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Because of unfavorable wetting conditions much residual oil is left when a porous material is flushed by water. Methods suggested to change reservoir wetting to improve oil displacement efficiency are generally expensive. The present laboratory study was undertaken to gain all understanding of the factors which determine reservoir wettability, and to find out if oil displacement efficiency might be improved by a wettability change accomplished at low cost in an oil reservoir. Contact angle measurements were made on mineral surfaces using several sets of reservoir oil and water samples. Results of the contact angle studies suggest that reservoir wettability may be primarily determined by natural surface-active substances present in the reservoir fluids. The effect of changing salinity and pH of the water phase was studied. The results suggest that gross changes in preferential wettability might be accomplished by injection of water containing simple chemicals to alter pH or salinity in the reservoir. Such treatment could be much less expensive than injection of commercial surface-active agents. Waterflood tests have also been made using synthetic cores and oil and water having wetting characteristics similar to those of reservoir fluids. Cores initially oil-wet were flooded in such a way that they were made preferentially water-wet by the advancing flood water. This reversal in preferential wettability achieved greater oil displacement efficiency than when either oil-wet or waterwet conditions were maintained throughout the flood. For the systems studied, the higher the oil viscosity the greater the percentage improvement obtained over conventional waterflood recovery. This suggests that a flooding process making use of wettability-reversal may extend the oil viscosity range over which water flooding is attractive. Because a precise adjustment of reservoir wettability does not seem to be required, and because altering the pH or salinity in some reservoirs may be inexpensive, it appears that a waterflooding process employing wettability- reversal could find successful field application.

Immiscible displacement tests were performed in a consolidated sandstone core over the interfacial tension range from less than 0.01 to 5 dynes/cm to better define how interfacial tension (IFT) reduction can lead to increased oil recovery. Data obtained were displacement efficiency at breakthrough vs IFT for both drainage and imbibition conditions. These tests simulate water flooding under oil-wet and water-wet conditions, respectively. Results of the study have shown that displacement efficiency under both oil-wet and water-wet conditions can be markedly improved by a sufficient reduction in IFT. In the particular porous media used and for the low pressure gradients employed, the IFT must be reduced to a value less than about 0.07 dynes/cm to achieve increased recovery at the time of breakthrough of the injected phase. Below 0.07 dynes/cm, further small reductions in IFT result in large increases in displacement efficiency. Observed increases in recovery were obtained at pressure gradients which are well below those which can exist in the interwell area of a reservoir under water flood. The effect of pressure gradient on recovery is discussed. INTRODUCTION A residual oil saturation remains in rock which has been water flooded because, under usual reservoir conditions, the driving force which can be generated is inadequate to expel oil trapped by capillary forces. Since these capillary forces can be reduced by reducing the IFT a frequently studied method 1-5 of increasing oil recovery has been the use of surfactants to reduce the water-oil interfacial tension. Mungan1 observed improved recovery in both water-wet and oil-wet systems at 1.1 dyne/cm IFT, finding that the amount of improvement was greater in oil-wet systems. Moore and Blum,2 working with visual flow cell micro-models, concluded that recovery cannot be improved in water-wet systems by injecting surfactant solutions. They calculated that, for a pressure gradient of 1 psi/ft in their model, the IFT must be reduced to 0.03 dynes/cm to release oil trapped under water-wet conditions. Berkeley et al.3 indicated that, at representative field flooding rates, the IFT must be reduced to 0.001 dyne/cm to improve recovery. Thus, there is a wide variation of opinion about the IFT levels needed to improve recovery under field conditions. These previous investigators all have used as their experimental method the addition of surfactants to the injected water. The use of surfactant solutions to reduce IFT creates two experimental problems:the loss of surfactant through adsorption on reservoir rock can obscure the true IFT value which exists at the displacement front, andat very low values of IFT emulsification of oil and water commonly occurs. It is difficult, therefore, to determine whether the increased recovery is caused by IFT reduction as such, or is instead caused by emulsification. Also the properties of the available surfactants limit the IFT range which can be studied. In the present study the purpose is to better define how interfacial tension reduction can lead to increased oil recovery. A matter of great interest is the amount of recovery improvement potentially achievable in this way. The study was made using very low pressure gradients which was well within the range achievable by water flooding in the interwell region of petroleum reservoirs. A unique experimental approach was chosen to avoid adsorption and emulsification problems, and to allow convenient control of IFT. The fluid phases used were the equilibrium vapor and liquid phases of the methane-n-pentane system. The interfacial tension level was varied by changing the equilibrium pressure of the methane-pentane system over the range from 1,200 psia to near the critical pressure (2,420 psia). All tests were performed at a controlled temperature of 100F. The IFT-vs-pressure relationship for the methane-pentane system was based on the data of Stegemeier and Hough,6 and on new data obtained in this study. With this experimental approach it has been possible to study displacement at lower values of IFT than have been previously investigated.

Dedicated to Professor Gottfried Mark1 on the occasion of his 60th birthdayThe extremely rapid development of phosphaalkyne chemistry has focused above all on cycloaddition reactions and ligand properties.''I Of particular interest have been the cyclodimerization and cyclotrimerization reactions in the coordination sphere of metals. Thus, tert-butylphosphaacetylene 5 can be dimerized, for example, to give cobalt complexes containing diphosphacyclobutadiene (1 [' , 31) and diphosphabicyclo[l .I .O] butane ligands (Z14'). Complexes with cyclotrimer ligands derived from 1,3,5-triphosphabenzene 3IS1 and its Dewar derivatives 4161 are also known. + 1 2 3 4The use of metal complexes in cyclooligomerization reactions is not always necessary, as shown by the thermal cyclotetramerization of 5. Although most phosphaalkynes are not particularly thermally stable,['] the behavior of these species upon heating has not been studied.When neat phosphaalkyne 5 is heated for a long period of time at 130 "C and the product distilled, pale yellow crystals are obtained which exhibit a high melting point and are stable toward air. Elemental analysis and mass spectrometry revealed that the product is a tetramer of 5, compound 7. The stepwise decomposition of the tetramer to the monomer occurs upon electron-collision-induced fragmentation. ['] The N M R spectra of 7 support a tetraphosphacubane structure with alternating C and P atoms. The 13C{1H) N M R spectrum (100.62 MHz, C6D6) shows three multiplets at 6 = -29.07,21.57, and 30.64 (Fig. 1). These represent the [*I Prof. Dr. M. Regitz, Dip1.-Chem. T. Wettling, Dip1.-Chem. J. Schneider, Dip].-Chem. 0. Wagner, Prof.

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