Publication Details (including relevant citation information):
Welton, Thomas Donovan, Bryant, Jason, Funkhouser, Gary P. -
Abstract: Abstract Typical viscoelastic surfactant gels used for gravel packing and hydraulic fracturing have relied upon cationic, amphoteric, or zwitterionic surfactants as their primary gelling agent, complicating the stimulation of some wells due to wettability or formation fluid compatibility issues. This paper describes an anionic viscoelastic surfactant developed for fracturing and gravel packing applications. The performance of the gel as a function of temperature, salt type and concentration, and cosurfactant will be presented along with the chemical structure/activity relationship. Rheological measurements demonstrate that fluids made with the new anionic surfactant have effective performance over a range of conditions encountered in treating wells. Introduction Surfactant-gelled fluids have been used for fracture acidizing, matrix acidizing, gravel packing, frac packing, and hydraulic fracturing since the mid-1970s.1-4 One of the perceived benefits of these fluids is that they are considered less damaging than polymer-based fluids. The fluid's rheology is heavily influenced by temperature, counter ions (salts), co-surfactants, and contamination. Because the processing of surfactants does not yield micron-sized impurities often observed in guar gum, and the molecular weights of surfactant molecules are much smaller than guar molecules, surfactant gels do not form a filter cake, and thus have higher leakoff rates into the reservoir. Surfactant gels typically lose their viscosity by contact with hydrocarbons or by dilution with formation fluids. However, this often also results in an undesirable emulsion being formed. Another potential problem of some cationic surfactant gels is that they may adversely alter the wettability of sandstone formations. Current gel-forming surfactants are predominantly cationic (such as a quaternary amine) or amphoteric/zwitterionic (such as betaine). A new class of anionic surfactants has been discovered to form surfactant gels.5-8 Methyl ester sulfonates (MES) have been commercially available since the mid-1960s; however, their ability to form surfactant gels was not discovered until recently. To date, the only known oilfield uses of MES are antisludging, dispersing, and demulsification.8, 9 MES is prepared by the addition of sulfur trioxide to the a-carbon of a methyl ester and subsequently neutralized with a base (Fig. 1). There are three drivers for commercialization of MES. First, it is less expensive than a-olefin sulfonate. Second, it is derived from renewable resources such as palm kernel oil. Finally, it has a better environmental profile than many surfactant gels because it is biodegradable and exhibits low aquatic toxicity.10 Rheology Preliminary rheological studies were performed on three laboratory samples prepared as follows: Sample 1 Mixing water with an MES surfactant B in an amount of approximately 5% by weight of the sample with approximately 5% sodium chloride, with the final sample pH not adjusted. Sample 2 Sample 1 in alkali form, with the pH adjusted to approximately 10 using sodium hydroxide. Sample 3 Sample 1 in acidic form, with the pH adjusted to approximately 4 using hydrochloric acid. Once the samples were prepared, rheological responses of each fluid were measured using a Haake RheoStress RS150 stress-controlled rheometer fitted with a 60-mm diameter, 2 cone and plate. The temperature was held constant at 77 F. A constant frequency (1 Hz) oscillatory stress sweep was performed over a broad stress range to obtain the storage modulus (G'), loss modulus (G''), and phase angle (d). Results are shown in Figs. 2, 3, and 4 for the unadjusted, alkaline, and acidic samples, respectively. In the linear viscoelastic limit, the storage modulus of the sample where the pH was unadjusted is about 20 Pa. Significantly higher degrees of elasticity were observed for the two samples that had the pH adjusted, because the storage moduli for the alkali and acidic solutions were approximately 38 Pa and 55 Pa, respectively.