Low Salinity Flooding: Experimental Evaluation and Numerical Interpretation

Abstract

Abstract Low Salinity Flooding (LSF) is an emerging technology to improve oil recovery for both sandstone and carbonate reservoirs. Extensive laboratory experiments investigating the effect of LSF are available in the literature. To quantify the low salinity effect, spontaneous imbibition and/or tertiary waterflooding experiments have been reported. In only a few published cases, the experimental data was interpreted using numerical simulation to derive relative permeability curves for both low and high salinity water, to be used in field simulation. A critical review of the literature data shows a wide spread in the LSF response in both pressure and recovery. Moreover, most of the flooding experiments reported in the literature are performed at a low flow rate, of ~1 ft/day, which may lead to a significant capillary end effect and, consequently, to a possible overestimation of the LSF effect. The focus of this paper is on: 1- The experimental procedures used for proper evaluation of the LSF effect; 2- Reporting experimental data performed on sandstone samples in both tertiary and secondary mode waterflood; 3-The numerical interpretation of the laboratory data to obtain relative permeability and capillary pressure curves for both high salinity (HS) and low salinity (LS) water, to be used in reservoir simulation to quantify the benefit of LSF on reservoir scale and 4- Investigating whether the tertiary flooding experiments can be used to derive relative permeability curves for both HS and LS waterflooding. The main conclusions of the study are: 1- While spontaneous imbibition (SI) experiments could provide an indication of a potential low salinity effect, they are not sufficient to quantify the effect in flooding experiments; 2- The LSF effect measured during low rate flooding experiments (i.e., field rate) is not representative for the field scale as it is usually dominated by capillary end effect. Therefore, the low rate (raw) coreflood data will suggest a larger LSF benefit than would actually be the case; 3- The tertiary mode experiments cannot be used to derive the LS relative permeability curves as it only spans a narrow saturation range during LSF and 4- Both tertiary and secondary mode corefloods performed using multi-rates are required to obtain relative permeability curves for HS and LS water. 1. Introduction Low salinity waterflooding (LSF) is an emerging technology in which the salinity of the injected water is optimized to improve oil recovery over conventional waterflooding. Over the last two decades several groups have published laboratory and field data which show extra oil recovery upon injection of LS water. However, a wide range of responses in the extra oil recovery is reported in the literature, from 0 to more than 20%. To extrapolate laboratory results to the field scale and to separate several potential underlying LSF mechanisms, measuring and accounting for the pressure drop over the core during flooding experiments is essential. In a number of cases reported in the literature no pressure data was shown. In the cases when the pressure data was available, a wide range of pressure responses was observed. In some cases a pressure increase was observed once low salinity was injected. The effect of such pressure increases on laboratory results was rarely discussed.