The fluids commonly used in frac-pack treatments were initially designed for either fracturing low permeability formations or for conventional gravel pack operations. These fluids do not address the unique problems found when fracturing higher permeability formations. Conventional fracpack fluids (borate crosslinked guars and HEC) undergo whole polymer leakoff, and create a deep invasion bank of concentrated polymer that is very difficult to break and can lead to high formation damage.
Viscoelastic surfactant (VES) fracturing fluids have been developed which exhibit excellent rheological properties and still maintain low formation damage characteristics even in high permeability formations. These fluids are completely polymer free and do not rely on internal chemical breakers to degrade the fluid viscosity.
This paper will detail the results of recent laboratory studies on viscoelasitic surfactant fracturing fluids, and will compare the rheological and formation damage properties to other conventional fracturing fluids. The laboratory findings will be substantiated by several field case studies.
For many years fatty amine quaternary ammonium salts have been used as thickeners for consumer products (e.g., bleach, liquid dishwasher detergent). Such materials, called viscoelastic surfactants, are a class of compounds that form micelles in an aqueous system containing certain cations, and impart viscoelastic properties to the liquid. The deformation of such systems is time dependent (i.e., the system acts as a solid unless a sufficient amount of shear has been applied for a certain length of time). When the system deforms, the rheological behavior is nearly Newtonian (Fig. 1).
The texture of such fluids at rest is similar to that of gelatin: therefore, they are excellent particle-suspension media, and have been used for several years in gravel packing applications.1 The surfactant is added to common completion brines, and the resulting fluid effectively suspends sand/proppant. The concentration of surfactant required to provide adequate suspension varies from 2.5% to 6% (by volume), depending on the anticipated fluid temperature. VES fluids are very easy to prepare in the field. A simple dilution of the concentrate in brine, with minimal agitation, is all that is required.
Viscoelastic (VES) fluids have more recently found application as a fracturing fluid for high permeability formations.2 As shown in Fig. 2, the leakoff behavior of VES fluids is less dependent upon differential pressure than HEC fluids. At fracturing pressures, the fluid-loss rates of VES fluids are lower than those observed with HEC fluids. At production rates, VES fluids should be more mobile than HEC fluids.
The viscosity of VES fluids can be broken via two mechanisms:
contact with oil or condensate: and
reduction of the salt concentration. Since one or both scenarios occur during cleanup in any well, no additional breaker chemicals are required.
The principal advantage of a VES fluid is that, unlike polymer-based systems such as HEC or guar, very little residue is left upon breaking. As a result, less formation damage is observed. Typical VES fluid performance is illustrated in this paper, including rheology, fluid-loss and core-cleanup data.