ABSTRACT:

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").

INTRODUCTION

The concept of injecting low salinity water into an oil reservoir is not a new topic. Jadhunandan (1990) has shown that injection of brine can lead to improvement of oil recovery in sandstone oil reservoirs. Webb et al. (2004) and Seccombe et al. (2008) have demonstrated the effect in the laboratory and in field tests. Initially, the mechanism was not very clear. Lager et al. (2008) give a very comprehensive overview of the historical development of a deeper understanding of the effect. Since the effect occurred on sandstone but not in fired or acidized sandstones, Tang and Morrow (1996) and Lager et al. (2006) concluded that the effect is related to the presence of clays. Lager et al. (2006) give a comprehensive overview of the different mechanisms including the most relevant bibliography. Lager et al. (2006, 2008) warn that some of the above listed mechanisms are rather effects than causes like pH induced IFT reduction, which has been ruled out as cause by Ligthelm et al. (2009). In Lager et al. (2006) and most of the other studies, the effects of low salinity flooding are studied in core flooding experiments where observations are indirect via an increase of oil production in a recovery curve. Many microscopic mechanisms would be seen macroscopically from a reservoir engineering perspective as wettability modification. Jerauld et al. (2006) argue that the effect of low salinity flooding on a recovery curve can then be modeled as a modification of the relative permeability and Maas et al. (2001) had already proposed a modification of the capillary pressure saturation functions.

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