Contribution of Feldspar Minerals to pH during Smart Water EOR Processes in Sandstones

Abstract

It has previously been suggested that a local pH increase at pore surfaces is responsible for desorption of polar oil components and wettability alteration in sandstone reservoirs, leading to increased oil recovery during smart water or low salinity injection. The pH increase can be a result of cation exchanges at clay surfaces or at other mineral surfaces present in sandstone reservoirs, such as feldspars, which is the topic of this paper. In this study, static batch tests and dynamic sandpack flooding tests have been performed to investigate the pH development in the brine phase in contact with three common feldspars, Ab-feldspar (albite), An-feldspar (anorthite), and Or-feldspar (microcline). Temperature varied between 23 and 130 °C, and brine salinity varied from 0 ppm (deionized water) to 100 000 ppm NaCl in batch tests. The sandpacks were composed of 10 wt % feldspar and 90 wt % quartz and flooded with 100 000 and 1000 ppm brines. In all tests, equilibrated brine pH was monitored to study cation-exchange reactions between the feldspars and the brine phase. The main results in this study showed that all three feldspars tested, Ab-feldspar, An-feldspar, and Or-feldspar, caused a pH increase in the brine phase at all temperatures and salinities tested and can therefore influence pH during waterflooding of sandstone reservoirs. There was a general decrease in ion exchange and pH increment when the test temperature increased for all three feldspars. The least stable feldspar, or the most reactive feldspar, in cation-exchange processes seemed to be An-feldspar, causing the highest pH at most conditions, also being least affected by increasing brine salinity.