Polymer-enhanced low-salinity brine to control in situ mixing and salt dispersion in low-salinity waterflooding

Although viability of low-salinity waterflooding (LSWF) at the laboratory scale has been proven, there are some challenges associated with its field application, which sheds uncertainties on its economic success. One of the challenges is the minimum required volume of low-salinity water, which should be injected to the reservoir due to the salt dispersion in porous media. Once the low-saline brine is injected into the reservoir, mixing of injected (low-salinity) and resident (high-salinity) brines occurs and the developed mixing zone grows continuously as the front moves from the injection well toward the production well. Increase in the salinity of the front reduces the efficiency of LSWF. In this paper, we demonstrate experimentally that if low-salinity brine is augmented with a small amount of a polymer (as a viscosifying and mobility control agent), salt dispersion can be significantly suppressed. In this regard, a systematic series of single-phase sandpack flooding experiments was designed and performed. The impacts of salinity of resident high-salinity brine, salinity of low-salinity brine, and polymer concentration on mixing (dispersion) control were investigated. Analytical and numerical simulation methods were implemented to analyze the experimental data and infer dispersivity. The results show that adding 200 ppm of partially hydrolyzed polyacrylamide (HPAM) to the injection brine reduces the dispersivity by more than 70%. Once the dispersivity is reduced, the salinity profile becomes sharper; thus, significantly less volume of low-salinity brine will be required to establish low-salinity conditions in the whole core. Additionally, the analysis of variance shows that polymer concentration and salinity of high-salinity brine are the main factors affecting the dispersivity. The total salinity of low-salinity brine was found to be comparatively less important.