This report covers the August 15 to November 15, 2000 period and summarizes our current IPEC studies. Surfactant enhanced soil washing can result from two distinct mechanisms that are an outgrowth of fundamental surfactant properties: one occurs above the critical micelle concentration (solubilization) and the other occurs below the critical micelle concentration (roll-up mechanism). Above the CMC (critical micelle concentration), surfactant molecules aggregate into micelles, which are basically spheres with a water-like exterior and an oil-like interior. Surfactant enhanced solubilization results from contaminant partitioning into the hydrophobic interior of these surfactant micelles.
In the roll-up mechanism, surfactant accumulation at the oil-water interface increases the contact angle between the oil and the cuttings. The repulsion between the monomer head group and the solid surface promotes separation of the oil from the cuttings. This mechanism is analogous to what we use in washing our clothes and can be carried out using standard oil field equipment (trommels, sand screws, shaker tables, and centrifuges). Convective currents created by agitation and abrasion provide the energy necessary to create additional surface area of the oil phase and thus displace the oil from the cuttings. The roll-up mechanism is augmented by reduction in oil/water interfacial tension because the oil droplet becomes elongated and is more easily ruptured by macroscopic forces like agitation or shear from scrubbing. Since ease of emulsification is inversely correlated to oil/water interfacial tension, interfacial tension reduction generally enhances detergency by the roll-up mechanism. Figure 1 shows oil/water interfacial tension for low surfactant concentrations (0.025 wt %) as a function of electrolyte (NaCl) concentration for three surfactants. The Isofol 145- 4PO is a branched alcohol propoxylate sulfate purchased from Condea Vista. Dowfax 8390 is an alkyl diphenyl oxide disulfonate purchased from Dow Chemical. The surfactant purchased from Lubrizol is an alkyl succinic anhydride-taurine adduct. Steol is a propoxylated sulfate with a linear alkyl tail. The isofol 145-4 PO effected the lowest oil/water interfacial tensions for sub-cmc concentrations, and it was therefore chosen for sub-CMC washing experiments using the roll-up mechanism. Figure 2 shows results of washing drill cuttings with a low concentration of isofol (0.1 wt %). Washing experiments compare the low [isofol] solution to water alone (no surfactant) for two initial oil contents of 10 wt. % and 20 wt. %. The vast majority of liberated oil using sub-CMC [surfactant] is free phase as opposed to solubilized/emulsified when using supra-CMC [surfactant]. For both initial oil contents of 10 wt % and 20 wt %, the final oil contents are the same (8.0 wt %). The enhancement of sub-CMC [isofol] over water alone is evident for both initial oil contents.
Figure 1. Oil ((-olefins)/water IFT for sub-CMC surfactants as a function of [NaCl].
Figure 2. Washing results using the roll-up mechanism for two initial oil contents. The red are results for sub-CMC [isofol] = 0.1%. The blue is for water only. The final oil content of the cuttings is 8.0 wt% independent of the initial oil contents.
The roll-up mechanism using only sub-CMC [surfactant] and appropriate [electrolyte] reduces the oil content of the cuttings to 8% while solubilization reduces the oil content of the cuttings to 0.1%. Thus, solubilization reduces the oil content to a value that is two orders of magnitude lower than the roll-up mechanism using only surfactant and electrolyte. For an initial oil content of 20 wt %, water-only washing reduces the oil content to 14% demonstrating that sub-CMC [surfactant] is a great improvement over water alone.