Tracing Fluid Flow in Flooded Chalk under Long Term Test Conditions

Injection of MgCl2 into a Campanian chalk sample from Liege (Belgium) for enhanced oil recovery over 516d revealed mineralogical changes. A new mineral phase grew massively in the first centimeters of the tested core characterised by an increase of MgO about 90x the original composition and a depletion of CaO by more than 1/3 based on whole-rock geochemistry. Two new phases could be revealed so far in the tested core: magnesite and dolomite. MLA studies demonstrated that different Mg-enriched carbonate minerals could be observed in the different flooded slices of the chalk. This is of importance as O-isotope values are disturbed by the injected fluid and even reflect the injection temperature of 130°C when using δ18O values for temperature calculations. In this test, fluid injection and flow could not be monitored by REE. XRD analyses could only reveal the existence of magnesite. In contrast, O-isotope values show a strong variation and are most probably fixed in the newly formed mineral magnesite and dolomite. With these results it is possible to develop a fine tool to trace fluid flow, which changes the mineralogical composition of chalk. These results are of importance for peer groups involved in EOR processes.