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This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 113533, "Performance Enhancements of Viscoelastic- Surfactant Stimulation Fluids With Nanoparticles," by James B. Crews, SPE, and Tianping Huang, SPE, Baker Hughes, originally prepared for the 2008 SPE Europec/EAGE Conference and Exhibition, Rome, 9-12 June. The paper has not been peer reviewed. The full-length paper introduces newly developed, select nanosized crystals with unique surface charges and explains how nanoparticle technology pseudocrosslinks viscoelastic surfactant (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 a crosslinked-polymer fluid (CPF). The nanoparticle-pseudocrosslinked VES-micelle fluid develops a wall-building pseudofilter cake on the face of porous media to control fluid loss. Introduction CPFs are the most common type of fluid used for hydraulic fracturing. These fluids can achieve high viscosities with low leakoff rates for a wide range of reservoir temperatures and permeabilities. With their efficient leakoff control, CPFs can be used to generate excellent fracture geometry in most reservoirs. They also have excellent proppant-suspension and -placement capability. However, one weakness of CPFs is the fracture-conductivity dam-age that occurs as a result of incomplete crosslinked-polymer filter-cake removal from the fracture. Over the past decade, classical VES-fluid systems have been used for frac packs and conventional hydraulic fracturing. The composition of these fluid systems typically has been fresh water, salt [such as 4% potassium chloride (KCl)], and VES product. The classical VES-fluid systems have viscosity-dependant leakoff control into porous media and do not develop or leave filter cake on a fracture face. The advantage of this type of leakoff control is that no filter-cake damage occurs in the fracture. However, a disadvantage is that a significantly high amount of whole-gel leakoff occurs into the formation during a treatment, and most often insufficient treatment fluid remains in the fracture for generating proper fracture geometry. Additionally, no internal breaker technology has existed for VES systems until recently. The reliance on the external breaking mechanism has resulted in too frequent poor and incomplete VES-fluid cleanup from the treated reservoir. Crosslinked Polymer vs. Pseudocrosslinked Micelles Two key parameters for crosslinking polymers in an aqueous medium is the molecular weight of the polymer and the degree of polymer overlap. The more polymer overlap there is, the more intrapolymer (polymer-to-polymer) crosslinking that can occur. In almost all cases, crosslinking polymers significantly improves performance properties for hydraulic fracturing, such as much higher fluid viscosity, improved thermal stability, better proppant transport, and lower rate of fluid leakoff.
Abstract 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
Pulsed neutron carbon/oxygen measurement is often the preferred tool in cased hole logging to distinguish hydrocarbon from water in freshwater or low salinity formation environments. However, the existence of production tubing and pipe requires that the d