A New Method to Quantify Carbonate Rock Weathering

The structure and composition of carbonate rocks are modified greatly when they are subjected to phenomena that lead to their weathering. These processes result in the production of residual alterite whose petrophysical, mechanical, and hydrological properties differ completely from those of the unweathered rock. From a geotechnical perspective, it is important that such changes are fully understood as they affect reservoir behavior and rock mass stability. This paper presents a quantitative method of calculating a weathering index for carbonate rock samples based on a range of petrophysical models. In total, four models are proposed, each of which incorporates one or more of the processes involved in carbonate rock weathering (calcite dissolution, gravitational compaction, and the incorporation of inputs). The selected weathering processes are defined for each model along with theoretical laws that describe the development of the rock properties. Based on these laws, common properties such as rock density, porosity, and calcite carbonate content are estimated from the specific carbonate rock weathering index of the model. The propagation of measurement uncertainties through the calculations has been computed for each model in order to estimate their effects on the calculated weathering index. A new methodology is then proposed to determine the weathering index for carbonate rock samples taken from across a weathered feature and to constrain the most probable weathering scenario. This protocol is applied to a field dataset to illustrate how these petrophysical models can be used to quantify the weathering and to better understand the underlying weathering processes.