Dissolution rate variability of sandstone calcite cement

For a holistic understanding of the long-term usage and safety analysis of reservoir rocks it is crucial to understand the fundamental mineral reactions and its control mechanisms. Kinetic quantification of the processes involved with fluid-rock interactions are especially important for predicting the evolution of pore space in rocks subjected to fluid injection, such as in CO2-sequestration and hydrocarbon exploration techniques.
The calcite cement selected for this study belongs to a fluvial-aeolian Rotliegend succession exposed near Bebertal (Flechtinge High, Germany) that was deposited in the same conditions as those that form the prolific gas reservoirs of the Southern Permian Basin1,2. Optical microscopy, SEM-BSE images and Cathodoluminescence analysis of the unreacted samples showed that two types of cement were present, although the calcite cement patches were composed of single crystals. We hypothesized that these different types of cement would react differently to fluid input. We used polished thick-sections of plug samples for dissolution experiments in a flow-through cell using a 2 mmol Na2CO3 solution (pH = 8.6, T≈ 21°C), for 7 reaction intervals (3 to 32 hours). Before and after each experiment the sample’s topography changes were mapped using a vertical scanning interferometer (VSI). High-resolution surface maps are subsequently used to calculate surface dissolution rates3.
After experiments, VSI images revealed an increase of the surface roughness in the cement patches. Detailed analysis of the rate dissolution variability in between the calcite cement patches and the intravariability of each cement patch related to chemical composition variability in the samples will be presented.
1Fischer, C., Gaupp, R., Dimke, M., Sill, O., 2007. A 3D high resolution model of bounding surfaces in aelian-fluvial deposits: An outcrop analogue study from the Permian Rotliegend, Northern Germany. Journal of Petroleum Geology, 30(3), 257–273.
2Fischer, C.; Dunkl, I.; von Eynatten, H.; Wijbrans, J. R.; Gaupp, R., 2012. Products and timing of diagenetic processes in Upper Rotliegend sandstones from Bebertal (North German Basin, Parchim Formation, Flechtingen High, Germany). Geological Magazine, 149 (5), 827-840.
3Luttge, A., and Bolton, E., 1999. An interferometric study of the dissolution kinetics of anorthite : The role of reactive surface area. American Journal of Science, 299, 652–678.