Abstract:

For carbonate reservoirs, it is common that oil recovery is relatively low and water cut rises quickly due to the complex underground conditions, such as a wide range of fractures and caves. Low salinity waterflooding is an enhanced oil recovery （EOR） technique with low cost and high effects, and it has been extensively researched since 1990s. The stimulation mechanism, laboratory methods and field operation conditions for low salinity waterflooding of carbonate reservoir were analyzed. Experimental results reveal that wettability alteration and enhancement of connectivity among different pore systems are responsible for the improved recovery. The wettability alteration can be attributed to surface charges alteration or microscopic dissolution of minerals, which is in fact the reaction between main determining ions （SO₄^2-，Ca²⁺ ，Mg²⁺ ）and rock surfaces. However, the enhancement of connectivity among different pore systems is caused mainly by microscopic dissolution of minerals. Numerous laboratory methods are applied to obtain the main mechanisms for low salinity waterflooding on carbonate oil reservoirs, such as coreflooding study, imbibition test, interfacial tension measurement, contact-angle measurement, nuclear magnetic resonance technique, ion analysis and Zeta potential measurement. When performing low salinity waterflooding, higher oil recovery will be achieved if ions composition can be optimized. In addition, the combination of low salinity waterflooding and polymer flooding will also remarkably improve oil recovery. In China, there is large number of carbonate reservoirs, therefore, it is of significance to conduct theoretical and applied researches related to low salinity waterflooding.

Key words: multiple regression , curve reconstruction , model optimization , Engineering application , nonlinear models