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.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
15 March 2007
- Meeting paper published:
30 May 2007
- Revised manuscript received:
23 January 2008
- Manuscript approved:
7 May 2008
- Published online:
2 March 2009
- Version of record:
26 February 2009
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