Pore-scale imaging and analysis of carbonate dissolution during reservoir-condition CO2-acidified brine flow : influence of chemical and physical heterogeneity

The sequestration of carbon dioxide (CO2) into geologic formations is one of the long-term solutions proposed to mitigate atmospheric CO2 concentrations [1]. Potential storage sites include deep saline aquifers and depleted hydrocarbon reservoirs. In the case of carbonate reservoirs, the possibility of significant dissolution reaction taking place between CO2, in situ brines and rock is very real. Proposed in this thesis is a method to dynamically image reactive transport in carbonates at subsurface conditions, representing the movement of CO2 saturated brine in the reservoir. The work will focus on understanding reactive transport in carbonate reservoirs by imaging changes in the pore structure, porosity and permeability in representative rock samples induced by the simultaneous flow and reaction of CO2 saturated brine at reservoir conditions. X-ray micro-tomography will be the imaging tool of choice to investigate pore structure changes during reactive transport experiments. Effluent from the dissolution reactions will be collected for analysis via inductively coupled plasma mass spectrometry (ICP-MS) to monitor the preferential dissolution of different minerals in the rock. Direct simulation on 3D images from micro-tomography (μCT) and network modelling tools will be used for further analysis of the rock property evolution. In this thesis we study dissolution of carbonate minerals with an increasing level of complexity to observe the effect of chemical and physical heterogeneity. We start with a single mineral dolomite case where we investigate the impact of rock heterogeneity and flowrate on reaction rates and dissolution dynamics. Next we study dissolution in a chemically heterogeneous medium consisting of two minerals with contrasting initial structure and transport properties, but with uniform spatial distribution. Finally, we studied a heterogeneous reservoir sample consisting of dolomite and calcite (having the ratio 8 to 1) which had a non-uniform spatial distribution.