Oil Recovery by Alkaline Waterflooding

Abstract

When oil containing organic acids is flooded with alkaline water, the result can be a high oil recovery efficiency, provided a bank of viscous oil-in-water emulsion forms in situ. The amount of additional oil recovered depends on the pH and salinity of the water and the type and amount of organic acid it contains, as well as on the amount of fines in the porous medium. Introduction The method described in this paper is based on the fact that organic acids, naturally occurring in some crude oils, will react with alkaline water to produce soaps at the oil/water interface. The soaps thus formed lower the interfacial tension between crude oil and flood water by a factor of several hundred, and under the proper conditions of salinity, pH, and temperature, they change the wettability of the porous medium to preferentially oil-wet. When the proper alkaline water and acidic oil flow simultaneously in a porous medium, a viscous oil-external emulsion is formed. The flow properties of this type of emulsion permit a high, nonuniform pressure gradient to be permit a high, nonuniform pressure gradient to be generated across the narrow region in the vicinity of the emulsion front. The pressure gradients are sufficient to overcome the reduced capillary forces and displace the oil from the pore space. The displacement efficiency can be much improved over ordinary waterflood efficiencies. Other techniques of flooding with alkaline water to increase oil recovery have been reported in the literature. Almost 50 years ago, Nutting, proposed flooding an oil reservoir with alkaline sodium carbonate solutions, after it was observed that those solutions could remove oil from glass and silica surfaces by decreasing the wetting of the surface by oil. In more recent years, Leach et al. have reported the use of alkaline water to cause a reversal of wetting conditions in some naturally oil-wet reservoirs. Reisberg and Doscher also used alkaline water containing a surface-active chemical in some of their tests. The alkaline waterflooding method described in this paper differs in several ways from the other processes just mentioned. In this process, the alkaline water must be saline rather than fresh. The use of saline water causes the sand to be made oil-wet in the presence of the alkaline water. High salinity also leads to the formation of a water-in-oil type of emulsion, which does not form in the other processes. The alkaline water process can be considered primarily in reservoirs where the crude oil contains primarily in reservoirs where the crude oil contains organic acids. The most common organic acids in crude oil are naphthenic acids., The acid content of a crude oil tends to be higher when the base composition of the crude is high in naphthenic compounds. in cases where acid concentration is low, a bank of oil containing organic acids could be injected into a reservoir and followed by alkaline water. Mechanism of Displacement by Alkaline Water The mechanisms active at the front where alkaline water is displacing acidic crude oil includea drastic reduction of oil/water interfacial tension,wetting of the matrix grains by oil,formation of water drops inside the oil phase, anddrainage of oil from the volume between alkaline water drops to produce an emulsion containing very little oil. JPT P. 1365