Optimization of a Viscoelastic Surfactant (VES) Fracturing Fluid for Application in High-Permeability Formations

Abstract

Visco-Elastic Surfactant (VES) fluids are polymer-free fluids that generate viscosities suitable for fracturing operations without the use of polymer additives. VES fluids do not form polymer filter-cake, and thus, viscous resistance of the fluid flowing through the rock matrix primarily governs fluid loss. This has historically limited the application to fracturing reservoirs with low permeabilities. A new VES fracturing fluid has been developed for use in high permeability reservoirs and successfully pumped in the Gulf of Mexico. The fluid exhibits enhanced fluid efficiency while still maintaining the high proppant pack conductivity associated with the lack of polymer damage. In this paper, laboratory test results for the new fluid are presented along with three high-permeability case histories. The estimated reservoir permeabilities were as high as 167 mD and reservoir heights ranged from 30 -90 feet. In all cases the entire propped fracture design was successfully placed. Introduction Visco-elastic surfactants have been used in oil and gas wells for fracturing stimulation for over ten years1. During this time the technology has evolved from a niche application in gravel packing2 to a mainstream range of applications where clean proppant packs and gravel packs are desired3. In the latest form presented here, a VES fluid utilizing a new surfactant has been formulated and optimized for high permeability operations. Fluid Rheology Viscosity of a VES fluid is created by self-assembly of surfactant molecules in an aqueous solution. Hydrophobic tails of surfactants associate and orient to create rod shaped structures commonly referred to as micelles. Entanglement of these flexible micelles imparts viscosity to the solution as shown in Figure 1. A variety of surfactant types can be used for formulating VES fluids, including anionic surfactants, cationic surfactants, and zwitterionic surfactants4. The new VES fluid presented here utilizes a specially formulated zwitterionic surfactant that creates stable micelles with unusual high-temperature stability. Figure 2 presents the fluid rheology as a function of temperature for different levels of surfactant package concentration. The surfactant package creates useful rheology within concentrations ranging from 3.5% to 6%. The exact surfactant concentration depends on the bottom hole temperature and desired fluid viscosity. Fluid Breaker The VES fluid can break to water-like viscosity by exposure to liquid hydrocarbons or dilution with reservoir brines. Additionally, a new encapsulated breaker has been developed for the zwitterionic VES described here. The encapsulated material uses a polyelectrolyte to disrupt the surfactant micelles and lower the fluid viscosity even in dry gas wells where there is neither brine nor liquid hydrocarbon to assist the breaking process. The effectiveness of this breaker has been measured in a series of proppant pack conductivity tests demonstrating greater than or equal to 95% retained proppant pack permeability. Laboratory Fluid Loss Measurements To model the fluid loss properties, the VES system was injected into 12-inch long sandstone cores at a constant driving pressure of 1000 psi. The cumulative fluid volume flowing into the core was measured as a function of time and the total fluid loss coefficient vs. permeabilities was plotted in Figure 3. Also shown in Figure 3 is the typical trend for total fluid loss coefficient of crosslinked polymer fluids. The results indicate that the fluid loss coefficient for the VES system is comparable to that achieved with crosslinked polymers.

Performance and application of new anionic D3F-AS05 viscoelastic fracturing fluid

“…VES fluids have also been widely used for fracturing high-permeability formations; used as gravel-packing fluids and frac-packing fluids; they exhibit excellent rheological properties and maintain low formation damage characteristics compared with crosslinked-polymer fluids (McGowen et al, 1993;Mathis et al, 2002;Sullivan et al, 2006, Daren Journal of Petroleum Science and Engineering 78 (2011) 131-138 et al, 2008. The use of VES technology is now extended to other oilfield applications, such as filter cake removal and coiled tubing clean out (Parler et al, 1999).…”

“…Recently, Sullivan et al (2006) developed a new type of zwitterionic VES fluid that extends the initial fracturing temperature range from below 65°C to 120°C. Another investigation was undertaken by Daren et al (2008).…”

Recent Advances in Viscoelastic Surfactants for Improved Production From Hydrocarbon Reservoirs

“…Chemical diverting agents are also available for acid diversion. Use of foamed fluids like foamed KCl solutions, foamed ammonium chloride solutions, and gelled pills were reported by Zerhbouh (1993), Zeilinger et al (1995), and Taylor and Nasr-El-Din (2001). The main limitation of these foamed and gelled systems is their instability at temperatures greater than 200 F and their ineffectiveness in plugging formations with permeabilities greater than 500 md (Alleman et al 2003).…”

Enhancing the Oil Recovery from Naturally Fractured Reservoirs Using Viscoelastic Surfactant (VES) Flooding: A Field-Scale Simulation

“…Most of the research studies conducted for VES were for fracturing applications , while very limited studies were for EOR . However, limited simulation studies were performed to assess the effectiveness of VES treatment on a large scale, especially for naturally fractured reservoirs (NFRs).…”