Nanotechnology Applications in Viscoelastic Surfactant Stimulation Fluids

Abstract

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.

Analytical Evaluation of Nanoparticle Application To Mitigate Fines Migration in Porous Media

“…To prevent this phenomenon, pretreating of the porous medium (e.g., frac pack) with nanofluid before fine detachment/suspension exposure may lead to an effective remedy to fixate the injected or mobile fines near their original location. This prevents the fines from moving downstream (Huang et al 2008a). In Fig.…”

“…To verify the MOC solutions of two different scenarios, we apply both numerical simulations with STARS, the CMG's three-phase multicomponent simulator, and laboratory experiments (Huang et al 2008a). Motivated by the similar treatments to expand conventional simulator's capabilities to address shale gas problems , this paper defines the mutual relationships among nanoparticles, fines particles, and the complex of nanofine particles as two fictitious chemical reactions.…”

“…For the verification of the Scenario II, we apply our analytical solutions to match the existing laboratory experimental results from Huang et al (2008a). Inferred from their laboratory experiments, some experimental parameters are concluded as follows (i.e., flow velocity 1.7 Â 10 À4 m/s, length of permeable medium 0.33 m, the injected-fines concentration 5.2 Â 10 À5 m 3 /m 3 , and nanoparticle concentration 3.0 Â 10 À6 m 3 /m 3 ).…”

“…The experimental works (e.g., Huang et al 2008a) have proved an excess capability of rock to capture fines particles in the presence of nanoparticles. We model this observation with a two-step process: (1) adsorption of nanoparticles onto the particle/grain surface and (2) additional attachment of fines; doing so, we are able to use MOC to study the problem.…”

Synthesis and application of nonionic polyacrylamide with controlled molecular weight for fracturing in low permeability oil reservoirs

“…8,9 In high-temperature or ultra-deep wells, foamed and viscoelastic surfactant fluids have low viscosities and cannot effectively transport proppant without the use of a crosslinker, resulting in treatment failures. [10][11][12] To overcome these problems, gelled fluids based on synthetic polymers with good proppant transport properties, shear and high temperature resistance, low fluid loss properties, and good cleanup properties have been studied. Generally, low-cost zirconium crosslinked polymer gel fluids with lower toxicity are used in low permeability reservoirs.…”