Surfactant Flooding 2 *: The Effect of Alkaline Additives on Permeability and Sweep Efficiency

This paper is the second of a series of papers reporting our examinations of the effects alkaline additives have on dilute surfactant systems for low-tension waterflooding (LTWF). The first paper outlined the effects on interfacial tension (IFT), hardness removal, and surfactant retention by the core material. and how these parameters then affect overall recovery of oil from watered-out cores containing, high-hardness brines. This study examines the effects of those chemicals on permeability, sweep efficiency. and sweep symmetry through multipermeable noncommunicating zones. Correlations and possible mechanisms are offered that relate these findings to the earlier work on surfactant retention and hardness removal. The results of these studies indicate that each alkali behaves differently, but all are capable of enhancing the action of the dilute surfactant treatment. Sweep efficiency in three-dimensional (3D) patterns and sweep symmetry through multipermeable noncommunicating zones is increased by the alkaline chemicals. Selective permeability reduction, caused by the reaction with the residual hardness ions. is suspected as a mechanism. Overall, sodium silicate addition to the surfactant flood as a builder was found to produce the best performance because of its ability to inhibit surfactant retention, thereby increasing the recovery of crude before selective permeability reduction occurs. Overall permeability loss is only about 20 to 25% in a core initially containing 4.800 ppm of hardness as CaCO under our experimental conditions. Introduction The effect of surfactants in enhanced oil-recovery (EOR) systems is of great interest to those concerned with designing cost-effective processes to recover residual oil after waterflooding. Earlier work in this area shows surfactants playing, a role in three types of processes: alkaline flooding, where an alkali is added to a reservoir to form in-situ petroleum surfactants; LTWF, where a dilute surfactant solution containing a sacrificial inorganic agent is injected to form a tertiary-oil bank, and finally micellar/polymer flooding, where a surfactant/crude-oil slug, miscible with reservoir crude, is injected into a formation. In each case, a primary consideration is optimization of the effectiveness of the surfactant. Surfactant performance is impaired by any or all of these phenomena: complexation with multivalent metal ions in the reservoir or injection water, association with other surfactant- molecules and/or "sorption" into the reservoir substrate. This latter effect can be enhanced by the sorption of reservoir metal ions onto the substrate surface. providing added active sites for anionic surfactant interaction. It is obvious that surfactant and metal ions play an antagonistic role in contributing to the effectiveness of the oil-recovery process, and, therefore, considerable efforts are expended in development of reservoir conditioning stages (preflushes) before injection of a more expensive surfactant-containing or -generating slug. Detergency technology teaches that certain inorganic chemicals, most predominantly sodium silicate, sodium phosphate, and sodium carbonate (usually called builders) can improve surfactant's performance by minimizing the harmful effects of multivalent metal ions. Holm and Robertson have shown that sodium orthosilicate, when used as a preflush in their micellar/polymer system, improves residual oil recovery significantly over that by NaCl. Feuerbacher and Smith developed the use of these builders, preferably NaOH or sodium metasilicate, as preflush agents before LTWF. The use of the more alkaline builders-e.g., sodium orthosilicate in alkaline flooding -has been known since Nutting's) work in 1925. SPEJ P. 983^