This chapter presents the constitutive equations necessary to interpret laboratory and field data when both solid and pore volume evolve through time due to chemical and mechanical processes. The equations for the porosity evolution that are developed are generic, but the examples presented are acquired from chalk core studies. The processes at play when porosity is subject to change due to volumetric compaction and fluid-rock interactions when porous chalks are continuously flooded are presented here. As the overall solid mass is a conserved quantity, the void space is not. Constitutive equations are therefore required to estimate the time-evolution of the porosity. Laboratory triaxial tests were performed on high-porosity outcrop chalks from Obourg, Liegè, and Mons (Belgium). These tests are being compacted and continuously flooded with MgCl2 brine at elevated temperature and at high stresses. As calcite is replaced by magnesite, the overall mass and solid density change, thereby changing the volume of the solid. At the same time, the bulk volume is changing. Taking both effects into consideration, the pore volume evolution can be determined. We find that the porosity changes in nonintuitive ways as the relative importance of bulk compaction and chemical interaction may vary over time.
Part of the book: Porosity