Time-lapse acoustic, transport, and NMR measurements to characterize microstructural changes of carbonate rocks during injection of CO2-rich water

We investigated how changes induced in the microstructure of carbonate rocks by the injection of CO2-rich water affect pore-network properties. In particular, we investigated from multiple perspectives the microstructural changes and types of porosity that alter the observable geophysical properties. We thereby refined our understanding of induced modification of the pore network. Our experimental protocol included a suite of time-lapse acoustic, transport, and nuclear magnetic resonance (NMR) measurements, along with scanning electron microscopy (SEM) and CT-scan images; these gave us complementary sensitivity to changes in different properties of the pore space. Induced porosity variations were smaller than in previous reported results because of chemomechanical compaction resulting from dissolution under pressure. No porosity enhancements larger than 0.8 pu were observed. Results indicated that dissolution occured primarily in the grain-coating cement and the microporosity of the micritic phase. Both caused the formation of cracklike pores around larger grains leading to a more compliant frame, causing both. velocity reductions and an increased sensitivity of velocity to pressure. Chalky micritic facies exhibited velocity reductions of similar to 9%, whereas micritic limestones, less. prone to compaction and grain sliding, experienced smaller velocity reductions (similar to 5%). Because porosity enhancement was minimal, we hypothesized that the reductions were due to injection-induced reduction of grain-contact stiffness. Dissolution-induced compaction played an integral role also in the permeability response during injection. Compaction in pressure-sensitive chalky facies strongly counteracted the effects of dissolution, leading to negligible permeability and NMR response changes. In contrast, stiff micritic limestones with little dissolution-induced compaction exhibited larger permeability increases (>100%). This work demonstrated the advantages of utilizing a suite of concurrent and independent measurements to build a more comprehensive interpretation of microstructure changes induced by injecting fluids that are in chemical disequilibrium with the host formation.