Possible Mechanisms for Oil Recovery from Chalk and Sandstone Rocks by Low Salinity Water (LSW) Available to Purchase

Low salinity water flooding is gaining much of attention for its potential in increasing oil recovery, in spite of the debatable working mechanism for both sandstone (SS) and chalk. Various mechanisms have been suggested in literature.

The objective of this work is to address oil/brine/rock (COBR) interaction and deduce thermodynamically possible product of the interaction, then verify with the experimental results. The experiments were designed to have most of the flooding with LSW as a secondary recovery method following seawater (SSW). This is to mimic the situation in most oil fields, which gone through seawater flooding and to contribute to the debatable discussion on the rule of LSW to enhance oil recovery as secondary or primary fluid.

From previous work in our laboratory, sulfate and magnesium have been identified as active ions in the seawater in altering chalk wettability to more water wet; hence they were also tested as single ions water flooding and imbibing fluids for both SS and chalk. This approach has contributed to our understanding of the possible reactions that occurs due to COBR.

From the SS part of the experiments, the results show indications of ion exchange, mineral dissolution processes and rock weakening causing fine migration. Mineral dissolution and ion exchange are not unexpected in presence of mineral such as, in general, kaolinite. The experimental results confirmed the simulated reaction between LSW and kaolinite from the increase of the pH and the resulted ions. In addition, the increase of the pressure drop detected during the flooding, could be related to the fine migration in addition to visual observation.

From the work performed on chalk, the results indicate fine detachment, which was enhanced in presence of Mg²⁺ ions as imbibing fluid at elevated temperature (70°C) for chalk. An increase of the pH was also observed for all flooding and spontaneous imbibition tests, especially with SO₄^2- (for SS and chalk). In contrast to SS, chalk flooded with LSW showed reduction of pressure drop across the cores.