Nanotechnology for Oilfield Applications: Challenges and Effects
Nanotechnology is the design and application of engineered or naturally occurring nanoparticles with at least one dimension on the order of 1 to 100 nm to accomplish specific purposes. The unique properties of nanoparticles allow them to be used for many purposes in the oil field. This paper presents a critical review of the recent literature to determine the status of research and development and field application of nanotechnology in the oil field.
Nanotechnology is the manipulation, control, and integration of atoms and molecules to form materials, structures, components, devices, and systems at the nanoscale. One nanometer is 1-billionth of a meter. A water molecule is approximately one-tenth of a nanometer. A glucose molecule is approximately 1 nm. So, a nanometer approaches the size of molecules.
Nanoparticles possess three unique properties. First, their small size enables them to be transported into formation pores not accessible to larger particles. Second, at nanoscale, material properties are size-dependent because of the large surface-area-to-volume ratio. Therefore, nanoparticles can be engineered to contain specific optical, magnetic, interfacial, electrical, or chemical properties to perform specific functions.
Most of the proposed applications of nanotechnology in the oil field can be classified into the following six areas: sensing or imaging, enhanced oil recovery (EOR), gas mobility control, drilling and completion, produced-fluid treatment, and tight-reservoir application. A review of the literature showed that much of the current research is focused on the performance of nanoparticles in the reservoir. Some work is being conducted on the propagation of nanoparticles, and very little work is being conducted on the delivery and recovery of nanoparticles.
Of the six application areas, the authors ranked imaging, drilling through unstable zones, and tight-reservoir applications as having the biggest potential effect. Using nanoparticles to detect hydrocarbon saturation in a reservoir can have a significant effect on field development planning, such as well placement. Similarly, using nanoparticle-enhanced drilling fluid to stabilize and drill through unstable zones can increase the rate of penetration, reduce drilling costs, and minimize environmental effects. Furthermore, using specially designed nanoparticles to image and prop up induced and naturally occurring fractures in tight reservoirs can lead to sweet-spot identification and more prolific wells.
Sensing and Imaging
Contrast Agent for Crosswell Electromagnetic Imaging. Magnetic nanoparticles (MNPs) can be used as contrast agents for mapping the flood front in a reservoir. The principle behind this technology is that the speed of electromagnetic (EM) waves decreases when they pass through a magnetic medium. In waterflooding, water containing MNPs is injected into the reservoir. By placing an EM wave source at the injector and an EM wave receiver at the producer, a nearby observation well, or the surface, one can conduct crosswell EM imaging to locate the MNPs and, therefore, the fluid front.
Downhole Powerless Sensors. Nanoparticles can be designed to perform specific functions, such as sensing temperature, pressure, and the presence of hydrocarbons. The sensors contained in the nanoparticles do not need a supply of power; they perform their tasks through chemical or quantum effects. They can be injected into the reservoir, recovered, and analyzed.
Chemical EOR. Applications of nanoparticles in chemical EOR have been extensively studied. They are designed to achieve one or more of the following.
- Alteration of Rock Wettability and Reduction of Oil/Water Interfacial Tension (IFT). Nanoparticles are small enough to pass through pore throats in typical reservoirs and, thus, can access residual oil microscopically. They reduce oil/water IFT and alter rock surface wettability because of their surface reactivity, hence reducing the capillary force that the oil phase needs to overcome to be mobilized.
- Reduction of Oil Viscosity and Enhancement of Injection-Fluid Viscosity. Better conformance and mobility control, important in improving macroscopic recovery efficiency, is achieved by modifying the injection-fluid viscosity to match that of the oil. Polymer has been widely used to increase water viscosity. Nanoparticles, for example copper oxide, are capable of enhancing injection-water viscosity, as well. Nanoparticles can stabilize emulsions or foam because of their surface activities, which also can lead to enhanced injection-fluid viscosity.
Heavy Oil EOR. Recovery of heavy oil can be enhanced by reducing the oil viscosity. Nanoparticles can achieve this through two mechanisms.
- Enhancement of Heavy Oil Thermal Conductivity. Certain nanoparticles exhibit the capability to improve reservoir thermal conductivity and specific heat, in addition to their ability to enhance density and viscosity of treatment fluids. Thermal processes take place in reservoir rock by EM heating, which takes advantage of injected metal-oxide nanoparticles. When exposed to high-frequency EM radiation, these nanoparticles align themselves with the EM field, resulting in high-frequency particle movements that heat up the surrounding region through friction.
- In-Situ Upgrading of Heavy Oil. Nanoparticles can crack heavy-oil molecules chemically to upgrade heavy oil in situ by acting as nanoscale catalysts. The large surface area of nanoparticles improves the catalytic performances of hydrogenation and hydrocracking. Compared with micro-sized catalysts, nanoscale particles can be dispersed easily with minimal effect on injectivity.
Gas Mobility Control
Typical gas EOR processes that use the injection of a gaseous phase such as steam or carbon dioxide suffer from adverse mobility ratios and gravity segregation. Viscous instability leads to poor areal sweep, while gravity segregation leads to poor vertical sweep. Foam has been used to mitigate these effects. Foam bubbles are stabilized by a surfactant, which resides at the gas/aqueous interface. The presence of foam lamellae increases the effective viscosity of the injected gas phase and, thus, improves the mobility ratio and reduces gravity override.
However, surfactant propagation in a porous medium is hindered by adsorption at the rock surface and precipitation out of solution because of divalent-ion exchange with the rock. Also, the presence of oil has an adverse effect on foam stability. Nanoparticles can adsorb at the gas/liquid interface, depending on their wettability. A fully water-wet nanoparticle will stay in the liquid phase. A gas-wet nanoparticle will prefer to stay in the gaseous phase. Nanoparticles that possess both hydrophilic and hydrophobic properties will preferably adsorb at the gas/liquid interface like a surfactant molecule. However, the difference between the two is that the amount of energy required to detach a nanoparticle from the interface is a few orders of magnitude greater than the thermal energy of the nanoparticle. Therefore, the adsorption of a nanoparticle on the interface is, for all practical purposes, irreversible.
Drilling and Completion
Drilling and completion fluids that contain at least one additive with nanoscale particle size (1–100 nm) are considered nano-based fluids. Benefiting from the huge surface area, as well as the predominance of interparticular van der Waals and electrical forces over gravity, nano-based fluids are expected to exhibit game-changing fluid properties at very low concentrations.
Fluid-Loss Property Enhancement. Nanoscale particles can be used in drilling and fracturing fluids to achieve better wellbore stability and fluid-loss control. Experiments found that the addition of nanoparticles to clay dispersions containing a fixed electrolyte concentration resulted in a greater reduction in clay swelling than was achieved by the electrolyte alone. The addition of engineered nanoscale materials helps form thin mudcake and eliminate spurt loss. On the other hand, nanoscale additives can enhance rheological properties of surfactant and polymer-based nano fluids. In experimental study and field trials, improvement in lubricity and thermal stability of water-based drilling fluid was achieved by the addition of nanoscale graphene.
Cement Property Enhancement. Research showed an improvement in the sensing property of smart cement by 16% through the addition of nanoscale iron particles, which facilitate the monitoring of cement height during cementing operations. Other research demonstrated enhanced protection of smart cement by nanoscale calcium carbonate particles against oil-based mud contamination. Moreover, the research showed that the compressive strength and elasticity, along with chemical resistance, of American Petroleum Institute Class G cement can be enhanced by the addition of nanoparticles, which fundamentally change the crystallization structure of the cement.
Fracturing-Fluid Property Enhancement. Researchers have observed that the use of a nanoscale crosslinker resulted in lower polymer-loading in fracturing fluids, which was useful in reducing formation damage for post-fracture gas flow. Furthermore, rheology and thermal stability can be enhanced by the addition of nanoscale silica.
MNPs can be used to treat produced fluids by removing undesirable chemicals. Studies have examined the use of surface-modified MNPs to remove polymer from produced water. These particular MNPs are attracted to the negatively charged polymer molecules. When a magnetic field is applied to the mixture, the polymer/MNP settles out of solution and can be separated from the water phase. By adjusting the pH, the MNPs can be made to be negatively charged. Consequently, the polymer will desorb from the MNPs, which can then be reused. Because the strength of magnetic fields can be orders of magnitude greater than that of gravity, this method allows faster separation of polymer from water than traditional physical separation processes.
The application of nanotechnology in tight reservoirs has been gaining more attention because of the ability of nanoparticles to enter tight pores.
Fluid-Loss Control Additive for Drilling Fluid. Nanoscale silica was reported to be an effective fluid-loss additive in xathan-assisted water-based drilling fluid for shale drilling. The silica acts as a bridging material that facilitates quick formation of thin mud filter cake.
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