Chemo-mechanical evolution of pore space in carbonate microstructures upon dissolution: linking pore geometry to bulk elasticity

Abstract

One of the challenges faced today in a variety of geophysical applications is the need to understand the changes of elastic properties due to time-variant chemo-mechanical processes. The objective of this work is to model carbonate rock elastic properties as functions of pore geometry changes that occur when the solid matrix is dissolved by carbon dioxide. We compared two carbonate microstructures: porous micrite (“mudstone”) and grain-supported carbonate (“packstone”). We formulated a mathematical model that distinguishes the effects of micro- and macro- porosity on stiffness changes. We used measures of mechanical and chemical porosity changes recorded during injection tests to compute elastic moduli and compare them to moduli obtained from wave velocity measurements. In mudstones, both experimental and numerical results indicate that bulk moduli change by less than 5%. The evolution of elastic moduli is controlled by macropore enlargement. In packstones, model predictions under-estimate changes of elastic moduli with total porosity by 10% to 80%. The total porosity variation is 60% to 75% smaller than the chemical porosity variation, which indicates that pore expansion due to dissolution is counter-balanced by pore shrinkage due to compaction. Packstone elastic properties are controlled by grain sliding. The methodology presented in this paper can be generalized to other chemo-mechanical processes studied in rocks, such as dislocations, glide, diffusive mass transfer, recrystallization and precipitation.