Thomas Welton - Application of Cationic Surfactant-Based Fluids for Acid Diversion

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      Nasr-El-Din, Hisham A., Al-Nakhli, Ayman R., Al-Driweesh, Saad   M., Welton, Thomas D., Sierra, Leopoldo, Van Domelen, Mary S. -

      Abstract: Summary This paper examines the use of   surfactant gels during matrix acid treatments and describes field   trials of these fluids. Unlike available viscoelastic surfactants   used today in the field, this surfactant is cationic. If used in   live acids, the fluid has a relatively low viscosity when pumped.   Once the acid is spent, however, the surfactant molecules   increase its viscosity significantly. To enhance diversion   further, the acidic fluids or brines can be foamed with this   surfactant. Rheological measurements were conducted on   Hastelloy -fitted rotational viscometers at temperatures ranging   from 70 to 300 F. The effects of surfactant concentration, shear   rate, temperature, and acid additives on the apparent viscosity   of various surfactant-based fluids were investigated in detail.   The surfactant was stable thermally and hydrolytically with most   acid additives. While it was compatible (i.e., still formed a   viscosifying gel), some additives adversely affected the apparent   viscosity of surfactant solutions at a given temperature. The   apparent viscosity of surfactant solutions increased with salt   concentration and can be predicted by use of the Carreau-Yasuda   model. Coreflood tests indicated that the surfactant delayed acid   breakthrough in calcite cores. Acceptable corrosion rates were   obtained when this surfactant was added to the acid. The   performance of this surfactant was validated with field   treatments. The surfactant was used in more than 100 matrix acid   treatments (oil producers and water injectors). It was used to   increase the viscosity of acids in situ and enhance the stability   of foams used for diversion. All wells responded positively to   these treatments, and no operational problems were encountered.   Downhole gauges confirmed the ability of surfactant-based fluids   to divert the acid into various zones. Introduction Surfactant   gels have been used in matrix acidizing, fracture-pack, and   hydraulic-fracturing treatments since late 1970s (Norman 1978;   Leggett et al. 1982; Samuel et al. 2001). During acidizing   treatments of carbonate reservoirs, the acid will flow into the   most-permeable or least-damaged zones. The acid will form highly   conductive channels or "wormholes" (Schechter 1991; Samuel et al.   2001). Most of the fluid will flow into the path of least   resistance, leaving large portions of the target zones untreated.   Therefore, diversion plays a key role in matrix acid treatments.   This diversion can be accomplished through mechanical or chemical   means, or both (Mohammed et al. 2005; Chang et al. 2007;   Nasr-El-Din and Samuel 2007; Nasr-El-Din et al. 2006a, 2006b,   2006c; Baheiri and Nasr-El-Din 2007). Polymer gels, foams,   oil-soluble resins, rock salts, and surfactant gels are some of   the chemical means available (Chang et al. 2007). Surfactant gels   have become an increasingly popular choice for viscosifying   acidic fluids and brines (Nasr-El-Din et al. 2003; Mohammed et   al. 2005; Fu and Chang 2005; Nasr-El-Din and Sammuel 2007; Chang   et al. 2007; Nasr-El-Din et al. 2006a, b, c; Baheiri and   Nasr-El-Din 2007; Cawiezel and Dawson 2007). Extensive laboratory   testing was conducted to understand better how viscoelastic   surfactants work in the field. It was found that the rheological   properties depend on numerous factors such as surfactant   concentration, shear rate, temperature, acid concentration,   solvents, salt type and concentration, and other acid additives   such as corrosion inhibitors (Nasr-El-Din et al. 2008).   Surfactant gels can be prepared with anionic, cationic, or   amphoteric surfactants (Nasr-El-Din et al. 2003). Amphoteric   surfactants have been the subject of several laboratory   (Nasr-El-Din et al. 2008) and field studies (Nasr-El-Din et al.   2006b,c; Nasr-El-Din and Samuel 2007). The present study will   focus on cationic viscoelastic surfactants, which present several   advantages to carbonate acidizing. Unlike amphoteric viscoelastic   surfactants, cationic surfactants have a much higher activity   (nearly 75 wt% as compared with 30-40 wt% for amphoterics), do   not require cosurfactants or additives to perform adequately, and   are safe because no methanol is needed to enhance performance.   Cationic surfactants are compatible with most corrosion   inhibitors. Finally, by virtue of being cationic, this surfactant   will propagate in carbonate formations with minimum loss   resulting from adsorption onto the rock surface. The objectives   of this study are to (1) examine the effects of simple inorganic   salts and additives on the rheological properties of various   solutions that contain a cationic-surfactant gelling agent, (2)   assess its impact on acid propagation in carbonate cores, and (3)   validate their effectiveness with field trials.

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