Samples of chalk are flooded with different brines to observe the reactivity of the material with determining and quantifying the mineralogical changes. The type of chalk, the composition of the fluid and the pressure and temperature conditions are varied to understand how these parameters impact fluid flow and compaction which surely is an important drive mechanism for enhanced oil recovery (EOR). Changes in mineralogy affect porosity and permeability and controls compaction of a rock. This compaction is, in itself, an important drive for increased production of oil and the rock-fluid interaction is also believed to have a positive effect on altering the wettability of the rock towards more water-wet. We chose on-shore chalk (from Belgium, Denmark, USA) to compare results with reservoir chalk and to prepare pilot studies in chalk reservoirs and a homogenized, artificial core of 99.95% CaCO3-powder with micron-like grain sizes. The use of MgCl2 brines injected under reservoir conditions (130°C; 10-14 MPa effective stress) into drilled chalk plugs (diameter: 3,8cm; length: 7cm) produced significant effects in terms of mineralogical changes in chalk, which are studied by optical petrography, X-ray diffraction, whole-rock geochemistry, C-O isotope geochemistry, Mineral Liberation Analyzer (MLA), conventional scanning electron microscopy coupled with energy dispersive systems (SEM-EDS) methodology, electron microprobe analysis, nano-secondary ion mass spectrometry, micro-Raman and tip enhanced Raman spectroscopy coupled with atomic force microscopy and transmission electron microscopy (TEM). Tests have been carried out from weeks to a length of three years. Generally, we observe a mineralogical change from calcite to magnesite. Processes of dissolution, precipitation and re-precipitation are beyond doubt recognised. Mineral growth takes place after a short time of flooding with nano-scale Mg-rich carbonates, mostly as magnesite. Crystal boundaries between magnesite and calcite are sharp even on nano-scale (TEM). MLA shows that the mineralogical changes take place in two stages, one causes a mixture of Mg-rich carbonates (or magnesite) and calcite, while a second stage changes gradually the entire sample to nearly pure magnesite. Whole-rock geochemistry, TEM-EDS show that still c. 4wt.% of CaO is left in the altered chalk even after three years testing. MLA showed that the type of paleontological material alters in different velocities from calcite to Mg-rich carbonate (respectively magnesite) and that fossil debris hampers fluid flow. The observed mineralogical changes have a significant effect on porosity calculations, which implies that estimations of porosity in chalk without taking mineralogical changes into account are misleading. Results also showed that micro-Raman spectroscopy is capable of estimating MgO concentration in carbonates and identifying new grown mineral phases in a very quick, non-destructive, cheap, and effective way and that c. 4wt.% of CaO is left in the entirely altered chalk after three years of flooding.