Upscaling calcite dissolution rates in a tight reservoir sandstone

Calcite is a highly abundant mineral in the Earth's crust and occurs as a cement phase in numerous siliciclastic sediments, where it often represents the most reactive component when a fluid percolates through the rock. Hence, the objective of this study is to derive calcite dissolution rates on different scales in a reservoir sandstone using mineral surface experiments combined with vertical scanning interferometry (VSI) and two types of core plug experiments. The 3D geometry of the calcite cement phase inside the rock cores was characterized by X-ray micro-computed tomography (mu XCT) and was used to attempt dissolution rate upscaling from the mineral surface to the core scale. Initially (without upscaling), our comparison of the far-from-equilibrium dissolution rates at the mineral surface (mu m-mm-scale, low fluid residence time) and the surface normalized dissolution rates obtained from the core experiments (cm-scale, high fluid residence time) revealed differences of 0.5-2 orders of magnitude. The mu XCT geometric surface area connected to the open pore space (GSA(Cc,open)) considers the fluid accessibility of the heterogeneously distributed calcite cement that can largely vary between individual samples, but greatly affects the effective dissolution rates. Using this parameter to upscale the rates from the mu m- to the cm-scale, the deviation of the upscaled total dissolution rates from the measured total dissolution rates was less than one order of magnitude for all investigated rock cores. Thus, GSA(Cc,open) pen showed to be reasonably suitable for upscaling the mineral surface rates to the core scale.

Automated summary

This study aims to derive calcite dissolution rates in a reservoir sandstone by combining mineral surface experiments with scanning interferometry and core plug experiments. The 3D geometry of calcite cement phase inside rock cores was characterized using X-ray micro-computed tomography and was used to upscale the rates from mineral surface to core scale. The results show that the upscaled total dissolution rates were reasonably accurate, indicating the potential suitability of this method for upscaling the mineral surface rates to the core scale.