Low salinity waterflood (LSF) is a promising improved oil recovery (IOR) technology. Although, it has been demonstrated that LSF is an efficient IOR method for many sandstone reservoirs, the potential of LSF in tight oil reservoir is not well-established. This paper presents a systematic evaluation of the potential of low salinity waterfloding for the tight reservoirs in Jiyuan Oilfield, China. This investigation pushes the application envelope of low salinity waterflooding towards the reservoir with low permeability (lower than 0.5mD), formation salinity of up to 45,180ppm, reservoir temperature of 70°C and in-situ oil viscosity of 0.6 cp.
Our laboratory evaluation included zeta potential tests for interface of oil/brine and brine/rock, thermodynamic analysis through disjoining pressure calculation, corefloods using representative core samples.
Thermodynamic analysis showed that decreasing divalent cations and salinity makes the electrical charges at both oil/brine and brine/rock interfaces become strongly negative, which enhanced the repulsive forces between oil and rock due to the double electric layer expansion. As a result, the rock turns more water-wet. Secondary corefloods were conducted with two different brines, which include shallow aquifer water and ion tuning water with consideration of field application. Coreflooding Experimental results were history matched to obtain the relative permeability curves. Results showed that compared to shallow aquifer water, low-salinity water exhibited a higher oil relative permeability and lower water relative permeability at the same water saturation and a lower residual oil saturation to water.
Laboratory results were input into a reservoir simulator to investigate the potential of low-salinity water flood in Jiyuan oilfield. It showed that suitably formulated ion tuning water (ITW) has the potential to accelerate oil production and improve displacement efficiency, thus resulting in a higher recovery factor with only a fraction of pore volume of low-salinity water injected.
To conclude, this paper demonstrates that ITWF has a good potential as an IOR/EOR technology in tight reservoirs, the key points are described as follows. Firstly, the mechanism of ITWF was interpreted by thermodynamics of wettability. Secondly, laboratory experiments have shown that ITWF could improve oil recovery by accelerating the oil production rate and decrease the residual oil production. Thirdly, the potential of ITWF in a tight oil reservoir in Jiyuan oilfield is investigated using a mechanistic model based on input data of laboratory experiments.