DIRECT EXPERIMENTAL EVIDENCE OF WETTABILITY MODIFICATION BY LOW SALINITY

摘要

It has been observed by several parties in the past years that injection of low salinity water leads to the production of additional oil compared to injection of water at reservoir salinity or higher. While this process is already applied in the field, so far the microscopic mechanism is not fully clear. A mechanism proposed by several researchers is that low salinity water leads to a wettability change of the sandstone rock, which then causes release of oil that previously was attached mostly to clay minerals. So far all experimental evidence for this mechanism is macroscopic and indirect, i.e. in the form of an increase in produced oil. The main motivation for this study is to provide direct evidence and to directly visualize detachment of crude oil from clay minerals. In a small lab scale experimental study in a flow cell we observed the release of crude oil from a substrate covered with solidly attached clay particles when changing the brine from high salinity to low salinity. For very low salinity we observed a massive release of crude oil (up to 80%) but also a decomposition of the Montmorillonite clay minerals and release of fines (formation damage). For a brine composition with somewhat higher salinity and in addition presence of divalent ions, we observed release of oil but only insignificant amount of clay swelling ("controlled formation damage").