Abstract

Low-salinity waterflooding (LSW) and its effect on oil recovery factor have been studied around the world during the last 30 years. Low-salinity/fresh water was used in the initial stage of waterflooding because of deficiency of produced water. The Pervomaiskoye oil field in Republic of Tatarstan is one of the examples. Waterflooding started in 1966. Water was taken from the nearest water source, the Kama River. After more than 40 years, about two PVI has been injected, and 30.1% of this injected water was low-salinity.

This paper presents the results of laboratory study of double core flooding using high-salinity (formation) and low-salinity (river) water. Two plugs were taken with the clay content of about 0.8% and 0.1% and the permeabilities of 432 and 291 mD, respectively. Dead oil was used with the viscosity of 37.5 mPa.s. TDS values were about 252,738 ppm for high-salinity water and 848 ppm for low-salinity water. Dry and vacuumed plugs were saturated, first, with high-salinity water, then, with oil, to simulate connate water saturation; then, oil was displaced with the same water. After this, the cores wre restored to the connate water by a secondary oil flood, and finally, LSW was simulated. In both cases, oil recovery was recorded and fines release was analyzed.

Processing of the low-salinity corefloods data by a numerical model shows a two-fold decrease of water relative permeability for the first plug and 1.5 times reduce of residual oil saturation for the second plug. Fines release and formation damage was observed in the first plug and wettability alteration in the second plug. Oil relative permeability didn't change. Also, the unusual behavior of water relative permeability in response to low-salinity waterflood was observed, namely, it decreased at water saturation values more than 0.4.

To investigate the low-salinity EOR effect seven pilots on the Pervomaiskoye field were analyzed, with high-salinity waterflooding following LSW. Incremental oil recovery for these pilots, that has been achieved by now, is approximately 5-9%. Calculations of low-salinity EOR effect in the field show that LSW through 96 injectors ensures to date the incremental recovery factor of 3-4%.

The reservoir 3D model of five-spot pattern for the Pervomaiskoye field for fines-assisted waterflooding gave incremental oil recovery by 5-11%. The laboratory relative permeabilities were used in the model. Incremental oil recovery depends on the number of the flooded layers.

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