Experimental investigation of changes in petrophysical properties during CO2 injection into dolomite-rich rocks

Carbon dioxide may be injected into an underground geological structure for mere geo-sequestration purposes or as a means of enhanced hydrocarbon recovery. During such an operation, CO2 is expectedto dissolve in the in-situ fluids (primarily consisting of brine) generating a reactive in-situ solute (i.e. carbonated brine). Subsequently, a series of consecutive chemical reactions may occur between the solute and the host rock. While these reactions are generally known from a qualitative perspective, to what extent they may impact on the host formation's petrophysical properties requires extensive evaluation on a case by case basis. Due to the presence of highly reactive minerals in their composition, carbonate rocks (e.g. dolostone) present a more complex system to evaluate in terms of the above mentioned chemical reactions. This experimental study has been carried out to evaluate changes in the petrophysical properties of a number of heterogeneous dolostone samples after undergoing carbonated brine flooding under in-situreservoir conditions. In this study, the core-flood experiments are complemented by pre-and post-floodporosity, permeability and NMR (nuclear magnetic resonance) measurements, X-ray CT scanning and Xray Diffraction (XRD) and Energy-Dispersive X-ray (EDX) analysis. Overall, a slight increase in the porosity was observed in most samples, most likely, caused by the dissolution of dolomite (CaMg(CO3)(2)), calcite(CaCO3) and/ or anhydrite (CaSO4). The results also show an increase in the permeability of some samples which again could be attributed to dissolution of the minerals. The X-ray CT images show signs of excessive dissolution of minerals and the creation of dissolution patterns (i.e. wormholes). On the other hand, reductions in permeability and porosity by 57% and 12%, respectively, were also observed in a sample. This is believed to be due to the combined effects of the mineral precipitation and mechanical compaction mechanisms dominating over the mineral dissolution. A small shift in the pore size distribution of the samples towards smaller pore sizes was also observed which is believed to have been caused by mechanical compaction. (C) 2017 Elsevier Ltd. All rights reserved.