 5961 (January 1, 2010): 68–72.

Cole-Hamilton DJ: “Janus Catalysts Direct Nanoparticle Reactivity,” Science 327, no. 5961 (January 1, 2010): 41–42.

24. Villamizar L, Lohateeraparp P, Harwell J, Resasco DE and Shiau B: “Interfacially Active SWNT/Silica Nanohybrid Used in Enhanced Oil Recovery,” paper SPE 129901, presented at the SPE Improved Oil Recovery Symposium, Tulsa, April 24–28, 2010.

25. Hartstein A, Kusskraa V and Godec M: “Recovering ‘Stranded Oil’ Can Substantially Add to U.S. Oil Supplies,” US Department of Energy Office of Fossil Energy, Project Fact Sheet (February 2006), http://fossil.energy.gov/programs/oilgas/publications/eor_co2/C_-_10_Basin_Studies_Fact_Sheet.pdf (accessed November 8, 2010).