Investigating Rock Material Impact on Low Salinity Water Flooding: Experimental Study with Chalk Samples of a Danish North Sea Oil Reservoir

Abstract

Abstract The low salinity water-flooding technique, an enhanced oil recovery (EOR) method, alters the initial crude oil/brine/rock (COBR) equilibrium, enhancing oil mobilization and potentially reducing CO2 footprint of oil production. Uncertainties arise with natural crude oils and reactive rock surfaces, such as chalk. A study on low salinity waterflooding in Danish Chalk reservoirs yielded results differing from existing literature. To further investigate these results, core flooding experiments were conducted using diverse chalk samples, including outcrop material, dry-stored reservoir core plugs, and preserved reservoir core samples. It's crucial to highlight that existing literature data predominantly relies on outcrop sample experiments, potentially limiting their relevance to reservoir conditions. Computed tomography (CT) images were captured before the experiments to assess rock integrity. Nuclear Magnetic Resonance (NMR) measurements post-experiments gauged the average wettability of the core plug. Contact angle measurements assessed wettability alteration, and ion chromatography analyzed the effluent brine samples. Coreflooding outcomes revealed that preserved core samples initially exhibited an oil-wet state, reflective of reservoir conditions, but underwent changes post-waterflooding and cleaning. This renders direct comparisons challenging, making these cores non-reusable. Conversely, dry-stored reservoir core plugs maintained consistent wetting states across multiple experiments, ensuring reusability and facilitating repeatable, comparable results. However, outcrop samples yielded fundamentally dissimilar results from reservoir samples, proving inadequate analogs. Contact angle measurements highlighted significant differences in wetting conditions and wettability alteration between outcrop and reservoir samples. This discrepancy explains why the negative impact of sulfate ions was evident only in reservoir samples. Ion chromatography results revealed calcite dissolution and concurrent magnesium precipitation during low salinity water injection. While extensively demonstrated in sandstones and some carbonates, the application of low salinity water flooding faces significant challenges in carbonate reservoirs, especially chalk formations. Moreover, the current reported findings regarding chalk samples rely on experiments conducted using outcrop samples. Consequently, there is a lack of experimental data on real reservoir material in existing literature. The conflicting outcomes compared to the literature underscore the significance of conducting experiments on actual reservoir material.