Summary Viscoelastic surfactant (VES) fluids have been used widely in the oil industry as completion and stimulation fluids. The surfactants arrange structurally to form rod-like micelles that increase VES-fluid viscosity for regular fracturing and fracture-packing fluids. However, high fluid leakoff and low viscosities at elevated temperatures have limited VES fluids to hydraulic fracturing and fracture-packing applications. This paper will introduce a nanotechnology application for maintaining viscosity at high temperatures and controlling the loss of VES fluid without generating formation damage. The nanoparticles studied are 35-nm inorganic crystals that display unique surface morphology and surface reactivity. These nanometer-scale particles associate with VES micelles through chemisorption and surface-charge attraction to stabilize fluid viscosity at high temperatures and to produce a pseudofilter cake of viscous VES fluid that reduces significantly the rate of fluid loss and improves fluid efficiency. When internal breakers are used to break the VES micelles, the fluid will lose its viscosity dramatically and the pseudofilter cake will then break into brine and nanoparticles. Because the particles are small enough to pass through the pore throat of producing formations, they will be flowed back with the producing fluids, and no damage will be generated. The results of rheology, leakoff, and core-flow tests will be presented for the VES-fluid systems at temperatures 150 and 250°F. Introduction VES fluids have been used widely as gravel-packing, fracture-packing, and fracturing fluids for more than a decade because the fluids exhibit excellent rheological properties and maintain low-formation-damage characteristics compared with crosslinked-polymer fluids. VES fluids are composed of low-molecular-weight surfactants that form elongated micelle structures that exhibit viscoelastic behavior to increase fluid viscosity (Nehmer 1988; Brown et al. 1996; Samuel et al. 1999). Traditionally, the industry depends on external breakers (or reservoir conditions) to break VES fluids after treatment is completed. The two primary external conditions have been (1) contact with reservoir hydrocarbons and (2) contact and dilution with reservoir brine (Samuel et al. 1999). But, relying on the external or reservoir conditions to break down the leaked off VES fluid to achieve quick and complete treatment-fluid flowback has been a point of contention and is questionable, especially for dry-gas reservoirs (Crews et al. 2006). In a broad sense, internal breakers are hydrophilic compounds placed within the VES elongated micelles during surface mixing that will go wherever the fluid goes, ensure that the VES fluid breaks, and break the VES fluid so that it cleans up easily, enabling oil and gas to flow to the wellbore to be produced. Internal breakers generate VES-breaking compounds over time, which penetrate and collapse the viscous, rod-like VES micelles into nonviscous, more-spherical micelles, and the technology enables the VES breaker to accompany the VES fluid during a fracture-pack or regular fracturing treatment to enhance and ensure breaking and cleanup of the VES fluid from the reservoir (Crews 2005; Crews and Huang 2007). VES fluids are unlike polymer-based systems in that they are nonwall-building and do not form filter cake on the formation face during hydraulic fracturing and fracture-packing treatments. Without filter cake development, the amount of VES fluid leaked off from the fracture into formation during a fracturing treatment is primarily dependent on fluid viscosity. Because of its nonwall-building property, VES fluid exhibits high fluid leakoff from the fracture during a treatment and "screening out" is a common problem. Because of poor fluid efficiency of VES fluid, (1) the permeability of a reservoir is less than 400 md for most cases, (2) more total fluid volume is required for a given treatment, and (3) a larger amount of leaked off fluid within the reservoir matrix occurs, which needs to be removed (cleaned up) after the treatment. VES micelles are not stable at high temperatures and will rearrange thermally into nonviscous structures. The stability at high temperatures and fluid-loss property of VES fluids have limited their applications to fracturing and fracture-packing treatments. Nanoscience and nanotechnology have been used in many application areas, such as biomedical, pharmaceutical, space, and information technology. Nanotechnology represents the development and applications of materials, methods, and devices in which critical length scale is on the order of 1 to 100 nm and in which critical functionality is not a direct manifestation of the atomic or macroscale properties (Mokhatab et al. 2006). The laws that govern materials at nanoscale are different from those that have been accepted widely in larger scales. Some nanoparticles have been used in drilling fluids and have exhibited extraordinary rheological properties. Those advanced drilling fluids based on polymers that are physically or chemically associated with nanoparticles, as well as with with amphiphilic surfactants or polymers have been developed as stimuli-sensitive fluids. The fluid flow properties can be altered in response to a change in stimuli, such as temperature, salinity, and pH (Krishnamoorti 2006). The nanoparticles we used in this paper are less than 100 nm in size, with one select product with an average size of 35 nm. The nanoparticles are inorganic crystals with no solubility in water, oil, or solvent. This paper presents a nanotechnology application for maintaining viscosity at high temperatures and controlling the fluid loss of VES stimulation fluid without generating formation damage.

High-molecular-weight crosslinked polymer fluids have been used to stimulate oil and gas wells for decades. These fluids exhibit exceptional viscosity, thermal stability, proppant transportability, and fluid leak-off control. However, a major drawback of crosslinked polymer fluids is the amount of polymer residue they leave behind. Polymer residue has been shown to significantly damage formation permeability and fracture conductivity. 1–3 Recently, viscoelastic surfactant (VES) fluids composed of low-molecular-weight surfactants have been used as hydraulic fracturing and frac-packing fluids. The surfactants structurally arrange in brine to form rod-like micelles that exhibit viscoelastic fluid behavior. VES fluids, once broken, leave very little residue or production damage. However, excessive fluid leak-off and poor thermal stability has significantly limited their use. This paper will introduce newly developed, select nano-size crystals with unique surface charges and will explain how nanoparticle technology pseudo-crosslinks VES rod-like micelles together to improve the fluid loss control and proppant transport of VES fluids to a performance level similar to that of crosslinked polymer fluid. The nanoparticle pseudocrosslinked VES micelle fluid develops a wall-building pseudo-filtercake on the face of porous media to control fluid loss. When internal breakers are used to degrade the VES micelle structures the leaked-off VES fluid and the pseudo-filtercake breaks into brine water and nanoparticles. Since the nanoparticles are very small and readily pass through the pores of greater than 0.1 md formations, they are flowed back with the produced fluids, and no internal or external "solids" damage is generated. This paper will present laboratory data that shows how uniquely charged nanoparticles improve VES fluid rheology, leak-off control, and proppant suspension. Also presented are test results comparing nanoparticle enhanced VES to borate crosslinked guar polymer fluids. The mechanisms that enhance the performance of these fluids also will be discussed. Introduction Crosslinked polymer fluids (CPF) are the most common type of fluid used for hydraulic fracturing. These fluids can achieve high viscosities with low leak-off rates for a wide range of reservoir temperatures and permeabilities. With their efficient leak-off control, CPF can be used to generate excellent fracture geometry in most reservoirs. They also have excellent proppant suspension and placement capability. However, CPF have an inherent weakness that decades of developing internal breaker technologies have not been able to resolve: this weakness is the amount of fracture conductivity damage that occurs due to incomplete crosslinked-polymer filtercake removal from the fracture. A recent Joint Industry Project study has showed that polymeric filtercake thickness, and its yield stress, is one of the primary culprits to poor fracture cleanup and fracture conductivity loss when using crosslinked polymer fluids. 4

Over the past two decades, viscoelastic surfactant (VES) fluids used for gravel packing, frac-packing and conventional hydraulic fracturing have primarily relied on external or reservoir conditions to break the fluid's viscosity. Unlike polymeric fluids, no internal breakers have been used. Relying on external conditions to break VES fluid has been a point of contention and questionable, especially for dry gas applications. This paper describes how new internal breaker technology has been developed that allows VES fluids to be broken into easily producible fluids, without the need for contacting reservoir hydrocarbons. The reservoir pressure required to produce VES fluid is no longer compromised: very little pressure or time is required when an internal breaker is present. Mechanisms for internally breaking VES viscosity are discussed. New internal breakers have been found to work over a wide fluid temperature and mix water salinity range, and have compatible with newly developed VES stabilizers and fluid loss control agents. Presented are laboratory results that compare VES fluids with and without an internal breaker. Rheological data is presented that show the performance of internal breakers in degrading VES fluid viscosity, in particular at low fluid shear rates. Core clean-up tests were performed at 150°F, using 3% KCl brine and nitrogen gas as the displacement (clean-up) fluid. The results show that use of an internal breaker significantly improves VES fluid clean-up. This paper also presents VES system compatibility and proppant conductivity results. Introduction During the 1980's a VES fluid was developed for gravel packs1. Introduced with this fluid was the concept that no internal breaker was required. This same concept was also applied to VES fluids developed for frac-packing and conventional hydraulic fracturing during the 1990's2,3. The stated reason for not requiring internal breaker was that the VES micelles which impart fluid viscosity are sensitive to and will break upon contacting reservoir hydrocarbons, including gas hydrocarbons like methane, ethane and propane. During production the reservoir hydrocarbons are said to come in contact with the VES fluid in the pores of the reservoir and thereby will readily break the VES fluid viscosity and allow the once viscous fluid to then be easily removed. This concept of externally breaking VES fluids is still promoted today4. However, it should be noted that viable internal type breaker technology has not existed for VES fluids until recently5. With the introduction of internal VES breaker technology has come the ability to evaluate the need for such technology. This paper presents laboratory data for evaluating the cleanup of VES fluids with and without internal breaker. The data shows internal breakers may substantially improve VES fluid cleanup without the need for relying on an external breaking mechanism, and how VES fluids without internal breaker under certain conditions may be difficult to move and cleanup. Internal Breakers In a broad sense internal breakers are compounds placed within the VES fluid during surface mixing that will:go wherever the fluid goes;ensure the VES fluid breaks; andbreak the VES fluid into an easily producible fluid. From one point of view, using internal breaker is like adding the required amount of reservoir hydrocarbons to the VES fluid at the surface, ahead of time, so complete break in VES fluid viscosity will occur over time at reservoir temperature without the need for contacting reservoir hydrocarbons. The use of internal breakers should also improve:the rate and ease of VES fluid cleanup; andprevent viscous emulsions from forming when problematic reservoir hydrocarbons contact, mix with, and break VES-laden fluid in the reservoir.

Micellar fluids with viscoelastic behavior, or viscoelastic surfactant fluids, are used in the oil industry as completion and stimulation fluids. The viscoelastic (i.e. viscous and elastic) behavior of these fluids is based on the overlap and entanglement of very long worm-like-micelles. High fluid leak-off has, however, limited their application for hydraulic fracturing and frac-packing applications. Recent developments have found that micellar fluids can have wall-building leak-off control similar to crosslinked-polymer fluids (CPF) by the addition of a small amount of inorganic crystals that are less than 100 nanometers in size (i.e. nanoparticle). This paper will investigate the mechanism of fluid loss control for the particle-micellar fluid system. The tiny particles have high surface force, including van der Waals and electrostatic forces. The nanoparticle surface forces pseudo-crosslink the elongated micelles, which is similar to crosslinking polymers. The unique pseudo-crosslinking of surfactant micelles and particles has demonstrated improved viscosity, the formation of a filter cake, and enhanced thermal stability. Laboratory tests show the development of a filter cake significantly reduces the rate of fluid loss and demonstrates wall-building rather than viscosity-dependant leak-off control. Two models of static fluid loss control for particle-polymer fluid system will be reviewed. Fluid leakoff characteristics will be compared between the particle-micellar fluid system and particle-polymer fluid system. Uniqueness of this system is that the fluid loss control was achieved without compromising the internal breaking efficacy. Ordinarily additives 'plasticize' the micellar or colloidal aggregates. However, through our patented processes, we were able to successfully place the internal breakers inside the elongated micelles so as to not interfere with particles' pseudo-crosslinking. When internal breakers are activated by temperature, the elongated micelle structure collapses thereby significantly reducing the fluid viscosity and thus breaking down the filter cake into tiny particles. This results in clean breaking gel with reduced damage to formation permeability and fracture conductivity.

Viscoelastic surfactant (VES) based fluids are used in many applications in the oil industry. Their viscoelastic behavior is due to the overlap and entanglement of long wormlike micelles. The growth of these wormlike micelles depends on the charge of the head group, salt concentration, temperature, and the presence of other interacting components. The problem with these surfactants is that they are expensive and used at temperatures less than 200°F.The viscoelasticity of nanoparticle-networked VES fluid systems were analyzed by rotational and oscillatory viscometers. Apparent fluid viscosities were measured by using 2-4 vol% amidoamine oxide surfactant in 13 to 14.2 ppg CaBr 2 brines and 10.8 to 11.6 ppg CaCl 2 brines at different temperatures up to 275°F and a shear rate of 10 s -1 . The nanoparticles evaluated were MgO and ZnO at 6 pptg concentration. In addition, the effect of different nanoparticle concentrations (0.5 to 8 pptg) and particle size on the viscosity of VES fluid was investigated. The oscillatory shear rate sweep (100 to 1 s -1 ) was performed for the 4 vol% VES in 14.2 ppg CaBr 2 from 100 to 250°F.This study showed that the addition of nanoparticles improved the thermal stability of VES micellar structures in CaBr 2 and CaCl 2 brines up to 275°F and showed an improved viscosity yield at different shear rates. Micron and nano-size particles have potential to improve the viscosity of VES fluids. Lab tests show for VES micellar systems without nanoparticles, the dominant factor is the viscous modulus but when nanoparticles are added to the system at 275°F the elastic modulus becomes the dominant factor. These positive effects of nanoparticles on VES fluid characteristics suggest that these particles can reduce treatment cost and will extend the temperature range of the surfactants to 275°F.