Effects Of Connate Water Composition On Interfacial Tension Behaviour Of Surfactant Solutions
Authors: Kumar A. , Neale G. , Hornof V. 
Published: March 2010

Abstract
The present work establishes that the composition of the connate waters occurring in petroleum reservoirs can have significant effects on the interfacial tension behaviour between crude oil and surfactant solutions. Synthetic connate waters comprising binary and ternary mixtures of NaCI, CaCI2 and MgCI2 have been employed and their effects on the interfacial tension behaviour of crude oil have been studied for two different surfactant formulations (one containing petroleum sulfonate alone, the other containing petroleum sulfonate and lignosulfonate).It has been found that Ca++ and Mg++ ions are very detrimental to the interfacial tension behaviour of surfactant solutions, and that the data obtained are easier to interpret in terms of Ionic Strength than in terms of ionic concentrations. It is believed that these findings will have significant implications in the design of laboratory experiments which seek to elucidate the interactions which occur between crude oil and surfactant solutions in real petroleum reservoirs.

Introduction
The use of surfactant solutions in, Enhanced Oil Recovery (EOR) operations has been studied extensively in the past(3–4) and some comprehensive review articles have recently been published(5–8). Foster(9) established that for an efficient EOR process, the capillary number should be of the order of 0.01; this can be readily achieved by reducing to an ultra-low value the IFT at the oil/water interface. Various formulations (containing petroleum sulfonate surfactants, alcohols and salt) which are capable of reducing IFT have been reported in the literature(I-4). Work in our laboratories has centred on replacing part of the expensive petroleum sulfonate by cheaper lignosulfonates which are obtained as by-products from the pulp and paper industry. It has been observed that lignosulfonates can interact synergistically with petroleum sulfonates producing significant additional lowerings in IFT (10–12). More recent studies(l3,14) have shown that surfactant formulations containing spent sulfite liquor solids (SSLS) can likewise generate ultra-low 1FT's against crude oils. These solids, which contain lignosulfonate as their principal constituent, are obtained from the spent sulfite liquors which are discharged in the effluent streams from many Canadian pulp and paper mills.
Most previous workers have studied the IFT behaviour of surfactant formulations in the presence only of Na + and CI- ions, despite the fact that the connate (or interstitial) water in real petroleum reservoirs contains numerous other ions (notably Ca+ + and Mg+ +). In this context, Jennings(15) and Cooke(6) have reported that the presence of Ca+ + and/or Mg + + ions leads to drastic increases in IFT. Because the IFT behaviour of surfactant formulations can change significantly in the presence of even small amounts of divalent ions, the effects of connate water composition should be taken into consideration in all laboratory studies involving-phase behaviour and IFT measurements. In the related area of laboratory oil displacement studies, several workers have employed synthetic brines containing divalent ions (17–19). Dranchuk et al. emphasized the importance of connate water composition and chose to use original field waters in their displacement experiments(11).