Nanotechnology Applications in Viscoelastic Surfactant Stimulation Fluids

Abstract

Abstract Viscoelastic surfactant (VES) fluids have been widely used in the oil industry as completion and stimulation fluids. The surfactants structurally arrange to form rod-like micelles that increase VES fluid viscosity for regular fracturing and frac-packing fluids. However, high fluid leak-off and low viscosities at elevated temperatures have limited VES fluids for hydraulic fracturing and frac-packing applications. This paper will introduce a nanotechnology application for maintaining viscosity at high temperatures and controlling the fluid loss of VES fluid, without generating formation damage. The nanometer-scale particles studied display unusual surface morphologies and have high surface reactivity. These nanometer-scale particles, through chemisorption and surface charge attraction, associate with VES micelles to:stabilize fluid viscosity at high temperatures; andproduce a pseudo-filtercake of viscous VES fluid that significantly reduces the rate of fluid loss and improves fluid efficiency. When internal breakers are used to break the VES micelles, the fluid will dramatically lose its viscosity and the pseudo-filtercake will then break into nanometer-sized particles. Since 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, leak-off and core flow tests will be presented for the VES fluid systems at temperatures 150°F and 250°F. Introduction Viscoelastic surfactant (VES) fluids have been widely used as gravel-packing, frac-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 which exhibit viscoelastic behavior to increase fluid viscosity 1~3. Traditionally industry depends on external breakers (or reservoir conditions) to break VES fluid after treatment is completed. The two primary external conditions have been:contact with reservoir hydrocarbons; andcontact and dilution with reservoir brine 3. 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 4. 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 so that it easily cleans up, allowing oil and gas to flow to wellbore to be produced. Internal breakers generate VES breaking compounds over time which penetrate and collapse the viscous, rod-like VES micelles into non-viscous, more spherical micelles, and the technology allows the VES breaker to accompany the VES fluid during a frac-pack or regular fracturing treatment to enhance and ensure breaking and cleanup of the VES fluid from the reservoir 5~6. VES fluids are unlike polymer-based systems in that they are non-wall-building and do not form filtercake on the formation face during hydraulic fracturing and frac-packing treatments. Without filtercake development, the amount of VES fluid leaked off from the fracture into formation during a fracturing treatment is primarily fluid viscosity dependent. Due to its non-wall-building property, VES fluid exhibits very high fluid leak-off from the fracture during a treatment and "screening out" is a common problem. Because of very poor fluid efficiency of VES fluid:the permeability of a reservoir is less than 400 md for most cases ;more total fluid volume is required for a given treatment;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 thermally rearrange into non-viscous structures. The stability at high temperatures and fluid loss property of VES fluids have limited their applications for fracturing and frac-packing treatments.