Summary
Waterflooding recovers little oil from naturally fractured carbonate
reservoirs if the matrix is oil-wet and fracture intensity is high. Laboratory
experiments and mechanistic simulations have been conducted to understand the
injection of dilute anionic surfactant solutions into oil-wet, fractured
reservoirs. In this process, surfactant diffuses into the matrix, lowering the
interfacial tension (IFT) and contact angle, which decreases the capillary
pressure and increases oil relative permeability, enabling gravity to drain up
the oil. The rate of oil recovery increases with an increase in matrix
permeability, a decrease in initial gas saturation, a decrease of fracture
height or spacing, and an increase in the wettability-altering capabilities of
the surfactant. Increasing the surfactant concentration does not necessarily
enhance the oil recovery rate, because IFT and wettability alterations are not
linearly related to surfactant concentration. Adsorption of anionic surfactants
on calcite can be suppressed with an increase in pH and a decrease in
salinity.
Introduction
Approximately 60% of the world’s oil is found in carbonate reservoirs (Akbar
et al. 2000). Recovery from reservoirs depends on reservoir heterogeneity, oil
quality, drive mechanisms, and reservoir management. Many carbonate reservoirs
are naturally fractured and oil-wet/mixed-wet (Roehl and Choquette 1985,
Chillenger and Yen 1983). Such reservoirs are difficult to produce after the
primary production if the fractures form a connected network (Allan and Sun
2003). Waterflooding is effective only if the formation is water-wet. Flooding
processes do not work in general, because large, viscous gradients cannot be
imposed. Gravity drainage (surfactant, gas, and thermal) techniques can be
applied, but the recovery is slow. Surfactant-enhanced gravity drainage and
imbibition processes are being developed (Yang and Wadleigh 2000; Austad and
Milter 1997; Standnes and Austad 2000a, 2000b, 2003a, 2003b, 2003c; Xie et al.
2005; Seethepalli et al. 2004; Hirasaki and Zhang 2004) to improve oil recovery
from oil-wet/mixed-wet, fractured carbonate formations and are the subject of
this study.
Cationic surfactants of the type alkyl trimethyl ammonium bromide, CnTAB,
are effective [recovery approximately 70% original oil in place (OOIP)] in
imbibing water into originally oil-wet chalks at concentrations greater than
their critical micellar concentration (approximately 1 wt%) (Austad and Milter
1997; Standnes and Austad 2000a, 2000b, 2003b). Cationic surfactants form ion
pairs with adsorbed organic carboxylates of the crude oil, and solubilize them
into the oil thereby changing the rock surface to be water-wet. This
wettability alteration can lead to countercurrent imbibition of brine and,
thus, to oil recovery. The IFT between the surfactant solution and oil are not
low (> 0.1 dynes/cm). Several cheaper cationic surfactants of the form
C10NH2 and bioderivatives from the coconut palm, termed Arquad and Dodigen
(priced at 3 USD/kg), have been identified (Standnes and Austad 2003c, 2003a;
Strand et al. 2003). The two key problems with this method are still the high
concentration and the high surfactant cost.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
17 February 2006
- Meeting paper published:
22 April 2006
- Revised manuscript received:
27 May 2007
- Manuscript approved:
10 June 2007
- Version of record:
25 February 2008
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